阅读说明：本技术 相干/im-dd双操作光收发器 (coherent/IM-DD dual operation optical transceiver ) 是由 李文藻 浦田良平 周翔 利芬·维尔斯莱格斯 于 2020-02-13 设计创作，主要内容包括：本文公开了一种双模式光收发器。双模式光收发器包括：接收器部分,被配置成接收经相干调制和经强度调制的光信号两者,并且在用于直接检测的第一接收器模式和用于相干检测的第二接收器模式之间进行光切换；以及包括嵌套式马赫曾德尔调制器或偏振复用四马赫曾德尔调制器的发射器部分,被配置成在第一传输模式下操作以输出经强度调制的光信号,在第二传输模式下操作以输出经相干调制的光信号。在一些实施方式中,双模式接收器包括光开关,光开关被配置成基于施加到光开关的控制信号选择性地将接收到的光信号向下引导到直接检测光电路或相干检测光电路。(A dual mode optical transceiver is disclosed. The dual mode optical transceiver includes: a receiver portion configured to receive both the coherently modulated and intensity modulated optical signals and to optically switch between a first receiver mode for direct detection and a second receiver mode for coherent detection; and a transmitter section comprising nested mach-zehnder modulators or polarization multiplexed four-mach-zehnder modulators configured to operate in a first transmission mode to output an intensity modulated optical signal and in a second transmission mode to output a coherently modulated optical signal. In some embodiments, a dual mode receiver includes an optical switch configured to selectively direct a received optical signal down to a direct detection optical circuit or a coherent detection optical circuit based on a control signal applied to the optical switch.)

1. A photonic integrated chip, PIC, comprising:

a receiver portion configured to receive both the coherently modulated and intensity modulated optical signals and to optically switch between a first receiver mode for direct detection and a second receiver mode for coherent detection;

a transmitter section comprising nested mach-zehnder modulators or polarization multiplexed four-mach-zehnder modulators configured to operate in a first transmission mode to output an intensity modulated optical signal and in a second transmission mode to output a coherently modulated optical signal.

2. The PIC of claim 1, wherein the receiver portion detects both the coherently modulated and intensity modulated optical signals using at least one common photodiode.

3. The PIC of claim 2, wherein the photodiode comprises a waveguide photodiode.

4. The PIC of claim 1, coupled to a Digital Signal Processor (DSP) to decode the received coherently modulated and intensity modulated optical signals.

5. The PIC of claim 1, comprising an optical switch configured to selectively direct a received optical signal down to a direct detection optical circuit or a coherent detection optical circuit based on a control signal applied to the optical switch.

6. The PIC of claim 1, wherein the nested mach-zehnder modulators comprise a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in the first transmission mode the controllable phase shifter achieves a first phase shift and in the second transmission mode the controllable phase shifter achieves a second phase shift.

7. The PIC of claim 6, wherein the controllable phase shifter comprises a heater configured to introduce a thermo-optic phase shift in the output of the one Mach-Zehnder modulator.

8. The PIC of claim 6, wherein the first phase shift is a zero phase shift and the second phase shift is a pi/2 phase shift.

9. The PIC of claim 1, comprising: at least one demultiplexer coupled to the receiver portion to receive the coarse wavelength division multiplexed, intensity modulated optical signals; and a multiplexer coupled to the transmitter portion to transmit the coarse wavelength division multiplexed, intensity modulated optical signal.

10. The PIC of claim 1, wherein the PIC is coupled to a controller configured to switch the PIC between a receiver mode and a transmission mode.

11. A method of operation, comprising:

providing a source configured to generate an optical signal;

providing a transceiver, the transceiver comprising:

a receiver portion configured to receive both the coherently modulated and intensity modulated optical signals and to optically switch between a first receiver mode and a second receiver mode, an

A transmitter section comprising nested Mach-Zehnder modulators or polarization multiplexed four Mach-Zehnder modulators configured to operate in a first transmission mode and a second transmission mode;

transmitting, via the transmitter portion, an intensity-modulated optical signal in the first transmission mode or a coherently-modulated optical signal in the second transmission mode; and

receiving, via the receiver portion, an optical signal in the first receiver mode for direct detection or in the second receiver mode for coherent detection.

12. The method of claim 11, wherein receiving comprises detecting both the coherently modulated and intensity modulated optical signals using at least one common photodiode.

13. The method of claim 12, wherein the photodiode comprises a waveguide photodiode.

14. The method of claim 11, comprising decoding the received coherently modulated and intensity modulated optical signals using a Digital Signal Processor (DSP).

15. The method of claim 11, comprising selectively directing the received optical signal down to a direct detection optical circuit or a coherent detection optical circuit using the optical switch based on a control signal applied to the optical switch.

16. The method of claim 11, wherein the nested mach-zehnder modulators include a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in the first transmission mode the controllable phase shifter achieves a first phase shift and in the second transmission mode the controllable phase shifter achieves a second phase shift.

