Sagnac photo-thermal spectrum measurement device and method for square wave-phase coordinated regulation and control

Abstract

The invention relates to the technical field of gas detection, and discloses a square wave-phase coordinated regulation Sagnac photo-thermal spectrum measurement device and method.The measurement device comprises a photo-thermal excitation module, a photo-thermal detection module and a signal processing and closed-loop control module; the photo-thermal excitation module comprises a pump light laser, an erbium-doped optical fiber amplifier, a polarization controller, a first lens, a first wavelength division multiplexer, a second lens, a second wavelength division multiplexer and a hollow-core anti-resonance optical fiber. The output end of the first wavelength division multiplexer is connected with the first end of the hollow-core anti-resonance optical fiber. The first end of the hollow-core anti-resonance optical fiber is connected with the first wavelength division multiplexer and the first lens in sequence, the second end of the hollow-core anti-resonance optical fiber is connected with the second wavelength division multiplexer and the second lens in sequence, the pump light laser emits pump light, characteristic wavelength scanning and dynamic modulation are achieved under the control of the laser driver, and the pump light enters the erbium-doped optical fiber amplifier to provide enough photo-thermal excitation power. The system is high in anti-interference performance, good in stability, high in response speed and high in sensitivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas detection, and in particular to a square wave-phase synergistic regulation Sagnac optothermal spectroscopy measurement device and method. BACKGROUND

[0002] In the field of gas detection, although the traditional spectral absorption method is widely used in trace gas measurement, its core bottleneck is that it is difficult to achieve both sensitivity and environmental interference resistance. In order to achieve high sensitivity, it is often necessary to rely on a long optical path gas chamber or a precise temperature control environment, resulting in a heavy and unstable system. Existing enhancement technologies such as laser tuning absorption method achieve one-stage signal amplification through harmonic detection, and optothermal spectroscopy and photoacoustic spectroscopy all have the defects of insufficient single-stage gain and lack of noise suppression. Although the Sagnac vernier enhancement or intracavity laser detection technology developed in recent years improves the sensitivity through interference structure or intracavity amplification, it is still limited by the design of static working point and cannot dynamically suppress temperature drift / vibration, so the response speed is slow.

[0003] At present, there are also some related researches on trace gas detection and measurement in the prior art, such as the invention patent application for a double-parameter optical fiber sensor based on the vernier effect and Sagnac ring with application number 202510856225.5 and the name of a kind of double-parameter optical fiber sensor. The application proposes a double-parameter optical fiber sensor, which uses the combination of the vernier effect and the Sagnac interferometer to write a fiber Bragg grating (FBG) in a few-mode optical fiber, forming two transmission peaks, so as to simultaneously measure the gas concentration and temperature. The application improves the sensitivity of gas concentration and temperature measurement through the superposition of the vernier effect and the Sagnac interferometer, but it does not involve the real-time compensation and locking mechanism of phase disturbance, so the stability, response speed and sensitivity of the system are limited in actual application.

[0004] For example, the invention patent application for a trace gas detection system and method with application number 202510560533.3 and the name of a trace gas detection system, which uses a resonant cavity structure combined with a hollow optical fiber. The gas interacts with the probe light beam, accumulates phase changes, and enhances the sensitivity of the signal through phase-locked amplification technology. The system is mainly used for detecting low-concentration gas, but it relies more on phase accumulation and phase-locked technology, and the effect of low-frequency drift suppression is limited.

[0005] Although the research in the prior art solves some problems in complex alkane detection, it does not solve the core contradiction of complex alkane detection: achieving the unification of ppb-level sensitivity, fast response and anti-interference ability in harsh industrial scenes, and urgently needs breakthroughs in multi-stage synergistic enhancement mechanism and active noise suppression technology. SUMMARY

[0006] In order to overcome the deficiencies of the prior art, the purpose of the present application is to provide a square wave-phase collaborative regulation Sagnac optical thermal spectrum measurement device and method, which has strong anti-interference, good stability, fast response speed and high sensitivity.

