Coherent receiver with low VOA-induced phase change

Abstract

A coherent receiver (10) comprising: a receive signal (SIG) path (14) comprising: i) an input configured to connect a receive signal, ii) one or more signal paths connected to the input and one or more optical mixers (32), and iii) at least one variable optical attenuator (VOA) (12); a local oscillator (LO) signal path (16) comprising an input configured to be connected to the LO and one or more optical mixers (32); and a VOA-induced phase change compensation mechanism that reduces any VOA-induced phase change in the SIG-LO beat signal due to the at least one VOA (12).

Description

Technical Field

[0001] The present disclosure relates generally to components of optical networking. More specifically, the present disclosure relates to systems and methods for coherent receivers with low variable optical attenuator (VOA) induced phase variations. Background Art

[0002] An intradyne coherent receiver (ICR) provides advanced demodulation to analyze the amplitude and optical phase of amplitude modulated and phase modulated signals (SIG) relative to a local oscillator (LO) optical reference, respectively, in two orthogonal polarization states. It is generally desirable to be able to attenuate the SIG-LO beat signal. This is typically achieved by adding a variable optical attenuator in the signal path (i.e., the path carrying the modulated optical signal). Including a single VOA in an ICR to attenuate the SIG-LO beat signal can result in a phase shift in the beat signal, which can be problematic. Short pin junction VOAs (i.e., CI-VOAs) that provide attenuation of light by carrier injection will, in addition to providing attenuation, also change the phase of the incoming signal (i.e., introduce chirp). Another existing approach involves the use of a Mach-Zehnder interferometer (MZI) with a phase-varying element in each arm (i.e., one each for the X and Y channels). By varying the relative phase difference between the arms of the MZI, a variable attenuator can be created. CI-VOAs placed in each arm can create just enough phase shift (at least π) without causing too much intrinsic absorption. Unfortunately, due to the nature of the MZI transfer function, there is still a phase shift (of π / 2). This causes problems with beat signals in coherent receivers. It is advantageous to use a thermal phase shifter (TPS) as opposed to a current injection phase shifter, but for applications that require high-speed ICRs, such as high-speed optical communications, TPSs are too slow.

[0003] In order not to affect the system performance, it is necessary to integrate a fast VOA function in the coherent receiver while compensating for any phase changes caused by the VOA. Summary of the invention

[0004] The present disclosure relates in various aspects to systems and methods for a coherent receiver with low VOA-induced phase variations. In an embodiment, the present disclosure relates to systems and methods for a coherent receiver configured to eliminate phase variations caused by VOAs. In order to eliminate the phase variations caused by VOAs in the signal (SIG), we introduce a complementary VOA in the LO path to avoid phase variations in the beat signal. The method provides a simplified compensation scheme to eliminate any need for circuit-based compensation schemes. In another embodiment, the present disclosure includes a high-speed VOA that minimizes optical phase shifts during its operation while having minimal insertion loss at a fully open setting. This includes an MZI structure that combines low-speed and high-speed phase shifters to achieve a variable optical attenuator with low phase shift and low insertion at a full "bright" setting.

[0005] In one embodiment, a coherent receiver includes: a receive signal (SIG) path including i) an input configured to connect to a receive signal, ii) one or more signal paths connected to the input and one or more optical hybrids, and iii) at least one variable optical attenuator (VOA); a local oscillator (LO) signal path including an input configured to connect to the LO and the one or more optical hybrids; and a VOA-induced phase variation compensation mechanism that reduces any VOA-induced phase variation in a SIG-LO beat signal due to the at least one VOA.

[0006] The phase change compensation mechanism caused by the VOA may include at least one VOA in series in the LO path. The at least one VOA and the at least one VOA in series may both be the same type of VOA. The at least one VOA and the at least one VOA in series may each be a different type of VOA. The at least one VOA and the at least one VOA in series may be any one of a current injection VOA (CI-VOA) and a chirped Mach-Zehnder (MZ) interferometer (MZI) VOA (MZI-VOA), respectively. The at least one VOA and the at least one VOA in series are both controlled by the same control signal. The at least one VOA and the at least one VOA in series are respectively controlled by independent control signals. The at least one VOA and the at least one VOA in series may each include different phase contrast attenuation characteristics that are appropriately configured. The at least one VOA and the at least one VOA in series may both set their attenuation to provide the same amplitude of the corresponding beat signal.

[0007] The one or more signal paths may include two signal paths, one for X polarization and one for Y polarization, wherein the VOA in each of the one or more signal paths includes VOA (X) and VOA (Y). The phase change compensation mechanism caused by the VOA may include at least one series VOA in the LO path. The at least one series VOA may include two VOAs, the two VOAs including LO VOA (X) and LO VOA (Y) in the LO signal path.

[0008] The VOA-induced phase variation compensation mechanism may include the at least one VOA, the at least one VOA being a chirp-free Mach-Zehnder (MZ) interferometer. The chirp-free Mach-Zehnder (MZ) interferometer may include a pin junction phase shifter for fast phase control, and a thermal phase shifter (TPS) for slow phase control.

[0009] The phase change compensation mechanism caused by the VOA can reduce the phase change caused by the VOA to within tens of thousands or less. and .

[0010] In another embodiment, a coherent receiver is formed by a process comprising: forming a receive signal (SIG) path comprising i) an input configured to connect to a receive signal, ii) one or more signal paths connected to the input and one or more optical mixers, and iii) at least one variable optical attenuator (VOA); forming a local oscillator (LO) signal path comprising an input configured to connect to the LO and the one or more optical mixers; and providing a VOA-induced phase variation compensation mechanism that reduces any VOA-induced phase variation in the SIG-LO beat signal due to the at least one VOA. The VOA-induced phase variation compensation mechanism may include at least one cascaded VOA in the LO path. The VOA-induced phase variation compensation mechanism may include the at least one VOA, the at least one VOA being a chirp-free Mach-Zehnder (MZ) interferometer.

