Patent Publication Number: US-2020280373-A1

Title: Optical transceiver with integrated optical loopback

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 15/217,152, filed Jul. 7, 2016, now allowed which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to optical communication systems and components thereof, and more particularly relates to an integrated optical transceiver. 
     BACKGROUND 
     Broad-band optical communications typically require high data rate optoelectronic transceivers incorporating an optical receiver capable of converting high data rate signals from optical to electrical domain and an optical transmitter capable of converting high data rate signals from electrical to optical domain. An optoelectronic transceiver may include an optical transmitter (Tx) circuit and an optical receiver (Rx) circuit, which may be implemented with one or more photonic chips, which may be mounted on a circuit board with driver and signal processing electronics. An optical Tx circuit may include an electro-optic (EO) converter, typically an optical modulator such as a waveguide Mach Zehnder modulator (MZM) for devices operating in the GHz frequency range and beyond. An optical Rx circuit may include one or more optical couplers and/or optical mixers and may further include one or more opto-electric (OE) converters, such as one or more photodetectors, for example in the form of PIN photodiodes. 
     High-data-rate optoelectronic transceiver modules may require device-specific precise optimization of various parameters of the transmitter and the receiver. These parameters may include, for example, skew-compensation values of the RF channels of the transmitter and of the receiver, filter settings for digital pre-compensation of bandwidth limitations of the hardware, and/or channel power level adjustment, e.g. for squareness correction of quadrature amplitude modulated signals, and alike. In particular higher-order modulation formats, for example 16QAM and 64QAM, and high symbol rates, for example 60 GBaud, require highly optimized transceiver settings, such as precise skew compensation values. However, module settings that are found during factory calibration under laboratory conditions may not match optimal settings in the field. The need for module calibration in the field further increases with the rise of new transceiver platforms that separate the digital electrical data processing unit from an analogue electro-optic conversion engine. For example CFP2-ACO hot-pluggable optical transceiver modules may be hosted in the field on printed circuit boards (PCB) that are sourced separately from the pluggables. Consequently, the digital host unit and the analogue pluggable optical transceiver are calibrated in their respective factory independently, and their optimal settings may not match. However calibration of conventional transceiver modules typically requires on-site technical personal and thus may be too costly to perform in the field. 
     SUMMARY 
     An aspect of the present disclosure relates to an optical transceiver comprising a photonic transceiver chip with an integrated optical loopback. 
     An aspect of the present disclosure relates to a photonic integrated circuit (PIC) for an optical transceiver, the PIC comprising: an input PIC port for receiving an input optical signal; an output PIC port for transmitting an output optical signal; an optical receiver (Rx) circuit optically coupled to the input PIC port and configured to process the input optical signal for conversion into one or more electrical Rx signals; an optical transmitter (Tx) circuit optically coupled to the output PIC port, the optical Tx circuit configured to provide the output optical signal and a loopback optical signal responsive to one or more electrical Tx signals; and, a loopback optical circuit (LOC) configured to switchably couple the loopback optical signal into the optical Rx circuit for testing and/or tuning one or more parameters of the optical transceiver. 
     An aspect of the present disclosure relates to a method for characterizing an optoelectronic transceiver comprising a PIC, the PIC comprising an optical Rx circuit and an optical Tx circuit, the method comprising: switchably directing, within the PIC, a loopback optical signal from an output port of the optical Tx circuit to an input port of the optical Rx circuit; and, processing one or more electrical signals obtained from an output of the optical Rx circuit to estimate one or more characteristics of the optoelectronic transceiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments disclosed herein will be described in greater detail with reference to the accompanying drawings which represent example embodiments thereof, in which like elements are indicated with like reference numerals, and wherein: 
         FIG. 1  is a schematic block diagram of an optoelectronic transceiver system; 
         FIG. 2  is a schematic block diagram of an optoelectronic transceiver system with integrated optical loopback; 
         FIG. 3  is a schematic block diagram of an example optical front-end of an optoelectronic transceiver system with integrated optical loopback and a local oscillator dephasing; 
         FIG. 4  is a schematic block diagram of an example optical front-end of an optoelectronic transceiver system with channel mixing in the integrated optical loopback; 
         FIG. 5  is a schematic circuit diagram of a PIC implementing a dual-polarization diversity coherent optical transceiver with an integrated optical loopback; 
         FIG. 6  is a schematic diagram of a nested Mach-Zehnder modulator (MZM) with an output 2×2 coupler providing an optical loopback signal from a second output port; 
         FIG. 7  is a schematic diagram of an MZM output circuit having an optical loopback signal tapped off from a main output signal of the MZM; 
         FIG. 8  is a schematic block diagram of an example implementation of the optoelectronic transceiver system of  FIG. 3  incorporating the PIC of  FIG. 5  as an optical front-end and configured to perform self-test using the integrated optical loopback; 
         FIG. 9  is a schematic graphical representation of the IQ timing skew in a transmitter—receiver signal path. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that principles described herein may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the example embodiments. Statements herein reciting principles, aspects, and example embodiments are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. Thus, for example, it will be appreciated by those skilled in the art that block diagrams herein can represent conceptual views of illustrative circuitry embodying the principles of the technology. 
     Furthermore, the following abbreviations and acronyms may be used in the present document:
         PIC Photonic Integrated Circuit   RF Radio Frequency   OSNR Optical Signal to Noise Ratio   ROSNR Required Optical Signal to Noise Ratio   BER Bit Error Ratio   MZM Mach-Zehnder Modulator   MZI Mach-Zehnder Interferometer   PD Photo Detector   PCB Printed Circuit Board   QAM Quadrature Amplitude Modulation   CMOS Complementary Metal-Oxide-Semiconductor   GaAs Gallium Arsenide   InP Indium Phosphide   SOI Silicon on Insulator   SiP Silicon Photonics   ASIC Application Specific Integrated Circuit   FPGA Field Programmable Gate Array   LOC Loopback Optical Circuit   Tx Transmitter   Rx Receiver   OMC Optical Modulator Circuit   PAM Pulse-Amplitude-Modulation   PBSR Polarization Beam Splitter Rotator   PBRC Polarization Beam Rotator Combiner   QPSK Quadrature-Phase-Shift-Keying   ASK Amplitude-Shift-Keying   WDM Wavelength Division Multiplexing       

     Note that as used herein, the terms “first”, “second” and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated. The terms “connected”, “coupled”, and their variants are intended to encompass both direct connections and indirect connection through one or more intermediate elements, unless specifically stated otherwise. Radio frequency (RF) may refer to any frequency in the range from kilohertz (kHz) to hundreds of gigahertz (GHz). 
     One or more aspects of the present disclosure relates to a photonic integrated circuit (PIC) of an optical transceiver comprising an optical transmitter (Tx) circuit, an optical receiver (Rx) circuit, and an integrated optical loopback from the optical Tx circuit to the optical Rx circuit that can be switched on or off. In some implementations the optical transceiver may include a shared local oscillator (LO) circuit with an integrated optical phase shifter for dephasing an LO signal to the optical Rx circuit relative to a loopback optical signal from the optical Tx circuit. In some implementations the optical transceiver may be a dual polarization transceiver, and the integrated optical loopback may include an optical mixer for polarization channel mixing. In some implementations the optical Tx circuit, the optical Rx circuit, and the integrated optical loopback may be implemented in a same photonic chip. 
