Abstract:
An apparatus for separating polarization of light includes a rotatable beam splitter including an input for receiving light with a first polarization and an output for outputting light with a second polarization different from the first polarization. Rotation of the rotatable beam splitter changes the first polarization of the input light to the second polarization of the output light. An optical network and method are also set forth.

Description:
PRIORITY APPLICATION 
       [0001]    This application claims priority from U.S. provisional patent application serial number 61/489,124, filed on May 23, 2011, the contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The technology relates to polarization control and may be used for example in optical fiber applications. 
       INTRODUCTION 
       [0003]    Controlling the polarization state of light in an optical fiber is often required in optical fiber networks. Polarization control allows tasks such as alignment of fiber optic networks, balancing of power between a set of optical detectors, and probing a device under test with different launch polarization states to be performed. The polarization response of an optical network can be monitored using an industry technique known as polarization diverse detection where the polarization response of the light is split into one or more states and detected at respective optical detectors. Adding a polarization controller into this scheme permits monitoring of the polarization response of the optical network. In other words, as the input polarization is varied by a polarization controller, the response of the fiber can be separated and detected to gain insight to the polarization response of the optical network. Thus, it is common in optical fiber applications to both control the polarization at a point in an optical network and separate the response of the optical fiber into separate polarization states. 
         [0004]    Polarization control can be performed by inducing a phase shift between light transmitted along the fast and slow axis in an optical fiber. Applying the phase shift in a controlled manner permits predictable control of the output polarization of the transmitted light. This can be done with fiber retarders in which a phase shift is induced by a birefringent material. An example of a fiber retarder is an optical waveplate. A quarter waveplate converts circular polarized light into linearly polarized light while a half waveplate can change the polarization direction of linearly polarized light. Waveplates can be constructed with various materials, or they can be simulated by placing fiber in a loop polarization controller. A waveplate within a loop polarization controller is often referred to as a “paddle.” 
         [0005]    A loop polarization controller uses the strain induced by bending the fiber to provide polarization control. Strain deforms the fiber, and the length that light must travel along a transmission axis varies. Hence, bending an optical fiber induces a phase shift between the transmission axes of the optical fiber, similar to a fiber retarder. The amount of phase shift accumulated between the two transmission axes for a bend of a given diameter may be determined based on the diameter of the optical fiber and the wavelength of the light. The number of loops that the fiber is bent at that diameter may be increased in order to increase the accumulated phase shift. Thus, within a “paddle” of the loop polarization controller, several loops of fiber may exist. 
         [0006]    To provide a polarization rotator, a bend diameter and a number of loops are calculated to generate a first quarter waveplate/first paddle to transform the light into linear polarized light. Then the number of loops may be doubled in a second paddle to generate a half waveplate that rotates the polarized light. The position of the second paddle is thus changed in order to vary the rotation of the transmitted light.  FIG. 1  shows an example polarization controller  10  with two paddles  14  and  16 . An input optical fiber  12  directs light to a quarter waveplate  14  formed from N fiber loops having a diameter D, with N being a positive integer. The phase shift induced in a paddle/waveplate is controlled by the number of loops at a given diameter. The output of the quarter waveplate  14  is provided to a half waveplate  16  that includes 2N fiber loops with diameter D. The position of the half waveplate  16  is rotated to vary the output polarization of the light carried by output fiber  18 . 
         [0007]    Greater levels of polarization control can be achieved by adding a third control paddle in a quarter waveplate configuration to the example scheme shown in  FIG. 1 . A three paddle device can achieve any arbitrary polarization state from any incident polarization state by adjusting the paddles. Other approaches exist for polarization control in fiber optics such generating multiple effective waveplates by applying pressure to the fiber with piezoelectric actuators. 
         [0008]    But the inventors recognized that not all applications require full control of the polarization state in the optical fiber. For example, consider the case in which the polarization response of the optical network is required to be measured. A polarization diverse detection scheme is implemented in which light from a single optical fiber is split between two optical detectors. Standard optical fiber can transmit light along two principle axis designated as a fast (P) axis and a slow (S) axis. By splitting light to two optical detectors, an S detector and a P detector, the response of light transmitted along both of these axes may be monitored. This may be done using a polarization beam splitter (PBS) placed before the optical detectors. A PBS is composed of a birefringent crystal, where one axis of the crystal has a different index of refraction compared to the other axis of the crystal. Incident light is bent depending on which axis of the crystal it is incident. 
