5540 User Manual RevC
Contents
1. the compensator is rotated around its axis Rotate the compensator The compensator is rotated by turning the Orientation knurled ring When the initial alignment has been done properly the polarization compensator transmits the incident polarization unchanged whatever the orientation angle The indicator setting for Retardation is NOT the actual value of the tilt angle See Appendix Determining Compensator Settings for the relationship between Retardation indicator setting and tilt angle NEW FOCUS Figure 1 Initial alignment of the Berek compensator Model 5540 Patent 5 245 478 Retardation Orientation This ring controls the tilt angle See E This ring rotates the waveplate Appendix 1 for the relationship It is usually convenient to set the between tilt angle and the settings waveplate so the zero for this of this indicator Set this control to control is horizontal or vertical zero for initial alignment St To align the compensator force the light reflecting back from the compensator to propagate along the direction of the incident beam Brake This setscrew locks the orientation ring to prevent accidental re adjustment after the compensator has been set Setting the Actual Retardance The Retardation indicator controls the compensator plate tilt angle and hence the actual retardance value Setting the Retardation indica2. Model 5540 In the event that the compensator malfunctions or becomes damaged please contact New Focus for a return authorization number and instructions on shipping the unit back for evaluation and repair Customer Service e 19 20 e Customer Service NEW FOCUS Appendix Determining Compensator Settings The most general application which uses a compensator is one in which the input and desired output polarization states are known and the compensator settings need to be determined The actual desired retardance and orientation angle can be determined in a straightforward manner by first defining the input and output states in terms of the ellipticity e minor axis a major axis b and the orientation a of the major axis in the lab frame Figure 5 Arbitrary elliptically polarized light can be defined in terms of e anda Model 5540 Appendix Determining Compensator Settings e 21 From these parameters the retardance and orientation can be determined using the equations associated with the Poincar sphere A 2a A 2Q X cosA cosE x cosA cosE y sinA cosE y sin A cosE X X k tan 2 1 Ya Ni do x Yor v cd h sinE h sin hahah M P h h reun M Ya The solutions to these equations can then be used to determine the actual retardance R and orientation Z pen Z 05k 2m These results are completely general and can be used to determine the settings to conve
3. USER S GUIDE The Berek Polarization Compensator Model 5540 U S Patent 5 245 478 p New Focus A Newport Corporation Brand 3635 Peterson Way Santa Clara CA 95054 USA phone 408 980 5903 fax 408 987 3178 e mail techsupport newfocus com www newfocus com Warranty Newport Corporation guarantees its products to be free of defects for one year from the date of shipment This is in lieu of all other guarantees expressed or implied and does not cover incidental or consequential loss Information in this document is subject to change without notice Copyright 2012 2001 1998 Newport Corporation All rights reserved The New Focus logo and symbol are registered trademarks of Newport Corporation Document Number 554000 Rev C Contents Operation Introduction RUE Using the Compensator THEON mer Applications Characteristics Compensator Characteristics Customer Service Technical Support i ria Service ria fata TR nd xU rais Appendix I Determining Compensator Settings Model 5540 Contents e 3 4 e Contents NEW FOCUS Operation Introduction Model 5540 The Model 5540 Berek polarization compensator can convert the polarization of a collimated laser beam from any state to any other state with the flexibility and precision previously characteristic only of expensive and lossy Soleil Babinet compensators Used as a variable waveplate the Berek compe
4. acing a linear polarizer after the Berek compensator you can conveniently vary the attenuation of a beam without changing its polarization hi me RE A _ SIEHE li ti ice fh l i lout E input e b output polarizer 0 varies 0 r n 9 4 2 By varying the retardance and orientation you can produce arbitrary states of elliptical polarization which is especially useful in spectroscopy T User Defined 0 User Defined NEW FOCUS Characteristics Compensator Characteristics Model 5540 Model 5540 Wavelength Range 200 1600 nm Clear Aperture 10 mm Wavefront Distortion lt 1 8 wave Retardance 0 5 87 300 nm 0 n 1600 nm Resolution 0 001 wave null 0 01 wave 2x Characteristics e 17 18 e Characteristics NEW FOCUS Customer Service Technical Support Information and advice about the operation of any New Focus product is available from our applications engineers For quickest response ask for Technical Support and know the model and serial numbers for your product Hours 8 00 5 00 PST Monday through Friday excluding holidays Toll Free 1 866 NUFOCUS 1 866 683 6287 from the USA amp Canada only Phone 408 980 5903 Support is also available by fax and email Fax 408 987 3178 Email techsupport newfocus com We typically respond to faxes and email within one business day Service
