Patent Publication Number: US-9835800-B2

Title: Polarization control for a photonic platform

Description:
TECHNICAL FIELD 
     The current disclosure relates to polarization control and in particular to polarization control for photonic platform based components and modules. 
     BACKGROUND 
     Photonic platform based devices such as silicon photonic circuits typically require a particular state of polarization (SOP) of the incoming light, for example the transverse-electric (TE) SOP. However, in practice the incoming light may have a different SOP. Accordingly, only a portion of the incoming light may provide a usable signal to the silicon photonic component. The use of silicon photonics in datacenters may benefit from controlling the SOP of incoming light. 
     SUMMARY 
     In accordance with the present disclosure there is provided a state of polarization (SOP) controller comprising: an optical beam splitter for splitting a randomly polarized input beam into mutually orthogonal first and second polarization components; a polarization rotator disposed in an optical path of the second polarization component for providing a rotated second polarization component parallel to the first polarization component; a first variable phase shifter coupled to the optical beam splitter or the polarization rotator for reducing a phase difference between the first polarization component and the rotated second polarization component; an optical coupler for combining the first polarization component with the rotated second polarization component to provide an SOP controller output beam having a pre-determined SOP; and a reset controller configured for resetting the SOP controller during a reset period. 
     In a further embodiment of the SOP controller, the polarization rotator is coupled to the optical beam splitter. The polarization rotator and the optical beam splitter can also be a single polarizing beam splitter and rotator component. 
     In a further embodiment of the SOP controller, the first variable phase shifter is coupled to the optical beam splitter. 
     In a further embodiment, the SOP controller further comprises a second variable phase shifter coupled to the polarization rotator for shifting a phase of the rotated second polarization component. 
     In a further embodiment, the SOP controller further comprises a controller for controlling the first variable phase shifter and the second variable phase shifter if available, to reduce the phase difference between the first polarization component and the second rotated polarization component. 
     In a further embodiment of the SOP controller, the reset controller resets the SOP controller when a reset is required and an indication of the reset period is received. 
     In a further embodiment of the SOP controller, the indication of the reset period is an indication of an inter-frame gap received from a data layer controller. 
     In a further embodiment of the SOP controller, resetting the SOP controller comprises resetting the first variable phase shifter from providing a phase shift of π+α to provide a phase shift of α. 
     In a further embodiment, the SOP controller further comprises a Mach-Zehnder Interferometer (MZI) based optical switch, wherein the MZI based optical switch comprises the optical coupler. 
     In a further embodiment of the SOP controller, the MZI based optical switch further comprises the first variable phase shifter. 
     In a further embodiment, the SOP controller further comprises a polarization measurement component for determining an SOP of the randomly polarized input beam to control the first variable phase shifter to reduce the phase difference between the first polarization component and the rotated second polarization component. 