         [0009]      FIG. 2  depicts an example polarization beam splitter. An optical fiber  12  inputs light via a collimating lens  20  to a birefringent crystal  22 . A perpendicular polarization beam  24  is collimated by collimating lens  28  into output fiber  32 , and a parallel polarization beam  26  is collimated by collimating lens  30  into output fiber  34 . 
         [0010]    When implementing a polarization beam splitter (PBS) into an optical network, like the example shown in  FIG. 3 , light is split based on the orientation of the fast axis and the slow axis of the optical fiber with respect to the parallel and perpendicular axis of the birefringent crystal in the fixed PBS  36 . A polarization controller  10  is often desired to reorient the response of the fiber to the PBS. In the example shown in  FIG. 3 , the polarization controller  10  is a loop type polarization controller with two loops, and the perpendicular light is detected using an S photodetector  38 A and a P photodetector  38 B. 
         [0011]    Although a two loop polarization controller can be used to achieve polarization rotation about an axis, or even more sophisticated solutions can be used, these conventional approaches to polarization control suffer several disadvantages. For example, since standard single mode fiber suffers loss when bent tightly, the two loop paddles must either be large or use low bend loss fiber. Even with low bend loss fiber, each loop requires a minimum diameter based on the mechanical strength of the fiber in bending. So polarization controllers can be larger than desired. The large size issue is compounded by the fact that paddles also rotate on an edge so that the swept area during rotation is large, thereby necessitating a large enclosure for this kind of polarization control product. A minimum of two paddles is required when balancing power between two output detectors; three are required to produce all polarization states. This requires additional axis of motion for automated solutions further adding to size, but also adding to cost and complexity. Polarization control can also be achieved by pinching the fiber using piezo driven assemblies. Although these devices can be smaller than the conventional paddle-based polarization controllers, they are costly and require special high voltage power supplies. 
         [0012]    What is needed is technology that overcomes these limitations and provides for a less complex, less expensive, and smaller size polarization control that still provides polarization diverse detection. 
       SUMMARY 
       [0013]    An apparatus for separating polarization of light includes a rotatable beam splitter including an input for receiving light with a first polarization and an output for outputting light with a second polarization different from the first polarization. Rotation of the rotatable beam splitter changes the first polarization of the input light to the second polarization of the output light. The rotatable beam splitter may be configured for manual rotation and/or automatic rotation. In the latter case, a beam splitter rotator controller may be provided to controllably rotate the rotatable beam splitter. 
         [0014]    In one example embodiment, the rotatable beam splitter includes a gear assembly and an electric motor. A motor controller may be configured to control an amount of rotation of the rotatable beam splitter based on current drawn by the electric motor. 
         [0015]    In another example embodiment, the rotatable beam splitter includes a rotation limiter. The rotation limiter may further be configured to progressively stop rotation of the rotatable beam splitter. Example rotation limiters include a spring, one or more limit switches, a magnet and one or more Hall effect sensors, etc. 
         [0016]    The rotatable beam splitter outputs light with two different polarization states. In an embodiment where the rotatable beam splitter includes a gearbox assembly operable by an electric motor and a motor controller configured to detect motor power levels associated with power levels associated with the polarization states, the motor controller may be configured to control an amount of rotation of the rotatable beam splitter based on the detected power levels. An example first polarization is a parallel polarization and an example second polarization is a perpendicular polarization. An example change in polarization is a rotation of the polarization. 
         [0017]    In one example implementation, the rotatable beam splitter is a birefringent crystal having an X axis perpendicular to a Y axis such that the rotatable beam splitter is rotatable to a first position to align the X axis with the parallel polarization and the Y axis with the perpendicular polarization and rotatable to a second position to align the Y axis with the parallel polarization and the X axis with the perpendicular polarization. 
         [0018]    Another aspect of the technology relates to an optical network that includes an interferometer for mixing input light from a laser and light from a device or fiber under test and generating an interferometric light signal. A rotatable beam splitter in the network is configured to receive the interferometric light signal with a first light polarization and output light with a second light polarization different from the first light polarization. Rotation of the rotatable beam splitter changes the first light polarization to the second light polarization. Preferably, the rotatable beam splitter is configured to perform a polarization rotation operation without using a paddle-based polarization controller. 
