Abstract:
An optical beam having a randomly and unpredictably variable input polarization state propagates through an optical system containing a first optical surface, for example a diffraction grating, a first adjustable mirror, and means for rotating the respective parallel (P) and perpendicular (S) plane polarization components of the optical beam relative to the first optical surface by ninety degrees, thereby generating a polarization rotated optical beam having reversed orientations of the S and P polarization components relative to the input. The optical beam is reflected from the first adjustable mirror, which redirects the optical beam onto an optimized location on the first optical surface, thereby reducing PDL due to propagation through the optical system. In some embodiments the optical beam location on the first adjustable mirror remains substantially constant during adjustment.

Description:
TECHNICAL FIELD 
     The present invention relates to polarization dependent loss compensation, and particularly to an adjustable mirror assembly for polarization dependent loss compensation in an optical instrument. 
     BACKGROUND OF THE INVENTION 
     Polarization dependent loss (PDL), in which optical power transmitted along a propagation path changes as a function of polarization state of the light beam, is a well-known phenomenon in optical systems and instruments, for example optical spectrum analyzers. The maximum difference in power over all possible polarization states is termed polarization dependent loss (PDL). Because changes in polarization state of an input beam occur at an unpredictable time and rate, the optical spectrum analyzer (OSA) or other instrument preferably uses some type of static PDL correction that is not dependent on time or the exact state of polarization of the light that enters the system. 
       FIG. 1  illustrates schematically a current Agilent Technologies 8614x series OSA instrument (see Agilent Technologies data sheet 8614xB Optical Spectrum Analyzer Family Technical Specifications). This instrument incorporates a monochromator  10  having diffraction grating  17  as a dispersive element. An input beam  12   a  typically entering through input fiber  11  is directed by first mirror  13   a  and second mirror  13   b  through collimating element  16  along two passes  12   a  and  12   c  onto diffraction grating  17 . Between the two passes, the beam is directed through a resolution defining aperture that is normally incorporated into slit wheel  14  to provide a range of aperture sizes. Collimating element  16 , typically a lens, refocuses diffracted light from the surface of grating  17  in each pass  12   b  and  12   d  back onto the optical plane of slit wheel  14 . Output beam  12   e  is deflected by output mirror  13   c  into output fiber  18 . 
     The current method used to reduce PDL induced changes in power measured by the OSA as the input light source polarization state changes is to rotate the state of polarization of the input beam through monochromator  10  by 90 degrees between the first pass and the second pass. In the current instrument, this is implemented by inserting half-wave plate  15  in second pass  12   c  immediately after reflection from second mirror  13   b . This balancing technique effectively rotates the S and P states of polarization, which by definition are orthogonal, and reverses their state between first pass  12   a ,  12   b  and second pass  12   c ,  12   d  through the optical system. Any arbitrary state of polarization can be made up of a superposition of the orthogonal S and P states. For example, if the input beam state were S, after traveling through the first pass of the instrument it would be rotated to the orthogonal state P, and vice versa. Rotating the states so that both orthogonal states exist in the double-pass system regardless of the input state means that the output power of the OSA ideally does not change, even though the input polarization state changes. For this technique to work most effectively, the net reflectance for S polarization on first pass  12   a ,  12   b  multiplied by the net reflectance for P polarization on second pass  12   c ,  12   d  equals the net reflectance of P polarization on first pass  12   a ,  12   b  multiplied by the net reflectance for S polarization on second pass  12   c ,  12   d . Orthogonal states S and P are used to analyze this system because they are additionally the worst case states for this system. 
     To determine the power that is transmitted through the optical spectrum analyzer for the two worst case polarization states the following relation may be used.
 