[0007] The present application adopts the following technical scheme: a square wave-phase collaborative regulation Sagnac optical thermal spectrum measurement device, comprising an optical thermal excitation module, an optical thermal detection module and a signal processing and closed-loop control module, wherein the optical thermal excitation module comprises a pump light laser, an erbium-doped fiber amplifier, a polarization controller, a first lens, a first wavelength division multiplexer, a second lens, a second wavelength division multiplexer and a hollow-core anti-resonant fiber, the output end of the first wavelength division multiplexer is connected to the first end of the hollow-core anti-resonant fiber; the second end of the hollow-core anti-resonant fiber is sequentially connected to the second wavelength division multiplexer and the second lens, the pump light laser emits pump light, and under the control of a laser driver, realizes feature wavelength scanning and dynamic modulation, enters the erbium-doped fiber amplifier to provide sufficient optical thermal excitation power; then the polarization state is adjusted to be consistent with the main polarization state of the Sagnac interference ring through the polarization controller; after the pump light shaped by the first lens is combined with the probe light at the first wavelength division multiplexer, it enters the hollow-core anti-resonant fiber to form an optical thermal action area; the hollow-core part of the hollow-core anti-resonant fiber is connected to the gas to be measured, the absorption of the pump light by the gas to be measured causes local temperature rise and refractive index change, thereby forming an optical thermal phase disturbance related to the gas concentration in the Sagnac interference ring; downstream of the hollow-core anti-resonant fiber, the combined light enters the second wavelength division multiplexer, and the second wavelength division multiplexer is used to separate the pump light from the probe light channel and lead it out; the light splitting end of the second wavelength division multiplexer is electrically connected to the first detector, the separated pump light is coupled to the first detector through the second lens, which is used to monitor the pump power in real time and serves as the basis for subsequent signal normalization, while the probe light continues to propagate along the Sagnac interference ring from the probe light end of the second wavelength division multiplexer. The optical thermal detection module comprises a probe light laser, a circulator, a lithium niobate Y waveguide phase modulator, a delay optical fiber and a second detector, the output of the probe light laser is divided into clockwise and counterclockwise two paths through the circulator and the lithium niobate Y waveguide phase modulator, respectively coupled into the second end and the first end of the hollow-core anti-resonant fiber through the second lens and the first lens, and together with the hollow-core anti-resonant fiber, the lithium niobate Y waveguide phase modulator and the delay optical fiber forms a Sagnac interference ring, the second detector is arranged at the output end of the circulator and is used to receive interference output light and convert it into an electrical signal. The signal processing and closed-loop control module comprises a signal generator, a phase-locked amplifier, a data acquisition card, a field programmable gate array and an upper computer signal processing unit, wherein the signal generator is electrically connected with a lithium niobate Y waveguide phase modulator and a pump light laser respectively; the phase-locked amplifier and / or the field programmable gate array are used for second harmonic phase-locked demodulation of the electrical signal from the second detector and output of the demodulation result; the field programmable gate array realizes phase closed-loop control based on the demodulation result and the pump monitoring signal output by the first detector, outputs a phase modulation driving voltage to the lithium niobate Y waveguide phase modulator, locks the Sagnac interference ring working point in the π / 2 linear region, realizes digital phase-locked demodulation and closed-loop control, and is used for high-sensitivity reading out and noise suppression of weak light thermal phase disturbance; the upper computer signal processing unit communicates with the data acquisition card and the field programmable gate array, and is used for receiving, storing and collecting data and gas concentration inversion, display and parameter configuration.

[0008] Further, the signal generator can output a square wave phase bias signal to the lithium niobate Y waveguide phase modulator, and superimpose a small-amplitude sinusoidal modulation signal of the same frequency to the pump light laser.

[0009] Further, the length of the delay optical fiber is Satisfying:

[0010] wherein c is the speed of light, is the refractive index of the optical fiber, and the length of the delay optical fiber is to match the frequency of the square wave phase bias signal f PT to match the frequency of the square wave phase bias signal

[0011] Further, the gas to be measured is acetylene, ethane, ethylene, methane or other hydrocarbon gases with near-infrared absorption characteristics.

[0012] A square wave-phase collaborative regulation Sagnac optothermal spectrum measurement method using the square wave-phase collaborative regulation Sagnac optothermal spectrum measurement device, comprising the following steps: Step 1: optothermal excitation and phase difference generation; The pump light and the probe light are coupled to the hollow core anti-resonant optical fiber through the first wavelength division multiplexer, interact with the gas to be measured in the hollow core of the hollow core anti-resonant optical fiber, the gas to be measured generates an optothermal effect after absorbing the pump light, forms a thermal refractive index disturbance, and causes a change in the phase difference of clockwise and counterclockwise light in the Sagnac interference ring ; Step 2: square wave bias and frequency matching; To construct a time-domain non-reciprocal differential and suppress low-frequency drift, a lithium niobate Y-waveguide phase modulator and a delay fiber are introduced within the Sagnac interferometer loop. A square wave phase bias signal is applied to the lithium niobate Y-waveguide phase modulator via a signal generator and a field-programmable gate array. The square wave phase bias frequency applied to the lithium niobate Y-waveguide phase modulator is... f PT Delay fiber length satisfy: , in Here, c is the refractive index of the optical fiber, and c is the speed of light. The clockwise and counterclockwise paths sample the phase gain of each other as mirror images in the two half-cycles of the square wave, and the low-frequency common drift is canceled out by the differential. Since the Sagnac interference loop is a common path structure, the delay line introduces the arrival time misalignment rather than the geometric optical path difference, and is not limited by the coherence length. Step 3: Superposition of pump-frequency sinusoidal signals with 2 f Phase-locked demodulation; Step 4: Closed-loop phase compensation locks the π / 2 linear region and achieves high-slope readout.

[0013] Furthermore, the specific process of step 3 is as follows: In addition to outputting a square wave bias, the signal generator also outputs a signal that is... f PT A small-amplitude sinusoidal signal of the same frequency is superimposed on the driving current of the pump laser as a frequency modulation source. At the same time, the sinusoidal signal is sent to the lock-in amplifier as a reference input. The output of the Sagnac interference loop is converted into an electrical signal by the second detector, the 2f component is extracted by the lock-in amplifier, and the signal is sent to the signal processing and closed-loop control module through the data acquisition card.

[0014] Furthermore, the specific process of step 4 is as follows: the signal processing and closed-loop control module, according to 2 f Amplitude, interference DC component, and pump monitoring channel construction error signal are used to calculate the compensation phase. The driving voltage of the lithium niobate Y-waveguide phase modulator is adjusted in real time, and phase compensation is applied through closed-loop feedback. .