[0011] In another embodiment, a method includes the steps of receiving a receive signal at an input of a receive signal path having one or more signal paths, attenuating the receive signal through at least one variable optical attenuator (VOA), and providing the attenuated receive signal to one or more optical mixers; receiving a local oscillator (LO) signal and providing the attenuated LO signal to the one or more optical mixers; and compensating for any VOA-induced phase variation in a SIG-LO beat signal due to the at least one VOA. The VOA-induced phase variation compensation mechanism can reduce the phase variation caused by the VOA to within tens of thousands or less. and . BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present disclosure is herein shown and described with reference to the various drawings, wherein like figure numerals are used to represent like system components / method steps when appropriate, and wherein: Figure 1 is a block diagram of a coherent receiver 10 having internal VOAs in the X and Y receive signal paths.

[0013] Figure 2A-2D Figure 2 is a typical VOA characteristic diagram using a pin junction in carrier injection mode (forward voltage operation).

[0014] Figure 3 is a block diagram of a coherent optical receiver with internal VOAs in the X and Y receive signal paths, and a complementary VOA in the LO signal path.

[0015] Figure 4 is a block diagram of a coherent optical receiver with internal VOAs in the X and Y receive signal paths, and a complementary VOA after a 1×2 MMI in the LO signal path.

[0016] Figure 5 Plot of two VOAs with mismatched phase contrast decay responses.

[0017] Figure 6 Figure 2 is a diagram of two VOAs that can be phase matched if independent control and calibration can be used for the X and Y polarizations.

[0018] Figure 7 Figure 2 is a diagram of the process used to match the phase of two VOAs.

[0019] Figure 8 is a flow chart of a process for eliminating phase variations caused by a VOA.

[0020] Fig. 9 Schematic diagram of VOA in silicon photonics platform.

[0021] Fig.10 and Fig.11 is a graph showing the relationship between phase and amplitude for a balanced MZI with an ideal 3dB coupler.

[0022] Fig.12 FIG. 1 is a block diagram of an MZI VOA with two-stage phase shifters on each arm, where the two-stage phase shifters are separated.

[0023] Fig.13 FIG. 1 is a block diagram of an MZI VOA with two-stage phase shifters in each arm, where the two-stage phase shifters are overlapped.

[0024] Fig.14 and Fig.15 A top view of an embodiment of a single arm of an MZI VOA in which the secondary phase shifter is separated from the high-speed phase shifter ( Fig.14 ) and side view ( Fig.15 ) schematic diagram.

[0025] Fig.16 and Fig.17 A top view of an embodiment of a single arm of an MZI VOA in which a secondary phase shifter covers a high-speed phase shifter ( Fig.14 ) and cross-sectional views ( Fig.15 ) schematic diagram.

[0026] Fig.18 A diagram showing an embodiment of a single arm of a MZI VOA with a combined pin and resistive heater (electrically connected).

[0027] Fig.19 Diagram showing an embodiment of a single arm of an MZI VOA with a combined pin and resistive heater (AC / DC decoupling).

[0028] Fig. 20 Flowchart of a process for a coherent receiver with low phase variation caused by a variable optical attenuator (VOA). DETAILED DESCRIPTION

[0029] In other words, the present disclosure is generally directed to systems and methods for a coherent receiver with low phase changes caused by VOAs. In an embodiment, the present disclosure is directed to systems and methods for a coherent receiver configured to eliminate phase changes caused by VOAs. In order to eliminate the phase changes caused by VOAs in the signal (SIG) path, we introduce a complementary VOA in the LO path to avoid phase changes in the beat signal. The method provides a simplified compensation scheme to eliminate any need for circuit-based compensation schemes. In another embodiment, the present disclosure includes a high-speed VOA that minimizes optical phase shifts during its operation while having minimal insertion loss in a fully open setting. This includes an MZ structure that combines low-speed and high-speed phase shifters to achieve a variable optical attenuator with low phase shift and low insertion loss in a full "bright" setting. Coherent Receiver

[0030] Figure 1 1 is a block diagram of a coherent optical receiver 10 having internal VOAs 12X, 12Y in the X and Y receive signal paths. The coherent optical receiver 10 may be referred to as an ultra-high bandwidth (UHB) ICR. The coherent optical receiver 10 includes a receive signal (SIG) path 14 and a LO signal path 16. The receive signal path 14 includes an X polarization signal path 18X and a Y polarization signal path 18Y.

[0031] In one embodiment, the receive signal path 14 includes a spot size converter (SSC) 20, which is used to optically couple light from the receive (Rx) signal (SIG input) into the photonic chip. Next, there is a polarization beam splitter (PBS) 22, which splits the X polarization and the Y polarization onto the respective signal paths 18X and 18Y, and a polarization rotator (PR) 24 on the signal path 18Y, which is only required when the polarization needs to be rotated. Next, each of the signal paths 18X, 18Y has similar components, including a tap coupler 24, which is connected to the photodetector 26 and the VOA 18X, 18Y. After the VOA 12X, 12Y, there is another tap device 28, which is connected to the photodetector 30 and the 90-degree optical hybrid 32 (note that the term "optical hybrid" described herein refers to a 90-degree optical hybrid). Note that the photodetectors 26, 30 are used to monitor the VOAs 12X, 12Y to control the VOAs 12X, 12Y. The photodetectors 26, 30 are optional.

[0032] The optical mixer 32 also includes an input from the LO signal path 16. The LO signal path 16 includes an SSC 36 connected to the LO input and to a multi-mode interferometer (MMI) 38 configured as a 1×2 beam splitter, which provides an LO signal to the optical mixer 32 for coherent detection of the Rx signal. Each optical mixer 32 output is connected to a balanced pair of high-speed photodetectors (HSPDs) 40, i.e., each pair corresponds to an in-phase (I) and quadrature (Q) signal for each X polarization and Y polarization, i.e., XI, XQ, YI, YQ.