Power out=Power In *T   i input fiber *R   i  grating *R   i  mirror 1 *R i  mirror 2 * R   i ′ grating* R   i ′ mirror 3 * T   i ′ output_fiber
 
     In this expression, R and T represent respective reflection and transmission percentages, subscript i represents the polarization state, e.g., S or P, at the specified surface, and subscript i′ represents the rotated polarization state as the beam propagates from the first pass to the second. Because each surface has a different orientation, what would be considered S for one surface could be P for another, so to remove any misunderstanding, the rule applied in the following discussion is that the input polarization state is identified relative to the grating surface. Referring to the coordinate axes in  FIG. 1 , the grating dispersion direction and the P polarization direction are parallel to the y-axis, which is perpendicular to the plane of the figure, whereas the non-dispersion direction and the S polarization direction are parallel to the x-axis, pointing upward parallel to the plane of the figure. Both polarizations are mutually perpendicular to the z-axis, which is essentially parallel to the dominant propagation direction of light beam passes  12   a ,  12   b ,  12   c , and  12   d . Because any polarization state or unpolarized state of the field can be represented as a superposition of the orthogonal basis set of S and P, which also happen to be the worst case polarization states, only these two electric field states are required to define the worst case polarization dependent loss. The following example uses common reflectance values and an input power of one milliwatt. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 P input state (P relative to Grating) 
                   
                   
               
               
                   
                 = 1.0 mW*0.964*0.5*0.986*0.986 
                 * 
                 0.8*0.992*0.95 
               
               
                   
                 first pass 
                   
                 second pass 
               
               
                   
                 = 0.4686 
                 * 
                 0.754 
               
               
                   
                 = 0.353 mW 
               
               
                   
                 S input state (S relative to Grating) 
               
               
                   
                 = 1.0 mW*0.966*0.8*0.992*0.992 
                 * 
                 0.49*0.986*0.94 
               
               
                   
                 first pass 
                   
                 second pass 
               
               
                   
                 = 0.7604 
                 * 
                 0.454 
               
               
                   
                 = 0.345 mW 
               
               
                   
                   
               
             
          
         
       
