Patent Publication Number: US-2023139848-A1

Title: Systems and Methods for Bidirectional Polarization Signaling

Description:
TECHNICAL FIELD 
     The subject matter presented herein relates generally methods and systems for conveying and receiving data through optical signals. 
     BACKGROUND 
     Light propagates through optical fibers with little attenuation and supports high-bandwidth communication. Photonic signals—modulated beams of light—do not interfere with one another so a single fiber can carry many independent optical signals in the same or opposite directions. 
     Photonic signals conveyed together are generally distinguished by wavelength (color). To produce a photonic signal, a beam from a laser made to issue light of a specific wavelength is modulated to impress information on the beam. Many such signals, each of a different wavelength, can be injected concurrently into the same fiber to be later separated by wavelength at their respective destinations. The information impressed on each beam can then be recovered by demodulation. In similar fashion, information may be independently encoded onto multiple polarization modes of a single fiber, subsequently separating them to recover the disparate information. 
     Combining signals of different wavelengths into a single channel, and subsequently separating them to recover the disparate information, is commonly referred to as wavelength-division multiplexing (WDM). WDM enables bidirectional communications over a single strand of fiber, also called wavelength-division duplexing, as well as multiplication of capacity. Each signal requires a laser of a corresponding wavelength, however, and lasers are difficult to integrate and relatively expensive. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    depicts a photonic communication system  100  in which a host  105  and target  110  communicate bidirectionally over a single waveguide  115  using light of the same wavelength and from the same light source. 
     
    
    
     The illustrations are by way of example, and not by way of limitation. Like reference numerals similar elements. 
     DETAILED DESCRIPTION 
       FIG.  1    depicts a photonic communication system  100  in which a host  105  and target  110  communicate bidirectionally over a single waveguide  115  (e.g. an optical fiber) using light of the same wavelength and from the same light source. Using the same fiber and light source in both directions reduces cost, complexity, and power consumption. 
     For this illustration, host  105  is assumed to be a photonic transceiver (transmitter/receiver) within or in support of a memory controller and target  110  a photonic transceiver within or in support of a memory. Host  105  includes some controller core  120  controlling a multiplexer  121  that sends write data Wr or host clock HCk to target  110  via waveguide  115 , either selection timed to a local clock Ck from an oscillator  127  that serves as a reference for both host  105  and target  110 . Write data Wr is for storage in a memory core  125 , while host clock HCk communicates timing to target  110  for synchronizing the receipt of read data Rd from memory core  125 . All write, read, and timing signals are conveyed via the same waveguide  115 . Host  105  also conveys memory commands and addresses over the same fiber, but memory messaging is well known to those of skill in the art, so a detailed discussion is omitted. 
     Host  105  includes a laser  130  that delivers light of a desired bandwidth and polarization into an optical waveguide  135  (e.g., a silicon waveguide or optical fiber). The light consists of electric and magnetic fields that oscillate perpendicular to one another. By convention, “polarization” refers to the direction in which the electric field oscillates. Polarized waves can be transverse, meaning that the direction of oscillation is transverse to the direction of wave propagation. The light from laser  130  is polarized in the transverse magnetic (TM) mode, which is to say that the magnetic field is parallel to the plane of the substrate. Light can also be polarized in the transverse electric (TE) mode in which the electric field is parallel to the plane of the substrate. In silicon photonics, the substrate typically comprises a silicon wafer with waveguides formed using silicon oxide cladding and monocrystalline silicon cores. TM or TE polarized light can be obtained by reflection, refraction, birefringence, and selective absorption. 