[0015] Furthermore, + = π / 2, thereby locking the interferometric operating point at the linear center of the maximum slope of the interferometric curve, achieving high-slope readout of photothermal phase perturbations and low-frequency drift suppression.

[0016] Furthermore, f PT The inflection point frequency of the first-order amplitude-frequency response of the photothermal interaction region of the hollow anti-resonant fiber f cThe order of magnitude matching is matched to limit the amplitude frequency roll-off, and the light-thermal first-order amplitude frequency response inflection point frequency of the light-thermal action region of the hollow core anti-resonant optical fiber f c : f c ≈1 / ( ), wherein is a light-thermal characteristic time constant, , wherein r eff is an equivalent thermal diffusion radius of the hollow core anti-resonant optical fiber, is the thermal diffusion capacity of the gas to be measured in the hollow core light-thermal action region of the hollow core anti-resonant optical fiber, and the thermal diffusion rate is used, , wherein k is the thermal conductivity, the unit is W / (m·K); ρ is the density of the gas to be measured, the unit is kg / m 3 ; c p is the specific heat capacity, the unit is J / (kg·K).

[0017] Compared with the prior art, the beneficial effects of the present application are: 1. The square wave-phase synergistic regulation Sagnac photothermal spectrum measurement device and method, through the synergistic design of the light-thermal excitation module, the light-thermal detection module, and the signal processing and closed-loop control module, the low-frequency noise caused by environmental temperature fluctuation and mechanical vibration is actively inhibited, and the baseline drift and zero drift problems in traditional photothermal measurement can be effectively solved.

[0018] 2. The square wave-phase synergistic regulation Sagnac photothermal spectrum measurement device and method, by integrating a lithium niobate Y waveguide phase modulator, a phase-intensity closed-loop control loop is constructed, the interference response function is dynamically locked in the linear region of the sinusoidal curve from the nonlinear region of the cosine, and the response slope of the interference output to the gas refractive index change is significantly improved, the linear region width of the system is expanded, and the sensitivity can be improved by about 1-2 orders of magnitude.

[0019] 3. The square wave-phase synergistic regulation Sagnac photothermal spectrum measurement device and method, by the synergistic effect of square wave bias modulation and phase feedback demodulation, the working point of the Sagnac interferometer is stabilized at the π / 2 phase position, the response slope is maximized, the high-sensitivity detection of the small change of the photothermal refractive index is realized, the phase noise caused by environmental vibration and temperature drift can be simultaneously inhibited, and the sensitivity and anti-interference performance are considered.

[0020] 4. The square wave-phase collaborative regulation Sagnac photothermal spectrum measurement device and method of the present application can realize ppb detection limit of acetylene / ethylene (2 orders of magnitude higher than that of the traditional scheme) through closed-loop Sagnac phase locking combined with HC-ARF photothermal enhancement, and improves stability, and phase feedback inhibits environmental vibration / temperature drift, and is suitable for complex industrial application environment. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a structural schematic diagram of the square wave-phase collaborative regulation Sagnac photothermal spectrum measurement device of the present application; Figure 2 is a flow chart of the square wave-phase collaborative regulation Sagnac photothermal spectrum measurement method of the present application; Figure 3 is a curve showing output difference of output interference light intensity of the Sagnac interference ring 10 under open-loop and closed-loop conditions.

[0022] In the figure: pump light laser-11; erbium-doped fiber amplifier-12; polarization controller-13; first lens-14; first wavelength division multiplexer-15; second lens-16; second wavelength division multiplexer-17; hollow-core anti-resonant fiber-18; probe light laser-21; circulator-22; lithium niobate Y-waveguide phase modulator-23; delay optical fiber-24; second detector-25; first detector-26; signal generator-31; lock-in amplifier-32; data acquisition card-33; field programmable gate array-34; upper computer signal processing unit-35. DETAILED DESCRIPTION

[0023] The advantages and effects of the present application can be easily understood by those skilled in the art from the description. The present application can also be implemented or applied in different specific embodiments, and the details in the description can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the following examples and features in the examples can be combined with each other without conflict.

[0024] The purpose of the present application is to provide a square wave-phase collaborative regulation Sagnac photothermal spectrum measurement device and method to overcome the defects of the prior art.

[0025] Example 1 The square wave-phase collaborative regulation Sagnac photothermal spectrum measurement device provided in this embodiment is suitable for trace gas detection in industrial temperature and vibration environment, and can be used for detecting acetylene, ethane, ethylene, methane or other hydrocarbon gases with near-infrared absorption characteristics. In this embodiment, acetylene measurement is taken as an example.