[0033] Note that there are VOAs 12X, 12Y on the receive signal path 14, but not on the LO signal path 16, so when the VOA attenuation changes, a phase difference will result between the two paths 14, 16. At the same time, Figure 1 The components in FIG. 1 are example embodiments and one skilled in the art will recognize that there may be different components. Figure 1 The key aspect of this is that the VOAs 12X, 12Y cause the phase difference between the two paths 14, 16 to change as the attenuation of the VOAs changes. This can be disadvantageous in applications and depends on the magnitude and speed of the phase change caused by the VOAs. VOA

[0034] Figure 2A-2D The characteristic diagram of a typical VOA using a pin junction in carrier injection mode (forward voltage operation). These are referred to herein as current injection VOAs (CI-VOAs). When the junction is forward biased, the VOA provides light attenuation by carrier injection. The intrinsic region is located in the center of the waveguide, and light propagates in the center of the waveguide. However, the presence of injected carriers in the waveguide has the consequence of changing the refractive index (as implied by Soref's equation, see Soref et al. "Electrooptical effects in silicon." IEEE J. Quantum Electron., 23.1 (1987): 123-129, referred to herein as Soref et al.). According to Figure 2A The relationship shown, for an attenuation of 20 dB, will result in a significant phase change of nearly 10 rad.

[0035] This phase change caused by the VOA also has the undesirable consequence of changing the phase of the SIG-LO beat product. This beat signal is generated by a given balanced pair. The beat signal is generated by the difference in the photocurrent of each photodiode (i.e., high-speed photodiode (HSPD)). The beat signal can be expressed by the following equation:

[0036]

[0037] Where R is the photodiode responsivity, Ps is the optical power of the balanced pair entering the HSPD (high speed photodiode) from the SIG port, and P LO is the optical power from the local oscillator (LO) port, and are the optical (angular) frequencies of the incident signal and LO, respectively, and are the phases of these optical signals. Note that these phases include the phase noise of both the SIG and LO lasers, the phase modulation of the signal received from the SIG port, and the phase variation caused by the VOA.

[0038] It can be seen that changing the optical attenuation in the SIG path causes a corresponding phase change in the beat signal, which is amplified by the TIA and directed to the output of the coherent receiver 10. State-of-the-art ASICs can compensate for this phase change to some extent, depending on the magnitude and speed at which the change occurs. Ideally, when attenuating the optical signal from the SIG port, no such phase change would be produced, eliminating the need for any compensation. Coherent receiver eliminates phase variations caused by VOA

[0039] The present disclosure proposes adding VOAs 42, 42X, 42Y in the LO signal path 16, which may be identical to, substantially similar to, or different from but configured similarly to the VOAs used in the SIG path, such as Figure 3 and Figure 4 As shown, the amplitude of the beat signal is adjusted using SIG-VOA and LO-VOA simultaneously. Figure 3 1 is a block diagram of a coherent optical receiver 10 having internal VOAs 12X, 12Y in the X and Y receive signal paths 14 and a complementary VOA 42 in the LO signal path 16. Figure 4 1 is a block diagram of a coherent optical receiver 10 having internal VOAs 12X, 12Y in the X and Y receive signal paths 14 and complementary VOAs 42X, 42Y after a 1×2 MMI 36 in the LO signal path 16 .

[0040] Controlling the "LO-VOA" 42 with the same voltage as that used for the "SIG-VOA" 12 results in the same phase change being induced in the LO signal path 16 and in the receive signal path 14 (i.e. ), so that at all times, the change in both the LO VOA state and the SIG VOA state Related difference is eliminated so that the phase of the beat signal is not affected.

[0041] Of course, due to the use of LO-VOA 42, the attenuation needs to be adjusted differently to produce the same amplitude of the beat signal, because the beat signal is proportional to the product of the SIG and LO optical powers (see the above equation). Therefore, the attenuation provided by both SIG VOA 12 and LO VOA 42 needs to be divided by 2 (in dB) compared to the case where only SIG VOA 12 is used to provide attenuation.

[0042] The disadvantage of using the LO-VOA 42 is the additional loss associated with introducing the VOA in the LO signal path 16 - typically a few tenths of a dB at most. This additional loss can be partially compensated by the fact that the SIG-VOA 12 can be relatively short (compared to a solution without the LO-VOA) since the total attenuation is now provided by two elements. Overall, the losses in the SIG receive signal path 14 are more critical since the SIG receive signal path 14 carries the optical signal with information, which must maintain a high signal-to-noise ratio (SNR).

[0043] The control signal for the LO-VOA 42 can be the same voltage used to control the SIG-VOA 12. For high speed VOA operation, this allows time matching of the control signals for dynamic cancellation of phase. Alternatively, another control scheme is to connect all VOAs in series to ensure that the same current flows in each VOA so that each VOA will produce exactly the same phase shift. However, the latter option requires a higher control voltage.

[0044] Independent signals may also be sent to each VOA 12, 42. This ensures that if the LO-VOA 42 and SIG-VOA 12 have slight manufacturing differences, and therefore different "attenuation vs. voltage" or "attenuation vs. current" characteristics (depending on whether the VOA is controlled with a voltage source or a current source), phase maintenance is fully compensated. These independent control signals will take into account these different individual characteristics and adjust the LO-VOA and SIG-VOA voltage or current differently. Independent controls may also be used to adjust the LO-VOA and SIG-VOA differently when the ambient (chassis) temperature changes, to compensate for the effects of local temperature differences between the LO-VOA and SIG-VOA when the ambient (enclosure) temperature changes.