     In accordance with the present disclosure there is provided a silicon-based photonic switch comprising: a plurality of input ports; a plurality of output ports; an optical switching fabric for coupling one or more of the plurality of input ports to one or more of the plurality of output ports; and a plurality of state of polarization (SOP) controllers, each associated with a respective one of the plurality of input ports and converting a random polarization of an incoming beam to a pre-determined linear polarization, each of the SOP controllers comprising: an optical beam splitter for splitting the incoming beam into a transverse-electric (TE) component and a transverse-magnetic (TM) component; a polarization rotator for rotating the TM component to provide a rotated TM component parallel to the TE component; a first variable phase shifter for lessening a phase difference between the TE and rotated TM components; an optical coupler for combining the TE and rotated TM components into a single output beam having the pre-determined polarization; and a reset controller for resetting the SOP controller during a reset period. 
     In a further embodiment, the silicon-based photonic switch further comprises a data layer controller for configuring the optical switching fabric and providing an indication of the reset period to each of the reset controllers. 
     In a further embodiment of the silicon-based photonic switch, the reset period comprises an inter-frame gap within transmitted data. 
     In a further embodiment of the silicon-based photonic switch, wherein each of the SOP controllers further comprises a controller for controlling the first variable phase shifter to reduce the phase difference between the TE and rotated TM components. 
     In accordance with the present disclosure there is provided a method of controlling a state of polarization (SOP) of incoming light, the method comprising: splitting the incoming light into first and second orthogonal polarization components; rotating the second polarization component to be parallel to the first polarization component; reducing a phase difference between the first and rotated second polarization components; combining the first and rotated second polarization components into an output beam having a pre-determined SOP; and resetting a phase shift used in reducing the phase difference during a reset period. 
     In a further embodiment, the method further comprises determining that the reset period has occurred; and providing a reset signal upon detection of the reset period if a reset is required. 
     In a further embodiment of the method, resetting the phase shift during the reset period comprises: determining that the reset is required based on the phase shift; receiving an indication of an inter-frame gap in transmitted data; and resetting the phase shift when it is determined that the reset is required and the indication of the inter-frame gap is received. 
     In a further embodiment, the method further comprises receiving the indication of the inter-frame gap in transmitted data from a data layer controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are described herein with references to the appended drawings, in which: 
         FIG. 1  depicts a schematic of a datacenter including an optical network portion; 
         FIG. 2  depicts components of a state of polarization (SOP) controller for datacenter applications; 
         FIG. 3  depicts components of a further SOP controller for datacenter applications; 
         FIG. 4  depicts components of a reset controller for use with the polarization controller of  FIG. 2 . 
         FIG. 5  depicts components of a polarization measurement component for use in the SOP controller of  FIG. 3 ; 
         FIG. 6  depicts components of a further polarization measurement component for use in the SOP controller of  FIG. 3 ; 
         FIG. 7  depicts a Poincaré sphere representation of the SOP control; and 
         FIG. 8  depicts a method of SOP control. 
     
