Patent Publication Number: US-2020280372-A1

Title: Coherent/im-dd dual operation optical transceiver

Description:
RELATED APPLICATIONS 
     The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/812,584, titled “COHERENT/IM-DD DUAL OPERATION OPTICAL TRANSCEIVER” and filed on Mar. 1, 2019, the entire contents of which are hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND 
     Fiber-optics provides high bandwidth data center interconnection (DCI) for data center networks. Existing intra-data center interconnection technologies use intensity modulation (IM) and direction detection (DD) technology. However, the existing IM-DD approach does not scale well with bandwidth growth as data consumption rises. Continuous growth in DCI bandwidth demand and consumption, for example, to support over 100 Gb/s of data transmission per wavelength using IM-DD, can be technologically challenging and expensive to implement. 
     Coherent optics is an alternative approach suitable for high bandwidth DCI for data center networks. However, due to the evolutionary nature of data center networks, the un-proven next generation technologies, such as those based on coherent optics, must be backward compatible with the existing technology for a large scale datacenter network. Implementing the network to be partially and seamlessly upgraded, without needing to upgrade the entire data center all at once can be both cost prohibitive and impractical. As a result, a challenge remains on how to make the coherent optics technology backward compatible with the existing IM-DD technology to better bridge the current and future technologies. 
     SUMMARY 
     At least one aspect is directed to a photonic integrated chip (PIC). The PIC includes a receiver section configured to receive both coherently modulated and intensity modulated optical signals and to be optically switched between a first receiver mode for direct detection and a second receiver mode for coherent detection. The PIC also includes a transmitter section including a nested Mach-Zehnder Modulator or a polarization multiplexed quad Mach-Zehnder Modulator configured to be operated in a first transmission mode to output an intensity modulated optical signal and a second transmission mode to output a coherently modulated optical signal. 
     In some implementations, the receiver section detects both the coherently modulated and intensity modulated optical signals using at least one common photodiode. In some implementations, the photodiode comprises a waveguide photodiode. 
     In some implementations, the PIC is coupled to a digital signal processor (DSP) for decoding the received coherently modulated and intensity modulated optical signals. In some implementations, the PIC further includes an optical switch configured to selectively direct a received optical signal down a direct detection optical circuit or a coherent detection optical circuit based on a control signal applied to the optical switch. 
     In some implementations, the nested Mach-Zehnder Modulator includes a controllable phase shifter coupled to one Mach-Zehnder Modulator in the nested Mach-Zehnder Modulator, wherein in the first transmission mode, the controllable phase shifter implements a first phase shift and in the second transmission mode, the controllable phase shifter implements a second phase shift. 
     In some implementations, the controllable phase shifter comprises a heater configured to introduce a thermo-optic phase shift in the output of the one Mach-Zehnder Modulator. In some implementations, the first phase shift is a zero phase shift and the second phase shift is a π/2 phase shift. 
     In some implementations, the PIC further includes at least one demultiplexer coupled to the receiver section for receiving coarse wavelength division multiplexed, intensity modulated optical signals and a multiplexer coupled to the transmitter section to transmit wavelength division multiplexed, intensity modulated optical signals. 
     In some implementations, the PIC is coupled to a controller configured to cause the PIC to switch between receiver and transmission modes. 
     At least one aspect is directed to a method operating. The method includes providing a source configured for generating optical signals and providing a transceiver. The transceiver includes a receiver section configured to receive both coherently modulated and intensity modulated optical signals and to be optically switched between a first receiver mode and a second receiver mode, and a transmitter section comprising a nested Mach-Zehnder Modulator or a polarization multiplexed quad Mach-Zehnder Modulator configured to be operated in a first transmission mode and a second transmission mode. The method also includes transmitting, via the transmitter section, in the first transmission mode an intensity modulated optical signal or in the second transmission mode a coherently modulated optical signal. The method further includes receiving an optical signal, via the receiver section, in the first receiver mode for direct detection or in the second receiver mode for coherent detection. 
     In some implementations, receiving includes detecting both the coherently modulated and intensity modulated optical signals using at least one common photodiode. In some implementations, the photodiode comprises a waveguide photodiode. 
     In some implementations, the method further includes decoding the received coherently modulated and intensity modulated optical signals using a digital signal processor (DSP). In some implementations, the method also includes selectively directing using an optical switch a received optical signal down a direct detection optical circuit or a coherent detection optical circuit based on a control signal applied to the optical switch. 