         [0019]    In an example embodiment of the optical network, the first light polarization is a parallel polarization, the second light polarization is a perpendicular polarization, and the rotatable beam splitter is a birefringent crystal having an X axis perpendicular to a Y axis such that the rotatable beam splitter is rotatable to a first position to align the X axis with the parallel polarization and the Y axis with the perpendicular polarization and rotatable to a second position to align the Y axis with the parallel polarization and the X axis with the perpendicular polarization. 
         [0020]    Another aspect of the technology relates to a method for controlling polarization of an input light signal with a first light polarization. The method includes:
       rotating a rotatable beam splitter to cause polarization rotation to a second light polarization (the rotating may be performed manually and/or automatically);   receiving the input light signal at the rotatable beam splitter; and   outputing from the rotatable beam splitter light with the second light polarization.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows an example polarization controller with two paddles; 
           [0025]      FIG. 2  depicts an example polarization beam splitter (PBS); 
           [0026]      FIG. 3  shows an example polarization beam splitter used in an optical network; 
           [0027]      FIG. 4  shows an example rotatable beam splitter to achieve polarization control in accordance with a first example embodiment; 
           [0028]      FIGS. 5A-5C  are graphs that illustrate the concepts of slow and fast light axes, polarization states, and rotated polarization; 
           [0029]      FIG. 6  shows an example rotatable beam splitter used in an optical network to achieve polarization control; 
           [0030]      FIG. 7  shows an example rotatable beam splitter implemented using a motor assembly; 
           [0031]      FIG. 8  shows a second example embodiment of an example rotatable beam splitter to achieve polarization control; 
           [0032]      FIG. 9  shows an example rotatable beam splitter implemented using a motor assembly in accordance with the second embodiment; 
           [0033]      FIGS. 10A and 10B  show another example rotatable beam splitter implemented using a motor assembly in accordance with the second embodiment; and 
           [0034]      FIG. 11  is a graph illustrating power level of two polarization states versus beam splitter rotation angle for a non-limiting example test device. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The following description sets forth specific details, such as particular embodiments for purposes of explanation and not limitation. But it will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. In some instances, detailed descriptions of well known methods, interfaces, circuits, and devices are omitted so as not to obscure the description with unnecessary detail. Individual blocks are shown in the figures corresponding to various nodes. Those skilled in the art will appreciate that the functions of those blocks may be implemented using individual hardware circuits, using software programs and data in conjunction with a suitably programmed digital microprocessor or general purpose computer, and/or using applications specific integrated circuitry (ASIC), and/or using one or more digital signal processors (DSPs). Software program instructions and data may be stored on a non-transitory, computer-readable storage medium, and when the instructions are executed by a computer or other suitable processor control, the computer or processor performs the functions associated with those instructions. 
         [0036]    Thus, for example, it will be appreciated by those skilled in the art that diagrams herein can represent conceptual views of illustrative circuitry or other functional units. Similarly, it will be appreciated that any flow charts, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer-readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
         [0037]    The functions of the various illustrated elements may be provided through the use of hardware such as circuit hardware and/or hardware capable of executing software in the form of coded instructions stored on computer-readable medium. Thus, such functions and illustrated functional blocks are to be understood as being either hardware-implemented and/or computer-implemented, and thus machine-implemented. 
         [0038]    In terms of hardware implementation, the functional blocks may include or encompass, without limitation, a digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions. 
         [0039]    In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer, processor, and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term “processor” or “controller” also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above. 
         [0040]    The polarization control technology described here provides a low cost, low complexity, and small size way to rotate the polarization of light and at the same time ensure that the optical power between S and P channels in a polarization diverse detection scheme is balanced. This new technology does so without the use of paddle polarization controllers or piezoelectric devices and only requires one degree of freedom for full polarization power balancing capability. In contrast to conventional polarization control mechanisms where a rotated polarization state is achieved by rotation with respect to a fixed beam splitter, the inventors discovered that rotational polarization control may be obtained in a more advantageous way by rotating a beam splitter with respect to the fast axis and the slow axis of the optical fiber. 
         [0041]      FIG. 4  shows an example rotatable beam splitter to achieve polarization control in accordance with a first example embodiment. A rotatable beam splitter  42  is rotated by a beam splitter rotator  40 . By rotating the rotatable beam splitter  42 , the optical power traveling to the S and P polarization output fibers can be changed. The beam splitter rotator  40  may be implemented manually or automatically. For manual rotation, a human may rotate the rotatable beam splitter  42 , and in that case, the rotatable beam splitter  42  includes some structure that may be manipulated by the human to effect the rotation. Automatic rotation may, for example, be accomplished by a motor in which case it may be desirable to also use a suitable rotator controller  44 . (Dashed lines are used to indicate that the rotator controller  44  is optional). For example, if the beam splitter rotator  40  uses an electric motor, the rotator controller  44  may be an electric motor controller. Other types of motors may be used. Indeed, any type of beam splitter rotator  40  technique or mechanism may be used. 