[0026] Referring to Figure 1 As shown, the optical-thermal measurement device based on Sagnac interference includes an optical-thermal excitation module, an optical-thermal detection module, and a signal processing and closed-loop control module. The optical-thermal excitation module includes a pump light laser 11, an erbium-doped fiber amplifier 12 (EDFA), a polarization controller 13, a first lens 14, a first wavelength division multiplexer 15 (WDM1), a second lens 16, a second wavelength division multiplexer 17, and a hollow-core anti-resonant fiber 18 (HC-ARF). The output end of the first wavelength division multiplexer 15 is connected to the first end of the hollow-core anti-resonant fiber 18. The second end of the hollow-core anti-resonant fiber 18 is sequentially connected to the second wavelength division multiplexer 17 (WDM2) and the second lens 16. The pump light laser 11 in the embodiment is a distributed feedback laser (DFB), and the center wavelength is determined by the absorption spectrum of the measured gas. For example, for acetylene, a DFB laser with a wavelength of about 1.532 μm and a line width of less than 5 MHz is selected. The pump light laser 11 emits pump light, which, under the control of a laser driver, realizes feature wavelength scanning and dynamic modulation, enters the erbium-doped fiber amplifier 12 (EDFA) to provide sufficient optical-thermal excitation power, and then passes through the polarization controller 13 to adjust the polarization state to be consistent with the main polarization state of the Sagnac interference ring 10. After the pump light and the probe light are combined at the first wavelength division multiplexer 15 (WDM1) after being shaped by the first lens 14, they enter the hollow-core anti-resonant fiber 18 (HC-ARF) to form an optical-thermal action area. In the embodiment, a WDM1 with a wavelength of 1530 / 1550 nm is preferably used to realize coaxial transmission and high-isolation combining and splitting of the pump light and the probe light. The hollow-core portion of the hollow-core anti-resonant fiber 18 (HC-ARF) is connected to the measured gas containing acetylene. The absorption of acetylene to the pump light leads to local temperature rise and refractive index change, thereby forming an optical-thermal phase disturbance related to the gas concentration in the Sagnac interference ring 10. Downstream of the hollow-core anti-resonant fiber 18 (HC-ARF), the combined light enters the second wavelength division multiplexer 17 (WDM2), which is used to separate the pump light from the probe light channel and lead it out. The light splitting end of the second wavelength division multiplexer 17 is electrically connected to the first detector 26. The separated pump light is coupled to the first detector 26 through the second lens 16, which is used to monitor the pump power in real time and serves as a basis for subsequent signal normalization. The probe light continues to propagate along the Sagnac interference ring 10 from the probe light end of the second wavelength division multiplexer 17.

[0027] The photothermal detection module comprises a detection light laser 21, a circulator 22, a lithium niobate Y waveguide phase modulator 23 (Y waveguide), a delay optical fiber 24, a first detector 26 and a second detector 25. In the embodiment, the detection light source emitted by the detection light laser 21 can be a broadband light source with a center wavelength of 1550nm±20nm or 1310nm±20nm, or a narrow linewidth continuous wave light source. The output of the detection light laser 21 is injected into the input end of the circulator 22 after optical isolation, and is connected to the lithium niobate Y waveguide phase modulator 23 (Y waveguide) from the output end of the circulator 22. The lithium niobate Y waveguide phase modulator 23 is divided into clockwise and counterclockwise two paths, enters the optical fiber ring structure, and constitutes a Sagnac interference ring 10. The Sagnac interference ring 10 is connected in series with the hollow-core anti-resonant fiber 18, so that the two paths of detection light pass through the gas action area; the Y waveguide and the delay optical fiber 24 are connected in series downstream of the hollow-core anti-resonant fiber 18, and the Y waveguide is used for receiving the square wave bias and the closed-loop electro-optical compensation signal.

[0028] The length of the delay optical fiber 24 is configured to be matched with the square wave frequency . f PT The precise matching introduces a non-reciprocal differential phase sampling effect in the time domain; the two paths of light return to the Y waveguide to recombine to generate an interference signal, which is output to the second optical detector through the circulator 22, and the interference light intensity is converted into an electrical signal.

[0029] The signal processing and closed-loop control module comprises a signal generator 31, a lock-in amplifier 32, a data acquisition card 33 (DAQ), a field programmable gate array 34 (FPGA) and an upper computer signal processing unit 35. The signal generator 31 is electrically connected to the Y waveguide and the pump light laser 11, respectively, for outputting a square wave phase bias signal with a frequency of f PT to the lithium niobate Y waveguide phase modulator 23, for the Y waveguide to realize square wave phase biasing, and for superimposing a small-amplitude sinusoidal modulation with the same frequency on the driving current of the pump light laser 11, which is used as an optical thermal frequency domain fine tuning signal and a lock-in reference. The lock-in amplifier 32 and / or the FPGA are used for second harmonic (2 f ) lock-in demodulation of the electrical signal from the second detector 25 and output of the demodulation result; the FPGA realizes phase closed-loop control based on the demodulation result and the pump monitoring signal output by the first detector 26, and outputs a phase modulation driving voltage to the lithium niobate Y waveguide phase modulator 23, so as to lock the working point of the Sagnac interference ring 10 in the π / 2 linear region, realize digital lock-in demodulation and closed-loop control algorithm, and be used for high-sensitivity readout and noise suppression of weak optical thermal phase disturbance; the upper computer signal processing unit 35 communicates with the data acquisition card 33 and the FPGA, and is used for receiving, storing and collecting data, and realizing gas concentration inversion, display and parameter configuration.

[0030] The signal processing and closed-loop control module calculates the phase compensation amount in real time according to the preset control law, sends the corresponding driving voltage into the Y waveguide after digital-to-analog conversion, and makes the phase disturbance caused by the gas and the electro-optical compensation phase stabilize near π / 2, thereby locking the interference operating point in the linear region of the response curve. The FPGA is also responsible for coordinating the signal generator 31 trigger, DAQ sampling, phase-locked time constant adjustment, and data packaging and uploading, etc., to ensure that the photo-thermal excitation, Sagnac interference, phase-locked demodulation and phase closed loop work cooperatively in time and frequency.