     In some implementations of the method, the nested Mach-Zehnder Modulator includes a controllable phase shifter coupled to one Mach-Zehnder Modulator in the nested Mach-Zehnder Modulator, wherein in the first transmission mode, the controllable phase shifter implements a first phase shift and in the second transmission mode, the controllable phase shifter implements a second phase shift. 
     In some implementations of the method, the controllable phase shifter comprises a heater configured to introduce a thermo-optic phase shift in the output of the one Mach-Zehnder Modulator. In some implementations, the first phase shift is a zero phase shift and the second phase shift is a π/2 phase shift. 
     In some implementations, the method further includes demultiplexing received coarse wavelength division multiplexed, intensity modulated optical signals and transmitting coarse wavelength division multiplexed intensity modulated optical signals. In some implementations, the method further includes switching between receiver and transmission modes in response to a controller command. 
     At least one aspect is directed to a transceiver. The transceiver includes a receiver section configured to receive both coherently modulated and intensity modulated optical signals, a transmitter section comprising an optical modulator configured to be operated in a first transmission mode to output an intensity modulated optical signal and a second transmission mode to output a coherently modulated optical signal, and a digital signal processor (DSP) for decoding the received coherently modulated and intensity modulated optical signals. 
     In some implementations, the optical modulator includes a nested Mach-Zehnder Modulator or a polarization multiplexed quad Mach-Zehnder Modulator. In some implementations, the nested Mach-Zehnder Modulator comprises a controllable phase shifter coupled to one Mach-Zehnder Modulator in the nested Mach-Zehnder Modulator, wherein in the first transmission mode, the controllable phase shifter implements a first phase shift and in the second transmission mode, the controllable phase shifter implements a second phase shift. 
     In some implementations, the receiver section is configured to optically switch between a first receiver mode for direct detection and a second receiver mode for coherent detection. In some implementations, the transceiver is coupled to a controller configured to cause the transceiver to switch between receiver and transmission modes. 
     These and other aspects and implementations are discussed in detail below. The foregoing information and the following detailed description include illustrative examples of various aspects and implementations, and provide an overview or framework for understanding the nature and character of the claimed aspects and implementations. The drawings provide illustration and a further understanding of the various aspects and implementations, and are incorporated in and constitute a part of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. Like reference numbers and designations in the various drawings indicate like elements. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  shows a schematic diagram of an implementation of a single channel dual-mode optical transceiver, according to an illustrative implementation; 
         FIG. 2  shows a schematic diagram of another implementation of a wavelength division multiplexed dual-mode optical transceiver, according to an illustrative implementation; 
         FIGS. 3A, 3B, and 3C  show schematic diagrams showing operational schemes of a nested Mach-Zehnder Modulator, according to an illustrative implementation; 
         FIG. 4  shows a schematic diagram of another implementation of a wavelength division multiplexed dual-mode optical transceiver, according to an illustrative implementation; 
         FIGS. 5A and 5B  show schematic diagrams of example implementations of Mach-Zehnder Interferometer optical switches; and 
         FIG. 6  is a flowchart of an example method of operating a dual-mode optical transceiver, according to an illustrative implementation. 
     
    
    
     DETAILED DESCRIPTION 
     In the IM-DD approach currently deployed in intra-datacenter networks, a transmitted optical signal is modulated with a non-return-to-zero (NRZ) on-off-keying (OOK) format. In some IM-DD schemes recently developed, a 4-level pulse-amplitude modulation (PAM-4) format is used for modulating the optical signal. Within the current schemes, the transmitted optical signal intensity is typically received by a photodetector (PD) in the optical network. Most current IM-DD approaches use a simple and low cost solution for long-reach (LR) optical links using wavelengths in the O-band (around 1310 nm), typically covering up to a distance of 10 km. 
     Unlike the IM-DD approach, the coherent optical approach is based on modulation of a signal onto the amplitude and phase of the optical wave, instead of optical intensity. In the coherent approach, the transmitted optical signal is received by coherent detection, where the signal is mixed with a second optical wave called a local oscillator (LO) through a combination of different phase delays between the signal and LO, and finally detected on a plurality of PDs. The detected photocurrents in the PDs are subsequently processed by a digital signal processor (DSP) to demodulate the signal in the receiver unit. 
