Optical fiber frequency modulation interference cantilever beam micro-sound sensor

Abstract

The invention discloses an optical fiber frequency modulation interference cantilever beam micro-sound sensor, and belongs to the technical field of gas detection. In the optical fiber frequency modulation interference cantilever beam micro-sound sensor, an incident light beam enters a sensing unit after passing through a circulator; the sensing unit comprises a single-mode fiber and an SOI sheet, and a cantilever beam structure is processed in the SOI sheet; one end face of the single-mode optical fiber right faces the tail end of the cantilever beam, an F-P cavity is formed between the end face and the surface of the tail end of the cantilever beam, part of light beams entering the sensing unit are reflected by the end face after passing through the single-mode optical fiber, part of the light beams enter the F-P cavity and then are reflected to the single-mode optical fiber, and the two parts of the light beams reflected to the single-mode optical fiber generate interference. The interference light enters the photoelectric detector after passing through the circulator; the photoelectric detector converts the interference light signal into an electric signal and outputs the electric signal. Compared with an existing gas sensor, the optical fiber frequency modulation interference cantilever beam micro-sound sensor is better in sensitivity, precision and anti-interference capability.

Description

Technical Field

[0001] This application belongs to the technical field of gas detection, and more specifically, relates to a fiber optic frequency modulation interference cantilever microphonic sensor. Background Art

[0002] Currently, the transformers adopted in domestic and foreign power systems are generally oil-filled. The insulation system in such transformers mainly consists of a composite insulation structure composed of insulating materials such as insulating oil, insulating paper or cardboard. During the long-term operation of the transformer, faults such as overheating of the oil, partial discharge, and spark discharge may occur inside the transformer, causing the insulating oil (paper) to crack and generate characteristic gases such as CO, CO2, CH4, C2H6, C2H4, C2H2, and H2 to dissolve in the transformer insulating oil. When an overheating fault, a discharge fault, or internal insulation moisture occurs inside the transformer, the content of the decomposed gases will increase rapidly. Most of these gases dissolve in the insulating oil, and a small part rises to the surface of the insulating oil and enters the gas relay. It has been verified that the amount of various components of the gases in the oil is directly related to the nature and degree of the fault. According to the Dissolved Gas Analysis (DGA), the components and concentrations of the dissolved gases in the oil can be used to judge the type of internal faults in the transformer, and the judgment criteria usually refer to the national standard GB / T7252-2001. By quantitatively analyzing the components and contents of the dissolved gases in the oil, potential faults can be detected in a timely manner, and the transformer can be taken out of service for inspection and repair, avoiding the deterioration of latent faults in the transformer and reducing major losses.

[0003] The monitoring device for dissolved gases in transformer oil has been widely applied in domestic transformers, especially extra-high voltage large transformers. After long-term engineering applications, it has been found that the existing on-line monitoring devices for dissolved gases in oil have problems such as insufficient detection accuracy and frequent false alarms and missed alarms, and cannot meet the performance requirements of the power grid system for the monitoring devices of dissolved gases in oil. It is urgent to tackle the key technologies for rapid and high-precision detection of dissolved gases in oil and improve the reliability and timeliness of the on-line oil chromatograph monitoring device. Summary of the Invention

[0004] In view of the above defects or improvement requirements of the prior art, this application provides a fiber optic frequency modulation interference cantilever microphonic sensor, aiming to detect the photoacoustic effect excited by the gas to be measured, and judge the gas type and concentration according to the detection signal, thereby solving the technical problem of insufficient detection accuracy of the existing dissolved gas detection device in oil.

[0005] To achieve the above object, in a first aspect, the present application provides a fiber optic frequency modulation interference cantilever microphonic sensor, including a first circulator, a first sensing unit, a first photodetector, and an optoacoustic cell; the first output end of the first circulator is connected to the first sensing unit, the second output end of the first circulator is connected to the input end of the first photodetector, and the first sensing unit is fixed in the optoacoustic cell; The incident light beam enters the first sensing unit along the optical path after passing through the first circulator; The optoacoustic cell is used to excite the gas to be measured therein to generate an optoacoustic effect; The first sensing unit includes a light beam incident end and a SOI wafer; a cantilever beam is processed in the SOI wafer; the light beam incident end faces the end of the cantilever beam, and an F-P cavity is formed between the end surface of the cantilever beam; the end of the cantilever beam generates periodic vibration under the action of the optoacoustic effect of the gas to be measured, changing the cavity length of the F-P cavity; After the incident light beam enters the first sensing unit, a part of the light beam is reflected back to the optical path by the light beam incident end; another part of the light beam enters the F-P cavity through the light beam incident end, is reflected by the end of the cantilever beam, and then returns to the optical path through the light beam incident end. The two parts of the light beam reflected back to the optical path interfere, and the interference light enters the first photodetector after passing through the first circulator; The first photodetector converts the interference light into a first electrical signal and outputs it.