[0031] The photo-thermal measurement device based on Sagnac interference of the present application, the pump light laser 11 in the photo-thermal excitation module emits pump light, realizes characteristic wavelength scanning and dynamic modulation under the control of the laser driver, enters the erbium-doped fiber amplifier 12 (EDFA) to provide sufficient photo-thermal excitation power; then the polarization state is adjusted to be consistent with the main polarization state of the Sagnac interference ring 10 through the polarization controller 13; after the pump light is shaped by the first lens 14, the pump light and the probe light are combined at the first wavelength division multiplexer 15 (WDM1) and then enter the hollow core anti-resonant fiber 18 (HC-ARF) to form a photo-thermal action area; the hollow core part of the hollow core anti-resonant fiber 18 (HC-ARF) is connected to the measured gas containing acetylene, and the absorption of acetylene to the pump light leads to local temperature rise and refractive index change, thereby forming a photo-thermal phase disturbance related to the gas concentration in the Sagnac interference ring 10; downstream of the hollow core anti-resonant fiber 18 (HC-ARF), the combined light enters the second wavelength division multiplexer 17 (WDM2), which is used to separate and lead out the pump light from the probe light channel; the separated pump light is coupled to the first probe 26 through the second lens 16 for real-time monitoring of the pump power and as a basis for subsequent signal normalization, while the probe light continues to propagate along the Sagnac interference ring 10 from the probe light end of the second wavelength division multiplexer 17. The output of the probe light laser 21 in the photo-thermal detection module is divided into clockwise and counterclockwise two paths through the circulator 22 and the Y waveguide, respectively coupled into the second end and the first end of the hollow core anti-resonant fiber 18 (HC-ARF) through the second lens 16 and the first lens 14, and together with the hollow core anti-resonant fiber 18, the Y waveguide and the delay fiber 24 form the Sagnac interference ring 10, and the second probe 25 receives the interference output light and converts it into an electrical signal; the interference signal output by the second probe 25 is sent into the phase-locked amplifier 32 or the digital phase-locked unit inside the FPGA after preamplification, extracts the amplitude component corresponding to the second harmonic (2f), and is sent to the signal processing and closed-loop control unit through the data acquisition card 33 for gas concentration inversion and system state monitoring.

[0032] Embodiment two The working frequency of the square wave-phase cooperative regulation Sagnac optothermal spectrum measurement device and method f PT The square wave phase bias frequency applied to the Y waveguide is also the same frequency small amplitude sinusoidal modulation frequency superimposed on the pump laser driving current, and serves as a phase-locked reference. f PT The light-thermal first-order amplitude-frequency response inflection point frequency of the light-thermal interaction region of the hollow core anti-resonant fiber 18 f c The same order of magnitude is matched to limit the amplitude-frequency roll-off. The light-thermal first-order amplitude-frequency response inflection point frequency of the light-thermal interaction region of the hollow core anti-resonant fiber 18 f c : f c ≈1 / ( ), where is the light-thermal characteristic time constant, , where r eff The equivalent thermal diffusion radius of the hollow core anti-resonant fiber 18, is the thermal diffusivity of the gas to be measured in the hollow core light-thermal interaction region of the hollow core anti-resonant fiber 18, , wherein k is the thermal conductivity, the unit is W / (m·K); ρ is the density of the gas to be measured, the unit is kg / m 3 ; c p is the specific heat capacity, the unit is J / (kg·K).

[0033] In this embodiment, taking 25℃, 1atm acetylene as an example, k is the thermal conductivity 0.018 W / (m·K), ρ is the density of the target gas acetylene about 1 kg / m 3 , c p is the specific heat capacity 1690 J / (kg·K), and the thermal diffusivity =1×10 -5 m 2 / s is calculated.

[0034] The thermal diffusivity characterizes the thermal diffusion capacity of the measured gas acetylene in the HC-ARF hollow core light-thermal interaction region of the embodiment (the effective thermal diffusivity under the given temperature and pressure conditions), which is used to characterize the amplitude-frequency roll-off characteristic of the light-thermal effect with the modulation frequency. In this embodiment, the equivalent thermal diffusion radius of the HC-ARF is 35μm, the light-thermal characteristic time constant τ is calculated as τ≈12.4μs, and the inflection point frequency f c≈12.8kHz, the inflection frequency reflects the turning point from the flat region to the roll-off region of the photo-thermal response caused by the thermal diffusion of the gas to be measured, according to f PT The inflection frequency f c is selected according to the principle of matching the same order of magnitude, and in this embodiment f PT 20kHz is taken.

[0035] In this embodiment, τ and f c are calculated from the thermal diffusivity and the equivalent radius, and f PT and f c are selected according to the principle of matching the same order of magnitude to limit the amplitude-frequency roll-off f PT =20kHz is taken to balance between resisting low-frequency drift and maintaining the amplitude of the photo-thermal response.

