CN115900920A - An Extrinsic Fabry-Perot Acoustic Sensor Based on Cantilever Beam Structure - Google Patents

Abstract

The invention discloses an extrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure. The device comprises a cantilever beam and a 45-degree total reflection structure. The basic principle is that an extrinsic Fabry-Perot cavity is formed between a cantilever beam and an optical fiber by utilizing a 45 total reflection structure to change the propagation direction of transmission light. The cantilever beam is arranged in a free sound field, sound pressure with certain frequency and amplitude is applied to push the cantilever beam to generate synchronous deflection deformation, so that the interference cavity length is changed, the interference cavity length variation is obtained by demodulating the interference spectrum, and measurement of a sound pressure signal at the position to be measured is realized by combining a cantilever beam theoretical deformation model. The cantilever beam structure provided by the invention has the advantages of high sensitivity, large dynamic range and the like.

Description

An extrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure

Technical Field

The invention belongs to the technical field of optical fiber sensing, and in particular relates to a non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure.

Background Art

Acoustic sensors are a common tool for collecting sound signals in daily life. Traditional acoustic sensors mainly convert sound signals into electrical signals based on the piezoelectric properties of materials. However, this structure has great limitations, such as being susceptible to external noise and electromagnetic interference, and cannot be used in some complex environments, such as magnetic fields and underwater environments.

Compared with traditional electrical sensors, optical sensors have the advantages of high sensitivity, strong anti-interference ability, corrosion resistance, and low loss, so they have attracted widespread attention. As a new type of acoustic sensor, fiber optic acoustic sensor uses sound pressure signals to modulate optical signal parameters, and then obtains information about the sound pressure signal by demodulating the optical signal. According to the different modulation parameters of the optical signal, we mainly divide optical acoustic sensors into: intensity modulation type, wavelength modulation type, and phase modulation type.

Fiber Fabry-Perot acoustic sensor based on phase modulation is widely used due to its compact structure, high sensitivity, simple demodulation method, etc. Fiber Fabry-Perot acoustic sensor is a high-sensitivity optical interference detector based on the fiber Fabry-Perot interference principle. The sound pressure signal of a certain frequency and amplitude drives the cantilever beam to produce periodic deflection deformation, that is, the interference cavity length changes. Combined with the theoretical deformation model of the cantilever beam, the sound pressure signal at the measured position can be measured.

The fiber Fabry-Perot acoustic sensor currently used is mainly a diaphragm structure, with two reflective surfaces consisting of the fiber end face and a diaphragm made of materials such as stainless steel, silicon, quartz, and graphene. The diaphragm structure detects acoustic signals through the vibration of the diaphragm, which mainly depends on the mechanical properties of the diaphragm material. The sensitivity of its acoustic sensor is generally low, and because the diaphragm structure is generally a fully enclosed structure, it is easily affected by the external ambient temperature, causing the interference spectrum to drift, and the stability is poor.

Summary of the invention

In order to solve the problems of the prior art, the purpose of the present invention is to overcome the shortcomings of the existing technology and provide a non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure. Compared with the traditional diaphragm structure, the deformation of the cantilever beam structure proposed in the present invention under the same sound pressure can be increased by two orders of magnitude, achieving a higher sensitivity of sound pressure signal detection. In addition, unlike the fully enclosed structure of the diaphragm type, the cantilever beam structure can form an "open cavity" structure while forming a non-intrinsic Fabry-Perot cavity, which can avoid the influence of the temperature difference inside and outside the cavity of the diaphragm structure on the interference spectrum.

The present invention is a non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure. Cantilever beams with different size parameters are prepared by micromachining technology, and the cantilever beam is placed in a free sound field. Sound pressure of a certain frequency and amplitude is applied to promote the periodic deflection deformation, thereby causing the interference cavity length to change. The change in the interference cavity length can be obtained by using a demodulation system, and combined with the deformation theoretical model of the cantilever beam structure, the sound pressure signal at the measured position can be measured.