[0006] Preferably, the first sensing unit further includes a cantilever beam housing, the cantilever beam housing is a hollow structure, at one end of the hollow structure, a fixing groove is provided perpendicular to the optical path direction; the SOI wafer is installed in the fixing groove; the light beam incident end is fixed at the other end of the hollow structure; the F-P cavity is located in the hollow structure.

[0007] Preferably, the cantilever beam is processed in the SOI wafer by using MEMS technology.

[0008] Preferably, the size range of the cantilever beam is in to between.

[0009] Preferably, the size of the cantilever beam is preferably .

[0010] Preferably, a gold film is plated on the side of the cantilever beam facing the light beam incident end.

[0011] Preferably, it further includes a second circulator, a second sensing unit, a second photodetector, a coupler, and a signal analyzer; The first output end of the coupler is connected to the input end of the first circulator, and the second output end of the coupler is connected to the input end of the second circulator; the first output end of the second circulator is connected to the second sensing unit, and the second output end of the second circulator is connected to the input end of the second photodetector; the output ends of the first photodetector and the second photodetector are connected to the signal analyzer; The second sensing unit has the same structure as the first sensing unit, and the second sensing unit and the first sensing unit are symmetrically fixed at both ends of the photoacoustic cell; The coupler divides the incident light beam into two sub-beams which respectively enter the first circulator and the second circulator. The sub-beam passing through the second circulator enters the second sensing unit. The second sensing unit generates interference light under the influence of the photoacoustic effect of the gas to be measured and enters the second photodetector; the second photodetector converts the interference light into a second electrical signal and outputs it; The signal analyzer receives the first electrical signal and the second electrical signal, performs differential processing, and then demodulates to obtain the type and concentration of the gas to be measured.

[0012] Preferably, it further includes a first collimator and a second collimator. The first collimator is located between the first circulator and the first sensing unit and is used to collimate the incident light beam entering the first sensing unit; the second collimator is located between the second circulator and the second sensing unit and is used to collimate the incident light beam entering the second sensing unit.

[0013] In a second aspect, the present application provides a gas detection method based on a fiber optic frequency modulation interferometric cantilever microphone sensor. The gas detection method includes the following steps: The gas to be measured is introduced into the photoacoustic cell and sealed; The laser in the photoacoustic cell excites the gas to be measured to generate a photoacoustic effect; The photoacoustic effect drives the F-P cavity length in the first sensing unit to change periodically; The incident light beam passes through the first circulator and then enters the first sensing unit along the optical path to obtain interference light with periodically changing interference fringes; The interference light passes through the first circulator and then enters the first photodetector and is converted into a first electrical signal; The first electrical signal is demodulated to obtain the type and concentration of the gas to be measured; The fiber optic frequency modulation interferometric cantilever microphone sensor is the fiber optic frequency modulation interferometric cantilever microphone sensor described in the first aspect of the claims.

[0014] In a third aspect, the present application provides a gas detection method based on a fiber optic frequency modulation interferometric cantilever microphone sensor. The gas detection method includes the following steps: The gas to be measured is introduced into the photoacoustic cell and sealed; The laser in the photoacoustic cell excites the gas to be measured to generate a photoacoustic effect; The photoacoustic effect drives the periodic change of the F-P cavity length in the first sensing unit and the second sensing unit; After the incident light beam enters the coupler, it is divided into a first sub-beam and a second sub-beam; The first sub-beam passes through the first circulator and then enters the first sensing unit along the optical path to obtain a first interference light with periodic change of interference fringes; The second sub-beam passes through the second circulator and then enters the second sensing unit along the optical path to obtain a second interference light with periodic change of interference fringes; The first interference light passes through the first circulator and then enters the first photodetector to be converted into a first electrical signal; The second interference light passes through the second circulator and then enters the second photodetector to be converted into a second electrical signal; The signal analyzer performs differential processing on the first electrical signal and the second electrical signal and then demodulates to obtain the type and concentration of the gas to be measured; The fiber optic frequency modulation interferometric cantilever microphone sensor is the fiber optic frequency modulation interferometric cantilever microphone sensor described in the first aspect of the claims.