[0045] Using only one VOA 42 in the LO signal path 16 (e.g. Figure 3 Error! Reference source not found) in some way constrains the attenuation on the X and Y channels to be the same. Figure 4As shown, if one wants to maintain independent adjustment of the attenuation of the X and Y channels, two VOAs 42X, 42Y can be used in the LO path after the XY beam splitter (1×2 MMI 38) in the LO signal path 16. VOA Control

[0046] Figure 5 is a graph of two VOAs 42X, 42Y having mismatched phase contrast decay responses. Figure 6 Graph of two VOAs 42X, 42Y having their phase matched to a single control value. Figure 7 is a schematic diagram of a process 50 for matching the phases of two VOAs 42X, 42Y.

[0047] As described herein, VOAs 42, 42X, 42Y may be referred to as "tandem" VOAs, meaning that they are used in conjunction with VOAs 12X, 12Y, i.e., VOAs 12X, 12Y in the SIG path and VOAs 42, 42X, 42Y in the LO path. That is, tandem refers to the operation of the VOAs, working in conjunction with each other.

[0048] The attenuation of the series VOA is controlled by a digital optoelectronic automatic gain control (AGC) loop. Figure 5 The two VOAs shown in the example do not have perfectly matched phase contrast attenuation characteristics, and the digital AGC loop controlling the VOAs may have "gain and offset" and / or "random access memory (RAM lookup table (LUT)" compensation features to match the phase contrast attenuation response of the two VOAs 42X, 42Y.

[0049] exist Figure 7 In the example shown, both VOAs are controlled by a single integrator of the AGC. The output of the AGC is digital and has a range. The VOA attenuation is set by a high current DAC (HCDAC). The functions of the "Gain and Offset" and / or "RAM LUT" elements are:

[0050] (1) If Figure 6 As shown, the gain and offset components are used to map the integrator output OAGC_OUT range so that at a given OAGC value, the two HCDACs will set their respective VOA attenuations so that they get close to the same phase. The difference in attenuation settings does not affect the performance of the AGC and receiver.

[0051] (2) If a RAM LUT table exists, it can be programmed to perform the “gain and offset” function described in (1) above; in addition, the RAM LUT table can also be programmed to The slope of the decay-HCDAC is linearized to a constant. Linearization is ~ constant, which is required for AGC performance.

[0052] Of course, various methods are contemplated for controlling the VOAs 42X, 42Y to ensure that the phase of the LO-VOA matches the phase of the SIG-VOA. This control is advantageous for achieving different VOA characteristics on the same silicon photonics (SiP) die, and also for their differences over temperature. VOA Phase Cancellation Process

[0053] Figure 8 Flow chart of process 80 for eliminating phase changes caused by VOAs. Process 80 may be implemented on a coherent receiver 10 or the like. Process 80 includes receiving a receive signal at an input of a receive signal path having one or more signal paths, attenuating the receive signal by a variable optical attenuator (VOA) in each of the one or more signal paths, and providing the attenuated receive signal to one or more optical mixers (step 82); receiving a local oscillator (LO) signal, attenuating the LO signal with one or more complementary VOAs, and providing the attenuated LO signal to one or more optical mixers (step 84); and setting one or more complementary VOAs to substantially cancel any phase changes caused by the VOA from each of the one or more signal paths (step 86). Note that the term "substantially" means that the elimination of phase changes is set to eliminate all phase changes or eliminate most phase changes. Those skilled in the art will recognize that there may be slight phase differences, i.e. That is, although the term "substantially" is not used in conjunction with the phase change cancellation, those skilled in the art will understand that this means most of the phase change.

[0054] The VOA and the one or more complementary VOAs in each of the one or more signal paths may be pin junctions. The VOA and the one or more complementary VOAs in each of the one or more signal paths may each be the same type of VOA. The one or more signal paths may include two signal paths, one for X polarization and one for Y polarization, and the VOAs in each of the one or more signal paths include VOA (X) and VOA (Y). The one or more complementary VOAs may include a VOA in the LO signal path. The one or more complementary VOAs may include two VOAs, including the LO VOA (X) and the LO VOA (Y) in the LO signal path.

[0055] Process 80 may also include controlling the VOA and the one or more complementary VOAs in each of the one or more signal paths with the same control signal. Process 80 may also include controlling the VOA and the one or more complementary VOAs in each of the one or more signal paths with independent control signals. Process 80 may also include compensating for different phase-attenuation characteristics between any pair of the VOA and the one or more complementary VOAs in each of the one or more signal paths. Process 80 may also include setting the attenuation or VOA in the VOA and the one or more complementary VOAs in each of the one or more signal paths to provide the same amplitude as the corresponding beat signal. Coherent Receiver with Complementary VOA

[0056] In one embodiment, a coherent receiver includes a receive signal path and a local oscillator (LO) signal path, wherein the receive signal path includes: i) an input terminal configured to connect to a receive signal, ii) one or more signal paths connected to the input terminal and one or more optical mixers, and iii) a variable optical attenuator (VOA) in each of the one or more signal paths; the local oscillator (LO) signal path includes: i) an input terminal configured to connect to the LO and one or more optical mixers, and ii) one or more complementary VOAs located between the input terminal and the one or more optical mixers, wherein the one or more complementary VOAs are configured to substantially cancel any phase variation from the VOA in each of the one or more signal paths.

[0057] The VOA and the one or more complementary VOAs in each of the one or more signal paths may be pin junctions, i.e., all are CI-VOAs described herein. The VOA and the one or more complementary VOAs in each of the one or more signal paths may all be VOAs of the same type. The same type may be a CI-VOA, a chirped MZ-VOA, and the like. The chirped MZ VOA is a single-ended drive on the upper MZ arm 14 or the lower MZ arm 16. In another embodiment, the VOA and the one or more complementary VOAs in each of the one or more signal paths may each be a different type of VOA. In addition, the different types may be any one of a CI-VOA, a chirped MZ-VOA, and the like.