In order to achieve the above invention purpose, the present invention is conceived as follows:

Two 45-degree total reflection structures of 45-degree angled single-mode optical fiber and 45-degree reflector were designed. The propagation direction of the transmitted light was changed by using this structure, so as to form a Fabry-Perot interference cavity between the single-mode optical fiber and the two reflection surfaces of the cantilever beam. Furthermore, the deformation of the cantilever beam, that is, the change in the length of the interference cavity, was used to demodulate the deformation of the cantilever beam, thereby realizing the sound pressure measurement.

According to the above inventive concept, the present invention adopts the following technical solution:

A non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure includes a cantilever beam and a 45-degree total reflection structure, wherein the 45-degree total reflection structure is mainly obtained in two forms: a 45-degree angled single-mode optical fiber and a 45-degree reflector. The 45-degree total reflection structure is used to change the propagation direction of the light beam, thereby forming a non-intrinsic Fabry-Perot cavity composed of a single-mode optical fiber and a cantilever beam.

Preferably, the single-mode optical fiber is a 45-degree angle optical fiber, whose side wall is a first reflection surface, and the lower side of the cantilever beam is a second reflection surface, and a non-intrinsic Fabry-Perot cavity is formed between the two reflection surfaces.

Preferably, the single-mode optical fiber is a single-mode optical fiber with a common flat end face, whose end face is the first reflection surface, and the lower side of the cantilever beam is the second reflection surface. A 45-degree reflector is used to change the propagation direction of the transmitted light, so that a Fabry-Perot interference cavity is formed between the two reflection surfaces.

Preferably, the cantilever beam generates periodic deflection deformation under the influence of sound pressure of a certain frequency and amplitude, thereby causing the interference cavity length to change, and the change in the interference cavity length is obtained by demodulating the interference spectrum.

基于微加工技术的自由性，所述悬臂梁尺寸参数可调节以适应不同频率的声压信号测量；其本征频率计算公式为

式中E为杨氏模量，ρ为密度，长度为L，厚度为h，I为其转动惯量。Preferably, the deformation of the cantilever beam under uniform sound pressure is the change in the interference cavity length, and the relationship between the applied sound pressure and the deformation of the cantilever beam is:

Based on the freedom of micromachining technology, the cantilever beam size parameters can be adjusted to adapt to the measurement of sound pressure signals of different frequencies; the eigenfrequency calculation formula is:

Where E is Young's modulus, ρ is density, length is L, thickness is h, and I is its moment of inertia.

Compared with the prior art, the present invention has the following obvious outstanding substantial features and significant advantages:

1. Compared with the traditional diaphragm acoustic sensor, the processed cantilever beam structure has better mechanical properties under the same sound pressure, the vibration displacement is two orders of magnitude higher, and the dynamic range is also better than the traditional diaphragm acoustic sensor;

2. The present invention adopts a 45-degree total reflection structure and a cantilever beam to form a non-intrinsic Fabry-Perot cavity. Different from a diaphragm-type fully enclosed structure, the Fabry-Perot cavity is an "open cavity" structure, thus being able to avoid the influence of the temperature difference inside and outside the cavity on the interference spectrum;

3. The present invention can utilize the micro-machining process with a certain degree of freedom, so it can process cantilever beam structures with different size parameters to achieve the adjustment of the acoustic frequency response range and sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a ) is a schematic diagram of a cantilever beam structure acoustic sensor based on a 45-degree angled optical fiber according to the present invention.

FIG1( b ) is a schematic diagram of the spectrum of the extrinsic Fabry-Perot interferometry (EFPI) principle based on a 45-degree angled optical fiber according to the present invention.

FIG. 2( a ) is a schematic diagram of a cantilever beam structure acoustic sensor based on a 45-degree reflector according to the present invention.

FIG. 2( b ) is a schematic diagram of the spectrum of the extrinsic Fabry-Perot interferometry (EFPI) principle based on a 45-degree reflector of the present invention.

FIG. 3 is a schematic structural diagram of a 45-degree total reflection structure of the present invention.

Figure 4 shows different cantilever beam structures.

FIG5( a ) is a mechanical deformation simulation diagram of a cantilever beam structure realized using COMSOL multi-physics field simulation software.