[0015] Generally speaking, compared with the prior art through the above technical solutions conceived in this application, the following beneficial effects are obtained: (1) In this application, the light beam incident end of the first sensing unit and the cantilever beam form an F-P cavity. After the gas to be measured excites the acousto-optic effect, pressure is generated to drive the cantilever beam to vibrate periodically. The periodic vibration of the cantilever beam changes the F-P cavity length periodically, and the periodic change of the F-P cavity length generates an interference light signal with periodic change of interference fringes. Demodulating the interference light signal can obtain the type and concentration of the gas to be measured. The gas sensing method in this application has extremely high precision.

[0016] (2) In this application, the main sensing component of the first sensing unit uses a cantilever beam. When the photoacoustic effect of the gas to be measured causes a pressure change, the cantilever beam will only bend and will not stretch. That is, under the same pressure change, the displacement generated by the cantilever beam is two orders of magnitude higher than that generated by the thin film, and its displacement change within 10um is a strictly linear change. That is to say, the cantilever beam is more sensitive to weak pressure changes and has higher detection sensitivity.

[0017] (3) In this application, SOI is used to process the cantilever beam. The natural frequency of the SOI material is close to the vibration frequency when the gas generates the acousto-optic effect, so its vibration sensitivity is high; in addition, the multi-layer structure of the SOI material can effectively reduce the internal stress of the material; finally, using the SOI material is convenient for manufacturing a cantilever beam with extremely small size using the MEMS process, and its processing technology is simple.

[0018] (4) In this application, the sensor composed of the cantilever beam requires fewer components, is small in size, and has a simple structure, which is convenient for miniaturization and integration.

[0019] (5) In this application, the size range and coating material of the cantilever beam are disclosed. Under this processing selection, the natural frequency of the cantilever beam obtained by processing is close to the vibration frequency when the gas produces the acousto-optic effect, so its vibration sensitivity is high.

[0020] (6) In the present application, two first and second sensing units with the same structure are symmetrically fixed at the two ends of the photoacoustic cell. As a result, the sound pressure generated by the gas to be measured in the photoacoustic cell will cause the two cantilever beams to move in opposite directions, while the environmental interference will cause the two cantilever beams to move in the same direction. Therefore, the installation layout method of the first and second sensing units can suppress environmental interference and amplify the signal strength through signal differential processing, so that the demodulated data is more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the composition of an optical fiber frequency modulation interference cantilever beam microphonic sensor provided in an embodiment of the present application.

[0022] Figure 2 It is a cross-sectional view of a cantilever beam shell provided in an embodiment of the present application.

[0023] Figure 3 It is a cross-sectional view of an SOI wafer provided in an embodiment of the present application.

[0024] Figure 4 This is a top view of an SOI wafer provided in an embodiment of the present application.

[0025] Figure 5 This is a cantilever beam natural frequency simulation diagram provided in an embodiment of the present application.

[0026] Figure 6 It is a schematic diagram of the composition of another fiber optic frequency modulation interference cantilever beam microphonic sensor provided in an embodiment of the present application.

[0027] Figure 7 This is a gas detection flow chart provided in an embodiment of the present application.

[0028] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is the first circulator; 2 is the first sensing unit; 3 is the first photodetector; 4 is the photoacoustic cell; 5 is the first collimator; 6 is the second circulator; 7 is the second sensing unit; 8 is the second photodetector; 9 is the second collimator; 10 is the signal analyzer; 11 is the coupler; 20 is the beam incident end; 21 is the SOI chip; 22 is the cantilever beam; 23 is the FP cavity; 24 is the cantilever beam housing; 241 is the fixing groove. DETAILED DESCRIPTION

[0029] To make the objectives, technical solutions, and advantages of this application more clearly understood, the following further elaborates on this application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0030] First, the technical terms involved in the embodiments of this application are introduced.

[0031] SOI (Silicon-On-Insulator), that is, silicon-on-insulator, is a silicon-based semiconductor material. By forming an insulating layer (such as silicon dioxide) on a silicon substrate and preparing a single-crystal silicon thin film on the insulating layer, high-performance semiconductor devices can be constructed.

[0032] MEMS (Micro-Electro-Mechanical Systems) process, that is, micro-electromechanical systems processing technology. The system combines microelectronics and micromachining technologies to manufacture components such as sensors, actuators, signal processing, and control circuits on a micrometer scale and integrates them on one or more chips.

[0033] F-P cavity (Fabry–Perot), Fabry-Perot resonator, also known as F-P cavity, is an optical resonator composed of two parallel plane mirrors. This structure was first invented by French physicists Charles Fabry and Alfred Perot in 1897 and is widely used in fields such as optical communication, laser technology, and spectroscopy to control or measure the optical wavelength.