         [0042]      FIGS. 5A-5C  are graphs that illustrate the concepts of slow and fast light axes, polarization states, and rotated polarization.  FIG. 5A  shows that the states of polarization in an optical fiber are arbitrary. It is helpful to establish a set of perpendicular axes to describe linear polarized light. In this case, two axes are established S (Slow) and P (Fast). Depicted in  FIG. 5A  is linear-polarized light being transmitted along the S axis. No light is transmitted along the P axis of the fiber. In  FIG. 5B , linear-polarized light aligned with the S axis of the optical fiber is incident on a birefringent crystal of a polarization beam splitter. The beam splitter is aligned so that its X (parallel) axis is aligned with the S transmission axis of the optical fiber. All light is transmitted to the X output of the beam splitter, and no light is translated to the Y (parallel) output. In  FIG. 5C , the beam splitter is rotated 90 degrees so that its X axis aligns with the P axis of the fiber. No light is transmitted along the P axis of the fiber, and no light is observed on X output of the beam splitter. The linear-polarized light in alignment with the S axis of the optical fiber is now aligned with the Y axis of the crystal, and all light is observed on the Y output. 
         [0043]      FIG. 6  shows an example rotatable beam splitter used in an optical network to achieve polarization control. The example network includes an interferometer connected to a rotatable polarization beam splitter  42 . Again, by rotating the rotatable beam splitter  42  by any suitable means, the optical power traveling to the S and P polarization output fibers coupled here to S and P photodetectors  38 A and  38 B can be changed. 
         [0044]    Another advantage of this new polarization rotation scheme is that it can be implemented in compact fashion. For example, in a manual rotation implementation, little if any additional structure is needed for a human to rotate the beam splitter. Automated rotation implementations can also be very small in size. Consider a micro gear motor assembly implementation such as the example shown in  FIG. 7 . The fiber  12  leading into the beam splitter  50  may be a standard single mode fiber so that the polarization state of the light does not follow the fiber birefringence axes as the beam splitter  50  is rotated. The beam splitter  50 , however, separates the polarization states into parallel and perpendicular output fibers S and P depending on how the incoming beam is incident on the structure. 
         [0045]    A micro motor connected to a micro planetary gearbox is shown at  52 , which in turn is coupled to the rotatable beam splitter  50  via a small spur gear set. The rotatable beam splitter  50  is mounted in a spur gear  56  that meshes with a spur gear  54  mounted on the micro planetary gearbox  52 . The assembly is supported with a compact, low profile housing  58 . Ninety degrees of rotation of the beam splitter  50  is needed to translate through the maxima for both S and P polarizations. 
         [0046]    An additional practical concern relates to limiting rotation of the beam splitter  50 . Without a limit on rotation, the rotated beam splitter can potentially wind up the fiber and damage the fiber network. Accordingly, mechanical features may be added that limit the rotation of the beam splitter, e.g., to less than one revolution. One way to do that is to use one or more mechanical stops coupled to the rotating beam splitter. The mechanical stop or stops are arranged with a stationary portion of the structure in a fashion to stop rotation in that direction. The motor controller monitors the current draw of the motor during operation. The controller may halt operation when the current drawn by the motor increases or increases by more than a predetermined amount, e.g., identifying that the end of desired rotation is reached. The power levels on the two polarization states may also be monitored, and if those power levels stop changing relative to each other, then the controller determines that the rotatable beam splitter  50  has reached an end of rotation. At this point, the motor controller only allows the motor to rotate in a direction away from the end of rotation. 
         [0047]    The end of rotation stop may be made progressive through the use of a spring to minimize wear on components. Reference numeral  70  in  FIG. 9  shows one example implementation of a progressive mechanical stop. In this example configuration, the mechanical stop is the end of the gear motor, and the progressive spring is a torsional spring mounted on the beam splitter housing. As the beam splitter rotates, one of the arms of the spring eventually contacts the exterior of the motor. As motion continues in that direction, the spring load increases, and therefore, the motor load and current increase. The progressive nature of this mechanical stop reduces shock on the system compared to a hard stop. 