     Current coherent optical communication technology is mainly used in long-haul, submarine, and more recently metro networks, due to its benefits of high spectral efficiency, high sensitivity and resilience to fiber transmission impairments. The optical wavelength is usually in the C-band (around 1550 nm), due to the availability of the erbium-doped fiber amplifier (EDFA). Despite the advantages of coherent optics, its high complexity, high cost and incompatible operating wavelength make it prohibitive for implementation in currently existing intra-data center interconnects. 
     Due to the advancement of photonic integrated circuit (PIC) technology, it is possible to develop a low cost, highly integrated coherent optical transceiver for intra data center interconnects. However, the integration of the PIC and coherent optical receiver technologies cannot be implemented in data center networks without ensuring backward compatibility with existing IM-DD technologies. The challenge to make the coherent optical technology backward compatible with the existing IM-DD technologies, and the need for bridging current and future technologies culminate into the development of the transceiver technology as disclosed herein. 
     As described in various embodiments and implementations herein, a dual-mode optical transceiver and related technologies disclosed in the application can be implemented in data center networks and can be seamlessly integrated with both existing IM-DD technologies and coherent optical technologies. The disclosure relates to a dual-mode optical transceiver that supports both IM-DD and coherent optical technologies (IM-DD/Coherent transceiver) and a method of operating the IM-DD/Coherent transceiver. Specifically, the disclosure relates to an IM-DD/Coherent transceiver having a receiver section configured to receive both intensity modulated (IM) and coherently modulated (CM) optical signals. In some implementations, the IM-DD/Coherent transceiver can be optically switched between a first receiver mode for direct detection (DD) and a second receiver mode for coherent detection (CD). The disclosure also relates to an IM-DD/Coherent transceiver having a transmitter section configured to transmit both IM and CM optical signals. In some implementations, the IM-DD/Coherent transceiver includes a nested Mach-Zehnder Modulator or a polarization multiplexed quad Mach-Zehnder Modulator that is configured operate in a first transmission mode to output an IM optical signal and a second transmission mode to output a CM optical signal. 
       FIG. 1  shows a schematic diagram of a single channel dual-mode optical transceiver  100 , according to an illustrative implementation. The transceiver  100  shown in  FIG. 1  includes a photonic integrated circuit (PIC)  110  that is connected to a digital signal processing (DSP)  108  via transimpedance amplifiers (TIAs)  106   a  and  106   b . As shown in  FIG. 1 , the PIC  110  of the transceiver  100  includes a transmitter  120  and a receiver  160 . The transmitter  120  further includes a laser source  130 , an optical switch  124 , a variable optical attenuator (VOA)  126 , a light splitter  122 , and an optical modulator  140 . The receiver  160  further includes a polarization splitter rotator (PSR)  162 , optical switches  164   a  and  164   b , VOAs  166   a  and  166   b , a PSR  165 , two 90 degree optical hybrids  180   a  and  180   b , four photodiodes (PDs)  190   a ( i - iv ), and four PDs  190   b ( i - iv ). 
     In some implementations, the PIC  110  can include the DSP  108  and/or TIAs  106   a  and  106   b , along with the components in the transmitter  120  and the receiver  160 . In other words, the transceiver  100  can include all the components shown in  FIG. 1 , according to some implementations. In some implementations, the PIC can include TIAs  106   a  and  106   b , the transmitter  120 , and the receiver  160 . 
     In some implementations, the TIAs  106   a  and  106   b  can include any impedance amplifier suitable for amplifying an electrical current. In some implementations, the TIAs  106   a  and  106   b  are quad channel TIAs for coherent receivers. 
     In some implementations, the DSP  108  can include any digital signal processor suitable for this application. The DSP  108  is configured to carry out processing functions used in both IM and CM transmission and reception, such as converting digital signals into modulator drive signals for both IM and CM transmission, as well as symbol recovery for IM and CM receiving. The transceiver  100 , in some implementations, includes a separate integrated circuit processor  115 , such as an ASIC, FPGA, or microprocessor to carry out the control functionality described herein. 
     In some implementations, the transmitter  120  of the transceiver  100  is configured to be operated in a first transmission mode to output an intensity modulated optical signal and a second transmission mode to output a coherently modulated optical signal. In some implementations, the receiver  160  of the transceiver  100  is configured to receive both coherently modulated and intensity modulated optical signals and to be optically switched between a first receiver mode for direct detection of intensity modulated signals and a second receiver mode for coherent detection of coherently modulated optical signals. 