     The integrated optical loopback enables a flexible and precise in-field calibration of the transceiver, for example during an automatic start-up sequence of a transponder incorporating the transceiver, which results in an overall improvement of the transponder performance. The in-field transceiver calibration, as enabled by the integrated optical loopback, avoids the propagation of uncertainty of calibration data in traditional transceivers, in which digital and analog interfaces, although hosted on a same PCB, are calibrated separately at their respective manufacturing facilities. 
     Referring to  FIG. 1 , an optical transceiver  10  may include a transmitter (Tx) signal path  20  and a receiver (Rx) signal path  30 . The Tx signal path  20  may be formed of a data source  26 , an electrical amplifier  24 , and an optical Tx circuit  22  that may be in the form of, or include, an electro-optic (EO) converter. The Rx signal path  30  may be formed of a data sink  36 , an electrical amplifier  34 , and an optical Rx circuit  32  that may include an opto-electric (OE) converter. The electrical amplifier  34  may be in the form of, or include, one or more trans-impedance amplifiers (TIA) and may be referred to hereinafter as TIA  34 ; however any suitable amplifier may be used. The data source  26  and the data sink  36  may be embodied with a digital signal processor  50 , for example but not exclusively a microprocessor, an ASIC, an FPGA, or a combination thereof. 
     In operation the data source  26  encodes data to be transmitted in accordance with chosen modulation and transmission formats to form one or more electrical Tx signals  25 . These one or more electrical Tx signals  25  may be first generated digitally and then converted to the analog domain using a digital to analog converter (DAC), not shown. The one or more electrical Tx signals  25  are passed to the optical Tx circuit  22  after optionally amplified by an electrical amplifier  24 . The optical Tx circuit  22  converts the one or more electrical Tx signals  25  into an output optical signal  21  that may be coupled into an optical fiber or otherwise transmitted to a destination over an optical communication link. The optical Rx circuit  32  receives an input optical signal  31 , for example from a return optical fiber of the communication link, and converts it into one or more electrical Rx signals  35  in cooperation with the TIA  34 . The data sink  36  performs digital signal processing on the electrical Rx signal or signals  35 , after their conversion into the digital form by a suitable analog to digital converter (ADC, not shown), to extract transmitted data therefrom. The digital signal processing performed by the data sink  36  may include clock and/or frequency recovery, demodulation, error correction decoding, and other digital signal processing operations as known in the art. 
       FIG. 2  illustrates a modification of the optical transceiver  10  of  FIG. 1 , which may be generally referred to as the optical transceiver  100 . In the optical transceiver  100 , the optical Tx circuit  22  and the optical Rx circuit  32 , or at least a portion thereof, are embodied within a same photonic integrated circuit (PIC)  90 , which also includes an integrated loopback optical circuit (LOC)  40 . The integrated LOC  40 , which may also be referred to herein simply as LOC  40 , is configured to switchably direct a loopback optical signal  41  from the output of the optical Tx circuit  22  to an input of the optical Rx circuit  32 , to facilitate testing the Tx signal path  20  and the Rx signal path  30  of the optical transceiver  100 . The loopback optical signal  41  may be in the form of, or include, the output optical signal  21 , or a signal related thereto. PIC  90  has an input PIC port  92  for receiving the input optical signal  31 , and an output PIC port  91  for transmitting the output optical signal  21 . The optical Rx circuit  32  is optically coupled to the input PIC port  92  and is configured to process the input optical signal  31  for conversion into one or more electrical Rx signals  35 . The optical Tx circuit  22  is optically coupled to the output PIC port  91  and is configured to provide the output optical signal  21  and/or the loopback optical signal  41  responsive to the one or more electrical Tx signals  25 . The optical Rx circuit  32 , the optical Tx circuit  22 , and LOC  40  may be implemented in a same photonic chip  95 . Optical paths between various elements of PIC  90  are schematically illustrated with solid lines and may be embodied with one or more optical waveguides formed within the photonic chip  95 . By way of example, the photonic chip  95  may be a silicon photonics chip, such as a SOI based chip; in other embodiments it may be a GaAs based chip, an InP based chip, or a chip of any other suitable material system. 
     LOC  40  is configured to switchably couple the loopback optical signal  41  into the optical Rx circuit  32  for testing and/or tuning one or more parameters of the optical transceiver  100 . The testing may include measuring and/or tuning one or more parameters related to one of: the optical Rx circuit  32 , the optical Tx circuit  22 , the one or more electrical Rx signals  35 , or the one or more electrical Tx signals  25 . LOC  40  may include an optical switching circuit  47  configured to switchably block the optical Rx circuit  32  from receiving the loopback optical signal  41 . The optical switching circuit  47  may be electrically controlled from a controller  70 . The optical switching circuit  47  may be for example in the form of one or more electrically controlled optical shutters. An optical shutter may be implemented for example with a variable optical attenuator (VOA), such as but not exclusively a p-i-n diode-based VOA, a Mach-Zehnder-Interferometer (MZI), a ring resonator, a micro-electro-mechanical system (MEMS), a combinations thereof, or as any other suitable optical shutter that may be integrated in a same PIC with the optical Rx and Tx circuits and which may be electronically controlled. One or more optical couplers  45 ,  49  may be used to couple the loopback optical signal  41  from an output of the optical Tx circuit  22  into the optical Rx circuit  32 . In some embodiments the optical coupler  45  providing the optical loopback signal  41  may be an output coupler of the optical Tx circuit  22 . In some embodiments the output optical coupler  49  of LOC  40  may be an input coupler or mixer of the optical Rx circuit  32 . 
     The optical Tx circuit  22  may be implemented using one or more optical modulators, such as for example, but not exclusively, one or more MZMs. Embodiments wherein the optical Tx circuit  22  is implemented with one or more directly modulated lasers would also be within the scope of the present disclosure. The optical Rx circuit  32  may include one or more photodetectors (PDs), and may also include one or more optical couplers and/or one or more optical mixers such as optical hybrids. A loopback controller  70  may be provided to control the optical switching circuit  47  to couple the loopback optical signal  41  into the optical Rx circuit  32  when the optical transceiver is in a loopback mode, and to block the loopback optical signal  41  from being received by the optical Rx circuit  32  during regular operation of the optical transceiver  100 , when the optical transceiver  100  is operable to receive and transmit optical signals from an optical communication link. In some embodiments the loopback controller  70  may be automatically activated at a transceiver start-up to perform loopback measurements. In some embodiments the loopback controller  70  may be remotely controlled over a network to enable remote transceiver testing using the integrated loopback. The loopback controller  70  may be in communication with a digital signal processor  150 , which may be an embodiment of processor  50  described above, and may be further configured to perform signal processing operations for transceiver testing when the optical transceiver is in the loopback mode of operation. Processor  150  may also communicate with the loopback controller  70  to switch between the loopback mode and a regular mode of transceiver operation. 
       FIG. 3  schematically illustrates a PIC  190 , which may be an embodiment of PIC  90  and may operate as an optical front end of an optical transceiver such as that illustrated in  FIGS. 1 and 2 . Similarly to PIC  90 , PIC  190  includes an integrated LOC  40  which may be as generally described above with reference to PIC  90  and  FIG. 2 , an optical Rx circuit  130  coupled to an input PIC port  92 , and an optical Tx circuit  120  coupled to an output PIC port  91 . In some embodiments PIC  190  may be implemented in one photonic chip  195 . Optical paths between various elements of PIC  190  are schematically illustrated with solid lines and may be embodied with one or more optical waveguides formed in the photonic chip  195 . 
