5540 User Manual RevA
Contents
1. indicator setting and the relationship between tilt angle and Retardation indicator setting Fig 2a 2 50 Retardance in waves versus 7 00 indicator setting for the New Focus Berek compen 150 sator at 488 nm R 632 8 nm and 1060 nm 100 0 50 1060 nm 0 00 0 0 5 0 10 0 15 0 15 I 13 Fig 2b 11 Indicator setting versus wavelength 13 for 72 and 4 K retardance 7 F 5 NOTE Please see Appendix 1 for the rela tionship between tilt angle and Retardation 3 indicator setting 0 4 0 8 1 2 1 6 Vv Operation 1 Initial alignment Fig 3 Set the Retardation indicator on the Model 5540 Berek compensator to zero this also sets the tilt angle to zero Place the compensator in your setup making sure that the collimated laser beam is centered on the 12 mm diameter aper ture We recommend mounting the Model 5540 in a gimbal type mirror mount such as the Model 9852 shown in Fig 3 The compensator housing should be mounted such that the 0 on the Orientation indicator is at a convenient angle Now leaving the Retardation indicator set to 0 use the two tilt controls of the mirror mount to align the compensator so that it is nor mal to the propagation direction of the light by finding the reflection from the compensator plate and forcing it to propagate back along the direction of the incident beam When the compensator is properly aligned and the Reta2. where N is an integer can be verified in a way similar to circular polarization but with the light making N passes through the compensator plate 13 PO v Applications 7 a tke 7 P a z output The Model 5540 polarization compensator can be used as a 1 2 wave plate to rotate the plane of polarization of a linearly polarized laser beam Q ii IT ae paa p Penner pia il input 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 isola tion which protects the laser from feedback 14 Vv out input 0 x 0 By placing a linear polarizer after the Berek com pensator one can conveniently vary the attenua tion of a beam without changing its polarization By varying the retardance and orientation one can produce arbitrary states of elliptical polarization which is especially useful in spectroscopy 15 Vv Appendix 1 Determinin a The most general application which uses a com Compensator f f pensator is one in which the input and desired out Settings for Arbi lazat ki h eg ai put polarization states are known and the compen J sator settings need to be determined The actual Output States desired retardance and orientation angle can be determined in a straight
3. Model 5540 User s Manual The Berek Polarization Compensator Patent 5 245 478 v Warranty Contents 554000 Rev A 2 Vv New Focus Inc 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 inci dental or consequential loss Warranty 2 Introduction 3 Theory 4 Operation 8 Applications 14 Appendix 1 Determining Compensator 16 Settings for Arbitrary Input and Output States Specifications 19 References 20 v Is a registered trademark of New Focus Inc Vv Introduction 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 compen sators Used as a variable waveplate the Berek compensator can impose 1 4 wave or 1 2 wave retardance at any wavelength between 200 nm and 1600 nm This dramatically reduces 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 12 mm aperture this compensator will accept beams from almost any Patent Pending commercially available laser system Vv Theory Invented in 1913 the Berek polarization compen sator has long been u
4. forward manner by first defining the input and output states in terms of the ellipticity e minor axis a major axis b and the orientation of the major axis in the lab frame Arbitrary elliptically polarized light can be defined in terms of e the ratio of the minor to the major axis and a the orientation of the ellipse From these parameters the retardance and orienta tion can be determined using the equations associ ated with the Poincar sphere A 20 A 2Q E 2tan E R e X cos A cos X COS A COS E y sin A cos E V sin A cos E e u VaT V Vv L Jenn n l c0s4 i h sin h sin E h yh h M 2 o 4 a M r 2sin 2I The solutions to these equations can then be used to determine the actual retardance R and orien tation 2 R Z 0 5k T These results are completely general and can be used to determine the settings to convert any input polarization into any output polarization assum ing the states can be defined in terms of their ellip ticity and orientation The relationship between retardance R and tilt angle can be determined from the angular depen dence of the birefringence in MgF The extraordi nary index of refraction as seen by the optical beam is given by 2 1 _ cos Or sin Opr 2 2 2 N n n where is the tilt angle and 7 and 7 are the 17 18 Vv indices of refraction These are in genera
5. l wave length dependent and their dispersion relations are given by 2 2 2 048764 0 39884 231204 n Il a 0 0434 f2 0 0946 a 23 7936 04134 0 50502 2 49054 n l i f2 0 0368 oo 2 237720 Thus the retardance R in waves is related to the tilt angle by _ 2000 2 l n sin Op 7 R n sin Op 1 2 3 1 7 sin Op The relationship between tilt angle and Retardation indicator setting is given by 0 sin 22 4 71 or I o2 Z0 Specifications Wavelength Range Aperture Wavefront Distortion Retardance Resolution Vv 200 1600 nm 12mm lt 1 8 wave 0 5 87 300 nm 0 7 1600 nm 0 001 wave null 0 01 wave 27 19 20 References Vv 1 Berek M Zbl Miner Geol Paldont 388 427 464 580 1913 quoted in Born M and Wolf E Principles of Optics 6th edition Pergamon Press London 1980 p 694 2 Dodge M Appl Opt 23 12 1980 85 1984