     As shown in  FIG. 1 , the transmitter  120  includes the laser source  130  configured to provide a laser light. The transmitter  120  also includes the optical switch  124 , the VOA  126 , the light splitter  122 , and the optical modulator  140  where the laser light is manipulated as it transits through the components. In some implementations, the laser light from the laser source  130  is transmitted into the optical switch  124  that is configured for controllably distributing the optical power of the laser according to the operating mode of the transmitter  120 . For example, when operating in the IM mode (also referred to herein as “pulse-amplitude modulation (PAM) mode”), the optical switch can be controlled to direct all of the light output by the laser source  130  to the modulator  140 . When operating in the CM mode (also referred to herein as “quadrature-amplitude modulation (QAM) mode”), the optical switch can be controlled to split the light emitted by the laser source  130  between the modulator  140  and the optical hybrids  180   a  and  180   b  to serve as a local oscillator. In some implementations, in the CM mode, the optical switch  124  distributes the optical power to the modulator  140  and the 90° optical hybrids  180   a  and  180   b  according to any of the ratios 10:90, 30:70, 50:50, 70:30, or 90:10 or any ratios therebetween. 
     In some implementations, the laser source  130  is integrated in the transmitter  120 . In some implementations, the laser source  130  is a standalone unit, die, or module that is attached to the transmitter  120 . In some implementations, the laser source  130  is integrated in the PIC  110 . In some implementations, the laser source  130  is a standalone unit, die, or module that is attached to the PIC  110 . 
     In some implementations, the optical hybrids  180   a  and  180   b  can include a 90° optical hybrid. As would be understood by a person of ordinary skill in the art, a 90° optical hybrid is an optical component that generates four interference signals by combining two optical signals together, imparting on one of the signals four different phase delays, each separated by 90°, hence the inclusion of the four photodiodes  190   a ( i - iv ) and  190   b ( i - iv ) coupled to each optical hybrid  180   a  and  180   b . In some implementations, a 90° optical hybrid with single ended detection may be used, instead, yielding two interference signals. In such implementations, only two photodiodes may be needed per optical hybrid. 
     In some implementations, the optical modulator  140  is a nested Mach-Zehnder modulator (n-MZM). In some implementations, a Mach-Zehnder modulator (MZM) can be used for intensity modulation. In some implementations, two MZMs can be used in parallel as a n-MZM, and the n-MZM can be used for coherent modulation. In some implementations, the optical modulator  140 , e.g., a n-MZM, is configured to perform both IM and CM. 
     As shown in  FIG. 1 , the receiver  160  includes the PSR  162  configured for receiving transmitted optical signal and splitting the optical signal into two polarization components. The receiver  160  also includes the optical switches  164   a  and  164   b , the VOAs  166   a  and  166   b , the PSR  165 , and the optical hybrids  180   a  and  180   b . The optical hybrid  180   a  is connected to the PDs  190   a ( i - iv ), and the optical hybrid  180   b  is connected to the PDs  190   b ( i - iv ). As shown in  FIG. 1 , each of the four PDs  190   a ( i - iv ) and four PDs  190   b ( i - iv ) are connected to one of the TIAs  106   a  or  106   b  that are connected to the DSP  108 . In some embodiments, the PDs can be balanced dual-input waveguide photodetectors. In some implementations, the PDs can be single-ended photodiodes. The use of single-ended photodiodes can improve yield, but may come at the cost of sensitivity (about 3 dB) and lack of common mode suppression provided by balanced photodetectors. 
     As disclosed herein and illustrated in  FIG. 1 , the transceiver  100  is configured to operate in both an IM mode and a CM mode. When the receiver  100  is configured to operate in the IM mode, the optical paths through the PIC  110  are referred to as IM (for transmission) or DD (for receiving) optical paths. When the transceiver  100  is configured to operate in the CM mode, the optical paths through the PIC  110  are referred to as coherent optical paths. 
     As shown in  FIG. 1 , when operating in the CM mode, the coherent path for transmission begins at the laser source  130 , which generates and outputs laser light to the optical switch  124 . The optical switch splits the beam and sends a portion of the optical energy of the laser beam of light to the optical hybrids  180   a  and  180   b  via the VOA  126  and the light splitter  122 . This laser light is used as a local oscillator for coherent detection of coherently modulated received optical signals. The remainder of the optical energy output by the laser source  130  travels through the PIC  110  to the optical modulator  140 . The modulator  140  coherently modulates the light and transmits the modulated light outward to an output fiber. 
