FBG grating demodulator based on silicon nitride photonic filter chip

Abstract

This invention relates to an FBG grating demodulator based on a silicon nitride photonic filter chip, belonging to the field of fiber optic sensing technology. The demodulator includes a broadband light source, an optical circulator, an FBG sensor array, a silicon nitride photonic filter chip, a photodetector, an electrical amplifier, an ADC data acquisition card, a digital filter (FPGA), and a computer processing unit. The silicon nitride photonic filter chip integrates a micro-ring resonant cavity, a subwavelength grating dispersion modulation structure, and a thermo-optical tuning electrode, achieving continuous, high-speed, and linear wide-range wavelength coordination through the thermo-optical effect. Furthermore, it employs peak deconvolution and lookup table algorithms for wavelength demodulation. This invention enables multi-channel distributed measurement on a single fiber channel. With no moving mechanical parts, this invention is small in size, has a long lifespan, is resistant to static electricity, offers high detection accuracy, good stability, good vibration resistance, a high tolerance threshold, and low manufacturing cost. It is suitable for various fields such as infrastructure safety monitoring, the energy industry, aerospace, and optical communication, and has broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, and specifically relates to a fiber Bragg grating (FBG) demodulator based on a silicon nitride photonic filter chip. Background Technology

[0002] Fiber Bragg grating (FBG) sensing technology, with its advantages of electromagnetic interference resistance, small size, strong multiplexing capability, and ease of networking, has become the mainstream technology for safety monitoring of large-scale infrastructure and high-end equipment. The FBG demodulator, as the core equipment of the sensing system, mainly realizes high-precision detection and physical quantity inversion of the FBG reflection center wavelength; its performance directly determines the reliability and accuracy of the entire monitoring system.

[0003] Currently, FBG demodulators generally use tunable Fabry-Perot (FP) filters as the core wavelength selection unit, and achieve wavelength scanning by driving the fiber end face to move and changing the cavity length through piezoelectric ceramics (PZT). This technical approach faces significant bottlenecks: core components are highly dependent on imports, the high-end all-fiber FP filter market is monopolized by foreign manufacturers, core chips account for more than 70% of the total cost, and domestically produced alternatives lag behind in key indicators such as precision, modulation rate, maximum withstand optical power, and operating temperature range, creating a "bottleneck" problem; technical and process pain points are prominent, traditional FP filters require high-precision coating and mechanical alignment, high-end coating equipment relies on imports, there is a lack of experience in film thickness control and low-loss processes, real-time active alignment is complex, cavity parallelism and loss require extremely high precision in fiber end-face processing, PZT is susceptible to electrostatic or power frequency breakdown, the devices are fragile, maintenance frequency is high, and it is difficult to adapt to long-term monitoring scenarios such as dams and long-distance pipelines; performance and application scenarios are limited, traditional solutions only support 30-50 parallel measurements, the number of channels is limited, demodulation accuracy is generally around 3pm, resolution is insufficient, and the equipment is bulky, making it difficult to meet the needs of portable inspection, deployment in confined spaces, and low-load platform applications.

[0004] Silicon nitride photonics, as an emerging integrated photonics technology, boasts significant advantages such as ultra-high refractive index difference, low loss, and CMOS process compatibility. Wavelength demodulation schemes based on silicon nitride photonic filter chips can achieve ultra-compact, highly stable, and high-speed wavelength selection and demodulation, while simultaneously achieving miniaturization, high resolution, large bandwidth, multi-channel multiplexing, and high stability. This provides a new technological path for improving the performance of FBG sensing systems and has significant engineering value and strategic importance for breaking foreign technological monopolies and ensuring the security monitoring of major national infrastructure. Summary of the Invention

[0005] To address the issues of existing FBG demodulators, such as reliance on imported core components, complex technology, and limitations in performance and application scenarios, this invention proposes an FBG grating demodulator based on a silicon nitride photonic filter chip. By replacing the FP cavity with a ring cavity, using subwavelength grating dispersion modulation, peak deconvolution, and lookup table algorithms, it achieves high-resolution, high-bandwidth, and chip-level miniaturized FBG wavelength demodulation, breaking the foreign technology monopoly and meeting my country's needs for independent and controllable infrastructure security monitoring.

[0006] An FBG grating demodulator based on a silicon nitride photonic filter chip includes: a broadband light source, an optical circulator, an FBG sensor array, a silicon nitride photonic filter chip, a photodetector, an electrical amplifier, an ADC data acquisition card, a digital filter (FPGA), and a computer processing unit.