6. n have been optimized consider reducing the retardance by one full wave or improving the collimation The smaller the retardance the less sensitive is the polarization to imperfect collimation due to the angular dependence of the birefringence Vv B To verify circular polarization reflect the output light back through the compensator The polarization of the wave that goes through the compensator twice will be exactly orthogonal to the incident polarization for the circularly polarized light If the incident polarization is linear this can best be achieved by setting a cal cite polarizer in front of the Model 5540 polar ization compensator oriented to transmit the incident beam perfectly If the compensator is set to produce perfect circular 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 preci sion the plane containing the forward and backward propagating beams must be orthogo nal to the plane of incidence of the incident beam on the tilt plate C To verify elliptical polarization you must measure the major and minor axes of the polar ization ellipse or use a second compensator as is found in an ellipsometer Special cases such as produced by V 2 waves of retardance
7. nce The Retardation indicator controls the compen sator plate tilt angle and hence the actual retar dance value Setting the Retardation indicator to 0 gives zero tilt and zero retardance At nonzero values the Berek polarization compen sator can be set to either a single order or a mul tiple order waveplate The smaller the retar dance the more accurately it can be set To a good approximation the tilt angle that gives retardance R in waves is p sin 0 284 AR where J is the wavelength in microns The Retardation indicator setting is related to the tilt angle by j 02 7 2 0 Approximate Retardation indicator settings can be read from Fig 2a More accurate values can be obtained using the formulas in Appendix 1 When setting the Retardation make sure that the entire compensator housing and therefore the orientation angle does not rotate Vv 3 Setting the orientation angle The knurled ring marked Orientation rotates the entire housing to properly orient the slow axis of the compensator plate with respect to the inci dent polarization If the compensator has been mounted with the 0 on the orientation scale vertical one can read the orientation angle directly To make circular polarization from lin ear polarization one must set the retardance to 1 4 wave and the orientation angle to the inci dent linear polarization direction plus 45 If the reta
8. rdance 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 rotation causes the plane of polarization to rotate by twice the orien tation 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 Fig 3 7 12 Vv 4 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 A Since linear polarization can be blocked 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 Simply place a calcite polariz er such as a Model 5521 or 5523 after it and orient the polarizer to block the desired output polarization Both the tilt angle see Setting the retardance pg 10 and orientation angle of the compensator can be adjusted to fine tune the polarization At this stage do not use the mount ing to adjust the tilt of the compensator housing If the 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 trans mitted through the polarizer even when tilt and orientatio
9. rdance is tuned to 90 Patent Pending 6 Vv The Model 5540 Berek compensator allows the retardance and the orientation of the plane of inci dence to be adjusted conveniently and indepen dently 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 conve niently 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 Fig 2a shows the actual retar dance versus the Retardation indicator setting for the New Focus Berek compensator at 488 nm 632 8 nm and 1060 nm Fig 2b shows Retardation indicator setting versus wavelength to achieve precise A 2 and A 4 retardance For an in depth discussion on calculating these curves see Appendix 1 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
10. rdation indicator is set to 0 the reflected beam will not move away from the incident beam direction when the compensator is rotated around its axis 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 orienta tion angle NOTE The indicator setting for Retardation is NOT the actual value of the tilt angle See Appendix 1 for the relationship between Retardation indicator setting and tilt angle Vv Fig 3 Initial alignment of the Berek compensator Patent 5 245 478 Retardation Orientation This ring controls the tilt angle This ring rotates the waveplate See Appendix 1 for the relation It is usually convenient to set the ship between tilt angle and the set tings of this indicator Set this con trol to zero for initial alignment waveplate so the zero for this control is horizontal or vertical lt Hl E a To align the compensator force bey the light reflecting back from the I i compensator to propagate along B a 7 Brake the direction of the incident beam i 4 K ORENTATION This setscrew locks the orien tation ring to prevent acciden tal re adjustment after the compensator has been set 10 Vv 2 Setting the actual retarda
11. sed 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 compen sator has no effect on its polarization 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 retard ed 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 of tilt angle Thus the light in the two planes of polarization accumulates a relative phase shift or retardance For example Fig 1 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 v Fig 1 Conversion of linearly polarized to circularly polarized light A Linearly polarized light at the input of the Berek compensator B Circular polarization at the output of the Berek compensator when the reta
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