[0034] Now, the technical solutions of this application are further described in combination with the embodiments and the drawings. As Figure 1 shown, it is a schematic diagram of the composition of a fiber optic frequency modulation interference cantilever beam microphone sensor provided by the embodiment of this application. Among them, the fiber optic frequency modulation interference cantilever beam microphone sensor includes: a first circulator 1, a first sensing unit 2, a first photodetector 3, and a photoacoustic cell 4; The first output end of the first circulator 1 is connected to the first sensing unit 2 through an optical fiber, the second output end of the first circulator 1 is connected to the first photodetector 3 through an optical fiber, and the first sensing unit 2 is fixed in the photoacoustic cell 4.

[0035] The photoacoustic cell 4 is sealed with the gas to be measured, and a laser is emitted in the photoacoustic cell 4 to excite the gas to be measured to produce a photoacoustic effect.

[0036] The incident light beam for gas detection enters the circulator 1 from the input end of the circulator 1 through a single-mode optical fiber and then enters the first sensing unit 2 along the optical path. Inside the first sensing unit 2, there are a light beam incident end 20, an SOI wafer 21, a cantilever beam 22, and a cantilever beam housing 24. Among them, the cantilever beam housing 24 is a hollow structure. At one end of the hollow structure, a fixing groove 241 is provided perpendicular to the optical path direction; the SOI wafer 21 is installed in the fixing groove 241, and a cantilever beam 22 structure is processed in the middle of the SOI wafer 21 by MEMS technology. The light beam incident end 20 is fixed at the other end of the hollow structure, and the light beam incident end 20 faces the end of the cantilever beam 22. An F-P cavity 23 is formed between the surface of the light beam incident end 20 and the end of the cantilever beam 22, and the F-P cavity 23 is located inside the hollow structure.

[0037] After the incident light beam for gas detection enters the first sensing unit 2, a part of the light beam is reflected back to the optical path by the light beam incident end 20; another part of the light beam enters the F-P cavity 23 through the light beam incident end 20, is reflected by the end of the cantilever beam 22, and then returns to the optical path through the light beam incident end 20. The two parts of the light beam reflected back to the optical path interfere, and the interference light enters the first photodetector 3 after passing through the circulator 1.

[0038] The first photodetector 3 converts the interference light signal into an electrical signal and outputs it.

[0039] Demodulating the electrical signal can obtain the type and concentration of the gas to be measured in the photoacoustic cell.

[0040] In this embodiment, the main sensing component of the first sensing unit 2 uses a cantilever beam 22. When the photoacoustic effect of the gas to be measured causes a pressure change, the cantilever beam 22 only bends and does not stretch. That is, under the same pressure change, the displacement generated by the cantilever beam 22 is two orders of magnitude higher than that generated by the thin film, and its displacement change within 10 um is a strictly linear change. That is to say, the cantilever beam 22 is more sensitive to weak pressure changes and has a higher detection sensitivity.

[0041] Figure 2 This is a cross-sectional view of the cantilever beam housing 24 in the embodiment of the present application. As shown in the figure, the cantilever beam housing 24 is a cylindrical hollow structure. A ceramic ferrule is placed at one end of the hollow structure, and the light beam incident end 20 is fixed inside the ceramic ferrule. A fixing groove 241 is provided at the other end of the hollow structure perpendicular to the optical path. The SOI wafer 21 is fixed in the fixing groove 241 by pasting to ensure that the light beam incident through the light beam incident end 20 can be emitted to the end of the cantilever beam 22 in the SOI wafer 21. An F-P cavity 23 is formed between the surface of the light beam incident end 20 and the end of the cantilever beam 22, and the F-P cavity 23 is located inside the hollow structure.

[0042] A cylindrical metal protective shell is installed outside the cantilever beam housing. In this embodiment, the sensor composed of the cantilever beam has few components, small size, and simple structure, which is convenient for miniaturization and integration.

[0043] Such asFigure 3 The following is a cross-sectional view of a SOI wafer provided by an embodiment of the present application. Figure 3 As can be seen, in the middle part of the SOI wafer 21, a cantilever beam 22 is processed, and the thickness of the cantilever beam 22 is . The thickness of the cantilever beam 22 is much smaller than the thickness of the SOI wafer 21.

[0044] As Figure 4 is a top view of a SOI wafer provided by an embodiment of the present application. As can be seen from Figure 4 , in the middle part of the SOI wafer 21, a gate-shaped cantilever beam 22 is processed by MEMS technology. The upper, left, and right sides of the cantilever beam 22 are suspended, and the lower side is connected to the SOI wafer 21. As can be seen from Figure 4 , the length of the cantilever beam 22 is , the width is , and there is a gap of between the upper, left, and right sides of the cantilever beam 22 and the SOI wafer 21.

