Coherent receiver with low phase change caused by VOA

Abstract

A coherent receiver (10) includes: a received signal SIG path (14) including: i) an input terminal configured to connect a received signal, ii) one or more signal paths connecting 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) including an input terminal configured to connect to the LO and one or more optical mixers (32); and a phase change compensation mechanism caused by the VOA that reduces any VOA-induced phase change in the SIG-LO beat signal due to the at least one VOA (12).

Description

Field of the Invention

[0001] The present disclosure generally relates to components for optical networking. More specifically, the present disclosure relates to systems and methods for coherent receivers with low phase change caused by variable optical attenuators (VOAs). Background of the Invention

[0002] An intradyne coherent receiver (ICR) provides advanced demodulation to analyze the amplitude and optical phase of an amplitude-modulated and phase-modulated signal (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). Adding a single VOA in the ICR to attenuate the SIG-LO beat signal may cause a phase change in the beat signal, which can be problematic. A short p-i-n junction VOA that provides optical attenuation by carrier injection (i.e., CI-VOA) will, in addition to providing attenuation, also change the phase of the incoming signal (i.e., introduce chirp). Another existing method involves using a Mach-Zehnder interferometer (MZI) that has phase-changing elements on each arm (i.e., one each for the X and Y channels). By changing the relative phase difference between the arms of the MZI, a variable attenuator can be produced. CI-VOAs placed in each arm can produce just enough phase shift (at least π) without causing excessive 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 the beat signal in the coherent receiver. In contrast to current injection phase shifters, using a thermal phase shifter (TPS) is advantageous, but for applications that require high-speed ICRs, such as high-speed optical communications, the TPS is too slow.

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

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

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

[0006] The phase change compensation mechanism caused by VOA may include at least one series VOA in the LO path. The at least one VOA and the at least one series VOA may both be the same type of VOA. The at least one VOA and the at least one series VOA may each be a different type of VOA. The at least one VOA and the at least one series VOA 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 series VOA are both controlled by the same control signal. The at least one VOA and the at least one series VOA are controlled by independent control signals, respectively. The at least one VOA and the at least one series VOA may each include different phase contrast attenuation characteristics that are appropriately configured. The at least one VOA and the at least one series VOA may both set their attenuations to provide the same amplitude for 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 each signal path in the one or more signal paths includes a VOA(X) and a 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, and the two VOAs include the LO VOA(X) and the LO VOA(Y) in the LO signal path.

[0008] The phase change compensation mechanism caused by the VOA may include the at least one VOA, and the at least one VOA is a non-chirped Mach-Zehnder (MZ) interferometer. The non-chirped Mach-Zehnder (MZ) interferometer may include a p-i-n 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 may reduce the phase change caused by the VOA to within or less than tens of thousands and .

[0010] In another embodiment, a coherent receiver is formed by a process including the steps of: forming a received signal (SIG) path including i) an input terminal configured to connect to a received signal, ii) one or more signal paths connected to the input terminal and one or more optical mixers, and iii) at least one variable optical attenuator (VOA); forming a local oscillator (LO) signal path including an input terminal configured to connect to the LO and the one or more optical mixers; and providing a phase change compensation mechanism caused by the VOA that reduces any phase change caused by the VOA in the SIG-LO beat signal due to the at least one VOA. The phase change compensation mechanism caused by the VOA may include at least one series VOA in the LO path. The phase change compensation mechanism caused by the VOA may include the at least one VOA, and the at least one VOA is a non-chirped Mach-Zehnder (MZ) interferometer.

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

[0012] The present disclosure is illustrated and described with reference to various drawings, wherein, where appropriate, the same reference numerals are used to denote the same system components / method steps, and wherein:

[0013] Figure 1 FIG. is a block diagram of a coherent receiver 10 having internal VOA in X and Y receive signal paths.

[0014] Figures 2A - 2D FIG. is a characteristic diagram of a typical VOA using a p-i-n junction in carrier injection mode (forward voltage operation).

[0015] Figure 3 FIG. is a block diagram of a coherent optical receiver having internal VOA in X and Y receive signal paths and complementary VOA in the LO signal path.

[0016] Figure 4 FIG. is a block diagram of a coherent optical receiver having internal VOA in X and Y receive signal paths and complementary VOA after a 1×2 MMI in the LO signal path.

[0017] Figure 5 FIG. is a diagram of two VOAs having mismatched phase contrast attenuation responses.

[0018] Figure 6 FIG. is a diagram of two VOAs whose phases can be matched if independent control and calibration can be used for X and Y polarizations.

[0019] Figure 7 FIG. is a process diagram for matching the phases of two VOAs.

[0020] Figure 8 FIG. is a flowchart of a process for VOA-induced phase change cancellation.