     As shown in  FIG. 1 , when operating in the IM mode for transmission in the transceiver  100 , the optical switch  124  is controlled to pass all of the optical energy of the light emitted by the laser source  130  to the optical modulator  140 . The optical modulator  140  modulates the laser light via intensity modulation and then transmits the modulated light outward to an output fiber. Accordingly, the light output onto the output fiber in both the IM and CM modes travels the same optical path, through the same optical components. 
     With respect to optical signals received by the transceiver  100 , as shown in  FIG. 1 , in both the IM and CM operating modes, the respective optical paths begins at the PSR  162 , which splits the light into its two constituent polarizations, passing each polarization component to a respective optical switch  164   a  or  164   b . At the optical switches  164   a  and  164   b , the paths light traverses varies based on the operating mode of the transceiver  100 . In the IM mode, the optical switches  164   a  and  164   b  are controlled to direct light received at each optical switch to the PSR  165 , which serves to recombine the optical two polarization components of the received signal and direct them to one of the photodiodes (e.g.,  190   a ( i )) of one of the optical hybrids (e.g.,  180   a ). In the CM mode, optical switches  164   a  and  164   b  are controlled to direct the light at respective optical switches  164   a  and  164   b  to respective optical hybrids  180   a  and  180   b  via VOAs  166   a  and  166   b . At the optical hybrids, each polarization component signal is combined with local oscillation optical signals delayed by various phase delays. For example, in the implementation shown in  FIG. 1 , in which the optical hybrids are 90° optical hybrids, the polarization component signals are combined with  4  local oscillator signals, each separated in phase by 90°. The resultant interference signals are detected by respective photodiodes  190   a ( i - iv ) and  190   b ( i - iv ). The electrical outputs of the photodiodes are fed through the TIAs  106   a  and  106   b  to the DSP  108  for symbol recovery. 
     As mentioned above, in some implementations, the PIC  110  is coupled to a controller separate from the DSP and one or more drivers configured to control the optical switches  124 ,  164   a  and  164   b , the VOAs  126 ,  166   a , and  166   b , and the optical modulator  140  of the PIC  110 . In some implementations, the controller can be implemented, e.g., as a microcontroller unit, an integrated circuit logic unit, or as a software-controlled microprocessor. 
     In some implementations, the dual-mode optical transceiver  100  that operates in a single channel IM-DD/Coherent transceiver configuration shown in  FIG. 1  can be used as the basis for a multi-channel dual-mode transceiver using parallel single mode (PSM) or wavelength division multiplexing (WDM) technologies. 
       FIG. 2  shows a schematic diagram of one implementation of a multi-channel dual-mode optical transceiver  200 , according to an illustrative implementation. As shown in  FIG. 2 , the dual-mode transceiver  200  is configured to operate as a n-wavelength (nλ) WDM transceiver implemented in a multi-channel IM-DD/Coherent transceiver configuration. As shown in  FIG. 2 , the transceiver  200  includes a plurality of PICs  210 A,  210 B,  210 C, and  210 D (collectively referred to as PICs  210 ). In some implementations, a multi-channel or WDM transceiver configuration can be implemented on a single PIC, instead of multiple PICs, or one PIC per channel. As shown in  FIG. 2 , each of the PICs  210  in the transceiver  200  is configured to transmit and receive optical signals at wavelengths that differ from the other PICs  210  in the transceiver. As shown in  FIG. 2 , each of the PICs  210 A,  210 B,  210 C, and  210 D is configured substantially similar to the PIC  110  shown in  FIG. 1 , with similar reference numerals corresponding to substantially similar components. For example, the optical switches  224 ,  264   a , and  264   b  in  FIG. 2  correspond to the optical switches  124 ,  164   a  and  164   b  in  FIG. 1 . Similarly, the VOAs  226 ,  266   a , and  266   b  correspond to the VOAs  126 ,  166   a  and  166   b  in  FIG. 1 ; the optical modulator  240  in  FIG. 2  corresponds to the optical modulator  140  in  FIG. 1 , and so forth. While each PIC  210  has its own laser source  230  associated with it (integrated or optically coupled), each laser source  230  is configured to output a different wavelength of light. Accordingly, the transceiver  200  can be controllably switched between operating in a CM mode or an IM mode. In some implementations, some PICs may operate in the IM mode while other PICs operate in the CM mode. In some other implementations, all PICs operate in the same mode at any given time, IM mode or CM mode. 