    
    
     DETAILED DESCRIPTION 
     Datacenters may include one or more optical networks or optical portions of a larger network that include silicon-based photonic components. Generally, optical transceivers used in datacenters are non-coherent where the information is not coded in the polarization. Silicon photonic components, such as photonic switches, may operate optimally with a particular linear polarization of light, such as transverse-electric (TE) polarization. The silicon photonics may incur losses when operating with non-TE polarizations of incoming light. A state of polarization (SOP) controller that can convert a random polarization of incoming light to a linear polarization, such as TE polarization, for particular use in datacenter applications is described further herein. The SOP controller is able to convert the random polarization to the linear polarization and transfer the total power of the incoming signal to a single, linearly polarized beam. The SOP controller splits the incoming beam into orthogonal polarization components and transfers the power from one component to the other component. The SOP controller described further herein may use control signals from higher data control layers in order to provide a simple architecture that can be efficiently implemented in silicon. SOP controllers may be provided in a silicon photonic switch to control the SOP of incoming light and improve the performance of the optical component. 
       FIG. 1  depicts a schematic of a datacenter including an optical network portion. It will be appreciated that the datacenter schematic  100  is only illustrative of a possible network hierarchy, and similar components may be deployed to provide different network architectures. The datacenter  100  comprises a number of servers  102 - 1 - 102 - n  (referred to collectively as servers  102 ) which may be arranged into a number of physical groups, cabinets or racks  104 - 1 - 104 - m  (referred to collectively as racks  104 ). Each of the racks  104  may be associated with one or more switches  106 - 1 - 106 - p  (referred to collectively as switches  106 ) for switching network traffic between the servers  102  connected to the same one of the switches  106 , as well as other network locations. 
     A group of switches  106  may be connected to one or more aggregation nodes  108 - 1 - 108 - q  (referred to collectively as aggregation nodes  108 ). Although depicted as separate from the switches  106 , one or more of the aggregation nodes  108  may be provided within the switches  106 . The aggregation nodes  108  can aggregate a number of data packets from the switches  106  together into an optical frame and transmit the optical frame over optical cables  110 - 1 - 110 - q  (referred to collectively as optical cables  110 ). Similarly, the aggregation nodes  108  can separate the individual packets from a received optical frame and transmit the packets to the correct destination. A silicon photonic switch  112  is connected to the optical cables  110  of the aggregation nodes  108  and can switch the transmitted optical frames between different aggregation nodes  108 , or other optical components. The silicon photonic switch  112  may comprise a controller  114  for controlling the optical switching between different ports. As depicted in  FIG. 1 , the silicon photonic switch  112  may include state of polarization (SOP) controllers  116 - 1 - 116 - q  (referred to collectively as SOP controllers  116 ) for converting the polarization of incoming light from the respective optical cables  110  to a linear polarization optimized for the silicon photonic switch  112 . 
     The SOP controllers  116  of the photonic switches  112  in the datacenter  100  minimize, or at least reduce, polarization loss resulting from the silicon switch fabric&#39;s optimization for operation with one polarization, such as transverse-electric. As described in further detail below, the SOP controllers  116  convert the polarization in two stages. In the first stage, the randomly polarized light is split into two components, referred to as a transverse-electric (TE), E x  or horizontal, component and a transverse-magnetic (TM), E y  or vertical, component. Once split, the TM component is rotated to be parallel to the TE component and the phase difference between the two components is eliminated or at least reduced. In the second stage the phase corrected components are combined together into a single beam so that all, or at least most, of the optical power from the TE and rotated TM components are combined together into the single output beam. With the phase difference eliminated or reduced between the TE and rotated TM components, when the two components are re-combined the resulting beam will be linearly polarized and all, or substantially all, power from the incoming beam is transferred to the single polarization. 
     The SOP controllers  116  convert any incoming polarization to the desired SOP for the silicon photonic switch  112 . In order to convert the polarization, the SOP of the incoming light may be measured and tracked at a sufficient speed to account for polarization drifts in the incoming light. If the SOP is not measured, the convergence and tracking speed of the SOP controllers  116  may be reduced. The amount of the polarization drift may be affected by a number of factors, including for example a length of the optical cables  110 , temperature changes, pressure changes, applied stresses as well as other factors. In datacenter applications, the optical cables  110  may be relatively short, for example 2 km or less and located in a stable environment so that the temperature and pressures remain relatively constant. As such, the polarization drift in datacenter applications may be relatively small. Due to the relatively small polarization drift in datacenters, the SOP controllers  116  do not need to provide endless, or reset-free control. 