[0045] Combining Figure 3 and Figure 4 , it can be known that the length, width, and thickness dimensions of the cantilever beam 22 in this embodiment are .

[0046] In the photoacoustic cell, the photoacoustic effect is generated by laser excitation of the gas to be measured, driving the periodic vibration of the cantilever beam 22 in the first sensing unit 2. The periodic vibration of the cantilever beam 22 is converted into a periodic change in the cavity length of the F-P cavity 23, and the periodic change in the cavity length of the F-P cavity generates an interference light signal with a periodic change in interference fringes. The interference light signals generated by different gases at different concentrations are different. Therefore, by demodulating the interference light signal, the type and concentration of the gas to be measured can be obtained.

[0047] The natural frequency of SOI is relatively high. Therefore, it is relatively well-matched with the vibration frequency generated after the photoacoustic effect occurs with the gas, and has a relatively high quality factor. At the same time, it is easy to obtain and process, and is particularly suitable for etching processing.

[0048] In the embodiment of the present application, the cantilever beam 22 is fabricated on the SOI wafer by MEMS technology. Its processing process mainly includes: Spin coating: That is, covering a smooth, appropriately thick, defect-free, and evenly coated photoresist film on the surface of the SOI material to be etched; Exposure: Expose the photoresist on the substrate of the part to be removed according to the designed shape of the cantilever beam, so that it is easy to be dissolved by the developer; Development: Use the developer to dissolve the exposed photoresist and expose the substrate; Etching: Use the etchant to etch away the exposed substrate part; Cleaning: Clean off the excess photoresist and the residual etching solution.

[0049] Thereby, the processed SOI is obtained, and the SOI has a cantilever beam structure.

[0050] Finally, coat the surface of the cantilever beam, and coat a 200-nm-thick gold film on one side of the cantilever beam to improve the reflectivity.

[0051] In this embodiment, SOI is used to process the cantilever beam. The natural frequency of the SOI material is close to the vibration frequency when the acousto-optic effect occurs in the gas, so its vibration sensitivity is high; in addition, the multi-layer structure of the SOI material can effectively reduce the internal stress of the material.

[0052] Calculate the natural frequency of the processed cantilever beam through the following formula:

[0053] In the formula, L, E, d, respectively represent the length, elastic modulus, thickness and density of the cantilever beam. After simulation, as Figure 5 is the simulation diagram of the natural frequency distribution of the cantilever beam in this embodiment. The darker the color in the figure, the higher the natural frequency of that part. It can be seen from Figure 5 that the natural frequency at the end of the cantilever beam 22 is the largest.

[0054] After calculation, the size of the cantilever beam 22 in this embodiment is The natural frequency at the end of the cantilever beam 22 is 748 Hz. After combining the coating, the natural frequency at the end of the cantilever beam 22 is 574 Hz. This frequency is very close to the vibration frequency when the acousto-optic effect occurs in the gas. Therefore, it has a very high sensitivity to the vibration of the gas. Using the cantilever beam with this size, material and structure, it has a very high sensitivity to the vibration of the gas to be measured.

[0055] This application also provides two other embodiments. The size of one cantilever beam is , and the size of the other cantilever beam is , and both are coated with a gold film. After simulation calculation, the size range of the cantilever beam is to Between them, the natural frequency at their ends is relatively close to the vibration frequency when the acousto-optic effect occurs in the gas. Therefore, using the cantilever beam with this size range, material and structure, it has a relatively high sensitivity to the vibration of the gas to be measured.

[0056] Figure 6It is a schematic diagram of the composition of another fiber optic frequency modulation interference cantilever beam microphone sensor provided by an embodiment of the present application. As shown in the figure, it specifically includes: a first circulator 1, a first sensing unit 2, a first photodetector 3, a photoacoustic cell 4, a first collimator 5, a second circulator 6, a second sensing unit 7, a second photodetector 8, a second collimator 9, a signal analyzer 10, and a coupler 11.

[0057] The incident end of the coupler 11 is connected to a laser, the first output end of the coupler 11 is connected to the incident end of the first circulator 1, and the second output end of the coupler 11 is connected to the incident end of the second circulator 6.

[0058] The first output end of the first circulator 1 is connected to the incident end of the first collimator 5 through an optical fiber, the output end of the first collimator 5 is connected to the first sensing unit 2, the second output end of the first circulator 1 is connected to the incident end of the first photodetector 3 through an optical fiber, the output end of the first photodetector 3 is connected to the signal analyzer 10, and the first sensing unit 2 is fixed in the photoacoustic cell 4.