[0058] The one or more signal paths may include two signal paths, one for X polarization and one for Y polarization, wherein the VOA in each of the one or more signal paths includes VOA (X) and VOA (Y). The one or more complementary VOAs may include one VOA in the LO signal path. The one or more complementary VOAs may include two VOAs, including LO VOA (X) and LO VOA (Y) in the LO signal path.

[0059] The VOA and the one or more complementary VOAs in each of the one or more signal paths may be controlled with the same control signal.The VOA and the one or more complementary VOAs in each of the one or more signal paths may each be controlled with an independent control signal.

[0060] The VOA and the one or more complementary VOAs in each of the one or more signal paths can each include different phase contrast attenuation characteristics and thus be compensated. The VOA and the one or more complementary VOAs in each of the one or more signal paths can each set their attenuation to provide the same amplitude of the corresponding beat signal. Mach-Zehnder interferometer structure for high-speed, low phase variation and low insertion loss VOA

[0061] The VOA 12, 42 may include a pin junction operated in forward bias. The MZ structure itself is capable of providing a VOA (or more precisely, a switch with a continuous control signal using only one port) on a platform lacking the necessary dopants, without having to consider the phase, see, for example, Alireza Tabatabaei Mashayekh, et al., "Silicon nitride PIC-based multi-color laser engines for life science applications," Opt. Express 29, 8635-8653 (2021), the contents of which are incorporated herein by reference.

[0062] Furthermore, as described in this paper, for a given attenuation, standard VOAs induce a certain amount of phase shift that will exceed what can be compensated for while keeping the device compact. MZI VOAs using pin junctions are inherently lossy in their bright state due to manufacturing uncertainties that force the use of pin junctions to shift the operating point of the MZI to its fully open position.

[0063] In one embodiment, the present disclosure includes a high-speed VOA that minimizes optical phase shifts during its operation while having minimal insertion loss at the fully open setting. A Mach-Zehnder interferometer structure combining both low-speed and high-speed phase shifters is used to produce high attenuation while limiting phase shifts. We describe the implementation of phase shifters to limit insertion loss (IL), control loop complexity, and maximize compactness.

[0064] Low phase variation VOAs are useful for coherent receivers. Demodulation involves comparing the time-varying amplitude and / or phase of the input signal (SIG) with a phase-stable continuous wave (CW) laser (local oscillator - LO). During high-speed operation of the VOA (e.g., to compensate for rapid amplitude perturbations from a fiber link), any additional phase delivered by the VOA may introduce errors in the demodulation. Although state-of-the-art ASICs can compensate for this phase variation to a limited extent, it is still useful and, in some cases, necessary to minimize phase offsets.

[0065] Fig. 9 Schematic diagram of a VOA on a silicon photonics platform 100. The silicon photonics platform 100 includes a substrate 102, a buried oxide (BOX) layer 104, doped portions 106, 108 connected to metal contacts 110, 112, respectively, a waveguide core 114, and a top oxide cladding 116. Fast (e.g., on the order of 10-100 MHz) VOAs in the silicon photonics platform 100 are most commonly based on forward biased pin junction operation. The attenuation mechanism is based on light absorption provided by injected carriers in the intrinsic region, which also acts as a waveguide structure for light to pass through.

[0066] The relationship between carrier concentration, phase shift, and light absorption near 1550 nm optical wavelength is given by the empirical Soref and Bennet equation, see Soref et al. and Nedeljkovic et al. "Free-Carrier Electrorefraction and Electroabsorption Modulation Predictions for Silicon Over the 1–14-um Infrared Wavelength Range." IEEE Photonics Journal 3.6 (2011): 1171-1180, the contents of which are incorporated herein by reference:

[0067] where Δα is the additional material absorption, Δn is the shift in the silicon refractive index, and ΔN e and ΔN hare the free electron and hole concentrations, respectively. To perform the VOA function, a change in absorption means a change in the light phase through a change in the refractive index of the photoconductive material. This is also measured experimentally, as shown above. Figure 2A-2D shown.

[0068] It is noteworthy that the theoretical equations predict that the attenuation decreases for increasing phase shift, which is also observed experimentally ( Figure 2A ). It is possible to reduce the large phase shift caused by a specific attenuation target to some extent by making the VOA longer. However, this has obvious disadvantages in terms of integration of longer waveguides in larger structures and optical insertion loss (IL).

[0069] Due to the physical reality that current injection into a VOA always produces a phase shift, this fact can be exploited to embed the current injection VOA into an interferometer system, the simplest of which is the Mach-Zehnder interferometer (MZI). Fig.10 and Fig.11 The relationship between phase and amplitude for a balanced MZI with an ideal 3dB coupler is shown in . The phase shift is applied to a single arm of the MZI.

[0070] Transmission across the port goes from full brightness to full attenuation by inducing a π phase shift between the two arms, while inducing a maximum phase shift of π / 2 in the output. In practice, this is significantly lower (14-20 times lower) than a conventional pi-nVOA kept to a reasonable length (≤1mm). Resonant systems (e.g., a ring resonator including a phase shifter) still require the same total π phase shift to go from full brightness to full attenuation, while also being highly sensitive to the operating wavelength (and therefore incompatible with multichannel input signals), and more likely to incur large excess losses.

[0071] In order to maintain a fast-acting VOA, a fast phase shifter is still needed to induce this π phase shift. A forward-biased pin is also a preferred implementation of the MZI structure. It needs to be kept at a certain length in order not to induce too much parasitic attenuation. For single-ended operation, the additional attenuation of one arm reduces the extinction ratio of the MZI at full attenuation - the extinction ratio is defined as the ratio of the maximum transmission to the minimum transmission on the same port (e.g., a crossover port).