     The optical Tx circuit  120  may be in the form of, or include, an optical modulator configured to modulate input light  111  responsive to an electrical Tx signal  125 . An input optical port  121  of the optical Tx circuit  120  is coupled to a light source port  115  configured to receive the input light  111  from a suitable light source  105 , such as for example a single-frequency semiconductor laser. In some embodiments the light source  105  providing the input light  111  may be incorporated in PIC  190 . In some embodiments the light source  105  may be external to PIC  190 . 
     The optical Rx circuit  130  may include a local oscillator (LO) port  132  and may be configured for coherent optical detection, wherein the input optical signal  31  is coherently mixed with LO light received in the LO port  132 . In some embodiments the LO port  132  of the optical Rx circuit  130  may be optically coupled to the light source port  115 , with a fraction of the input light  111  directed into the LO port  132  as the LO light, for mixing with an optical signal received into a signal port  131 . 
     When PIC  190  operates in a loopback mode, the optical switching circuit  47  is switched to a “pass-through” state, allowing a loopback optical signal  41  from an output port of the optical Tx circuit  120  to be coupled into the signal port  131  of the optical Rx circuit  130 , where it is coherently mixed with the LO light. An electrical or optical signal at the output of the optical Rx circuit  130  may be sensitive to an optical phase difference Δϕ sLO =(ϕ s −ϕ LO ) between an optical phase ϕ s  of light received in the signal port  131  and an optical phase ϕ LO  of light received in the LO port  132 , which may be referred to as the signal-to-LO phase difference. 
     In regular operation, when the optical signal received in the signal port  131  of the optical Rx circuit  130  is the input optical signal  31  received in the input PIC port  92 , e.g. from an optical fiber link, the phase difference Δϕ sLO  is generally unknown and may fluctuate. To account for this unknown phase the Rx signal path  30  of the transceiver may have to perform phase recovery processing, which may be generally designed to recover one or more transmitter generated signals or signal channels from the received input optical signal  31  when the phase difference Δϕ sLO  is most unfavorable for the coherent signal detection. 
     In order to reproduce the signal—LO mixing conditions in the loopback mode that emulate conditions during normal transceiver operation, including operating conditions which may be less favorable for the coherent signal detection, in some embodiments PIC  190  may include an optical phase shifter  135  in the optical path between the light source port  115  and the LO port  132 . In some embodiments, the optical phase shifter  135  may be dynamically tunable during a test in the loopback mode of operation to evaluate the operation of the phase recovery processing in the Rx signal path of the transceiver in dependence on the signal—LO phase difference ΔϕsLO. In some embodiments the optical phase shifter  135  may be factory pre-set to a state that corresponds to a less-favorable, or the least-favorable, LO-signal mixing in the loopback mode of operation of the transceiver. In some embodiments the least-favorable LO-signal mixing may correspond, for example, to ΔϕsLO=45°; in some embodiments a range of ΔϕsLO corresponding to less-favorable LO-signal mixing conditions may be explored, such as for example from about 20° to 70°. These unfavorable LO-signal mixing conditions, which may be termed “LO dephasing”, may in some embodiments correspond to mixing of different transmitter-generated signal channels in the output signal or signals  137  of the optical Rx circuit  130 . 
       FIG. 4  schematically illustrates PIC  290 , which may be an embodiment of PIC  90  and may operate as an optical front end of an optoelectronic transceiver such as that illustrated in  FIGS. 1 and 2 . Similarly to PIC  90  and PIC  190 , PIC  290  includes an integrated LOC  240 , an optical Rx circuit  230  coupled to an input PIC port  92 , and an optical Tx circuit  220  coupled to an output PIC port  91 . An optical Rx circuit  230  may be an embodiment of an optical Rx circuit  32  or  130  as generally described above with reference to PIC  90  of  FIG. 2  and PIC  190  of  FIG. 3 . An optical Tx circuit  220  may be an embodiment of an optical Tx circuit  22  or  120  as generally described above with reference to PIC  90  of  FIG. 2  and PIC  190  of  FIG. 3 . LOC  240  may be an embodiment of LOC  40  described above. 
     The optical Tx circuit  220  may be configured to provide, responsive to an electrical Tx signal  225 , an output optical signal  221  that includes two optical signal channels, which may be differently modulated and may be referred to herein as a first optical channel or as the “X” channel, and a second optical channel that may be referred to as the “Y” channel. Similarly, the Rx circuit  230  may be configured to receive an input optical signal  231  that may also include two optical signal channels “X” and “Y”. The Rx signal path of the optical transceiver in which PIC  290  is used may be configured to de-multiplex the “X” and “Y” channels from the received optical signal. LOC  40  may be configured to switchably couple a loopback optical signal  241  from the optical Tx circuit  220  into the optical Rx circuit  230 . The loopback optical signal  241  may include the “X” and “Y” channels generated by the optical Tx circuit  220 . In some embodiments wherein the optical Tx circuit  220  includes an optical modulator, PIC  290  may include an optical source port (not shown) coupled to an input port of the optical modulator as described above with reference to  FIG. 3 . In some embodiments the optical Rx circuit  230  may be configured for coherent detection and include an LO port that is coupled to the optical source port of the PIC via a phase shifter as described above with reference to  FIG. 3 . 
     During regular operation of the transceiver in a communication link, the “X” and “Y” channels may be intermixed in the received input optical signal  231 , which may complicate the de-multiplexing of these two channels from the received optical signal by the transceiver. In order to reproduce the operating conditions in the field, in some embodiments LOC  240  may incorporate a channel mixer  280  configured to intermix the Tx-generated “X” and “Y” optical channels of the loopback optical signal  241  prior to coupling the loopback optical signal  241  into the optical Rx circuit  230 . In some embodiments, the “X” and “Y” channels may represent two polarization channels of the input optical signal  231 , as described below in further detail. In some embodiments PIC  290  may be implemented in one photonic chip  295 , which may be an embodiment of the photonic chip  95  or the photonic chip  195  described above. Optical paths between various elements of PIC  290  are schematically illustrated with solid lines and may be embodied with one or more optical waveguides formed within the photonic chip  295 . 
     A first optical switching or coupling element  244  may be disposed in an optical path from the optical Tx circuit  220  to the output PIC port  91 , and configured to couple at least a portion of the output optical signal  221 , or a signal related thereto, into the channel mixer  280  as the loopback optical signal  241 . A second optical switching or coupling element  246  may be disposed in an optical path from the input PIC port  92  to the optical Rx circuit  230 , and configured to couple the loopback optical signal  241  with intermixed optical channels into the optical Rx circuit  230 . At least one of the first optical switching or coupling element  244 , or the second optical switching or coupling element  246  may be electronically switchable between a state in which LOC  240  allows the loopback optical signal  241  to be coupled into the optical Rx circuit  230 , and a state in which LOC  240  substantially blocks the loopback optical signal  241  from being coupled into the optical Rx circuit  230 . In some embodiments one of the first optical switching or coupling element  244  and the second optical switching or coupling element  246  may be in the form of, or include, an electrically-controlled optical switch, while the other of the first optical switching or coupling element  244  and the second optical switching or coupling element  246  may be a passive optical coupler. In some embodiments each of the first optical switching or coupling element  244  and the second optical switching or coupling element  246  may be in the form of, or include, an electrically-controlled optical switch. 
