Demodulator chip with phase noise compensation function and optical fiber acoustic wave sensing system

Abstract

The application relates to the technical field of sensing, and discloses a demodulator chip with a phase noise compensation function and an optical fiber acoustic wave sensing system, which comprises an optical port module, a first polarization filter, a beam splitter module, a modulator, a coherent receiving module and a phase noise extraction module; the optical port module is used for acquiring an incident laser signal, outputting detection light, acquiring scattered light and coupling with a delay optical fiber; the first polarization filter is connected with the optical port module; the beam splitter module is connected with the first polarization filter; the beam splitter module is connected with the optical port module through the modulator; the coherent receiving module is connected with the beam splitter module; the coherent receiving module is connected with the optical port module; the beam splitter module is connected with the phase noise extraction module, and the phase noise extraction module is connected to the delay optical fiber; the phase noise extraction module is used for extracting phase noise through a delay self-coherence mode; the laser linewidth demand can be reduced, and the sensing performance under the same laser linewidth level can be improved.

Description

Technical Field

[0001] This invention relates to the field of sensing technology, and in particular to a demodulator chip with phase noise compensation function and an optical fiber acoustic wave sensing system. Background Technology

[0002] Distributed Acoustic Sensing (DAS) is a distributed sensing method based on optical fibers. It achieves real-time sensing of physical quantities such as vibration and temperature by detecting phase changes in the backscattered Rayleigh signal within the optical fiber. This technology not only possesses the general advantages of fiber optic sensing, such as light weight and immunity to electromagnetic interference, but also enables high-fidelity information acquisition and high-precision sensing over long distances. It has been widely applied in fields such as security, oil and gas pipeline monitoring, geological disaster early warning, earthquake monitoring, and marine observation.

[0003] Existing DAS systems typically rely on bulky and costly discrete optical components, limiting their widespread adoption in practical applications. To address this issue, photonic integration technology has been used to integrate discrete optical devices onto a chip, thereby reducing size and cost while improving system consistency. However, laser integration remains challenging. Furthermore, the performance of a DAS system is closely related to the phase noise of the laser, typically requiring lasers with linewidths of kHz or lower to meet sensing requirements. However, achieving narrow-linewidth lasers is difficult; commercially available DFB laser chips generally have linewidths on the order of 10kHz or higher, and even narrower linewidths require complex designs.

[0004] Phase noise compensation offers a novel solution to this problem. Unlike directly reducing the laser linewidth, phase noise compensation extracts phase noise and compensates for it digitally, achieving an equivalent narrow-linewidth laser effect. For lasers with linewidths in the 10–100 kHz range, phase noise compensation can transform a system from undemodulable to demodulable; for lasers with linewidths in the 1 kHz range, it can further optimize sensing performance.

[0005] Chinese patent application CN202210077291.9 discloses a laser linewidth measurement system based on delayed self-zero difference detection of a coherent receiving module, but its function is limited to measuring the laser linewidth and does not propose a scheme for further suppressing the influence of the measured phase noise and linewidth. Chinese patent application CN202311484887.1 discloses a phase noise compensation method and an optical fiber acoustic wave sensing system, but this scheme suffers from insufficient stability; its optical fiber-based interference structure is prone to coupling with environmental noise, making it difficult to achieve the expected compensation effect. Summary of the Invention

[0006] The purpose of this invention is to at least solve one of the technical problems existing in the prior art, and to provide a demodulator chip with phase noise compensation function and a fiber optic acoustic wave sensing system, which can reduce the laser linewidth requirement and improve the sensing performance at the same laser linewidth level.

[0007] In a first aspect, embodiments of the present invention provide a demodulator chip with phase noise compensation function, comprising: an optical port module, a first polarization filter, a beam splitter module, a modulator, a coherent receiving module, and a phase noise extraction module, wherein:

[0008] The optical port module is fixedly coupled to the fiber array and is used to acquire the incident laser signal provided by the laser, output probe light, acquire scattered light, and couple with the time-delay fiber, which is a polarization-maintaining fiber.

[0009] The first polarization filter is connected to the optical port module to acquire the incident laser signal;

[0010] The beam splitter module is connected to the output of the first polarization filter; one output of the beam splitter module is connected to the optical port module through a modulator to output the probe light;

[0011] The coherent receiving module is connected to another output terminal of the beam splitter module to obtain local light; the coherent receiving module is connected to the optical port module to obtain the scattered light;

[0012] Another output of the beam splitter module is connected to the phase noise extraction module to provide input light to the phase noise extraction module. The phase noise extraction module is connected to the optical port module to connect to the delay fiber. The phase noise extraction module is used to extract phase noise in a delayed self-coherent manner.

[0013] According to some embodiments of the present invention, the demodulator chip includes a first optical port, a second optical port, a third optical port, a fourth optical port, and a fifth optical port.

[0014] The first optical port is used to acquire the scattered light and is connected to the input terminal of the coherent receiving module;

[0015] The second optical port is used to acquire the incident laser signal and is connected to the input of the first polarization filter;

[0016] The third optical port is connected to the output of the modulator and is used to output the probe light;

[0017] The fourth and fifth optical ports are used to couple with the delay fiber and connect to the phase noise extraction module.

[0018] According to some embodiments of the present invention, the demodulator chip includes a first beam splitter and a second beam splitter;

[0019] The input terminal of the first beam splitter is connected to the output terminal of the first polarization filter, the two output terminals of the first beam splitter are respectively connected to the input terminal of the second beam splitter and the input terminal of the modulator, and the output terminal of the modulator is connected to the third optical port;

[0020] The two outputs of the second beam splitter are respectively connected to the input of the coherent receiving module and the input of the phase noise extraction module.

[0021] According to some embodiments of the present invention, the demodulator chip includes a phase noise extraction module comprising a 1×2 multimode interference coupler, a second polarization filter, a 2×4 multimode interference coupler, a first balanced detector, and a second balanced detector.

[0022] The input end of the 1×2 multimode interference coupler is connected to one output end of the second beam splitter; the two output ends of the 1×2 multimode interference coupler are respectively connected to the fourth optical port and one input end of the 2×4 multimode interference coupler.

