An Introduction to the New Focus TA
Contents
1. DS 1014012. ATION NOTE TA 7600 VAMP Tapered Amplifier To illustrate the effect of seed wavelength on VAMP TA performance we took the setup in Fig 5 and measured the output power from the VAMP TA as aii the seed wavelength was tuned from 945 975 nm All else equal we would Gti expect that a graph of Pout vs A would qualitatively resemble the tapered 1000 diode spectrum in Fig 6 with the greatest amplification occurring near the maximum in the spectrum Figure 7 summarizes what was observed after 800 600 correcting for variations in the seed power with wavelength Indeed we see Output Power mW that the greatest amplification occurs in the vicinity of 950 nm near the maximum in the output spectrum of the tapered diode 950 960 nm In Am addition the shape of the amplification curve resembles the spectrum of the 0 tapered diode above 945 nm Therefore although reasonable amplification Sead Pawei mi can be achieved at other wavelengths best results are obtained when the Figure 8 Seeding of VAMP TA by Velocity laser at 960 nm seed wavelength matches the wavelength corresponding to the maximum in the output spectrum of the tapered diode 6 Summary In applications requiring more power than can be provided by an ECDL we have seen that the New Focus TA 7600 VAMP TA system is a robust solution Its thoughtfully designed controller and safety features are user oriented and it can be paired with New Focus lasers as well as
3. TA head is shown in Fig 3 along with callouts to some key features On the left is the FC APC input fiber connection a standard on all VAMP TAs This allows the user to simply connect the FC APC patch cable carrying the seed beam to the head In this way the seed beam is easily coupled into an internal fiber b that has been carefully pre aligned to the input facet of the tapered amplifier diode and permanently fixed in place This pre aligned internal fiber ensures consistent and trouble free alignment of the seed beam into the diode every time The seed and output power are monitored using two pick off Si photodiodes c The amplifier output can either be fiber coupled d or free space A 35 dB isolator at the output not shown in the figure is standard with every amplifier Fluctuations in the temperature of the tapered amplifier diode can adversely affect the VAMP TA performance therefore temperature control is critical Thermal isolation of the laser head is achieved by enclosing with thermally insulating foam e and the aluminum base of the tapered amplifier housing f acts as a dissipative heat sink An on board temperature sensor g is used to monitor the diode temperature and provides the input to the active temperature control closed loop Finally a thermoelectric cooler affixed to the bottom of the diode block maintains a constant diode temperature Figure 3 Glimpse inside a VAMP TA head Tapered Amplitier Contr
4. TECHNICAL APPLICATION NOTE An Introduction to the New Focus TA 7600 VAMP Tapered Amplifier 1 Introduction The output power of an external cavity diode laser ECDL can often range from 10 100 mW depending on such factors as the capability of the laser diode the external cavity optics and their configuration and the drive electronics However some applications require much higher laser power and in these cases an optical amplifier can be used to directly amplify the ECDL output The amount by which the ECDL output is amplified is often referred to as the power gain and is usually measured in units of decibel dB The power gain of an optical amplifier which takes a seed power Pooag to give an amplified output power Pout can be obtained using the equation below For a hypothetical optical amplifier which takes a seed power of 10 mW to give an output power of 1 W the power gain has a magnitude of 20 dB Peed Power gain dB 10xLog z out Depending on the ECDL wavelength different optical amplification techniques can be used In the near IR and short wavelength IR rare earth doped fiber amplification is often used Here the gain medium is a glass optical fiber that is doped with rare earth ions of Er Nd or Yb By first optically pumping the ions a laser beam propagating down the fiber can subsequently be amplified by way of stimulated emission In Raman or Brillouin scattering amplification the gain medium is usual
5. d by pressing the Power Lock button In this way the output power can be highly stabilized with fluctuations exceeding no more than 1 TECHNICAL APPLICATION NOTE TA 7600 VAMP Tapered Amplifier Although the instructions above will be more than enough for most applications detailed information on the operation of the VAMP TA can be obtained from the User s Manual 5 Effect of Seed Laser Wavelength and Power On the Output of a VAMP TA System Currently each VAMP TA is built and tested using a New Focus Vortex Plus laser However any laser not exceeding the 100 mW maximum input power at the FC APC connector can be used to seed a VAMP TA However not all seed lasers are created equal and the performance of a VAMP TA system will depend on a couple of seed laser parameters including the wavelength and power To achieve optimal amplification the seed laser wavelength should match as closely as possible the center wavelength in the spectrum of the tapered amplifier diode The example setup of Fig 5 includes a VAMP TA The spectrum for the tapered diode inside this VAMP TA is shown in Fig 6 below and it shows when being driven at a diode current of 3 3 A a maximum in output intensity near 950 960 nm Intensity arbitrary units 920 930 940 950 960 970 980 Wavelength nm Figure 6 Output spectrum from 920 980 nm of the tapered diode inside the VAMP TA of Figure 5 Vv New Focus A Newport Company TECHNICAL APPLIC