5. collimation The smaller the retardance the less sensitive is the polarization to imperfect collimation due to the angular dependence ofthe birefringence Circular Polarization To verify circular polarization reflect the output light back through the compensator The polarization ofthe wave that goes through the compensator twice will be exactly orthogonal to the incident polarization for the circularly polarized light Ifthe incident polarization is linear this can best be achieved by setting a calcite polarizer in front of the Model 5540 polarization compensator oriented to transmit the incident beam perfectly Ifthe compensator is set to produce perfect circular Operation 9 polarization the reflected wave should be blocked by the calcite polarizer to 0 01 Slight adjustments to the compensator Orientation and Retardation and hence to the tilt angle can improve the purity of the circular polarization and reduce the transmitted intensity in this test For best precision the plane containing the forward and backward propagating beams should be orthogonal to the plane of incidence of the incident beam on the tilt plate Elliptical Polarization To verify elliptical polarization you must measure the major and minor axes of the polarization ellipse or use a second compensator as is found in an ellipsometer Special cases such as produced by M2 waves of retardance where Nis an integer can be verified in a way similar
6. dance Foran in depth discussion on calculating these curves see Appendix Determining Compensator Settings It contains the equations necessary to determine the actual retardance required to convert an arbitrary input polarization state into a specified output polarization state It also contains both the exact expression for actual retardance versus Retardation indicator setting and the relationship between tilt angle and Retardation indicator setting 12 Operation NEW FOCUS Figure 3 2 50 Retardance in waves versus indicator 2 00 setting at 488 nm 632 8 nm and 1060 nm 1 50 1 00 0 50 1060 nm 0 00 0 0 5 0 10 0 15 0 Figure 4 15 Indicator setting versus A 13 2 wavelength for v 4 2 and 2 4 retardance T 9 A 7 D 5 3 04 0 8 12 16 Model 5540 Operation e 13 14 Operation NEW FOCUS Applications The Model 5540 polarization compensator can be used as a1 2 wave plate to rotate the plane of polarization of a linearly polarized laser beam Lae a a E output It can be used as a 1 4 wave plate to make a plane polarized laser beam circularly polarized When used with a linear polarizer this will provide isolation which protects the laser from feedback hi RI ESRI ERA t o Less POZA i mn input output 2 ga 4 Model 5540 Applications e 15 16 e Applications By pl
7. ft or retardance For example Figure 2 shows the slow axis being retarded by 90 relative to the fast axis resulting in linearly polarized light being converted to circularly polarized light when the device is oriented at 0 A Linearly polarized light at the input of the Berek compensator B Circular polarization at the output of the Berek compensator when the retardance is tuned to 90 Operation e 11 The Model 5540 Berek compensator allows the retardance and the orientation of the plane of incidence to be adjusted conveniently and independently using two knurled rings one on each end of the housing Once the tilt angle is set correctly with the Retardation knob rotating the housing with the Orientation knob rotates the variable wave plate just like a conventional compensator or retarder Precision scales allow both tilt angle and orientation angle to be set accurately The axis of rotation for the plane of incidence can be conveniently set parallel to the propagation direction using a conventional optic mount Near normal incidence the retardance increases quadratically with tilt angle allowing very precise compensation of small phase shifts due to other optical elements Figure 3 shows the actual retardance versus the Retardation indicator setting for the New Focus Berek compensator at 488 nm 632 8 nm and 1060 nm Figure 4 shows Retardation indicator setting versus wavelength to achieve precise A 2 and 4 retar
8. nsator can impose 1 4 wave or 1 2 wave retardance at any wavelength between 200 nm and 1600 nm dramatically reducing the number of different plates necessary in the laboratory The Model 5540 polarization compensator can fit in a variety of 2 mirror mounts or you can mount it directly to a post With its 10 mm aperture this compensator will accept beams from almost any commercially available laser system Operation e 5 Using the Compensator 6 Operation Initial Alignment 1 Set the Retardation indicator on the compen sator to zero This also sets the tilt angle to zero Place the compensator in your setup Make sure that the collimated laser beam is centered on the 12 mm diameter aperture Mount the compensator housing It should be mounted such that the o on the Orientation indicator is at a convenient angle We recommend mounting the Model 5540 in a gimbal type mirror mount such as the Model 9852 shown in Figure 1 Align the compensator Leaving the Retardation indicator set to o use the two tilt controls of the mirror mount to align the compensator so that it is normal to the propagation direction of the light To do this find the reflection from the compensator plate and force it to propagate back along the direction of the incident beam When the compensator is properly aligned and the Retardation indicator is set to O the reflected beam will not move away from the incident beam direction when