[0023] The fifth optical port is connected to the input of the second polarization filter, and the output of the second polarization filter is connected to the other input of the 2×4 multimode interference coupler.

[0024] The two outputs of the 2×4 multimode interference coupler are respectively connected to the two inputs of the first balanced detector, and the other two outputs of the 2×4 multimode interference coupler are respectively connected to the two inputs of the second balanced detector.

[0025] According to some embodiments of the present invention, the first optical port, the second optical port, the third optical port, the fourth optical port and the fifth optical port are implemented using edge couplers.

[0026] According to some embodiments of the present invention, the demodulator chip is provided in which the first polarization filter and the second polarization filter are implemented by a polarization beam splitter to filter out the TM component of the input light so that the output light is pure TE mode light.

[0027] According to some embodiments of the present invention, the demodulator chip is provided in which the first beam splitter and the second beam splitter are adjustable beam splitters. The first beam splitter is used to adjust the intensity of the probe light output from the third optical port. The first beam splitter and the second beam splitter jointly adjust the intensity of the optical signal entering the coherent receiving module and the intensity of the optical signal entering the phase noise extraction module.

[0028] According to some embodiments of the present invention, the demodulator chip is an optical intensity modulator used to generate a carrier-suppressed double-sideband modulated signal.

[0029] According to some embodiments of the present invention, the demodulator chip is a coherent receiving module with polarization diversity and phase diversity.

[0030] According to some embodiments of the demodulator chip provided by the present invention, the phase noise extraction module is used to perform delayed self-zero difference detection on the optical signal provided by the second beam splitter, and outputs the I / Q components of the autocoherent signal through the first balanced detector and the second balanced detector respectively; the I / Q signal combination can be expressed as:

[0031]

[0032] in, and The I / Q signal output by the phase noise extraction module. A constant related to the power of the optical signal entering the phase noise extraction module. Represents a complex exponential function. The angular frequency of the optical signal entering the phase noise extraction module. The phase noise of the optical signal entering the phase noise extraction module. This represents the time delay corresponding to the delay fiber.

[0033] According to some embodiments of the present invention, the demodulator chip is fabricated using a 220nm SOI process, and the fiber array includes single-mode fiber and polarization-maintaining fiber; wherein: the first optical port and the third optical port correspond to the single-mode fiber, and the second optical port, the fourth optical port and the fifth optical port correspond to the polarization-maintaining fiber.

[0034] Secondly, embodiments of the present invention provide an optical fiber acoustic wave sensing system, including discrete optical components, a signal generation circuit, a signal conditioning circuit, and a demodulator chip with phase noise compensation function as described in the first aspect embodiment above.

[0035] in:

[0036] The discrete optical components include a laser, a circulator, an optical amplifier, a delay fiber, and a sensing fiber; the output of the laser is connected to the second optical port, the third optical port is connected to the input of the optical amplifier, the output of the optical amplifier is connected to port 1 of the circulator, port 2 of the circulator is connected to the sensing fiber, and port 3 of the circulator is connected to the first optical port; the upper limit of the linewidth of the laser is 1MHz.

[0037] The signal generation circuit includes a multi-channel low-speed signal generator, a high-speed signal generator, and an electrical amplifier; the signal output from the high-speed signal generator is amplified by the electrical amplifier and then output to the RF port of the modulator; the multi-channel low-speed signal generator provides tuning signals and bias signals to the demodulator chip.

[0038] The signal conditioning circuit includes a first transimpedance amplifier, a second transimpedance amplifier, an analog-to-digital converter, and a data processing module; the output terminal of the coherent receiving module is connected to the input terminal of the first transimpedance amplifier, which is AC coupled; the output terminals of the first balanced detector and the second balanced detector are connected to the input terminal of the second transimpedance amplifier, which is DC coupled; the output terminals of the first transimpedance amplifier and the second transimpedance amplifier are connected to the input terminal of the analog-to-digital converter, and the output terminal of the analog-to-digital converter is connected to the input terminal of the data processing module.

[0039] According to some embodiments of the present invention, the fiber optic acoustic wave sensing system uses the phase noise extracted by the phase noise extraction module to compensate the sensing signal output by the coherent receiving module, and achieves phase noise suppression through two methods: direct compensation of the sensing signal and reconstruction of the matched filter.

[0040] The demodulator chip with phase noise compensation function and the fiber optic acoustic wave sensing system provided by the embodiments of the present invention have at least the following beneficial effects: By introducing a phase noise extraction module, the dependence on narrow linewidth lasers in the 1kHz or even 100Hz range is avoided, so that commercial DFB laser chips with linewidths in the 10–100kHz range can meet the application requirements, thereby significantly reducing the integration difficulty and cost of the laser; under the same linewidth conditions, better sensing performance can be achieved through phase noise compensation. For example, for lasers in the 1kHz or even 10kHz range, a sensing effect close to that of a 100Hz laser can be obtained after compensation; in addition, chip-level integration has significant advantages in terms of size, weight and cost compared to completely discrete implementation, and it is also easier to achieve stability control; the delay line function is undertaken by an external optical fiber, which shows advantages in terms of loss control, replacement and adjustment and implementation difficulty.

[0041] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0042] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the technical solutions of the present invention, and do not constitute a limitation on the technical solutions of the present invention.

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0044] Figure 1 This is a structural diagram of a demodulator chip with phase noise compensation function provided in an embodiment of the present invention;

[0045] Figure 2 This is a structural diagram of the fiber optic acoustic wave sensing system provided in an embodiment of the present invention;

[0046] Figure 3 This is a comparison chart of the vibration signals obtained by demodulation with and without phase noise compensation;

[0047] Figure 4 This is a comparison of the distribution of vibration phase standard deviation along the optical fiber obtained by demodulation with and without phase noise compensation. Detailed Implementation

[0048] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0049] In the description of the embodiments of the present invention, "several" means one or more, "multiple" means two or more, "greater than," "less than," "exceeding," etc. are understood to exclude the number itself, while "above," "below," "within," etc. are understood to include the number itself. "At least one" refers to one or more, and "at least one of the following" and similar expressions refer to any combination of these items, including any combination of single or multiple items. If "first," "second," etc., are used in the description, they are only for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance or implicitly indicating the number of indicated technical features or the order of the indicated technical features.