     The TM-polarized light in waveguide  135  is conveyed to a transmitter  137 , an optical modulator that includes an electronic signal driver  140  and associated photonic ring  145 , a circular waveguide. Light entering waveguide  135  exhibits a range of wavelengths, or spectrum. Some of that light couples into adjacent ring  145  via the small space separating the two waveguides via a process known as evanescent coupling. Ring  145  has an optical wavelength that is an integer multiple of some wavelengths within the spectrum of the beam traveling through waveguide  135 . These wavelengths couple more efficiently into ring  145  and are thus removed from the beam to leave a pattern of relatively dark notches at wavelengths that are a function of ring  145 . Write data Wr from controller core  120  is expressed as patterns of electrical signals that stimulate ring  145  to vary the notch wavelengths and thus modulate the light passing through waveguide  135 . Light exiting waveguide  135  will therefore exhibits relatively dim notches, the wavelengths of which change responsive to write data Wr. (By analogy, one might imagine sending a signal by shining a rainbow of light and periodically darkening either the red or blue light in a recognizable pattern.) Other forms of modulators (e.g., Mach-Zehnder modulators) and modulation schemes can be used. Host  105  can include additional transmitters  137  to support multiple propagation modes through waveguide  115 . 
     The modulated, TM-mode light from waveguide  135  enters a polarization splitter  150 , an optical element that divides polarized light into TM and TE modes. There being little to no TE-mode component of the modulated beam, splitter  150  simply forwards the TM-mode light from host  105  to target  110  via waveguide  115 , a polarization-maintaining fiber that maintains the TM mode to a polarization splitter  153  at target  110 . Host  105  and target  110  have respective optical ports  171  and  172  that are keyed to maintain the physical orientation of waveguide  115  relative to the orientations of polarizations of splitters  150  and  153 . Polarization splitter  153  passes the modulated TM-mode beam to a 3 dB splitter  155 , which divides the beam into a pair of identical modulated TM-mode beams on waveguides  157  and  169 . 
     Waveguide  157  conveys the TM-mode beam to an optical receiver  158 , a demodulator that includes a photonic ring  160  of an optical length and consequent resonant wavelengths matched to the notches induced by transmitter  137 . The modulation at the transmitter changes the notch wavelengths so that they couple more or less well with ring  160 . Light from ring  160  thus varies in intensity as a function of the information (e.g. write data Wr) used to modulate the transmitted beam. A waveguide  163  conveys the fluctuating beam from ring  160  to an opto-electric sensor  165  (e.g. a photodiode), which converts light intensity into an electrical signal. In other embodiments optical receiver  158  is simplified by omission of ring  160  and waveguide  163 . Intensity fluctuations are instead monitored from waveguide  157 . Photonic rings of different optical lengths are more often used in sets to distinguish between wavelengths WDM systems. System  100  is shown with a single ring-based receiver for ease of illustration. 
     The electrical signal is fed to a clock-and-data recovery circuit (CDR)  167  that, as the name implies, recovers a clock signal RCk and the original write-data signal Wr from the output of sensor  165 . CDRs are well known circuits so a detailed discussion is omitted. 
     Clock signal RCk and write-data signal Wr are conveyed to memory core  125 , the clock signal timing memory-core operations, such as the arrival, sampling, and storing of write data and related command and address signals. This illustration is a simplification, as timing can be different in frequency and phase across different functional areas of a circuit or system. 
     A waveform diagram  168  illustrates how a write-data signal Wr is conveyed from host  105  to target  110 . The waveforms are depicted as idealized non-return-to-zero (NRZ) signals, NRZ being a binary code in which logical ones are represented by a relatively high intensity over a symbol period and logical zeroes a relatively low intensity for the symbol period, with no other neutral or rest condition. Controller core  120  at host  105  conveys write data Wr timed to reference clock signal Ck. Transmitter  137  impresses the write-data signal onto a TM-mode carrier beam passing though waveguide  135 . The modulated beam is conveyed to receiver  158 , as noted previously, where CDR  167  recovers clock signal RCk and the write data pattern as write data Wr. CDR  167  passes write data Wr to memory core  125 , which stores write data in time with recovered clock signal RCk to complete a write transaction for system  100 . 
     Read transactions move read data Rd from target  110  to host  105  using a modulated beam. Target  110  lacks a light source, however, instead relying on host  105  for both light and timing for the conveyance of the read signal. Controller core  120  and multiplexer  121  in host  105  issue a periodic host clock signal HCk in lieu of write data to modulate the H-mode beam in waveguide  135 . At the target side, CDR  167  recovers clock signal RCk from the periodically modulated T-mode beam for timing the read operation with core  125 . Memory core  125 , responsive to a read command, ignores recovered data signal Wr and instead presents the requested read data Rd to a modulator  170  that includes an electronic signal driver  173  and associated photonic ring  174 . 
