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DEVELOPMENT OF CONTINUOUS WAVE AND MODE LOCKED

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1. Figure 2 2 Loss modulation for active mode locking 2 2 1 2 Passive mode locking Passive mode locking is the most common method to generate ultrashort pulse Since it uses the pulse to modulate itself therefore it has some advantage Firstly it required no external synchronization Secondly the response of the modulator can be extremely fast Passive mode locking is acquired by inserting a saturable absorber into the laser cavity preferably close to one of the mirrors as shown in Figure 2 3 A saturable absorber is a medium whose absorption coefficient decrease as the intensity of light passing through it increase Thus it transmits intense pulse with relatively little absorption and absorb weak one When a saturable absorber is used to mode lock a laser the laser is simultaneously Q switched 12 saturable absorber gain y f Figure 2 3 Schematic of insertion a saturable absorber in cavity Haus 2000 2 2 1 3 Kerr Lens mode locking Kerr lens mode locking KLM is a process based on the nonlinear effect of self focusing This effect will produce an intensity dependent change in the refractive index of material When a Gaussian beam passing through a material with the beam more intense at the center than the edge the index of refraction of the material will become higher at the center Aschom 2003 Therefore creating a lens which is focused the beam within the material such as shown in Figure 2 4 This process will
2. Figure 2 9 Prism pair operation In this project prism pair was used as a correction for Group velocity Dispersion GVD to reduce the effect of first order dispersion in the Ti sapphire laser The method was based upon the earlier work of Fork et al Pearson and Whiton 1993 The dispersion of the cavity d P d is given in Equation 2 4 Taal Stan S ine 2 all Where is the distance between the apexes of the prism n is the refractive index of the prisms is the free space wavelength of interest and p is the propagation angle of a ray with respect to a reference line drawn between the apexes of the two prisms Distance required for correction GVD passing through the length of material in this case Ti sapphire crystal Distance is calculated by dividing dispersion in cavity 20 d P d by the product of second order dispersion of the Ti sapphire crystal d Nerf and the thickness of the crystal t Then Equation 2 5 was produced d coe d P 2 t 2 d d 2 5 By solving the Equation 2 4 and 2 5 with n 1 71125 dn dd 0 04958 um d n di 0 1755 pm for prism made of SF10 glass at wavelength of 800 nm t 13 mm and Ad Nell 0 1745 um The index refraction and properties for sapphire were shown in Appendix A From this calculation the prism pair separation is determined as 19 cm 2 4 Laser oscillator Basically a laser consists of a pumped amplifying medium positi
3. Overall experimental setup of Ti sapphire laser 33 3 3 General Setup Prior to operate the laser either in continuous mode or pulse mode several major alignments need to be discussed This including the alignment of pumping source focusing beam linear cavity and cooling system 3 3 1 Pumping Source Alignment The first step to develop Ti sapphire laser is to aligned a diode pumped solid state laser as a pumping source In this alignment two high reflective mirrors at 532 nm known as pumped mirrors PM1 and PM2 were used to locate the pumping beam The beam can be positioned easier by using two mirrors rather than just one mirror Only small power of DPSS laser was operated during this alignment that is about 2 W The mirror PM1 was placed at an angle upon DPSS laser beam such as shown in Figure 3 2 The beam was reflected to the second mirror of PM2 The output of the DPSS laser is in vertical plane of polarization The polarization beam was rotated to the horizontal plane in order to prepare a Brewster angle condition of the Ti sapphire crystal Thus a polarization rotator PR was employed and placing into the beam path after PM2 The best condition for the beam to become horizontal is identified when only a small portion of the reflected beam from PR was detected near the DPSS laser window It is better to note that the beam from the PM2 must always at horizontal or having the same level This condition can be achieved by using tw
4. 14 The self focusing also describes as self phase modulation SPM and often termed the Kerr effect Kuhn 1998 As discussed before as the intensity of the pulse increases the index of refraction of the material will increase and the pulse focused Temporal SPM is time dependent phase shift that occurs as the pulse sweep through the dispersive material The rising intensity on the front edge of the pulse increases the index of refraction This will delay the individual oscillation and thus red shift the rising edge The reverse effect occurs on the trailing edge Thus temporal SPM chirp the pulse For ultrashort pulse generation the round trip time in the resonator for all frequency component of the light must be the same Otherwise frequency components with different phase shift will longer add coherently and the mode locking will break down In the normal laser operation temporal SPM will cause a red shift of the pulse and the Group velocity dispersion GVD will also cause a red shift on the pulse Thus in order to achieve transform limited pulse width it is necessary to incorporate some type of dispersion compensation that blue shifts the pulse Prism pair commonly used to introduce the GVD compensation 2 3 Dispersion Dispersion is the phenomenon that happens to the light wave and also sound wave Through this phenomenon the wave become separated into spectral component with different frequencies In the ultrashort pulse genera
5. By solving the Equation 2 9 and 2 10 we can get the angle is 9 arccos C VC 1 2 11 where C t n 1 n 1 2 12 nR Where t and n are the thickness and the refractive index of the Brewster angled medium R is the radius of curvature of the folding mirrors With refractive index of n 1 76 a 28 thickness of t 13 mm and curvature radius of R 100 mm From the calculation is obtained as 19 24 2 5 Optical Pumping Energy input by pumping Input Output Amplifying medium p Figure 2 16 Optical pumping In laser terminology the process of energizing the amplifying medium is known as pumping Pumping an amplifying medium by irradiating it with intense light is referred to as optical pumping Figure 2 16 The source of pumping can be flashlamp or other laser In this project Diode Pumped Solid State DPSS laser was used as a pumping source using End Pumping Method When the laser material is pumped by certain pumping source the light will be absorbed by the active medium This process will lead to the emission in the laser material such as shown in Figure 2 17 29 Before After Before After Co M A O E i atom atom atom atom a C SCL A Absorption Emission Figure 2 17 Absorption and emission process There are two types of the emissions which are spontaneous emission and stimulate emission For the spontaneous emission the direction and phase of the emitted ph
6. 8 2 9 2 10 LIST OF FIGURES TITLE The improvement of ultrashort pulse generation Schematic of a modulator insertion in cavity Loss modulation for active mode locking Schematic of insertion a saturable absorber in cavity Kerr Lens mode locking process a Physical aperture and b gain aperture The effect of the dispersion to the pulse Dispersion due to slab geometry Grating pair operation Prism pair operation Laser oscillator PAGE 10 11 12 12 13 15 17 18 19 21 xii 2 11 2 12 2 13 2 14 2 15 2 16 2 17 2 18 3 1 3 2 3 3 3 4 3 5 3 6 3 7 Commonly used cavity for Ti sapphire Oscillator Typical cavity for Ti sapphire oscillator Schematic diagram of Ti sapphire oscillator Schematic diagram of the tightly focused four mirror resonator configurations Beam diameter as a function of the stability parameter Optical pumping system Absorption and emission process Lifetime of the upper laser level of Ti sapphire as a function of temperature Overall experimental setup of Ti sapphire laser Alignment of the pumping source Alignment for focusing the DPSS beam Alignment of linear cavity New focal point formations after passing mirror M1 Crystal holder The pipe installation at crystal holder 21 22 23 24 26 28 29 30 32 34 35 36 36 37 38 xiii 3 8 3 9 3 10 3 11
7. duration of the mode locked Ti Sapphire laser is 30 53 femtosecond vi ABSTRAK Pengayun laser Ti nilam telah dibangunkan berdasarkan teknik mod terkunci sendiri menggunakan rongga lipatan Z Diode pam laser keadaan pepejal Verdi 5 telah digunakan sebagai sumber pengepaman dengan panjang gelombang asas 532 nm sesuai untuk jalur penyerapan bagi hablur Ti nilam Rongga laser disusun atur melalui satu set cermin yang terdiri daripada cermin pantulan tinggi 99 8 untuk memantulkan alur dalam julat 720 nm hingga 820 nm dan pengganding keluaran dengan penghantaran 5 Sepasang prisma untuk mengawal sebaran digunakan untuk menghasilkan denyut femtosaat Denyut dicetuskan melalui gangguan luaran Kestabilan laser dikekalkan dengan membekalkan sistem air penyejukan Laser dioperasi dalam dua mod iaitu mod selanjar dan mod denyut dengan mekanisma mod terkunci Kuasa keluaran maksimum laser selanjar Ti nilam ialah 1 12 W sepadan dengan kuasa pengepaman 5 5 W dan kecekapan 26 Kuasa purata optimum bagi laser Ti nilam mod terkunci ialah 577 nm sepadan dengan kuasa pengepaman yang sama iaitu 5 5 W dengan kecekapan yang lebih rendah 18 Frekuensi laser denyut mod terkunci ialah 96 43 MHz Spektrum sinaran laser berpusat pada 806 74 nm dengan lebar jalur 22 37 nm pada lebar penuh separuh maksimum Tempoh denyut bagi laser Ti nilam mod terkunci ialah 30 53 femtosaat CHAPTER TABLE OF CONTENTS TITLE DECLARATION DEDICATION ACKNOWLEDGEMENTS A
8. form and effective fast saturable absorber Compared to the bleachable dyes the Kerr effect is extremely fast wavelength independent and allows the generation of a continuous train of mode locked pulsed from Continuous Wave CW pumped laser An n3 lfr t Nonlinear medium Kerr lens Aperture Incident beam Intense pulse Low intensity light Figure 2 4 Kerr Lens mode locking process Keller 2003 13 The self focusing effect will create pulse mode locked set of mode that generates short pulses regime and CW mode regime Two techniques can be used to achieve KLM which is hard aperture and soft aperture technique Figure 2 5 a shows KLM on physical aperture also known as physical aperture or hard aperture The aperture placed between the laser gain medium and the mirror is small enough in diameter to provide a relatively high loss for the CW mode However if a light pulse with higher intensity than the CW beam is generated within the gain medium thereby providing a more favorable environment for a pulsed laser than for a CW laser The same aperturing effect can be achieved by making a smaller diameter pump beam than the CW mode size as shown in Figure 2 5 b call as gain aperture or soft aperture cw mode Mirror Aperture to restrict cw mode a cw mode Mirror j Pump beam ASS a P f Narrow pump beam to favor pulsed mode b Figure 2 5 a Physical aperture and b gain aperture
9. maximum FWHM The pulse duration can be estimated as Donnelly and Grossman 1998 C Af sjm 4 4 Af is the frequency of the pulse c is speed of light is the center wavelength and A is the bandwidth at FWHM of the pulse By knowing the frequency Af the pulse duration of Ti sapphire output can be calculated by using the following equation At z 4 5 However the Equation 6 4 is not suitable to calculate the pulse duration for the femtosecond pulse because the femtosecond pulse is in the formation of Sech pulse shape Schneider et al 2000 and Keller et al 1991 Therefore the following equation Rudolf and Wilhelmi 1989 Vasil ev 1995 and Diels and Rudolph 1996 needs to be used Af At The typical spectrum of mode locked signal is shown in Figure 4 17 Such signal is very difficult to obtain It is required clean environment and vibration free The mode locked signal would not be able to grab and display unless the critical condition could be 90 fulfilled Matrox Inspector Vesion 2 1 was employed to measure the bandwidth The signal in Figure 4 17 shows that the center wavelength of the spectrum is obtained as 806 74 nm The bandwidth for this particular signal is Af 24 80 nm measured at Full Wave Half Maximum FWHM The information obtained from this particular signal is used to compute the pulse duration of femto signal Equation 4 4 and 4 7 are used to estimate the pulse duration Th
10. order to initiate the femtosecond pulse generation 4 4 2 Cleaning Factors The cleanness of the optical component play an important role in determining the output power of the Ti sapphire laser In order to verify this factor an experiment was carried out to test the different production of laser output before and after cleaning In this matter the optical components in the cavity were exposed in the air Later without any cleaning the output power of the Ti sapphire laser was directly measured with respect to the pumping power For comparison another experiment was carried out but all the optical components were cleaned using alcohol solution like acetone and lens tissue The other tools employed in the cleaning process are bulb blower and hemostat Entire tools used in cleaning process are shown in Appendix B Firstly the dust needs to be cleaned using bulb blower The aim for using bulb blower is to remove the particle gently The surface of optical component is very sensitive because of coating layer available and to maintain the smoothness and the flatness of the surface Otherwise rough rapping and washing could cause scratching incising and removing the coating from the surface The right technique to clean optical component is by wetting the folded or unfolded lens tissue in the acetone or alcohol solution In this case acetone was used The wetting tissue was placed gently on the 85 surface of optical component Hemostat was u
11. same time mirror M2 was adjusted until the appearance of pulses The femtosecond pulse should appear near the end of stability region Sometimes it s very hard to get the femtosecond because of the dust on the optical component and along the beam path Therefore all the optical components must always clean The way 47 to solve the problem is by covering the entire optical components Typical example of femtosecond pulse obtained is shown in Figure 3 14 Time ns Figure 3 14 Femtosecond pulse detect using fast photodetector Lin et al 2002 Spectrometer was employed to detect the spectrum of femtosecond pulse The spectral bandwidth of the femto pulse taken at Full Wave Half Maximum FWHM should be at least 7 nm and at the center wavelength of around 800 nm This value is corresponding to pulse duration of 100 femtosecond Figure 3 15 shows the typical example of the femtosecond pulse spectrum with FWHM of 8 nm 48 1 0 IN 08 0 4L intensity a u 1 i 780 790 800 810 820 wavelength nm Figure 3 15 Femtosecond pulse spectrum Schneider et al 2000 3 6 Output measurement The output of the femtosecond laser was characterized by using power meter spectrometer and fast photodetector It is better to notice that the output coupler is in wedge shape In this configuration the output laser could be divided into three major rays namely P3 P2 and P1 The other beam can t be used because t