[0021] Figure 9 FIG. is a schematic diagram of a VOA in a silicon photonics platform.

[0022] Figure 10 and Figure 11 is a graph showing the relationship between the phase and amplitude of a balanced MZI with an ideal 3 dB coupler.

[0023] Figure 12 is a block diagram of an MZI VOA having secondary phase shifters on each arm, where the secondary phase shifters are separated.

[0024] Figure 13 is a block diagram of an MZI VOA having secondary phase shifters on each arm, where the secondary phase shifters overlap.

[0025] Figure 14 and Figure 15 is a top view ( Figure 14 ) and a side view ( Figure 15 ) schematic diagram 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.

[0026] Figure 16 and Figure 17 is a top view ( Figure 14 ) and a cross-sectional view ( Figure 15 ) schematic diagram of an embodiment of a single arm of an MZI VOA in which the secondary phase shifter covers the high-speed phase shifter.

[0027] Figure 18 shows a diagram of an embodiment of a single arm of an MZI VOA having a combination of a p-i-n and a resistive heater (electrically connected).

[0028] Figure 19 shows a diagram of an embodiment of a single arm of an MZI VOA having a combination of a p-i-n and a resistive heater (AC / DC decoupled).

[0029] Figure 20 is a flowchart of a process for a coherent receiver having a low phase change caused by a variable optical attenuator (VOA). Detailed Description

[0030] In other words, the present disclosure relates in various aspects to systems and methods for coherent receivers with low phase changes caused by VOA. In an embodiment, the present disclosure relates to systems and methods for coherent receivers configured to eliminate phase changes caused by VOA. To eliminate the phase changes caused by VOA in the signal (SIG) path, we introduce a complementary VOA in the LO path to avoid phase changes in the beat signal. This method provides a simplified compensation scheme to eliminate any need for a circuit-based compensation scheme. In another embodiment, the present disclosure includes a high-speed VOA that minimizes optical phase shift during its operation while having minimal insertion loss at full-open settings. This includes an MZ structure incorporating low-speed and high-speed phase shifters for implementing a variable optical attenuator with low phase shift and low insertion loss at full "bright" settings.

[0031] Coherent receiver

[0032] Figure 1 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 an LO signal path 16. The receive signal path 14 includes an X-polarized signal path 18X and a Y-polarized signal path 18Y.

[0033] In one embodiment, the receive signal path 14 includes a spot size converter (SSC) 20 for optically coupling light from the received (Rx) signal (SIG input) into the photon chip. Next, there is a polarization beam splitter (PBS) 22 that splits the X and Y polarizations onto respective signal paths 18X and 18Y, and a polarization rotator (PR) 24 on the signal path 18Y, which is only needed when it is necessary to rotate the polarization. Next, each of the signal paths 18X, 18Y has similar components, including a tap coupler 24 that is connected to a photodetector 26 and VOAs 18X, 18Y. After the VOAs 12X, 12Y, there is another tap device 28 that is connected to a photodetector 30 and a 90-degree optical hybrid 32 (note that the term "optical hybrid" as used 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.

[0034] The optical mixer 32 also includes an input from the LO signal path 16. The LO signal path 16 includes an SSC 36 that is connected to the LO input and to a multimode interferometer (MMI) 38 configured as a 1×2 beam splitter, and the multimode interferometer 38 provides an LO signal to the optical mixer 32 for coherent detection of the Rx signal. Each output of the optical mixer 32 is connected to a balanced pair of high-speed photodetectors (HSPDs) 40, namely, each pair corresponding to the in-phase (I) and quadrature (Q) signals of each X polarization and Y polarization, i.e., XI, XQ, YI, YQ.

[0035] Note that there are VOAs 12X, 12Y on the received signal path 14, but not on the LO signal path 16. Therefore, when the VOA attenuation changes, a phase difference will be generated between these two paths 14, 16. At the same time, Figure 1 The components in are example embodiments, and those skilled in the art will recognize that there can be different components. Figure 1 The key aspect is that when the attenuation of the VOA changes, the VOAs 12X, 12Y cause the phase difference between these two paths 14, 16 to change. This is disadvantageous in applications and depends on the magnitude and speed of the phase change caused by the VOA.

[0036] VOA

[0037] Figures 2A - 2D Figure showing the characteristics of a typical VOA using a p-i-n junction in the 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 optical attenuation through carrier injection. The intrinsic region is located at the center of the waveguide, and light propagates in the center of the waveguide. However, the presence of carriers injected into the waveguide has the consequence of changing the refractive index (as implied by the equation of Soref, 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 20 dB of attenuation, a significant phase change of nearly 10 rad will be obtained.

[0038] 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 the difference in the photocurrents of each photodiode (i.e., high-speed photodetector (HSPD)) in a given balanced pair The beat signal can be represented by the following equation:

[0039]

[0040] where R is the responsivity of the photodiode, 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 the 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 change caused by the VOA.

[0041] 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 based on the magnitude and speed at which the change occurs. Ideally, when attenuating the optical signal from the SIG port, no such phase change occurs, eliminating the need for any compensation.

[0042] The coherent receiver eliminates the phase change caused by the VOA

[0043] The present disclosure proposes adding VOAs 42, 42X, 42Y in the LO signal path 16, which can be identical, substantially similar, or different from the VOA used in the SIG path but configured to be similar to the VOA used in the SIG path, as Figure 3 and Figure 4 shown, and adjusting the beat signal amplitude using both the SIG-VOA and the LO-VOA simultaneously. Figure 3 Block diagram of the coherent optical receiver 10 having internal VOAs 12X, 12Y in the X and Y receive signal paths 14 and complementary VOAs 42 in the LO signal path 16. Figure 4 Block diagram of the coherent optical receiver 10 having internal VOAs 12X, 12Y in the X and Y receive signal paths 14 and complementary VOAs 42X, 42Y after the 1×2 MMI 36 in the LO signal path 16.

[0044] Using the same voltage to control the "LO-VOA" 42 as is used for the "SIG-VOA" 12 causes the same phase change in the LO signal path 16 and in the receive signal path 14 (i.e., ), such that at all times, the difference Associated between the changes is eliminated so that the phase of the beat signal is not affected.

[0045] Of course, since the LO-VOA 42 is used, different attenuation adjustments are required to generate beat signals of the same amplitude because the beat signal is proportional to the product of the SIG and LO optical powers (see the equation above). Therefore, compared to the case where attenuation is provided only by the SIG VOA 12, the attenuation provided by both the SIG VOA 12 and the LO VOA 42 needs to be divided by 2 (in dB).

[0046] The disadvantage of using the LO-VOA 42 is the associated additional loss introduced by the VOA in the LO signal path 16 - generally up to a few tenths of a dB at most. Since the total attenuation is now provided by two components, this additional loss can be partially compensated by the relatively short part of the SIG-VOA 12 (compared to the scheme without the LO-VOA). Overall, since the SIG receive signal path 14 carries an optical signal with information and this must maintain a high signal-to-noise ratio (SNR), the loss in the SIG receive signal path 14 is more critical.

[0047] The control signal for the LO-VOA 42 can be the same voltage as that used to control the SIG-VOA 12. For the operation of high-speed VOAs, this allows the time matching of the control signals for the dynamic cancellation of the phase. Alternatively, optionally, another control scheme is to connect all the VOAs in series to ensure that the same current flows through each VOA, so that each VOA generates exactly the same phase shift. However, the latter option requires a higher control voltage.

[0048] Independent signals can also be sent to each of the VOAs 12, 42. This can ensure that if there are slight manufacturing differences between the LO-VOA 42 and the SIG-VOA 12 and thus different "attenuation vs. voltage" or "attenuation vs. current" characteristics (depending on whether the VOA is controlled by a voltage source or a current source), the phase remains fully compensated. These independent control signals will take into account these different individual characteristics and differentially adjust the LO-VOA and SIG-VOA voltages or currents. When the ambient (chassis) temperature changes, independent control can also be used to differentially adjust the LO-VOA and SIG-VOA to compensate for the effect of the local temperature difference between the LO-VOA and the SIG-VOA when the ambient (enclosure) temperature changes.

[0049] Using only one VOA 42 in the LO signal path 16 (as Figure 3 shown) somewhat limits the same attenuation on the X and Y channels. AsFigure 4 As shown, if one wants to independently adjust the attenuation of the X and Y channels, two VOAs 42X and 42Y can be used in the LO path after the X-Y beam splitter (1×2 MMI 38) in the LO signal path 16.

[0050] VOA control

[0051] Figure 5 It is a graph of two VOAs 42X and 42Y with mismatched phase-contrast attenuation responses. Figure 6 It is a graph of two VOAs 42X and 42Y with phases matched to a single control value. Figure 7 It is a schematic diagram of a process 50 for matching the phases of two VOAs 42X and 42Y.

[0052] As described herein, the VOAs 42, 42X, and 42Y can be referred to as "series" VOAs, meaning they are used in conjunction with the VOAs 12X and 12Y, i.e., the VOAs 12X and 12Y in the SIG path and the VOAs 42, 42X, and 42Y in the LO path are used in conjunction. That is to say, series refers to the operation of the VOAs, working in conjunction with each other.

[0053] The control of the attenuation of the series VOAs is performed by a digital optoelectronic automatic gain control (AGC) loop. To address Figure 5 the fact that the two VOAs shown in the example do not have perfectly matched phase-contrast attenuation characteristics, the digital AGC loop for controlling the VOAs can have "gain and offset" and / or "random access memory (RAM look-up table (LUT))" compensation features to match the phase-contrast attenuation responses of the two VOAs 42X and 42Y.

[0054] In Figure 7 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:

[0055] (1) As Figure 6 shown, the gain and offset elements are used to map the range of the integrator output OAGC_OUT such that at a given OAGC value, the two HCDACs will set their respective VOA attenuations so that they obtain nearly the same phase. The difference in the attenuation settings does not affect the performance of the AGC and the receiver.

[0056] (2) If there is a RAM LUT table, it can be programmed to perform the "gain and offset" function as described in (1) above; in addition, the RAM LUT table can also be programmed to make The slope is linearized to a constant. By doing so, the slope of the attenuation - HCDAC can also be linearized to a constant, which is required for AGC performance.

[0057] Of course, consider various methods for controlling VOA 42X and 42Y to ensure that the phase of the LO - VOA matches the phase of the SIG - VOA. This control is beneficial for implementing different VOA characteristics on the same silicon photonics (SiP) die and also for their differences with temperature changes.

[0058] VOA phase cancellation process

[0059] Figure 8 FIG. 80 is a flowchart of process 80 for phase change cancellation caused by VOA. Process 80 can be implemented on coherent receiver 10 or the like. Process 80 includes receiving a received signal at an input of a received signal path having one or more signal paths, attenuating the received signal by a variable optical attenuator (VOA) in each of the one or more signal paths, and providing the attenuated received signal to one or more optical mixers (step 82); receiving a local oscillator (LO) signal, attenuating the LO signal by one or more complementary VOA, and providing the attenuated LO signal to one or more optical mixers (step 84); and setting one or more complementary VOA to substantially cancel any phase change from each of the one or more signal paths due to the VOA (step 86). Note that the term "substantially" means that the cancellation of the phase change is set to eliminate all phase changes or to eliminate most of the phase changes. Those skilled in the art will recognize that there may be a small phase difference, i.e., it may not be exactly equal to zero. That is, although the term "substantially" is not used in conjunction with phase change cancellation, those skilled in the art will understand that it means most of the phase changes.

[0060] The VOA in each of the one or more signal paths and the one or more complementary VOA can be p - i - n junctions. The VOA in each of the one or more signal paths and the one or more complementary VOA can each be the same type of VOA. The one or more signal paths can include two signal paths, one for X polarization and one for Y polarization, and the VOA in each of the one or more signal paths includes VOA(X) and VOA(Y). The one or more complementary VOA can include one VOA in the LO signal path. The one or more complementary VOA can include two VOA, including LO VOA(X) and LO VOA(Y) in the LO signal path.

[0061] Procedure 80 may further include controlling the VOA and one or more complementary VOAs in each of one or more signal paths with the same control signal. Procedure 80 may further include controlling the VOA and one or more complementary VOAs in each of one or more signal paths with independent control signals. Procedure 80 may further include compensating for different phase-attenuation characteristics between any pair of the VOA and one or more complementary VOAs in each of one or more signal paths. Procedure 80 may further include setting attenuation or VOA in the VOA and one or more complementary VOAs in each of one or more signal paths to provide the same amplitude as the corresponding beat signal.

[0062] Coherent receiver with complementary VOA

[0063] In one embodiment, a coherent receiver includes a received signal path and a local oscillator (LO) signal path. The received signal path includes: i) an input terminal configured to connect to a received signal, ii) one or more signal paths connecting 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 eliminate any phase change from the VOA in each of the one or more signal paths.

[0064] The VOA and one or more complementary VOAs in each of one or more signal paths may be p-i-n junctions, i.e., all are CI-VOAs as described herein. The VOA and one or more complementary VOAs in each of one or more signal paths may all be of the same type of VOA. The same type may be CI-VOA, chirped MZ-VOA, and the like. The chirped MZ VOA is single-ended driven on the upper MZ arm 14 or the lower MZ arm 16. In another embodiment, the VOA and one or more complementary VOAs in each of one or more signal paths may each be a different type of VOA. Additionally, the different type may be any one of CI-VOA, chirped MZ-VOA, and the like.

[0065] One or more signal paths may include two signal paths, one for X polarization and one for Y polarization, where each path in the one or more signal paths includes a VOA(X) and a VOA(Y). One or more complementary VOA may include one VOA in the LO signal path. One or more complementary VOA may include two VOAs, including an LO VOA(X) and an LO VOA(Y) in the LO signal path.

[0066] The VOA in each of the one or more signal paths and the one or more complementary VOA may be controlled with the same control signal. The VOA in each of the one or more signal paths and the one or more complementary VOA may each be controlled with an independent control signal.

[0067] The VOA in each of the one or more signal paths and the one or more complementary VOA may each include different phase contrast attenuation characteristics and thus be compensated. The VOA in each of the one or more signal paths and the one or more complementary VOA may each set their attenuation to provide the same amplitude of the corresponding beat signal.

[0068] Mach - Zehnder interferometer structure of high - speed, low - phase - change and low - insertion - loss VOA

[0069] VOAs 12, 42 may include a p-i-n junction operating in forward bias. The MZ structure itself is capable of providing a VOA (or rather, a switch with a continuous control signal using only one port) on a platform lacking the necessary dopants, regardless of 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 content of which is incorporated herein by reference.

[0070] Furthermore, as described herein, for a given attenuation, a standard VOA causes a certain amount of phase shift, which will exceed the phase shift that can be compensated while keeping the device compact. Due to manufacturing uncertainties, the MZI VOA using a p-i-n junction is inherently lossy in its bright state, and manufacturing uncertainties force the use of the p-i-n junction to shift the operating point of the MZI to its fully open position.

[0071] In one embodiment, the present disclosure includes a high-speed VOA that minimizes optical phase shift during its operation while having a minimum insertion loss at the fully open setting. A Mach-Zehnder interferometer structure incorporating both a low-speed phase shifter and a high-speed phase shifter is used to generate high attenuation while limiting the phase shift. We describe the implementation of phase shifters for limiting insertion loss (IL), controlling the complexity of the control loop, and maximizing compactness.

[0072] A low-phase-change VOA is useful for coherent receivers. Demodulation involves comparing the time-varying amplitude and / or phase of an 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 an optical fiber link), any additional phase imparted by the VOA can introduce errors in demodulation. Although state-of-the-art ASICs can compensate for such phase variations to a limited extent, it is still useful and in some cases necessary to minimize the phase shift as much as possible.

[0073] Figure 9 FIG. 7 is a 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 respectively connected to metal contacts 110, 112, a waveguide core 114, and a top oxide cladding 116. The fast (e.g., on the order of 10 - 100 MHz) VOA in the silicon photonics platform 100 most commonly operates based on a forward-biased p-i-n junction. The attenuation mechanism is based on the optical absorption provided by carriers injected into the intrinsic region, which also serves as the waveguide structure through which light passes.

[0074] The relationship between carrier concentration, phase shift, and optical absorption near the optical wavelength of 1550 nm is given by the empirical Soref and Bennet equations, 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 content of which is incorporated herein by reference:

[0075]

[0076] where Δα is the additional material absorption, Δn is the shift in the silicon refractive index, and ΔN e and ΔNh They are the free electron and hole concentrations respectively. To perform the VOA function, the absorption change means changing the optical phase by changing the refractive index of the photoconductive material. This is also measured experimentally, as shown previously Figures 2A - 2D as shown

[0077] It is worth noting that for the increasing phase shift predicted by the theoretical equation, the attenuation decreases instead, which can also be seen in the experiment ( Figure 2A ). By making the VOA longer, it is possible to reduce to some extent the large phase shift caused by a specific attenuation target. However, this has obvious disadvantages for the integration of longer waveguides in larger structures and the optical insertion loss (IL).

[0078] Due to the physical reality that current injection into the VOA always generates a phase shift, this fact can be utilized by embedding the current injection into the VOA in an interferometer system, and the simplest interferometer system is the Mach-Zehnder interferometer (MZI). Figure 10 and Figure 11 show the relationship between the phase and amplitude of a balanced MZI with ideal 3dB couplers. The said phase shift is applied to a single arm of the MZI.

[0079] By causing a π phase shift between the two arms, the cross-port transmission changes from fully bright to fully attenuated, while causing a maximum phase shift of π / 2 at the output. In practice, this is much lower (14 - 20 times lower) than that of a conventional p-i-n VOA maintained at a reasonable length (≤1mm). Resonant systems (e.g., ring resonators including phase shifters) still require the same total π phase shift to go from fully bright to fully attenuated, are highly sensitive to the operating wavelength (thus incompatible with multi-channel input signals), and are more likely to generate larger excess losses.

[0080] To maintain a fast-acting VOA, a fast phase shifter is still required to cause this π phase shift. A forward-biased p-i-n is also a preferred embodiment of the MZI structure. To not cause too much parasitic attenuation, it needs to maintain a certain length. 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., the cross port).

[0081] The differential operation of the high-speed phase shifter will allow a net zero phase shift of the output signal, but will require operating a dual phase shifter at the midpoint, where these high-speed phase shifters will result in a higher base insertion loss at maximum transmission.

[0082] Secondary phase shifters in each arm

[0083] Figure 12Block diagram of an MZI VOA 200 having secondary phase shifters 202 on each arm 204, 206, where the secondary phase shifters are separate. Figure 13 Block diagram of an MZI VOA having secondary phase shifters 202 on each arm 204, 206, where the secondary phase shifters 202 are overlapping. The MZI VOA 200 includes a coupler 200 that splits the input into each arm 204, 206. In Figure 12 it, the MZI VOA 200 includes secondary phase shifters 202 separate from high-speed phase shifters (HSPS) 212. In Figure 13 it, the MZI VOA 200 includes secondary phase shifters 202 stacked on high-speed phase shifters (HSPS) 212. The secondary phase shifters 202 can be thermal phase shifters (TPS). The high-speed phase shifters 212 can be p-i-n structures. Two arms 204, 206 are combined together by a coupler 314 and an output is provided.

[0084] Introducing secondary phase shifters 202 in each arm 204, 206 is advantageous. The secondary phase shifters can operate at a slower speed but without causing any (significant) attenuation to the induced phase shift. This allows the MZI VOA 200 to be properly biased at the transmission maximum when the high-speed phase shifters 212 are not operating. Due to manufacturing defects, the fabricated device will have an initial random phase difference between the two arms 204, 206, which can be compensated by the secondary phase shifters 202. A common mechanism utilized to fabricate such phase shifters 202 in a silicon photonics platform is the thermo-optic effect by locally heating the waveguide. Another mechanism is to use microelectromechanical actuators, such as described in "Silicon photonic microelectromechanical phase shifters for scalable programmable photonics." Opt. Lett. 46.22 (2021):5671 - 5674, the content of which is incorporated herein by reference.

[0085] Figure 12 and Figure 12 The couplers 210, 214 shown in it are 2×2 couplers, but can 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. The silicon photonics implementation of such couplers 210, 214 can be a multimode interference (MMI) coupler, a directional coupler, an adiabatic coupler, or any other compact device that achieves such a function.

[0086] Other arrangements combining high - speed p - i - n phase shifters with thermal phase shifters

[0087] Various other arrangements for combining a high-speed p-i-n phase shifter with a thermal phase shifter in an MZ structure are described below.

[0088] Figure 14 And Figure 15 Top view ( Figure 14 ) and side view ( Figure 15 ) schematic diagrams of an embodiment in which the secondary phase shifter 202 of the single arms 204, 206 of the MZI VOA 200 is separated from the high-speed phase shifter 212. The secondary phase shifter 202 can be titanium nitride (TiN), or resistively doped silicon can be used on both sides of the optical waveguide. Deep trenches can be etched to increase power efficiency by increasing thermal isolation from the environment, thereby increasing the heat trapped around the waveguide.

[0089] Figure 16 And Figure 17 Top view ( Figure 14 ) and cross-sectional view ( Figure 15 ) schematic diagrams of an embodiment in which the secondary phase shifter 202 of the single arms 204, 206 of the MZI VOA covers the high-speed phase shifter 212. Here, the TiN TPS is placed directly on top of the p-i-n junction. This saves space (and reduces IL), but heats the (translation note: p-i-n) junction during operation, which must be taken into account in the control loop.

[0090] Figure 18 Diagrams showing an embodiment of the single arms 204, 206 of the MZI VOA 200 having a combined p-i-n and resistive heater (electrically connected). The side doped portions (p++ and n++) forming the p-i-n junction are reused as the resistive heater. The resistive portion can extend over the p-i-n portion. Now, the junction and heater controls 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 shape factor (which affects the sheet resistance value). The resistance values of the resistors do not have to be exactly the same.

[0091] Figure 19A diagram showing an embodiment of a combined p-i-n and resistive heater (AC / DC decoupled) for a single arm 204, 206 of an MZI VOA 200. 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. To quickly compensate for static perturbations, the two signals are combined, and after the RC decay of the fast signal, the slow signal takes over. Other AC / DC schemes where the two are not fully decoupled can also be envisioned.

[0092] More generally, the p-i-n and resistive portions can be directly combined ( Figure 18 ), or segmented ( Figure 19 ). The current-injected p-i-n junction combines a resistive element for thermo-optic excitation. This can include segmentation and AC / DC decoupling. This is done to address the requirements of a low-phase-change VOA in the coherent receiver 10.

[0093] The MZI VOA 200 describes a chirp-free Mach-Zehnder interferometer that includes

[0094] (1) a p-i-n junction phase shifter (current-injected VOA) for fast phase control, and

[0095] (2) a thermal phase shifter (TPS) for slow phase control.

[0096] The combination of these two structures achieves a VOA with low phase shift and low insertion loss in the all "bright" setting.

[0097] Combination

[0098] The VOA 12, 42 in the coherent receiver 10 can use the MZI VOA 200. In other words, the coherent receiver 10 includes a VOA 42 that generates very low [SIG × LO] phase perturbations when the VOA 42 is excited.

[0099] A coherent receiver 10 with low [SIG × LO] phase perturbations from VOA excitation can be achieved using:

[0100] (1) SIG and LO series VOAs (VOAs on the SIG path and the LO path)

[0101] (2) CI-VOAs on the SIG path and CI-VOAs on the LO path

[0102] (3) A chirped MZ-VOA on the SIG path and a CI-VOA on the LO path. The chirped MZ VOA is driven single-endedly on either the upper MZ arm or the lower MZ arm.

[0103] (4) A chirped MZ-VOA on the SIG path and a chirped MZ-VOA on the LO path

[0104] (5) A CI-VOA on the SIG path and a chirped MZ-VOA on the LO path

[0105] The focus of these two methods is to generate phase excitations on both the SIG path and the LO path, such that the [SIG × LO] phase perturbation caused by the VOA is close to zero.

[0106] (6) A zero-chirp MZ VOA 200 (VOA only on the SIG path)

[0107] This MZ VOA is driven differentially on the upper MZ arm and the lower MZ arm.

[0108] In one embodiment, a coherent receiver includes a received signal (SIG) path and a local oscillator (LO) signal path. The received signal (SIG) path includes: i) an input terminal configured to connect to a received signal, ii) one or more signal paths connecting to the input terminal and one or more optical mixers, and iii) at least one variable optical attenuator (VOA); the local oscillator (LO) signal path includes an input terminal configured to connect to the LO and one or more optical mixers; and a phase change compensation mechanism caused by the VOA, which reduces any phase change caused by the VOA 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 phase change compensation mechanism caused by the VOA is configured to remove or reduce any phase change caused by the VOA in the SIG-LO beat signal.

[0109] In one embodiment, the phase change compensation mechanism caused by the VOA includes at least one series VOA in the LO path. The at least one VOA and the at least one series VOA can be respectively the same type of VOA or respectively different types of VOA. The at least one VOA and the at least one series VOA can be respectively any one of a current injection VOA (CI-VOA) and a chirped Mach-Zehnder (MZ) interferometer (MZI) VOA (MZI-VOA). The at least one VOA and the at least one series VOA can be controlled by the same control signal respectively. The at least one VOA and the at least one series VOA can also be controlled by independent control signals respectively. The at least one VOA and the at least one series VOA can respectively include different phase contrast attenuation characteristics, which can be compensated. The at least one VOA and the at least one series VOA can both set their attenuation to the same amplitude that provides a corresponding beat signal. As described herein, a "series" VOA is the same (e.g., the same part number), substantially similar, or different but configured to operate in a substantially similar manner, etc.

[0110] 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 path of the one or more signal paths includes VOA(X) and VOA(Y). The phase change compensation mechanism caused by the VOA can include at least one series VOA in the LO path. The at least one series VOA can include two VOAs, and the two VOAs include LO VOA(X) and LO VOA(Y) in the LO signal path.

[0111] In yet another embodiment, the phase change compensation mechanism caused by the VOA includes at least one VOA, and the at least one VOA is a non-chirped Mach-Zehnder (MZ) interferometer. The non-chirped Mach-Zehnder (MZ) interferometer can include a p-i-n junction phase shifter for fast phase control and a thermal phase shifter (TPS) for slow phase control.

[0112] Figure 20Flowchart of process 300 for a coherent receiver having low phase change caused by a variable optical attenuator (VOA). Process 300 includes: receiving a received signal at an input of a receive signal path having one or more signal paths, attenuating the received signal by at least one variable optical attenuator (VOA), and providing the attenuated received 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 change in the SIG-LO beat signal due to the at least one VOA (step 306). The compensation may include (1) a series VOA for attenuating the LO signal such that the LO signal undergoes a VOA-induced phase change similar to that of the attenuated received signal, or (2) attenuation using a non-chirped Mach-Zehnder (MZ) interferometer.

[0113] Experimental results

[0114] The phase change caused by the VOA can be expressed in or units, and these units can be related, for example can be 2 - 4 times larger than . As is known to those skilled in the art, RMS is root mean square (RMS), RAD is radian, etc. In the Figure 1 method described herein, i.e., with the VOA only on the SIG path and on the LO path, the VOA-induced phase change can be on the order of hundreds of thousands of these values, and this may exceed the tracking ability of the coherent DSP. The chirped Mach-Zehnder (MZ) interferometer on the SIG path described herein can have a VOA-induced phase change on the order of tens of thousands of these values, which is a significant improvement over the CI-VOA only in the SIG path and is generally within the tracking ability of the coherent DSP. The series VOA method, i.e., with VOA on both the SIG path and the LO path, and the non-chirped MZ method on the SIG path, can have a VOA-induced phase change on the order of thousands of these values.

[0115] Although the chirped MZ on the SIG path has a phase change of less than tens of thousands of radians per second, when the Rx tracks the optical input power transient, using the DSP phase change ability is not desirable. When an optical transient occurs, there will also be a corresponding SOP phase rotation. The advantages of the series VOA and non-chirped MZ methods described herein include keeping the phase change caused by the VOA as low as possible, so that most of the phase change tracking budget in the DSP is allocated to any optical line perturbation.

[0116] Conclusion

[0117] It should be appreciated that some of the 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 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 the same, etc., in order to implement some, most, or all of the functions of the methods and / or systems 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 circuitry. Of course, combinations of the foregoing methods may be used. For some of the 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 for the various embodiments described herein.

[0118] In addition, some embodiments may include a non-transitory computer-readable medium having instructions stored thereon for performing the functions described and claimed herein on a computer, server, apparatus, device, 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 memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically EPROM (EEPROM), 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 circuit or logic), and 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 the various embodiments.

[0119] Although the present disclosure has been illustrated and described with reference to preferred embodiments and specific examples thereof, it will be apparent to those of ordinary skill in the art that other embodiments and examples can 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 hereby contemplated, 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 can be used in any and all combinations with each other.

Claims

1. A coherent receiver, comprising: a received signal SIG path including i) an input terminal configured to connect to a received optical signal, ii) one or more signal paths connecting to the input terminal and one or more optical mixers, and iii) at least one first variable optical attenuator VOA; and a local oscillator LO signal path including an input terminal configured to connect to the LO and the one or more optical mixers; wherein the coherent receiver includes a phase change compensation mechanism caused by the VOA, the phase change compensation mechanism caused by the VOA being configured to reduce any phase change caused by the VOA in the SIG-LO beat signal due to the at least one first VOA in the received signal SIG path, wherein the phase change compensation mechanism caused by the VOA includes one or more of the following two items: (1) at least one second VOA in the LO signal path before the one or more optical mixers, the at least one second VOA being configured to add an LO phase change to the LO signal path to at least partially compensate for the phase change caused by the VOA in the received signal SIG path due to the at least one first VOA; and (2) a chirp-free Mach-Zehnder MZ interferometer forming at least one first VOA in the received signal SIG path.

2. The coherent receiver according to claim 1, wherein the phase change compensation mechanism caused by the VOA includes the at least one second VOA in the LO signal path before the one or more optical mixers.

3. The coherent receiver according to claim 2, wherein the at least one first VOA and the at least one second VOA are both the same type of VOA.

4. The coherent receiver according to claim 2, wherein the at least one first VOA and the at least one second VOA are each different types of VOA.

5. The coherent receiver according to any one of claims 2 to 4, wherein the at least one first VOA and the at least one second VOA are respectively any one of a current injection VOA, i.e., a CI-VOA, and a chirp Mach-Zehnder MZ interferometer MZI VOA, i.e., an MZI-VOA.

6. The coherent receiver according to any one of claims 2 to 5, wherein the at least one first VOA and the at least one second VOA are both configured to be controlled by one of a same control signal and an independent control signal.

7. The coherent receiver according to any one of claims 2 to 6, wherein the at least one first VOA and the at least one second VOA each include appropriately configured different phase contrast attenuation characteristics.

8. The coherent receiver according to any one of claims 2 to 7, wherein the at least one first VOA and the at least one second VOA both set their attenuation to provide the same amplitude of the corresponding beat signal.

9. The coherent receiver according to any one of claims 2 to 8, wherein 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).

10. The coherent receiver according to claim 9, wherein the at least one second VOA in the LO signal path includes two VOAs, the two VOAs including LO VOA(X) and LO VOA(Y).

11. The coherent receiver according to claim 1, wherein the chirp-free Mach-Zehnder (MZ) interferometer comprises: a p-i-n junction phase shifter for fast phase control, and a thermal phase shifter TPS for slow phase control.

12. The coherent receiver according to any one of claims 1 to 10, wherein the phase change compensation mechanism caused by the VOA is configured to reduce the phase change caused by the VOA to within thousands of and 13. A method comprising the steps of: receiving a received signal at an input of a received signal SIG path having one or more signal paths, attenuating the received signal by at least one first variable optical attenuator (VOA), and providing the attenuated received 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 via an LO signal path; and compensating for any VOA-induced phase change in the SIG-LO beat signal due to the at least one first VOA in the received signal SIG path by one or more of the following: (1) at least one second VOA in the LO signal path before the one or more optical mixers, the at least one second VOA configured to add an LO phase change to the LO signal path to at least partially compensate for the VOA-induced phase change in the signal path due to the at least one first VOA, and (2) a chirp-free Mach-Zehnder MZ interferometer forming the at least one first VOA in the received signal SIG path.

Images