     
     This example shows that for the first pass with a P input state the net output power is 0.4686 mW. For the second pass with an S input state, which is actually P relative to the grating having had the polarization rotated by half-waveplate  15 , the net power is 0.454 mW. This shows that the net effects of the two halves of the optical spectrum analyzer are closely but not perfectly balanced. If the first pass with an S input state were compared to the second pass with a P input state, the same result occurs. If these passes are multiplied together, both input states will result in roughly the same power through the optical spectrum analyzer at approximately 0.35 mW. The transmitted powers are not exactly the same for the two different polarization states. In this example, the net system PDL is defined by the relation, PDL=10*LOG(Power for P input state/Power for S input state), which in this example evaluates to slightly less than 0.1 dB polarization dependent loss. If great care is not taken in the selection of the optical system components this value for PDL can be as large as 1 dB. 
     These differences are important, because the input polarization changes continually and unpredictably with time, due for example to fluctuations in optical source and/or fiber polarization states. These can produce instrument output power fluctuations that may be incorrectly attributed, for example, to source power fluctuations, but in reality may be artifacts produced by instrument PDL. It would therefore be advantageous for an optical system or instrument to be substantially immune to performance changes arising from input polarization fluctuations. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a system and a method for compensating polarization dependent loss (PDL) due to propagation through an optical system of an optical beam having a randomly and unpredictably variable input polarization state. The optical system contains a first optical surface, for example a diffraction grating, a first adjustable mirror, and a polarization shifter for interchanging the respective parallel (P) and perpendicular (S) polarization axes of the optical beam relative to the first optical surface, thereby generating a polarization interchanged optical beam having reversed orientations of the S and P polarization components relative to the input. The optical beam is reflected from the first adjustable mirror, which reoptimizes by redirecting the optical beam location on the first optical surface from first pass to second pass, thereby reducing PDL due to propagation through the optical system. In this regard the first optical surface may be considered a variable PDL component. In some embodiments the location of the optical beam relative to the first adjustable mirror remains substantially constant. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  illustrates schematically a current Agilent Technologies 8614x series Optical Spectrum Analyzer (OSA) instrument; 
         FIG. 2  illustrates schematically as an example the surface of a diffraction grating illuminated over a first spatial extent by first pass beam and over an offset spatial extent by a second pass beam; 
         FIG. 3  is a perspective view illustrating a mirror assembly embodiment that makes PDL adjustment possible; 
         FIGS. 4A and 4B  illustrate the mirror assembly of  FIG. 3  in accordance with the present embodiment, viewed respectively from the top and from the bottom; 
         FIGS. 5A-5B  and  FIGS. 6A-6B  are component details illustrating respective first and second flexure mirror mounts; and 
         FIG. 7  depicts an optionally included adjustment procedure for PDL compensation, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain system properties facilitate the polarization rotation scheme described in connection with FIG.  1 . One such property is that the first pass  12   a ,  12   b  of the system physically have the same effective total component reflectance ratio of S/P polarizations as the second pass  12   c ,  12   d . Individual component reflectance values may vary from first pass to second pass, but their aggregate value as used in the ratio is the same. One way of implementing this method is to make the net component reflectance values from the first pass for S and P polarizations respectively the same as in the second pass. Embedded in this property is the desirability of component spatial uniformity. Any optical beam traveling through the OSA will have some spatial extent over which the optical power is distributed. The term spatial extent means the length and width or physical area of the optical component surface where the light is incident. The term spatial uniformity means that the optical component will have the same optical properties over the entire spatial extent. 
     Because of spatial variations in the grating reflectance and in the input light beam power distribution, the net PDL value for any OSA system is considered a weighted sum of the products of local power concentrations and local reflectance of each infinitesimal area of the spatial extent of a reflective component illuminated by a light beam. The net transmitted power is found by multiplying the local power concentration by the local reflectance associated with that location in the beam cross-section and then summing all of those contributions over the full spatial extent of the beam.  FIG. 2  illustrates schematically as an example the surface of diffraction grating  17  illuminated over spatial extent  21  by first pass beam  12   a  and over offset spatial extent  23  by second pass beam  12   c . For example, point A,  22 , within spatial extent  21  of first pass beam  12   a  lies in a region of local variation  25  of the grating reflectance. As second pass beam  12   c  returns to grating  17 , corresponding point A,  24  within spatial extent  23  of beam  12   c  will not necessarily contact grating  17  in the same region of local reflectance variation  25 . This means that an S state beam at the grating for the first pass will encounter a local variation  25  that a second pass rotated P state will not necessarily encounter, and consequently the sum of the products of local power concentrations multiplied by the local reflectance values results in different values for first pass  12   a  relative to second pass  12   c . This typically leaves some portion of polarization dependent loss unbalanced. In this regard grating  17  can be considered a variable PDL component. 