[0059] The first output end of the second circulator 6 is connected to the incident end of the second collimator 9 through an optical fiber, the output end of the second collimator 9 is connected to the second sensing unit 7, the second output end of the second circulator 6 is connected to the second photodetector 8 through an optical fiber; the output end of the second photodetector 8 is connected to the signal analyzer 10, and the second sensing unit 7 is fixed in the photoacoustic cell 4.

[0060] Wherein the first sensing unit 2 and the second sensing unit 7 have the same structure and are symmetrically fixed at both ends of the photoacoustic cell 4. The photoacoustic cell 4 contains the gas to be measured, and the photoacoustic effect of the gas to be measured is excited in the photoacoustic cell 4 by a laser.

[0061] After the detection laser beam is emitted and passes through the coupler 11, it is divided into two sub-beams, and the two sub-beams respectively enter the first circulator 1 and the second circulator 6.

[0062] The sub-beam passing through the first circulator 1 enters the first sensing unit 2. The first sensing unit 2 generates interference light under the influence of the photoacoustic effect of the gas to be measured and enters the first photodetector 3; the first photodetector 3 converts the interference light into a first electrical signal and outputs it.

[0063] The sub-beam passing through the second circulator 6 enters the second sensing unit 7. The second sensing unit 7 generates interference light under the influence of the photoacoustic effect of the gas to be measured and enters the second photodetector 8; the second photodetector 8 converts the interference light into a second electrical signal and outputs it.

[0064] The signal analyzer 10 receives the first electrical signal and the second electrical signal, performs differential processing, and then demodulates to obtain the type and concentration of the gas to be measured.

[0065] Among them, the detection beam is emitted by a DFB laser, with a wavelength of 1550 nm, a line width reaching 3 MHz, an output power of 30 mW, a tuning current of 0.01 nm / mA, a temperature control of 0.1 nm / °C, and the interface is a standard single-mode fiber SMF output.

[0066] Coupler 11 is a broadband single-mode fiber 1*3 coupler, and its four ends are all FC / APC interfaces, which are adapted to the DFB laser. The working bandwidth of this single-mode fiber is 1450~1650 nm, which can well cover the working band of the DFB laser, and the interface optical loss is small. The wavelength range of the single-mode fiber jumper is 1260~1625 nm, which is adapted to the overall requirements.

[0067] The wavelength ranges of the first circulator 1 and the second circulator 6 are 1525~1610 nm. It is a three-port device. During its transmission process, light can only propagate in one direction, that is, from port 1 to port 2, and from port 2 to port 3. If it is not in this order, it will cause great loss to the light.

[0068] The first photodetector 3 and the second photodetector 8 are another important devices. Since the measured carrier frequency is relatively high, the bandwidth of the photodetector should be large enough. It uses a fiber interface and can directly access the interference signal. The bandwidth is 1.2 GHz, the peak wavelength is 1550 nm, which is very suitable for this embodiment, and it has a small volume and is convenient for installation.

[0069] It uses a reverse-biased detector to produce a linear response with the applied input light. The generated photocurrent changes with the incident light intensity and wavelength, and the photocurrent can be converted into a voltage by adding a load resistor at the output end, which is convenient for the subsequent AD conversion.

[0070] In order to improve the detection sensitivity and accuracy, improve the reliability of the signal, two sensing units are used to collect the signal at the same time, and the signal of the target gas is obtained by the differential method.

[0071] In this embodiment, the two identical first and second sensing units are symmetrically placed at the symmetric ends inside the cylindrical photoacoustic cell to collect the signal generated by the photoacoustic effect. Thus, the sound pressure generated by the gas to be measured in the photoacoustic cell will cause the two cantilever beams to move in opposite directions, while the environmental interference will cause the two cantilever beams to move in the same direction. Therefore, the installation layout method of the first and second sensing units can suppress the environmental interference through signal differential processing, amplify the signal intensity, and thus the demodulated data is more accurate.

[0072] Figure 7 It is the flow chart of gas detection. As can be seen from the figure, it includes the following steps: During the detection process, the excitation laser in the photoacoustic cell 4 irradiates the gas to be measured sealed in the photoacoustic cell. The gas to be measured absorbs light energy, then vibrates under excitation and releases heat energy to de-excite. At the same time, the released heat energy causes the gas to be measured and the surrounding medium to be periodically heated according to the modulation frequency of the laser, thereby causing periodic pressure fluctuations in the gas to be measured and the medium. Finally, this periodic pressure fluctuation causes periodic vibration of the microcantilever beam, resulting in periodic change of the F-P cavity length; The detection laser passes through the first circulator 1 and then enters the first sensing unit 2 along the optical path to obtain interference light with periodic change of interference fringes; The interference light passes through the first circulator 1 and then enters the first photodetector 3 and is converted into a first electrical signal; The first electrical signal is demodulated to obtain the type and concentration of the gas to be measured.