[0050] It should be noted that the terms "setting," "installing," and "connecting" in the embodiments of this invention should be interpreted broadly. Those skilled in the art can reasonably determine the specific meaning of the above terms in the embodiments of this invention in conjunction with the specific content of the technical solution. For example, the term "connection" can be a mechanical connection, an electrical connection, or a connection that allows for mutual communication; it can be a direct connection or an indirect connection through an intermediate medium.

[0051] It should be noted that the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0052] This invention provides a demodulator chip with phase noise compensation function and an optical fiber acoustic wave sensing system, which can reduce the laser linewidth requirement and improve sensing performance at the same laser linewidth level.

[0053] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0054] Reference Figure 1 A first aspect of the present invention provides a demodulator chip with phase noise compensation function, comprising: an optical port module, a first polarization filter, a beam splitter module, a modulator, a coherent receiving module, and a phase noise extraction module, wherein:

[0055] The optical port module is fixedly coupled to the fiber array and is used to acquire the incident laser signal provided by the laser, output probe light, acquire scattered light, and couple with the time-delay fiber, which is a polarization-maintaining fiber.

[0056] The first polarization filter is connected to the optical port module to acquire the incident laser signal;

[0057] The beam splitter module is connected to the output of the first polarization filter; one output of the beam splitter module is connected to the optical port module through a modulator to output probe light;

[0058] The coherent receiver module is connected to another output of the beam splitter module to obtain local light; the coherent receiver module is connected to the optical port module to obtain scattered light.

[0059] The other output of the beam splitter module is connected to the phase noise extraction module to provide input light to the phase noise extraction module. The phase noise extraction module is connected to the optical port module to connect to the delay fiber. The phase noise extraction module is used to extract phase noise in a delayed self-coherent manner.

[0060] Reference Figure 1 In some embodiments of the present invention, the demodulator chip includes a first optical port, a second optical port, a third optical port, a fourth optical port, and a fifth optical port.

[0061] The first optical port is used to acquire scattered light and connect to the input of the coherent receiving module;

[0062] The second optical port is used to acquire the incident laser signal and is connected to the input of the first polarization filter;

[0063] The third optical port is connected to the output of the modulator and is used to output probe light;

[0064] The fourth and fifth optical ports are used for coupling with the delay fiber and connecting to the phase noise extraction module.

[0065] Reference Figure 1 In some embodiments of the present invention, the demodulator chip includes a first beam splitter and a second beam splitter.

[0066] The input of the first beam splitter is connected to the output of the first polarization filter. The two outputs of the first beam splitter are connected to the input of the second beam splitter and the input of the modulator, respectively. The output of the modulator is connected to the third optical port.

[0067] The two outputs of the second beam splitter are connected to the input of the coherent receiving module and the input of the phase noise extraction module, respectively.

[0068] Reference Figure 1 In some embodiments of the present invention, the demodulator chip includes a phase noise extraction module comprising a 1×2 multimode interference coupler, a second polarization filter, a 2×4 multimode interference coupler, a first balanced detector, and a second balanced detector.

[0069] The input of the 1×2 multimode interference coupler is connected to one output of the second beam splitter; the two outputs of the 1×2 multimode interference coupler are respectively connected to the fourth optical port and one input of the 2×4 multimode interference coupler.

[0070] The fifth optical port is connected to the input of the second polarization filter, and the output of the second polarization filter is connected to the other input of the 2×4 multimode interference coupler.

[0071] The two outputs of the 2×4 multimode interference coupler are connected to the two inputs of the first balanced detector, and the other two outputs of the 2×4 multimode interference coupler are connected to the two inputs of the second balanced detector.

[0072] In the demodulator chip provided in some embodiments of the present invention, the first optical port, the second optical port, the third optical port, the fourth optical port and the fifth optical port are implemented using edge couplers.

[0073] It should be noted that, in addition to edge couplers, optical ports can also be implemented using grating couplers, fiber taper couplers, free-space couplers, and other methods. These methods have their own suitability for different scenarios such as silicon photonics chip testing and packaging.

[0074] Grating couplers are a common optical port implementation method in silicon photonics chips. They achieve vertical coupling of light by utilizing the diffraction effect of gratings, meaning the optical fiber is typically located above the chip to handle optical signal input and output. They are compatible with automated large-scale testing at the wafer and chip levels, can be arrayed on the chip, and have strong compatibility with CMOS processes, reducing the difficulty of large-scale integration. However, their shortcomings are also significant, including narrow bandwidth, relatively high coupling loss, and the coupling effect being affected by polarization state, making packaging more challenging.

[0075] Fiber taper coupling achieves optical coupling through a specially designed fiber taper. During fabrication, the fiber is heated with a flame and simultaneously stretched, reducing its diameter from 125 micrometers to approximately 1 micrometer. This narrow fiber taper allows the evanescent tail of light to be brought close to devices under test, such as rings and photonic crystal resonators, thereby completing optical signal transmission. It is suitable for wafer-scale automated testing and can achieve high-density coupling, but it has stringent requirements for the device packaging environment. It requires silicon photonic devices to use air as a cladding, while conventional silicon photonics manufacturing processes often use oxides to encapsulate silicon, limiting its application scenarios.

[0076] Free-space coupling relies on optical components such as lenses and mirrors to transmit light signals between optical fibers and chips in free space, and is often used in special scenarios. These include low-temperature experiments where samples are placed in a vacuum, biosensors that need to operate in solvents, and situations where multiple light outputs need to be measured simultaneously using an infrared camera. In practical applications, light signals can be scanned using mirrors on a galvanometer, or images of the chip's emitted light can be captured by a camera. However, using a camera limits the imaging scanning rate, and the overall system is susceptible to interference from external factors such as dust and vibration.