12. takes the expectation value of some operator to change by one standard deviation Since for photon E h Equation 2 1 can be written as Aa At 2 2 Nl In the context of laser pulse Aw refers to the spectral bandwidth of the pulse and At describes the pulse s temporal duration Therefore smaller At demands a larger Aw or frequency range Heisenberg uncertainty is fundamental to understand the generation of ultrashort light pulses The more precise a photon can be placed in time the more uncertain the photon energy is In other word the pulsed laser light must be broadband and contain a spectrum of colors Wang 1999 2 2 Mode Locking There are many uses of very short duration laser pulses in the fields of digital communications diagnostics of ultrafast processes and ablation of materials without causing significant heating of the material Much effort has been made to develop the techniques for generating short pulses Q switched process can produce pulses that are limited to minimum pulse durations of a few nanoseconds Another technique that has allowed the generation of the optical pulses as short as 5 fs 5 x 10 s is known as mode locking Silfvast 2004 The technique of laser mode locking has been around for more than 35 years The first mode locking was demonstrated in 1964 and has since then developed into a very active research area Koumans 2001 Ultrashort pulses with pulsewidth in picosecond or femtoseco
13. the beam incident to the crystal While vertical adjustment is to adjust the crystallographic of the crystal Lastly about the mode locked operation For the mode locked operation many factors need to be considered Firstly the environment factor since the cavity very sensitive to the environment factors such as the vibration and dust therefore a few things need to be done Initially any vibration source needs to remove Then every time modification introduced to the cavity the optical components need to clean as mention in Chapter 4 Second factor is the dispersion the dispersion need to be controlled by careful adjustment of the prism pair in cavity The beam insertion in the prism needs to adjust until stable pulse output produced 5 3 Suggestions The main problem that affects the cavity is the dust and vibration Therefore it is recommended to build proper housing for the laser to avoid the dust expose on the optical components The cover also can block the green beam reflected by the optical components to the end user In order to reduce or eliminate the vibration it is suggest that to use using anti vibration table system This can be done by installing the stabilizer pneumatic isolator to every corner of the table Another thing can be done is by removing the DPSS laser power supply from the optical table because the fan existed in the power supply will produce the vibration 95 Another problem is to make the mode locked pulse s
14. to the measurement the software needs to be calibrated according the spectrum scale By using this software the bandwidth of the spectrum can be determined precisely 3 7 4 2 ToptiCale V25 ToptiCalce V25 is TOPTICA Photonics AG s scientific calculator especially designed for using in optic laboratories The special features of ToptiCalc is mathematical expressions which entered in a simple intuitive syntax dots and commas are accepted as decimal separators a history list collects all entered expressions and the calculated results Furthermore it contains important fundamental constants are available 69 in SI units spectroscopy constants are available in convenient lab units arbitrary functions and variables can be defined by the user and some optics and spectroscopy calculations are implemented CHAPTER 4 CHARACTERIZATION OF TITANIUM SAPPHIRE LASER OUTPUT 4 1 Introduction In this section the output Ti sapphire laser will be discussed This is started from the fluorescence process of the Ti sapphire crystal The characterization is included the continuous wave CW and mode locking operation The output both cases are measured in term of the spectrum and the power produced from the system The mode locked pulse properties will also discuss in detail in this section 4 2 The fluorescence Ti sapphire crystal Titanium sapphire crystal was pumped by DPSS laser The schematic diagram of the experiment setup for this pum
15. used Huang 1995 The KLM of Ti sapphire lasers was discovered in 1991 and capable to produce the shortest pulse which is less than 6 fs duration However shorter sub 5 fs pulse has been demonstrated with external cavity pulse compression Fermann ef al 2001 and Xu et al 1998 The KLM process will be discussed in detail in Chapter 2 1 3 Problem Statement A Ti sapphire crystal is the most important solid state medium to generate femtosecond pulse laser This is because its posses a broad gain bandwidth However it is not an easy task to generate femtosecond laser Only knowledgeable and experience scientist will be able to take the challenge The difficulties arise due to the precision optical components and procedure alignment Therefore this work has been carried out in order to study the design of femtosecond laser 1 4 Research Objective The objective of this research is to study the construction of femtosecond laser by using Ti sapphire crystal based on KLM technique The study includes the identification of optical components gain medium and pumping source The crucial part of the work is the alignment of the laser cavity Finally the laser output obtained will be characterized 1 5 Research Scope In this research Ti sapphire crystal was employed as gain medium The crystal was pumped by green laser In this case Diode Pumped Solid State DPSS laser Verdi 5 was employed The fluorescence of the crystal immediately prod
16. 0 840 850 860 Wavelength rm 3 5 W o 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 820 830 840 850 860 Wavelength inm 4 0 W treaty Counts Intensity counts 1000 1000 900 900 800 800 700 700 600 Bon 500 500 400 400 300 300 200 200 100 100 0 o 600 610 620 630 640 650 650 670 680 690 700 710 720 730 740 750 760 770 780 790 BOO 810 920 830 B40 850 360 800 610 620 630 640 650 650 670 680 690 TOO 710 720 730 740 750 760 770 780 790 800 810 820 830 640 850 860 Wavelength nm 4 5 W Figure 4 3 Wavelength nm 5 0 W The fluorescence intensity at different pumped power 74 1000 900 800 700 600 500 400 300 200 100 Intensity au 0 1 2 3 4 5 6 Pumped power W Figure 4 4 The fluorescence intensity as a function of pumping power 4 2 1 Estimation of pulse duration Since a very narrow pulse required a very wide spectrum Pearson and Withon 1993 therefore Ti sapphire is suitable for producing short pulses lasers With the bandwidth of 235 nm the Ti sapphire crystal was able to support pulses in femtosecond range An estimation to limit the gain bandwidth of Ti sapphire impose in pulse duration in a Kerr Lens Mode Locked KLM laser transform limited Gaussian zero chirp pulse relation can be used _ 0 441 AV 4 1 P Where T is the pulse width and Av is the gain bandwidth of Ti sapphire In this particular case t
17. 3 12 3 13 3 14 3 15 3 16 3 17 3 18 3 19 3 20 3 21 3 22 The schematic of cooling system Alignments of Z folded cavity Beam alignment method Laser output detected using IR card Alignment setup of femtosecond operation Alignment of the prism pair and M4 Femtosecond pulse detect using fast photodetector Femtosecond pulse spectrum Setup for the output detection Absorption and fluorescence spectra of the Ti Sapphire Octahedral configuration of Ti A1203 Crystal structure of sapphire at crystallographic c axis Energy level diagram for Ti in sapphire Schematic diagram of Brewster angle experiment Brewster angle determination 38 40 41 42 44 46 47 48 49 51 52 52 53 54 55 xiv 3 23 3 24 3 25 3 26 3 27 3 28 3 29 4 1 4 2 4 3 4 4 4 5 4 6 4 7 4 8 The DPSS laser system The optical components in the laser cavity Power supply Front panel control 3D a and 2D b beam profile of Verdi 5 DPSS laser a Horizontal cursor profile b Vertical cursor profile The spectrum of Verdi 5 DPSS laser output Ti sapphire gain cross section The experimental setup for the fluorescence detection The fluorescence intensity as a function of wavelength The fluorescence intensity at different pumped power The fluorescence intensity as a function of pumping power The Ti sapphire laser output spectrum The
18. 88 It can be lased over the entire band from 660 to 1100 nm The Ti sapphire crystal also become as the breakthrough of ultrafast solid state lasers because it is first solid state laser medium was able to support ultrashort pulses without cryogenic cooling Ultrafast laser was first generated in 1965 by passive mode locking of a ruby laser Shapiro 1977 Then one year later Nd glass laser was successfully produce pulse duration of some picoseconds by using the same technique In 1981 the first light pulse with duration less than 0 1 picoseconds or 100 femtosecond was generated by improvements of the passively mode locked dye laser Rudolf and Wilhelmi 1989 This progress of femtosecond pulses generation by solid state laser have followed from the self mode locking in a Ti sapphire laser by Sibbett group in 1991 Keller 2003 The self mode locking behavior has known as Kerr Lens Mode locking KLM It is the basis for femtosecond pulse generation in a wide variety of solid state laser system Nevertheless several mode locking methods for Ti sapphire laser were reported which including active mode locking with an acoustic optical modulator additive pulse mode locking APM passive mode locking using organic dyes or semiconductor doped glass as saturable absorber and resonant passive mode locking RPM Keller et al 1991 and Sarukura and Ishida 1992 Perhaps among all of the various schemes KLM is most famous and simplest technique
19. BSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLE LIST OF FIGURES LIST OF SYMBOLS LIST OF APPENDICES INTRODUCTION 1 1 1 2 1 3 1 4 1 5 1 6 Introduction Literature Survey Problem Statement Research Objective Research Scope Thesis Outline vii PAGE ii iii iv vi vii xi xii xvii XIX NH ann N el THEORY 2 1 Ultrashort laser pulse 2 2 Mode Locking 2 2 1 Mode locking Technique 2 2 1 1 Active mode locking 2 2 1 2 Passive mode locking 2 2 1 3 Kerr Lens mode locking 2 3 Dispersion 2 3 1 Source of dispersion 2 4 Laser Oscillator 2 4 1 Cavity Configuration 2 4 2 Cavity Optimization 2 4 3 Astigmatism correction 2 5 Optical Pumping System 2 6 Temperature Control RESEARCH METHODOLOGY 3 1 Introduction 3 2 Alignment of the cavity 3 3 General setup 3 3 1 Pumping Source Alignment 3 3 2 Focusing DPSS beam 3 3 3 Linear cavity alignment 3 3 4 The cooling system 3 4 Continuous Wave operation 3 4 1 Z folded cavity 3 4 2 Testing the CW output 3 4 3 Optimum CW operation 3 5 Femtosecond operation 3 5 1 Dispersion Control 10 10 11 12 14 17 20 21 23 27 28 29 31 31 33 33 34 35 37 39 39 40 42 43 43 viii 3 6 3 7 3 3 2 3 5 3 Optimization of femtosecond pulse operation Starting the femtosecond pulse operation Output measurement Laser component and equipment 3 7 1 31 2 3 7 3 3 7 4 3 7 5 Active Medium Ti sapphire Verdi 5 Diode Pumped S
20. DEVELOPMENT OF CONTINUOUS WAVE AND MODE LOCKED TITANIUM SAPPHIRE LASER WAN AIZUDDIN BIN WAN RAZALI A thesis submitted in fulfillment of the requirements for the award of the degree of Master of Science Physics Faculty of Science Universiti Teknologi Malaysia APRIL 2008 To my beloved Ayahanda and Bonda Wan Razali bin Wan Ismail and Zainab binti Hassan and my sweet brother and sister Wan Lukman and Fatimah Zahra ill iv ACKNOWLEDGEMENT In the name of Allah Most Gracious Most Merciful Praise be to Allah the Cherisher and Sustainer of the worlds For His Mercy has given me the strength and time to complete this project I would like to express my appreciation to my respected supervisor Associate Professor Dr Mohamad Khairi Saidin and Associate Professor Dr Noriah Bidin for their supervision guidance enjoyable discussion and motivation throughout this study Beside them I have much pleasure to those who have assisted me in various ways in carrying out the experimental works They are my late technician Mr Nyan Abu Bakar my new technician Mr Ab Rasid Ithnin and all staff that involve during my research My thanks are also due to Government of Malaysia through IRPA grant vote 74533 for giving us financial support Thanks are also due to Universiti Teknologi Malaysia for giving me the opportunity to pursue my master here Last but not least my appreciations go to my friends and my family for contin
21. Figure 4 2 The spectrum dispersed in the range between visible regime of 613 61 nm to infrared regime of 855 28 nm with roughly bandwidth of 241 67 nm The spectrum is particularly produced after the Ti sapphire crystal pumped by DPSS laser at power of 3 5 W The 72 precise measurement of bandwidth could be performed by using Matrox Inspector Version 2 1 Software The fluorescence supposes to produce broader bandwidth But with the application of coated mirrors M1 and M2 that limit the broadening bandwidth of the spectrum Intensity counts 1000 900 800 TOOT je Spal oy Why doi 600 nA 5007 4007 hal 300 2004 100 i i I a ee sr er re 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 820 830 840 850 860 Wavelength nm Figure 4 2 The fluorescence intensity as a function of wavelength The fluorescence phenomenon was investigated by further increasing the power of the DPSS laser as a pumping source The typical results obtained are shown in Figure 4 3 The spectrograms are arranged in the increasing order of pumped power No significant spectrum is observed at low pump power As the pumped power increases the peak intensity obviously increases too The maximum intensity obtained is 831 43 in arbitrary corresponding to 5 W pumping powers The collected data of intensity measurement are used to plot graph as shown in Figure 4 4 The intensity of the fluorescence bea
22. From the calculation the refractive index of Ti Sapphire crystal was obtained as 1 73 with a small deviation of 1 70 from the actual value which is 1 76 which very close to the value given by manufactured as 1 76 paralllel component y o perpendicular component D ol O oO oO oO L L 1 a oO 1 reflectivity 20 4 0 10 20 30 40 50 60 70 80 90 angle of incident Figure 3 22 Brewster angle determination 3 7 2 Verdi 5 Diode Pumped Solid State Laser Basically Verdi 5 DPSS laser consists of the laser diode as a pumping source Nd Y VO as a gain medium and LBO as a frequency doubler Figure 3 23 illustrated the simple schematic of the DPSS laser operation According to Figure 3 23 the Diode Laser with 808 nm wavelength pumped the Nd Y VO crystal to produce 1064 nm beam Then LBO crystal is strike by 1064 nm beam The LBO acts as a frequency doubler to produce 532 nm beam 56 Pumping source Gain medium Frequency doubler 1064 nm 532 nm Diode Laser Figure 3 23 The DPSS laser system In the actual alignment more optical components are provided in the laser cavity Figure 3 24 shows the entire optical alignment included in the laser cavity of Verdi 5 The major element in the laser cavity is Nd YVO Neodymium Vanadate as the gain medium LBO Lithium tribonate as the frequency doubling crystal etalon as the single frequency optic optical di