     While most of the components shown in  FIG. 1  are replicated in each of the PICs  210 A- 210 D, the transceiver  200 , in some implementations may only include a single PSR  262  for an optical signal, which splits the received WDM signal into its constituent polarization components prior to those polarization components being separated by wavelength by respective demultiplexers  263   a  and  263   b , which direct the wavelength specific signals to corresponding PICs  210 . In some other implementations, the transceiver  200  may include a single demultiplexer and a separate PSR  262  for each PIC. In addition to the demultiplexers  263   a  and  263   b , the transceiver  200  includes a multiplexer  228  to combine the outputs of the modulators  240  of the respective PICs  210  into a combined WDM output optical signal. The transceiver can have a single DSP to process the amplified electrical outputs of all of the PICs  210 , or it can include multiple DSPs to process the electrical outputs of individual or subsets of the PICs  210 . 
       FIGS. 3A, 3B, and 3C  show schematic diagrams showing operational schemes of a nested Mach-Zehnder Modulator (n-MZM)  340 , that be used as the optical modulators  140  or  240  shown in  FIGS. 1 and 2 .  FIG. 3A  shows an operational scheme  300   a  for the n-MZM  340  for standard coherent modulation of a laser. As shown in  FIG. 3A , a continuous wave signal is input across MZM  342  and MZM  344  of the n-MZM  340 . Both MZMs  342  and  344  are biased at their null point and are driven by respective I and Q drive signals. The output of the MZM  344  driven by the Q drive signal passes through a phase delay, e.g., a π/2 phase delay) before being combined with the output of the MZM  342 , which is driven by the I drive signal. 
       FIG. 3B  shows an example operational scheme  300   b  to generate an IM signal using the same n-MZM  340 . The operational scheme  300   b  uses the same electrical driving signals (a pulse amplitude modulation (PAM) drive signal) to drive both MZMs  342  and  344 . Both MZMs  342  and  344  are biased at their respective quadrature points. Therefore, the outputs from both MZMs  342  and  344  are identical IM signals. As shown in  FIG. 3B , the relative phase shift between the outputs from the two MZMs  342  and  344  is 0. Therefore, the two identical IM signals are constructively combined to form a single IM signal as shown in  FIG. 3B . 
       FIG. 3C  shows an alternative operational scheme  300   c  to generate the IM signal using the same n-MZM  340 . The operational scheme  300   c  drives one of the MZMs  342  or  344  with a PAM drive signal and a null bias. The other MZM  342  or  344  is not driven, and is based at its maximum. The relative phase shift between the outputs from the two MZMs  342  and  344  is 0. The combined output is again a single PAM signal. 
     In some implementations, the phase shifter on the MZM  344  can be implemented by placing a heater close to the optical waveguide, and the phase of the optical signal is then controlled by the heater, leveraging the thermo-optic effect. As indicated above, in the CM mode, the phase shifter is controlled to achieve a phase shift of π/2 between the two MZMs  342  and  344 . In some implementations, in the IM mode, the phase shifter is controlled to achieve a phase shift of 0 between the two MZMs  342  and  344 . 
       FIG. 4  shows a schematic diagram of another implementation of a wavelength division multiplexed dual-mode optical transceiver  400 , according to an illustrative implementation. The transceiver  400  is configured for dual operations in a 1-channel PM-xQAM coherent mode and a 4-channel Coarse Wavelength Division Multiplexing (CWDM4) IM-DD mode. As shown in  FIG. 4 , the transceiver includes a transmitter  420  and a receiver  460  that is connected to a DSP  408  via TIAs  406   a  and  406   b . The receiver  460  is similar to the receiver  160 . However, the transmitter  420  is configured differently than the transmitters  120  and  220 . 
     As shown in  FIG. 4 , the transmitter  420  includes four laser sources  430   a ,  430   b ,  430   c  and  430   d  with each outputting a different wavelength, an optical switch  424 , an optical splitter  422 , an optical modulator  440  that includes four MZMs  442 ,  444 ,  446 , and  448 , a MUX  428 , two more optical switches  452  and  454 , and a PSR  456 . In particular, the transmitter  420  uses the optical modulator  440 , which is a polarization multiplexed quad Mach-Zehnder modulator (PM-QMZM)  440 , instead of the n-MZM  140 ,  240  or  340  as shown in  FIGS. 1-3 . The configuration illustrated in  FIG. 4  utilizes a scheme where each of the 4 MZMs  442 ,  444 ,  446 , and  448  in the PM-QMZM  440  has 2 input ports and 2 output ports. 