[0077] The glass waveguide evanescent wave coupling method first forms a surface optical waveguide in the glass through ion exchange, and then uses the evanescent wave coupling principle to achieve optical field transfer between the glass waveguide and the silicon nitride chip waveguide. One end of the glass chip is coupled to the optical chip, and the other end is fabricated into a fan-out waveguide adapted to the fiber spacing and connected to the optical fiber. It can also be combined with mechanical structures and connectors to achieve pluggable functionality. Its coupling stability is relatively good, but the typical coupling loss at a wavelength of 1310nm is about 1.5dB, and the overall loss is relatively high.

[0078] The microprism + concave mirror combination coupling method modifies the beam propagation direction by mounting prefabricated microprisms on a silicon photonic wafer, and then uses a concave mirror in the Photonic-Plug to expand the beam, reducing the coupling alignment accuracy requirements. The beam is then reflected multiple times before finally entering the fiber array. This method transforms the coupling alignment problem of optical ports into a wafer-level processing problem, adapting to the requirements of pluggable optical ports. However, it requires high processing and assembly accuracy for the microprisms and concave mirrors.

[0079] Lens array-assisted coupling involves several methods. Some manufacturers glue lenses onto the fiber array to form MOLA connectors, which expand the beam and couple it to the optical structure on the chip's V-groove, significantly reducing alignment tolerance. Other manufacturers glue lenses directly to the end face of the optical chip and the fiber array. This method can improve the alignment tolerance of the optical port in the X / Y direction to ±6µm and keep insertion loss at a low level. However, the lens bonding process and packaging sealing design are more difficult, requiring additional sealing caps and other structures to prevent dust from affecting the coupling effect.

[0080] In the demodulator chip provided in some embodiments of the present invention, the first polarization filter and the second polarization filter are implemented using a polarization beam splitter to filter out the TM component of the input light so that the output light is pure TE mode light.

[0081] For example, the 1550nm incident light enters from the second optical port and first passes through the first polarization filter; the polarization filter is a passive device with a pure waveguide structure, which filters out the TM component of the input light through an asymmetric directional coupler structure, so that the output light is pure TE mode light.

[0082] Understandably, the core requirement of polarization filters is to "allow only specific polarization states to pass through while suppressing orthogonal polarization states." Besides polarization beam splitters (PBS, essentially a combination of "splitting + filtering"), more common implementations are specifically designed for "single-path polarization screening," encompassing different principles such as material absorption, crystal birefringence, micro / nano structures, and electrical modulation. These are adaptable to various scenarios ranging from consumer electronics to optical communication and chip integration.

[0083] The core principle of an absorption-type polarization filter is to use materials with "polarization-selective absorption," which have almost no absorption for the target polarization state (such as horizontal H) and allow it to pass through smoothly, while strongly absorbing (extincting) orthogonal polarization states (such as vertical V), thus directly achieving polarization selection. Its advantages are simple structure, low cost, and small size; its limitations are a moderate extinction ratio (usually 100:1 to 1000:1), and some materials (such as organic dyes) have poor high-temperature resistance, making them unsuitable for high-power laser scenarios.

[0084] The core principle of a birefringent crystal polarization filter is to utilize the "birefringence effect" of a crystal—natural light, upon incident, decomposes into o-ray (ordinary ray) and e-ray (extraordinary ray), which have different refractive indices. Through crystal cutting, phase matching, or combination structures, one of the polarization states is deflected out of the light path (or canceled out by interference), retaining only the target polarization state. Its advantages are high extinction ratio, high power tolerance, and strong stability; its limitations are higher cost, larger size, sensitivity to incident angle, and unsuitability for compact integration scenarios.

[0085] The core principle of micro / nano structure polarization filters is to utilize the "structure polarization dependence"—light of the target polarization state can pass smoothly through the structure using a periodic structure at the micro / nano scale (period ≤ wavelength of light), while orthogonal polarization states are suppressed due to scattering, reflection, or mode cutoff. This allows for integration onto chips, making them compatible with CMOS processes. Their advantages include extremely high integration density, compact size, and compatibility with semiconductor processes; limitations include high processing precision requirements (requiring nanoscale photolithography) and relatively narrow bandwidth in some structures.

[0086] The core principle of electrically controlled polarization filters is to dynamically switch the "allowed polarization state" by controlling the polarization response of the material through an electric field (such as the orientation of liquid crystal molecules or the birefringence of ferroelectric materials), thus achieving "switchable polarization filtering" rather than fixedly selecting a single polarization. Its advantages include dynamic switching of polarization states and high flexibility; its limitations include complex structure, high cost, and some types (such as liquid crystals) are not suitable for high-power applications.

[0087] The core principle of thin-film interferometric polarization filters is to deposit multiple dielectric thin films on a substrate. By utilizing the difference in reflection / transmission interference between different polarization states in the thin film, the target polarization state is almost fully transmitted, while orthogonal polarization states are suppressed due to destructive interference, thus meeting the requirements for high bandwidth and low loss. Its advantages are wide bandwidth, low loss, and strong stability; its limitations are complex thin-film deposition processes, high cost, and the need for special design to withstand temperature changes.

[0088] In the demodulator chip provided in some embodiments of the present invention, the first beam splitter and the second beam splitter are adjustable beam splitters. The first beam splitter is used to adjust the intensity of the probe light output from the third optical port. The first beam splitter and the second beam splitter jointly adjust the intensity of the optical signal entering the coherent receiving module and the intensity of the optical signal entering the phase noise extraction module.

[0089] In this embodiment, the TE mode light after passing through the first polarization filter is split into two beams by the first beam splitter. One beam enters the transmit branch as the probe light, and the other beam enters the second beam splitter. The second beam splitter splits the incident light into two beams, one of which serves as the local light for the coherent receiving module, and the other as the input light for the phase noise extraction module. The beam splitter is adjustable and, based on a 1-input 2-output Mach-Zehnder interferometer structure, the beam splitting ratio can be adjusted through the thermo-optical effect.