23. Star is available with interchangeable filters which together with the variable shutter speed allow to easily and accurately measure any type of source from fractions of a microwatt to watts in intensity Fiber adapters are available to connect to fiber sources 3 7 4 4 Oscilloscope Tektronix Oscilloscope Model TDS3054B with 500 MHz bandwidths sample rates up to 5GSa s and having 4 Channels was employed in this work It contains Full VGA Color LCD 25 Automatic Measurements 9 Bit Vertical Resolution FFT Standard Multi language User Interface QuickMenu Graphical User Interface for Easy Operation and WaveAlert Automatic Waveform Anomaly Detection The oscilloscope delivers 3 600 wfms s continuous waveform capture rate to capture glitches and infrequent events three times faster than comparable oscilloscopes In addition the oscilloscope s real time intensity grading highlights the details about the history of a 67 signal s activity making it easier to understand the characteristics of the waveforms captured In this research the oscilloscope was used to display the mode locked pulse train captured using fast photodetetor 3 7 4 5 Photodetector The photodetector Model PD24K001 was used to detect the fluorescence produced by the excited Ti sapphire crystal Initially photodiode is used to observe the fluorescence increment because it more sensitive and have faster respond compared by using power meter The rise time of the photodetec
24. Ti sapphire laser 24 The simple schematic for the folded cavity is shown in Figure 2 14 which is equivalent with lens system for the matrix analysis However this diagram is not considering the astigmatism produce by the cavity Kerr medium 2wi gt lt 2w d d f d2 Figure 2 14 Schematic diagram of the tightly focused four mirror resonator configurations The spherical mirror will act as a focusing element which is provide a tightly focused resonator mode in the gain medium if the condition of d fi gt gt 1 and d f2 gt gt l1 Therefore in this setup d and dz are chosen to equal with 60 cm and 80 cm respectively to satisfy the tightly focused condition The range value for stable resonator given by 0 lt 6 lt 6 and 2 SO lt Oras 2 6 where and mx 6 2 7 25 Therefore d fi tfi 2 8 Where is stability parameter and f is focal length By substituting the Equation 2 8 with f 5 cm d 60 cm and dz 80 cm the 62 and max is equal to 0 33 0 45 and 0 79 respectively The position of 62 and max are shown in Figure 2 15 The stability region is separated by forbidden zone For symmetric design whereby d equal to d2 forbidden zone will be removed in other word there is just a single stability region In this design unequal arm was used which is known as asymmetric design Therefore the stability region is split into two as shown in Figure 2 15 When d is short
25. Z configuration is commonly used for Ti sapphire laser as well as for Cr LiSAF and Cr LiAF Kalashnikov et al 1997 The schematic layout for typical cavity is illustrated in Figure 2 12 It is consists of output coupler M1 end mirror M2 two curve mirrors M3 M4 with 10 cm radius of curvature ROC focusing into a Ti sapphire crystal and a pair of intracavity prisms for dispersion compensation end mirror M2 curve mirror M4 Ti sapphire crystal curve mirror M3 output coupler M1 Figure 2 12 Typical cavity for Ti sapphire laser Huang 1995 However recent research had successful to replace the prism usage Sutter et al 1998 and Bartels et al 1999 with the negative dispersion mirrors that reflect light with longer wavelength from deeper region in the coating than light with shorter wavelength 23 This technique found to be more compact reliable and user friendly than any previous femtosecond laser Beside it is more easier to compensate the dispersion especially produce by the Ti sapphire crystal 2 4 2 Cavity Optimization In this part the calculation of the cavity is included to determine dj d2 and dy see Figure 2 13 The calculation for all parameters is referred to Xu et al 1998 The entire setup for such cavity is shown in Figure 2 13 end mirror M3 JZ d n curve mirror M2 curve mirror M1 Ti Sapphire P output coupler d Figure 2 13 Schematic diagram of
26. aperture mode locking uses a physical slit within the cavity to block the CW components of light Beside soft aperture mode locking uses the gain medium as the aperture Xu et al 1998 In general the cavity becomes a stable resonator when the curve mirrors separation d2 f f2 with f and fz is the focal length of the curve mirror Kowalevics 2004 As a result of using the Equation 2 6 2 7 and 2 8 the value of dj dz and df can be determined as d 60 cm dz 80 cm and dy 10 5 cm respectively 27 2 4 3 Astigmatism correction As a result of the presence of Brewster angle in the Ti sapphire crystal some astigmatism is introduced in the cavity which can lead a beam waist size difference between sagittal perpendicular to the plane of incident and tangential parallel to the plane of incident plane Kowalevicz 2004 Astigmatism needs to be reduced because it can cause unstable mode locking Lytle et al 2004 and also reduced output power Although the astigmatism correction calculated using the theoretical equation in contrast to real Z folded cavity astigmatism cannot be completely compensated The astigmatism just can be minimized by titling the folding mirrors M1 and M2 to a certain angle extension The optimal fold angle for the cavity to reduce astigmatism can be calculated using the following equation Kolgenik et al 1972 2Nt 2f sin 0 tan0 R sin 0 tan 0 2 9 where N n 1Wn 1 n 2 10
27. d Wang Q 1999 Suppression of amplified spontaneous emission in a femtosecond chirped pulse amplifier Optics amp Laser Technology 31 425 430 Index refraction for sapphire Wavelength nm 488 515 694 755 780 800 820 860 Physical constants for sapphire Physical Constant Melting point Density Thermal expansion coefficient Thermal conductivity Heat capacity Thermal Dispersion Hardness APPENDIX A Extra ordinary index ne 1 76711 1 76486 1 75542 1 75346 1 75274 1 75220 1 75168 1 75072 Value 2 015 3 965 8 4 10 6 35 79 5 1 4 1075 12 103 Ordinary index no 1 77533 1 77304 1 76341 1 76141 1 76068 1 76013 1 75961 1 75863 Units gr cm m e W m C J mol K C 1 Modified mohs 104 APPENDIX B Tools for cleaning optical component Bulb blower we Acetone alcohol and lens tissue O Hemostat 105 PRESENTATIONS Wan Aizuddin W R Mohamad Khairi S Noriah B Diagnostic Of Diode Pumped Solid State Laser Annual Fundamental Science Seminar 2005 AFSS 2005 4 5 July 2005 Ibnu Sina Institute UTM Wan Aizuddin W R Mohamad Khairi S Noriah B Experimental Study Of Thermoelectric Cooler For DPSS Laser International Meeting on Frontiers of Physics IMFP 2005 25 29 July 2005 Kuala Lumpur Wan Aizuddin W R Mohamad Khairi S Noriah B Study The Absorption Of Diode Pumped Solid State Laser on Ti Sapphire Crystal 1 Natio
28. d operated the laser in continuous awe two final mirrors were inserted into the linear cavity The last two mirrors are consisted of output coupler OC and rear mirror M3 The insertion of these two mirrors changed the linear cavity to become Z folded cavity configuration 3 4 1 Z folded cavity The procedure to complete the development is describe First the focusing lens L was determined to be at the best position This was performed by introducing a white paper behind mirror M2 The beam spot was ensured to be existed at the center of the mirror M2 This is similar procedure as implemented in Figure 3 3 After optimizing the best position of the focusing lens L an output coupler OC was inserted into the system Similarly the OC also need to be aligned at the best location The procedure for this alignment is entirely different with the focusing lens In this case luminescence of the Ti sapphire crystal was utilized The luminescence was illuminated at the center of the output coupler OC The presence of the beam can be detected using infrared card An aperture was employed to enhance the alignment The luminescence beam was incident on the aperture The beam from the output coupler was ensured to be reflected back on the aperture Finally the configuration of Z folded cavity was completed by inserting the last mirror of M3 The position of M3 is shown in Figure 3 9 In order to avoid astigmatism phenomenon the mirr
29. duced to only a few femtosecond pulses of 20 100 fs are common in many laboratories The reduction of pulse duration has been accompanied by large increases in the peak pulse intensity from 10 10 W cm in the mid 1980 s up to 10 10 W cm in 2004 Tate 2004 The improvement of the ultrashort pulse laser is shown in Figure 1 1 KLM Chirped mirror CEO control First ML laser Ti sapphire 1960 1970 1980 1990 2000 Dye laser 27 fs with 10 mW 10 ps Ti sapphire laser ES 5 fs with gt 100 mW 100 fs FWHM pulse width oO f o wd Compressed Ai 1960 1970 1980 1990 2000 Year Figure 1 1 The improvement of ultrashort pulse generation Keller 2003 Figure 1 1 illustrated the improvement in pulse generation since the first demonstration of a laser in 1960 Until the end of the 1980s ultrashort pulse generation was dominated by dye lasers and pulses as short as 27 fs with an average power of 10 mW was achieved Valdmanis and Fork 1986 After external pulse compression a pulses as short as 6 fs was produced However this situation changed with the discovery of the Ti sapphire lasers Since the discovery of laser action in Ti sapphire in 1982 Ti sapphire become one of the most widely used solid state laser material Kuhn 1998 It combines the excellent thermal physical and optical properties of sapphire with the broadest tunable range of any known material Eggleston et al 19
30. dwidth Light speed xviii XIX LIST OF APPENDICES APPENDIX TITLE PAGE A Index refraction for sapphire 103 B Tools for cleaning optical component 104 CHAPTER 1 INTRODUCTION 1 1 Introduction Through the improvement of lasers currently it is possible to observe motion in nature with unprecedented temporal resolution With the ultrafast 107 laser usage exploring physical phenomena is possible Ultrafast laser are currently following the path already taken by many physic invention The continuing development of ultrafast laser technology have led to many new and fascinating application in physics engineering chemistry biology and medicine Sutter et al 1998 Among the ultrafast lasers Ti sapphire laser is the most popular laser used Current areas of activity using Ti sapphire lasers include nonlinear conversion high repetition rate systems extended operating range and novel resonators The widespread applications of Ti sapphire include LIDAR Rodriguez et al 2004 dual wavelength DIAL systems fundamental research spectroscopy as well as tunable Optical Parametric Oscillators OPO pumping and simulating diode pumping in solid state lasers McKinnie et al 1997 and Xu et al 1998 The development of ultrafast laser technology has shown the rapidly progress over the past decade This is due to the great feature of the lasers that give superior performance for many applications There are four features of th
31. e Washington State Ph D Thesis Hecht E and Zajac A 1982 Optics Addision Wesley Publishing Company Jung I D Kartner F X Matusche K N Sutter D H Morier Genoud F Shi Z Scheuer V Tilsch M Tschudi T and Keller U 1997 Semiconductor saturable absorber mirrors supporting sub 10 fs pulses Appl Phys B 65 137 150 Kalashnikov V L Kalosha V P Poloyko I G and Mikhailov V P 1997 Optimal resonators for self mode locking of continuous wave solid state lasers J Opt Soc Am B 14 4 964 969 98 Keller U tHooft G W Knox W H and Cunningham J E 1991 Femtoseond pulse from continuously self starting passively mode locked Ti sapphire laser Optics Letter 16 13 1022 1024 Keller U 2003 Recent developments in compact ultrafast lasers Nature 424 831 838 Kuhn K 1998 Laser Engineering United State Prentice Hall Inc Koechner W and Bass M 2003 Solid State Lasers A graduate Text New York Springer Verlag Kolgenik H W Ippen E P Dienes A and Shank C V 1972 Astigmatically Compensated Cavities for CW Dye Lasers IEEE J of Quantum Electronic 8 373 379 Koumans R G M P 2001 Semiconductor Mode locked laser Mode locking Characterization and Application California Institude of Technology Pasedena Ph D Thesis Kowalevicz A M 2004 Novel Femtosecond laser development with application in Biomedical Imaging and Photon
32. e calculation result showed the pulse duration of the femtosecond pulse is 27 56 fs by assuming this signal having Sech shape The pulse duration can be easily calculated by using ToptiCalc V25 software Intensity counts 750 760 770 780 790 800 810 820 830 840 850 Wavelength nm Figure 4 17 Spectrogram of mode locked pulse CHAPTER 5 CONCLUSIONS AND SUGGESTIONS 5 1 Conclusions The femtosecond of Ti sapphire laser was successfully studied Two types of Ti sapphire laser mode which are continuous wave and mode locked operation have been demonstrated The laser was pumped using DPSS laser Verdi 5 with wavelength of 532 nm The Ti sapphire crystal was used an active medium for the laser to produce output laser at IR region For the cavity alignment a set of mirror with reflection of 99 8 and coating with broad range of 720 nm to 820 nm was employed This includes the flat and spherical mirrors Output coupler with transmission of 5 was used to transmit the laser For the mode locked Ti sapphire laser a prism pair was conducted to compensate the dispersion in order to produce the femtosecond pulse The Z folded cavity type was set up in this project The total length of laser cavity is 150 5 cm The separation between adjacent mirror M1 M2 M2 M 4 and M1 OC was 10 5 cm 60 cm and 80 cm respectively 92 Both types of laser mode have been successful analyzed The fluorescence spectrum output s
33. e it an excellent replacement for several common dye lasing materials The Ti sapphire has a wide absorption band extend about 200 nm center at near 490 nm Gan 1995 as shown in Figure 3 17 Ti sapphire can be pumped by variety of sources operating in the green argon ion copper vapour frequency doubled Nd YAG and dye lasers are routinely used However for the commercial Ti sapphire laser frequency doubled Nd YAG or Nd YLF lasers are used as a pumping source to pump Ti sapphire crystal Song et al 2005 tw Polarization Intensity arb units o o D o r Oo N l f Fluorescence Absorbance 0 400 500 600 700 800 900 Wavelength nm Figure 3 17 Absorption and fluorescence spectra of the Ti Sapphire To make Ti sapphire TiO is doped into a crystal of Al2O3 typical concentrations range between 0 1 0 5 by weight so that Ti ions occupy some of the Al tion sites in the lattice The Ti ion possesses the simplest electronic configuration among transition ions only one electron being left in the 3d shell The second 3d electron and the two 4s electrons of the Ti atom are in fact used for ionic binding to 52 oxygen anions When Ti is substituted for an Al ion the Ti ion is situated at the center of an octahedral site whose six apexes are occupied by O ions as shown in Figure 3 18 Figure 3 19 shows the crystal structure of sapphire at crystallographic c axis The new technique to growth hi
34. e ultrafast laser that makes it so special The first feature is the ultrashort pulse duration Through this feature this laser allows very fast temporal resolution Therefore this kind of laser can freeze the motion of fast moving object including molecules and electrons Professor Ahmed Zewail has won a Nobel Prize in chemistry by observing the molecule reaction in slow motion using ultrafast laser Smith 1999 The second feature of ultrafast laser is high pulse repetition rate With multi gigahertz repetition rates this laser was used in high capacity telecommunication systems photonic switching devices optical interconnection and for clock distribution The third feature is ultrafast laser have broad spectrum which supports good spatial resolution for optical coherence tomography OCT OCT is a technique for non invasive cross sectional imaging in biological systems Lastly the ultrafast laser has high peak intensity This high intensity source makes non thermal ablation without increase temperature is possible The ability of intense ultrashort pulse lasers to fabricate microstructures in solid targets is very promising and the quality of ablated holes and pattern is much better using femtosecond laser 1 2 Literature survey Over the last two decades there have been a series of impressive achievements in the technology of short pulse lasers From tens of picoseconds in the mid 1970 s laser pulse durations have now been re