FIG5( b ) is a numerical analysis diagram of the frequency response of the cantilever beam structure calculated using COMSOL multi-physics field simulation software.

FIG6 is a diagram of the demodulation system for acoustic sensing testing.

FIG. 7( a ) is an interference spectrum diagram of the cantilever beam acoustic sensor proposed in the present invention under different sound pressures.

FIG. 7( b ) is a diagram showing the cantilever beam deformation amplitude response of the cantilever beam acoustic sensor proposed in the present invention under different sound pressures.

Description of reference numerals:

1-single-mode optical fiber; 11-45-degree angled optical fiber side wall, which is the first reflection surface of the cantilever beam structure acoustic sensor based on the 45-degree angled optical fiber provided by the present invention; 12-flat end face of the optical fiber, which is the first reflection surface of the cantilever beam structure acoustic sensor based on the 45-degree reflector provided by the present invention; 6-45-degree reflector; 61-45-degree reflector surface; 2-cantilever beam; 21-lower side of the cantilever beam, which is the second reflection surface; 3-base, used for assembling and fixing components; 31-boss, used for forming the initial interference cavity length; 4-cantilever beam fixture; the circular holes appearing on each module in the attached drawings are all threaded holes with a diameter of 2 mm, which are used to assemble and fix each module.

DETAILED DESCRIPTION

In order to more clearly illustrate the features and technical contents of the embodiments of the present invention, the embodiments of the present invention will be described below in conjunction with the accompanying drawings.

The embodiment of the present invention combines the principle of non-intrinsic Fabry-Perot interference and provides an acoustic sensor structure based on a 45-degree total reflection structure and a cantilever beam. The periodic deflection deformation of the cantilever beam driven by the sound pressure signal can be converted into the change in the interference cavity length, and the change in the interference cavity length can be obtained by demodulation to determine the information of the sound pressure applied at the measured position.

The above scheme is further described below in conjunction with specific implementation examples. The preferred embodiments of the present invention are described in detail as follows:

Embodiment 1:

In this embodiment, referring to Figures 1(a) and (b), a non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure includes a cantilever beam 2 and a 45-degree total reflection structure, characterized in that: the 45-degree total reflection structure is obtained by a 45-degree angled single-mode optical fiber 1; the 45-degree total reflection structure is used to change the propagation direction of the light beam, thereby forming a non-intrinsic Fabry-Perot cavity composed of the single-mode optical fiber 1 and the cantilever beam 2.

In this embodiment, the single-mode optical fiber 1 is a 45-degree angle optical fiber, the side wall 11 of which is a first reflection surface, and the lower side surface 21 of the cantilever beam is a second reflection surface, and a Fabry-Perot interference cavity is formed between the two reflection surfaces.

In this embodiment, the single-mode optical fiber 1 is a single-mode optical fiber with a common flat end face, whose end face 12 is the first reflection surface, and the lower side surface 21 of the cantilever beam is the second reflection surface. A 45-degree reflector 6 is used to change the propagation direction of the transmitted light, so that a Fabry-Perot interference cavity is formed between the two reflection surfaces.

In this embodiment, the 45-degree bevel angle single-mode optical fiber is prepared by a grinder platform and finally processed by a polishing sheet to increase its bevel reflectivity.

In this embodiment, the incident light is totally reflected at the bevel of the 45-degree bevel optical fiber, and the propagation direction is deflected by 90 degrees, thereby forming reflections at the side wall of the bevel optical fiber and the lower end surface of the cantilever beam, respectively, to obtain an interference light signal.

Embodiment 2:

This embodiment is basically the same as the above embodiment, see Figure 2 (a) and (b), with the special features that:

In this embodiment, a common single-mode optical fiber 1 with a flat end face is used, and a 45-degree reflector is mainly provided to form a total reflection structure to change the propagation direction of the transmission light.

In this embodiment, the incident light is emitted from the single-mode optical fiber 1 and is totally reflected at the interface of the 45-degree reflector 6. The transmitted light is deflected 90 degrees and reflected back at the position of the cantilever beam 2 to obtain an interference light signal.