[0090] It is understandable that the beam splitting ratio of an tunable beamsplitter can be adjusted in real time through electrical, optical, or mechanical means. Its core principle is to adjust the ratio of reflected to transmitted light by changing the physical properties of the device (such as refractive index, phase, and coupling length). Typical types include electro-optic tunable beamsplitters (such as those based on lithium niobate or silicon-based electro-optic effects), thermo-optic tunable beamsplitters (commonly used in silicon-based chips), and mechanically tunable beamsplitters (such as rotating thin-film beamsplitters). Among these, electro-optic tunable beamsplitters have a fast response speed (nanosecond level) and are suitable for optical communication and optical switching; thermo-optic tunable beamsplitters change the refractive index of the waveguide by heating, and have low cost and moderate response speed (millisecond level); mechanically tunable beamsplitters have a simple structure, are suitable for experimental scenarios, but have a slow response speed.

[0091] In some embodiments of the present invention, the demodulator chip is a light intensity modulator used to generate a carrier-suppressed double-sideband modulated signal.

[0092] In the transmit branch, the modulator modulates the input light to generate a pulse sequence that is output from the third optical port. In this embodiment, the modulator is an optical intensity modulator that achieves high-speed modulation based on the Mach-Zehnder structure and carrier dispersion effect to generate a carrier-suppressed double-sideband modulated signal.

[0093] Understandably, the core of a modulator is "loading an electrical / optical signal onto an optical carrier." Classified by the modulated object (the physical parameters of light), besides the most commonly used intensity modulator, there are core types such as phase modulators, frequency modulators, polarization modulators, and wavelength modulators. Furthermore, there are composite modulators that combine multiple parameters, adaptable to different scenarios such as optical communication, lidar, and spectral analysis. Among these:

[0094] The core principle of a phase modulator (PM) is to change the refractive index of the modulation medium using an electrical signal (or optical signal) (e.g., through electro-optic, thermo-optic, or acousto-optic effects), causing the phase of the optical carrier to change with the modulation signal (the amplitude and intensity of the light remain unchanged, only the phase is modulated). For example, in an electro-optic phase modulator based on lithium niobate (LN), the refractive index of LN changes after a voltage is applied, and the phase delay of light propagating in the medium changes accordingly. Silicon-based phase modulators achieve phase modulation through thermo-optic effects (heating changes the refractive index of the silicon waveguide) or carrier injection effects. Its advantages include a wide modulation bandwidth (up to tens of GHz), low insertion loss, and good linearity; its limitations are that it cannot be directly received by a detector (the signal needs to be restored through interferometric demodulation), and the system complexity is slightly higher.

[0095] The core principle of a frequency modulator (FM) is to linearly change the frequency of an optical carrier with respect to a modulating signal; essentially, it's "differential phase modulation" (the rate of change of phase over time is the frequency). Implementation methods include electro-optic frequency modulation (EMM), acousto-optic frequency modulation (AFM), and direct frequency modulation (FM). EFM utilizes the electro-optic effect to change the refractive index of a medium, indirectly altering the propagation frequency of light. AFM involves ultrasonic waves acting on an acousto-optic crystal, causing a periodic change in the crystal's refractive index, resulting in a Doppler frequency shift as light passes through, with the frequency changing according to the ultrasonic signal. Direct frequency modulation directly changes the oscillation frequency of a laser by controlling its injection current (this is "integrated light source modulation," requiring no additional independent modulator). The advantages of frequency modulators are strong anti-interference capabilities and high measurement accuracy (suitable for radar / sensing); the limitations are a relatively narrow modulation bandwidth (acousto-optic MFM is typically <1 GHz) and complex demodulation circuitry.

[0096] The core principle of a polarization modulator (POLM) is to control the birefringence of the modulation medium through electrical signals, thereby changing the polarization state of the optical carrier (e.g., switching from linear polarization to elliptic polarization or orthogonal linear polarization), so that the polarization state changes with the modulation signal. Common implementations include lithium niobate polarization modulators (using electro-optic effects to control birefringence), liquid crystal polarization modulators (using electric fields to control the orientation of liquid crystal molecules, changing the polarization state), and silicon-based polarization modulators (using carrier injection to control waveguide birefringence). Its advantage is that it can be combined with polarization multiplexing technology to improve communication capacity; its limitation is that it is sensitive to polarization stability and requires the use of a polarization controller.

[0097] The core principle of a wavelength modulator (WM) is to change the wavelength (color) of an optical carrier according to the modulation signal. Essentially, it's a "wavelength representation of frequency modulation" (wavelength is inversely proportional to frequency). Implementation methods include indirect modulation and direct modulation. Indirect modulation changes the refractive index of the modulation medium through electro-optic / thermo-optic effects, shifting the resonant wavelength of the light (e.g., wavelength modulators based on microring resonators). Direct modulation controls the injection current or temperature of the laser, directly changing the output wavelength of the laser (e.g., integrated modulation of distributed feedback DFB lasers). The advantages of wavelength modulators are adaptability to multi-channel transmission and high detection sensitivity; limitations include a limited wavelength tuning range (typically in the nanometer range) and relatively slow response speeds for some types (thermo-optic modulation in the millisecond range).

[0098] The core principle of a pulse modulator is to control the on / off state of light using electrical signals, converting continuous light into pulsed light. Essentially, it's a "special form of intensity modulation" (focusing only on the binary state of "light / no light"), but emphasizing the timing and width control of the pulses. Common implementations include electro-optic switches (fast on / off, nanosecond response), acousto-optic switches (medium-speed on / off, microsecond level), and mechanical shutters (low-speed on / off, millisecond level). Its advantages are simple structure and clear modulation logic; its limitations are that it is only suitable for pulsed signal scenarios and has weak continuous modulation capabilities.

[0099] In some embodiments of the present invention, the demodulator chip is a coherent receiving module with polarization diversity and phase diversity.

[0100] In the receiving branch, the returned Rayleigh scattered light enters the demodulator chip through the first optical port, serving as the signal light for the coherent receiving module. Here, it performs coherent reception together with the local light. In this embodiment, the coherent receiving module is a polarization diversity and phase diversity coherent receiving module. Except for the photodetector, it is entirely based on a passive waveguide structure. In particular, a multimode interference coupler is used to support greater manufacturing tolerance and resistance to external disturbances. The polarization diversity and phase diversity coherent receiving module outputs the I / Q components of the TE mode and TM mode signals, respectively.

