Abstract:
Polarimeters based on transversal division of the input beam and use of different polarization elements in different polarization states to change polarizations of different portions of the input beam so that the power levels of the different portions of the input beam can be measured to determine the polarization state of the input beam. A wedged substrate can be used to direct the different portions of the input beam at different directions and a lens can be used to focus these different portions at different locations at a plane.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/693,354 entitled “Low Cost Polametric Detector” and filed Jun. 22, 2005, the disclosure of which is incorporated by reference as part of the specification of this application. 

   BACKGROUND 
   This application is related to devices and techniques for measuring optical polarization of light. 
   Optical polarization is an important parameter of an optical signal in various optical systems. For example, in fiber optic communication systems, polarization-dependent effects in fibers and other devices, such as polarization-dependent loss (PDL) and polarization-mode dispersion (PMD), can have significant impacts on performance and proper operations of optical devices or systems. Hence, it may be desirable to measure and monitor the state of polarization (SOP) and the degree of polarization (DOP) of an optical signal in these and other systems. 
   Optical polarimeters are devices designed to measure polarization of light and can be implemented in various configurations. Some commercial polarimeters use rotating waveplates to control the polarization of the input light and measure optical power levels of the controlled input light for determine the Stokes parameters of the input light. Such polarimeters can be bulky, expensive, slow, and have a relatively short life time, and therefore may not suitable for certain applications such as system applications in fiber networks. Some other polarimeters use four optical detectors and require complicated beam-splitting optics. Therefore, these polarimeters can also be bulky, expensive and difficult to align and calibrate. 
   SUMMARY 
   This application, among others, disclose implementations of polarimeters based on transversal division of the input beam. The disclosed polarimeters can be implemented to have simple structures that may be of a low cost and packaged in a relatively compact size. Low cost polarimeters may be important to certain applications, such as applications with polarization analysis, applications for network monitoring and sensor readout. 
   In one implementation, a device is described to include a substrate and polarization elements located on the substrate. The polarization elements are configured at different polarization states and spatially separated from one another to receive different portions of a common input optical beam to produce transmitted light beams in different polarization states. The device further includes an optical detector that includes active detector sensing areas, each of which corresponds to a respective polarization element and receives a transmitted light beam from the respective polarization element. 
   In another implementation, a device is described to include polarization elements configured at different polarization states and spatially separated from one another to receive different portions of a common input optical beam to produce transmitted light beams in different polarization states. A wedged substrate is included in this device on which the polarization elements are located. The wedged substrate transmits light and has angled surfaces to cause the transmitted light beams to propagate at different directions. A lens is positioned to receive transmitted light beams from the polarization elements placed on the wedged substrate and to focus the transmitted light beams. The device further includes fibers located to receive focused beams from the lens, respectively. 
   In yet another implementation, a device includes polarization elements at different polarization states that are spatially separated from one another to receive different portions of a common input optical beam to produce transmitted light beams, and optical detectors, each of which corresponds to a designated polarization element, to respectively receive transmitted light beams from the polarization elements and to produce detector output signals that in combination contain information on a state of polarization of the common input beam. 
   These and other implementations are described in greater detail in the drawings, the detailed description and the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows one example of a polarimeter. 
       FIG. 2  shows one example of a polarimeter using a wedged substrate to change directions of different beams separated from a common input beam. 
       FIG. 3  illustrates operation of the wedged substrate shown in  FIG. 2 . 
       FIG. 4  shows one example of a fiber pigtail implementation of a polarimeter using a wedged substrate to change directions of different beams separated from a common input beam. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows one exemplary implementation of a polarimeter  100 . This device  100  includes a polarization unit  110  that has four different optical polarizers  111 ,  112 ,  113  and  114  that are spatially separated from one another to receive different portions of a common input beam  101  whose polarization is to be measured. The four different optical polarizers  111 ,  112 ,  113  and  114  modify the input optical polarization of the different portions of the input beam  101  to produce different polarization states in the different portions of the input beam  101 , respectively. The polarization unit  110  can be placed on a substrate  120  on which the four different optical polarizers  111 ,  112 ,  113  and  114  are positioned. An optical detector  130  having a 4-detector array (or a quard detector) with four different optical sensing regions or optical detectors  131 ,  132 ,  133  and  134  is placed in the optical path of the light output from the polarization unit  110  to receive and detect the corresponding four different portions, respectively, of the input beam  101  produced by the polarization unit  110 . The detector outputs from the optical detectors  131 ,  132 ,  133  and  134  and the polarization states controlled by the four different optical polarizers  111 ,  112 ,  113  and  114  can be used to determine the Stokes parameters of the polarization of the input beam  101 . This processing can be performed in a controller of the polarimeter  100 . 
   Many polarizer combinations are possible for the polarization unit  110  with different polarizers as long as at least four different portions of the input beam have different polarizations after passing through the polarization unit  110 . In  FIG. 1 , the four optical polarizers  111 ,  112 ,  113  and  114  are shown to be a linear vertical polarizer, a linear horizontal polarizer, a right hand circular polarizer and a 45-degree linear polarizer. In other implementations, for example, a LHC (left hand circular) polarizer can be used to replace the RHC (right hand circular) polarizer  113 . The orientations of the exemplary polarizers can be different what is shown in  FIG. 1 . In yet other implementations, three polarizers  112 ,  113  and  114  are sufficient and the position occupied by the polarizer  111  can be left empty without any polarization element as long as the four portions of the input beam  101 , after passing through the polarization unit  110 , have different polarizations. The locations of the polarizers in the polarizer arrangement in  FIG. 1  can also be exchanged. The polarizers  111 ,  112 ,  113  and  114  can be affixed to the substrate  120 , either on the first side of substrate  120  to receive the input beam  101  or on the second side to face the detector  130  or a lens  220  as in  FIG. 2 . Certainly, the polarization unit  110  may be designed to divide the input beam  101  into more than four portions. 