FIG3 is a schematic diagram of the overall structure of the present invention, as shown in the figure, including: a cantilever beam 2, a base 3, a cantilever beam fixture 4, and a 45-degree reflector 6. In Example 1, the 45-degree reflector 6 is not placed, and it is only used in Example 2. The other structures remain unchanged and are applicable to the two embodiments mentioned above. The cantilever beam 2 structure refers to the rectangular module on the left, which is 10 mm long and 3 mm wide, and the other side is used for installation and placement. A boss 31 with a height of 0.2 mm is designed at one end of the base 3 to form the initial cavity length of the non-intrinsic Fabry-Perot cavity; and a rectangular groove with a length of 0.5 mm and a width of 0.2 mm is reserved in the middle of the boss for placing the 45-degree angled optical fiber 1 and the single-mode optical fiber 5 with a flat end face. The cantilever beam fixture 4 is used to fix the prepared cantilever beam structure 2. The 45-degree reflector 6 is used to change the propagation direction of light in Example 2 to form an interference cavity. It can be observed that each module contains a circular threaded hole with a diameter of 2 mm for installing and fixing each module.

Commonly used cantilever beam structures mainly include: rectangular cantilever beam, rectangular cantilever beam + mass block, triangular cantilever beam. Figure 4 shows two other cantilever beam structures besides the rectangular cantilever beam structure proposed in the present invention: one is to process a circular structure at the free end of the rectangular cantilever beam, which is equivalent to adding a mass block structure; the other is a triangular cantilever beam structure. The above cantilever beam structures are all suitable for the cantilever beam structure acoustic sensor proposed in the present invention, and can also be applied to the field of physical and chemical parameter sensing such as acceleration, pressure, humidity, and temperature.

The thickness of the cantilever beams proposed in the embodiments of the present invention is 0.5 mm, which can ensure that in a free sound field, even a weak sound pressure can cause the cantilever beam to produce a certain deformation and be detected.

The rectangular cantilever beam 2 used in the present invention is made of stainless steel, with a Young's modulus of E and a density of ρ. Its eigenfrequency f is (length L, thickness h):

The rectangular cantilever beam 2 used in the present invention is subjected to a uniform sound pressure with an amplitude of P, and the displacement at the edge of its free end is (I is its moment of inertia):

Figure 5(a) is a mechanical simulation of the proposed cantilever beam structure using COMSOL multi-physics simulation software. A uniform boundary load of 5Pa is applied to the upper side of the cantilever beam, and the deformation diagram of the designed cantilever beam is obtained. It can be seen that the deformation at the free end of the cantilever beam is the largest. Therefore, in order to obtain the maximum sound pressure detection sensitivity, the optical fiber tip is placed parallel to the free end of the cantilever beam during the experiment.

Figure 5(b) shows the frequency response analysis of the cantilever beam structure using COMSOL multi-physics simulation software. It can be seen from the figure that the eigenfrequency of the machined cantilever beam structure is around 4KHz, which is basically consistent with the result obtained by the calculation formula of the eigenfrequency f. Based on the freedom of micromachining technology, cantilever beam structures of different sizes can be machined to change the eigenfrequency, thereby realizing the measurement of sound pressure signals of different frequencies.

FIG6 is a test diagram of the sound pressure demodulation system built during the experiment. The signal demodulation system mainly includes a broadband light source, a photodetector, an oscilloscope, a circulator, a commercial optical microphone (B&K4966), and a cantilever beam structure acoustic sensor proposed in the present invention. The assembled cantilever beam structure acoustic sensor and the commercial optical microphone (B&K 4966) are placed in a free sound field generated by a function generator driving a loudspeaker. The cantilever beam produces periodic deflection and deformation under the action of sound pressure of a certain frequency and amplitude, and the interference cavity length will change. The demodulated interference cavity length change is combined with the cantilever beam deformation theoretical model to achieve the measurement of the sound pressure signal at the measured position. The specific demodulation system is shown in FIG6: the broadband light source is input from port 1 of the optical fiber circulator, and output from port 2 to the cantilever beam structure acoustic sensor proposed in the present invention. Under the action of free field sound pressure, the cantilever beam undergoes periodic deflection and deformation, and the interference cavity length changes; the reflected light signal is input through port 2, output from port 3, and converted into an electrical signal by the photodetector and received by the oscilloscope, and sent to the computer for demodulation processing.