17. The method of claim 16, wherein the controllable phase shifter comprises a heater configured to introduce a thermo-optic phase shift in the output of the one mach-zehnder modulator.

18. The method of claim 16, wherein the first phase shift is a zero phase shift and the second phase shift is a pi/2 phase shift.

19. The method of claim 11, comprising: demultiplexing the received coarsely wavelength division multiplexed, intensity modulated optical signals; and transmitting the coarse wavelength division multiplexed, intensity modulated optical signal.

20. The method of claim 11, further comprising switching between a receiver mode and a transmission mode in response to a controller command.

21. A transceiver, comprising:

a receiver portion configured to receive both coherently modulated and intensity modulated optical signals;

a transmitter portion comprising an optical modulator configured to operate in a first transmission mode to output an intensity modulated optical signal and to operate in 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.

22. The transceiver of claim 21, wherein the optical modulator comprises a nested mach-zehnder modulator or a polarization multiplexed four mach-zehnder modulator.

23. The transceiver of claim 22, wherein the nested mach-zehnder modulators comprise a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in the first transmission mode the controllable phase shifter achieves a first phase shift and in the second transmission mode the controllable phase shifter achieves a second phase shift.

24. The transceiver of claim 21, the receiver portion configured to optically switch between a first receiver mode for direct detection and a second receiver mode for coherent detection.

25. The transceiver of claim 21, wherein the transceiver is coupled to a controller configured to switch the transceiver between a receiver mode and a transmission mode.

Background

The optical fiber provides high bandwidth Data Center Interconnect (DCI) for the data center network. Existing intra-data center interconnect technologies use Intensity Modulation (IM) and Direction Detection (DD) techniques. However, as data consumption increases, existing IM-DD methods do not scale well as bandwidth grows. The continued increase in DCI bandwidth requirements and consumption (e.g., using IM-DD to support data transmission in excess of 100Gb/s per wavelength) can be technically challenging and expensive to implement.

Coherent optics is an alternative approach to high bandwidth DCI suitable for data center networks. However, due to the evolving nature of data center networks, unverified next generation technologies (such as those based on coherent optics) must be backward compatible with existing technologies for large scale data center networks. It can be expensive and impractical to implement a partial and seamless upgrade of a network without having to upgrade the entire data center all at once. As a result, it remains a challenge how to make coherent optical technology backward compatible with existing IM-DD technology to better bridge current and future technologies.

Disclosure of Invention

At least one aspect relates to a Photonic Integrated Chip (PIC). The PIC includes a receiver portion configured to receive both the coherently modulated and intensity modulated optical signals and to optically switch between a first receiver mode for direct detection and a second receiver mode for coherent detection. The PIC also includes a transmitter section comprising nested mach-zehnder modulators or polarization multiplexed quad mach-zehnder modulators configured to operate in a first transmission mode to output an intensity modulated optical signal and to operate in a second transmission mode to output a coherently modulated optical signal.

In some embodiments, the receiver portion detects both the coherently modulated and intensity modulated optical signals using at least one common photodiode. In some embodiments, the photodiode comprises a waveguide photodiode.

In some embodiments, the PIC is coupled to a Digital Signal Processor (DSP) to decode the received coherently modulated and intensity modulated optical signals. In some embodiments, the PIC further includes an optical switch configured to selectively direct a received optical signal down to either the direct detection optical circuit or the coherent detection optical circuit based on a control signal applied to the optical switch.

In some embodiments, the nested mach-zehnder modulators include a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in a first transmission mode the controllable phase shifter achieves a first phase shift and in a second transmission mode the controllable phase shifter achieves a second phase shift.

In some embodiments, the controllable phase shifter includes a heater configured to introduce a thermo-optic phase shift in the output of one of the mach-zehnder modulators. In some embodiments, the first phase shift is a zero phase shift and the second phase shift is a pi/2 phase shift.

In some embodiments, the PIC further comprises: at least one demultiplexer coupled to the receiver section to receive the coarse wavelength division multiplexed, intensity modulated optical signals; and a multiplexer coupled to the transmitter portion to transmit the wavelength division multiplexed, intensity modulated optical signals.

In some embodiments, the PIC is coupled to a controller configured to switch the PIC between a receiver mode and a transmission mode.

At least one aspect relates to a method of operation. The method comprises the following steps: a source configured to generate an optical signal is provided, and a transceiver is provided. The transceiver includes: a receiver portion configured to receive both coherently modulated and intensity modulated optical signals and to optically switch between a first receiver mode and a second receiver mode; a transmitter section comprising nested Mach-Zehnder modulators or polarization multiplexed four Mach-Zehnder modulators configured to operate in a first transmission mode and a second transmission mode. The method also includes transmitting, via the transmitter portion, the intensity-modulated optical signal in a first transmission mode or the coherently-modulated optical signal in a second transmission mode. The method further includes receiving, via the receiver portion, the optical signal in a first receiver mode for direct detection or in a second receiver mode for coherent detection.

In some embodiments, receiving comprises detecting both the coherently modulated and intensity modulated optical signals using at least one common photodiode. In some embodiments, the photodiode comprises a waveguide photodiode.

In some embodiments, the method further comprises decoding the received coherently modulated and intensity modulated optical signals using a Digital Signal Processor (DSP). In some embodiments, the method further comprises selectively directing the received optical signal down to a direct detection optical circuit or a coherent detection optical circuit using the optical switch based on a control signal applied to the optical switch.

In some embodiments of the method, the nested mach-zehnder modulators include a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in the first transmission mode the controllable phase shifter achieves a first phase shift and in the second transmission mode the controllable phase shifter achieves a second phase shift.

In some embodiments of the method, the controllable phase shifter includes a heater configured to introduce a thermo-optic phase shift in an output of one of the mach-zehnder modulators. In some embodiments, the first phase shift is a zero phase shift and the second phase shift is a pi/2 phase shift.

In some embodiments, the method further comprises: demultiplexing the received coarsely wavelength division multiplexed, intensity modulated optical signals; and transmits the coarse wavelength division multiplexed, intensity modulated optical signal. In some embodiments, the method further comprises switching between the receiver mode and the transmission mode in response to a controller command.

At least one aspect relates to a transceiver. The transceiver includes: a receiver portion configured to receive both coherently modulated and intensity modulated optical signals; a transmitter portion comprising an optical modulator configured to operate in a first transmission mode to output an intensity modulated optical signal and to operate in a second transmission mode to output a coherently modulated optical signal; and a Digital Signal Processor (DSP) to decode the received coherently modulated and intensity modulated optical signals.

In some embodiments, the optical modulator comprises nested mach-zehnder modulators or polarization multiplexed four-mach-zehnder modulators. In some embodiments, the nested mach-zehnder modulators include a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in a first transmission mode the controllable phase shifter achieves a first phase shift and in a second transmission mode the controllable phase shifter achieves a second phase shift.

In some embodiments, the receiver portion is configured to optically switch between a first receiver mode for direct detection and a second receiver mode for coherent detection. In some embodiments, the transceiver is coupled to a controller configured to switch the transceiver between a receiver mode and a transmission mode.

These and other aspects and embodiments are discussed in detail below. The foregoing information and the following detailed description include illustrative examples of various aspects and embodiments, and provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. The accompanying drawings provide an illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification.

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 illustrates a schematic diagram of an embodiment of a single channel dual mode optical transceiver in accordance with an illustrative embodiment;

FIG. 2 illustrates a schematic diagram of another embodiment of a wavelength division multiplexed dual mode optical transceiver in accordance with an illustrative embodiment;

FIGS. 3A, 3B, and 3C show schematic diagrams illustrating operating schemes of nested Mach-Zehnder modulators in accordance with an illustrative embodiment;

FIG. 4 illustrates a schematic diagram of another embodiment of a wavelength division multiplexing dual mode optical transceiver in accordance with an illustrative embodiment;

FIGS. 5A and 5B show schematic diagrams of example embodiments of Mach-Zehnder interferometer optical switches; and

FIG. 6 is a flowchart of an example method of operating a dual mode optical transceiver in accordance with an illustrative embodiment.

Detailed Description

In the IM-DD method currently deployed in a data center network, a transmitted optical signal is modulated in a non-return-to-zero (NRZ) on-off keying (OOK) format. In some recently developed IM-DD schemes, the optical signal is modulated using a 4-level pulse amplitude modulation (PAM-4) format. In current solutions, the transmitted optical signal strength is typically received by a Photodetector (PD) in the optical network. For long-range (LR) optical links using wavelengths in the O-band (about 1310nm), typically covering a distance of 10km, most current IM-DD methods use a simple and low cost solution.

Unlike the IM-DD method, the coherent optical method is based on modulating the signal onto the amplitude and phase of the optical wave, rather than on the optical intensity. In the coherent method, the transmitted optical signal is received by coherent detection, where the signal is mixed with a second optical wave, called Local Oscillator (LO), by a combination of different phase delays between the signal and the LO, and finally detected on multiple PDs. The photocurrent detected in the PD is then processed by a Digital Signal Processor (DSP) to demodulate the signal in the receiver unit.

Current coherent optical communication technologies are mainly used in long-haul, undersea and, more recently, metropolitan networks due to their benefits of high spectral efficiency, high sensitivity and resiliency to fiber transmission impairments. Due to the availability of Erbium Doped Fiber Amplifiers (EDFAs), optical wavelengths are typically within the C-band (around 1550 nm). Despite the advantages of coherent optics, its high complexity, high cost and incompatible operating wavelengths make it impractical for implementation in currently existing intra-data center interconnects.

Due to advances in Photonic Integrated Circuit (PIC) technology, a low cost, highly integrated coherent optical transceiver may be developed for intra-data center interconnects. However, if backward compatibility with existing IM-DD technology cannot be ensured, integration of PIC and coherent optical receiver technology cannot be implemented in a data center network. The challenge of making coherent optical technology backward compatible with existing IM-DD technology and the need to bridge current and future technologies has ultimately led to the development of the transceiver technology disclosed herein.

As described in various embodiments and implementations herein, the dual mode optical transceiver and related techniques disclosed in the present application may be implemented in a data center network and may be seamlessly integrated with both existing IM-DD technology and coherent optical technology. The present disclosure relates to a dual mode optical transceiver supporting both IM-DD and coherent optical technologies (IM-DD/coherent transceiver), and a method of operating an IM-DD/coherent transceiver. In particular, the present disclosure relates to an IM-DD/coherent transceiver having a receiver portion configured to receive both Intensity Modulated (IM) and Coherently Modulated (CM) optical signals. In some embodiments, an IM-DD/coherent transceiver may be optically switched between a first receiver mode for Direct Detection (DD) and a second receiver mode for Coherent Detection (CD). The present disclosure also relates to an IM-DD/coherent transceiver having a transmitter portion configured to transmit both IM and CM optical signals. In some embodiments, the IM-DD/coherent transceiver includes nested mach-zehnder modulators or polarization multiplexed quad mach-zehnder modulators configured to operate in a first transmission mode to output the IM optical signal and in a second transmission mode to output the CM optical signal.

Fig. 1 shows a schematic diagram of a single channel dual mode optical transceiver 100 in accordance with an illustrative embodiment. The transceiver 100 shown in fig. 1 includes a Photonic Integrated Circuit (PIC)110, the PIC 110 being connected to a Digital Signal Processor (DSP)108 via transimpedance amplifiers (TIAs) 106a 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, an optical splitter 122, and an optical modulator 140. Receiver 160 further includes a polarization beam splitting rotator (PSR)162, optical switches 164a and 164b, VOAs 166a and 166b, PSR 165, two 90 degree optical hybrids 180a and 180b, four Photodiodes (PDs) 190a (i-iv), and four PDs 190b (i-iv).

In some implementations, the PIC 110 may include components in the DSP 108 and/or TIAs 106a and 106b, as well as the transmitter 120 and receiver 160. In other words, according to some embodiments, the transmitter 100 may include all of the components shown in fig. 1. In some implementations, the PIC may include TIAs 106a and 106b, transmitter 120, and receiver 160.

In some implementations, the TIAs 106a and 106b may include any impedance amplifier suitable for amplifying current. In some implementations, TIAs 106a and 106b are four-channel TIAs for coherent receivers.

In some implementations, the DSP 108 may include any digital signal processor suitable for the application. The DSP 108 is configured to perform processing functions used in both IM and CM transmissions and receptions, such as converting digital signals to modulator drive signals for both IM and CM transmissions, and symbol recovery for IM and CM receptions. In some embodiments, transceiver 100 includes a separate integrated circuit processor 115, such as an ASIC, FPGA, or microprocessor, to perform the control functions described herein.

In some embodiments, the transmitter 120 of the transceiver 100 is configured to operate in a first transmission mode to output an intensity modulated optical signal and in a second transmission mode to output a coherently modulated optical signal. In some embodiments, the receiver 160 of the transceiver 100 is configured to receive both the coherently modulated and intensity modulated optical signals and to optically switch between a first receiver mode for directly detecting the intensity modulated signals and a second receiver mode for coherently detecting the coherently modulated optical signals.

As shown in fig. 1, the emitter 120 includes a laser source 130 configured to provide laser light. Transmitter 120 also includes an optical switch 124, a VOA 126, an optical splitter 122, and an optical modulator 140 in which the laser light is manipulated as it is transmitted through these components. In some embodiments, laser light from the laser source 130 is transmitted into the optical switch 124, and the optical switch 124 is configured to controllably distribute the optical power of the laser according to the operating mode of the transmitter 120. For example, when operating in an IM mode (also referred to herein as a "Pulse Amplitude Modulation (PAM) mode"), the optical switch may be controlled to direct all light output by laser source 130 to modulator 140. When operating in CM mode (also referred to herein as "Quadrature Amplitude Modulation (QAM) mode"), the optical switch may be controlled to split the light emitted by the laser source 130 between the modulator 140 and the optical hybrids 180a and 180b to serve as local oscillators. In some embodiments, in CM mode, optical switch 124: 90. 30: 70. 50: 50. 70: either 30 or 90:10 or any ratio therebetween, distributes the optical power to the modulator 140 and the 90 ° optical hybrids 180a and 180 b.

In some embodiments, the laser source 130 is integrated into the emitter 120. In some implementations, the laser source 130 is a separate unit, die, or module attached to the emitter 120. In some implementations, the laser source 130 is integrated in the PIC 110. In some implementations, the laser source 130 is a separate unit, die, or module attached to the PIC 110.

In some embodiments, the light mixers 180a and 180b may comprise 90 ° light mixers. Those skilled in the art will appreciate that a 90 ° optical hybrid is an optical assembly that produces four interference signals (each 90 ° apart) by combining two optical signals together, imparting four different phase delays on one of the signals, and thus, includes four photodiodes 190a (i-iv) and 190b (i-iv) coupled to each optical hybrid 180a and 180 b. In some embodiments, a 90 ° optical hybrid with single-ended detection may be used instead, thereby generating two interference signals. In such embodiments, only two photodiodes may be required per optical hybrid.

In some embodiments, optical modulator 140 is a nested Mach-Zehnder modulator (n-MZM). In some embodiments, a mach-zehnder modulator (MZM) may be used for intensity modulation. In some embodiments, two MZMs may be used in parallel as n-MZMs, and the n-MZMs may be used for coherent modulation. In some implementations, optical modulator 140 (e.g., n-MZM) is configured to perform both IM and CM.

As shown in fig. 1, the receiver 160 includes a PSR 162, the PSR 162 configured to receive the transmitted optical signal and to separate the optical signal into two polarization components. The receiver 160 also includes optical switches 164a and 164b, VOAs 166a and 166b, PSR 165, and optical hybrids 180a and 180 b. The optical hybrid 180a is connected to the PD 190a (i-iv), and the optical hybrid 180b is connected to the PD 190b (i-iv). As shown in fig. 1, each of the four PDs 190a (i-iv) and 190b (i-iv) is connected to one of the TIAs 106a or 106b connected to the DSP 108. In some embodiments, the PD may be a balanced dual input waveguide photodetector. In some embodiments, the PD may be a single-ended photodiode. The use of single-ended photodiodes may improve yield, but may be at the expense of sensitivity (about 3dB), and lack the common mode rejection capability provided by balanced photodetectors.

As disclosed herein and shown in fig. 1, transceiver 100 is configured to operate in both IM mode and CM mode. When receiver 100 is configured to operate in IM mode, the optical path through PIC 110 is referred to as an IM (for transmission) or DD (for reception) optical path. When the transceiver 100 is configured to operate in CM mode, the optical path through the PIC 110 is referred to as a coherent optical path.

As shown in fig. 1, when operating in CM mode, the coherent path for transmission begins with a laser source 130, the laser source 130 generating and outputting 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 to the optical mixers 180a and 180b via the VOA 126 and the beam splitter 122. The laser is used as a local oscillator for coherent detection of the coherently modulated received optical signal. The remaining light output by the laser source 130 can travel through the PIC 110 to the optical modulator 140. The modulator 140 coherently modulates the light and transmits the modulated light out to an output fiber.

As shown in fig. 1, when operating in IM mode for transmission in transceiver 100, optical switch 124 is controlled to pass all of the optical energy of the light emitted by laser source 130 to optical modulator 140. The optical modulator 140 modulates the laser light via intensity modulation and then transmits the modulated light out to an output optical fiber. Thus, light output onto the output fiber in both IM and CM modes travels the same optical path through the same optical components.

With respect to the optical signals received by transceiver 100, as shown in fig. 1, in the IM and CM modes of operation, the corresponding optical paths begin at PSR 162, PSR 162 splits the light into two constituent polarizations, passing each polarization component to a corresponding optical switch 164a or 164 b. At the optical switches 164a and 164b, the path traversed by the light varies based on the mode of operation of the transceiver 100. In the IM mode, the optical switches 164a and 164b are controlled to direct the light received at each optical switch to the PSR 165, the PSR 165 serving to recombine the two polarization components of the light of the received signal and direct them to one of the photodiodes (e.g., 190a (i)) of one of the optical mixers (e.g., 180 a). In CM mode, the control light switches 164a and 164b are controlled to direct light at the corresponding light switches 164a and 164b to the corresponding light mixers 180a and 180b via VOAs 166a and 166 b. In the optical hybrid, each polarization component signal is combined with a local oscillation optical signal that is delayed by various phase delays. For example, in the embodiment shown in fig. 1, where the optical hybrid is a 90 ° optical hybrid, the polarization component signals are combined with 4 local oscillator signals, each 90 ° apart in phase. The resulting interference signals are detected by corresponding photodiodes 190a (i-iv) and 190b (i-iv). The electrical output of the photodiode is fed through TIAs 106a and 106b to DSP 108 for symbol recovery.

As described above, in some embodiments, the PIC 110 is coupled to a controller separate from the DSP, and one or more drivers configured to control the optical switches 124, 164a, and 164b, the VOAs 126, 166a, and 166b, and the optical modulators of the PIC 110. In some embodiments, the controller may be implemented as, for example, a microcontroller unit, an integrated circuit logic unit, or a microprocessor controlled by software.

In some embodiments, the dual mode optical transceiver 100 operating in the single channel IM-DD/coherent transceiver configuration shown in fig. 1 may be used as the basis for a multi-channel dual mode transceiver using Parallel Single Mode (PSM) or Wavelength Division Multiplexing (WDM) techniques.

FIG. 2 illustrates a schematic diagram of one embodiment of a multi-channel dual mode optical transceiver 200 in accordance with an illustrative embodiment. As shown in fig. 2, dual mode transceiver 200 is configured to operate as an n-wavelength (n λ) WDM transceiver implemented in a multi-channel IM-DD/coherent transceiver configuration. As shown in fig. 2, transceiver 200 includes a plurality of PICs 210A, 210B, 210C, and 210D (collectively PICs 210). In some embodiments, a multi-channel or WDM transceiver configuration may be implemented on a single PIC rather than on multiple PICs, or on one PIC per channel. As shown in fig. 2, each PIC 210 in transceiver 200 is configured to transmit and receive optical signals at a different wavelength than the other PICs 210 in the transceiver. As shown in fig. 2, each of the PICs 210A, 210B, 210C, and 210D is configured substantially similar to the PIC 110 shown in fig. 1, with like reference numbers corresponding to substantially similar components. For example, optical switches 224, 264a, and 264b in fig. 2 correspond to optical switches 124, 164a, and 164b in fig. 1. Similarly, VOAs 226, 266a, and 266b correspond to VOAs 126, 166a, and 166b in fig. 1; light modulator 240 in fig. 2 corresponds to light modulator 140 in fig. 1, and so on. Although each PIC 210 has its own laser source 230 associated with it (integrated or optically coupled), each laser source 230 is configured to output light at a different wavelength. Thus, transceiver 200 may controllably switch between operating in CM mode or IM mode. In some embodiments, some PICs may operate in IM mode, while other PICs may operate in CM mode. In some other embodiments, all PICs operate in the same mode, i.e., either IM mode or CM mode, at any given time.

Although most of the components shown in fig. 1 are duplicated in each of the PICs 210A-210D, in some embodiments, the transceiver 200 may include only a single PSR 262 for the optical signal, which separates the received WDM signal into its constituent polarization components, which are then separated by wavelength by corresponding demultiplexers 263a and 263b, which direct the wavelength-specific signals to the corresponding PICs 210. In some embodiments, transceiver 200 may include a single demultiplexer and separate PSRs 262 for each PIC. In addition to demultiplexers 263a and 263b, transceiver 200 also includes multiplexer 228 to combine the outputs of modulators 240 of corresponding PICs 210 into a combined WDM output optical signal. The transceiver may have a single DSP to process the amplified electrical outputs of all the PICs 210, or it may include multiple DSPs to process the electrical outputs of individual PICs 210 or a subset thereof.

Fig. 3A, 3B, and 3C show schematic diagrams illustrating an operating scheme of a nested mach-zehnder modulator (n-MZM)340 used as the optical modulator 140 or 240 shown in fig. 1 and 2. FIG. 3A shows an operational scheme 300a for an n-MZM 340 for standard coherent modulation of a laser. As shown in FIG. 3A, continuous wave signals are input across MZM 342 and MZM 344 of n-MZM 340. Both MZMs 342 and 344 are biased at their zero points and are driven by corresponding I and Q drive signals. The output of the MZM 344 driven by the Q drive signal experiences a phase delay (e.g., a π/2 phase delay) before being combined with the output of the MZM 342 driven by the I drive signal.

FIG. 3B shows an example operating scheme 300B for generating an IM signal using the same n-MZM 340. Operating scheme 300b drives both MZMs 342 and 344 using the same electrical drive signal, a Pulse Amplitude Modulation (PAM) drive signal. Both MZMs 342 and 344 are biased at their corresponding quadrature points. Thus, the outputs from both MZMs 342 and 344 are the same IM signal. As shown in FIG. 3B, the relative phase shift between the outputs of the two MZMs 342 and 344 is 0. Thus, two identical IM signals are constructively combined to form a single IM signal, as shown in fig. 3B.

FIG. 3C shows an alternative operating scheme 300C for generating an IM signal using the same n-MZM 340. The operating scheme 300c drives one of the MZMs 342 and 344 with a PAM drive signal and zero bias. The other of the MZMs 342 and 344 is not driven and is based on its maximum value. The relative phase shift between the outputs of the two MZMs 342 and 344 is 0. The combined output is again a single PAM signal.

In some embodiments, the phase shifter on the MZM 344 may be implemented by placing a heater near the optical waveguide, and then using the thermo-optic effect by the heater to control the phase of the optical signal. As described above, in the CM mode, the phase shifter is controlled to achieve a pi/2 phase shift between the two MZMs 342 and 344. In some embodiments, in the IM mode, the phase shifter is controlled to achieve a 0 phase shift between the two MZMs 342 and 344.

Fig. 4 illustrates a schematic diagram of another embodiment of a wavelength division multiplexing dual mode optical transceiver 400 in accordance with an illustrative embodiment. Transceiver 400 is configured for dual operation 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 receiver 460 connected to the DSP 408 via the TIAs 406a and 406 b. Receiver 460 is similar to receiver 160. However, the configuration of the transmitter 420 is different from the transmitters 120 and 220.

As shown in fig. 4, transmitter 420 includes four laser sources 430a, 430b, 430c, and 430d (each outputting a different wavelength), an optical switch 424, an optical splitter 422, an optical modulator 440 including four MZMs 442, 444, 446, and 448, a MUX 428, two additional optical switches 452 and 454, and a PSR 456. In particular, transmitter 420 uses optical modulator 440, which is a polarization-multiplexed four mach zehnder modulator (PM-QMZM)440, in place of n-MZM 140, 240, or 340 as shown in fig. 1-3. The configuration shown in fig. 4 utilizes a scheme where each of the 4 MZMs 442, 444, 446 and 448 in 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 mixers, and eight PDs. In particular, the PD is a dual input waveguide PD, and four of them have one end connected to the coherent signal input and the other end connected to the intensity modulated signal input.

When operating in the CM mode of transceiver 400, a single laser (e.g., 430b) is turned on and its output is split between the four MZMs of PM-QMZM 440 via an optically switched optical splitter. For each constituent polarization of the laser optical output, one portion is sent to the upper arm of the MZM and one portion is sent to the lower portion of the other MZM. The phase delay is established on the output of the MZM that receives the optical signal at the upper arm of the MZM. Each of the four MZMs is then driven with a respective I or Q drive signal while being biased at their respective zero points. The outputs of the MZMs are then combined for output on the optical fibre. In CM mode, a portion of the light output by laser source 430b is also directed to receiver portion 460 of transceiver 400 to be used as a local oscillator signal. In this example 430a, 430c, and 430d, the remaining three lasers may remain off.

When operating in the IM-DD CWDM4 mode, all four lasers of different wavelengths are input into the four MZMs of PM-QMZM 440, which are driven by independent PAM drive signals while being biased at quadrature points like a conventional IM MZM. The outputs of the MZMs are then multiplexed together via multiplexer 428 and then switched onto the fibre via optical switch 452.

As shown in fig. 4, when operating in CM mode, receiver 460 operates similarly to receiver 160 shown in fig. 1. That is, each polarization component of the light of a single wavelength is switched to the optical hybrid to be mixed with the local oscillator, thereby generating a plurality of interference signals. The interference signal is detected using a photodetector, the output of which is forwarded to the DSP for symbol decoding.

When operating in IM mode, receiver 460 operates in a hybrid manner between receiver 160 and receiver 260 shown in fig. 1 and 2. In contrast to the receiver 260, which comprises a set of optical mixers and PDs for each wavelength channel, in the receiver 460 there is a set of two 90 ° mixers and eight PDs for all four wavelength channels. Specifically, the receiver 460 receives the WDM optical signal, and after the WDM optical signal is separated into its constituent polarization components, the receiver 460 is further separated into its component wavelengths using a demultiplexer. The two component polarization components for each wavelength are then directed to a polarization combiner via an optical switch. These combined signals are then directed to the corresponding photodiodes so that each different recombined optical signal having its corresponding wavelength is received by a different photodiode. The photodiode outputs are output to one or more DSPs for symbol detection. Thus, like receiver 260, receiver 460 may perform direct detection on WDM optical signals, but like receiver 160, such detection may be performed on a single PIC without the need for an additional optical hybrid and photodetector.

As discussed above, various embodiments of the dual mode transceivers described herein include a plurality of optical switches. In some embodiments, one or more such switches may be implemented using active mach-zehnder interferometer (MZI) switches that include symmetric MZIs and heaters (or other phase shifters). Heater-based phase shifters actively control and change the refractive index of the waveguide through the thermo-optic effect. The optical switch is configured to change optical interference at an output coupler (e.g., a 3dB coupler) using thermo-optic effects, thereby switching optical power from one output port to another. For coherent operation, the optical switches in the transmitter portion of the transceiver may be controlled such that half of the laser source power is output to the modulator and the other half is output as a local oscillator signal to the optical hybrid. For transmitter operation in IM mode, the optical switch can be controlled to direct all laser power to the MZM.