[0036] A square wave-phase collaborative control Sagnac photo-thermal spectrum measurement method of this embodiment, using the square wave-phase collaborative control Sagnac photo-thermal spectrum measurement device in embodiment one, referring to Figure 2 , includes the following steps: Step 1: photo-thermal excitation and phase difference generation; The pump light and the probe light are coupled to the hollow core anti-resonant optical fiber 18 (HC-ARF) through the first wavelength division multiplexer 15, interact with the gas to be measured in the hollow core of the HC-ARF, and after the gas to be measured absorbs the pump light, the photo-thermal effect is generated, the thermal refractive index perturbation is formed, and the phase difference change of the clockwise and counterclockwise light in the Sagnac interference ring 10 is caused .

[0037] Step 2: square wave bias and frequency matching; In order to construct a time-domain non-reciprocal differential and suppress low-frequency drift, a lithium niobate Y waveguide phase modulator 23 (Y waveguide) and a delay optical fiber 24 are introduced into the Sagnac interference ring 10, a square wave phase bias signal with a duty cycle of 50% is applied to the Y waveguide through a signal generator 31 and an FPGA, and the square wave phase bias frequency applied to the Y waveguide is f PT 20KHz is taken, and the length of the delay optical fiber 24 is satisfies: , wherein is the refractive index of the optical fiber, ≈1.468, c is the speed of light, and by substitution ≈5.1km.

[0038] fPT and f c After matching of the same magnitude, the clockwise and counterclockwise paths sample mirror images of each other's phase gain in the two half-cycles of the square wave, and the low-frequency common drift is differentially canceled out. Since the Sagnac interferometer ring 10 has a common-path structure, the delay line introduces an arrival time misalignment rather than a geometric optical path difference, and is not limited by the coherence length. Furthermore, the delay line length can be shortened by increasing the modulation frequency, and adjustments can be made according to the application scenario. The first-order amplitude-frequency selection for photothermal applications follows... f PT and f C The principle of similar magnitude aims to achieve a compromise between resisting low-frequency drift and maintaining sufficient amplitude.

[0039] Step 3: Superposition of pump-frequency sinusoidal signals with 2 f Phase-locked demodulation; In addition to outputting a square wave bias, signal generator 31 also outputs a signal with... f PT A small-amplitude sinusoidal signal of the same frequency is superimposed on the driving current of the pump laser 11 as a frequency modulation source. At the same time, the sinusoidal signal is sent to the lock-in amplifier 32 as a reference input. The output of the Sagnac interference loop is converted into an electrical signal by the second detector 25, the 2f component is extracted by the lock-in amplifier 32, and the signal is sent to the FPGA-based signal processing and closed-loop control module through the data acquisition card 33.

[0040] Step 4: Closed-loop phase compensation locks the π / 2 linear region and achieves high-slope readout. The signal processing and closed-loop control module is based on 2 f Amplitude, interference DC component, and pump monitoring channel construction error signal are used to calculate the compensation phase. The driving voltage of the lithium niobate Y-waveguide phase modulator 23 is adjusted in real time, and phase compensation is applied through closed-loop feedback. Phase perturbation caused by gas + = π / 2, thereby locking the interferometric working point at the linear center of the maximum slope of the interferometric curve, realizing high-slope readout of photothermal phase perturbations and low-frequency drift suppression, and realizing high-sensitivity, low-drift online measurement of acetylene concentration.

[0041] In this embodiment, the modulation and demodulation system adopts a collaborative scheme of "square wave phase bias combined with same-frequency sine and second harmonic (2f) phase lock": the signal generator 31 outputs a 20kHz square wave to the phase modulator, which is equivalent to constructing time-domain differential and non-reciprocal sampling in the Sagnac interference loop 10; on the other hand, it superimposes a small-amplitude sine wave of the same frequency onto the pump DFB to form a frequency domain fine-tuning around the center of the spectral line. The lock-in amplifier 32 uses this sine wave as a reference to achieve a phase lock-in of 2kHz. fThe channel is phase-locked demodulated to obtain a second harmonic amplitude in a monotonous relationship with the gas absorption intensity. f The modulation depth is selected by calibration experiments to make the amplitude close to the maximum value, so that the signal-to-noise ratio is as high as possible under the premise of ensuring linearity.

[0042] In this embodiment, the interference response function I=0.5I0[1-cos ( + )] corresponding to the output interference light intensity I of the Sagnac interference ring 10 is also established, which describes the relationship between I and the total phase difference (determined by the gas optical thermal phase disturbance and the closed-loop compensation phase), and the present application determines the π / 2 linear working point and realizes closed-loop locking by using the interference response function. Figure 3 As shown in the figure, the horizontal coordinate is the phase of the Sagnac interference ring 10, and the vertical coordinate is the output light power / intensity I of the detector, P 0 、P 1 It is shown that under the two phase states of square wave bias, the output falls on two different intensity levels. The closed loop stabilizes the working point at the linear center (about π / 2) of the interference curve, which can significantly improve the readout slope and suppress low-frequency drift. In order to realize fast capture and stable maintenance of the linear center, the present embodiment applies a square wave phase bias to the Y waveguide, so that the interference output forms a synchronous difference between the two bias states, When the difference changes, the difference simultaneously serves as an effective readout signal and a closed-loop error signal, which drives real-time compensation, so that the working point is continuously locked in the maximum slope linear region, so that the weak change is converted into a larger I change.