[0073] The type and concentration information of the gas to be measured can be obtained by demodulating the interference fringes.

[0074] Demodulation is performed by frequency modulation interference technology. Frequency modulation interference technology is a small-sized high-resolution fiber optic interference high-precision displacement measurement technology. It mainly realizes the modulation of the optical path difference by modulating the laser wavelength. Since the accuracy of measuring displacement by this method is high (experimental verification shows sub-nanometer resolution), it is very suitable for measuring the vibration displacement of the microcantilever beam.

[0075] It should be understood that expressions such as "including" and "may include" that can be used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as "including" and / or "having" can be interpreted as indicating a specific characteristic, number, operation, constituent element, component, or a combination thereof, but cannot be interpreted as excluding the existence or possibility of addition of one or more other characteristics, numbers, operations, constituent elements, components, or a combination thereof.

[0076] In the description of the embodiments of the present application, it should be noted that unless otherwise clearly specified and defined, the term "connection" should be understood in a broad sense. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium. Among them, "fixed connection" means that the two are connected and the relative position relationship after connection remains unchanged. "Rotational connection" means that the two are connected and can rotate relative to each other after connection. "Sliding connection" means that the two are connected and can slide relative to each other after connection. The orientation terms mentioned in the embodiments of the present application, such as "top", "bottom", "inner", "outer", "left", "right", etc., are only with reference to the direction of the accompanying drawings. Therefore, the orientation terms used are for better and clearer description and understanding of the embodiments of the present application, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the embodiments of the present application.

[0077] In addition, in the embodiments of the present application, mathematical concepts such as symmetry, equality, parallelism, and perpendicularity are mentioned. These definitions are all in view of the current technological level, rather than absolute strict definitions in the mathematical sense, allowing for a small amount of deviation. Approximate symmetry, approximate equality, approximate parallelism, approximate perpendicularity, etc. are all acceptable. For example, when it is said that A is parallel to B, it means that A is parallel to B or approximately parallel to B, and the included angle between A and B can be between 0 degrees and 10 degrees. When it is said that A is perpendicular to B, it means that A is perpendicular to B or approximately perpendicular to B, and the included angle between A and B can be between 80 degrees and 100 degrees.

[0078] It can be understood that the various numerical numbers involved in the embodiments of the present application are only for the convenience of description and are not used to limit the scope of the embodiments of the present application.

[0079] As mentioned above, the above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present application can easily think of changes or substitutions, which should all be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A fiber optic frequency modulation interference cantilever microphonic sensor, characterized in that, It includes a first circulator (1), a first sensing unit (2), a first photodetector (3), and a photoacoustic cell (4); the first output end of the first circulator (1) is connected to the first sensing unit (2), the second output end of the first circulator (1) is connected to the input end of the first photodetector (3), and the first sensing unit (2) is fixed inside the photoacoustic cell (4); The incident light beam enters the first sensing unit (2) along the optical path after passing through the first circulator (1); The photoacoustic cell (4) is used to excite the gas to be measured therein to generate a photoacoustic effect; The first sensing unit (2) includes a light beam incident end (20) and a SOI wafer (21); a cantilever beam (22) is processed in the SOI wafer (21); the light beam incident end (20) is opposite to the end of the cantilever beam (22), and an F-P cavity (23) is formed between the light beam incident end (20) and the end surface of the cantilever beam (22); the end of the cantilever beam (22) generates periodic vibration under the action of the photoacoustic effect of the gas to be measured, changing the cavity length of the F-P cavity (23); After the incident light beam enters the first sensing unit (2), a part of the light beam is reflected back to the optical path by the light beam incident end (20); another part of the light beam enters the F-P cavity (23) through the light beam incident end (20), is reflected by the end of the cantilever beam (22), and then returns to the optical path through the light beam incident end (20). The two parts of the light beams reflected back to the optical path interfere, and the interference light enters the first photodetector (3) after passing through the first circulator (1); The first photodetector (3) converts the interference light into a first electrical signal and outputs it.

2. The fiber optic frequency modulated interference cantilever microphone sensor according to claim 1, wherein The first sensing unit (2) further includes a cantilever beam housing (24), the cantilever beam housing (24) is a hollow structure, and at one end of the hollow structure, a fixing groove (241) is provided perpendicular to the optical path direction; the SOI wafer (21) is installed in the fixing groove (241); the light beam incident end (20) is fixed at the other end of the hollow structure; the F-P cavity (23) is located inside the hollow structure.