     With the current polarization interchange technique, the optical component reflectance values are optimally uniform over their spatial extent for the PDL of that component to cancel the first pass with the second pass over that component or else fortuitously vary by the appropriate amount relative to some other component that is also nonuniform to make the weighted sum of the PDL balance to a minimum. Because spatial uniformity as well as the individual reflectance values of each optical component are difficult to control, there is almost always some unbalanced residual PDL. 
     In accordance with embodiments of the present invention, spatial non-uniformity of grating  17  used in the 8614x series OSA is nominally compensated by rotating the orientation of second mirror  13   b , in both the dispersion and non-dispersion directions, such that the beam path for second pass  12   c  through monochromator  10  is redirected to superimpose spatial extent  23  of second pass  12   c  substantially onto spatial extent  21  of first pass  12   a  at the surface of grating  17 . If residual PDL is found, then spatial extent  23  of the second pass is adjusted away from this substantial superposition to obtain a better matching condition for the system PDL. Moving the second pass spatial extent relative to the first pass changes the distribution of the reflected beam off grating  17  of second pass  12   c  relative to first pass  12   a , consequently altering the aggregate PDL value for the system. This change in reflectance is due to the nonuniformities found in the grating either by design or by typical manufacturing tolerances.  FIG. 3  is a perspective view illustrating a mirror assembly embodiment that implements this adjustment. 
     Mirror assembly  32  depicted in  FIG. 3  allows first mirror  33   a  and second mirror  33   b  each to be independently adjusted in the dispersion as well as the non-dispersion plane, using a set of four right-circular flexures whose axes are oriented nominally orthogonal to the planes of the dispersion and non-dispersion directions of the respective mirrors, as described in more detail below. First mirror  33   a  is supported by first flexure mirror mount  34   a , and second mirror  33   b  is similarly supported by second flexure mirror mount  34   b . In the present embodiment, first mirror  33   a  and second mirror  33   b  are oriented approximately at right angles with one another on opposite sides of slit wheel  14 . First flexure mirror mount  34   a  and second flexure mirror mount  34   b  are each attached through a flexure mechanism to base  35 . Although two-axis angular adjustment of second mirror  33   b  is sufficient to practice the technique of the present invention, the present embodiment for convenience depicts a similar adjustment mechanism for first mirror  33   a , which is useful for making certain optical system alignments that are unrelated to PDL compensation. Output mirror  33   c  deflecting the output beam into output fiber  18  and input fiber holder  31  positioning input fiber  11  are likewise shown for clarity in  FIG. 3  but are unrelated to the present PDL compensation technique. 
       FIGS. 4A and 4B  illustrate mirror assembly  32  in accordance with the present embodiment, viewed from the top in direction  4 A— 4 A and from the bottom in direction  4 B— 4 B respectively in FIG.  3 . First flexure mirror mount  34   a  supporting first mirror  33   a  is anchored to first movable mounting block  42   a  connected integrally to base  35  through first base flexure  45   a , which has a thinned section first flexure rotation axis  48   a . Likewise, second flexure mirror mount  34   b  supporting second mirror  33   b  is anchored to second movable mounting block  42   b  connected integrally to base  35  through second base flexure  45   b , which has a thinned section second flexure rotation axis  48   a . Set screws  47   a ,  47   b  preloaded in compression in respective threaded bores  46   a ,  46   b  actuate respective first and second movable mounting blocks  42   a ,  42   b  along respective angular arcs  41   a ,  41   b  about respective first and second flexure rotation axes  48   a ,  48   b , thereby providing adjustment in the non-dispersion direction for respective first and second mirrors  33   a ,  33   b . Through-bores  49  in base  35  provide precise alignment via locating pins with an instrument frame (not shown). 
       FIGS. 5A-5B  and  FIGS. 6A-6B  are component details illustrating respective first and second flexure mirror mounts  34   a  and  34   b . First mirror  33   a  attaches to surface  53  on movable member  52  of right circular flexure  55 , integral with first flexure mirror mount  34   a . A set screw (not shown) in threaded bore  56  bears against movable member  52 , actuating movable member  52  along an angular arc about first flexure rotation axis  58 , and thereby providing adjustment in the dispersion direction for first mirror  33   a . First flexure mirror mount  34   a  includes through bores  59  (preferably counterbored) for attaching as a unit to first movable mounting block  42   a  of base  35 . In the present embodiment, first movable mounting block  42   a  also contains slot  44 , which advantageously provides optical path clearance for input beam  12   a.    