[0101] It should be noted that in the phase noise extraction module, the input optical signal is first equally split into two beams by a 1×2 multimode interference coupler. One beam directly enters a 2×4 multimode interference coupler; the other beam passes sequentially through the fourth optical port, the delay fiber, the fifth optical port, and the second polarization filter before entering the 2×4 multimode interference coupler. This link is a fully polarization-maintaining optical transmission link, in which the light propagates in TE mode. This link is used to provide the time delay relative to the other link. This achieves zero-difference input light; the 2×4 multimode interferometer, based on the principle of optical self-image, can be used as a 90° mixer to mix the two signals; the output light of the 2×4 multimode interferometer coupler is received in a balanced manner by the first and second balanced detectors, thereby generating a delayed zero-difference signal.

[0102] It should be noted that the core function of a coherent receiver module is to reconstruct the complete information of the optical signal, such as amplitude, phase, and polarization, through the interference of the local oscillator (LO) light and the received signal light (Signal). Its classification is mainly based on four core dimensions: polarization processing capability, integration level, demodulation architecture, and application scenario. Different types are adapted to various coherent optical communication needs, from low-speed short-distance to high-speed long-distance. Among them:

[0103] The core characteristic of a single-polarization coherent receiver module is that it processes only a single polarization state (such as TE or TM polarization). The local oscillator light and the signal light must maintain strict polarization matching (usually requiring a polarization controller). It has a simple structure and low cost. The implementation principle involves the signal light and LO light being filtered to the same polarization state by a polarizer, then passing through a 90° mixer (Hybrid) to generate I / Q interference signals, which are then converted into electrical signals by a photodetector (PD), demodulating the amplitude and phase information. Its advantages are a simplified structure and fewer components (no polarization diversity devices are needed); its limitations include weak resistance to polarization disturbances, the need for an additional polarization control unit, and the inability to utilize polarization multiplexing to increase capacity.

[0104] The core feature of dual-polarization (DP) coherent receiver modules is their ability to simultaneously process two orthogonal polarization states (TE / TM or H / V). Essentially, they are "the integration of two single-polarization receiver channels." Combined with polarization multiplexing (PDM) technology, they can double the communication capacity, making them the core type of current mainstream coherent modules. The implementation principle is as follows: the signal light is split into two orthogonal polarization states by a polarization beamsplitter (PBS); each polarized light is then mixed with the LO light (synchronously incident after being split by the polarization beamsplitter) and fed into an independent 90° mixer; finally, four electrical signals (H-polarization I / Q + V-polarization I / Q) are output, and the complete information of the two polarization states is restored through digital signal processing (DSP). Advantages include strong resistance to polarization disturbances (no additional polarization controller required) and support for polarization multiplexing to increase capacity; limitations include complex structure, high integration requirements, and higher cost than single-polarization modules.

[0105] In the demodulator chip provided in some embodiments of the present invention, the phase noise extraction module is used to perform delayed self-zero difference detection on the optical signal provided by the second beam splitter, and outputs the I / Q components of the autocoherent signal through the first balanced detector and the second balanced detector respectively; the I / Q signal combination can be expressed as:

[0106]

[0107] in, and The I / Q signal output by the phase noise extraction module. A constant related to the power of the optical signal entering the phase noise extraction module. Represents a complex exponential function. The angular frequency of the optical signal entering the phase noise extraction module. The phase noise of the optical signal entering the phase noise extraction module. This represents the time delay corresponding to the delay fiber.

[0108] In some embodiments of the present invention, the demodulator chip is fabricated using a 220nm SOI process, and the fiber array includes single-mode fiber and polarization-maintaining fiber; wherein: the first optical port and the third optical port correspond to single-mode fiber, and the second optical port, the fourth optical port and the fifth optical port correspond to polarization-maintaining fiber.

[0109] In some embodiments of the present invention, the demodulator chip also includes a communication modulation module, which is used to superimpose a communication signal (such as a low-speed data signal modulated by OFDM) onto the probe light output from the third optical port, so as to transmit the sensing light and communication light simultaneously using the sensing fiber. Correspondingly, the coherent receiving module also includes a communication signal demodulation unit, which is used to separate the Rayleigh scattered sensing signal and the transmitted communication signal and demodulate them separately. The signal conditioning circuit and the data processing module include a communication protocol processing unit, which supports standard communication protocols and realizes real-time transmission of sensing data. Thus, it can realize the integration of sensing and data transmission, so there is no need to deploy additional communication fiber, which reduces the deployment cost of the system and is suitable for scenarios such as submarine optical cables and remote area sensor networks.

[0110] Reference Figure 2 A second aspect of the present invention provides an optical fiber acoustic wave sensing system, including discrete optical components, a signal generation circuit, a signal conditioning circuit, and a demodulator chip with phase noise compensation function as described in the first aspect embodiment above.

[0111] in:

[0112] The discrete optical components include a laser, a circulator, an optical amplifier, a delay fiber, and a sensing fiber; the output of the laser is connected to the second optical port, the third optical port is connected to the input of the optical amplifier, the output of the optical amplifier is connected to port 1 of the circulator, port 2 of the circulator is connected to the sensing fiber, and port 3 of the circulator is connected to the first optical port; the upper limit of the laser linewidth is 1MHz.

[0113] The signal generation circuit includes a multi-channel low-speed signal generator, a high-speed signal generator, and an electrical amplifier; the signal output from the high-speed signal generator is amplified by the electrical amplifier and then output to the RF port of the modulator; the multi-channel low-speed signal generator provides tuning and bias signals for the demodulator chip.

[0114] The signal conditioning circuit includes a first transimpedance amplifier, a second transimpedance amplifier, an analog-to-digital converter, and a data processing module; the output of the coherent receiving module is connected to the input of the first transimpedance amplifier, which is AC coupled; the outputs of the first and second balanced detectors are connected to the input of the second transimpedance amplifier, which is DC coupled; the outputs of the first and second transimpedance amplifiers are connected to the input of the analog-to-digital converter, and the output of the analog-to-digital converter is connected to the input of the data processing module.