[0043] In this embodiment, the square wave bias modulation is to apply a square wave bias jumping between two phases to one of the propagation directions of the Y waveguide (or an equivalent non-reciprocal quantity). The square wave repeatedly samples the interference response curve, so that the small phase disturbance is converted into a synchronous intensity difference, which is convenient for phase-locked loop / closed loop. I=0 and I≠0 are synchronous difference signals of zero / non-zero, not the total light intensity of zero.

[0044] The square wave-phase cooperative regulation Sagnac optothermal spectrum measurement device and method of the present application locks the working point at the π / 2 linear center, and the same size The maximum I change amount is generated near π / 2, so that the phase-voltage conversion gain is maximum and the signal-to-noise ratio is optimal; meanwhile, the output near π / 2 is approximately a first-order linear relationship with respect to phase, the concentration inversion is not easily affected by the cosine nonlinearity, and the closed-loop control is more stable; and, when the open loop, the slow change of the environmental temperature / stress / polarization state will make the working point slide along the cosine curve, once sliding to near 0 or π, the slope is close to 0, and the readout desensitization and significant drift are prone to occur, the closed loop adjusts in real time will be + Forced to lock at π / 2, equivalent compensation for slow disturbance, at this time, the (or its driving voltage) itself becomes a more stable measurement quantity.

[0045] The present application suppresses noise through the cooperation of three layers, and provides inherent common-mode rejection through common-path Sagnac interference; suppresses low-frequency drift through square wave-delay differential; Locking the working point at π / 2 in combination with 2 f The frequency domain gain of phase locking and linear readout. Under the same power and action length, compared with the open loop, the detection limit is significantly improved, and the stability of the interference spectrum line is significantly improved, and the equivalent response can reach milliseconds.

[0046] In addition, it should be understood that, although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that those skilled in the art can understand.

Claims

1. A Sagnac photothermal spectroscopy measurement device with square wave-phase coordinated modulation, characterized in that: The system includes a photothermal excitation module comprising a pump laser (11), an erbium-doped fiber amplifier (12), a polarization controller (13), a first lens (14), a first wavelength division multiplexer (15), a second lens (16), a second wavelength division multiplexer (17), and a hollow-core anti-resonant fiber (18). The output of the first wavelength division multiplexer (15) is connected to the first end of the hollow-core anti-resonant fiber (18). The second end of the hollow-core anti-resonant fiber (18) is connected to the second wavelength division multiplexer (17) and the second lens (16) in sequence. The pump laser (11) emits pump light, which is scanned and dynamically modulated under the control of the laser driver, and enters the erbium-doped fiber amplifier (12) to provide sufficient photothermal excitation power. Then, the polarization state is adjusted by the polarization controller (13) to be consistent with the main polarization state of the Sagnac interference ring 10. After being shaped by the first lens (14), the pump light and the probe light are combined at the first wavelength division multiplexer (15) and then enter the hollow anti-resonant fiber (18) to form a photothermal region. The hollow part of the hollow anti-resonant fiber (18) is filled with the gas to be measured. The absorption of the pump light by the gas to be measured leads to a local temperature increase and a change in refractive index, thereby forming a photothermal phase perturbation related to the gas concentration in the Sagnac interference ring 10. Downstream of the hollow anti-resonant fiber (18), the combined light enters the second Wavelength division multiplexer (17), the second wavelength division multiplexer (17) is used to separate and extract the pump light from the probe light channel; the splitting end of the second wavelength division multiplexer (17) is electrically connected to the first detector (26), the separated pump light is coupled to the first detector (26) through the second lens (16) for real-time monitoring of pump power and as the basis for subsequent signal normalization, while the probe light continues to propagate along the Sagnac interference ring (10) from the probe light end of the second wavelength division multiplexer (17).

2. A Sagnac photothermal spectroscopy measurement device with square wave-phase coordinated modulation, characterized in that: It also includes a photothermal detection module and a signal processing and closed-loop control module; the photothermal detection module includes a probe laser (21), a circulator (22), a lithium niobate Y-waveguide phase modulator (23), a time-delay fiber (24), a first detector (26) and a second detector (25). The output of the probe laser (21) is divided into clockwise and counterclockwise paths by the circulator (22) and the lithium niobate Y-waveguide phase modulator (23), and is coupled into the second end and the first end of the hollow anti-resonant fiber (18) through the second lens (16) and the first lens (14), respectively, and together with the hollow anti-resonant fiber (18), the lithium niobate Y-waveguide phase modulator (23) and the time-delay fiber (24), it forms a Sagnac interference ring (10). The second detector (25) is set at the output end of the circulator (22) to receive the interference output light and convert it into an electrical signal. The signal processing and closed-loop control module includes a signal generator (31), a lock-in amplifier (32), a data acquisition card (33), a field-programmable gate array (34), and a host computer signal processing unit (35). The signal generator (31) is electrically connected to the lithium niobate Y-waveguide phase modulator (23) and the pump laser (11), respectively; the lock-in amplifier (32) and / or the field-programmable gate array (34) are also connected. The signal is used to perform second harmonic phase-locked demodulation on the electrical signal from the second detector (25) and output the demodulation result; the field programmable gate array (34) realizes phase closed-loop control based on the demodulation result and the pump monitoring signal output by the first detector (26), outputs phase modulation drive voltage to the lithium niobate Y waveguide phase modulator (23), locks the operating point of the Sagnac interference ring (10) in the π / 2 linear region, realizes digital phase-locked demodulation and closed-loop control, and is used for high-sensitivity readout and noise suppression of weak photothermal phase disturbances; the host computer signal processing unit (35) communicates with the data acquisition card (33) and the field programmable gate array (34) to receive and store the acquired data and perform gas concentration inversion, display and parameter configuration.