       FIG. 5  schematically illustrates PIC  390 , which may be an embodiment of PIC  290  or PIC  190 , and may be operable as an optical front end of an optoelectronic transceiver, such as transceiver  100  of  FIG. 2 . In the illustrated embodiment PIC  390  is configured to transmit, and coherently receive, optical signals comprising two polarization multiplexed optical channels, and combines features described hereinabove with reference to  FIGS. 2-4  and PICs  90 ,  190 , and  290 . PIC  390  includes an integrated LOC  340 , an optical Rx circuit  330 , and an optical Tx circuit  320 , which may all be embodied in a same photonic chip, generally as described above with reference to  FIGS. 2-4 . Optical paths between various elements of PIC  390  are schematically illustrated with solid lines and may be embodied with planar optical waveguides formed in the photonic chip implementing the PIC. The optical Tx circuit  320  is configured to separately output two optical channels, which may be separately guided to a PIC output port  361  for combining into an output optical signal  301 . The PIC output port  361  may be in the form of, or include, a polarization beam rotator and combiner (PBRC)  361 . The optical Rx circuit  330 , which is configured as a dual coherent optical receiver, is coupled to a PIC input port  362 , which may be in the form of, or include, a polarization beam splitter and rotator (PBSR). 
     The dual optical Tx circuit  320  is embodied as an optical modulator circuit (OMC) formed of a first optical modulator  320 X and a second optical modulator  320 Y, which are connected in parallel between an optical splitter  312  and the PBRC  361 . The optical splitter  312  may be referred to herein as the OMC input splitter  312 , and may embody an input optical port of the OMC. The first optical modulator  320 X may be referred to as the X-channel modulator. The second optical modulator  320 Y may be referred to as the Y-channel modulator. Input optical ports of the two modulators  320 X and  320 Y are optically coupled to a light source port  303  via the OMC input splitter  312  and an optical splitter  351 . The optical splitter  351  may be referred to herein as the first LO optical splitter  351 . The light source port  303  may be an embodiment of the light source port  115  and may be configured to receive input light  311  from a suitable light source (not shown), such as for example a single frequency laser. In some embodiments the light source port  303  may be in the form of, or include, an edge coupler. In some embodiments the light source port  303  may be in the form of, or include, a grating coupler. In some embodiments the light source may be incorporated into the PIC  390 . In some embodiments the light source port  303  may be in the form of, or include, a suitable light source such as a single-frequency semiconductor laser. The optical splitter  351  is configured to split the input light  311  into two light fractions, one of which is guided to the input optical coupler  312  of the OMC  320 , and the other is guided to the optical Rx circuit  330  as the LO light, as described below. 
     In operation the first optical modulator  320 X modulates a first fraction of the input light  311  responsive to an X-channel electrical Tx signal or signals, and outputs an optical X-channel signal, which may also be referred to as a first optical channel. The second optical modulator  320 Y modulates a second fraction of the input light  311  responsive to a Y-channel electrical Tx signal or signals, and outputs an optical Y-channel signal, which may also be referred to as the second optical channel. The PBRC  361  may be configured to rotate the polarization of one of the Y-channel or X-channel optical signals received from the first optical modulators  320 X or the second optical modulator  320 Y, and combine them by polarization multiplexing to form the output optical signal  301 , in which the optical X-channel signals and Y-channel signals are combined in orthogonal polarizations. 
     The dual optical Rx circuit  330  includes a first optical mixer (OM)  330 X and a second optical mixer (OM)  330 Y, each of which having two or more input ports and one or more output ports, and configured to coherently mix light received into the input ports and to output mixed light from each of the one or more output ports. The OMs  330 X,  330 Y are connected to different output ports of the PBSR  362  to process two orthogonal polarizations of an input optical signal  302 . The input optical signal  302  may be generated by a remote transceiver that may be similar to the transceiver incorporating PIC  390 , and may be received by the PBSR  362  from an optical communication link. In the illustrated embodiment each of the first OM  330 X and the second OM  330 Y is embodied as a 90° optical hybrid (OH) having at least two input ports  331 ,  332  and four output ports. Embodiments in which the OMs  330 X,  330 Y are in the form of 120° optical hybrids, 2×2 optical couplers, 2×1 optical couplers are also within the scope of the present disclosure. In various embodiments each of the OMs  330 X,  330 Y may be in the form of one or more MMI couplers, one or more evanescent waveguide couplers, or a combination thereof. The input port  331  of each OM  330 X,  330 Y is configured as a signal port, while the input port  332  is configured as the LO port; these designations may be changed in other embodiments. The signal ports  331  may be referred to also as the first signal ports. Each of the output ports of the first and second OMs  330 X,  330 Y may be coupled to a different photodetector (PD)  336 , which together may form a PD circuit  338 . In some embodiments the PDs  336  may be incorporated in the PIC  390  and may be viewed as a part of the optical Rx circuit  330 . 
     In the illustrated embodiment the first signal ports  331  of each OM  330 X,  330 Y are optically coupled to the PBSR  362 , and the LO ports  332  are optically coupled to the light source port  303  via the first LO optical splitter  351  that provides the LO light, and a second LO optical splitter  318  which splits the LO light between the LO ports  332  of the first and second OM  330 X,  330 Y. Each of the first OM  330 X and the second OM  330 Y may also have a second signal port  333  that may be coupled to LOC  340  to receive a portion of the loopback optical signal  341 . In some embodiment a phase shifter  335  may be provided in the optical path of the LO light as described above with reference to  FIG. 3  and the optical phase shifter  135 . 
     LOC  340  may be configured to switchably couple output ports of the first and second optical modulators  320 X,  320 Y to the first signal ports  331  of the first and second OMs  330 X,  330 Y, so that the loopback optical signal  341  from the optical Tx circuit  320  reaches the signal ports of the OMs  330 X,  330 Y in the loopback mode of transceiver operation, but is blocked from the OMs  330 X,  330 Y during regular operation of the transceiver when the transceiver communicates with an external optical link. The loopback optical signal  341  is composed of an X-channel optical signal  341   x  received from an output port of the first optical modulator  320 X, and a Y-channel optical signal  341   y  received from an output port of the second optical modulator  320 Y. The X-channel and Y-channel optical signals  341   x ,  341   y  may also be referred to as the first and second optical channels. Two optical shutters  344  may be disposed in the optical paths of the X-channel and Y-channel optical signals  341   x ,  341   y  from the optical modulators  320 X,  320 Y to the OMs  330 X,  330 Y. The optical shutters  344  may be electrically switched between a “pass-through” state, in which the loopback optical signals  341   x ,  341   y  are allowed to reach the OMs  330 X,  330 Y, and a “blocking” state in which substantially no loopback signal reaches the optical Rx circuit  330 . Here “substantially” means an attenuation of the loopback signal in the blocking state that results in a residual loopback signal leaking into the Rx optical circuit that is sufficiently low in power so as not to increase the ROSNR for error-free detection, which may depended on a used modulation format. By way of example, for detection of QPSK optical signal with the ROSNR of 10 dB, during normal operation of the transceiver the ratio of the residual loopback signal power to an input optical signal power may be on the order of or below 1%, or for higher QAM-orders, such as 32 QAM or 64 QAM, on the order of or below 0.1%. Depending on the expected input optical signal power entering the PIC at the PIC input port  362  and depending on the loopback optical signal power at the optical blockers  344 , this may correspond to an attenuation at the blockers  344  in their “blocking” state in the range of 20 dB to 50 dB relative to the pass-through state. Optionally additional optical shutters  342  may be provided in the optical paths from the modulators  320 X and  320 Y to the output PBRC  361  to block the output optical signals of the modulators from reaching the PIC output port  361  in the loopback mode. Also optionally optical shutters  346  may be provided in the optical paths from the input PBSR  362  to the OMs  330 X and  330 Y to block any light signal that may be received in the PIC input port  362  from reaching the OMs  330 X,Y in the loopback mode while the transceiver is being tested. Optical shutters  342 ,  344 ,  346  may be implemented with a variety of optical devices as described above with reference to  FIGS. 2 and 3 . They may be electronically controlled by transceiver electronics to switch between a loopback mode of operation in which the loopback optical signal  341  is coupled into the optical Rx circuit  330 , and a regular mode of operation in which the loopback optical signal  341  is blocked from the optical Rx circuit  330 , while the input optical signal  302  may be received by the optical Rx circuit  330 . In some embodiments a 2×2 optical switch may be used in place of the optical shutters  342  and  344 . Optical shutters  344  may also be placed in the optical paths of the mixed optical loopback signals  345 ,  343  downstream of the loopback optical mixer  380 . 
     In some embodiments LOC  340  may include a first optical channel mixer  380  disposed in the optical path of the optical loopback signal  341  to intermix the optical signals or channels  341   x ,  341   y  prior to coupling into the signal ports of the OM  330 X and OM  330 Y. The first optical channel mixer  380  may also be referred to herein as the loopback optical mixer  380 . The presence of the loopback optical mixer  380  enables the transceiver to emulate, in the loopback mode of operation, signal reception conditions during normal operation of the transceiver when the polarization state of the input optical signal  302  is unknown, and light received in each of the OMs  330 X,  330 Y may be a mix of the X-channel and Y-channel signals of the remote transceiver. In the absence of an optical channel mixer in LOC  340 , the X-channel and Y-channel signals generated in the Tx signal path would be mapped one-to-one to the X-channel and Y-channel signals at the outputs of the OM  330 X and  330 Y; this could complicate testing polarization channel demultiplexing by the Rx signal path of the transceiver, and make it more difficult to separate Rx-related channel impairments from Tx-related channel impairments, and to determine the origin of such channel impairments as timing skews, power imbalance, S-parameters, and the like. 
     The optical channel mixer  380  may be embodied, for example, as a 2×2 optical coupler, and configured to mix the loopback optical channels  341   x  and  341   y  received from output ports of the first and second optical modulators  320 X,  320 Y, respectively, and to output a first mixed loopback optical signal  343  and a second mixed loopback optical signal  345 . In some embodiments the 2×2 optical coupler implementing the optical channel mixer  380  may have a 1:1 splitting ratio between output ports, so that each of the loopback optical channels  341   x  and  341   y  is equally split between the first and second mixed loopback optical signals  343 ,  345 . The first and second mixed loopback optical signal  343 ,  345  may be guided into the second signal ports  333  of the first OM  330 X and the second OM  330 Y, respectively, for example using optical waveguides. In other embodiments the mixed loopback optical signal  343 ,  345  may be coupled into the first signal ports  331  of the first OM  330 X and the second OM  330 Y using optical couplers and/or optical switches, for example as described above with reference to  FIGS. 3 and 4 . 
     In some embodiments PIC  390  may be configured to operate with quadrature modulated (QM) optical signals, wherein the X and Y optical channels at both the Rx and Tx sides comprise each an in-phase (I) signal component and a quadrature-phase (Q) signal component. This yields four signal channels in total, or per wavelength, which may be denoted as the XI, XQ, YI, and YQ channels. In such embodiments the first optical modulator  320 X may be an IQ modulator configured to convert an XI-channel electrical Tx signal “XI_Tx” and an XQ-channel electrical Tx signal “XQ_Tx” into an optical XI-channel signal “XIopt” and an optical XQ-channel signal “XQopt”, respectively, and combine them in quadrature into an X-channel optical signal “Xopt”. The second optical modulator  320 Y may be an IQ modulator configured to convert a YI-channel electrical Tx signal “YI_Tx” and a YQ-channel electrical Tx signal “YQ_Tx” into an YI-channel optical signal “YIopt” and an YQ-channel optical signal “YQopt”, respectively, and combine them in quadrature into a Y-channel optical signal “Yopt”. The optical signals XIopt, XQopt, YIopt, and YQopt may be referred to as the XI optical signal, the XQ optical signal, the YI optical signal, and the YQ optical signal, respectively. 
     Accordingly, in some embodiments the first loopback optical channel  341   x , which is sourced from an output of the X-channel optical modulator  320 X, may include the XI-channel optical signal “XIopt” and the XQ-channel optical signal “XQopt” that are produced by the first optical modulator  320 X. The second loopback optical channel  341   y , which is sourced from an output of the Y-channel optical modulator  320 Y, may include the YI-channel optical signal “YIopt” and the YQ-channel optical signal “YQopt” that are generated by the second optical modulator  320 Y. 
       FIG. 6  illustrates an example IQ modulator  420  that may implement each one of the first and second optical modulators  320 X and  320 Y in some embodiments. The IQ modulator  420  is a nested MZM formed of two inner MZMs  4201  and  420 Q connected optically in parallel between an input optical coupler  405  and an output optical coupler  407 . In operation electrical “I” and “Q” signals are applied to the inner MZMs  4201  and  420 Q, respectively, to modulate input light  401  therewith. Modulated light at the output of the inner MZMs  4201  and  420 Q is summed in quadrature at the output coupler  407  to produce a QM optical signal  411 , which may be guided to an output optical port of the PIC in which the IQ modulator  420  is implemented, for example to the PBRC  361  of PIC  390 . In some embodiments the output coupler  407  may be a 2×2 coupler with two output coupler ports, each of which may output a QM optical signal comprising I and Q optical channels modulated by the electrical “I” and “Q” signals, respectively, that are combined in quadrature. In some embodiments a first output coupler port of the output optical coupler  407  may be optically coupled to the output PIC port, for example to the PBRC  361  of PIC  390 , and a second output coupler port may be optically coupled to a loopback optical circuit, such as for example LOC  340 , to provide a loopback optical signal  441 . The loopback optical signal  441  may represent either one of the first loopback optical signal  341   x  or the second loopback optical signal  341   y  in the embodiment of  FIG. 5 . In some embodiments a loopback optical signal may be a tapped off fraction of the output optical signal  411  obtained using a tap-off coupler disposed in an optical path from the output optical coupler  407  of the modulator to the output PIC port. This is schematically illustrated in  FIG. 7  wherein a loopback optical signal  541  is tapped-off from a same output port of an output coupler  507  of an optical modulator, for example an MZM, using an optical tap coupler  531 . The loopback optical signal  541  may represent either one of the first loopback optical signal  341   x  or the second loopback optical signal  341   y  in the embodiment of  FIG. 5 . 
     Referring back to  FIG. 5 , the optical Rx circuit  330  in such embodiments may be in the form of a dual phase-diversity coherent optical receiver configured to process an input optical signal  302  carrying two QM optical channels in each of two orthogonal polarizations, i.e. the XI, XQ, YI, and YQ channels generated in a Tx signal path of a remote transceiver (not shown). The phase diversity reception may be accomplished for example by embodying each of the OMs  330 X,  330 Y as a 90° hybrid, and differentially connecting the PDs  336  in the PD circuit  338  in pairs, as known in the art. Responsive to receiving QM optical signals into signal ports  331  or  333  and the LO light into the LO ports  332 , the OMs  330 X,  330 Y may cooperate with the PD circuit  338  to provide two quadrature electrical signals in each of the X and Y channels of the Rx signal path, outputting in total four Rx electrical signals that may be denoted XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx. 
     In regular operation the polarization state of the input optical signal  302  and the LO-signal phase difference Δϕ sLO  at each OM  330 Y,  330 X are generally unknown and may fluctuate, and the electrical Rx signals XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx at the output of the PD circuit  338  do not generally map one-to-one to the XI, XQ, YI, and YQ signal channels of the input optical signal, but include each a mixture of two or more transmitter-defined channels. To account for this, the Rx signal path of the transceiver may perform a number of signal processing steps, such as polarization demultiplexing and phase recovery processing, which may be generally designed to recover the transmitter generated XI, XQ, YI, and YQ signal channels from the received input optical signal  302  when the polarization and IQ channels are strongly intermixed. The loopback optical mixer  380  mixes the X- and Y-channels of the loopback optical signal  341 , but the I and Q channels thereof may remain not mixed unless the phase of the LO light entering the OMs  330 X,  330 Y is suitably controlled. The mapping of, for example, the XI_Tx and XQ_Tx signals at the input of the optical Tx circuit  320  to the XI_Rx and XQ_Rx signals at the output of the optical Rx circuit  330  in the loopback mode depends on the phase difference Δϕ XsLO  between the signal and LO light at the inputs of the X-channel OM  330 X. Similarly, the mapping of the YI_Tx and YQ_Tx signals at the input of the optical Tx circuit  320  to the YI_Rx and YQ_Rx signals at the output of the optical Rx circuit  330  in the loopback mode depends on the phase difference Δϕ YsLO  between the signal and LO light at the inputs of the Y-channel OM  330 Y. 
     By way of example, in the absence of the loopback optical mixer  380 , if Δϕ XsLO  is 0 or 180°, XI_Tx will be mapped to XI_Rx, and XQ_Tx will be mapped to XQ_Rx. If Δϕ XsLO  is +\−90°, XI_Tx will be mapped to XQ_Rx, and XQ_Tx will be mapped to XI_Rx. If Δϕ XsLO  is neither one of 0, +\−90°, or 180°, for example Δϕ XsLO =+\−45°, XI_Tx will be mapped to a mixture of XI_Rx and XQ_Rx, and XQ_Tx will be mapped to a mixture of XQ_Rx and XI_Rx. Similar relationships hold for the mapping of the YI_Tx and YQ_Tx signals to the YI_Rx and YQ_Rx signals in dependence on Δϕ YsLO . 
     Thus, the optical phase tuner  335  may effectively function as an IQ channel mixer, and may be referred to as the second optical channel mixer. In some embodiments, the optical phase tuner  335  may be configured to provide an effective mixing of the Tx-generated I and Q channels at the output of the PD circuit  338 . In some embodiments the optical phase tuner  335  may be configured to be tuned over a suitably broad range of the optical phase shift to explore various degrees of the IQ channel mixing, for example it may be configured to be tuned over 90° optical phase shift range or more, or 180° degrees or more. The ability to tune the degree of IQ channel mixing may facilitate estimating signal impairments in the Rx signal path and the Tx signal path of the transceiver separately from each other. 
     Referring to  FIG. 8 , there is schematically illustrated an optoelectronic transceiver  300  which may include PIC  390  as its optical front-end. The optoelectronic transceiver  300 , which may also be referred to as the host transceiver, may also include a digital signal processor  350  and analog electrical circuitry  360  interfacing processor  350  and PIC  390 . Processor  350  may be an embodiment of processor  50  or  150 , and may include a data source module  26 , a data sink module  36 , and loopback test logic (LTL)  46 . The analog electrical circuitry  360  may include electrical amplifiers in at least one of the Rx and Tx signal paths, and a loopback controller  70  for switching LOC  340  between the loopback state and the regular operation state, as described above. LTL  46  may be configured to perform, in cooperation with the data source  26  and the data sink  36 , signal processing operations for testing the host transceiver  300  when the host transceiver is in the loopback mode, and LOC control operations. The data source module  26  is configured to generate, from input data signals, electrical Tx signals  371  that may comprise the XI_Tx, XQ_Tx, YI_Tx, and YQ_Tx signals. The data sink module  26  is configured to receive electrical Rx signals  373  that may comprise the electrical Rx signals XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx, and to process these signals to recover transmitter-generated XI, XQ, YI and YQ signal channels. 
     In some embodiments PIC  390  may be installed in a PCB “B” that may also host the analog electrical circuitry  360 . In some embodiments the digital signal processor  350  may be installed in the same PCB “B”, or in a different PCB “A” that is connected to PCB “B” by signal lines suitable to transmit the Tx signals  371  and the electrical Rx signals  373  therebetween. 
     Referring to  FIGS. 5 and 8 , during regular operation of the host transceiver  300  the loopback is blocked, and the optical Rx circuit  330  may receive a dual-polarization (DP) QM optical signal from a remote transceiver (not shown) as the input optical signal  302 . Due to channel mixing related to polarization rotation, chromatic dispersion, polarization mode dispersion in the optical link, and due to an unknown and possibly varying phase offset between the LO light and the received optical signal in the optical Rx circuit  330 , each of the electrical Rx signals  373  XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx may be some combination of the XI, XQ, YI, and YQ signal channels of the remote transceiver, which may also be somewhat distorted during propagation. The data sink module  36  of processor  350  of the host transceiver may be configured to recover the transmitter-generated XI, XQ, YI, and YQ signal channel from the electrical Rx signals XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx using known in the art techniques. 
     The data signals recovered in the Rx signal path  30  may however be somewhat distorted relative to those initially generated in the Tx signal path; examples of the distortions include pulse shape distortions due to bandwidth limitations in the signal path, power imbalance between channels, and timing skew between channels. At least some of these and other distortions may originate in the Tx and/or Rx signal paths of communicating transceivers, and the ability to identify contributions of the Tx and Rx signal path to the distortion would facilitate transceiver calibration and optimization. 
     When PIC  390  operates in the loopback mode, in the absence of the optical channel mixer  380 , the electrical Tx signals XI_Tx, XQ_Tx, YI_Tx, YQ_Tx could be mapped one-to-one to the electrical Rx signals XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx at the output of the PD circuit  338 . This one-to-one mapping of the electrical Tx signals to the electrical Rx signals could complicate or make impossible separating signal impairments in the Rx signal path  30  of the host transceiver  300  from those in the Tx signal path  20  of the transceiver, and also complicate or make impossible characterizing aspects of the transmitter signal recovery process in the data sink  36  of the digital signal processor  350  of the host transceiver. 
     Advantageously, in the presence of the optical channel mixer  380  in LOC  340  and the LO-signal “dephasing” action of the optical phase shifter  335 , in the loopback mode of operation each of the electrical Rx signals  373  XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx comprises some combination of two or more of the electrical Tx signals  371  XI_Tx, XQ_Tx, YI_Tx, YQ_Tx generated by the data source  26  of the digital signal processor  350  of the host transceiver  300 . This mixing of the Tx signal channels enables separating signal impairments in the Rx signal path of the transceiver from those in the Tx signal path  20  of the transceiver, and their separate characterization and tuning. 
       FIG. 9  schematically illustrates, by way of example, a transformation of data signals in the XI and XQ channels from the time they are generated by a data source of a signal processor of a transceiver to the time they are recovered by a data sink of a signal processor of a same or different transceiver. The top panel of  FIG. 9  illustrates example electrical Tx signals XI  601  and XQ  602  which may be generated by the data source of a transmitting transceiver. The lower panel of  FIG. 6  illustrates corresponding digital signals  611  (XI) and  612  (XQ) as may be recovered by the data sink of a receiving transceiver. One transmission quality parameter is a relative timing skew, i.e. a time delay, between the transmission channels XI, XQ, YI, and YQ. It is typically desired that a timing skew between these four transmission channels is sufficiently small, generally the smaller the better.  FIG. 9  illustrates by way of example a timing skew TIQ  622  between the XI and XQ transmission channels, and may be referred to as the IQ timing skew. A relative time delay between, for example the XI and YI signals may be referred to as the polarization timing skew, or the XY timing skew. Generally, non-idealities in both the Tx signal path and the Rx signal path may contribute in the timing skew. An approach to transceiver testing that is able to separate the Tx and Rx contributions to the timing skews and other transmission distortions is desirable for transceiver optimization and characterization. 
     In some embodiments the transceiver  300  may be configured to be switched between a loopback mode and a regular operation mode as described above. When operating in the loopback mode, the data source  26  of processor  350  of the host transceiver  300  may be configured to provide one or more electrical Tx signals  371  to the optical Tx circuit  320  of the host transceiver  300 , and the data sink  36  may be configured to recover the Tx signals  371  from the electrical Rx signal  373 , as it would during the regular operation of the host transceiver  300 . From here, LTL  46  provided in processor  350 , or another instrument, may be engaged to determine one or more transmission quality parameters, for example by comparing the electrical Tx signals  371  to the recovered Tx signals that are recovered by the data sink from the electrical Rx signals  373 . 
     For example, LTL  46  may be configured to detect a signal distortion parameter, such as for example one of the IQ skew and the XY skew accrued in the Tx and Rx signal paths combined. The Tx-path skew and the Rx-path skew of the host transceiver  300  may then be estimated separately, for example by separately varying skew compensation parameters in the Tx signal path and the Rx signal path. A transmission quality measure, such as for example the bit error ratio (BER), the Q-factor as a measure of the eye opening, the error vector magnitude (EVM), or any other suitable transmission quality measure, may be used to optimize the distortion parameter being measured. In some embodiments, the Tx-side and the Rx-side skews and other distortion parameters may be estimated by generating orthogonal frequency combs in the electrical transmit channels and analyzing the phase and amplitude of the mixed input into the receiver. Here the term “orthogonal frequency combs” refers to a frequency comb in which no frequency is a harmonic of any other frequency in the comb. 
     Other transmission parameters of the optical transceiver that may also be measured in the loopback mode include an electro-optic (EO) S21 parameter of a Tx and/or Rx signal path. The EO S21 parameter of the Tx signal path characterizes the combined electro-optic frequency response of the data source  26 , the amplification circuitry of the Tx signal path, and the electro-optic converter stage  120 ,  220 , or  320 . The OE S21 parameter of the Rx signal path characterizes the combined electro-optic frequency response of the OE converter of the optical Rx circuit  130 ,  230 ,  330 , the analog amplification circuitry of the Rx signal path, and the data sink  36 . By comparing the electrical Rx signals  373  XI_Rx, XQ_Rx, YI_Rx, and YQ_Rx to the electrical Tx signals  371  XI_Tx, XQ_Tx, YI_Tx, YQ_Tx, the EO S21 parameters for the Rx signal path and the Tx signal path can be separately estimated. In some embodiments S21 parameter information, such as its frequency dependence, may be extracted from saved electrical signal traces, and may then be used for example to optimize Tx signal pre-emphasis. 
     Although the description hereinabove with reference to  FIGS. 5-9  was focused primarily on PICs configured to transmit and receive quadrature modulated light signals, in other embodiments PIC  390  and the transceiver  300  may be configured to operate with other modulation formats, such as for example PAM-4, PAM-8, 4-ASK, and the like. In such embodiments each of the optical modulators  320 X and  320 Y may be for example in the form of a single MZM driven by multi-level electrical PAM signals, or a nested MZM, each driven by a binary signal. The dual optical Rx circuit  330  may remain substantially the same, with a suitably modified PD circuit  338  as would be evident to a skilled reader. 
     Accordingly, an aspect of the present disclosure provides a method for characterizing an optoelectronic transceiver (e.g.  100 ,  300 ) comprising a PIC (e.g.  90 ,  190 ,  290 ,  390 ), the PIC comprising an optical receiver (Rx) circuit and an optical transmitter (Tx) circuit, the method comprising: switchably directing, within the PIC, a loopback optical signal from an output port of the optical Tx to an input port of the optical Rx; and, processing one or more electrical signals obtained from an output of the optical Rx to estimate one or more characteristics of the optoelectronic transceiver. 
     In some implementations of the method the loopback optical signal comprises a first optical channel (e.g. I and/or X) and a second optical channel (e.g. Q and/or Y), and the method comprises mixing the first and second optical channels within the PIC using an integrated optical mixer (e.g.  280 ,  380 ) and/or an LO phase tuner (e.g.  135 ,  335 ). 
     In some implementations wherein the optical Rx circuit is configured to coherently mix the loopback optical signal with local oscillator (LO) light, the method comprises tuning an optical phase of the LO light in the loopback mode of PIC operation. 
     In some implementations, wherein the optical Tx circuit (e.g.  220 ,  320 ) comprises a quadrature optical modulator (e.g.  320 X,  320 Y), the method may further comprise: providing the loopback optical signal comprising an in-phase (I) optical signal and a quadrature-phase (Q) optical signal responsive to an I-channel electrical Tx signal (e.g. XI_Tx or YI_Tx) and a Q-channel electrical Tx signal (e.g. XQ_Tx or YQ_Tx); obtaining, using the optical Rx circuit, an I-channel electrical Rx signal (e.g. XI_Rx or YI_Rx) and a Q-channel electrical Rx signal (e.g. XQ_Rx or YQ_Rx) responsive to the loopback optical signal; and, comparing the I-channel and Q-channel electrical Tx signals and the I-channel and Q-channel electrical Rx signals to estimate at least one of: an IQ skew of an Rx signal path of the optoelectronic transceiver, an IQ skew of a Tx signal path of the optoelectronic transceiver, an electro-optic (EO) S21 parameter of the Tx signal path of the optoelectronic transceiver, or an OE S21 parameter of the Rx signal path of the optoelectronic transceiver. 
     In some implementations wherein the optical Tx circuit (e.g.  320 ) is configured to output a polarization multiplexed optical signal responsive to an X-channel electrical Tx signal (e.g. XI_Tx and/or XQ_Tx) and a Y-channel electrical Tx signal (e.g. YI_Tx and/or YQ_Tx), the method may further comprise: providing the loopback optical signal (e.g.  341   x ) comprising an X-channel optical signal responsive to the X-channel electrical Tx signal and a Y-channel optical signal (e.g.  341   y ) responsive to the Y-channel electrical Tx signal; mixing the X-channel optical signal and the Y-channel optical signal to provide two mixed optical signals (e.g.  345 ,  343 ) to the optical Rx circuit (e.g.  330 ); obtaining, using the optical Rx circuit, an X-channel electrical Rx signal (e.g. XI_Rx and/or XQ_Rx) and a Y-channel electrical Rx signals (e.g. YI_Rx and/or YQ_Rx); and, comparing the X-channel and Y-channel electrical Tx signals and the X-channel and Y-channel electrical Rx signals to separately estimate at least one of: an XY or IQ skew of an Rx signal path of the optoelectronic transceiver, an XY or IQ skew of a Tx signal path of the optoelectronic transceiver, an EO S21 parameter of the Tx signal path of the optoelectronic transceiver, or an OE S21 parameter of the Rx signal path of the optoelectronic transceiver. 
     A further aspect of the present disclosure provides a PIC for an optical transceiver, the PIC comprising: an input PIC port for receiving an input optical signal; an output PIC port for transmitting an output optical signal; an optical receiver (Rx) circuit optically coupled to the input PIC port and configured to process the input optical signal for conversion into one or more electrical Rx signals; an optical transmitter (Tx) circuit optically coupled to the output PIC port, the optical Tx circuit configured to provide the output optical signal and a loopback optical signal responsive to one or more electrical Tx signals; and, a loopback optical circuit (LOC) configured to switchably couple the loopback optical signal into the optical Rx circuit for testing and/or tuning one or more parameters of the optical transceiver. 
     In some implementations the loopback optical circuit, the optical Rx circuit, and the optical Tx circuit may be implemented in a same optical chip. 
     In some implementations the loopback optical circuit may comprise a switching circuit configured to switchably block the optical Rx circuit from receiving the loopback optical signal. In some implementations the switching circuit may be configured to switchably block an optical connection between the optical Rx circuit and the input PIC port when the loopback optical signal is coupled into the optical Rx circuit. In some implementations the switching circuit may be configured to switchably block an optical connection between the optical Tx circuit and the output PIC port when the loopback optical signal is coupled into the optical Rx circuit. 
     In some implementations the loopback optical signal may comprise a first optical channel and a second optical channel, and the loopback optical circuit may comprise a loopback optical mixer configured to mix the first optical channel and the second optical channel. In some implementations the output PIC port comprises a polarization combiner for combining the first and second optical channels in orthogonal polarizations. 
     In some implementations the optical Tx circuit comprises an optical modulator circuit (OMC) (e.g.  320  in  FIG. 5 ) comprising an input optical port (e.g.  312 ), and the optical Rx circuit (e.g.  330 ) comprises a first Rx optical mixer (e.g.  330 X). The first Rx optical mixer may comprise one or more signal ports (e.g.  331 ,  333 ) switchably coupled to the input PIC port (e.g.  362 ) and the loopback optical circuit (e.g.  340 ), and a local oscillator (LO) port (e.g.  332 ). The PIC may further comprise a light source port for receiving input light from a light source, the light source port coupled to the input optical port of the OMC and the LO port of the first optical mixer. 
     In some implementations the PIC (e.g.  190 ,  390 ) may further comprise a phase tuner (e.g.  135 ,  335 ) disposed in an optical path from the light source port (e.g.  115 ,  303 ) to the LO port of the first optical mixer and configured to tune an optical phase of the input light received in the LO port. 
     In some implementations the first Rx optical mixer comprises a first optical hybrid (OH) having the LO port coupled to the light source port; the one or more signal ports of the first Rx optical mixer may comprise a first signal port optically coupled to the input PIC port and a second signal port optically coupled to the loopback optical circuit. 
     In some implementations the OMC (e.g.  320  in  FIG. 5 ) may comprise a first optical modulator (e.g.  320 X) configured to output a first optical channel and a second optical modulator (e.g.  320 Y) configured to output a second optical channel; the LOC (e.g.  340 ) may comprise a loopback optical mixer (e.g.  380 ) configured to mix the first optical channel and the second optical channel and to output a first mixed optical signal and a second mixed optical signal; the one or more signal ports of the first Rx optical mixer may be optically coupled to the loopback optical mixer for receiving the first mixed optical signal. The optical Rx circuit may further comprise a second Rx optical mixer, the second Rx optical mixer comprising: an LO port coupled to the light source port, and one or more signal ports optically coupled to the loopback optical mixer for receiving the second mixed optical channel and to the input PIC port. In some implementations each of the first optical mixer and the second optical mixer may comprise a 90° optical hybrid (OH), and the one or more signal ports of each of the first optical mixer and the second optical mixer may comprise a first signal port coupled to the input PIC port and a second signal port coupled to the loopback optical mixer. 
     In some implementations the optical Tx circuit may comprise a first output optical coupler (e.g.  407  in  FIG. 6 ) having a first output coupler port optically coupled to the output PIC port and a second output coupler port optically coupled to the loopback optical circuit. 
     In some implementations the switching circuit may be configured to switchably couple the output optical signal from the optical Tx circuit to either the optical PIC port or to the optical Rx circuit as the loopback optical signal. In some implementations the switching circuit may comprise one or more optical shutters. 
     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Indeed, various other embodiments and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. 
     For example, it will be appreciated that different dielectric materials and semiconductor materials including but not limited to silicon (Si), Silicon on Insulator (SOI), and compound semiconductor materials of groups commonly referred to as A3B5 and A2B4, such as GaAs, InP, and their alloys and compounds may be used to fabricate elements of the optical transceiver example embodiments of which are described hereinabove. Furthermore in some embodiments the optical Tx circuit may be embodied using integrated devices other than MZMs, including but not limited to other types of optical modulators, such as for example optical phase modulators or optical modulators based on micro-ring or micro-disk resonators, or as directly modulated lasers. In embodiments where the data source, e.g. 26, provides more than one channel, those channels could serve as source for modulated optical signals exploiting polarization multiplexing, quadrature modulation, or multi-level amplitude modulation (e.g. PAM-4, PAM-8, 4-ASK, and the like). Principles described above can be exploited for both single-optical-channel transceivers and WDM transceivers. 
     Furthermore, elements or features described hereinabove with reference to a particular example embodiment may also be incorporated in other described embodiments or their variants. Furthermore, some of the elements described hereinabove with reference to one or more embodiments may be omitted or replaced with another elements capable of similar functions, and another elements added. 
     Furthermore in the description above, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present disclosure. In some instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description with unnecessary detail. Thus, for example, it will be appreciated by those skilled in the art that block diagrams herein can represent conceptual views of illustrative circuitry embodying the principles of the technology. Furthermore, it will be appreciated that each of the example embodiments described hereinabove may include features described with reference to other example embodiments. 
     Thus, while the present invention has been particularly shown and described with reference to example embodiments as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.