9. on causes the plane of polarization to rotate by twice the orientation angle After adjusting the orientation angle it may be convenient to lock the orientation adjustment ring This can be done by tightening the set screw marked Brake on the ring itself See Figure 1 Verifying the Output Polarization When using a waveplate one should always verify that it has produced the desired polarization There are various methods some of extreme precision NEW FOCUS Model 5540 Linear Polarization Since linear polarization can beblocked to better than 0 01 with a calcite polarizer it is easy to verify that the output of the Model 5540 Berek polarization compensator is linear and oriented properly 1 Placeacalcite polarizer such as a Model 5524 or 5525 after the compensator 2 Orientthe polarizer to block the desired output polarization 3 Fine tune the polarization Both the tilt angle see Setting the Actual Retardance on page 7 and orientation angle of the compensator can be used to do this At this stage do not use the mounting to adjust the tilt of the compensator housing Ifthe laser beam is not perfectly collimated you may see a narrow absolutely dark line through the center of a very dim beam transmitted through the polarizer If too much light is transmitted through the polarizer even when tilt and orientation have been optimized consider reducing the retardance by one full wave or improving the
10. rt any input polarization into any output polarization assuming the states can be defined in terms of their ellipticity and orientation 22 Appendix Determining Compensator Settings NEW FOCUS The relationship between retardance A and tilt angle can be determined from the angular dependence of the birefringence in MgF The extraordinary index of refraction as seen by the optical beam is given by where Opis the tilt angle and n and me are the indices of refraction These are in general wavelength dependent and their dispersion relations are given by 0 48762 0 398822 2 31202 no 1 2 2 t 2 2 2 b 00434 l 00946 b 23796 poo qa 04134 050504 249054 0036 2 00908 2 270 Thus the retardance A in waves is related to the tilt angle by _ 2000 2 1 n 2 2 2 sin Op ny sin Op 2 sin 8 e 1 n The relationship between tilt angle 64 and Retardation indicator setting is given by 50 22 71 0 M or 25022 Tisi 7 67 Dodge M Appl Opt 23 12 1980 85 1984 Model 5540 Appendix Determining Compensator Settings e 23 24 e Appendix Determining Compensator Settings NEW FOCUS
11. to circular polarization but with the light making passes through the compensator plate Theory 10 Operation Invented in 1913 the Berek polarization compensator has long been used in microscopy and other technical applications It consists of a single plate of a uniaxial material with its extraordinary axis perpendicular to the plate Therefore when light is at normal incidence to the plate it propagates through the device with a velocity independent of polarization It sees an isotropic material in this configuration and the Berek polarization compensator has no effect on its polarization Berek M Zbl Miner Geol Pal ont 388 427 464 580 1913 quoted in Born M and Wolf E Princi ples of Optics 6th edition Pergamon Press London 1980 p 694 NEW FOCUS Figure 2 Conversion of linearly polarized to circularly polarized light Model 5540 However when the plate is tilted with respect to the direction of the incident light the plane of incidence becomes the plane of the extraordinary index of refraction Light polarized in this plane propagates at a different velocity The wave is slowed or retarded by an amount that depends on the angle of tilt and on the wavelength Light in the polarization perpendicular to the plane of incidence continues to propagate as an ordinary wave with velocity independent oftilt angle Thus the light in the two planes of polarization accumulates a relative phase shi
12. tor to o gives zero tilt and zero retardance At nonzero values the Berek polarization compensator can be set to either a single order or a multiple order waveplate The smaller the retardance the more accurately it can be set To a good approximation the tilt angle R that gives retardance R in waves is da sin 0284 2R where A is the wavelength in microns Operation e 7 8 e Operation The Retardation indicator setting I is related to the tilt angle by 5022 71sin 6 Approximate Retardation indicator settings are shown in Figure 3 More accurate values can be obtained using the formulas in Appendix Determining Compensator Settings When setting the Retardation make sure that the entire compensator housing and therefore the orientation angle does not rotate Setting the Orientation Angle The knurled Orientation ring rotates the entire housing to properly orient the slow axis of the compensator plate with respect to the incident polarization If the compensator has been mounted with the o on the orientation scale vertical you can read the orientation angle directly To make circular polarization from linear polarization you must set the retardance to 1 4 wave and the orientation angle to the incident linear polarization direction plus 45 Ifthe retardance is set to 1 2 wave a 45 orientation angle will rotate the plane of polarization by 90 In general a A 2 wave plate rotati
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