6. ly an optical fiber but can be a crystal or cell filled with gas or liquid For applications in the visible and near IR regions tapered semiconductor diodes are often used as the gain media And as we will see the tapered semiconductor diode is at the heart of the New Focus TA 7600 VAMP Tapered Amplifier system This Note begins with a brief introduction to optical amplification by way of the tapered semiconductor diode We then concentrate on the New Focus TA 7600 VAMP Tapered Amplifier VAMP TA system with an emphasis on its design features and ease of use In the last section we look at how to implement the VAMP TA system in amplifying the output of a seed laser and will see how such seed laser parameters as wavelength and power can ultimately affect its performance Figure 1 Tapered amplifier chip on standard C mount Arrow points to chip 2 What is a Tapered Amplifier As mentioned above a tapered amplifier can be used to optically amplify milliwatt radiation from a seed ECDL into watts all while preserving the spectral properties of the seed wavelength linewidth polarization etc At the heart of any tapered amplifier system is the amplifier chip an example is shown in Fig 1 and central to the chip itself is the tapered gain region This tapered gain region with an output facet much wider than the input facet is often made of a IIl V semiconductor such as GaAs By electrically pumping this region an inversion in
7. oller Figure 4 Easy to use front panel of the A 7600 LN Tapered Amplifier controller offers full controls over VAMP TA system The VAMP TA head is controlled using the TA 7600 LN Tapered Amplifier controller which is shown in Figure 4 The system can be easily controlled through the new easy to use multi line display front panel interface In addition the controller is capable of automatically recognizing any 7600 series VAMP TA head This makes its use turnkey as the controller will maintain a constant diode temperature and limit the maximum current to the tapered diode according to information programmed into the circuit board in the head Remote control of the VAMP TA is possible via either the RS232 or USB port on the rear panel Moreover an intuitive Windows based graphical user interface GUI application is included with each controller The GUI permits the full functionality of the front panel save for turning the key switch on off with the convenience of remote control The next section illustrates the use of a VAMP TA system laser head and controller in amplifying the output of a New Focus seed laser V New Focus A Newport Company 4 How To Use the New Focus VAMP TA System Figure 5 shows a ILB 6718 Velocity laser head a and its TLB 6700 LN controller b This is being used to seed the VAMP TA system head c and controller d As shown with the Velocity laser set to A 960 nm its fiber coupled output i
8. s connected to the FC APC input fiber connection of the VAMP TA head Figure 5 Seeding of a VAMP TA by a Velocity laser The VAMP TA system is turnkey Turn the keyswitch on the front panel to the I position as shown in the Fig 5 This will turn ON the system On startup the display screen will cycle through a series of informative screens and stop at the main screen through which the user can control all VAMP TA settings If Poaag lt 10 mW the nput Power LED will be lit red and you will not be able to turn the current ON to the tapered diode This is a safety feature which prevents the user from degrading the lifetime of the tapered diode through self lasing When Pseed 2 10 mW sufficient seed power the Input Power LED will be lit green allowing the current to the tapered diode to be turned ON The current to the tapered diode is turned on via the Current On Off button After the Current On Off LED blinks six times the control knob on the front panel can be turned to increase the diode current until the user reaches a desired diode current value or until an appropriate level of output power has been established Unlike some other tapered amplifier systems the New Focus VAMP TA system has been designed to prevent the user from unintentionally increasing the current to the tapered diode past its maximum operating current After reaching an appropriate power level the power lock feature of the VAMP TA system can be activate
9. the carrier population can be established Both facets are usually anti reflection coated Thus a beam of radiation of suitable wavelength and polarization subsequently directed into this gain region would experience amplification due to stimulated emission resulting from electron hole recombination In a Master Oscillator Power Amplifier MOPA configuration an external laser is used to provide the seed radiation to the tapered amplifier system An example schematic of a MOPA setup is shown in Fig 2 In this example a New Focus Vortex Plus Is being used to seed a VAMP TA represented by the dashed lines As shown in the figure internal optics shape and guide the beam Lens 1 helps focus and direct the seed beam into the diode The amplified output is then collimated by lenses 2 and 3 and passes through a single stage isolator which prevents light from being fed back into the diode V New Focus A Newport Company TA 7600 VAMP Tapered Amplifier SS ee ee Se ee ee ee eee I Lens 2 Lens 3 Isolator New Focus Vortex Plus mil r Tapered Diode Seed Laser VAMP Tapered Amplifier Figure 2 Optical amplification via MOPA configuration Seed New Focus Vortex Plus laser seeds VAMP TA represented by dashed lines Some internal elements are shown albeit not to scale Input and output optical fibers not shown 3 Design Features of the New Focus VAMP TA System A three dimensional rendition of a VAMP
10. third party lasers We have also looked at how the output power can be highly sensitive to the seed laser wavelength and power by taking as an example a Velocity VAMP TA pair Although each seed laser VAMP TA combination will lead to somewhat different output parameters we have drawn some general Amplification conclusions and can use these to predict roughly the outcome of a given seed laser VAMP TA pair The New Focus TA 7600 VAMP TA system frees 945 950 955 960 965 970 975 Wavel nath am your mind from having to worry about power and keeps you focused on the science Figure 7 VAMP TA amplification as a function of seed wavelength To illustrate the effect of the seed power the output power was measured now as a function of seed power i e Pout VS Popa The results are shown in Fig 8 The plot shows an interesting trend of nearly linearly increasing output power with increasing seed power below approximately Peeeg 9 mW Beyond this there appears to be a plateau in the output power to give a less than linear dependence saturating at a value of approximately 1300 mW Although the exact shape of this dependence will change with seed laser VAMP TA pair the overall trend in is clear there is a regime in which the output power will nearly linearly depend on the seed power however after a certain value of seed power you see only diminishing gains in the output until Pout saturates V New Focus A Newport Company
Download Pdf Manuals
Related Search