     As noted previously, 3db splitter  155  produces two similar beams. The TM-mode beam modulated with host-clock signal HCk is conveyed to a polarization rotator  175  that changes the TM-mode polarization to TE-mode polarization. A TE-mode beam therefore passes and is modulated by ring  174  to incorporate read data Rd, the read modulation appearing as present and missing pulses in the modulated beam from the host. Ring  174  can be placed on either side of rotator  175 , and more transmitters can be included in target  110  to support additional propagation modes through waveguide  115 . 
     Polarization splitter  153  merges the modulated, TE-mode beam with the TM-mode beam from host  105 . The TE-mode beam passes through waveguide  115 . Polarization splitter  150  splits the TE-mode beam from the combined beam and conveys the resulting TE-mode beam to a receiver  179 , a demodulator with a photonic ring  180  matched to ring  174  at target  110 . A waveguide  183  conveys a modulated beam from ring  180  to an opto-electric sensor  185 , which converts light intensity into an electrical version of read signal Rd. As noted in connection with receiver  158  of target  110 , light can be detected without ring  180  and waveguide  183  in other embodiments. 
     Host  105  is the source of the timing signal and thus does not require a CDR to recover the timing of read signal Rd. 
     A waveform diagram  190  illustrates how a simplified read-data signal Rd is conveyed from target  110  to host  105 . Host  105  issues a TM-mode beam modulated with host-clock signal HCk to target  110 , which demodulates the signal to recover clock signal RCk. Rotator  175  rotates the periodic TM-mode beam to the TE mode, and memory core  125  with modulator  170  modulates the TE-mode beam with read-data signal Rd. The resulting modulated TE-mode passes through polarization splitter  153  and waveguide  115  to host  105 . Polarization splitter  150  passes the modulated beam to receiver  179 , which recovers read-data signal RD provides it to controller core  120  to complete the read transaction. 
     In read direction, the TE-mode beam modulated by ring  174  had been modulated at host  105  using a timing reference (e.g. a periodic signal). The target-side modulator thus modulates an already modulated beam, in this embodiment by effectively deleting clock pulses in the polarized beam. Other embodiments use modulation schemes that support full duplex communication across waveguide  115 . For example, three-level pulse-amplitude modulation (PAM3) can be used to distinguish data transmitted from host  105  from data simultaneously transmitted from target  110 . Manchester coding, in which every data period has power in one half cycle, also supports full duplex communication. 
     System  100  communicates information bidirectionally, over the same waveguide, using a single light source. This simplification saves cost, area, and power, even more where the number of target nodes is greater than one. The identical signal paths to and from host  105  improves timing accuracy and reduces uncertainty, and the fact that the same light makes a round trip through target  110  minimizes the delay at the receiver. Using one passband for both directions reduces round-trip dispersion asymmetry and improves accuracy, though the methods can be extended to WDM (e.g., multiple bidirectional channels over a single fiber, each channel passing a TM-mode beam in one direction and a TE-mode beam in the other). For example, waveguide  135  can support multiple modulators each controlling a respective wavelength or set of wavelengths. 
     Photonic devices can be instantiated in silicon, and silicon-based photonic devices can be made using existing and very well-established semiconductor fabrication techniques. Electrical and photonic components of host  105  can thus be integrated on the same substrate. Specialized components, such as memory core  125  and laser  130 , may be better formed using processes that are incompatible or suboptimal for other components. Host  105  and target  110  may therefore include both integrated and discrete components. 
     Host  105  is shown communicating with a single target  110  but can service more and different targets in other embodiments. An optical switch disposed between host  105  and target  110  can allow host  105  to establish bidirectional communication with multiple targets, each target lacking a respective light source or sources. 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance the method of interconnection establishes some desired electrical communication between two or more circuit nodes, or terminals. Such interconnection may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. § 112.