Abstract:
Techniques and devices for monitoring polarization of light using at least two polarization elements where the difference between the outputs of the two polarization elements are used to monitor a change in polarization.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/440,007 entitled “Sensitive Polarization Monitoring and Controlling” and filed on Jan. 13, 2003, the entire disclosure of which is incorporated herein by reference as part of this application. 

   BACKGROUND 
   Optical polarization is an important parameter in many optical devices and systems. A change in optical polarization of light may modify behavior of light or operation of optical devices and systems. Hence, it is desirable to monitor optical polarization of light in various applications. For one example, various optical polarization stabilizers may use an optical polarizer for monitoring the polarization. 
   SUMMARY 
   This application includes techniques, devices and systems that use two polarization elements in a polarization monitoring device to monitor the polarization state of light. 
   In one implementation, a method for monitoring polarization of light includes the following steps. A first partial polarization beam splitter is used to split by reflection a fraction of light in one of first and second mutually orthogonal polarization directions from an input beam to produce a first monitor beam. A second partial polarization beam splitter is used to split by reflection a fraction of the light in the one of the first and second mutually orthogonal polarization directions from the input beam to produce a second monitor beam. The first and second partial polarization beam splitters are oriented to have their polarization axes to be 90 degrees with each other. The first and the second monitor beams are then converted into first, and second detector signals, respectively. A difference between the first and the second detector signals is used to indicate an amount and a direction of a deviation in a polarization of the light from a known direction. 
   This and other implementations and variations are described in greater detail with reference to the drawings, the detailed description, and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a transfer curve of a polarizer as a function of the polarizer angle. 
       FIG. 2  illustrates one exemplary implementation of a three-element polarization monitoring device. 
       FIG. 3  shows the variation of output signals V 1 , V 2 , and V 3  from three PBS elements in  FIG. 2  as a function of the input polarization angle when V 3  reaches maximum, V 1 =V 2  so that (V 1 −V 2 )=0. 
       FIG. 4  shows an exemplary implementation of the polarization control system based on the polarization monitoring device in  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   This application is in part based on the recognition that a polarizer when used as a polarization monitoring device may not be sufficiently sensitive to variations in polarizations when the polarizer is operated at angular locations A, B, or C in a transfer curve of the polarizer as a function of the polarizer angle. This is illustrated in  FIG. 1 . If the polarizer is set at the angle A (maximum transmission) or B (minimum transmission) in monitoring the polarization, the transmission power of the polarizer is least sensitive to a change in the polarization since the slope at A and B is zero. In addition, at these two operating angles A and B, the power change in the transmission of the polarizer does not directly indicate the direction of the polarization variation from the direction of the polarizer. 
   On the other hand, the transmission power of the polarizer would have a sensitive response to a change in the polarization angle when the polarizer is set to operate at the angle C at the waist of the transmission peak. However, the transmission loss in this operation mode is 3 dB and the power fluctuation in the input light, such as the power variation in the light source, may directly affect the accuracy of the polarization monitoring and thus the accuracy of the polarization control. 
   In addition, the operation at the point C presents another technical issue: the polarization under monitoring may not be unique for the same transmission output from the polarizer. For example, at the point C, two linear polarization sates orientated at ±45 degrees with respect to the passing axis of the polarizer and two circularly polarized lights (right and left hand) all have 50% of the peak power in the transmission power. 
     FIG. 2  illustrates one exemplary implementation of a three-element polarization monitoring device  200  of this application. The device  200  includes 3 partial polarization beam splitters (PBS)  210 ,  220 , and  230  that are disposed in the optical path of an input light beam  201 . The partial PBSs  210 ,  220 , and  230  are configured to partially reflect only one polarization of the two orthogonal polarizations, for example, “S” polarization, and do not reflect the other orthogonal “p” polarization. The orientations of the polarization directions P 1 , P 2 , and P 3  of the partial PBSs  210 ,  220 , and  230  are also illustrated in  FIG. 2 . The 1 st  and 2 nd  partial polarization beam splitters  210  and  220  are oriented at 90° from each other and they are also oriented ±45° from the 3 rd  partial PBS  230 . Three optical detectors  214 ,  224 , and  234  are respectively positioned to receive reflected beams  212 ,  222 , and  232  from the PBSs  210 ,  220 , and  230 , respectively, to produce detector output signals  216  (V 1 ),  226  (V 2 ), and  236  (V 3 ). A signal adder  240  is coupled to detectors  214  and  224  to produce a first signal  242  from the signals V 1  and V 2  and a signal subtracting device  250  is coupled to produce a second signal from signals V 1  and V 2 . 
   In operation of the device  200 , the signal gains G 1 , G 2 , and G 3  for the three detectors  214 ,  224 , and  234  may be adjusted so that V 1max =V 2max =v 3max  or the differences in the detectors may be electronically calibrated during the processing. The input polarization in the beam  201  may be aligned with the reflection axis of the 3 rd  partial PBS  230 . When this happens, the signal V 3  will be at its maximum and V 1  and V 2  will be equal. 
     FIG. 3  shows the variation of V 1 , V 2 , and V 3  as a function of the input polarization angle when V 3  reaches maximum, V 1 =V 2  so that (V 1 −V 2 )=0. When the input polarization angle deviates to the left side of the input polarization that produces the maximum output, V 3max , in the detector  232 , the detector outputs for the detectors  212  and  222  satisfy V 1 &gt;V 2  so that (V 1 −V 2 )&gt;0. When the input polarization angle is right at the angle for producing V 3max , V 1 &lt;V 2  so that (V 1 −V 2 )&lt;0. Therefore, the quantity (V 1 −V 2 ) can be used to indicate both the amount of the deviation and the direction of the deviation of the input polarization with respect to the direction of the 3 rd  partial PBS  230 . In particular, this differential signal (V 1 −V 2 ) can also tell the direction of the polarization mis-alignment: V 1 −V 2 &gt;0 indicates that an increase of polarization angle is required while V 1 =V 2 &lt;0 indicates that a decrease in polarization angle is required. 
   Furthermore, when V 3 =V 3max , V 1 −V 2  is most sensitive to polarization changes. Therefore using the differential signal of (V 1 −V 2 ) as a feedback signal to control the polarization is most sensitive and the feedback loop can produce a high gain. 
   In yet another aspect, the sum signal (V 1 +V 2 ) is a constant as a function of the polarization angle. Notably, the sum signal only changes when the optical power changes. Therefore, to eliminate power sensibility in the polarization monitoring in the device  200 , the quantity (V 1 −V 2 )/(V 1 +V 2 ) and V 3 /(V 1 +V 2 ) may be used. These two values are independent of optical power fluctuations. The signal V 3  ( 236 ) from the 3 rd  detector  234  may also be monitored because the signal  252  of (V 1 −V 2 ) alone may not indicate the difference between a linear polarization oriented along the 3 rd  partial PBS  230  and circularly polarized light (RCP and LCP). 
     FIG. 4  further shows a polarization control system  400  based on the above polarization monitoring device  200  in  FIG. 2 . A polarization controller  410  is disposed at the input of the beam  201  to adjustably control the polarization of the input light. The output of the controller  410  is then monitored by the device  200  in an in-line configuration to produce an output beam  202  with a desired output polarization. A feedback control unit  420  is implemented to produce a control signal for controlling the controller  410  based on the signals  242 ,  252 , and  236 . This system  400  may be operated to stabilize the output polarization in the output beam  202 . This system  400  has the advantages of being highly sensitive to polarization variation, capable of detecting the direction of polarization change, and being insensitive to laser power fluctuation of the input beam. 
   Other enhancements and variations of the described implementations may be made.