Fig. 5A and 5B show schematic diagrams of embodiments of mach-zehnder interferometer (MZI) switches used in the transmitter and receiver sections, respectively, in the dual-mode transceivers described herein. Fig. 5A shows an embodiment of an active MZI switch 600a for use in a transmitter section, such as transmitter section 160 of transceiver 100. MZI switch 600a comprises a laser 630a, a 3dB coupler 602a, a heater 612a, a 3dB coupler 604a, a modulator 640a, and a receiver 660 a. Laser 630a and modulator 640a are connected on the same side of the MZI switch (bottom path), while receiver 660a is connected on the other side, opposite laser 630a (cross path). According to some embodiments, the configuration shown in FIG. 5A improves or maximizes the Extinction Ratio (ER) of MZI switch 600a when operating in IM-DD mode, particularly when there are defects in 3dB couplers 602a and 604 a.

FIG. 5B shows an embodiment of MZI switch 600B according to an illustrative embodiment. Fig. 5B illustrates an embodiment of a MZI switch 600B suitable for use in a receiver portion of a dual-mode transceiver, such as the receiver 160 shown in fig. 1. MZI switch 600b includes a signal input, 3dB coupler 602b, heater 612b, 3dB coupler 604b, PD 690b, and optical hybrid 680. As shown in fig. 5B, the input signal and optical hybrid 680 is connected on the same side of MZI switch 600B (bottom path) and the direct detection path toward PD 690B is on the other side, opposite the input signal (cross path). According to some embodiments, the configuration shown in FIG. 5B improves or maximizes the ER of MZI switch 600B when operating in a coherent mode, particularly when there is a defect in the 3dB coupler.

Fig. 6 is a flowchart of an exemplary method 700 of operating a dual mode optical receiver in accordance with an illustrative embodiment. Method 700 includes, at stage 710, providing a source configured to generate an optical signal. The method 700 further includes providing a transceiver having a receiver portion and a transmitter portion, the receiver portion configured to receive the coherently modulated and intensity modulated optical signals and optically switch between a first receiver mode and a second receiver mode, and the transmitter portion configured to operate in a first transmission mode and a second transmission mode at stage 720. The transmitter section may include nested mach-zehnder modulators or polarization multiplexed four-mach-zehnder modulators.

The method 700 further comprises transmitting the intensity modulated optical signal in a first transmission mode or the coherently modulated optical signal in a second transmission mode via the transmitter portion at stage 730. Method 700 further includes receiving in a first receiver mode for direct detection via the receiver portion or receiving in a second receiver mode for coherent detection at stage 740. In some embodiments, receiving comprises detecting both the coherently modulated and intensity modulated optical signals using at least one common photodiode. In some embodiments, the photodiode comprises a waveguide photodiode.

In some embodiments, method 700 optionally includes providing a Digital Signal Processor (DSP) to decode the received coherently modulated and intensity modulated optical signals at stage 750. In some embodiments, method 700 optionally includes providing an optical switch at stage 760 to selectively direct a received optical signal down to a direct detection optical circuit or a coherent detection optical circuit based on a control signal applied to the optical switch.

In some embodiments of the method, the nested mach-zehnder modulators include a controllable phase shifter coupled to one of the nested mach-zehnder modulators, wherein in the first transmission mode the controllable phase shifter achieves a first phase shift and in the second transmission mode the controllable phase shifter achieves a second phase shift.

In some embodiments of the method, the controllable phase shifter includes a heater configured to introduce a thermo-optic phase shift in an output of one of the mach-zehnder modulators. In some embodiments, the first phase shift is a zero phase shift and the second phase shift is a pi/2 phase shift.

In some embodiments, method 700 optionally includes providing at stage 770: at least one demultiplexer coupled to the receiver section to receive the coarse wavelength division multiplexed, intensity modulated optical signal; and a multiplexer coupled to the transmitter section to transmit the coarse wavelength division multiplexed, intensity modulated optical signal. In some embodiments, the transceiver is coupled to a controller configured to cause the transceiver to switch between a receiver mode and a transmission mode.

The techniques described herein have advantageous benefits. For example, by creating backward compatible transceivers, the long term network cost of a data center can be significantly reduced. In addition, integrated optical switches are used to redirect optical power in photonic integrated circuits to achieve coherent and IM operation using shared components. The use of shared components allows for smaller form factors and further reduces costs. Furthermore, the techniques described herein include a fully integrated solution using a single photonic circuit that can be used as both a coherent transceiver and an IM-DD transceiver. As a result, the coherent transceiver may provide backward compatibility with conventional PAM transceivers.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention 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 embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. 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 subcombination or variation of a subcombination.

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 some cases, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the program components and systems can generally be integrated together in a single software product or can be packaged into multiple software products.

References to "or" may be construed as inclusive such that any term described using "or" may refer to any single, more than one, or all of the described terms. The labels "first", "second", "third", etc. do not necessarily denote an order, and are generally only used to distinguish between the same or similar items or elements.

Various modifications to the embodiments described in this disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the claims are not intended to be limited to the embodiments shown herein but are to be accorded the widest scope consistent with the present disclosure, principles and novel features disclosed herein.