35. er also can be used CHAPTER 3 RESEARCH METHODOLOGY 3 1 Introduction In this chapter the experimental techniques used in this study will be described These comprised the alignment of the cavity and laser components used in this project The equipments used in the experiment and software conducted in the analysis also included 3 2 Alignment of the cavity The alignment of the cavity is a very challenging task in the construction of the Ti sapphire laser Moreover for the mode locked laser operation requires extremely precise alignment compare to the CW operation It also needs to have careful design of the laser cavity Kowalevicz 2004 As discussed previously in chapter 2 the Z or 32 X folded is the best cavity for the standard KLM lasers In this chapter the technique to align Ti sapphire laser will be discussed Figure 3 1 shows the overall setup for Ti sapphire laser The step by step procedure of alignment for the Ti sapphire laser will be described in detail The optical components employed in each of alignment also will be included Prior to operate the laser either in continuous or pulse mode the general setup which will be utilized in both operations will be described P1 P2 is prism M1 M4 is mirror L is lens PR is polarization rotator OC is output coupler PM1 PM2 is pumped mirror Pl gee Beam M2 Dumper OC Figure 3 1 M4 L 19 P N DPSS laser
36. er than d2 the position of the outer stability region will shift away from the forbidden zone but maintain the beam diameter This shifting also occurs when dz is shorter than d The Kerr lens mode locking can be optimized by the adjustment of the arm length ratio For the symmetric cavity the maximum Kerr lens sensitivity occur when the laser operate at the center of stability with the crystal put at the center between two curve mirrors However the stability of the symmetric cavity is easy to effect by the environment Therefore asymmetric cavity is typically used for KLM laser For the best KLM performance the arm length ratio should in the range between 4 3 and 2 1 Kowalevicz 2004 26 First stability region I 1 L L 1 Second stability region Forbidden zone Beam diameter mm 0 d 0 33 d2 0 45 6 62 0 79 Stability parameter 6 mm Figure 2 15 Beam diameter as a function of the stability parameter 6 In order to get the KLM the hard aperture and soft aperture KLM can be used As discussed before hard aperture mode locking uses a physical slit within the cavity to block the CW components of light Beside soft aperture mode locking uses the gain medium as the aperture see Figure 2 5 For soft aperture Kerr Lens Mode locking an asymmetric cavity configuration is predicted to be most favorable for strong SAM Self Amplitude Modulation and 6 adjusted to 62 in second stability zone For hard
37. f dispersion is grating and prism pair which can produce negative dispersion For the grating pair operation as shown in Figure 2 8 the first grating diffract the beam as a result different wavelength travel in different direction Since the angle of diffraction for shorter wavelength is smaller than longer wavelength 18 therefore the transit time for longer wavelength is more than shorter wavelength After that the second grating the wavelength will merge together again Grating 1 Grating 2 Figure 2 8 Grating pair operation A prism pair also works similar to the grating pair In this case the dispersion also introduce by the geometry of the setup which illustrated in Figure 2 9 whereby the shorter wavelength goes through less prism material therefore have shorter optical path Beside longer wavelength goes through more prism material therefore have longer optical path Through this setup the dispersion can be compensated However the mode locked operation cannot self stating by using only prism pair in the cavity Self starting can be achieved by using broadband Semiconductor Saturable Absorber Mirror SESAM The SESAM is produced from new development arising from the need for a reliable starting mechanism for ultrashort pulses Jung et al 1997 19 Shorter wavelength light goes through less prism material shorter optical path Longer wavelength light goes through more prism material longer optical path
38. f the fast photodetector it s cannot displayed the pulse in femtosecond scale as compared to autocorrelator By using fast photodetector only mode locked pulse train can be traced therefore the pulse duration cannot be measured A fast photodiode was coupled to oscilloscope with bandwidth of 500 MHz The femtosecond pulse 88 formation during mode locked operation is shown in Figure 6 16 The result is in good agreement with the result of normal mode locked pulse obtained by Lin 2002 with the repetition rate of 93 3 MHz There are many types of the mode locking pulse including of regular mode locking period doubled mode locking quasi periodic mode locking and chaotic mode locking Xing et al 1999 Figure 6 16 is known as the regular mode locking This clarified based on similarity of the amplitude of the signal The frequency of the pulse is obtained as 96 43 MHz corresponding to pulse spacing of 10 37 ns Ta 20 omvo aT Ons rn Ch2 tf 133mv Figure 4 16 Oscillogram of mode locked pulses 89 4 6 Femtosecond pulse duration In stead of using an autocorrelator to measure the pulse duration the spectrometer also can be used to indirectly measure the pulse duration of femtosecond signal The spectrum detected by the spectrometer can be used to calculate the pulse duration The parameters used to calculate the pulse duration is the center wavelength and the bandwidth of the spectrum at full wave half
39. gh quality Ti sapphire laser crystal is Induction Field Up Shift method IFUS and Temperature Gradient technique TGT which differ from Czochralski CZ method and Heat Exchange Method HET Fuxi 1995 O 0 1 0 OO R Ti O O Figure 3 18 Octahedral configuration of Ti Al20 Svelto 1998 O Aluminum or titanium O Oxygen Figure 3 19 Crystal structure of sapphire at crystallographic c axis 53 When Ti ion is introduced into the Ti sapphire host the energy level split into two level as shown in Figure 3 20 When the light is absorbed by the titanium atom causing it excite to the highest energy level of En Then through a very fast phonon emission process the atom move to the metastable state of the lowest E The stimulated emission occurs between the E excited electronic state and T ground electronic state After that follow by multiple phonon emission which is transfers the ion back to the lowest energy position of the ground electronic state Elsayed 2002 Configuration coordinate a u Figure 3 20 Energy level diagram for Ti in sapphire Elsayed 2002 In this research the crystal was characterized by determining the refractive index using Brewster method The Brewster angle is important to investigate because by placing the crystal at the Brewster angle the loss of energy during pumping process can be minimized The schematic diagram for Brewster angle measurement is shown in 54 Fig
40. hat the beam almost perfectly overlapping with the normal distribution of theoretical Gaussian beam Figure 3 26 3D a and 2D b beam profile of Verdi 5 DPSS laser Vertical 0 2887 16357 DELTA 13 Figure 3 27 3 7 2 3 Laser spectrum of Verdi 5 80 04 60 04 40 04 20 04 61 Horizontal 0 0 3474 19684 DELTA 16 a Horizontal cursor profile b Vertical cursor profile The typical spectrum produced from Verdi 5 DPSS laser is shown in Figure 3 28 Tremendously only one line of 532 06 nm is appeared This indicates Verdi 5 DPSS laser has eliminate the pumping beam This is showed that Verdi 5 is suitable to be as a pumping source of Ti sapphire crystal 80 0 37 70 532 07 Figure 3 28 1000 1050 The spectrum of Verdi 5 DPSS laser output 62 3 7 3 Optical component of Ti sapphire laser In the development of Ti sapphire laser its comprised of many optical components beside active medium and the pumping source The optical components used in the development are including mirrors lens output coupler and prisms 3 7 3 1 Dielectric mirror There are 4 dielectric mirrors employed in the development of Ti sapphire laser namely M1 M2 M3 and M4 The dimension of the mirror is 20 mm in diameter with the thickness of 6 mm The reflectivity for the mirror at IR is about 99 8 Mirror M1 and M2 are concave mirrors but mirror M3 and M4 are flat m
41. he gain bandwidth can be estimated by knowing the range of the spectrum by 75 4 2 meee 1 A 4 3 By solve Equation 4 3 with 2 613 61 nm and 22 855 28 nm the Av is equal to 138 THz Therefore the shortest pulse can be determined by solving the Equation 4 1 with calculated Av From the calculation the shortest pulse duration Tp is roughly 3 27 fs However there are many other factors that affect the pulse duration produced such as dispersion bandwidth limit of the mirror and self focusing in nonlinear medium Pearson and Withon 1993 4 3 Characterization of Continuous Wave CW Laser After the fluorescence was detected all optical components in laser cavity needs to be aligned properly as discussed in chapter 3 in order to have the laser generation For these purpose two flat mirrors M3 and Output Coupler OC are added in the laser cavity such as shown in Figure 3 9 in chapter 3 Z folded cavity configuration is chosen in this experiment 4 3 1 Spectrum of continuous wave beam When the fluorescence signals are precisely aligned the gain in the cavity is higher than the losses As a result the light amplified and produced lasing from output coupler OC The spectrum of the radiation was measured using Ocean Optics 76 Spectrometer The result is shown in Figure 4 5 A narrow bandwidth is appeared which center at 784 23 nm This means the wavelength of the Ti Sapphire laser in continuous wave CW mode
42. he power is too small to detect The power of the beams produced becomes lower after reflected by output coupler surface P3 is lower than Pz and Pz is lower than P P3 lt P2 lt P To be able to synchronize the measurement two of the beam P2 and P3 are reflected by gold coated mirrors M and M2 The reflected beam P3 was detected by using fast photodetector Meanwhile beam P2 was observed by using spectrometer and the spectrum was displayed on the monitor Matrox Inspectror software was utilized to analyze the 49 spectrum Beam P1 was directly incident to a power meter to measure the output power of the laser The setup for output measurements is shown in Figure 3 16 Output coupler wedge shape Fast Photodetector Spectrometer Power meter Figure 3 16 Setup for the output detection M M 3 7 Laser component and equipment In this section the material and equipment used in this study will be described These include the laser material optical components for resonator development equipments used in the experiment and software utilized for analysis 50 3 7 1 Active medium Ti Sapphire All lasers contain an energized substance that can increase the intensity of light passing through it This substance is called amplifying medium or sometime the gain medium or active medium The active medium can be a solid a liquid or a gas In this project Ti sapphire crystal was used as an active medium Titanium d
43. ic Device Fabrication Harvard university Ph D Thesis Li M 1999 Design of an ultrshort pulse multipass amplifier and investigation of femtosecond breakdown University of Connecticut Ph D Thesis Lin J H Hsieha W F and Wu H H 2002 Harmonic mode locking and multiple pulsing in a soft aperture Kerr lens mode locked Ti sapphire laser Optics Communications 212 149 158 99 Lingyun Photoelectronic System Co Ltd 2005 Diode pumped Solid State Laser System LYDPG 1 China Operator s Manual Lytle A L Gershgoren E Tobey R I Murnane M M Kapteyn H C and M ller D 2004 Use of a simple cavity geometry for low and high repetition rate modelocked Ti sapphire lasers Optics Express 12 7 1409 1416 Matrox Electronic Sytems Ltd Matrox Inspector Version 2 1 Canada User manual McKinnie I T Oien A L Warrington D M Tonga P N Gloster L A W and King T A 1997 Ti Ion Concentration and Ti sapphire Laser Performance IEEE J Of Quantum Electronics 33 7 1221 1230 Mukhopadhyay P K Alsous M B Ranganathan K Sharma S K Gupta P K Gupta J George J and Nathan T P S 2003 Simultaneous Q swithing and mode locking in an intracavity frequency doubled diode pumped Nd Y VO4 KTP green laser with Cr4 Y AG Optics Communication 222 399 404 Newport 2006 The Newport Resource 2006 2007 USA Catalogue Newport 2004 818P Series High Power Detector USA U
44. irror In this work the curve mirrors M1 and M2 with radius of curvature ROC of 100 mm and coated with highly transmission at 488 to 540 nm and highly reflection at 672 to 887 nm However the flat mirrors M3 and M4 are coated with only highly reflection at 672 to 887 nm 3 7 3 2 Focusing lens The DPSS Laser beam needs to focus sharply to ensure high intensity of the beam exposed to the crystal The intensity need to be highly enough to produce a strong nonlinearity Therefore the beam needs to focus by using a particular focusing lens The lens used in this project is made from BK 7 glass and specially coating with highly transmission at 532 nm in order to maximize the pumping power incident to the crystal The focal length of the lens is 100 mm 63 3 7 3 3 Output coupler In the laser system a mirror which is partially reflect is necessary to emit the laser beam The 95 reflection mirror was used as an output coupler In this case only 5 of the beam will be lased out and the rest will be reflected and amplified in the cavity The output coupler in this experiment was coated with high transmission at 642 to 830 nm The percentage of the transmission of the output coupler is depends to the wavelength of the laser output desired to be produced Figure 3 29 For the wavelength at higher gain for example from 750 nm to 850nm output coupler with transmission around 10 to 15 is recommended At the lower gain in the range of 700 850 nm a
45. is at 784 23 nm Intensity counts 400 A 3 s i x call eee eee ae 200 p 1007 i i 740 750 760 770 780 790 800 810 820 Wavelength nm Figure 4 5 The Ti sapphire laser output spectrum The output power of the CW operation is verified by increasing the pumping power The typical results obtained are shown in Figure 4 6 The spectrums are arranged in the increasing order of the input power In general all spectrum lines are very narrow but yet the intensity is obviously increasing from low level power to highest level power tested in this experiment The intensity of the pulse is measured in arbitrary au The intensity is plotted against the pumping power The graph is shown in Figure 4 7 Initially no power was detected although the pumping power was increased meaning no lasing occurs At this stage the pumping power was not enough to produce stimulated emission As a result no emission of coherent beam consequently no spectrum observed As the pumping increases further up to 2 W stimulated emission started to be traced by the spectrometer The stimulate emission overcome the spontaneous emission Then the chain reaction took place which measured by the increasing intensity within 2 W to 5 W The output was found linearly increases with respect to the pumping after the threshold power of 2 W The fluctuation in the graph 77 shows that the CW output of Ti sapphire laser was unstable This arises due
46. ite paper TPSA SSSI PM2 Figure 3 3 Alignment for focusing the DPSS beam 3 3 3 Linear cavity alignment The focused beam of DPSS laser was used to excite active medium in this case Ti sapphire crystal The stimulated emission from the crystal is then amplified in a linear cavity The linear cavity was developed by using two concave mirrors M1 and M2 The configuration of cavity is shown in Figure 3 4 The procedure of the alignment was started by inserting concave mirror M1 in front of the lens L Introducing M1 in the path of focused beam subject to increase the focal length such as illustrated in Figure 3 5 The beam was focused about 110 mm away from the original focal point A Ti sapphire crystal was placed at a new position of focal point The crystal was adjusted until the focused beam passing through at the center The angle of the crystal was also adjusted to minimize the reflection The other crucial parameter desire to be considered in the performance of alignment is its crystallographic axis This was implemented by introducing a polarization cube PC 36 110 mm Figure 3 4 Alignment of linear cavity New focal Original point focal point Figure 3 5 New focal point formations after passing mirror M1 In order to find the suitable crystallographic axis a screw 1 on the crystal holder was manipulated such as depicted in Figure 3 6 The adjustment was continued until a minimum reflected power from po
47. larization cube PC was detected This point is referring as a dark field When the dark field condition was achieved the holder of the crystal was tightens Mirror M2 was then inserted into the system The distance between 37 mirror M1 and M2 surface was chosen approximately at 110 mm This distance will be optimized during the laser operation The beam dumper was put beyond the mirror M2 to block the beam Figure 3 6 Crystal holder 1 Screw for aligning crystallographic axis orientation 2 Spring 3 Fixing screw 3 3 4 The cooling system Gain medium Ti sapphire crystal will be pumped by high power diode laser The crystal was pumped using end pumping technique To avoid temperature gradient during pumping process the crystal was desired to be cooled In fact cooling the crystal will also give an advantage to stable the output laser The cooling was installed by providing the chilled water such as illustrated in Figure 3 7 The inlet and outlet of the pipe were connected to the crystal holder The temperature was maintained as ambient temperature However the water was pumped so 38 that it was circulated during the pumping process The schematic of cooling system is shown in Figure 3 8 a Figure 3 7 The pipe installation at crystal holder Crystal holder Water Outlet Water tank Figure 3 8 The schematic of cooling system 39 3 4 Continuous Wave operation Finally to complete the task an
48. late synchronously with the pulse in the cavity Eventually those wavelengths will experience more loss or less gain due to the Kerr lens mode locking mechanism and will die out Therefore without any dispersion control it is impossible to get the ultrashort 16 laser pulses Huang 1995 Equation 2 3 illustrates the theoretical effect of dispersion in the cavity Salin and Brun 1987 pla K 58 O 0 TOCE o 2 3 In Equation 2 3 is known as the absolute phase and describes the exact phase of the central frequency When considering dispersion is known as the group velocity and is the first derivative of the spectral phase with respect to frequency evaluated at the central frequency Non zero value of the Group Velocity describes a linear translation of the pulse in time The GVD is represented by and is the second derivative of the phase with respect to frequency evaluated at the central frequency Nonzero GVD values give a parabolic phase profile and are responsible for most of the temporal widening experienced by pulses Higher order dispersion is labeled as Third Order Dispersion TOD Walker 2006 As the higher order derivatives usually give very small contribution the third and fourth order dispersion TOD and FOD regularly are neglected Therefore the GVD need to be controlled to obtain ultrashort pulse In fact there are two main dispersive pro
49. llow full analysis of the laser beam s characteristics 3 7 4 3 Spectrometer In this project two kinds of spectrometer were used Those spectrometers are USB2000 Spectrometer and Wave Star CCD laser spectrum analyzer The USB 2000 spectrometer is used to measure the spectrum from Ti sapphire laser and Wave Star CCD laser spectrum analyzer is used to analyze the spectrum of DPSS laser 66 The USB2000 Miniature Fiber Optic Spectrometer is a small footprint plug and play version of spectrometer The USB2000 couples a low cost high performance 2048 element linear CCD array detector with an optical bench make it small enough compared to another spectrometer The USB2000 accepts light energy transmitted through single strand optical fiber and disperses it via a fixed grating across the linear CCD array detector which is responsive from 200 1100 nm Setting up the USB2000 Spectrometer is easy The USB2000 is easy to use by simply installs the Spectrometer Operating Software and then connects the USB cable from the spectrometer to the computer The Ophir Wave Star spectrum analyzer is a spectrometer which introduces a new level of accuracy and ease in spectral measurements This device can easily and accurately measure spectra from a wide variety of sources including continuous and pulsed sources from microwatts to watts in intensity The program automatically tags the peaks with the wavelength so the result is readily available The Wave
50. m is found to be proportional upon the pumping power This mean the higher power to pump the crystal the more intense fluorescence is produced In the fluorescence band a little drop was occurred at wavelength of 660 nm and 740 nm This happened because of the impurities and defect of the crystal such as point defect and line defect Duhr and Brauna 2005 13 Intensity courts Irtensity courts 1000 1000 900 900 800 800 700 700 600 600 500 500 400 400 300 300 200 200 100 100 0 a 800 610 620 630 640 650 660 670 68D 690 700 710 720 730 740 750 760 770 780 790 800 810 820 83D 840 850 860 Wavelength nm 1 5 W Intensity counts 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 760 790 800 810 820 830 640 850 850 Wavelength nm 2 0 W Intensity courts 1000 1000 900 900 800 800 700 700 600 600 500 500 400 400 300 200 200 200 100 100 a 0 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 20 830 840 850 860 Wavelength nm 2 5 W Intensity courts 1000 a00 800 700 600 500 400 300 200 100 600 610 620 630 640 850 660 670 630 590 700 710 720 730 740 750 780 770 780 790 500 810 620 830 840 850 860 Wavelength nm 3 0 W Intensity courts 1000 a00 800 700 600 500 400 300 200 100 o 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 820 83
51. n Without safety precaution during handling the DPPSS laser it will cause bad injury Figure 5 1 shows the DPSS laser during operation The DPSS laser beam is in vertical polarization Therefore the direction of the polarization must be changed to horizontal polarization in order get the Brewster angle condition This problem is solved 93 by insert the polarization rotator before strike the Ti sapphire crystal with the pump beam Figure 5 1 DPSS laser during operation The second problem was regarding the cavity of the laser The cavity was Z cavity type therefore it comprised of many optical components and need large space to set up Besides the cavity of the laser is very sensitive and need very precise alignment in order to get the laser beam Moreover small touch to the component in the laser cavity may cause the laser beam lost and need for other alignments The third problem is to generate the laser from the crystal In order to get lasing from the crystal firstly the suitable crystal need to be chosen The crystal characteristics that need to be considered is the doping dimension and crystal s crystallographic 94 Without an appropriate crystal the lasing could not be achieved Another critical factor is to determine the best position of the crystal Therefore the crystal s holder that has been used in this cavity can be adjusted in horizontal and vertical The horizontal adjustment is to adjust the angle of
52. n Figure 4 9 and 4 10 respectively Figure 4 10 shows that the beam spot having uniphase mode This indicates that the Ti sapphire laser posses a good beam quality Figure 4 9 3D beam profile in near field 81 Figure 4 10 2D beam profile in near field On the other hand the beam was also observed from the far field at distance between the detector and the beam of 70 cm corresponding to the same pumping power used in the near field The result of beam profile in far field for 3D and 2D are represented in Figure 4 11 and 4 12 respectively The beam profile in far field also shows that the laser still operated in TEMoo This result proved that the beam produced is in good quality beam Figure 4 11 3D beam profile in far field 82 Figure 4 12 2D beam profile in far field 4 4 Characterization of Mode locked Laser As explained previously in chapter 3 in order to have the mode locking operation the cavity such as shown in Figure 3 9 need to have some modification This is carried out by inserting a prism pair grating pair or Negative Dispersion Mirror in the cavity In this particular alignment a prism pair was chosen The prisms are used to compensate the dispersion in the cavity Without dispersion compensation a longer wavelength of the pulse propagate faster than shorter wavelength leading to pulse stretching The uncompensated dispersion will make the femtosecond pulse impossible The cavity for the mode locked operatio
53. n is shown in Figure 3 12 in chapter 3 83 4 4 1 Compensating Effect The laser output from mode locked cavity was measured using power meter at various pumping power The collected data of power measurement from mode locked Ti sapphire laser are used to plot graph of output against input power The graph obtained is shown in Figure 4 13 600 500 400 300 Output power mW Pumping power W Figure 4 13 Output power as a function of the pumping power Mode locked Ti sapphire laser has produced similar trend of graph as obtained by CW operation The threshold pumping power for mode locked cavity is 2 5 W which almost double than CW operation The insertion of the prism pair in the cavity caused more losses in the pumping power Beside that more power was dissipated by the reflection on the optical component especially on the prism pair This factor makes the 84 gain power in cavity become lower at the same time the loss become higher Therefore more pumping power is needed in order to make the laser gain higher than losses in the cavity The output power linearly increases after exceeding threshold power of 2 W The optimum power produced from this system is 577 mW corresponding to pumping power of 5 5 W The efficiency of mode locked Ti sapphire laser is 18 which lower compared to the CW operation Although the efficiency for mode locked cavity is lower but this cavity is very important in
54. nal Colloquium on Photonics 29 30 November 2005 Kajang Selangor Wan Aizuddin W R Mohamad Khairi S Noriah B Indentification Of Ti Sapphire Laser Oscillator Components Seminar Penyelidikan Advanced Optical Crystal for Electro Optics Application 2006 21 23 Mei 2006 Melaka Wan Aizuddin W R Mohamad Khairi S Noriah B The Configuration of Ti sapphire Crystal in Cavity Laser and Electro Optic Seminar LEOS 2006 June 28 29 2006 Senai Johor Wan Aizuddin W R Mohamad Khairi S Noriah B Study The Absorption And The Emission Of Ti Sapphire Crystal International Conference on Solid State Science and Technology 2006 ICSSST 2006 September 4 6 2006 Kuala Terengganu Terengganu
55. nd 850 900 nm low transmission around 3 to 7 is needed to be used At the range of 750 nm to 850 nm the transmission can be higher because in this range the laser gain is higher therefore with higher transmission the gain already enough to amplify in laser cavity Because of the output coupler in this work was coating with high transmission of 642 to 830 nm the transmission percentage suitable to use is about 5 Laser gain T 10 15 T 3 7 700 nm 750 nm 850 nm 900 nm Figure 3 29 Ti sapphire gain cross section 64 3 7 3 4 Prisms The main problem to produce the ultrashort pulse is the dispersion Dispersion can cause the different wavelength of light to experience different phase shift Poutous 1996 As a result the shorter wavelength to lag behind the longer wavelength The main source of dispersion is the laser crystal itself Prism pair can be used to compensate the dispersion by producing a negative dispersion as explained in Chapter 2 3 7 4 Detection Devices There are several devices use to detect and analyze the output of the Ti sapphire laser The devices are comprised of Newport power meter Beamstar CCD beam profiler Ocean Optics spectrometer Tektronic oscilloscope and photodiode 3 7 4 1 Power meter Newport power meter Model 841 PE is the newest version to Newport s power meter family This powerful tool is also easy to use and intuitive enough to master in minutes This meter can al
56. nd regime are obtained from solid state laser by mode locking Mode locking also has been proven can produce a train of the time domain requires a broad spectrum in the frequency domain 10 2 2 1 Mode locking Technique There are several techniques to achieve mode locking Practically there are three main techniques to produce mode locking namely active mode locking passive mode locking and Kerr lens mode locking 2 2 1 1 Active mode locking Active mode locked involves placing a very fast shutter in the laser cavity The mode locking take place if the shutter opens only for a very short period of time at every time the light pulse make a complete round trip in the cavity It can be explained by considering a laser with many modes oscillating simultaneously gain modulator E Figure 2 1 Schematic of a modulator insertion in cavity Consider a loss modulator such as acousto optics modulator inserted in the cavity as shown in Figure 2 1 Kowalevicz 2004 In this system locking of the modes will be made by modulating the loss at a period equal to cavity round trip time Tr 2L c Figure 2 2 If the loss is higher than gain the pulse cannot be produced Means no single mode can oscillate When the loss modulates in laser cavity the superposition of the modes 11 will occur Then construct a pulse that would arrive at the modulator just before it opened and pass through just before it closed intensity
57. ngineering Proc Of Large Engineering Systems Conference LESCOPE 139 145 Bartels A Dekorsy T and Kurz H 1999 Femtosecond Ti sapphire ring laser with a 2 GHz repetition rate and its application in time resolved spectroscopy Optics Letters 24 14 Carey J J 2002 Near field effect of Terahertz pulse University of Stratchlyde Ph D Thesis Coherent 2005 Verdi V 2 V 5 V 6 Diode Pumped Laser Canada Operator s Manual Diels J C and Rudolph W 1996 Ultrashort Laser Pulse Phenomena United Kingdom Optics and photonics Donnelly T D and Grossman C 1998 Ultrafast phenomena A laboratory experiment for undergraduates Am J Phys 66 8 677 685 97 Elsayed K A 2002 Development of an efficient Ti sapphire Laser Transmitter for Atmospheric Ozone LIDAR Measurement Old Dominion University Ph D Thesis Eggleston J M Deshazer L G and Kangas K W 1988 Characteristics and Kinetics of Laser Pumped Ti Sapphire Oscillators JEEE J of Quantum Electronics 24 6 1009 1015 Fermann M E Galvanauskas A and Sucha G 2001 Ultrafast Lasers Technology and Applications United State Marcel Dekker Gan F 1995 Laser Material Singapore World Scientific Haus H A 2000 Mode Locking of Lasers IEEE J of Quantum Electronics 6 6 1173 1185 Huang C P 1995 Generation amplification and characterization of ultrashort laser pulses generate by Titanium Doped Sapphir
58. ngth This laser is classified as Class IV which can harm the end user Safety goggle must always be used when operated this laser to protect the eye from potentially damaging exposure 59 The step by step instruction for operates the laser system is as follows Firstly make sure key switch is in the STANDBY position Figure 3 25 Then set power switch on the power supply rear panel to ON The AC power and LASER EMISSION indicator will light After that allow about 30 minutes for the heater and Thermoelectric cooler TEC to achieve operating temperature The display will show SYSTEM WARMING UP Once the process finished the display will indicate STANBY means the system now ready for key on Then put the key switch in ON position then open the shutter by pressing the SHUTTER OPEN push button on power supply front panel Then the laser light will emit from the laser head after the current ramp up Lastly the desire power can be adjusted using POWER ADJUST KNOB Figure 3 25 Power supply Front panel control 3 7 2 2 Beam profile of Verdi 5 Figure 3 26 shows the beam profile of the Verdi 5 DPSS laser in 3D and 2D The beam profile of the Verdi 5 DPSS laser shows the better quality beam Figure 3 26 b 60 shows the beam spot is in perfect TEMoo Figure 3 27 a shows the horizontal cursor profile and Figure 3 27 b shows vertical cursor profile of the Verdi 5 DPSS laser Both vertical and horizontal profile indicated t
59. o apertures A1 and A2 Firstly the DPSS beam needs to enter the first aperture Al Another aperture A2 was placed about 1 meter away Second mirror PM2 needs to be adjusted until the beam enters the second aperture A2 Both apertures were aligned in axis by drawing a line on the optical breadboard using pencil If the beam able to enter both apertures this indicates that the beam is properly aligned 34 DPSS laser Figure 3 2 Alignment of the pumping source 3 3 2 Focusing DPSS beam A focusing lens of focal length 100 mm was employed to focus DPSS laser beam such as shown in Figure 3 3 The beam from PM2 needs to be in the center of the lens L The lens s mounting should be tightened at the base properly to avoid misalignment Proper alignment of the lens is ensured by placing a screen In this case a white paper is introduced Placing at a distance away to see the beam shape after going through L as illustrated in Figure 3 3 The best position is noticed by moving the lens to pro and back but the beam spot does not move The aperture A2 is removed in order to observe beam spot In case the spot moving to the left side the lens mount was tilted clockwise slightly If after adjustment the spot not at the center the lens holder need to be adjusted perpendicular to the beam direction The beam from L is desired to be reflected back into aperture Al The same procedure was repeated several times until the spot will not move 35 Wh
60. o spots were appeared Ensure that the spots are parallel to each other by aligning the prisms The distance between two spots for P1 needs to pass through the apex of prism This can be ensured by setting the distance between two spots reflected by the prism approximately 1 2 mm Figure 3 13 The distance between two spots represented the beam insertion in the prism The lower beam inserted in the prism mean the higher positive dispersion produced Hence the beam insertion should be minimized However ensure that not loosing laser generation The same procedure was used to align the P2 The slit is used to remove the CW beam element in the femtosecond pulse 46 M4 a 2mm Figure 3 13 Alignment of the prism pair and M4 3 5 3 Starting the femtosecond pulse operation In this laser design the femtosecond regime does not self starting Initiating self starting femtosecond regime can be performed by using a Semiconductor Saturable Absorber Mirror SESAM Starting the femtosecond operation was carried out by disturbing the prism P2 using finger to give external perturbation Spence et al 1991 This disturbance would change the depth of insertion of the prism As a result an initial perturbation to generate an intracavity power fluctuation that builds up to a stable circulating femtosecond pulse was produced Ye and Cundiff 2005 The femtosecond pulse was detected by using fast photodetector which coupled to the oscilloscope At the
61. ode astigmatic compensator and three cavity mirrors Mirror Mirror Pump LBO Output coupler Mirror Astigmatic Compensator Etalon Optical Diode Green Output Figure 3 24 The optical components in the laser cavity Coherent 2005 The temperature of the Nd YVQOx crystal and etalon are controlled by thermoelectric cooler TEC which are capable of heating and cooling the optical 57 element The heat from the laser cavity is dissipated by heat sink mounted on the laser head base plate The nonlinear medium for the system is a Type I none critically phase matched LBO crystal held at approximately 150 C Optical diode was used to achieve unidirectional operation which is homogeneously broadened system Therefore it tends to naturally run single frequency with etalon reinforcing this behavior The laser diode was employed as a pumped source The laser diode bar efficiently converts low voltage high current electrical power into laser light Electrical to optical conversion efficiencies typically approach 50 The coupling efficiency obtained in launching light from the bar through the fiber array and into a single transport fiber is predictably specified to be not less than 80 with typical value exceeding 90 When the nominal wavelength is centered on the strong absorption band associated with neodymium ions in a vanadate host more than 90 of the incident diode laser light can be absorbed by the cry
62. of femtosecond pulse In third part describes the starting of femtosecond pulse 3 5 1 Dispersion Control The same Z folded cavity of CW Ti sapphire laser was employed to generate femtosecond pulse However to convert from CW to become pulse laser another components are required A pair of prism needs to be added in the cavity as shown in Figure 3 12 The usage of the prism is to control the dispersion in the cavity By controlling the dispersion in the cavity a femtosecond pulse laser could be achieved The procedure of alignment was started by inserting a prism P1 Firstly we need to insert a prism P1 near the deviation angle After the desired place is obtained the prism s holder was tightened to fix it on the optical table Adjustment knob of prism P1 was adjusted slowly in this manner prism was slightly moved into the beam path Just a few fraction of the beam was required to be inside the prism A stop point for correcting the alignment of the prism was then needs to be found Stop point is the condition whereby the beam starts to change direction as the prism rotated The prism was tightened after getting the stop point A second prism P2 was then aligned into the cavity using the same procedure of prism P1 The distance between the prisms is set around 19 cm as calculated using 44 Equation 2 4 and 2 5 The separation of the prisms will introduce negative dispersion in the cavity which is to compensate the disper
63. olid State Laser 3 7 2 1 Verdi 5 operation 3 7 2 2 Beam profile of Verdi 5 3 7 2 3 Laser spectrum of Verdi 5 Optical component of Ti sapphire laser 3 7 3 1 Dielectric Mirrors 3 7 3 2 Focusing Lens 3 7 3 3 Output coupler 3 7 3 4 Prism Detection Devices 3 7 4 1 Power meter 3 7 4 2 Beam Star CCD profiler 3 7 4 3 Spectrometer 3 7 4 4 Oscilloscope 3 7 4 5 Photodetector 3 7 4 6 Fast Photodetector Software 3 7 5 1 Matrox Inspector 2 1 3 7 5 2 ToptiCale V25 CHARACTERIZATION OF TITANIUM SAPPHIRE LASER BEAM 4 1 Introduction 4 2 The fluorescence of Ti sapphire crystal 4 2 1 Estimation of pulse duration 45 46 48 49 50 55 58 59 61 62 62 62 63 64 64 64 65 65 66 67 67 68 68 68 70 70 74 iX 4 3 Characterization of Continuous Wave CW Laser 4 3 1 Spectrum of continuous wave beam 4 3 2 The power of continuous wave beam 4 3 3 Beam profile of continuous wave beam 4 4 Characterization of Mode locked Laser 4 4 1 Compensating Effect 4 4 2 Cleaning Factors 4 4 3 Stability zone 4 5 Mode locked pulse 4 6 Femtosecond pulse duration CONCLUSION AND SUGGESTIONS 5 1 Conclusion 5 2 Problem 5 3 Suggestions REFERENCES APPENDIX A APPENDIX B PRESENTATIONS 75 75 78 80 82 83 84 86 87 89 91 92 94 96 103 104 105 TABLE NO 3 1 LIST OF TABLES TITLE Physical properties of Ti sapphire PAGE 50 xi FIGURE NO 1 1 2l 2 2 2 3 2 4 2 5 2 6 2l 2
64. oned between two mirrors as illustrated in Figure 2 10 The purpose of the mirrors is to provide what is described as positive feedback This means simply that some of the lights that emerge from the amplifying medium are reflected back into it for further amplification Laser mirrors usually do not reflect all wavelengths but the reflectivity is matched to the wavelength at which the laser operates An amplifier with positive feedback is known as an oscillator 21 Energy input by pumping Total Partial reflector reflector Output Amplifying medium lt q ____ Laser cavity ______ beam Figure 2 10 Laser oscillator 2 4 1 Cavity configuration In the construction of the Ti sapphire laser it is different than the regular setup as common laser design which is in straight line Basically there are two commonly used cavities for Ti sapphire laser as depicted in Figure 2 11 a Figure 2 11 Commonly used cavity for Ti sapphire laser Koumans 2001 22 The Ti sapphire laser design can be in an X or Z configuration The folded cavity suitable to obtain good mode matching with pump and to provide tight focusing in the mirror Mukhopadhyay et al 2003 Beside it can control the astigmatism produce from laser cavity by adjusting the angle of the arm Both types work equally well and usually selected based on considerations of available space in setting up the cavity Silfvast 2004 Nevertheless
65. oped sapphire Ti AlLO3 or Ti sapphire was developed late in laser evolution Since the discovery of laser action in Ti sapphire in 1982 it becomes one of the most widely used solid state laser material Kuhn 1998 The very important application of Ti sapphire lasers is the generation and amplification of femtosecond mode locked pulse Koechner and Bass 2003 It combines the excellent thermal physical and optical properties of Sapphire with the broadest tunable range of any known material Eggleston et al 1998 The physical properties of the crystal are listed listed in Table 3 1 Table 3 1 Physical properties of Ti sapphire Physical properties value Index of Refraction n 1 76 Fluorescent lifetime t 3 2 Us Fluorescent line width FWHM A 180 nm Peak emission wavelength Ap 790 nm Peak stimulated cross section e Parallel to c axis e Perpendicular to c axis 6 4 1x 10 1 cm ot 2 0 x 10 cm Stimulated emission cross section 5 2 8 x 10 cm Quantum efficiency No 1 Nonlinear coefficient 3 2 x 10 cm W Damage threshold 10 J cm Thermal conductivity 0 35 W cm K 51 Ti sapphire crystals has good operation in the pulsed periodic quasi CW and CW modes of operation Ti sapphire is a 4 level laser system with fluorescence lifetime of 3 2 us Koechner and Bass 2003 This crystal can be lased over the entire band from 660 to 1100 nm With broad tunability mak
66. or M3 and OC have to be aligned at an angle of 19 40 with respect to the optical axis of the pair spherical mirror The portion pumped beam was ensured to be located at the center of mirror M3 Figure 3 9 Alignments of Z folded cavity 3 4 2 Testing the CW output Now let we considered the propagation of the beam in the laser Ensure that the OC are reflected to M1 M2 and M3 Overlapping beam between OC and M3 was determined either by using IR viewer or IR card IR viewer is better to used but not available in our lab Therefore the testing was carried out using IR card The IR card needs to be placed in front of the M3 followed by a red filter which was used to block the green beam as shown in Figure 3 10 41 IR card M3 Red filter Figure 3 10 Beam alignment method The IR beam shows on the IR card should be overlapped The first IR at the M3 is came from the fluorescence of the Ti sapphire crystal at initial stage and the second IR beam was reflected by the output coupler OC The first IR beam need to be adjusted until illuminated beam fall at the center of mirror M3 After that the output coupler OC was adjusted until the second IR beam overlap with the first one This step was repeated at the output coupler OC without using red filter When the entire IR beams are overlapping this indicates that the mirrors were properly aligned If a correct alignment has been done the laser generation should be produced The lase
67. otons are random This type emission can t produce laser The laser will be produced only after the stimulate emission occur 2 6 Temperature Control Another aspect to consider on discussing Ti sapphire laser is temperature control It is very vital factor for the Ti sapphire laser system because it can effect the output of the laser such as the power and stability Moreover without proper temperature control Ti sapphire crystal possibly will be ruined The upper state lifetime of the Ti sapphire laser crystal is sensitive to temperature as shown in Figure 2 18 This lifetime has a constant value up to 200 K and then there is a clear reduction in the lifetime with increasing temperature At room temperature the upper state laser lifetime is 3 2 us To have efficient laser performance it is important to keep the temperature of the laser crystal near room temperature Lifetime p 30 4 3 2 1 0 100 00 00 400 00 600 Temperature K Figure 2 18 Lifetime of the upper laser level of Ti sapphire as a function of temperature Elsayed 2002 Due to excessive temperature will result in a reduction of the upper state lifetime and thereby will increase the laser threshold at the same time will reduce the output power To improve the situation cooling system need to be provided In this project water cooling system was utilized to control the crystal s temperature Alternatively Termoelectric cooler cooled by a Peltier cool
68. pectrum output power and beam profile were studied The maximum power produced by CW laser and mode locked laser are 1 12 W and 577 mW respectively The stability zone has been obtained by changing the separation between M1 and M2 The mode locked operation was detected by observing the pulse train captured using fast photodiode and clearly monitored using 500 MHz oscilloscope The mode locked pulses were identified as the regular mode locking This clarified based on similar of the amplitude of the signal The frequency of the pulse is 96 43 MHz The pulse duration was calculated through the mode locked spectrum The mode locked spectrum was detected using Ocean Optics spectrometer The spectrum was analyzed precisely by using Matrox Inspector Version 2 1 software The pulse duration obtained from the calculation according the spectrum is 27 56 fs with bandwidth of 24 80 nm at FWHM and at center wavelength of 806 74 nm 5 2 Problem In this work there are a few factors that obstructed the progress of research The first problem is regarding to the pumping source It arise when the DPSS laser have not enough pumping power Therefore the previous system has been replaced with the new one Verdi 5 DPSS laser A lot of time was spent waiting for the new laser and involved expensive budget Another problem is the DPSS laser have very bright and high power beam Therefore it is very dangerous and we need to follow safety procedure during laser operatio
69. perties applied in femtosecond pulse laser The first property is GVD which is the tendency of various frequencies of light to propagate at different speed in certain material In material with positive GVD the longer wavelength travel faster than the shorter one thus red wavelength shifting the pulse The second dispersive property which also plays important role is the self phase modulation SPM 17 2 3 1 Source of dispersion There are several types of the dispersion source First is dispersion from the material itself In the solid state laser the material dispersion is unavoidable This is because the presence of the lenses mirrors and crystals will introduce material dispersion Values for the dispersion can be calculated from the refractive index The second source of the dispersion is due to slab geometry as shown in Figure 2 7 With the same non zero incident angle light of different wavelength will travel in different optical paths after entering a dispersive material This effect happens in addition to the material dispersion For the broad bandwidth material this effect will be most severe for ultrashort laser pulse The Brewster cut of the Ti sapphire crystal which are commonly used in Ti sapphire laser will introduce such effect Longer length of the crystal will give more dispersion effect Incident angle Brewster s angle 800 nm 60 4 Figure 2 7 Dispersion due to slab geometry Another source o
70. ping process is shown in Figure 4 1 DPSS laser was employed as a pumping source Ti sapphire crystal was utilized as an active medium The green light of DPSS laser is used to excite the Ti sapphire crystal using end 71 pumping technique The pumped beam finally dumped into the beam dumper Immediately after the Ti sapphire crystal was pumped by green beam of DPSS laser the fluorescence was emitted The fluorescence beam is confined in the cavity comprised of two mirrors M1 and M2 The spherical mirrors of M1 and M2 were coated with highly reflective to the infrared IR beam and let through the visible light of green beam The fluorescence beam is incident at an angle of approximately 20 to mirror M1 The reflected the IR beam was detected by using spectrometer which coupled to personal computer A Matrox Inspector software was conducted to analyze the spectrum of fluorescence beam produced from the excited Ti sapphire crystal Beam dumper Me Ti hi i sapphire M1 Spectrometer Figure 4 1 The experimental setup for the fluorescence detection Basically when green light from DPSS laser hits Ti sapphire crystal that lead the titanium atoms into excited state Naturally de excitation occurs whereby the atoms return to the ground level and released energy spontaneously The spontaneous emission produces fluorescence or also known as luminescence The typical luminescence spectrum captured by an Ocean Optics spectrometer is depicted in
71. procedures to align the Ti sapphire laser This includes the alignment of the lens mirrors output coupler and prism pair for Continuous Wave and mode locked cavity In addition this part also discuss about the optimization of femtosecond operation Since the alignment of the cavity is very critical therefore this chapter is the most important part in this work The characterization of the Ti sapphire laser is explained in Chapter VI that have been constructed This includes the spectrum of the beam the output power the beam profile and the estimation of the pulse duration Finally the conclusion of the project is made in Chapter VII The summarization contains the synopsis of the project the problem involved during the performance of the project Last but not least further works to be carried out in the future are suggested CHAPTER 2 THEORY 2 1 Ultrashort laser pulse Ultrafast pulse occur on femtosecond 10 s or shorter time Because the energy in the amplifying material is released in such short pulse each pulse has a very high power output Peak powers of up to one terawatt 10 W have been achieved In order to generate ultrafast pulse medium with large number of frequency modes allow it to produce on a very short time scale According to Heisenberg uncertainty principle Donnelly and Grossman 1998 h AEAt gt 7 2 1 where AE is the standard deviation in the energy and Ar represents the amount of time it
72. r output detected using IR card such as shown in Figure 3 11 If no lasing appeared a photodetector could be used to detect the beam The photodetector is more sensitive compared to the power meter and gives faster response to the power changing The photodetector was coupled to an oscilloscope The output coupler OC and mirror M3 need slightly adjustment to achieve optimum power as manifested in the oscilloscope 42 Figure 3 11 Laser output detected using IR card 3 4 3 Optimum CW operation There are several steps to achieve the maximum power First small change is made upon the upper adjustment knob of M3 When the power decrease upper adjustment knob of output coupler OC need to optimize and see if the new optimum is higher or lower If it is higher continue move the upper adjustment knob of output coupler OC at the same direction If the power turn lower move the upper adjustment knob of output coupler OC in the opposite direction These steps are repeated if neither direction makes it go up it is at the maximum condition After that upper adjustment knob of M3 and output coupler OC was adjusted by using the same step as explained before This adjustment need to repeat several times until the optimum power achieved 43 3 5 Femtosecond operation In the alignment of femtosecond operation the discussion is divided into three sections The first part involves the dispersion control The second part explains the optimization
73. resonant intracavity second harmonic generation SHG The nonlinear optical medium used for SHG is birefringent crystal LBO The efficiency of the SHG process is determined by the crystal orientation as defined by the direction of light propagation polarization state of the incident light and nominal crystal temperature The LBO doubler is housed within an oven that is design to maintain the crystal temperature at a typical value of 150 C Given the proper crystal orientation and considering appropriate polarization states the refractive indices of the birefringent crystal can be arranged to be identical for both fundamental 1064 nm wavelength and its second harmonic at 532 nm When the phase matching condition between the two different wavelength implied by these consideration are satisfied substantial power flow from the fundamental to the second harmonic is obtainable during a single pass through the doubler The plane of polarization for the fundamental is parallel to the laser base plate while the green polarization is perpendicular to it The green light is then extracted from the resonator via a dichroic output coupling mirror which is coated to be highly reflecting at 1064 nm but essentially transparent at 532 nm 3 7 2 1 Verdi 5 operation Since the Verdi 5 can produce beam up to 5 5 W and have very bright green beam so it is very dangerous when operating this laser especially for our eye which absorb highest at green wavele
74. rt power meter The output power for the CW operation was measured as a function of the pumping power The result is represented in Figure 4 8 79 1200 Output power mW Pumping power W Figure 4 8 Output power as a function of the pumping power The trend of graph in Figure 4 8 is almost similar to the graph in Figure 4 7 However the distribution of measurement is better than spectrum measurement The output of laser starts to increase after the input power greater than 1 W Therefore the threshold power for CW of Ti sapphire laser is 1 3 W When the pumping power greater than 1 5 W the output power is proportionally increases with respect to the pumping power The maximum output power of the CW Ti sapphire laser is 1 12 W This obtained at corresponding pumping power of 5 5 W The efficiency of the CW Tusapphire laser is 26 which considered as higher compared to other laser system as obtained by Elsayed 2002 which have 22 efficiency 80 4 3 3 Beam profile of continuous wave beam Beamstar CCD beam profiler was employed to measure the profile of CW Ti sapphire laser The pumping power was set at 1 7 W The lower pumping power was conducted to avoid over exposed to the beam profiler detector The distance between the detector and the beam is 8 0 cm The beam capture at this distance is referred as near field observation The result of the beam profile in near field for 3D and 2D are represented i
75. second pulse laser balancing self phase modulation group velocity dispersion saturable absorption and saturable gain IEEE J of Quantum Electron 22 1 112 118 Vasil ev P 1995 Ultrafast Diode Lasers fundamental and application Boston Artech House Publishers Wang H W 1999 Development and application of high peak ultrafast laser University of Michigan Ph D Thesis 102 Walker S J 2006 Development and Characterization of a Regeneratively Amplified Ultrafast Laser System with an All Glass Stretcher and Compressor University of Waterloo Master Thesis Xing Q Chai L Zhang W And Wang C 1999 Regular period doubling quasi period and chaotic behaviour in a self mode locked Ti Sapphire laser Optics Communication 162 71 74 Xu L Tempea G Spielmann C Krausz Sting F A Ferencz K and Takano S 1998 Continuous wave mode locked Ti sapphire laser focusable to 5 x 10 W cm Optics Letters 23 10 789 791 Xu L Tempea G Poppe A Lenzner M Spielmann Ch Krausz F Sting A and Ferencz 1997 K High power sub 10 fs Ti sapphire oscillators J Appl Phys B 65 151 159 Ye J and Cundiff S 2005 Femtosecond Optical frequency Comb Technology Principle Operation and Application United State Springer Yong Y C 2002 Chartered Electro Optics Diode Pumped Solid State Lasers Electronic Review 16 1 10 11 Zhang W Wang Y Chai L Xing Q an
76. sed to hold the lens tissue The tissue was pulled slowly from the top to the bottom of surface The same procedure was repeated two or three times Each time a different tissue needs to be used not advisable to use the same one It s could cause damage to the surface Alternatively the alcohol solution could be spray on the surface of the component and then wipe it with lens tissue using the same method Excessive cleaning liquid are dried using bulb blower The comparison between the power before and after clean is shown in Figure 4 14 By cleaning the optical component the dust on the optical surface are removed Without the dust the light expose on the optical component can enter optimally This result proves that the cleanness give quite large impact to the output power D fo a 5 2 before cleaning a after cleaning 4 50 5 00 Pumping Power W Figure 4 14 Output power before and after cleaning 86 4 4 3 Stability zone The stability zone will influence the mode locked output of the Ti sapphire laser In fact the stability of the cavity was depended on the distance of the curve mirror In order to prove this one of the mirrors in the cavity that is M2 is provided in micro scale of adjustment to adjust the separation between the curve mirrors The power produced by adjusting of the mirror M2 or the spacing between two curve mirrors was measured The graph obtained in Figure 4 15
77. ser s Manual Newport 2004 841 PE Hand held Optical Power Energy Meter USA User s Manual Ng S P Tang D Y Kong J Xiong Z J Chen T Qin L J and Meng X L 2005 Quasi cw diode pumped Nd GdVO laser passively Q switched and mode locked by Cr YAG Saturable Absorber Optics Communications 250 168 173 Ocean Optics Inc 2005 Fiber Optic Spectrometer USB 2000 USA Installation and Operation Manual 100 Ophir Optronic Ltd 2002 BeamStar CCD Laser Beam Profiler for Windows Jerusalem User Manual Pearson and Whiton G 1993 Use of ZnS as a self Focusing element in Self Starting Kerr Lens mode locked Ti Sapphire laser Oklahoma State University Ph D Thesis Rodriguez M Bourayou R Mejean G Kasparian J Yu J Salmon E Scholz A Stecklum B Eisloffel J Laux U Hatzes A P Sauerbrey R Woste L and Jean P Wolf 2004 Kilometer range nonlinear propagation of femtosecond laser pulses Physical Review E 69 036607 1 5 Rudolf W and Wilhelmi B 1989 Light pulse compression United Kingdom Harwood Academic Publishers Salin F and Brun A 1987 Dispersion compensation for femtosecond pulses using high index Prisms J Appl Phys 61 10 4736 4739 Sarukura N and Ishida Y Ultrashort Pulse Generation from a Passively Mode Locked Ti Sapphire Laser Based System JEEE J Quantum Electronics 28 10 2134 2141 Schneider S Stockmann A and Sch
78. shows the curve of the second stability zone as explained in Figure 2 15 of the Chapter 2 The graph can be used to identify the occurrence of the mode locked pulse According to Xu 1998 the mode locking pulse is appeared at the closer boundary of stability zone The near boundary is measured to be in the range of 108 35 mm to 108 75 mm Thus in order to initiate the femtosecond pulse the perturbation need to provide to the prism P1 within 0 4 mm apart In this case it was in the M1 and M2 spacing range of 108 35 mm to 108 75 mm Near boundary region Therefore if there is no other factor affect the cavity such as the vibration and dust the femtosecond pulse should be appeared at this range However to get the femtosecond pulse is very difficult and requires patience Precise adjustment of the prisms and curve mirror spacing is very important in order to get a better result 87 350 Near boundary 300 l 250 200 150 Output power mW 100 50 0 108 0 108 2 108 4 108 6 108 8 109 0 109 2 109 4 109 6 109 8 110 0 M1 and M2 separation mm Figure 4 15 Output power by adjustment of the M1 and M2 spacing 4 5 Mode locked pulse In order to observe the mode locked pulse autocorrelator is appropriate device to be used Unless such device is not available fast photodetector can be used Lin et al 2002 as replacement to trace the femtosecond pulse formation However the disadvantage o
79. sion in cavity The best position is determined using the same procedure of prism P1 The prism P1 was adjusted until the beam from P1 strikes P2 Both prisms are ensured to be at the same level After that mirror M4 was aligned such as shown in Figure 3 12 The arm length between the M2 to M4 was set to be in the ratio of 4 3 of the M1 OC length With the alignment of mirror M4 the basic alignment of femtosecond operation was completed The mirror M3 was used only during alignment procedure but not included during femtosecond operation this illustrates by the dotted line in Figure 3 12 M4 Figure 3 12 Alignment setup of femtosecond operation 45 3 5 2 Optimization of femtosecond pulse operation Firstly prism P1 was inserted in the beam path All the beams need to strike on the prism P1 By a slight adjustment of mirror M4 the laser pulse generation through the prism would be produced The mirror M3 can be blocked because there is still green laser will be reflected back in the cavity After lasing generation achievable OC and M4 were aligned until a maximum output power was produced The power should at least 40 of output power without prism to ensure enough power was produced during the femtosecond operation Otherwise the first alignment setup of CW operation should be repeated When the maximum output power was obtained by a proper adjustment of the mirrors the reflected beam from the prism apex was aligned By using IR card tw
80. so be networked via USB or RS 232 The built in features of the 841 PE include a complete statistics package that have line plot and a histogram The Data sampling parameters depending on whether a power or energy detector is being used can be set by using the same screen The 841 PE meter is equipped with a DB15 input connector for direct compatibility with Newport s 65 new 818P and 818E Series High Power and Energy Detectors In this project 818P 020 12 series of high power detector was used This detector is very sensitive and has broadband flat spectral response from 0 19 to 11 um Besides that it has high peak power pulse damage resistance The power can be measured in the range of 1 mW to 20 W Typical applications of this detector is including the measurements for CW or pulsed Ion Nd YAG Ti sapphire CO2 high power laser diodes and Excimer laser 3 7 4 2 Beam Star CCD profiler The BeamStar CCD Laser Beam Profiler is a beam diagnostic measurement system for real time measurement of continuous or pulse laser beam It provides an extensive range of graphical presentations and analysis capabilities of laser beam parameters such as beam width shape position power and intensity profiles The CCD based laser beam profiler is fully utilized by powerful software that displays any structure larger than one pixel in vivid color calculates the beam distribution and profile as well as total beam intensity distribution in order to a
81. spectrum intensity at different pumped power The spectrum intensity as a function the power Output power as a function of the pumping power 56 56 59 60 61 61 63 71 72 13 74 76 11 78 79 XV 4 9 4 10 4 11 4 12 4 13 4 14 4 15 4 16 4 17 5 1 3D beam profile in near field 2D beam profile in near field 3D beam profile in far field 2D beam profile in far field Output power as a function of the pumping power Output power before and after cleaning Output power by adjustment of the M1 and M2 spacing Oscillogram of mode locked pulses Spectrogram of mode locked pulse DPSS laser during operation 80 81 81 82 83 85 88 90 93 xvi eRe vm Aw po p a aN a l n A f d P d A Nerd t a SO xvii LIST OF SYMBOLS Energy Planck constant Frequency Standard deviation in the energy Pulse s temporal duration Spectral bandwidth Absolute phase Group velocity Group velocity dispersion Third Order Dispersion Central frequency Wavelength Distance between the apexes of the prism Index of refraction of the prisms Free space wavelength of interest Propagation angle of a ray Dispersion in cavity Product of second order dispersion of crystal Thickness of the crystal Stability parameter Focal length Arm length Tp Av Radius of curvature Optimal fold angle Pulse width Gain ban
82. stal to generate high optical gain within a small volume The tuning rate of diode bars as a function of operating temperature is typically 0 3 nm C Mean that the higher temperature will imply longer wavelength Optimum efficiency for the laser diode is typically obtained when the temperature within the range of 5 C to 35 C Nd YVO was used as a gain medium because offers several significant advantages over alternative solid state laser media common to diode pumped lasers Neodymium ions doped into vanadate host exhibit a comparatively large absorption coefficient centered at a wavelength convenient for pump diode laser They are also spectrally broad therefore insensitive to the precise wavelength or bandwidth of an optical pump source Both of these characteristics contribute to ease of operation and the overall efficiency of diode pumping The characteristic lasing wavelength of Nd YVOx is nominally 1064 nm in the near infrared region of the optical spectrum This infra red light is in fact the oscillating of fundamental wavelength in the resonator rather than the visible green output associated with the device The multilayer dielectric coating of the mirrors that define the ring resonator are designed to provide high reflectance centered at 1064 nm to sustain circulating infrared power levels that typically exceed 100W 58 Converting a fraction of this circulating power into visible light is accomplished via the process of non
83. table and self starting In this case the laser cavity needs to have some modifications The first modification is by putting the SESAM mirror in laser cavity as explained by Jung 1997 By using SESAM no critical alignment is required and the mode locked operation is self starting The other alternative to make the mode locked operation self starting is by inserting another nonlinear material in the laser cavity such as ZnS as suggested by Pearson and Whiton 1993 By using this material it is possible to reduce pump requirement the laser is self starting and improve long stability of the laser Ti sapphire laser is a well known having a very wide tunability The wavelength can be easily tuned by using birefringent tuning element as reported by Keller et al 1991 Therefore it is suggested to insert the birefringent plate in the cavity However the laser tuning range is limited by a mirror reflectivity and birefringent plate itself Since femtosecond laser study is still a new field in Malaysia we hope this research will give benefit to the femtosecond laser studies in our country Last but not least hopefully this thesis will be a good reference for future studies 96 REFERENCES Aschom J B 2003 The role of focusing in the interaction of femtosecond laser pulses with transparent material Harvard University Ph D Thesis Alaoui C and Salameh Z M 2001 Solid State Heater Cooler Design and Evaluation Power E
84. tion the dispersion control is very vital because it can lead to temporal broadening of pulse limiting the generation of ultrashort pulse and also disable the mode locking Figure 2 6 shows the effect of the dispersion to the pulse 15 We can see the differences between pulse without dispersion top and the pulse after dispersion According to Figure 2 6 there are two aspects need to be considered First the center of the pulse is delayed with respect to a pulse traveling in air This is usually called the group delay which is not a broadening effect Second normally ce dispersive media such as glass impose a positive frequency sweep or chirp on the pulse meaning that the blue component is delayed with respect to the red and the pulse becomes broadened one This is due to presence of the positive Group Velocity Dispersion GVD or by other terms Group Delay Dispersion GDD Figure 2 6 The effect of the dispersion to the pulse Carey 2002 The dispersion will cause shorter wavelength to lag behind the longer wavelength This type of dispersion known as positive dispersion and the resulting broadened pulse is said to be positively chirped When dispersion broadens the bandwidth ultrashort pulses will have large group delay difference between the two ends of the spectrum Then limit the mode locked bandwidth Since the wavelength on either end of the spectrum will have different group delays they cannot oscil
85. to the spontaneous emission in the cavity and the sensitivity of the spectrum analyzer detector itself Intensity counts Intensity counts 400 400 300 i 300 i 200 i f i 200 i aK PEE PIERE PEE AIE eee ee EEA EETA E a ee pee ey SEG ey ee eee 100 100 g 0 ia Li a jr i m a i 749 150 760 mo 70 190 900 810 sa Wavelength nm Wavelength nm 2 0 W 2 5 W Intensity counts intensity counts 400 400 300 5 i 300 I 200 H 200 hi I IIN SENIA ENA D E ey eens Deed ere sinters ceases e aE Usha ketenes brads Ath ail icani pbietniaan 100 100 D D 740 750 750 770 780 70 00 10 820 sel m Ho ut ue La ag gi ne Wavelength am Wavelength nm Intensity counts Intensity counts i 400 400 300 ae 300 m i Messi oe IN jA RABI TAB ENEE INAL ARK RNA ee anne al E eee eee ee a Oe ie et Oe Ce See eee ani 100 100 0 0 740 750 760 770 780 790 800 810 820 740 750 760 770 780 790 800 810 820 Wavelength nm Wavelength nm 4 0 W 4 5 W Figure 4 6 The spectrum intensity at different pumped power 78 400 350 a Q O N e O Spectrum intensity no gi oO oO Q O oa O 1 2 3 4 5 6 Power W Figure 4 7 The spectrum intensity as a function of power 4 3 2 The power of continuous wave beam The output power of continuous wave CW Ti sapphire laser was measured using Newpo
86. tor is 3 5 ns The photodetector having active area of 10 mm with response wavelength in the range of 350 nm to 1100 nm 3 7 4 6 Fast photodetector A fast photodiode Model PIN 800 is possible to monitor the output of Q switch lasers and mode locked laser In this work the photodiode was utilized to capture a pulse train produced by the Ti sapphire laser during mode locked operation which couple to Tektronix oscilloscope The type of photodiode is a PIN detector The rise time of the photodetector is 200 ps It has an active area of 1 mm with response wavelength in the range of 350 nm to 1100 nm 68 3 7 5 Software In this work Matrox Inspector Version 2 1 and ToptiCalc V25 softwares were utilized for analysis The Matrox Inspector Version 2 1 was particularly used to measure the bandwidth of the spectrum produced by Ti sapphire laser Meanwhile ToptiCalc V25 software was used to calculate the pulse duration of the mode locked Ti sapphire laser signal 3 7 4 1 Matrox Inspector 2 1 Matrox Inspector Version 2 1 software have 32 bits application running under Windows 95 Windows NT or Window XP that allow to load grab process view analyze print organize and save images It does not need a frame grabber to run and comes with a number of sample images and scripts This software is used to analyze the spectrum which measured using spectrometer The typical spectrum are the result from the fluorescence and mode locked output Prior
87. uced after excited by DPSS laser was studied The configuration of the cavity was chosen to be in Z folded type The lasing was tested in two modes Firstly in continuous wave operation and secondly in mode locked operation A prism pair was conducted to compensate the dispersion High speed photodetector was utilized to detect the mode locked signal The spectrum analyzer was used to measure the wavelength of the output beam and estimate the pulse duration 1 6 Thesis outline The thesis is divided into seven chapters The first chapter will discuss about the ultrafast laser advantages and reviewing some improvement regarding ultrafast laser Chapter II reviews the theory related to the research This will explain the detail of the mode locking technique and theory behind the development of the Ti sapphire laser such as dispersion compensation and cavity design Chapter III describes the methodology of the project This would include entire materials used to setup the laser cavity such as active medium pumping source and optical components The measurement equipments and software for analysis utilize are also will be included Chapter IV explains about the pumping source used to excite the Ti sapphire crystal In this part all the specifications and procedure to handle the DPSS laser are provided Lastly the operation of the laser system and the characterization of the laser output will be discussed Chapter V discusses the
88. uing support patience throughout the present work and who have favored me with correspondence I have much pleasure in expressing my obligation May Allah bless those who have involved in this project ABSTRACT A Ti sapphire laser was developed based on self mode locking technique using a Z folded cavity Diode pumped solid state laser Verdi 5 was used as a pumping source with fundamental wavelength of 532 nm suitable for the absorption band in Ti sapphire crystal Laser cavity was aligned by a set of mirrors with a high reflectivity of 99 8 to reflect the beam within the range of 720 nm to 820 nm and an output coupler with a 5 transmission A pair of prism was employed to control the dispersion for producing femtosecond pulse The pulse was initiated via an external perturbation The stability of the laser was sustained by providing a water cooling system The laser operated in two modes which are continuous wave mode CW and pulse mode with mode locked ML mechanism The maximum output power of the CW Ti sapphire laser is 1 12 W corresponding to a pumping power of 5 5 W and the efficiency of 26 The optimum average power of mode locked Ti sapphire laser is 577 mW corresponding to the same pumping power of 5 5 W and a lower efficiency of 18 The frequency of mode locked laser pulse obtained is 96 43 MHz The spectrum of laser radiation is centered at 806 74 nm with a bandwidth of 22 37 nm at full width half maximum FWHM The pulse
89. ulbauer W 2000 Self starting mode locked cavity dumped femtosecond Ti sapphire laser Optics Express 6 11 220 226 Shapiro S L 1977 Ultrashort light pulses Picosecond Technique and Applications Germany Springer Verlag Silfvast W T 2004 Laser Fundamentals United State Cambridge University Press Smith D L 1999 Coherent Thinking Engineer and Science 4 6 17 101 Song C Hang Y Zhang C Xu J and Zhou W 2005 Growth of composite Ti sapphire by hydrothermal method J of Crystal Growth 277 200 204 Spence D E Kean P N and Sibbett W 1991 60 fsec pulse generation from a self mode locked Ti sapphire laser Optics Letters 16 1 42 44 Sun Z Li R Bi Y Hu C Kong Y Wang G Zhang H and Xu Z 2005 Experimental study of high power pulse side pumped Nd YAG laser Optics amp Laser Technology 37 163 166 Sutter D H Jung I D Kartner F X Matuschek N Genoud F M Scheuer V Tilsch M Tschudi T and Keller U 1998 Self Starting 6 5 fs Pulses from a Ti Sapphire Laser Using a Semiconductor Saturable Absorber and Double Chirped Mirrors IEEE J Of Selected Topics In Quantum Electronics 4 2 169 178 Svelto O 1998 Principles of Lasers Fourth Edition United State Springer Tate J L 2004 Intense Laser Propagation In Sapphire Ohio State University Ph D Thesis Valdmanis J A and Fork R L 1986 Design considerations for a femto
90. ure 3 21 He Ne laser was used as a source of illumination The Ti sapphire crystal was placed on a rotating stage He Ne laser was incident on to the crystal at an angle The reflection of the beam was detected by photodetector and displayed the power on power meter Power meter Rotating stage Ti sapphire crystal Figure 3 21 Schematic diagram of Brewster angle experiment In this experiment the power of the reflected beam from the crystal was measured The He Ne laser was placed in two polarization directions Firstly in the vertical polarization perpendicular component and secondly in the horizontal polarization parallel component The result obtained from the experiment was shown in Figure 3 22 For the perpendicular component the power was found gradually increase with the angle In contrast the parallel component gradually decreases until reach at 60 Immediately after the minimum point the reflectance power drastically increases with the increment of the angle The graph is in the good agreement with the result obtained by Hecht and Zajac 1982 The parallel component of the beam was used to calculate the Brewster angle The curve showed that Brewster angle of the crystal is equal to 60 From the result the refractive index also can be calculated using Equation 3 1 Ouseph et al 2001 55 n tan 0 3 1 ny where 0g is brewster angle n is air s refractive index and mis crystal s refractive index

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