         [0048]      FIG. 8  shows a second example embodiment of an example rotatable beam splitter to achieve polarization control. As in  FIG. 4 , a beam splitter rotator  40  and rotatable beam splitter  42  are provided. If the beam splitter rotator  40  is automated in some fashion, then a rotator controller  44  controls operation of the beam splitter rotator  40 . In addition, a rotation limiter  46  is shown associated by dashed lines to each of rotator controller  44 , beam splitter rotator  40 , and rotatable beam splitter  42  to indicate that the rotation limiter may be a stand alone component or that the rotation limitation operation may be performed by or as a part of any of the rotator controller  44 , beam splitter rotator  40 , and rotatable beam splitter  42 . 
         [0049]      FIG. 9  shows a non-limiting example rotatable beam splitter implemented using a motor assembly in accordance with the second embodiment. Here, a torsional spring  70  is used to limit rotation, e.g., to about 270 degrees. The components are similar to those described in  FIG. 6 . A rotatable beam splitter  60  is inserted through a housing  68  and includes an output gear  66  mounted on one end. The progressive end of rotation stop  70  also acts as a retainer to hold the rotatable beam splitter  60  in place. A micro gear motor  62  is used with a spur gear  64  mounted on the motor output shaft. The simple progressive end of rotation stop  70  reduces costs and simplifies wiring. 
         [0050]      FIGS. 10A  (cross section) and  10 B (perspective) show another example rotatable beam splitter implemented using a motor assembly in accordance with the second embodiment that makes use of hall sensors as limit switches. A magnet ring  72  is added to the beam splitter adjacent to the drive gear. This ring contains a magnet  74  that triggers one of two hall sensors  76  mounted on a small printed circuit board.  FIG. 10A  shows the assembly near one end of travel with the right hall sensor active. The assembly would rotate in the clockwise direction until the desired polarization state was reached or the left hall sensor was activated. This ring triggers one of two hall sensors that identify each end of travel. Although adding a few wires, this embodiment removes the need for a mechanical rotation limiter. 
         [0051]    A non-backdriving example embodiment is advantageous because it insures that the system stays at the desired polarization output state. This can be done by use of inherently non-backdriving gear trains, such as worm gear drives, or through the use of high reduction ratios. Other gearbox configurations may be used. There may also be cases where the polarization state is desired to be dynamically changing. In this case, a non-backdriving solution will likely be less desirable due to the higher inherent wear of these drivetrains. 
         [0052]    More generally, because the beam splitter is rotated rather than the fiber, the technology does not require low bend loss fiber, thereby eliminating one splice between low bend loss and standard fiber in most networks. Low bend loss fiber typically has a higher intrinsic loss per unit length than standard fiber, and so elimination of low bend loss fiber reduces optical loss. The technology is significantly less costly than paddle motor assemblies and only requires one degree of freedom to provide polarization rotation instead of two. Volumetrically the technology is significantly smaller than paddle motor assemblies. Thus, compared to conventional polarization rotation mechanisms, the technology described here is a robust, small size, and low cost alternative. 
         [0053]    Example and non-limiting tests were performed using a tunable laser coupled into the input of a polarizing beam splitter (PBS), and the two outputs were sent to detectors in a processing module. The PBS was fixed in a rotational stage, and power levels on the two output ports were monitored while the beam splitter was rotated with a constant power input. The power level of the two polarization states S and P varied from minimum to maximum 180 degrees out of phase as shown in  FIG. 11 . As may be seen in the graph, the variation in output power allows for power balancing between the two polarization states. 
         [0054]    It is also example embodiment implements dynamic polarization control using a high speed rotational device. A device such as a voice coil, limited angle motor, or any limited angle rotational device (preferably a high speed device) may be used to oscillate the beam splitter in a dynamic fashion. 
         [0055]    Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above description should be read as implying that any particular element, step, range, or function is essential such that it must be included in the claims scope. The scope of patented subject matter is defined only by the claims. The extent of legal protection is defined by the words recited in the allowed claims and their equivalents. All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the technology described, for it to be encompassed by the present claims. No claim is intended to invoke paragraph 6 of 35 USC §112 unless the words “means for” or “step for” are used. Furthermore, no embodiment, feature, component, or step in this specification is intended to be dedicated to the public regardless of whether the embodiment, feature, component, or step is recited in the claims.