[0007] The broadband light source is used to output a broadband optical signal;

[0008] The optical circulator has its input end connected to the output end of the broadband light source, and is used to guide the optical signal of the broadband light source into the FBG sensor array and guide the wavelength signal reflected by the FBG sensor array into the silicon nitride photonic filter chip.

[0009] The FBG sensor array is composed of multiple FBG sensors connected in series. Its input end is connected to the output end of the optical circulator and is used to reflect light signals of a specific wavelength.

[0010] The silicon nitride photonic filter chip has its input end connected to the reflector end of the optical circulator, and is used for on-chip filtering and wavelength selection of the wavelength signal reflected by the FBG.

[0011] The photodetector has its input end connected to the output end of the silicon nitride photonic filter chip, and is used to convert optical signals into electrical signals;

[0012] The electrical amplifier has its input terminal connected to the output terminal of the photodetector and is used to amplify the electrical signal.

[0013] The ADC data acquisition card has its input terminal connected to the output terminal of the electrical amplifier, and is used to convert analog electrical signals into digital signals.

[0014] The digital filter (FPGA) has its input terminal connected to the output terminal of the ADC data acquisition card, and is used to filter digital signals to remove noise, interference, high-frequency noise, etc.

[0015] The computer processing unit has its input terminal connected to the output terminal of the digital filter (FPGA) and is used to process, display, and store signals.

[0016] The silicon nitride photonic filter chip uses a thermal tuning method to achieve wavelength selection, with a tuning speed of not less than 1MHz and a wavelength resolution of not less than 1pm.

[0017] The FBG sensor array supports the cascading multiplexing of hundreds of sensors on a single fiber, enabling distributed monitoring at multiple measurement points. Each FBG sensor corresponds to a central reflection wavelength, used to sense external physical quantities and reflect light signals of a specific wavelength.

[0018] The demodulator includes a digital filtering processing module (FPGA), and the digital filter incorporates a peak deconvolution algorithm and a wavelength lookup table algorithm to achieve accurate positioning of spectral wavelength peaks and high-speed demodulation.

[0019] The peak deconvolution algorithm constructs an inversion model to perform deconvolution processing on the measured spectral data to achieve "high-definition restoration". At the same time, it uses the L-BFGS optimization algorithm for iterative solution. During the solution process, a regularization constraint term is introduced to improve the accuracy of the results. The regularization constraint term includes at least one of a smoothing constraint term and a sharpening constraint term.

[0020] The wavelength lookup table algorithm achieves rapid wavelength calculation by pre-establishing a one-to-one correspondence between spectral feature parameters and wavelengths; and it achieves proportional scaling of spectral amplitude by minimizing errors through a diagonal gain matrix, thereby improving the accuracy and reliability of wavelength demodulation.

[0021] The computer processing unit converts digital signals into physical quantity measurement results such as strain, temperature, and pressure through a wavelength inversion algorithm, and displays and stores them in real time, adapting to long-term online monitoring scenarios for large structures.

[0022] By changing the tuning parameters of the silicon nitride photonic filter chip, high-speed scanning and demodulation of the reflected wavelength of the FBG sensor array are achieved. The main parameters of the silicon nitride photonic filter chip include resonant wavelength, free spectral range (FSR), resonant rate, 3 dB bandwidth, and center wavelength, which can be precisely controlled through thermo-optical effects. During use, the filter continuously, rapidly, linearly, and over a wide range adjusts its channel center wavelength to match the reflected wavelength of each grating in the FBG sensor array sequentially within a predetermined wavelength range. When the filter passband coincides with the corresponding FBG reflection peak, a significant light intensity peak signal is generated. Combined with a high-speed signal acquisition circuit and peak extraction algorithm, the wavelength information of the light intensity peak is converted into a measurable electrical signal, enabling rapid scanning, wavelength demodulation, and dynamic monitoring of multi-channel FBG sensing signals.

[0023] During wavelength tuning, whenever the wavelength position changes by Δλ, the light intensity value detected by the photodetector is recorded. The light intensity corresponds to the FBG reflection signal intensity at the current wavelength. After scanning the entire working bandwidth, the FBG reflection spectrum is reconstructed using all detected light intensity values.

[0024] The silicon nitride photonic filter chip is mass-produced using wafer-level photolithography, reducing the overall size of the device by more than 90% compared to traditional FP demodulators.

[0025] Furthermore, the silicon nitride photonic filter chip is fabricated based on silicon-based photonic integration technology, integrating a silicon nitride ring resonant cavity, a subwavelength grating dispersion modulation structure, and a thermo-optical tuning electrode. It adopts an all-solid-state structure without mechanical moving parts, completely eliminating the optical coating, mechanical alignment, and PZT piezoelectric tuning structure required by traditional FP cavities, thereby fundamentally avoiding mechanical wear, temperature drift, hysteresis, and electrostatic breakdown problems.