     In datacenters, a millisecond SOP change is likely to be a worst case scenario. Accordingly, SOP controllers  116  capable of being reset every millisecond or less may be sufficient to provide desirable performance even in worst case scenarios. In data transmissions in datacenters it is likely that every millisecond there will be a period of time when a reset may be performed without impacting the data transmission, or when the impact on data transmission resulting from the reset would be acceptable. For example, photonic frames are generally larger than packets, while not exceeding a few microseconds. Accordingly, if the SOP controllers  116  are able to be reset between photonic frames, it is not necessary to provide an endless, or reset-free controller. Since photonic switches, such as the silicon photonic switch  112 , may be reconfigured before the start of transmitting photonic frames, the SOP controllers  116  can be reset, if necessary, during a gap time between transmission of photonic frames. Although the gap time between photonic frames is described as being a particularly suitable for performing a reset, resets may be performed during other reset periods when the reset has an acceptable impact on data transmission. The SOP controllers  116  may use higher level control information to identify the occurrence of reset periods, such as the gaps between transmitted frames, and use the identified reset period to perform an SOP reset when required. 
     As described, although the switch  112  may be reconfigured during an inter-frame gap time between transmission of photonic frames, other times may be used to reset the SOP controllers  116 . It is to be noted however that resets occurring during a packet transmission may result in loss of the packet. For example, rather than performing the reset during an inter-frame gap time, the reset may be performed during transmission of low priority packets, or during transmissions capable of dealing with lost packets. 
     The SOP controllers  116  can provide fast and reliable SOP control at minimum, or at least low, insertion losses. The use of higher level data layer control signals in the SOP controller  116  provides a simple SOP controller that can be implemented on-chip with the silicon photonic switching fabric. 
       FIG. 2  depicts components of a state of polarization (SOP) controller  200  for use in various applications, such as for example datacenter applications. As depicted in  FIG. 1 , the photonic packet switch  112  may have a number of incoming ports  110 - 1 - 110 - q  with an individual SOP controller  116 - 1 - 116 - q  associated with each of the incoming ports to convert the random polarization of the incoming light prior to impinging on the switching fabric. A single SOP controller  200 , which may be used as any of the SOP controllers  116 , may include both optical components as well as electrical components. The SOP controller  200  may be formed on the same photonic chip as the photonic switching fabric, or may be formed on separate photonic chips that are optically coupled together. 
     The SOP controller  200  receives a randomly polarized input beam  202 , which is provided to a polarizing beam splitter and rotator (PBSR)  204 . The PBSR  204  splits the input beam  202  into a first component  206   a  and a second component  206   b  that is orthogonal to the first component. The first component  206   a  may be referred to as a TE component and may be considered as being horizontally polarized. The second component  206   b  may be referred to as a TM component and may be considered as being vertically polarized. In addition to splitting the incoming beam, the PBSR  204  rotates one of the components, depicted as the second component  206   b , by 90° so that it is parallel with the other component. The rotated second component may be referred to as the rotated TM component or simply TM*. A phase difference between the TE and TM*, or TM, components may be eliminated or reduced by one or more variable phase shifters, such as phase shifter  208   a . The SOP controller  200  comprises a controller  214  for controlling the various components. The variable phase shifter  208   a , which is controlled by the controller  214 , is used to eliminate or reduce the phase difference between the two polarization components. The phase-aligned components may be provided to a 2×2 photonic coupler  210  that combines the two components together into a single output  212  that has a linear polarization suitable for use with silicon photonic components, such as a TE component. The 2×2 photonic coupler  210  may be provided in various ways including, for example by a Mach-Zehnder Interferometer (MZI)-based switch structure, or other non-MZI-based switch structures. The phase shifter  208   a  may be external to the photonic coupler  210  or may be combined with the photonic coupler  210  if, for example, the photonic coupler  210  is provided by an MZI-based switch structure, or other non-MZI-based switch structure. For example, if the 2×2 photonic coupler  210  is provided by an MZI-based switch structure, the phase shifter  208   a  may be incorporated into an arm of the MZI switch structure. The SOP controller  200  allows almost or all of the power in a randomly polarized input beam to be transferred to a polarization suitable for use with silicon photonic components, which may be a TE polarization. 