     As shown in  FIG. 4 , the receiver  460  includes five PSRs, two DEMUXs, two optical switches, two VOAs, two 90 degree optical hybrids, and eight PDs. In particular, the PDs are dual input waveguide PDs and four of them connect one end to coherent signal inputs and connect the other end to intensity modulated signal inputs. 
     When operating in CM mode of the transceiver  400 , a single laser, for example  430   b , is turned on, and its output is split between the four MZMs of the PM-QMZM  440  via the optical switch optical splitters. For each constituent polarization of the laser light output, a portion is sent an upper arm of an MZM and a portion is sent to a lower portion another MZM. Phase delays are instituted on the outputs of the MZMs receiving the optical signal at their upper arms. Each of the four MZM is then driven with a corresponding I or Q drive signal, while biased at their respective null points. The outputs of the MZMs are then combined for output on an optical fiber. In the CM mode, a portion of the light output by the laser source  430   b  is also directed to the receiver portion  460  of the transceiver  400  to serve as a local oscillator signal. The remaining three lasers, in this example  430   a ,  430   c , and  430   d , can remain off. 
     When operating in IM-DD CWDM4 mode, all four lasers at different wavelengths are input into the four MZMs of the PM-QMZM  440 , which are driven with independent PAM drive signals, while biased at quadrature points just like traditional IM MZMs. The outputs of the MZMs are then multiplexed together via the multiplexer  428  before being switched onto an optical fiber via optical switch  452 . 
     As shown in  FIG. 4 , the receiver  460 , when operating in CM mode, operates similarly to the receiver  160  shown in  FIG. 1 . That is, each polarization of component of a single wavelength of light is switched to an optical hybrid to be mixed with a local oscillator, yielding multiple interference signals. The interference signals are detected using photodetectors, the outputs of which are forwarded to a DSP for symbol decoding. 
     The receiver  460 , when operating in IM mode, operates in a fashion that is a hybrid between the receiver  160  and the receiver  260  shown in  FIGS. 1 and 2 . In contrast with the receiver  260 , which includes one set of optical hybrids and PDs for each wavelength channel, in the receiver  460 , there is one set of two 90° hybrid and eight PDs for all four wavelength channels. Specifically, the receiver  460  receives a WDM optical signal, which after it splits into its constituent polarization components, the receiver  460  further splits into its component wavelengths using demultiplexers. The two constituent polarization components of each wavelength are then directed to a polarization combiner via an optical switch. These combined signals are then directed to respective photodiodes, so a different photodiode receives each of the different recombined optical signals having their respective wavelengths. The photodiode outputs are output to one or more DSPs for symbol detection. Accordingly, like the receiver  260 , the receiver  460  can carry out direct detection on a WDM optical signal, but like receiver  160 , such detection can be carried out on a single PIC without additional optical hybrids and photodetectors. 
     As discussed above, the various implementations of the dual-mode transceiver described herein include multiple optical switches. In some implementations, one or more of such switches can be implemented using an active Mach-Zehnder interferometer (MZI) switch, which includes a symmetrical MZI and a heater (or other phase shifter). Heater-based phase shifters actively control and change the refractive index of a waveguide through the thermo-optic effect. Leveraging the thermo-optic effect, the optical switch is configured to alter the optical interference at the output coupler (e.g., a 3 dB coupler) thereby switching optical power from one output port to another. For coherent operation, the optical switch in the transceiver transmitter portion can be controlled such that half of the laser source power is output to the modulator, and the other half to is output to the optical hybrids as a local oscillator signal. For IM mode transmitter operation, the optical switch can be controlled to direct all the laser power to the MZM. 
       FIGS. 5A and 5B  show schematic diagrams of an implementation of a Mach-Zehnder Interferometer (MZI) switch used in the transmitter portion and receiver portion, respectively, of a dual mode transceiver as described herein.  FIG. 5A  shows an implementation of the active MZI switch  600   a  for use in a transmitter portion, such as the transmitter portion  160  of the transceiver  100 . The MZI switch  600   a  includes a laser  630   a , a 3 dB coupler  602   a , heaters  612   a , a 3 dB coupler  604   a , a modulator  640   a  and a receiver  660   a . The laser  630   a  and the modulator  640   a  are connected on the same side of the MZI switch (bottom path) and the receiver  660   a  is connected to the other side, across from the laser  630   a  (cross path). According to some implementations, the configuration shown in  FIG. 5A  improves or maximizes the extinction ratio (ER) of the MZI switch  600   a  when operating in IM-DD mode, especially when there are imperfections in the 3 dB couplers  602   a  and  604   a.    