   The input light beam  101  passes through the polarizer assembly  110  with the center of the beam  101  to be close to the center of the assembly  110  as possible to divide the input beam  101  into approximately equal portions. Either the substrate  120  or the polarizers  111 ,  112 ,  113  and  114  can face the input beam  101 . In  FIG. 1 , the polarizers  111 ,  112 ,  113  and  114  face the beam  101 . The gaps between the polarizers  111 ,  112 ,  113  and  114  may be filled with optically opaque material to prevent light from going through the gaps to reach the optical detectors  131 ,  132 ,  133  and  134  in part because the polarization of such light in the gaps is not modified and presences of such light at the detectors  131 ,  132 ,  133  and  134  reduces the signal to noise ratio of the detection. 
   In the design in  FIG. 1 , the total sensing area of the four detectors  131 ,  132 ,  133  and  134  is approximately the same as the total transversal or cross section area of the input beam  101 . In some applications, this design may require the size of the active area of the detector array to be relatively large, resulting a slower detection speed and an increased cost. 
   The exemplary polarimeter  200  in  FIG. 2  uses an alternative design to reduce the detector active area. This design includes a wedged substrate  210  to replace the planar substrate  120  in  FIG. 1  and an optical detector  230  with four detectors  231 ,  232 ,  233  and  234  with small active detector sensing areas. In the illustrated example, the wedged substrate  210  has a first flat side  215  to hold the polarizers  111 ,  112 ,  113  and  114  and a second wedged side with four wedged surfaces  211 ,  212 ,  213  and  214  to direct four portions of the input beam  101  passing through four different polarizers  111 ,  112 ,  113  and  114  into four different directions as four different beams. The polarizers  111 ,  112 ,  113  and  114  may also be placed on the four wedged surfaces  211 ,  212 ,  213  and  214 , respectively, on the wedged side. Each wedged surface is tilted at an angle and is not perpendicular to the optic axis of the device so that a light ray changes its direction due to refraction at the wedged surface. A focusing lens  220  is placed in the optical path between the wedged substrate  210  and the optical detector  230  to focus the four portions of the beam onto the four different detectors  231 ,  232 ,  233  and  234 . In the illustrated example, the wedged surfaces face the lens  220 . The focusing by the lens  220  reduces the beam size of each of the four beams output by the wedged substrate  210  and thus the size of the active detector sensing area of the detectors  231 ,  232 ,  233  and  234  can be reduced accordingly. The detectors  231 ,  232 ,  233  and  234  with smaller active detector areas can be used to achieve a higher detection speed and lowered photodetector cost in comparison with a detector  130  with four larger detectors  131 ,  132 ,  133  and  134 . 
     FIG. 3  illustrates the relationship between the wedge angle (or crossing angle) 2α of the wedge side of the wedged substrate  210 , the detector separation of the 4-detector array and the focal length f of the lens  220 . For example, in one implementation for a wedge with a crossing angle of 3.7 degrees (commonly used in other fiber optic devices) and a beam separation of 0.5 mm on the detector, the focal length of the lens  220  is 7.8 mm. Assume the refractive index of the wedged substrate material is n1, and each wedged surface forms an angle of α1 with the optic axis of the polarimeter. In the illustrated geometry in  FIG. 3 , n1 sin α1=n2 sine α2 where n2 is the refractive index of the air so that α2˜n1α1 when the angles are small and n2=1. Therefore, α=α2−α1=(n1−1)α1=0.5α1. The beam spot separation of different beams output from the wedged substrate is D=2f tan α. For the wedges with a crossing angle 2α of 3.7 degrees, the wedge angle is also 3.7 degrees. For a spot separation of D=0.5 mm, the focal length f is 7.8 mm. 
   For systems requiring pigtailed photodectors to achieve a high detection speed, or systems requiring remote detection of optical signals, coupling light in different portions of the beam into different fibers can be achieved by using an exemplary polametric detector  400  in  FIG. 4 . 
   The polametric detector  400  includes an optical collimator  410  at the input to expand an input beam  401  into a collimated beam  402  with a larger cross section. This collimated beam  402  lens may be implemented with different lenses, including a graded index lens, a c-lens, and others. A wedged substrate  210  is used for mounting the four polarizers  111 ,  112 ,  113  and  114  either on the flat side or the wedged side of the substrate  210  to direct the four different portions of the beam into four different directions. A lens  220  is used to focus the four beams produced by the wedged substrate  210 . A ferrule  430  containing four fibers  440  is placed at the focal plane of the lens  220  in order for the lens  220  to focus the four portions of the beam into different fibers  440 , respectively. The four fibers  440  are coupled to four different optical detectors which measure the power levels of the four beams in the fibers  440  for determining the Stokes parameters of the input polarization. A fiber collimator  420  may be used to hold the lens  220  and the fiber ferrule  430  at a fixed position relative to each other. As an example, for a fiber separation of 0.125 mm and a crossing angle of 3.7 degrees, the corresponding focal length is 1.95 mm. 
   Only a few implementations are disclosed. However, it is understood that variations and enhancements may be made.