[0026] Furthermore, this invention combines an FPGA parallel processing architecture with peak deconvolution and lookup table algorithms to complete noise removal and accurate peak fitting during spectral reconstruction. This can eliminate demodulation errors caused by factors such as crosstalk from multi-channel sensors and optical path scattering, ensuring measurement linearity and repeatability in multi-channel multiplexing scenarios.

[0027] Furthermore, this invention achieves high-speed scanning of the entire working bandwidth of the FBG sensor array by programmably adjusting the tuning parameters of the silicon nitride photonic filter chip; during wavelength tuning, light intensity signals are synchronously acquired with a fixed wavelength step size Δλ, and high-precision reconstruction of the FBG reflection spectrum is completed based on the full-band light intensity data, ensuring the accuracy of wavelength demodulation under multi-sensor multiplexing.

[0028] Furthermore, this demodulator adopts an all-solid-state chip architecture and standardized wafer fabrication process. The core components have no easily damaged or consumable parts, and it can still maintain stable demodulation performance under harsh conditions such as high and low temperatures, high humidity, and strong electromagnetic interference, which greatly extends the equipment's fault-free operating time and reduces the total life cycle maintenance cost.

[0029] Furthermore, this demodulator, through its chip-level miniaturization design and multi-channel multiplexing capability, can be flexibly adapted to diverse application scenarios such as handheld portable testing, deployment in confined underground spaces, and distributed long-term monitoring of large infrastructure, balancing the convenience of equipment deployment with the scalability of the measurement system.

[0030] The beneficial effects of this invention are as follows:

[0031] Independent and controllable: Breaking the foreign monopoly, the silicon nitride annular cavity is used to replace the FP cavity, eliminating the need for coating and mechanical alignment, and getting rid of dependence on imported high-precision coating equipment and PZT devices. Core technologies and devices have achieved independent research and development and mass production.

[0032] Improved accuracy: The spectral resolution reaches the sub-pm level, which is 4 times more accurate than traditional domestic demodulators, solving the pain points of insufficient resolution and data distortion in traditional products.

[0033] Reduced calibration and maintenance costs: No temperature drift, no hysteresis, no need for frequent calibration, one-time batch forming by photolithography, waveguide connection without subsequent adjustment, maintenance frequency is reduced by 90% compared to traditional products, suitable for long-term monitoring scenarios such as dams and long-distance pipelines.

[0034] Improved reliability: The high-damage-threshold silicon nitride waveguide is used, which is corrosion-resistant and resistant to high and low temperatures. It can work stably for more than 10 years when buried in dams, tunnels and other scenarios; it completely avoids the scrapping of PZT devices due to electrostatic or power frequency breakdown, improving reliability by more than 80%.

[0035] High integration and portability: Using silicon-based optical waveguide technology, core functional modules such as ring cavity, waveguide structure, and modulation unit are integrated into a single chip. The chip area is small, reducing the size by more than 90% compared to traditional discrete FP filter demodulators. Combined with wafer-level packaging technology, it is suitable for scenarios such as handheld inspection and deployment in confined spaces.

[0036] Reduced cabling costs: A single device can support hundreds of FBG measurement channels, and all measurement points can be connected in series with a single optical fiber, reducing cabling costs by more than 60% compared to traditional products; it is especially suitable for long-distance, multi-point monitoring scenarios, solving the problems of limited channels and complex cabling in traditional products. Attached Figure Description

[0037] Figure 1 This is a system block diagram of an FBG grating demodulator based on a silicon nitride photonic filter chip according to the present invention. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the technical solution.

[0039] Example: This example discloses an FBG grating demodulator based on a silicon nitride photonic filter chip, including a broadband light source, an optical circulator, an FBG sensor array, a silicon nitride photonic filter chip, a photodetector, an electrical amplifier, an ADC data acquisition card, a digital filter (FPGA), and a computer processing unit.

[0040] A broadband light source outputs C+L band broadband light, which enters the FBG sensor array via an optical circulator. The FBG array converts external physical quantities into wavelength shifts and reflects narrowband optical signals. The reflected light enters a silicon nitride photonic filter chip via the optical circulator. The chip achieves high-speed wavelength scanning and filtering through electronic control or thermal tuning. The filtered optical signal is converted into an electrical signal by a photodetector, amplified by an amplifier, and then acquired as a digital signal by an ADC. The digital signal enters an FPGA to perform noise reduction, clutter filtering, peak deconvolution, and wavelength lookup table calculation. Finally, a computer processing unit completes wavelength inversion, physical quantity conversion, data display, storage, and over-limit alarm.