3. The fiber optic frequency modulation interference cantilever microphonic sensor according to claim 1, characterized in that, The cantilever beam (22) is processed in the SOI wafer (21) by using MEMS technology.

4. The fiber optic frequency modulation interference cantilever beam microphone sensor according to claim 1, characterized in that, The dimensions of the cantilever beam (22) range from to inclusive.

5. The fiber optic frequency modulated interference cantilever microphone sensor according to claim 4, characterized in that, The dimensions of the cantilever beam (22) are preferably .

6. The fiber optic frequency modulated interference cantilever microphonic sensor according to claim 1, wherein A gold film is plated on the side of the cantilever beam (22) opposite to the light beam incident end (20).

7. The fiber optic frequency modulation interference cantilever beam microphone sensor according to claim 1, characterized in that, It further includes a second circulator (6), a second sensing unit (7), a second photodetector (8), a coupler (11), and a signal analyzer (10); The first output end of the coupler (11) is connected to the input end of the first circulator (1), and the second output end of the coupler (11) is connected to the input end of the second circulator (6); the first output end of the second circulator (6) is connected to the second sensing unit (7), and the second output end of the second circulator (6) is connected to the input end of the second photodetector (8); The output ends of the first photodetector (3) and the second photodetector (8) are connected to the signal analyzer (10); The second sensing unit (7) has the same structure as the first sensing unit (2), and the second sensing unit (7) and the first sensing unit (2) are symmetrically fixed at both ends of the photoacoustic cell (4); The coupler (11) divides the incident light beam into two sub-beams which respectively enter the first circulator (1) and the second circulator (6). The sub-beam passing through the second circulator (6) enters the second sensing unit (7). Under the influence of the photoacoustic effect of the gas to be measured, the second sensing unit (7) generates interference light and enters the second photodetector (8); the second photodetector (8) converts the interference light into a second electrical signal and outputs it. The signal analyzer (10) receives the first electrical signal and the second electrical signal, performs differential processing, and then demodulates to obtain the type and concentration of the gas to be measured.

8. The fiber optic frequency modulation interference cantilever beam microphone sensor according to claim 7, characterized in that, It further includes a first collimator (5) and a second collimator (9). The first collimator (5) is located between the first circulator (1) and the first sensing unit (2) and is used to collimate the incident light beam entering the first sensing unit (2); the second collimator (9) is located between the second circulator (6) and the second sensing unit (7) and is used to collimate the incident light beam entering the second sensing unit (7).

9. A gas detection method based on a fiber optic frequency modulation interference cantilever microphonic sensor, characterized in that, The gas detection method includes the following steps: The gas to be measured is introduced into the photoacoustic cell (4) and sealed. The laser in the photoacoustic cell (4) excites the gas to be measured to generate a photoacoustic effect. The photoacoustic effect drives the F-P cavity length in the first sensing unit (2) to change periodically. The incident light beam passes through the first circulator (1) and then enters the first sensing unit (2) along the optical path to obtain interference light with periodically changing interference fringes. The interference light passes through the first circulator (1) and then enters the first photodetector (3) and is converted into a first electrical signal. The first electrical signal is demodulated to obtain the type and concentration of the gas to be measured. The fiber optic frequency modulation interferometric cantilever microphone sensor is the fiber optic frequency modulation interferometric cantilever microphone sensor according to any one of claims 1-6.

10. A gas detection method based on a fiber optic frequency modulation interference cantilever microphonic sensor, characterized in that, The gas detection method includes the following steps: The gas to be measured is introduced into the photoacoustic cell (4) and sealed. The laser in the photoacoustic cell (4) excites the gas to be measured to generate a photoacoustic effect. The photoacoustic effect drives the F-P cavity lengths in the first sensing unit (2) and the second sensing unit (7) to change periodically. The incident light beam is incident on the coupler (11) and is divided into a first sub-beam and a second sub-beam. The first sub-beam passes through the first circulator (1) and then enters the first sensing unit (2) along the optical path to obtain the first interference light with periodically changing interference fringes. The second sub-beam passes through the second circulator (6) and then enters the second sensing unit (7) along the optical path to obtain the second interference light with periodically changing interference fringes. The first interference light passes through the first circulator (1) and then enters the first photodetector (3) and is converted into a first electrical signal. The second interference light passes through the second circulator (6) and then enters the second photodetector (8) and is converted into a second electrical signal. The signal analyzer (10) performs differential processing on the first electrical signal and the second electrical signal and then demodulates to obtain the type and concentration of the gas to be measured. The fiber optic frequency modulation interferometric cantilever microphone sensor is the fiber optic frequency modulation interferometric cantilever microphone sensor according to any one of claims 7-8.

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