[0115] In some embodiments, the laser is selected as a communication laser; the delay fiber is a standard panda-eye polarization-maintaining fiber; and the sensing fiber is a G.652.D single-mode fiber.

[0116] In the fiber optic acoustic wave sensing system provided in some embodiments of the present invention, the fiber optic acoustic wave sensing system uses the phase noise extracted by the phase noise extraction module to compensate the sensing signal output by the coherent receiving module, and achieves phase noise suppression through two methods: direct compensation of the sensing signal and reconstruction of the matched filter.

[0117] In some embodiments, the data is collected Then, by taking the argument, we can obtain:

[0118]

[0119] In the formula yes Remove integer numbers of 2 The residual frequency components are then automatically determined by the complex argument taking process; for The summation yields:

[0120]

[0121] right By performing linear fitting, calculating the slope, and removing the slope, the phase noise of the laser can be further obtained. In subsequent compensation, the following will be used. That's all.

[0122] For the output of the coherent receiving module Firstly, compensation can be provided directly:

[0123]

[0124] The actual applied modulation signal can be corrected to obtain:

[0125]

[0126] Amplitude correction can be further performed based on the measured envelope of the probe signal to obtain... The output envelope of the optical amplifier in the system is The ideal frequency domain envelope is a self-defined shape that meets the requirements, denoted as . Therefore, the envelope correction factor is:

[0127]

[0128] Then we have:

[0129]

[0130] Based on this, matched filtering can be used to further compensate the sensed signal. An example of the matching operation is as follows:

[0131]

[0132] Simulation verification of the fiber optic acoustic wave sensing system was performed using a 50kHz laser, a 20m delay fiber, and a 50km sensing fiber. A sinusoidal vibration was applied to the tail end, and the pulse width of the chirped pulse signal was... The frequency was scanned from 10MHz to 410MHz. Figure 3 The results of the demodulated vibration signal with and without phase noise compensation are shown. It can be seen that demodulation cannot be achieved without phase noise compensation, but sinusoidal vibration can be restored after it is applied. Figure 4 The comparison of the vibration phase standard deviation along the optical fiber obtained by demodulation with and without phase noise compensation is shown. The compensation effect along the optical fiber can be seen, achieving suppression of phase noise influence throughout the entire process.

[0133] The demodulator chip with phase noise compensation function and the fiber optic acoustic wave sensing system provided by the embodiments of the present invention, by introducing a phase noise extraction module, avoid the dependence on narrow linewidth lasers in the 1kHz or even 100Hz range, so that commercial DFB laser chips with linewidths in the 10–100kHz range can meet the application requirements, thereby significantly reducing the integration difficulty and cost of lasers; under the same linewidth conditions, better sensing performance can be achieved through phase noise compensation. For example, for lasers in the 1kHz or even 10kHz range, a sensing effect close to that of a 100Hz laser can be obtained after compensation; in addition, chip-level integration has significant advantages in terms of size, weight and cost compared to completely discrete implementation, and it is also easier to achieve stability control; the delay line function is undertaken by an external optical fiber, which shows advantages in terms of loss control, replacement and adjustment and implementation difficulty.

[0134] In some embodiments of the fiber optic acoustic wave sensing system of the present invention, the external time-delay fiber can be replaced with an adjustable time-delay module composed of an optical switch and multiple polarization-maintaining fibers, or an adjustable optical delay line based on a microelectromechanical system (MEMS). Furthermore, in some embodiments of the demodulator chip provided by the present invention, a time-delay monitoring unit is added between the chip's multimode interference coupler module and balanced detector module. This unit is a miniature optical power and phase detector used to monitor the phase difference of the output light from the 2×4 multimode interference coupler in real time and calculate the actual time delay τ_real of the current time-delay fiber. The data processing module is configured to run a time-delay correction algorithm, which compares τ_real with the target time delay τ_target (preset according to the laser linewidth), outputs a time-delay adjustment signal, and controls the switching of the external adjustable time-delay module.

[0135] Specifically, the optical switch is a 1×4 microelectromechanical system (MEMS) optical switch, with multiple polarization-maintaining fiber segments of lengths of 10m, 20m, 50m, and 100m, corresponding to time delays τ1=50ns, τ2=100ns, τ3=250ns, and τ4=500ns. The control terminal of the optical switch is connected to the time delay adjustment signal output terminal of the data processing module, and can switch the delay fiber of different lengths according to algorithm instructions. The correspondence between linewidth and time delay is preset: for example, when the laser linewidth is 100Hz~1kHz, τ4=500ns is selected; when the linewidth is 1kHz~10kHz, τ3=250ns is selected; when the linewidth is 10kHz~100kHz, τ2=100ns is selected; and when the linewidth is 100kHz~200kHz, τ1=50ns is selected. The data processing module triggers the optical switch switching by reading the laser linewidth parameter (user preset input) or automatically detecting the bandwidth of the phase noise signal.

[0136] When the system starts up, the data processing module reads the laser linewidth parameters or automatically identifies the linewidth range through the I / Q signal bandwidth output by the phase noise extraction module. Based on a preset correspondence, it outputs a delay adjustment signal to control the MEMS optical switch to switch to the delay fiber of the target length. The delay monitoring unit collects the phase difference of the output light from the 2×4 multimode interference coupler in real time, calculates the actual delay τ_real, and feeds it back to the data processing module. If |τ_real - τ_target| > 5%τ_target, the algorithm corrects the delay deviation by fine-tuning the driving voltage of the optical switch to ensure delay accuracy. The phase noise extraction module extracts the phase noise based on the adjusted delay τ according to the formula in the basic embodiment. Subsequent compensation processes only require the corrected τ for calculation. Therefore, through the adjustable delay module and delay correction algorithm, the system can be compatible with a wider range of laser linewidths. At the same time, the dynamic delay correction function avoids delay drift caused by changes in ambient temperature and fiber vibration, further improving the phase noise extraction accuracy, reducing user restrictions on laser selection, and enhancing the versatility of the solution.