[0041] The silicon nitride photonic filter chip integrates a ring resonant cavity and a subwavelength grating dispersion structure, with a tuning speed ≥1MHz, a resolution ≤1pm, no mechanical moving parts, and stability far exceeding that of traditional FP cavities.

[0042] During the scanning process, light intensity is collected at a fixed step size Δλ. After completing the full-band scan, the reflection spectrum is reconstructed to achieve crosstalk-free demodulation of multiple FBG sensors.

[0043] This embodiment can achieve simultaneous monitoring of hundreds of channels on a single optical fiber. The whole machine is small in size, high in precision, long in life, and maintenance-free, making it suitable for long-term safety monitoring of various large structures.

[0044] It should be noted that the above embodiments are not intended to limit the scope of protection of the present invention. Equivalent transformations or substitutions made based on the above technical solutions all fall within the scope of protection of the claims of the present invention.

Claims

1. An FBG grating demodulator based on a silicon nitride photonic filter chip, characterized in that, include: Broadband light source, optical circulator, FBG sensor array, silicon nitride photonic filter chip, photodetector, electrical amplifier, ADC data acquisition card, digital filter (FPGA) and computer processing unit, The broadband light source is used to output a broadband optical signal; The input end of the optical circulator is connected to the output end of the broadband light source, which is used to guide the optical signal into the FBG sensor array and guide the FBG reflected signal into the silicon nitride photonic filter chip. The FBG sensor array is composed of multiple FBG sensors connected in series. Its input end is connected to the output end of the optical circulator and is used to reflect light signals of a specific wavelength. The silicon nitride photonic filter chip has its input end connected to the reflective end of the optical circulator, and is used to filter and select the wavelength of the FBG reflected light. The photodetector has its input end connected to the output end of the silicon nitride photonic filter chip, and is used to convert optical signals into electrical signals; The electrical amplifier has its input terminal connected to the output terminal of the photodetector and is used to amplify the electrical signal. The ADC data acquisition card has its input terminal connected to the output terminal of the electrical amplifier, and is used to convert analog electrical signals into digital signals. The digital filter (FPGA) has its input terminal connected to the output terminal of the ADC data acquisition card, and is used to filter and extract peak values ​​from digital signals. The computer processing unit has its input terminal connected to the output terminal of the digital filter (FPGA) and is used to perform wavelength inversion and physical quantity calculation on the signal after the signal.

2. The demodulator according to claim 1, characterized in that, The silicon nitride photonic filter chip uses a thermal tuning method to achieve wavelength selection, with a tuning speed of not less than 1MHz and a wavelength resolution of not less than 1pm.

3. The demodulator according to claim 1, characterized in that, The FBG sensor array supports the series connection of hundreds of sensors on a single fiber to achieve distributed monitoring at multiple measurement points, with each FBG sensor corresponding to a specific center reflection wavelength.

4. The demodulator according to claim 1, characterized in that, The digital filter (FPGA) incorporates a peak deconvolution algorithm and a wavelength lookup table to achieve accurate wavelength peak location and high-speed demodulation.

5. The demodulator according to claim 1, characterized in that, The computer processing unit converts digital signals into physical quantity measurement results such as strain, temperature, and pressure through wavelength inversion algorithms, and displays and stores them in real time, adapting to long-term online monitoring scenarios for large structures.

6. The demodulator according to claim 1 or 2, characterized in that, By changing the tuning parameters of the silicon nitride photonic filter chip, high-speed scanning and demodulation of the reflected wavelength of the FBG sensor array can be achieved.

7. The demodulator according to claim 6, characterized in that, During the process of scrambling, the value of light intensity detected by the detector as a function of wavelength is recorded, and the FBG reflection spectrum is obtained by reconstructing the full-band light intensity data.

8. The demodulator according to claim 2, characterized in that, The silicon nitride photonic filter chip is fabricated in batches using wafer-level photolithography.

9. The demodulator according to claim 2, characterized in that, The waveguide structure in the silicon nitride photonic filter chip uses high-temperature purified silicon nitride material. The silicon nitride material contains extremely strong Si-N / Si-O covalent bonds, and the waveguide can withstand an optical power of 10W, which is 100 times that of fiber optic FP products.

10. The demodulator according to claim 2, characterized in that, The waveguide in the photonic filter chip is a subwavelength grating waveguide structure, with a grating period smaller than the effective propagation wavelength of the working light in the waveguide, which is used to control the effective refractive index and dispersion characteristics of the waveguide. The photonic filter chip is used to perform wave division processing on the reflected signals from hundreds of FBG sensors to achieve parallel demodulation of multi-channel signals.

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