     Although the PBSR  204  is depicted as a single component, the PBSR  204  may be provided by separate optical components. For example, a beam splitter may slit the input beam  202  into two polarization components and a rotator optically coupled to the beam splitter may rotate one of the polarization components. Further, the rotator may be arranged in an optical path between the beam splitter and the photonic coupler  210 , between the beam splitter and the phase shifter  208   a , or between the phase shifter and the photonic coupler  210 . That is, if the rotator is provided separately from the beam splitter, it is located in an optical path downstream from the beam splitter and upstream of the photonic coupler  210 . 
     As described above, the controller  214  may control the operation of the SOP controller components, including the variable phase shifter  208   a  as well as the 2×2 photonic coupler  210 . The controller  214  may receive an indication of an amount of power in a secondary output  216  of the 2×2 photonic coupler  210 . The power may be detected by a power detector such as a photo detector  218 , which provides an electrical signal  220  to the controller that provides an indication of the amount of power in the signal at the second output  216 . Various different control techniques may be provided by the controller  214  to control the phase shifters  208   a  and photonic coupler  210 . Regardless of the specific control techniques employed, each control technique attempts to maximize the power from the input beam  202  that is transferred to the output beam  212 . In order to maximize the power in the output beam  212 , the power in the second output  216  of the photonic coupler  210  is minimized. 
     In controlling the phase shifter  208   a , the controller  214  may require resetting the phase shift provided by the phase shifter  208   a . For example if a phase shift of 2π+α is required, the phase shifter may be reset to a, which requires a finite amount of time to be completed. The polarization controller  214  performs the reset, when required, during a time at which the impact on transmitted data will be the smallest, or at least acceptable. Since the SOP is slowly varying, the controller  214  may delay when the reset occurs until a reset period occurs. As an example, when a photonic frame ends transmission, there may be a gap time during which the reset may be performed without any impact on the transmitted signals. A data layer controller  114  provided by the switch associated with the SOP controller  200  may provide an indication of the occurrence of reset periods during which the reset can be performed. For example, the reset period may be during an inter-frame gap, during transmission of a low-priority packet, or any other time during which a corrupted transmission, which could result in packet loss, is acceptable. 
       FIG. 3  depicts components of a further SOP controller  300  suitable for datacenter applications. The SOP controller  300  of  FIG. 3  is similar in operation to the SOP controller  200  of  FIG. 2 ; however, the SOP controller  300  of  FIG. 3  includes a polarization measurement component  322 , as well as an additional phase shifter  308   b . Although a single phase shifter  208   a  may be used to eliminate the phase difference between the two polarization components, the additional phase shifter  308   b  provides greater flexibility in how the phase difference is eliminated, as well as possibly providing flexibility in when resets may be performed. The polarization measurement component  322  determines the SOP of the input light beam  202 , which may be provided to a polarization controller  314  for use in controlling the phase shifters  208   a ,  308   b  and the 2×2 photonic coupler  210 . The use of the SOP measurement allows the controller  314  to more quickly converge on transferring all of the power in the input beam  202  to the output  212 . The polarization measurement component  322  may utilize one or more different optical signals. The optical signals may be provided from optical taps  324   a ,  324   b ,  324   c ,  324   d  (referred to collectively as optical taps  324 ). The polarization measurement component  322  may use one or more of the optical signals from the optical taps  324 . As depicted, the polarization measurement component  322  may measure the SOP of a beam, or more particularly, between two components of the beam. By measuring the polarization between the TE and TM* components of the input beam  202  the polarization controller  314  can determine an amount of phase shift required to apply to the two polarization components in order to convert the random polarization to a linear polarization. As depicted, the polarization measurement component  322  may also measure the polarization between the phase-shifted TM* and TE components in order to provide real-time, or near real-time, feedback for correcting for polarization drift. That is, as the polarization drifts, the polarization of the phase-shifted TM* and TE components will vary from the desired linear polarization and the drift can be corrected by varying control of the phase shifters  208   a ,  308   b . The controller  314  receives the measurements from the polarization measurement component  322  and determines the phase shifts required by the phase shifters. In particular, the controller  314  determines the phase shifter settings according to: 
     