     Similarly, in  FIGS. 6A-6B  second mirror  33   b  attaches to surface  63  on movable member  62  of right circular flexure  65 , integral with second flexure mirror mount  34   b . A set screw (not shown) in threaded bore  66  bears against movable member  62 , actuating movable member  62  along an angular arc about second flexure rotation axis  68  and thereby providing adjustment in the dispersion direction for second mirror  33   b . Second flexure mirror mount  34   b  includes through bores  69  (preferably counterbored) for attaching as a unit to second movable mounting block  42   b  of base  35 . The mirror flexures are oriented for convenience to redirect the optical beam in two substantially orthogonal dispersion and non-dispersion directions. In an implementation of the embodiments, for example, the tolerances on orthogonality of the flexure axes are about +/−2 degrees. However, normally orthogonal means that some cross-coupling in the adjustment directions is allowed without compromising the functionality of the device, so long as the two axes of mirror adjustment permit the second pass beam to be redirected onto any given location on the grating. 
     In accordance with embodiments of the present invention, first and second mirrors  13   a  and  13   b  are actuated to rotate in the non-dispersion and dispersion directions without appreciably changing the location of the optical beam  12   b  on the surface of either mirror. If the mirrors were to translate relative to the optical beam, there can be undesirable lateral shifts in the beam path and/or changes in the focal plane of the instrument. For large enough mirror translations, the beam location can actually walk off the mirror surface. In the embodiment depicted in  FIGS. 3 ,  4 A- 4 B, the axes of the right-circular flexures are disposed to minimize translation of the mirror surface in all three axes as it is rotated during PDL compensation adjustment, such that the mirrors experience pure rotation in the non-dispersion direction and substantially pure rotation in the dispersion direction as well. Set screws are pre-loaded such that they remain under compression throughout the entire usable adjustment range of the mirror assembly. This is accomplished, for example, by configuring wedge angles such as wedge angles  51  and  61  between the normal to mirror mounting surfaces  53 ,  63  and the corresponding axes of set screw threaded bores  46   a ,  46   b ,  56 ,  66  in both dispersion and non-dispersion directions, forcing the set screws into compression in order to rotate the respective mirror surfaces into a nominally normal orientation. Alternatively to the flexure and set screw configuration described in connection with  FIGS. 3 ,  4 A- 4 B, mirror actuation can be accomplished by other rotational drivers known to those skilled in the art, for example mechanical drives such as pin connections with springs and set screws, gear drives, electromechanical drives such as motors and piezoelectric actuators and hydraulic drives, all of which are considered to be within the scope of the present invention 
       FIG. 7  depicts an optionally included mirror adjustment procedure for PDL compensation in an optical system, in accordance with embodiments of the present invention. Typically an adjustment procedure includes a coarse adjustment such as that depicted at step  701  using conventional techniques, typically involving an autocollimator, followed by a fine adjustment such as that starting at step  711 . Preferably the fine adjustment procedure starts with second mirror  33   b  in the non-dispersion direction, employing real time measurement of PDL using conventional techniques as an indicator of the degree of successful mirror adjustment. At step  712  it is determined if the measured PDL value is acceptable. If so, then at step  720  it is determined if the set screw adjustment has exceeded its limit. If so, then special troubleshooting procedures outside the scope of the present invention are performed at step  721 . If not, then at step  722  adjustment is terminated and the procedure ends. If measured PDL at step  712  is not acceptable, then at step  714  a tentative adjusting set screw (typically non-dispersion) and actuation direction are selected to reduce the measured PDL. These selections are then implemented at step  715 , following which it is determined at step  716  if the PDL decreased at step  715 . If so, then the procedure returns to step  712  for a next iteration. If not, then at step  717  the selected set screw is actuated slightly in the opposite direction, and at step  718  it is determined if the actuation of step  717  decreased the measured PDL. If so, then the adjustment procedure returns to step  712  for a next iteration. If not, then at step  719  a different set screw (typically dispersion direction) is selected and turned, following which the adjustment procedure returns to step  712  for a next iteration. In some embodiments, either coarse or fine adjustment can be omitted. Although the procedure depicted in  FIG. 7  can readily be performed by an operator manually, it can also be performed automatically and algorithmically using optical sensor feedback under hardware, software, or firmware control. In addition, it will be recognized by those having skill in the art that variations of the procedure of  FIG. 7  are considered to be within the scope of the present invention. 
     The PDL compensation technique in accordance with the present invention is useful for double pass or multiple pass systems where the light beam encounters some surfaces more than once. It would be most effective if the surface that is encountered more than once is also the dominant PDL contributing component in the system, such that its effect can be minimized. It is theoretically possible to use this technique, even if the light encounters the component only once. In that configuration, the grating and the adjustable mirror constitute essentially a variable PDL component used to try to compensate some other component in the system. This method would introduce spatial variation across the beam, which would need to be taken into account if implemented. It would also have a potentially limited PDL range dictated by the actual spatial non-uniformities for the S and P states on the component. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.