Figure 7(a) shows the interference spectrum obtained by the demodulation system shown in Figure 6 when applying uniform sound pressures of 1Pa and 1.5Pa respectively. It can be seen that the interference spectrum will drift when different sound pressures are applied. Based on the phase demodulation principle of the Fabry-Perot interferometer cavity: the change in the interference cavity length will cause the interference spectrum to shift. By tracking the wavelength shift corresponding to the peak (or trough) of the spectrum line, the change in the interference cavity length can be demodulated.

FIG7( b ) is an amplitude response diagram obtained by applying different sound pressures to the cantilever beam. It can be seen that within the deformation tolerance range of the cantilever beam, the deformation amount of the cantilever beam is linearly related to the magnitude of the applied sound pressure.

This embodiment is based on a non-intrinsic Fabry-Perot acoustic sensor with a cantilever beam structure. It includes a cantilever beam and a free sound field structure. The cantilever beam is placed in a free sound field, and a sound pressure of a certain frequency and amplitude is applied to it to drive it to produce synchronous deflection deformation, thereby causing the interference cavity length to change. The change in the interference cavity length is obtained by demodulating the interference spectrum, and combined with the cantilever beam theoretical deformation model, the sound pressure signal at the measured position is measured. The cantilever beam structure of this embodiment has the advantages of high sensitivity and large dynamic range.

The above describes the embodiments of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Various changes can be made according to the purpose of the invention. Any changes, modifications, substitutions, combinations or simplifications made according to the spirit and principle of the technical solution of the present invention should be equivalent replacement methods. As long as they meet the purpose of the invention and do not deviate from the technical principles and inventive concepts of the present invention, they belong to the protection scope of the present invention.

Claims (5)

1. A non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure, comprising a cantilever beam (2) and a 45-degree total reflection structure, characterized in that the 45-degree total reflection structure is mainly obtained in two forms: a 45-degree angled single-mode optical fiber (1) and a 45-degree reflector (6); the 45-degree total reflection structure is used to change the propagation direction of the light beam, thereby forming a non-intrinsic Fabry-Perot cavity composed of the single-mode optical fiber (1) and the cantilever beam (2). 2. The non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure according to claim 1 is characterized in that: the single-mode optical fiber (1) is a 45-degree angle optical fiber, its side wall (11) is a first reflection surface, and the lower side surface (21) of the cantilever beam is a second reflection surface, and a Fabry-Perot interference cavity is formed between the two reflection surfaces. 3. The non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure according to claim 1 is characterized in that: the single-mode optical fiber (1) is a single-mode optical fiber with a common flat end face, its end face (12) is the first reflection surface, and the lower side surface (21) of the cantilever beam is the second reflection surface, and a 45-degree reflector (6) is used to change the propagation direction of the transmitted light, so that a Fabry-Perot interference cavity is formed between the two reflection surfaces. 4. The non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure according to claim 2 or 3 is characterized in that: the cantilever beam (2) produces periodic deflection deformation under the influence of sound pressure of a certain frequency and amplitude, thereby causing the interference cavity length to change, and the change in the interference cavity length is obtained by demodulating the interference spectrum. 5. The non-intrinsic Fabry-Perot acoustic sensor based on a cantilever beam structure according to claim 4 is characterized in that: the deformation of the cantilever beam (2) under uniform sound pressure is the change in the interference cavity length, and the relationship between the applied sound pressure and the deformation of the cantilever beam is:

Based on the freedom of micro-machining technology, the size parameters of the cantilever beam (2) can be adjusted to adapt to the measurement of sound pressure signals of different frequencies; the eigenfrequency calculation formula is:

Where E is Young's modulus, ρ is density, length is L, thickness is h, and I is its moment of inertia.

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