       
         
           
             
               
                 
                   
                     
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     Where:
         e jφupper  is the phase shift of the first phase shifter  208   a  in  FIG. 3 ;   e jφlower  is the phase shift of the second phase shifter  308   b  in  FIG. 3 ;   E x     o   e jφ     x    is the first component;   E y     o   e jφ     y    is the second component; and       

               e     j   ⁢           ⁢   ϑ       (           E     x   0                 E     y   0             )         
is the combined phase-shifted first and second components.
 
     The controller  314  also controls the photonic switch  210  in order to combine the phase shifted TM* and TE components into the output beam while minimizing the second output of the photonic switch  314 . The polarization controller  314  controls the photonic switch  210  according to: 
     
       
         
           
             
               
                 
                   
                     
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     Where:
         φ is phase shift between first and second components provided by the phase shifter;   E x  is the amplitude of the output of the first variable phase shifter;   E y  is the amplitude of the output of the second variable phase shifter; and   E x   F  is the linearly polarized output beam.       

     The controller  314  may monitor the power of the second output of the photonic coupler  210  using, for example, a photo detector  218 , in order to adjust the phase shifters and switch to maintain the second output of the switch  216  at 0, which results in most or all of the power being transferred to the output beam  212 . 
     As depicted in  FIG. 3 , the controller  314  may receive signals from a data layer controller  114 . The data layer controller  114  may be a controller of the photonic packet switch  112  depicted in  FIG. 1 , and may provide an indication of when gaps between optical frames occurs. If a reset of the phase shifters  208   a ,  308   b  is required, it may occur during reset periods such as during the gaps between photonic frames or during other times that provide acceptable packet loss rates. For example, the reset may be performed during transmission of a low priority packet, or packet determined to be not important and as such capable of being corrupted and lost. Accordingly, by receiving the data from the packet switch, the SOP controller  300  is able to utilize a simpler architecture since the SOP controller  300  can perform resets at times that will not impact the data transmission. 
       FIG. 4  depicts components of a reset controller  402  for use with the controller of SOP controllers  200 ,  300  of  FIG. 2 or 3 . The controller  214  of  FIG. 2  is depicted in  FIG. 4 . The controller  214  and the reset controller  402  are depicted as separate components; however, the reset controller  402  components may be incorporated into the controller  214 . The controller  214  determines that a reset is required to be performed and sends a reset request  404  to the reset controller  402 . A reset may required when the phase shift increases past 2π, for example to 2π+α The reset request is latched into a flip-flop  406  and output  408  to an AND circuit  410 . The AND circuit performs an AND operation of the flip-flop output  408  and a signal  412  indicative of the occurrence of a reset period such as a gap between transmitted photonic frames. The signal  412  may be received from higher level controllers, not shown, of the switch. The reset period signal  412  is provided to the reset controller  402  from a data layer controller  114 . The AND circuit provides an SOP reset signal  414  when the flip-flop has latched a reset request and a reset period has occurred. When the SOP reset signal  414  is generated, a delay component  416  can provide a delay before applying a latch reset signal  418  to the flip-flop latch in order to reset the flip-flop so that there is no pending reset request. The SOP reset signal  414  is received by the polarization controller  214  and used as an indication of when to perform the reset. 
       FIG. 5  depicts components of a polarization measurement component  500  for use in the SOP controller  200  of  FIG. 2 . The polarization measurement component  500  measures a resultant polarization of two polarization components, depicted as the TM* component  502   b , which may be the rotated TM component, and the TE component  502   a . The TM* component  502   b  and TE component  502   a  may be provided by taps  324   a ,  324   b  off the inputs to the phase shifters  208   a ,  208   b  of the SOP controller  300  depicted in  FIG. 3 , or by taps  324   c ,  324   d  of the outputs of the phase shifters  208   a ,  308   b  of the SOP controller  300 . Providing measurements before and after the phase shifters may provide quicker convergence as well as eliminating any phase offset introduced by the phase shifters. 
     The polarization measurement may utilize only the input measurements or output measurements; however, the convergence speed may be lower. The different taps  324   a - 324   d  before and after the phase shifters  208   a ,  308   b  may be selectively coupled to the measurement component  500  through a pair of multiplexers or switches that can selectively couple one of the two components from before or after a phase shifter to the measurement components. Alternatively, the measurement components may be duplicated in order to provide polarization measurements of the TM* and TE components before the phase shifters  208   a    308   b  in  FIG. 3  and between the phase shifted TM* and TE components after the phase shifters  208   a    308   b.    
     The polarization measurement component  500  determines an SOP between a TE component  502   a  and a TM* component  502   b . The TE and TM* components may be split into different optical paths, one of which is provided to a 2×1 combiner  504 , whose output is provided to a photo detector  506 . The other split optical paths  508 ,  510  are provided to respective photo detectors  532 ,  534 . As depicted, the photo detector  532  may provide a first power indication P 1 , the photo detector  534  may provide a second power indication P 2  and the photo detector  506  may provide a third power indication P 3 . The phase difference between the two components, TE and TM*, may be determined according to the following equations:
 
 P 1=| E   x | 2   (3)
 
 P 2=| E   y | 2   (4)
 
 P 3=½ [P 1+ P 2+2√{square root over ( P 1 P 2)} cos(φ xy )]  (5)
 
     Where:
         E x  is an amplitude of the first component;   E y  is an amplitude of the second component; and   φ xy  is the phase difference between the two components.       

     Equation (5) above can be re-arranged to provide the phase difference φ xy  based on the three measurements P 1 , P 2 , P 3  provided by the respective photo detectors  532 ,  534 ,  506 . 
       FIG. 6  depicts components of a further polarization measurement component  600  for use in the SOP controller of  FIG. 3 . The measurement component  600  comprises a first 50/50 optical splitter  604  which splits the input beam of a TE  602   a  component into two equal beams  606 ,  608 . A second 50/50 optical splitter  610  splits the input beam of a TM* component  602   b  into two equal beams  612 ,  614 . One of the beams  606  from the first splitter  604  passes through a fixed phase shifter  616  that shifts the phase of the split signal  606  by π, or 180° degrees. Similarly, one of the beams  614  from the second splitter  610  passes through a fixed phase shifter  618  that shifts the phase of the split signal  614  by π, or 180° degrees. The phase shifted outputs are then combined with the un-shifted outputs of the opposite splitter. As depicted, a first combiner  620  combines the output from the first fixed phase shifter  616  with the un-shifted output  612  from the second splitter  610 . Similarly, a second combiner  622  combines the output from the second fixed phase shifter  618  with the output from the first splitter  604 . The outputs  624 ,  626  from the respective combiners  620 ,  622  are detected by a pair of photo detectors  628 ,  630 . The electrical signal at the node between the two photo detectors  628 ,  630  is proportional to the difference between the two output signals. It is noted that the two output signals that are compared are the 180° degree phase shifted TM* component combined with an un-shifted TE component and the 180° degree phase shifted TE component combined with an un-shifted TM* component. The polarization measurement component further includes a pair of photo detectors  632 ,  634  that detect an intensity of the TM* component, or more particular a tap of the TM* component  636 , and the TE component, or more particularly a tap of the TE component  638 . The outputs of the intensity photo detectors  632 ,  634  may each be detected or measured by circuitry (not shown), which may be made from, for example CMOS technologies, and represent the amplitude of the TM* component (E x0    640 ) and the amplitude of the TE component (E y0    642 ). Additionally, the signal  644  proportional to the difference signal between the two combiners  624 ,  626  can be detected or measured by circuitry. Based on the three measurements  640 ,  642 ,  644  the SOP of the signal being measured can be fully determined. 
       FIG. 7  depicts a Poincaré sphere representation  700  of the SOP control. Different states of polarization can be represented on the Poincaré sphere representation  700 . For example, a point  702  may represent an elliptical SOP. The result of the two-stage polarization control is represented graphically in  FIG. 5 . In the first stage, represented by arrow  704 , the random polarization, which in  FIG. 5  is elliptical, is converted to a linear polarization. The second stage, represented by arrow  706 , transfers all of the power of the linear polarization to the desired polarization, which in  FIG. 6  is depicted as a point on S 1  axis. 
       FIG. 8  depicts a method  800  of SOP control. The method  800  begins with initially measuring the SOP from the beam splitter at a step  802 . As described above, the measurement may determine the amplitude of the TM and TE components of the polarized beam as well as a phase shift between the two components. The method determines if a phase shift is required at a step  804 , which is required if there is a phase difference between the TM and TE components, or if the phase difference between the two components is greater than an allowable threshold. If a phase shift is required (Yes at step  804 ), the method eliminates the phase difference at a step  806  between the TM and TE components. The phase difference is eliminated by adjusting the control of variable phase shifters associated with the TM and TE components. With the phase difference eliminated, or if no phase shift is required (No at step  804 ), the polarization is transferred to the TE polarization at a step  808 . The TM and TE components, with the phase difference eliminated, are combined together into a single output having the desired polarization using a 2×2 photonic coupler, which may be provided by an MZI switch structure. 
     As described above, the polarization may drift from the initially measured SOP. Depending upon an amount of drift, it may be necessary to reset the SOP controller. The method determines if a reset is required at a step  810 . If the reset is not required (No at step  810 ) real-time SOP measurements of the phase shifter output is performed at a step  812  and again it is determined if a phase shift is required at the step  804 . The phase shift may be required based on the real-time measurements as a result of polarization drift. If it is determined that a reset is required (Yes at step  810 ), a reset is requested at a step  814 . A reset may be required if the phase shift required to eliminate the phase difference is greater than what can be provided by the variable phase shifter. For example, as a phase shifter approaches 2π, and additional phase shift of α is required resulting in a phase shift of 2π+α, the phase shifter may be reset to provide a phase shift of α. As described previously, once a reset is requested, the actual reset may occur at the next occurrence of the next gap between transmitted optical frames. The method  800  may determine if a gap was detected at a step  816 , for example based on control signals received from higher layers of the switch fabric. If no gap was detected (No at step  816 ) the method continues to wait until a gap is detected. If a gap was detected (Yes at step  816 ), the SOP controller is reset at a step  818  and the SOP from the beam splitter is measured at the step  802 . 
     The method  800  described above allows any polarization of incoming light to be converted to a desired linear polarization, such as the TE polarization. The method eliminates a measured phase difference between TM and TE components of the incoming light and then transfers all of the power of the incoming beam to the TE polarization, which the silicon photonic components operate efficiently with. 
     The present disclosure provided, for the purposes of explanation, numerous specific embodiments, implementations, examples and details in order to provide a thorough understanding of the invention. It is apparent, however, that the embodiments may be practiced without all of the specific details or with an equivalent arrangement. In other instances, some well-known structures and devices are shown in block diagram form, or omitted, in order to avoid unnecessarily obscuring the embodiments of the invention. The description should in no way be limited to the illustrative implementations, drawings, and techniques illustrated, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and components might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.