3. The Sagnac photothermal spectroscopy measurement device with square wave-phase coordinated modulation according to claim 2, characterized in that: The signal generator (31) can output a square wave phase bias signal to the lithium niobate Y waveguide phase modulator (23) and superimpose a small-amplitude sinusoidal modulation signal of the same frequency onto the pump laser (11).

4. The Sagnac photothermal spectroscopy measurement device with square wave-phase coordinated modulation according to claim 3, characterized in that: Length of delay fiber (24) satisfy: ， In the formula, c is the speed of light. The refractive index of the fiber is given by the length of the delay fiber (24). The frequency used to offset the square wave phase signal f PT Matching introduces a non-reciprocal differential phase sampling effect in the time domain.

5. The Sagnac photothermal spectroscopy measurement device with square wave-phase coordinated modulation according to claim 4, characterized in that: The gas to be tested is acetylene, ethane, ethylene, methane, or other hydrocarbon gases with near-infrared absorption characteristics.

6. A method for measuring Sagnac photothermal spectrometry with square wave-phase coordinated modulation, using the Sagnac photothermal spectrometry device with square wave-phase coordinated modulation as described in any one of claims 1-5, characterized in that: Includes the following steps: Step 1: Photothermal excitation and phase difference generation; The pump light and probe light are coupled to the hollow anti-resonant fiber (18) via the first wavelength division multiplexer (15). The pump light interacts with the gas to be tested in the hollow core of the hollow anti-resonant fiber (18). After absorbing the pump light, the gas to be tested generates a photothermal effect, forming a thermally induced refractive index perturbation, which causes a phase difference change between the clockwise and counterclockwise light in the Sagnac interference ring (10). ; Step 2: Square wave bias and frequency matching; To construct a time-domain non-reciprocal differential and suppress low-frequency drift, a lithium niobate Y-waveguide phase modulator (23) and a delay fiber (24) are introduced into the Sagnac interferometer loop (10). A square wave phase bias signal is applied to the lithium niobate Y-waveguide phase modulator (23) through a signal generator (31) and a field-programmable gate array (34). The square wave phase bias frequency applied to the lithium niobate Y-waveguide phase modulator (23) is... f PT The clockwise and counterclockwise paths sample the phase gain of each other as mirror images in the two half-cycles of the square wave, and the low-frequency common drift is canceled out by the differential. Since the Sagnac interference ring (10) is a common path structure, the delay line introduces the arrival time misalignment rather than the geometric optical path difference, and is not limited by the coherence length. Step 3: Superposition of pump-frequency sinusoidal signals with 2 f Phase-locked demodulation; Step 4: Closed-loop phase compensation locks the π / 2 linear region and achieves high-slope readout.

7. The Sagnac photothermal spectroscopy measurement method with square wave-phase coordinated modulation according to claim 6, characterized in that: The specific process of step 3 is as follows: In addition to outputting a square wave bias, the signal generator (31) also outputs a signal with... f PT A small-amplitude sinusoidal signal of the same frequency is superimposed on the driving current of the pump laser (11) as a frequency modulation source. At the same time, the sinusoidal signal is sent to the lock-in amplifier (32) as a reference input. The output of the Sagnac interference loop is converted into an electrical signal by the second detector (25), and the 2f component is extracted by the lock-in amplifier (32). The signal is then sent to the signal processing and closed-loop control module through the data acquisition card (33).

8. The Sagnac photothermal spectroscopy measurement method with square wave-phase coordinated modulation according to claim 7, characterized in that: The specific process of step 4 is as follows: the signal processing and closed-loop control module, according to 2 f Amplitude, interference DC component, and pump monitoring channel construction error signal are used to calculate the compensation phase. The driving voltage of the lithium niobate Y-waveguide phase modulator (23) is adjusted in real time, and a compensation phase is applied through closed-loop feedback. .

9. The Sagnac photothermal spectroscopy measurement method with square wave-phase coordinated modulation according to claim 8, characterized in that: + = π / 2, thereby locking the interferometric operating point at the linear center of the maximum slope of the interferometric curve, achieving high-slope readout of photothermal phase perturbations and low-frequency drift suppression.

10. The Sagnac photothermal spectroscopy measurement method with square wave-phase coordinated modulation according to claim 9, characterized in that: f PT The inflection point frequency of the first-order amplitude-frequency response of the photothermal region of the hollow anti-resonant fiber (18) f c Matching of the same magnitude to limit the amplitude roll-off, the inflection point frequency of the first-order amplitude-frequency response of the photothermal region of the hollow anti-resonant fiber (18) f c : f c ≈1 / ( )， In the formula, The photothermal characteristic time constant, ， In the formula, r eff Let be the equivalent thermal diffusion radius of the hollow antiresonant fiber (18). The thermal diffusivity is used to describe the thermal diffusion capability of the gas to be tested within the hollow photothermal region of the hollow anti-resonant fiber (18). ， In the formula, k is the thermal conductivity, with units of W / (m·K); ρ is the density of the gas being measured, with units of kg / m³. 3 ;c p Specific heat capacity, expressed in J / (kg·K).

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