Coupled dual microring modulator, dual polarization IQ modulator, coherent modulation method

Abstract

The application discloses a coupled double-microring modulator, a double-polarization IQ modulator and a coherent modulation method.The coupled double-microring modulator comprises an input-output waveguide, an active microring resonator, a passive microring resonator and a driving controller.The active microring resonator and the passive microring resonator constitute a non-Hermite coupled double-microring system with equivalent PT symmetry characteristics.The passive microring resonator has a constant attenuation coefficient gamma 1.The MZI adjustable coupler arranged in the active microring resonator can dynamically control the transmittance under the action of the driving control unit, so that the equivalent attenuation coefficient gamma 2 of the active microring resonator changes with the driving signal.The coupled double-microring modulator, the double-polarization IQ modulator and the coherent modulation method disclosed by the application solve the defects of the existing silicon-based microring modulator, such as serious amplitude-phase coupling, large chirp, poor process compatibility of the traditional PT symmetric structure, high-order coherent modulation constellation distortion and the like.

Description

Technical Field

[0001] This invention belongs to the field of photonic integrated circuits and coherent optical communication technology, specifically involving coupled dual micro-ring modulators, dual polarization IQ modulators, and coherent modulation methods. Background Technology

[0002] With the rapid development of cloud computing, generative artificial intelligence, and large-scale data processing technologies, the demand for optical interconnect networks in modern data centers is growing exponentially. Traditional optical transmission systems based on the direct modulation-direct detection (IM-DD) architecture have gradually become performance bottlenecks, struggling to meet the demands of future ultra-high-speed, low-latency, and high-energy-efficiency data center applications. The industry consensus is that coherent optical communication architecture, with its superior spectral and power efficiency, has become a key technology direction for next-generation data center optical interconnects. Coherent optical communication requires precise amplitude, phase, and polarization modulation of optical signals at the transmitting end, which directly places stringent requirements on the performance indicators of the modulation chip.

[0003] Commercial silicon optical modulators mainly include Mach-Zehnder interferometer (MZI) modulators based on carrier dispersion effects, micro-ring modulators, and electro-absorption modulators (EAMs) combined with absorbing materials such as germanium and silicon. EAM devices typically have advantages such as small size, low driving voltage, and ease of high-density integration, but their operating bandwidth, insertion loss, extinction ratio, chirp characteristics, and process compatibility require comprehensive optimization. For higher-order modulation formats, limiting factors are not only the extinction ratio, but also linearity, optical signal-to-noise ratio, bandwidth, and flatness. MZI modulators are mature technology with good linearity and wide optical bandwidth, making them an important solution for current high-speed intensity modulation and coherent modulation. However, their device length is relatively large, and driving power consumption and bias stability control remain key considerations for system integration.

[0004] Microring modulators have attracted widespread attention due to their small size, high modulation efficiency, and natural compatibility with wavelength division multiplexing (WDM) systems. However, single microring modulators rely on transmission variations near the resonant spectral lines for modulation, making their performance highly sensitive to laser wavelength, resonant peak position, and temperature drift. Higher Q values ​​are beneficial for reducing drive power consumption and enhancing modulation efficiency, but they also narrow the optical bandwidth and increase sensitivity to wavelength mismatch; lower Q values ​​are advantageous for high-speed and wideband operation, but may sacrifice modulation depth. Therefore, traditional microring modulators typically require trade-offs between bandwidth, loss, extinction ratio, modulation efficiency, and thermal stability. For higher-order modulation formats in coherent communication, in addition to the extinction ratio, the device's linear response, phase noise, chirp, and group delay characteristics are equally critical.

[0005] In recent years, non-Hermitian photonics has provided new design ideas for microcavity modulators. PT (Parity-Time) symmetric systems belong to a class of optical systems with gain-loss balance or equivalent non-Hermitian coupling. Under certain conditions, their eigenmodes can exhibit real eigenfrequency; when system parameters cross the vicinity of the singularity point (EP point), mode coupling, loss distribution, and spectral response change significantly. These characteristics can be used to control the linewidth, transmission spectrum, and mode selectivity of the resonator, providing possibilities for improving the dynamic response of microring modulators. In coupled microring structures, modulators with non-Hermitian or near-PT symmetric characteristics can be constructed by designing the coupling strength, resonant frequency detuning, and loss difference between the two rings. Compared to single microrings, these structures hold promise for altering the transmission response by adjusting the equivalent loss and coupled mode distribution, thereby alleviating to some extent the performance limitations of single-resonant modulation.

[0006] Research on modulators based on non-Hermitian theory and employing a coupled dual-microring architecture is extremely limited both domestically and internationally. Most existing coupled microring systems are applied in filtering, optical switching, and other fields, while their dynamic response design, temperature compensation mechanisms, and application strategies in coherent optical modulation remain unexplored. This invention proposes an innovative solution based on the aforementioned technical background. By combining non-Hermitian PT-symmetry theory with a coupled dual-microring structure, a novel electro-optic modulator is designed, aiming to overcome the bottleneck problems of dispersion distortion and insufficient extinction ratio in traditional microring modulators, while simultaneously achieving the goals of small size, low power consumption, and high integration, providing strong technical support for next-generation data center coherent optical interconnects. Summary of the Invention

[0007] The main objective of this invention is to provide a coupled dual-micro-ring modulator, a dual-polarization IQ modulator, and a coherent modulation method to address the shortcomings of existing silicon-based micro-ring modulators, such as severe amplitude-phase coupling, large chirp, poor process compatibility with traditional PT-symmetric structures, and distortion of high-order coherent modulation constellation diagrams. This invention relies on relative loss modulation to construct an equivalent PT-symmetric non-Hermitian system, eliminating the need for on-chip optical gain. It uses loss parameters as the core modulation quantity to suppress amplitude-phase coupling, achieving near-zero chirp amplitude modulation. Simultaneously, it integrates a closed-loop thermal compensation structure to ensure stable operating point, is compatible with standard silicon photonics processes, and is suitable for high-speed coherent optical interconnects and on-chip optical communication systems in data centers.