[0137] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A demodulator chip with phase noise compensation function, characterized in that, include: An optical port module is fixedly coupled to an optical fiber array and is used to acquire the incident laser signal provided by the laser, output probe light, acquire scattered light, and couple with a time-delay fiber, wherein the time-delay fiber is a polarization-maintaining fiber. A first polarization filter is connected to the optical port module to acquire the incident laser signal; A beam splitter module is connected to the output of the first polarization filter; one output of the beam splitter module is connected to the optical port module via a modulator to output the probe light. A coherent receiving module is connected to another output terminal of the beam splitter module to obtain local light; the coherent receiving module is also connected to the optical port module to obtain the scattered light. A phase noise extraction module is provided, with another output terminal of the beam splitter module connected to the phase noise extraction module to provide input light to the phase noise extraction module. The phase noise extraction module is connected to the optical port module to connect to the delay fiber. The phase noise extraction module is used to extract phase noise in a delayed self-coherent manner.

2. The demodulator chip with phase noise compensation function according to claim 1, wherein the optical port module includes a first optical port, a second optical port, a third optical port, a fourth optical port, and a fifth optical port; The first optical port is used to acquire the scattered light and is connected to the input terminal of the coherent receiving module; The second optical port is used to acquire the incident laser signal and is connected to the input of the first polarization filter; The third optical port is connected to the output of the modulator and is used to output the probe light; The fourth and fifth optical ports are used to couple with the delay fiber and connect to the phase noise extraction module.

3. The demodulator chip with phase noise compensation function according to claim 2, wherein the beam splitter module includes a first beam splitter and a second beam splitter; The input terminal of the first beam splitter is connected to the output terminal of the first polarization filter, the two output terminals of the first beam splitter are respectively connected to the input terminal of the second beam splitter and the input terminal of the modulator, and the output terminal of the modulator is connected to the third optical port; The two outputs of the second beam splitter are respectively connected to the input of the coherent receiving module and the input of the phase noise extraction module.

4. The demodulator chip with phase noise compensation function according to claim 3, wherein the phase noise extraction module includes a 1×2 multimode interference coupler, a second polarization filter, a 2×4 multimode interference coupler, a first balanced detector and a second balanced detector; The input end of the 1×2 multimode interference coupler is connected to one output end of the second beam splitter; the two output ends of the 1×2 multimode interference coupler are respectively connected to the fourth optical port and one input end of the 2×4 multimode interference coupler. The fifth optical port is connected to the input of the second polarization filter, and the output of the second polarization filter is connected to the other input of the 2×4 multimode interference coupler. The two outputs of the 2×4 multimode interference coupler are respectively connected to the two inputs of the first balanced detector, and the other two outputs of the 2×4 multimode interference coupler are respectively connected to the two inputs of the second balanced detector.

5. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The first optical port, the second optical port, the third optical port, the fourth optical port and the fifth optical port are implemented using an edge coupler.

6. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The first polarization filter and the second polarization filter are implemented using polarization beam splitters to filter out the TM component of the input light so that the output light is pure TE mode light.

7. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The first beam splitter and the second beam splitter are adjustable beam splitters. The first beam splitter is used to adjust the intensity of the probe light output from the third optical port. The first beam splitter and the second beam splitter jointly adjust the intensity of the optical signal entering the coherent receiving module and the intensity of the optical signal entering the phase noise extraction module.

8. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The modulator is an optical intensity modulator used to generate a carrier-suppressed double-sideband modulated signal.

9. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The coherent receiving module is a polarization diversity and phase diversity coherent receiving module.

10. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The phase noise extraction module is used to perform delayed self-zero difference detection on the optical signal provided by the second beam splitter, and outputs the I / Q components of the autocoherent signal through the first balanced detector and the second balanced detector respectively; the I / Q signal combination can be expressed as: ； ； in, and The I / Q signal output by the phase noise extraction module. A constant related to the power of the optical signal entering the phase noise extraction module. Represents a complex exponential function. The angular frequency of the optical signal entering the phase noise extraction module. The phase noise of the optical signal entering the phase noise extraction module. This represents the time delay corresponding to the delay fiber.

11. The demodulator chip with phase noise compensation function according to claim 4, characterized in that, The demodulator chip is fabricated using a 220nm SOI process, and the fiber array includes single-mode fiber and polarization-maintaining fiber; wherein: the first optical port and the third optical port correspond to the single-mode fiber, and the second optical port, the fourth optical port and the fifth optical port correspond to the polarization-maintaining fiber.

12. A fiber optic acoustic wave sensing system, characterized in that, Includes discrete optical components, signal generation circuit, signal conditioning circuit, and a demodulator chip with phase noise compensation function as described in any one of claims 4 to 11. in: The discrete optical components include a laser, a circulator, an optical amplifier, a delay fiber, and a sensing fiber; the output of the laser is connected to the second optical port, the third optical port is connected to the input of the optical amplifier, the output of the optical amplifier is connected to port 1 of the circulator, port 2 of the circulator is connected to the sensing fiber, and port 3 of the circulator is connected to the first optical port; the upper limit of the linewidth of the laser is 1MHz. The signal generation circuit includes a multi-channel low-speed signal generator, a high-speed signal generator, and an electrical amplifier; the signal output from the high-speed signal generator is amplified by the electrical amplifier and then output to the RF port of the modulator; the multi-channel low-speed signal generator provides tuning signals and bias signals to the demodulator chip. The signal conditioning circuit includes a first transimpedance amplifier, a second transimpedance amplifier, an analog-to-digital converter, and a data processing module; the output terminal of the coherent receiving module is connected to the input terminal of the first transimpedance amplifier, which is AC coupled; the output terminals of the first balanced detector and the second balanced detector are connected to the input terminal of the second transimpedance amplifier, which is DC coupled; the output terminals of the first transimpedance amplifier and the second transimpedance amplifier are connected to the input terminal of the analog-to-digital converter, and the output terminal of the analog-to-digital converter is connected to the input terminal of the data processing module.

13. The fiber optic acoustic wave sensing system according to claim 12, characterized in that, The fiber optic acoustic wave sensing system uses the phase noise extracted by the phase noise extraction module to compensate the sensing signal output by the coherent receiving module, and achieves phase noise suppression through two methods: direct compensation of the sensing signal and reconstruction of the matched filter.

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