[0072] Differential operation of the high speed phase shifters would allow for a net zero phase shift of the output signal, but would require operating the dual phase shifters at a midpoint where these high speed phase shifters would result in a higher base insertion loss at maximum transmission. Two-stage phase shifter in each arm

[0073] Fig.12is a block diagram of an MZI VOA 200 having a secondary phase shifter 202 on each arm 204, 206, wherein the secondary phase shifters are separate. Fig.13 FIG. 2 is a block diagram of an MZI VOA having a two-stage phase shifter 202 on each arm 204, 206, wherein the two-stage phase shifters 202 are overlapped. The MZI VOA 200 includes a coupler 200 that splits the input into each arm 204, 206. Fig.12 In FIG. 1 , the MZI VOA 200 includes a secondary phase shifter 202 separated from a high speed phase shifter (HSPS) 212. Fig.13 In the embodiment, the MZI VOA 200 includes a secondary phase shifter 202 stacked on a high speed phase shifter (HSPS) 212. The secondary phase shifter 202 may be a thermal phase shifter (TPS). The high speed phase shifter 212 may be a pin structure. The two arms 204, 206 are combined by a coupler 314 and provide an output.

[0074] It is advantageous to introduce a secondary phase shifter 202 in each arm 204, 206, which can operate at a slower speed but does not cause any (significant) attenuation of the induced phase shift. This allows the MZIVOA 200 to be appropriately biased at the transmission maximum when the high-speed phase shifter 212 is not operating. Due to manufacturing imperfections, the manufactured device will have an initial random phase difference between the two arms 204, 206, which can be compensated by the secondary phase shifter 202. A common mechanism used to manufacture such phase shifters 202 in silicon photonic platforms is the thermo-optic effect through local heating of the waveguide. Another mechanism is to use microelectromechanical actuators, such as described in Edinger et al. "Silicon photonic microelectromechanical phase shifters for scalable programmable photonics." Opt. Lett. 46.22 (2021): 5671-5674, the contents of which are incorporated herein by reference.

[0075] Fig.12 and Fig.12 The couplers 210, 214 shown in FIG. 2 are 2×2 couplers, but may also be three-port devices, i.e., without a secondary input or without a dump / monitor port. To maximize the extinction ratio, the splitting ratio is typically as close to 3 dB as possible. Silicon photonic implementations of such couplers 210, 214 may be multimode interference (MMI) couplers, directional couplers, adiabatic couplers, or any other compact device that achieves such functionality. Other arrangements combining high-speed pin phase shifters with thermal phase shifters

[0076] Various other arrangements combining high speed pin phase shifters with thermal phase shifters in an MZ structure are described below.

[0077] Fig.14 and Fig.15 FIG. 2 is a top view of a single arm 204, 206 of the MZI VOA 200 in which the secondary phase shifter 202 is separated from the high-speed phase shifter 212 in an embodiment ( Fig.14 ) and side view ( Fig.15 ). The secondary phase shifter 202 may be titanium nitride (TiN) or may utilize resistively doped silicon on both sides of the optical waveguide. Deep trenches may be etched to increase power efficiency by increasing thermal isolation from the environment, thereby increasing the heat trapped around the waveguide.

[0078] Fig.16 and Fig.17 FIG. 2 is a top view of an embodiment of a single arm 204, 206 of an MZI VOA in which the secondary phase shifter 202 covers the high-speed phase shifter 212 ( Fig.14 ) and cross-sectional views ( Fig.15 ). Here, the TiN TPS is placed directly on top of the pin junction. This saves space (and reduces IL), but it heats the junction during operation, which must be accounted for in the control loop.

[0079] Fig.18 A diagram of an embodiment of a single arm 204, 206 of an MZI VOA 200 with a combined pin and resistive heater (electrically connected) is shown. The side doped portions (p++ and n++) that form the pin junction are reused as resistive heaters. The resistive portion can extend past the pin portion. Now, the junction and heater control are interconnected, increasing the complexity of the control loop but simplifying the implementation of silicon photonics. The resistance value of the resistor is carefully designed by adjusting the doping level (e.g., using multiple dopings) and the form factor (affecting the sheet resistance value). The resistance value of the resistors does not have to be exactly the same.

[0080] Fig.19A diagram of an embodiment of a single arm 204, 206 of an MZI VOA 200 with a combined pin and resistive heater (AC / DC decoupling) is shown. The side doped portion is also used as a resistive heater, but the low-speed phase shifter and the high-speed phase shifter are decoupled. The AC signal controls the high-speed junction. This is because the high-speed junction is most useful for compensating fast line perturbations, which, as described above, can be decomposed into their Fourier components and compensated with an AC signal. The low-speed TPS 202 is limited to frequencies near DC, but can also excite the phase, thereby changing the state of the MZI independently of the AC signal. In order to quickly compensate for static perturbations, the two signals are combined together, and after RC decay of the fast signal, the slow signal takes over. Other AC / DC schemes can also be imagined where the two are not completely decoupled.

[0081] More generally, the pin and resistor parts can be combined directly ( Fig.18 ) or segmented ( Fig.19 ). The current injected pin junction is combined with a resistive element for thermo-optical excitation. This can include segmentation and AC / DC decoupling. This is done to address the requirement for low phase variation VOA in the coherent receiver 10.

[0082] MZI VOA 200 describes a chirp-free Mach-Zehnder interferometer consisting of

[0083] (1) pin junction phase shifter for fast phase control (current injection VOA), and

[0084] (2) Thermal phase shifter (TPS) for slow phase control.

[0085] The combination of these two structures achieves a VOA with low phase shift and low insertion loss at the full “bright” setting. combination

[0086] The VOAs 12, 42 in the coherent receiver 10 may use the MZI VOA 200. In other words, the coherent receiver 10 includes the VOA 42, which generates very low [SIG × LO] phase disturbances when the VOA 42 is excited.

[0087] A coherent receiver 10 with low [SIG × LO] phase perturbation from VOA excitation can be implemented using:

[0088] (1) SIG and LO series VOA (VOA on the SIG path and LO path)

[0089] (2) CI-VOA on the SIG path and CI-VOA on the LO path

[0090] (3) Chirped MZ-VOA on the SIG path and CI-VOA on the LO path. The chirped MZ VOA is driven by a single end on the upper MZ arm or the lower MZ arm.

[0091] (4) Chirp MZ-VOA on the SIG path and Chirp MZ-VOA on the LO path

[0092] (5) CI-VOA on the SIG path and chirp MZ-VOA on the LO path

[0093] The key point of both methods is to generate phase excitation on both SIG and LO paths so that the [SIG × LO] phase perturbation caused by the VOA is close to zero.

[0094] (6) Zero chirp MZ VOA 200 (VOA on SIG path only)

[0095] This MZ VOA is a differential drive on the upper and lower MZ arms.

[0096] In one embodiment, a coherent receiver includes a receive signal (SIG) path and a local oscillator (LO) signal path, the receive signal (SIG) path including: i) an input configured to connect to a receive signal, ii) one or more signal paths connected to the input and one or more optical mixers, and iii) at least one variable optical attenuator (VOA); the local oscillator (LO) signal path including an input configured to connect to the LO and one or more optical mixers; and a VOA-induced phase variation compensation mechanism that reduces any VOA-induced phase variation in the SIG-LO beat signal due to the at least one VOA. Note that the at least one VOA is optically integrated into the coherent receiver, and the VOA-induced phase variation compensation mechanism is configured to remove or reduce any VOA-induced phase variation in the SIG-LO beat signal.

[0097] In one embodiment, the phase variation compensation mechanism caused by the VOA includes at least one VOA in series in the LO path. The at least one VOA and the at least one VOA in series can be VOAs of the same type or VOAs of different types, respectively. The at least one VOA and the at least one VOA in series can be any one of a current injection VOA (CI-VOA) and a chirped Mach-Zehnder (MZ) interferometer (MZI) VOA (MZI-VOA), respectively. The at least one VOA and the at least one VOA in series can be controlled with the same control signal, respectively. The at least one VOA and the at least one VOA in series can also be controlled with independent control signals, respectively. The at least one VOA and the at least one VOA in series can include different phase contrast attenuation characteristics, respectively, which can be compensated. The at least one VOA and the at least one VOA in series can both set their attenuation to set the same amplitude of the corresponding beat signal. As described herein, "series" VOAs are the same (e.g., same part number), substantially similar, or different but configured to operate in a substantially similar manner, etc.

[0098] In another embodiment, the one or more signal paths include two signal paths, one for X polarization and one for Y polarization, wherein the VOA in each of the one or more signal paths includes VOA (X) and VOA (Y). The phase variation compensation mechanism caused by the VOA may include at least one series VOA in the LO path. The at least one series VOA may include two VOAs, the two VOAs including LO VOA (X) and LO VOA (Y) in the LO signal path.

[0099] In yet another embodiment, the phase variation compensation mechanism caused by the VOA includes at least one VOA, and the at least one VOA is a chirp-free Mach-Zehnder (MZ) interferometer. The chirp-free Mach-Zehnder (MZ) interferometer may include a pin junction phase shifter for fast phase control and a thermal phase shifter (TPS) for slow phase control.

[0100] Fig. 20A flow chart of a process 300 for a coherent receiver having low variable optical attenuator (VOA) induced phase variation. The process 300 includes: receiving a receive signal at an input of a receive signal path having one or more signal paths, attenuating the receive signal via at least one variable optical attenuator (VOA), and providing the attenuated receive signal to one or more optical mixers (step 302); receiving a local oscillator (LO) signal and providing the attenuated LO signal to the one or more optical mixers (step 304); and compensating for any VOA induced phase variation in a SIG-LO beat signal due to at least one VOA (step 306). The compensation may include (1) a series VOA for attenuating the LO signal such that the LO signal experiences a VOA induced phase variation similar to that of the attenuated receive signal, or (2) attenuation using an unchirped Mach-Zehnder (MZ) interferometer. Experimental Results

[0101] Can be or The units represent the phase change caused by the VOA, and these units can be related, for example Can be compared As is known to those skilled in the art, RMS is root mean square (RMS), RAD is radians, and so on. Figure 1 The approach described, i.e., VOA only on the SIG path and not on the LO path, can have VOA-induced phase variations on the order of hundreds of thousands of these values, and this can be beyond the tracking capabilities of a coherent DSP. The chirped Mach-Zehnder (MZ) interferometer on the SIG path described herein can have VOA-induced phase variations on the order of tens of thousands of these values, which is a significant improvement over CI-VOA only in the SIG path, and is generally within the range of coherent DSP tracking capabilities. The tandem VOA approach, i.e., VOA on both the SIG path and the LO path, and the unchirped MZ approach on the SIG path can have VOA-induced phase variations on the order of thousands of these values.

[0102] Although the chirp MZ on the SIG path is only within a few tens of kilo-radians / second of phase variation, it is not desirable to use the DSP phase variation capability when the Rx is tracking the optical input power transient. When an optical transient occurs, there will also be a corresponding SOP phase rotation. The advantages of the tandem VOA and chirp-free MZ approach described in this article include keeping the phase variation caused by the VOA as low as possible, so that most of the phase variation tracking budget in the DSP is allocated to any optical line disturbance. in conclusion

[0103] It should be appreciated that some embodiments described herein may include or utilize one or more general or special purpose processors ("one or more processors"), such as a microprocessor; a central processing unit (CPU); a digital signal processor (DSP); a custom processor, such as a network processor (NP) or a network processing unit (NPU), a graphics processing unit (GPU), etc.; a field programmable gate array (FPGA); and unique stored program instructions (including software and firmware) for controlling it, etc., so as to implement part, most or all of the functions of the method and / or system described herein in combination with certain non-processor circuits. Alternatively, some or all of the functions may be implemented by a state machine without stored program instructions, or in one or more application specific integrated circuits (ASICs), where each function or certain combinations of certain functions are implemented as custom logic or circuits. Of course, a combination of the foregoing methods may be used. For some embodiments described herein, the corresponding devices in hardware and optionally with software, firmware, and combinations thereof can be referred to as "circuits configured to...", "logic configured to...", etc., to perform a set of operations, steps, methods, processes, algorithms, functions, techniques, etc. on digital and / or analog signals of the various embodiments described herein.

[0104] In addition, some embodiments may include a non-transitory computer-readable medium having stored thereon instructions for performing the functions as described and claimed herein on a computer, server, device, apparatus, at least one processor, circuit / circuitry, etc. Examples of such non-transitory computer-readable media include, but are not limited to, hard disks, optical storage devices, magnetic storage devices, read-only memories (ROMs), programmable ROMs (PROMs), erasable PROMs (EPROMs), electrical EPROMs (EEPROMs), flash memory, and the like. When stored in a non-transitory computer-readable medium, software can include instructions executable by one or more processors (e.g., any type of programmable circuitry or logic) that, in response to such execution, cause the one or more processors to perform a set of operations, steps, methods, processes, algorithms, functions, techniques, etc. as described herein for various embodiments.

[0105] Although the present disclosure has been illustrated and described herein with reference to preferred embodiments and specific examples thereof, it is apparent to one of ordinary skill in the art that other embodiments and examples may perform similar functions and / or achieve similar results. All such equivalent embodiments and examples are within the spirit and scope of the present disclosure, are contemplated thereby, and are intended to be covered by the appended claims. In addition, it should be noted that the various elements, operations, steps, methods, processes, algorithms, functions, techniques, etc. described herein may be used in any and all combinations with each other.

Claims

1. A coherent receiver (10), comprising: a receiving signal SIG path (14) comprising i) an input terminal configured to connect a receiving signal, ii) one or more signal paths connected to the input terminal and one or more optical mixers (32), and iii) at least one variable optical attenuator VOA (12); a local oscillator (LO) signal path (16) comprising an input configured to be connected to the LO and the one or more optical mixers (32); and A VOA-induced phase variation compensation mechanism reduces any VOA-induced phase variation in the SIG-LO beat signal due to the at least one VOA (12).

2. The coherent receiver (10) of claim 1, wherein the VOA-induced phase variation compensation mechanism comprises at least one series VOA (42) in the LO path (14).

3. The coherent receiver (10) of claim 2, wherein the at least one VOA (12) and the at least one tandem VOA (42) are both VOAs of the same type.

4. The coherent receiver (10) of claim 2, wherein the at least one VOA (12) and the at least one tandem VOA (42) are each different types of VOAs.

5. The coherent receiver (10) according to any one of claims 2 to 3, wherein the at least one VOA (12) and the at least one cascade VOA (42) are any one of a current injection VOA, i.e., CI-VOA, and a chirped Mach-Zehnder interferometer, MZI VOA, i.e., MZI-VOA, respectively.

6. The coherent receiver (10) according to any one of claims 2 to 4, wherein the at least one VOA (12) and the at least one tandem VOA (42) are both controlled by one of the same control signal and independent control signals.

7. The coherent receiver (10) according to any one of claims 2 to 6, wherein the at least one VOA (12) and the at least one tandem VOA (42) each comprise different phase contrast attenuation characteristics that are appropriately configured.

8. The coherent receiver (10) according to any one of claims 2 to 7, wherein the at least one VOA (12) and the at least one tandem VOA (42) both set their attenuation to provide the same amplitude of the respective beat signals.

9. The coherent receiver (10) according to any one of claims 1 to 8, wherein the one or more signal paths (14, 16) comprise two signal paths, one for X polarization and one for Y polarization, The VOA in each of the one or more signal paths includes VOA(X) and VOA(Y).

10. The coherent receiver (10) of claim 9, wherein the mechanism for compensating for phase variations caused by VOAs comprises at least one series VOA (42) in the LO path (14).

11. The coherent receiver (10) of claim 9, wherein the at least one cascaded VOA (42) in the LO signal path (14) comprises two VOAs, the two VOAs comprising LO VOA (X) and LO VOA (Y).

12. The coherent receiver (10) according to any one of claims 1 to 11, wherein the VOA-induced phase variation compensation mechanism comprises the at least one VOA (12), the at least one VOA (12) being a chirp-free Mach-Zehnder (MZ) interferometer.

13. The coherent receiver (10) of claim 12, wherein the unchirped Mach-Zehnder (MZ) interferometer comprises: a pin junction phase shifter (212) for fast phase control, and A thermal phase shifter TPS (202) for slow phase control.

14. The coherent receiver (10) according to any one of claims 1 to 13, wherein the phase variation compensation mechanism caused by the VOA reduces the phase variation caused by the VOA to within several thousand or less. and .

15. A method (300) comprising the following steps: receiving (302) a receive signal at an input of a receive signal path having one or more signal paths, attenuating the receive signal via at least one variable optical attenuator (VOA), and providing the attenuated receive signal to one or more optical hybrids; receiving (304) a local oscillator (LO) signal and providing an attenuated LO signal to the one or more optical hybrids; as well as Any VOA-induced phase variation in the SIG-LO beat signal due to the at least one VOA is compensated (306).

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