       FIG. 5B  shows the implementation of a MZI switch  600   b , according to an illustrative implementation.  FIG. 5B  shows an implementation of the MZI switch  600   b  suitable for use in the receiver portion of a dual-mode transceiver, such as the receiver  160  shown in  FIG. 1 . The MZI switch  600   b  includes a signal input, a 3 dB coupler  602   b , heaters  612   b , a 3 dB coupler  604   b , a PD  690   b , and an optical hybrid  680 . As shown in  FIG. 5B , an input signal and the optical hybrid  680  are connected on the same side of the MZI switch  600   b  (bottom path) and the direct detection path towards the PD  690   b  is on the other side, across from the input signal (cross path). According to some implementations, the configuration shown in  FIG. 5B  improves or maximizes the ER of the MZI switch  600   b  when operating in coherent mode, especially when there are imperfections in the 3 dB couplers. 
       FIG. 6  is a flowchart of an example method  700  of operating a dual-mode optical transceiver, according to an illustrative implementation. The method  700  includes providing a source configured for generating optical signals at stage  710 . The method  700  also includes providing a transceiver having a receiver section configured to receive both coherently modulated and intensity modulated optical signals and to be optically switched between a first receiver mode and a second receiver mode, and a transmitter section configured to be operated in a first transmission mode and a second transmission mode at stage  720 . The transmitter section can include a nested Mach-Zehnder Modulator or a polarization multiplexed quad Mach-Zehnder Modulator. 
     The method  700  also includes transmitting, via the transmitter section, in the first transmission mode an intensity modulated optical signal or in the second transmission mode a coherently modulated optical signal at stage  730 . The method  700  further includes receiving, via the receiver section, in the first receiver mode for direct detection or in the second receiver mode for coherent detection at stage  740 . In some implementations, receiving includes detecting both the coherently modulated and intensity modulated optical signals using at least one common photodiode. In some implementations, the photodiode comprises a waveguide photodiode. 
     In some implementations, the method  700  optionally includes providing a digital signal processor (DSP) for decoding the received coherently modulated and intensity modulated optical signals at stage  750 . In some implementations, the method  700  optionally includes providing an optical switch configured to selectively direct a received optical signal down a direct detection optical circuit or a coherent detection optical circuit based on a control signal applied to the optical switch at stage  760 . 
     In some implementations of the method, the nested Mach-Zehnder Modulator includes a controllable phase shifter coupled to one Mach-Zehnder Modulator in the nested Mach-Zehnder Modulator, wherein in the first transmission mode, the controllable phase shifter implements a first phase shift and in the second transmission mode, the controllable phase shifter implements a second phase shift. 
     In some implementations of the method, the controllable phase shifter comprises a heater configured to introduce a thermo-optic phase shift in the output of the one Mach-Zehnder Modulator. In some implementations, the first phase shift is a zero phase shift and the second phase shift is a π/2 phase shift. 
     In some implementations, the method  700  optionally includes providing at least one demultiplexer coupled to the receiver section for receiving a coarse wavelength division multiplexed, intensity modulated optical signal and a multiplexer coupled to the transmitter section to transmit coarse wavelength division multiplexed, intensity modulated optical signals at stage  770 . In some implementations, the transceiver is coupled to a controller configured to cause the transceiver to switch between receiver and transmission modes. 
     The technology described herein has advantageous benefits. For example, by creating a transceiver that is backward compatible, data center networking costs can be significantly reduced over the long term. In addition, an integrated optical switch is employed to redirect optical power in a photonic integrated circuit to achieve coherent and IM operations using shared components. The use of shared components allows for smaller form factors and further reduced cost. Moreover, the technology described herein includes a fully integrated solution using a single photonic circuit that can function both as a coherent transceiver and as a IM-DD transceiver. As a result, this coherent transceiver can offer backward compatibility with a traditional PAM transceiver. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, 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. 
     References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. The labels “first,” “second,” “third,” and so forth are not necessarily meant to indicate an ordering and are generally used merely to distinguish between like or similar items or elements. 
     Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein.