Patent Publication Number: US-10330960-B2

Title: Silicon photonic hybrid polarization demultiplexer

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/326,309 filed on Jul. 8, 2014, the contents of which is incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an apparatus and method for providing a low-loss silicon photonic polarization demultiplexer having a high-bandwidth control loop. 
     BACKGROUND 
     Silicon photonics is an evolving technology that transmits data as light pulses along optical fibers. Multiplexers combine the light pulses into a single signal that is transmitted along an optical fiber where a demultiplexer divides the signal back into separate channels. Although active demultiplexing exists in silicon photonics systems, the tracking speed is far too slow to be of practical use. Tracking speed is limited by the bandwidth, i.e., the response time, exhibited by state of the art demultiplexers. Further, other types of demultiplexers exhibit large insertion loss that has proven to be unacceptable. Therefore, there is a need for a demultiplexer that provides both sufficient control bandwidth and low insertion loss and that is applicable for silicon photonics systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown in the drawings embodiments that are presently preferred it being understood that the disclosure is not limited to the arrangements and instrumentalities shown, wherein: 
         FIG. 1  illustrates is an example of a silicon photonics network utilizing the silicon photonic hybrid polarization demultiplexer of the present disclosure; 
         FIG. 2  illustrates an example of a hybrid phase shifter of the present disclosure using both thermal and electro-optic phase shifters; 
         FIG. 3  illustrates an example of the hybrid polarization demultiplexer of the present disclosure including a series of mixing stages; and 
         FIG. 4  illustrates a method incorporating the silicon photonic hybrid polarization demultiplexer of the present disclosure. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The detailed description set forth below is intended as a description of various configurations of the subject technology and is not intended to represent the only configurations in which the subject technology can be practiced. The appended drawings are incorporated herein and constitute a part of the detailed description. The detailed description includes specific details for the purpose of providing a more thorough understanding of the subject technology. However, it will be clear and apparent that the subject technology is not limited to the specific details set forth herein and may be practiced without these details. In some instances, structures and components are shown in block diagram form in order to avoid obscuring the concepts of the subject technology. 
     Overview 
     In one aspect of the present disclosure, a polarization demultiplexer is provided. The demultiplexer includes at least one a hybrid phase shifter configured to receive a light signal over a fiber element, the light signal including polarized optical signals. Each phase shifter includes a thermo-optic phase shifter configured to phase shift the polarized optical signal, an electro-optic phase shifter configured to phase shift the polarized optical signal, and control circuitry configured to regulate the thermo-optic and electro-optic phase shifters. 
     In another aspect of the present disclosure, a hybrid phase shifter is provided. The hybrid phase shifter includes a thermo-optic phase shifter configured to phase shift polarized optical signals of a light signal, where the thermo-optic phase shifter receives a control signal from control circuitry. The hybrid phase shifter also includes an electro-optic phase shifter configured to phase shift the polarized optical signals, where the electro-optic phase shifter receives a dither signal from the control circuitry, the control signal and the dither signal having different amplitudes. 
     In yet another aspect of the present disclosure, a method of demultiplexing a polarization-multiplexed optical signal is provided. The method includes receiving a light signal over a fiber element, the light signal including polarized optical signals, and separating the polarized optical signals of the light signal received over the fiber element. The separating includes providing a plurality of mixing stages, each mixing stage comprising a thermo-optic phase shifter and an electro-optic phase shifter adapted to phase shift the polarized optical signals. 
     Detailed Description 
     The present disclosure describes a low-loss active polarization demultiplexer using pre-existing silicon photonics building blocks. Specifically, a hybrid polarization multiplexer that combines both electro-optic and thermo-optic phase shift elements is used in an optical network. An example of an optical network  100  utilizing the hybrid demultiplexer of the present disclosure is shown in  FIG. 1 . Optical network  100  includes a transmitter  102 , which includes a signal generator  104 , having a laser  106  and a modulator  108 , and a multiplexer  110 . Laser  106  creates a beam of light having light pulses  107  of different wavelengths. Modulator  108  encodes data onto each light pulse  107 . The modulated light is then passed on to multiplexer  110 , which combines each of the multiple wavelengths of the modulated light onto a glass fiber  112 , and on to receiver  119 . Receiver  119  includes hybrid demultiplexer  114  of the present disclosure, which splits off the individual wavelengths where they are sent to photodetectors  115 , or coherent receivers, that convert the optical data into electrical data. Logic  118  then processes the data. The optical network  100  of  FIG. 1  is illustrative only, and the hybrid demultiplexer  114  of the present disclosure is equally applicable to other optical network designs. 
       FIG. 2  is an example of a hybrid phase shifter  121  that is one of the components of hybrid polarization demultiplexer  114 . Hybrid phase shifter  121  includes both a thermal phase shifter  122  and an electro-optic phase shifter  124 . The combination of thermal phase shifter  122  and electro-optic phase shifter  124  provides advantages that demultiplexers using only thermal phase shifters or only electro-optic phase shifter do not provide. Typical thermo-optic phase shifters require a control bandwidth of approximately 10 kHz. This requires dither tones to be applied at frequencies much larger than 10 kHz, which is insufficient for implementation of a control loop. Thus, the use of thermo-optic phase shifters alone cannot provide the adequate bandwidth necessary for implementation of a control loop. On the other hand, the use of only electro-optic phase shifters, due to their high loss, typically over 2 dB for a π phase shift, result in an unacceptable insertion loss. In other words, the bandwidth of the thermo-optic phase shifter is insufficient to support a dither signal and the electro-optic phase shifter has too high of an optical insertion loss to be effective. Advantageously, the use of hybrid elements, e.g., thermal phase shifter  122  and electro-optic phase shifter  124 , results in a high phase excursion, low loss, and sufficient bandwidth. A coupler  126 , such as, for example a 2×2 coupler, can be used in connection with hybrid phase shifter  121 , to form a mixing stage  120 , that maintains the polarization of the multiplexed signal. 
       FIG. 3  illustrates a series of cascaded mixing stages  120 , forming demultiplexer  114 , as part of a polarization multiplexed network. Each mixing stage  120  includes hybrid phase shifter  121 , which includes thermo phase shifter  122 , electro-optic phase shifter  124 , and coupler  126 . In the illustrated example, four mixing stages  120  are cascaded in order to effectively separate the polarized signals of the light beam, as described above. Although four cascaded mixing stages  120  are shown in  FIG. 3 , it is within the spirit of the invention to include less or more than the number of mixing stages  120  shown in  FIG. 3 . In the example shown in  FIG. 3 , a light signal, comprised of two polarized optical signals, X and Y, are transmitted along a fiber. The fiber rotates and mixes these signals. A polarization splitter  128  separates the light signal into an X′ signal and a Y′ signal, each of which represents a rotated and/or mixed version of the transmitted X and Y polarized signals. Polarization demultiplexer  114  of the present disclosure is needed to recover the original signals X and Y from the mixed X′ and Y′ signals. This is accomplished by application of appropriate control voltages or currents, as discussed in further detail below. 
     Feedback control logic  130  applies the appropriate voltages to continuously regulate and adjust the signal applied to each mixing stage  120 . Feedback control logic  130  regulates each mixing stage  120  by applying separate control and dither signals, rather than a combined signal. Error signals for feedback control logic  130  can be generated from a variety of sources. For example, in one example, pilot tones or pilot signals are applied on one or both polarizations at transmitter  102 . In another example, an error signal can be generated from a bit error rate (“BER”) measurement. 
     Specifically, feedback control logic  130  applies control signals  132  to each thermal-optic shifter  122  and separate dither signals  134  to each electro-optic shifter  124 . In order to avoid interference between control signal  132  and dither signal  134 , each dither signal  134  has a smaller amplitude than the amplitude of each control signals  132 . In one example, dither signals  134  are on the order of a few percent of the control signals  132 . Thus, in this example, control signals  132  have a bandwidth on the order of 10 s of kHz while dither signals  134  have a bandwidth in the MHz range. Therefore, the length of each electro-optic phase shifter is relatively short, for example, on the order of 10 microns or less, in comparison to the length of the thermal phase shifter  122 .  FIGS. 2 and 3  illustrate this relative difference in phase shift by depicting a longer thermal phase shifter  122  and a shorter electro-optic phase shifter  124 . 
     In the example shown in  FIG. 3 , where four mixing stages  120  are provided in a cascaded fashion, if electro-optic phase elements alone were used, the insertion loss would be approximately 8 dB (four stages at a 2 dB loss per stage). If the polarization demultiplexer is based solely on thermo-optic phase shift elements, the insertion loss would be approximately less than 0.5 dB but would result in insufficient bandwidth to allow practical dither-based control algorithms. However, because the high frequency dither signal used in hybrid demultiplexer  114  of the present disclosure has a relatively small amplitude, the phase shift of the electro-optic phase shifter  124  is short, on the order of 10 s of microns, and the resulting insertion loss is less than 0.2 dB for each electro-optic phase shifter  124 . Thermo-optic phase shifters  122  can be sized to obtain a phase shift of approximately π or 2π, or on the order of approximately 100 microns. Thus, thermo-optic phase shifters  122  have a negligible insertion loss of approximately 0.1 decibels for a π shift. Therefore, when compared to an approach that is based entirely on electro-optic phase shifters, the hybrid demultiplexer  114  of the present disclosure results in a very low insertion loss, e.g., on the order of 0.3 dB per mixing stage  120  as compared to at least 8 dB when four electro-optic phase shifters, while providing sufficient bandwidth. 
       FIG. 4  illustrates an example of a method utilizing the hybrid polarization demultiplexer  114  of the present disclosure. A light signal is received over a fiber element, at step  136 . The light signal includes polarized optical signals. However, during transmission, these optical signals can become rotated and/or mixed. For example, a light signal containing orthogonally polarized optical signals X and Y may arrive at polarization splitter  128 . Polarization splitter  128  can separate the signal components into X′ and Y′ of the light signal, at step  138 . However, signals X′ and Y′ are only a combination of the originally-transmitted X and Y signal components due to the mixing that occurred during transmission along the fiber element. Demultiplexer  114  includes a series of mixing stages  120  which are provided in a cascaded manner, at step  140 . Each mixing stage  120  includes a thermo-optic phase shifter  122  and an electro-optic phase shifter  124 . Feedback control logic  130  provides separate control signals to the thermo-optic and electro-optic phase shifters, at step  142 . After the signals have passed through the mixing stages  120 , demultiplexer  114  outputs the originally transmitted polarized optical signals X and Y, at step  144 . 
     The hybrid demultiplexer of the present disclosure provides a combination of electro-optic and thermal-optic phase shift elements such that each phase shift element is separately controlled: a dither signal controls the electro-optic phase shifter and a separate demultiplexer control signal controls the thermo-optic phase shifter. This allows the use of dither tones having a frequency far above the control signal bandwidth. Advantageously, the hybrid demultiplexer of the present disclosure is applicable to silicon photonics networks because it utilizes only existing Silicon building blocks such as thermo-optic phase shifters, electro-optic phase shifters such as silicon-insulator-silicon capacitive (SISCAP) structures, and 2×2 couplers. 
     The hybrid demultiplexer of the present disclosure also provides low excess loss which is superior to an electro-optic only approach, and low power consumption. For example, each thermal phase shifter consumes approximately 25 mW while each electro-optic phase shifter consumes approximately less than 10 mW. Thus, in the four phase shifter scenario shown in the figures, less than 200 mW are consumed. Further, the hybrid demultiplexer described herein is compatible with pre-existing control algorithms used with other polarization demultiplexing methodologies, can be implemented without the need for coherent receivers or digital signal processors (DSPs), and can be implemented regardless of the type of modulation format and data rate. 
     It is understood that any specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged, or that only a portion of the illustrated steps be performed. Some of the steps may be performed simultaneously. For example, in certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the examples described above should not be understood as requiring such separation in all examples, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but are to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” 
     A phrase such as an “aspect” does not imply that such aspect is essential to the subject technology or that such aspect applies to all configurations of the subject technology. A disclosure relating to an aspect may apply to all configurations, or one or more configurations. A phrase such as an aspect may refer to one or more aspects and vice versa. A phrase such as a “configuration” does not imply that such configuration is essential to the subject technology or that such configuration applies to all configurations of the subject technology. A disclosure relating to a configuration may apply to all configurations, or one or more configurations. A phrase such as a configuration may refer to one or more configurations and vice versa. 
     The word “exemplary” is used herein to mean “serving as an example or illustration.” Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of various aspects of the disclosure as set forth in the claims.