[0008] Another objective of this invention is to provide a coupled dual-micro-ring modulator, a dual-polarization IQ modulator, and a coherent modulation method. This scheme utilizes loss modulation and mode coupling effects in the coupled micro-rings to achieve coordinated modulation of the output optical field amplitude and phase response without on-chip optical gain. By rationally designing the coupling coefficient, relative loss, resonant wavelength, and electro-optic modulation method of the two micro-rings, low-power, small-size, near-zero-chill electro-optic modulation functionality can be achieved on a single chip. Furthermore, it can be combined with an IQ modulation structure for use in silicon photonic integrated transmitters for short-range coherent interconnects in data centers. The technical value of this scheme lies not in simply replacing mature MZI coherent modulators, but in exploring a more compact and easily array-integrated micro-ring coherent modulation architecture.

[0009] To achieve the above objectives, the present invention provides a coupled dual-micro-ring modulator, comprising input and output waveguides, an active micro-ring resonator, a passive micro-ring resonator, and a drive controller, wherein: The active microring resonator and the passive microring resonator are directly coupled through an optical coupler to form a non-Hermitian coupled dual microring system with equivalent PT symmetry characteristics. The input and output waveguides are coupled to at least one microring resonator via an optical coupler; The passive microring resonator has a constant attenuation coefficient γ1, which is used to provide reference optical characteristics; The active microring resonator is equipped with an MZI adjustable coupler. The MZI adjustable coupler can dynamically adjust the transmittance under the action of the drive control unit, so that the equivalent attenuation coefficient γ2 of the active microring resonator changes with the drive signal. The coupled dual-microring system is configured to operate in a working region with non-Hermitian mode evolution characteristics. At the critical coupling point, the system changes the equivalent attenuation coefficient γ2 of the active microring resonator, enabling the system to cross the critical coupling point and switch between undercoupled and overcoupled states. This reduces the coupling between the amplitude response and phase response during modulation, generating a low-chill or near-zero-chill amplitude-modulated optical signal.

[0010] As a further preferred technical solution to the above technical solution, the passive micro-ring resonator adopts an undoped pure waveguide structure.

[0011] As a further preferred embodiment of the above technical solution, the MZI tunable coupler includes two directional couplers and an optoelectronic phase-shifting structure arranged on two interference arms, wherein: The photoelectric phase shift structure is a carrier injection PN junction, a carrier depletion PN junction, or a PIN junction, and adopts a push-pull driving method. While adjusting the equivalent attenuation coefficient γ2 of the active microring resonator, it suppresses the additional phase disturbance introduced by the driving signal.

[0012] As a further preferred technical solution to the above technical solution, it also includes a thermal tuning unit, an on-chip optical power detector, and a digital controller, wherein: The thermal tuning unit is a thin-film microheater, integrated on the surface of the active microring resonator and the passive microring resonator, used to adjust the microring resonant wavelength; The on-chip optical power detector is a germanium-silicon photodiode structure, used to collect the output optical power of the micro-ring in real time; The digital controller is electrically connected to the thermal tuning unit and the on-chip optical power detector. It adaptively adjusts the driving voltage of the thermal tuning unit based on the optical power feedback signal to lock the optimal operating point of the system.

[0013] As a further preferred embodiment of the above technical solution, the digital controller operates based on a disturbance observation algorithm, and the specific control steps are as follows: Step S1: Read the current optical power signal acquired by the on-chip optical power detector. ; Step S2: Calculate the optical power gradient ; Step S3: Determine the system operating point offset direction based on the power gradient direction; Step S4: Increase or decrease the driving voltage of the thermal tuning unit accordingly; Repeat the above steps until the system converges and stabilizes in its optimal operating state.

[0014] This invention also provides a dual-polarization IQ modulator based on polarization multiplexing, comprising a main beam splitter, a polarization beam combiner, and two sets of IQ coherent modulators corresponding to X-polarization and Y-polarization respectively, wherein: The main beam splitter splits a single continuous input light into two independent optical signals, which are then sent to two independent IQ coherent modulators for X-polarization and Y-polarization, respectively, to provide incident light sources. The two sets of IQ coherent modulators are combined by a polarization combiner to form a dual-polarization multiplexed coherent optical signal. The IQ coherent modulator includes an input beam splitter, an I modulation branch, a Q modulation branch, a quadrature phase shift unit, and an output beam combiner; each of the I modulation branch and the Q modulation branch is provided with at least one of the aforementioned coupled dual-micro-ring modulators. The orthogonal phase shift unit is used to introduce a π / 2 static phase difference between the I modulation branch and the Q modulation branch; the output beam combiner coherently combines the two modulated optical fields and outputs a complex optical field signal.

[0015] As a further preferred technical solution of the above technical solution, the modulator supports QPSK, 16-QAM, 32-QAM, 64-QAM and higher order coherent modulation formats; by adjusting the combination of driving signals of two sets of active micro-ring resonators, constellation points of the corresponding modulation format are generated in the complex plane, and the phase purity of the constellation points is improved by relying on the near-zero chirp characteristics, thereby suppressing the distortion of higher order coherent modulation signals.

[0016] This invention also provides a coherent modulation method based on coupled dual micro-rings, applied to the aforementioned dual-polarization IQ modulator, comprising the following steps: Step S1: System initialization, set the attenuation coefficient γ1 of the passive micro-ring resonator and the initial attenuation coefficient γ2 of the active micro-ring resonator, so that the coupled dual micro-ring system satisfies the PT-like broken state condition, and calibrate the system coupling coefficient and the optimal operating point; Step S2: Operating point closed-loop locking, on-chip optical power detector collects optical power signal in real time, digital controller uses perturbation observation algorithm to drive thermal tuning unit to perform temperature compensation, stabilize the system operating point at critical coupling point; Step S3: Modulation signal conversion, converting the externally input baseband modulation signal into a voltage drive signal, inputting it to the MZI adjustable coupler, and dynamically adjusting the equivalent attenuation coefficient γ2 of the active micro-ring resonator; Step S4: Amplitude modulation. By continuously changing γ2, the coupled dual micro-ring system is switched between undercoupled and overcoupled states to output a low-chill amplitude-modulated optical field. Step S5: IQ coherent modulation. During IQ modulation, the I modulation branch and the Q modulation branch execute steps S2-S4 independently. The orthogonal phase shift unit is used to make the two optical fields form a π / 2 phase difference, and finally the coherent beam combining is completed by the output beam combiner. Step S6: Real-time performance monitoring, continuously monitor optical power, operating point offset, and chip temperature parameters, dynamically correct control algorithm parameters, and ensure stable modulation performance.

[0017] As a further preferred embodiment of the above technical solution, the sampling and iteration period of the perturbation observation algorithm is based on the chip thermal relaxation characteristic time. The algorithm is set to wait until the optical power response corresponding to a single perturbation stabilizes before executing the next iteration.

[0018] As a further preferred technical solution of the above technical solution, when performing high-order coherent modulation, the driving voltages of the I modulation branch and the Q modulation branch are combined and adjusted to realize multiple modulation formats including QPSK, 16-QAM and 32-QAM; by utilizing the characteristics of low amplitude-phase coupling and low residual chirp, the phase error and amplitude error of the high-order modulation constellation point are reduced.

[0019] The beneficial effects of this invention are as follows: 1. Strong process compatibility: Adopting a pure silicon-based loss control scheme, it eliminates the need for on-chip optical gain and III-V heterogeneous integration, enabling mass production on standard silicon photonics lines and CMOS lines, reducing production costs and process complexity.

[0020] 2. Excellent modulation performance: Amplitude modulation is achieved mainly through loss control, which effectively suppresses amplitude-phase coupling and frequency chirp. The modulation signal has a high extinction ratio and good phase purity, which significantly improves the constellation diagram quality of high-order coherent modulation.

[0021] 3. High integration and low power consumption: It inherits the advantages of small size and high modulation efficiency of micro-ring resonators, and is easy to achieve high-density array integration; the driving voltage swing is small, and the overall power consumption is much lower than that of traditional MZI coherent modulators.

[0022] 4. Strong operational stability: The integrated on-chip optical power detection, thermal tuning unit and disturbance observation closed-loop control algorithm automatically compensate for temperature drift, thermal crosstalk and process deviation, greatly improving the device's environmental adaptability and long-term operational reliability.

[0023] 5. Wide range of applications: It can be used as an amplitude modulator on its own, or integrated into an IQ modulator or a dual polarization multiplexing modulator, and is suitable for scenarios such as short-distance coherent interconnects in data centers, on-chip optical networks, and high-speed silicon photonics emission arrays. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of an embodiment of the single silicon-based coupled dual-micro-ring coherent modulator of the present invention.

[0025] Figure 2 This is a schematic diagram of the distribution of PT-symmetric and broken regions in the coupled dual micro-ring system of the present invention.

[0026] Figure 3 This diagram illustrates the transmission spectrum and phase changes corresponding to modulation in different types of PT-symmetric broken state regions.

[0027] Figure 4 This is a schematic diagram of the electric field vectors in the complex plane of the output light wave corresponding to two PT-like symmetric broken state regions.

[0028] Figure 5 This is a schematic diagram of the structure of an embodiment of the single-chip dual-polarization IQ coherent modulator array of the present invention. Detailed Implementation

[0029] The following description is intended to disclose the present invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art. The basic principles of the invention defined in the following description can be applied to other embodiments, modifications, improvements, equivalents, and other technical solutions that do not depart from the spirit and scope of the invention.

[0030] In the preferred embodiments of the present invention, those skilled in the art should note that QPSK, 16-QAM, 32-QAM, 64-QAM, etc. involved in the present invention can be regarded as prior art.

[0031] Preferred embodiment.

[0032] A major challenge in using traditional silicon-based microring modulators for high-order coherent modulation is that their amplitude and phase responses are often difficult to control completely independently. Taking a common PN junction depletion-type microring modulator as an example, the applied driving voltage alters the depletion region width and carrier distribution, simultaneously causing changes in both the effective refractive index and free carrier absorption. Due to the plasma dispersion effect in silicon, changes in carrier concentration lead to changes in refractive index, accompanied by changes in absorption loss. Therefore, the resonant wavelength, transmission intensity, and output phase of the microring all vary with the driving signal. Because the microring itself has strong wavelength selectivity, its amplitude and phase responses often exhibit a nonlinear coupling relationship near the operating point, causing the trajectory of the modulated light field in the complex plane to deviate from an ideal linear distribution. When this traditional microring modulator is applied to IQ modulation, it results in constellation diagram distortion, decreased modulation linearity, and a certain degree of frequency chirp. For high-order coherent modulation formats such as 16QAM and 64QAM, these non-ideal effects increase the complexity of digital predistortion, bias control, and equalization compensation.

[0033] On the other hand, traditional PT-symmetric optical systems typically employ a gain-loss balance structure, where one resonator introduces optical gain and the other introduces equivalent loss, thus forming a non-Hermitian coupling system with PT-symmetric characteristics. However, achieving on-chip optical gain in standard silicon photonics platforms is not easy. Silicon itself is an indirect bandgap material, making efficient light emission difficult. Introducing gain through heterogeneous integration of III-V materials, doped gain media, or epitaxial processes increases process complexity, thermal management difficulty, and manufacturing costs, and may reduce compatibility with CMOS-compatible processes. Therefore, in silicon photonic modulators, a more practical approach is usually not to pursue strict gain-loss PT symmetry, but rather to utilize fully passive or loss-tunable non-Hermitian coupling structures to construct equivalent physical responses related to PT-symmetric phase transitions, EP points, or the evolution of non-Hermitian modes.

[0034] Based on the above understanding, this invention can be described as follows: By introducing designable and tunable relative losses into a silicon-based coupled dual microring, and combining this with the refractive index and loss changes caused by electro-optic modulation, the device operates near a specific non-Hermitian coupling state, thereby improving the amplitude-phase coupling characteristics of the microring modulator, reducing modulation chirp, and enhancing the constellation diagram quality during high-order coherent modulation. This approach avoids dependence on on-chip optical gain, better aligns with the practical conditions of silicon photonics integration processes, and retains the advantages of microring modulators such as small size, low capacitance, low power consumption, and high-density integration. Ultimately, this technology is expected to provide a modulator implementation scheme with practical application potential for data center coherent optical interconnects, on-chip optical networks, and high-density silicon photonics emission arrays.

[0035] This invention discloses a dual-micro-ring coherent modulator and a dual-polarization IQ modulator based on PT-like symmetric non-Hermitian coupling, along with their operating methods. The entire system is fabricated using standard silicon photonics technology, is compatible with CMOS process flows, and features high integration, low power consumption, and strong mass production capabilities. The preferred embodiments are described in detail below, module by module, focusing on device structure, optical path principle, control logic, workflow, and fabrication process.

[0036] This invention discloses a coupled dual-micro-ring modulator, such as Figure 1 As shown, the core basic unit of the entire technical solution includes input and output waveguides, active microring resonators, passive microring resonators, and a drive controller, wherein: Substrate and waveguide fabrication: In this embodiment, a 220nm thick single-crystal silicon film SOI wafer is used as the substrate material, and a silicon waveguide is fabricated using a 193nm deep ultraviolet lithography combined with reactive ion etching process. The waveguide cross-section is set to a width of 550nm and a height of 220nm. This structure ensures single-mode transmission in TE mode while providing strong lateral and longitudinal constraints on the optical field, reducing transmission loss. The passive microring resonator and the active microring resonator are independent of each other, with a spacing of 200nm between them. This spacing is compatible with standard silicon photonics processes and can stably achieve optical coupling between the two rings. By adjusting the length of the coupling region, the coupling coefficient K between the two rings can be precisely controlled. By combining the ratio of the passive microring attenuation coefficient γ1, the active microring equivalent attenuation coefficient γ2, and the coupling coefficient K, the PT-like symmetry breaking critical point can be flexibly adjusted, and the modulation working window can be optimized.

[0037] For passive micro-ring resonators: The passive microring resonator employs an undoped pure waveguide structure, preferably made of silicon waveguide, leveraging its low propagation loss characteristic of ≤1dB / cm. The passive microring resonator has a constant attenuation coefficient γ1, used to provide reference optical characteristics. For active micro-ring resonators: An active microring resonator and a passive microring resonator are directly coupled through an optical coupler to form a non-Hermitian coupled dual microring system with equivalent PT symmetry characteristics; the input and output waveguides are coupled to at least one microring resonator through an optical coupler to complete the input and output of optical signals.

[0038] An MZI adjustable coupler is installed inside the active microring resonator. Under the action of the drive control unit, the transmittance of the MZI adjustable coupler can be dynamically adjusted, so that the equivalent attenuation coefficient γ2 of the active microring resonator changes with the drive signal. The MZI adjustable coupler includes two directional couplers and photoelectric phase shift structures arranged on two interference arms. The photoelectric phase shift structures are carrier injection type PN junctions, carrier depletion type PN junctions, or PIN junctions, and adopt a push-pull drive method to adjust the equivalent attenuation coefficient γ2 of the active microring resonator while suppressing the additional phase disturbance introduced by the drive signal.

[0039] Specifically, a miniature Mach-Zehnder interferometer (MZI) tunable coupler is integrated at a designated location within the active microring (resonator) to dynamically change the equivalent attenuation coefficient of the active microring. For example... Figure 1 As shown, the MZI consists of two 3dB directional couplers. An S-shaped bent waveguide is used for routing between the two arms to minimize bending loss. The photoelectric phase shifters of the two interferometer arms of the MZI are configured as integrated carrier depletion-type PN junctions. The free carrier concentration is changed by reverse bias and push-pull modulation, thereby utilizing the carrier contribution in the Drude model to change the phase delay difference between the two arms, achieving an adjustable beam splitting ratio. This allows the equivalent attenuation coefficient γ2(t) of the active microring to vary with the driving signal. The operating state is as follows: When the phase delay difference between the two interference arms is π, the optical power flows back to the active microring to the maximum extent, and γ2(t) takes the minimum value; when the phase difference deviates from π, the additional loss of the active microring increases, and γ2(t) increases accordingly.

[0040] The push-pull drive mode can effectively suppress additional phase disturbances introduced by the drive signal while adjusting the loss, thus ensuring the phase purity of the optical field.

[0041] The coupled dual-microring system is configured to operate in a region exhibiting non-Hermitian mode evolution characteristics, preferably near a PT-like broken state or singularity. Near the critical coupling point, the system modulates the equivalent attenuation coefficient γ2 of the active microring resonator, allowing the system to cross the critical coupling point and transition between undercoupled and overcoupled states. This reduces the coupling between the amplitude and phase responses during modulation, generating a low-chill or near-zero-chill amplitude-modulated optical signal. Specifically... Figure 2 As shown, Figure 2The simulation results of the coupled dual-micro-ring coherent modulator are shown. Based on the relative relationship between the dual-ring attenuation coefficients (γ1, γ2(t)) and the coupling coefficient (K) between the two rings, the entire coordinate plane can be divided into three parts separated by dashed lines: the middle PT-symmetric region and the two PT-broken state regions in the upper left and lower right. The black dashed lines represent the EP point curve. In (a), different colors in the figure represent the resonance depth (extinction ratio), and the red line represents the resonance depth reaching its maximum value, i.e., the critical coupling state. In (b), different colors in the figure show the corresponding resonant optical bandwidth. The modulation of the micro-rings can be set near the critical coupling point in the PT-broken state. By adjusting the transmittance of the MZI coupler in the active ring, the attenuation coefficient γ2(t) of the active ring can be adjusted equivalently, thereby allowing the system to switch between undercoupled and overcoupled states. The working region can be selected from the upper left (PT-broken state region B) or the lower right (PT-broken state region A) in (a) figure, both of which can achieve near-zero chirp high extinction ratio modulation. Figure 3 The image shows the transmission spectrum and phase changes of the microrings in the two regions.

[0042] Figure 4 for Figure 3 The corresponding normalized output electric field vector diagrams on the complex plane during modulation in different broken-state regions are shown. When modulation is performed within the designed PT broken-state operating region, the output electric field vector mainly varies along the real axis, with a smaller change in the imaginary part, thus achieving low-chirp intensity modulation. Compared to broken-state region A, the modulation intensity change is more pronounced in broken-state region B, which is attributed to... Figure 2 The change in resonance depth caused by the same intensity change at region B in the middle region is greater. Therefore, combined with Figure 3 , Figure 4 It can be seen that when this scheme operates in the PT broken state region, the output optical field vector mainly changes along the real axis of the complex plane, and the phase component fluctuates very little; among them, the modulation amplitude change in the PT broken state region B is more significant, which is the preferred operating range.

[0043] It is worth mentioning that the technical features such as QPSK, 16-QAM, 32-QAM, and 64-QAM involved in this patent application should be regarded as prior art. The specific structure, working principle, and possible control methods and spatial arrangement methods of these technical features can be adopted using conventional choices in the field and should not be regarded as the inventive point of this patent. This patent will not be further elaborated in detail.

[0044] To address the issues of silicon microring resonators being susceptible to temperature drift and process variations, a thermal tuning unit, an on-chip optical power detector, and a digital controller are also included. The thermal tuning unit is a thin-film microheater, integrated on the surface of the active microring resonator and the passive microring resonator. The material is selected from any one of titanium nitride, tungsten, and platinum, preferably titanium nitride. The thickness is 50-200nm and the width is 1-5μm, which is used to adjust the microring resonant wavelength. The on-chip optical power detector is a germanium-silicon photodiode structure, used to collect the output optical power of the micro-ring in real time; The digital controller is electrically connected to the thermal tuning unit and the on-chip optical power detector. It adaptively adjusts the driving voltage of the thermal tuning unit based on the optical power feedback signal to lock the optimal operating point of the system.

[0045] Furthermore, the digital controller operates based on the Disturbance and Observation (P&O) algorithm, and the specific control steps are as follows: Step S1: Read the current optical power signal acquired by the on-chip optical power detector. ; Step S2: Calculate the optical power gradient ; Step S3: Determine the system operating point offset direction based on the power gradient direction; Step S4: Increase or decrease the driving voltage of the thermal tuning unit accordingly; Repeat the above steps until the system converges and stabilizes in its optimal operating state.

[0046] This invention also discloses a dual-polarization IQ modulator based on polarization multiplexing, comprising a main beam splitter, a polarization beam combiner, and two sets of IQ coherent modulators corresponding to X-polarization and Y-polarization respectively, wherein: The main beam splitter splits a single continuous input light into two independent optical signals, which are then sent to two independent IQ coherent modulators for X-polarization and Y-polarization, respectively, to provide incident light sources. The two sets of IQ coherent modulators are combined by a polarization combiner to form a dual-polarization multiplexed coherent optical signal (which combines two independent optical signals carrying X-polarization and Y-polarization information respectively into a single dual-polarization optical signal output, realizing polarization multiplexing). The IQ coherent modulator includes an input beam splitter, an I modulation branch, a Q modulation branch, a quadrature phase shift unit, and an output beam combiner; each of the I modulation branch and the Q modulation branch is provided with at least one of the aforementioned coupled dual-micro-ring modulators. The orthogonal phase shift unit is used to introduce a π / 2 static phase difference between the I modulation branch and the Q modulation branch; the output beam combiner coherently combines the two modulated optical fields and outputs a complex optical field signal.

[0047] Specifically, such as Figure 5 As shown, Figure 5This is a schematic diagram of the planar structure of a complete single-chip four-wavelength dual-polarization IQ coherent modulator, where each polarization single-wavelength channel integrates a pair of modulation units (I and Q paths).

[0048] A broadband tunable MZI is integrated as a 50 / 50 beam splitter at the optical input port of the chip. This coupler is designed with an adjustable structure, using a heater above it to regulate the splitting ratio between the two arms, maintaining a power distribution ratio close to 50% within the target operating wavelength. This design ensures power balance between the I-arm and Q-arm, avoiding constellation distortion caused by power inequality.

[0049] After beam splitting, the optical signals are routed along an S-shaped waveguide to the I-arm and Q-arm micro-rings respectively, minimizing bending loss. The entire distribution network adopts a split design, with the lower path used by the Q-arm and the upper path used by the I-arm. The two signals are finally combined by a polarization beam splitter (PSR) to output a single TE+TM dual-polarization IQ composite signal, which then enters the transmission link.

[0050] The modulation units of the I-arm and Q-arm have identical structures, each containing a coupled dual micro-loop and a corresponding driving circuit. The two modulation units receive independent driving signals V from an off-chip signal processor. I (t) and V Q (t), modulated at the MZI coupler integrated within their respective active microrings.

[0051] Each microring is equipped with a TiN thin-film microheater as a thermal tuning unit for temperature compensation. The heater is integrated above the active microring, covering the microring waveguide as much as possible to improve heating efficiency. An on-chip temperature-sensitive diode (based on the positive temperature coefficient of the PN junction) is integrated for real-time monitoring of the microring temperature. The feedback control system automatically adjusts the heater's drive voltage using a P&O algorithm based on signals from the temperature sensor and power monitor, maintaining the microring's operating point at a preset temperature or preset resonant wavelength.

[0052] To establish a precise π / 2 phase difference between the I and Q components, static phase shift structures are integrated before the convergence point of the I and Q arms, respectively. A TiN thin-film microheater design is employed, with an additional heater integrated on the Q-arm waveguide. A π / 2 phase delay is generated by applying a constant voltage to this heater during initialization. This static phase shift can be set during chip initialization and can be slowly calibrated based on temperature drift or system feedback.

[0053] The modulated light from the I-arm and Q-arm is vector-combined at an integrated 50 / 50 beam combiner. This beam combiner also employs an adjustable directional coupler design, allowing the power combining ratio of the two arms to be adjusted via an externally set fixed bias voltage, ensuring that the amplitudes of the two components on the complex plane are equal.

[0054] The complete process flow for silicon-based coupled dual-micro-ring coherent modulators can be completed on standard 8 / 12-inch silicon photonics lines, is fully compatible with CMOS processes, and can be mass-produced.

[0055] Specifically, the modulator supports QPSK, 16-QAM, 32-QAM, 64-QAM and higher-order coherent modulation formats. By adjusting the combination of driving signals of two sets of active micro-ring resonators, constellation points of the corresponding modulation format are generated in the complex plane. The phase purity of the constellation points is improved by relying on the near-zero chirp characteristics, and the distortion of higher-order coherent modulation signals is suppressed.

[0056] This invention also discloses a coherent modulation method based on coupled dual micro-rings, applied to a dual-polarization IQ modulator, comprising the following steps: Step S1: System initialization, set the attenuation coefficient γ1 of the passive micro-ring resonator and the initial attenuation coefficient γ2 of the active micro-ring resonator, so that the coupled dual micro-ring system satisfies the PT-like broken state condition, and calibrate the system coupling coefficient and the optimal operating point; Step S2: Operating point closed-loop locking. The on-chip optical power detector acquires the optical power signal in real time. The digital controller uses a perturbation-observation algorithm to drive the thermal tuning unit for temperature compensation, stabilizing the system operating point near the critical coupling point. The sampling and iteration period of the perturbation-observation algorithm is based on the chip's thermal relaxation characteristic time. The algorithm is set to wait until the optical power response corresponding to a single perturbation stabilizes before executing the next iteration.

[0057] Step S3: Modulation signal conversion, converting the externally input baseband modulation signal into a voltage drive signal, inputting it to the MZI adjustable coupler, and dynamically adjusting the equivalent attenuation coefficient γ2 of the active micro-ring resonator; Step S4: Amplitude modulation. By continuously changing γ2, the coupled dual micro-ring system is switched between undercoupled and overcoupled states to output a low-chill amplitude-modulated optical field. Step S5: IQ coherent modulation. During IQ modulation, the I modulation branch and the Q modulation branch execute steps S2-S4 independently. The orthogonal phase shift unit is used to make the two optical fields form a π / 2 phase difference, and finally the coherent beam combining is completed by the output beam combiner. Step S6: Real-time performance monitoring, continuously monitor optical power, operating point offset, and chip temperature parameters, dynamically correct control algorithm parameters, and ensure stable modulation performance.

[0058] More specifically, by combining and adjusting the driving voltages of the I modulation branch and the Q modulation branch, multiple modulation formats, including QPSK, 16-QAM, and 32-QAM, can be realized; by utilizing the low amplitude-phase coupling and low residual chirp characteristics, the phase error and amplitude error of the higher-order modulation constellation points can be reduced.

[0059] In summary, (1) this embodiment first proposes a silicon-based coupled dual-micro-ring modulator structure based on relative loss modulation, which is used to construct a coupled optical system with non-Hermitian characteristics without on-chip optical gain requirements. Traditional PT-symmetric optical structures usually adopt a gain-loss balanced configuration, that is, one resonator has equivalent gain and the other resonator has equivalent loss. However, directly introducing optical gain into the silicon photonics platform will significantly increase the process complexity, thermal management difficulty and system instability. Therefore, a more feasible solution for conventional silicon photonics platforms is to adopt a completely passive or loss-tunable structure, and achieve equivalent non-Hermitian modulation through the relative difference between low loss and high loss.

[0060] In a coupled dual-microring system, when the resonant frequencies of the two microrings are close to or equal, and the loss distribution and coupling strength satisfy a specific relationship, the system can exhibit modal evolution characteristics similar to those of a PT-symmetric system, such as eigenmode splitting, mode localization, and strong response characteristics near the EP point. Strictly speaking, PT symmetry requires that, after removing the common loss term, the system Hamiltonian can be equivalently expressed as a gain-loss balance. Therefore, this scheme does not rely on the actual optical gain, but rather achieves an equivalent non-Hermitian coupling control mechanism through the relative loss difference under the common loss background.

[0061] The advantage of this approach lies in its good compatibility with silicon photonics processes. Specifically, tunable losses can be introduced into one of the micro-rings, such as a tunable coupler based on an MZI structure, a carrier absorption modulation region, or a controllable coupling loss structure. The total attenuation coefficient of the micro-ring can be dynamically adjusted by changing the equivalent transmittance or additional losses within the ring. This eliminates the need to integrate complex gain media on a silicon-based platform, enabling the control of non-Hermitian coupling states. The entire device can be fabricated using conventional silicon photonics processes, including silicon waveguides, PN or PIN junction modulation regions, directional coupler structures, thermally tunable structures, and metal interconnects, thus possessing good process compatibility and potential for large-scale integration.

[0062] (2) Then, near-zero chirp amplitude modulation is proposed. Another core idea of ​​this invention is to utilize the transmission spectrum reconstruction caused by the change in loss parameters in the coupled dual micro-ring system to achieve an amplitude modulation method mainly based on loss control. Unlike traditional single micro-ring modulators that mainly rely on the change in refractive index to cause the resonant wavelength shift, this scheme adjusts the equivalent attenuation coefficient of one of the micro-rings to switch the system between different coupling states, thereby changing the output intensity of the through end. In terms of device design, the equivalent attenuation coefficient is dynamically changed by the adjustable loss unit integrated in the modulation micro-ring, so that the device can switch between undercoupled, critically coupled and overcoupled states. When the system meets the critical coupling condition, the input light is coupled into the resonant structure and effectively dissipated or transferred by the internal loss and coupling channel, and the light intensity output of the through end is reduced to 0, thereby obtaining a higher modulation depth.

[0063] In this scheme, modulation primarily affects the system's loss parameters, rather than achieving intensity changes mainly through shifting the microring's resonant wavelength. Therefore, compared to traditional refractive index modulated microrings, the continuous change in output phase with modulation voltage can be suppressed, which helps reduce frequency chirp during amplitude modulation. The system's transmission response is highly sensitive to loss variations, allowing for larger transmittance changes with smaller voltage swings, thereby reducing drive power consumption and improving modulation efficiency.

[0064] It is important to emphasize that "zero chirp" is usually an idealized goal in practical devices. Since the plasma dispersion effect in silicon materials causes both refractive index and absorption changes, this scheme achieves low or near-zero chirp amplitude modulation by reducing the coupling between amplitude and phase through a modulation mechanism mainly based on loss control, rather than completely eliminating chirp under all operating conditions.

[0065] (3) Then, two coupled dual-micro-ring modulation units with identical structures or matched parameters are integrated on the same silicon photonics chip, serving as I-path and Q-path amplitude modulation units respectively, and together with the on-chip beam splitter, phase shifter, and beam combiner, they form an IQ coherent modulator. This structure can independently control the amplitudes of the two orthogonal optical fields, thereby synthesizing the target electric field vector on the complex plane, supporting various coherent modulation formats such as QPSK, 16QAM, 32QAM, and higher-order QAM. The actual constellation order that can be supported also depends on the device linearity, extinction ratio, driving voltage accuracy, noise, bandwidth, temperature stability, and back-end DSP capabilities.

[0066] This integrated scheme mainly consists of four parts: input beam splitting, I / Q-path modulation, orthogonal phase establishment, and output beam combining. First, the input continuous light is distributed to the I-arm and Q-arm via an on-chip tunable beam splitter. By adjusting the splitting ratio to compensate for process errors and the difference in insertion loss between the two arms, the power of the two optical paths is matched as closely as possible. Second, two coupled dual-micro-ring modulation units are located in the I-arm and Q-arm respectively, and are driven by independent electrical signals. By adjusting the tunable loss units or equivalent coupling states in their respective micro-rings, low-chirp amplitude modulation can be achieved on both arms. Third, a static phase difference of π / 2 is introduced between the I-arm and Q-arm to establish an orthogonal optical field relationship. This phase difference can be achieved using structures such as TiN thin-film microheaters or silicon-doped thermal phase shifters. Finally, the on-chip beam combiner coherently superimposes the two orthogonally modulated optical fields, forming an optical signal carrying complete complex electric field information in a single output waveguide.

[0067] The advantage of this architecture lies in extending the low-power, small-size, and near-zero-chirp amplitude modulation characteristics of a single coupled dual-micro-ring modulation unit to IQ coherent modulation scenarios. By controlling the amplitude levels of the I and Q paths separately and combining them with a stable orthogonal phase relationship, constellation points with different amplitudes and phases can be generated in the complex plane. Since the two signals are ultimately coherently combined in the form of optical field vectors, the system needs to precisely control the amplitude matching, phase offset, and delay consistency of the two arms to reduce the impact of I / Q imbalance, orthogonal phase errors, and crosstalk on the constellation diagram quality. If the modulation unit can maintain a low residual phase modulation, it helps to improve the phase stability of the constellation points and reduce the error vector amplitude caused by chirp and amplitude-phase coupling in higher-order modulation formats.

[0068] (4) Finally, operating point locking and dynamic compensation are performed. Since silicon microring resonators are highly sensitive to temperature changes, fluctuations in ambient temperature, changes in on-chip power consumption, and thermal crosstalk between adjacent devices can all cause resonant wavelength drift, resulting in the modulation operating point deviating from the design state. To improve the long-term stability of the device in the actual system, this invention integrates a TiN thin film microheater or other thermally tuned structure compatible with silicon photonics near each coupled dual-microring modulation unit to actively adjust the microring resonant wavelength and phase bias. By changing the electric power of the heater, the effective refractive index of the waveguide can be adjusted using the thermo-optical effect of silicon, thereby compensating for the operating point shift caused by temperature drift or process deviation.

[0069] In terms of control, a closed-loop feedback system can be formed by an on-chip power monitor and an off-chip control circuit. The on-chip monitoring photodetector acquires the optical power signal from the download end in real time and inputs it as feedback to the digital controller. The controller uses a perturbation-observation algorithm to apply a small-amplitude adjustment signal to the TiN heater and determines the deviation of the current operating point from the target point based on the direction of change of the output power or error signal. Then, it gradually corrects the heater power to maintain the device at the preset operating point.

[0070] Through the aforementioned active thermal tuning and feedback compensation mechanism, this invention improves the temperature drift tolerance of the coupled dual-micro-ring IQ modulator, enabling it to maintain relatively stable extinction ratio, phase bias, and modulation response in practical operating environments. This mechanism reduces the impact of operating point drift on high-order coherent modulation performance, providing necessary assurance for the long-term stable operation of silicon-based micro-ring modulators in data center coherent optical interconnects.

[0071] Therefore, the innovation of this invention compared to existing silicon-based microring modulators lies in: Innovation Point 1: A non-Hermitian coupling control scheme for silicon photonics processes is proposed. This invention employs a relative loss control method to construct a coupled dual-micro-ring system with non-Hermitian characteristics without relying on on-chip optical gain. This system exhibits mode evolution and response characteristics similar to PT-symmetric systems on the silicon photonics platform. This scheme avoids the contradiction between traditional "gain-loss" symmetry and CMOS-compatible processes, possessing good process feasibility and integration potential.

[0072] Innovation Point Two: Reduced amplitude-phase coupling and frequency chirp in traditional micro-ring modulation. This invention primarily achieves amplitude modulation by adjusting the system's equivalent attenuation parameter, rather than simply relying on resonant wavelength shift. This mechanism helps suppress continuous changes in the output light phase with the modulation signal, reduces residual phase modulation and frequency chirp, thereby improving the constellation diagram quality in high-order coherent modulation.

[0073] Innovation Point 3: Achieving a single-chip integrated IQ coherent modulation structure. This invention uses two coupled dual-micro-ring modulation units for the I and Q paths respectively, and combines them with on-chip beam splitting, phase shifting, and beam combining structures to achieve vector synthesis of complex electric fields, supporting various coherent modulation formats.

[0074] Innovation Point Four: Introduction of Integrated Temperature Compensation and Operating Point Locking Mechanism. This invention integrates on-chip power monitoring at the coupled dual-micro-ring download end and utilizes micro-ring thermal tuning to construct closed-loop feedback control for dynamic compensation of the resonant wavelength. This mechanism improves the device's tolerance to environmental temperature changes and on-chip thermal crosstalk, enhancing the long-term operational stability of the system.

[0075] For those skilled in the art, modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the protection scope of this invention.

Claims

1. A coupled dual-micro-ring modulator, characterized in that, It includes input and output waveguides, active microring resonators, passive microring resonators, and a drive controller, wherein: The active microring resonator and the passive microring resonator are directly coupled through an optical coupler to form a non-Hermitian coupled dual microring system with equivalent PT symmetry characteristics. The input and output waveguides are coupled to at least one microring resonator via an optical coupler; The passive microring resonator has a constant attenuation coefficient γ1, which is used to provide reference optical characteristics; The active microring resonator is equipped with an MZI adjustable coupler. The MZI adjustable coupler can dynamically adjust the transmittance under the action of the drive control unit, so that the equivalent attenuation coefficient γ2 of the active microring resonator changes with the drive signal. The coupled dual-microring system is configured to operate in a working region with non-Hermitian mode evolution characteristics. At the critical coupling point, the system changes the equivalent attenuation coefficient γ2 of the active microring resonator, enabling the system to cross the critical coupling point and switch between undercoupled and overcoupled states. This reduces the coupling between the amplitude response and phase response during modulation, generating a low-chill or near-zero-chill amplitude-modulated optical signal.

2. The coupled dual-micro-ring modulator according to claim 1, characterized in that, The passive microring resonator adopts an undoped pure waveguide structure.

3. The coupled dual-micro-ring modulator according to claim 1, characterized in that, The MZI tunable coupler comprises two directional couplers and an optoelectronic phase-shifting structure arranged on two interferometer arms, wherein: The photoelectric phase shift structure is a carrier injection PN junction, a carrier depletion PN junction, or a PIN junction, and adopts a push-pull driving method. While adjusting the equivalent attenuation coefficient γ2 of the active microring resonator, it suppresses the additional phase disturbance introduced by the driving signal.

4. A coupled dual-micro-ring modulator according to claim 1, characterized in that, It also includes a thermal tuning unit, an on-chip optical power detector, and a digital controller, wherein: The thermal tuning unit is a thin-film microheater, integrated on the surface of the active microring resonator and the passive microring resonator, used to adjust the microring resonant wavelength; The on-chip optical power detector is a germanium-silicon photodiode structure used to collect the output optical power of the micro-ring in real time. The digital controller is electrically connected to the thermal tuning unit and the on-chip optical power detector. It adaptively adjusts the driving voltage of the thermal tuning unit based on the optical power feedback signal to lock the optimal operating point of the system.

5. A coupled dual-micro-ring modulator according to claim 4, characterized in that, The digital controller operates based on the disturbance observation algorithm, and the specific control steps are as follows: Step S1: Read the current optical power signal acquired by the on-chip optical power detector. ; Step S2: Calculate the optical power gradient ; Step S3: Determine the system operating point offset direction based on the power gradient direction; Step S4: Increase or decrease the driving voltage of the thermal tuning unit accordingly; Repeat the above steps until the system converges and stabilizes in its optimal operating state.

6. A dual-polarization IQ modulator based on polarization multiplexing, characterized in that, It includes a main beam splitter, a polarization beam combiner, and two sets of IQ coherent modulators corresponding to X-polarization and Y-polarization, respectively, wherein: The main beam splitter splits a single continuous input light into two independent optical signals, which are then sent to two independent IQ coherent modulators for X-polarization and Y-polarization, respectively, to provide incident light sources. The two sets of IQ coherent modulators are combined by a polarization combiner to form a dual-polarization multiplexed coherent optical signal. The IQ coherent modulator includes an input beam splitter, an I modulation branch, a Q modulation branch, a quadrature phase shift unit, and an output beam combiner; each of the I modulation branch and the Q modulation branch is provided with at least one coupled dual-micro-ring modulator as described in any one of claims 1-5. The quadrature phase shifter is used to introduce a π / 2 static phase difference between the I modulation branch and the Q modulation branch; the output beam combiner coherently combines the two modulated optical fields and outputs a complex optical field signal.

7. The dual-polarization IQ modulator according to claim 6, characterized in that, The modulator supports QPSK, 16-QAM, 32-QAM, 64-QAM and higher-order coherent modulation formats. By adjusting the combination of the driving signals of two sets of active micro-ring resonators, constellation points of the corresponding modulation format are generated in the complex plane. The phase purity of the constellation points is improved by relying on the near-zero chirp characteristics, and the distortion of higher-order coherent modulation signals is suppressed.

8. A coherent modulation method based on coupled dual micro-rings, characterized in that, The method applied to the dual-polarization IQ modulator of claim 6 includes the following steps: Step S1: System initialization, set the attenuation coefficient γ1 of the passive micro-ring resonator and the initial attenuation coefficient γ2 of the active micro-ring resonator, so that the coupled dual micro-ring system satisfies the PT-like broken state condition, and calibrate the system coupling coefficient and the optimal operating point; Step S2: Operating point closed-loop locking, on-chip optical power detector collects optical power signal in real time, digital controller uses perturbation observation algorithm to drive thermal tuning unit to perform temperature compensation, stabilize the system operating point at critical coupling point; Step S3: Modulation signal conversion, converting the externally input baseband modulation signal into a voltage drive signal, inputting it to the MZI adjustable coupler, and dynamically adjusting the equivalent attenuation coefficient γ2 of the active micro-ring resonator; Step S4: Amplitude modulation. By continuously changing γ2, the coupled dual micro-ring system is switched between undercoupled and overcoupled states to output a low-chill amplitude-modulated optical field. Step S5: IQ coherent modulation. During IQ modulation, the I modulation branch and the Q modulation branch execute steps S2-S4 independently. The orthogonal phase shift unit is used to make the two optical fields form a π / 2 phase difference, and finally the coherent beam combining is completed by the output beam combiner. Step S6: Real-time performance monitoring, continuously monitor optical power, operating point offset, and chip temperature parameters, dynamically correct control algorithm parameters, and ensure stable modulation performance.

9. The coherent modulation method according to claim 8, characterized in that, The sampling and iteration cycle of the perturbation observation algorithm is based on the chip's thermal relaxation characteristic time. The algorithm is set to wait until the optical power response corresponding to a single perturbation stabilizes before executing the next iteration.

10. The coherent modulation method according to claim 8, characterized in that, When performing high-order coherent modulation, various modulation formats, including QPSK, 16-QAM, and 32-QAM, are realized by combining and adjusting the driving voltages of the I modulation branch and the Q modulation branch. The low amplitude-phase coupling and low residual chirp characteristics are used to reduce the phase error and amplitude error of the high-order modulation constellation points.

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