A push-pull optical fiber MEMS accelerometer based on Michelson interferometer

Abstract

A push-pull type optical fiber MEMS accelerometer based on Michelson interferometer comprises a mechanical assembly and two sensing optical fibers, the mechanical assembly is composed of a fixed base and an acceleration sensing chip, the acceleration sensing chip comprises an outer frame, a mass block and two groups of spring cantilever beams, the outer frame is provided with a fine optical fiber slot, the mass block is provided with two convex structures as reflecting surfaces, the sensing optical fiber end face and the top surface of the mass block convex structure form two air cavities as sensing arms of the Michelson interferometer. When the accelerometer is subjected to external acceleration, the mass block moves relative to the outer frame, the cavity length of the two air cavities changes in a push-pull manner, thereby changing the optical path and generating an interference signal. The interference signal is collected and demodulated by a photoelectric detector, a data acquisition circuit and a computer, and a vibration acceleration signal can be obtained. The present application has the advantages of high sensitivity, strong noise common mode rejection capability and is suitable for various dynamic vibration response measurement occasions.

Description

Technical Field

[0001] The present invention relates to optical fiber sensing, in particular to a push-pull optical fiber MEMS accelerometer based on a Michelson interferometer for measuring vibration acceleration signals. Background Art

[0002] Fiber-optic sensing technology is a new technology that has emerged with advances in optical fiber and fiber-optic communications. Interferometric fiber-optic accelerometers, using optical fiber as a medium, inherit the advantages of optical fiber, such as immunity to electromagnetic interference, light weight, and ease of reuse. Compared to traditional electrical accelerometers, they offer extremely high sensitivity and excellent dynamic response. When used to demodulate dynamic vibration signals, they can achieve high resolution and a wide dynamic range. Consequently, this technology is widely used in a variety of fields, including earthquake and volcano monitoring, tsunami warning, and building structural health monitoring.

[0003] After years of development, microelectromechanical systems (MEMS) technology has achieved significant breakthroughs in multiple fields. MEMS accelerometers, manufactured using microelectronics and micromachining techniques, feature small size, light weight, low cost, low power consumption, and high reliability. Compared to traditional accelerometers, MEMS accelerometers are easier to integrate into various systems, enabling intelligent measurement. Furthermore, MEMS technology, with micron-scale feature sizes, enables functions that are impossible with traditional mechanical accelerometers.

[0004] To meet the growing demands of the measurement field, interferometric fiber optic MEMS accelerometers, which combine traditional interferometric fiber optic sensor technology with MEMS sensor system technology, have emerged. The Fabry-Perot (FP) interferometer has a simple structure and can produce significant interference signal changes in response to tiny changes in physical quantities. This is why most interferometric fiber optic MEMS accelerometers currently on the market are based on this principle. However, this type of sensor also has some shortcomings. First, the FP interferometer structure is prone to multi-beam interference, which introduces additional interference signals during the measurement process, affecting the accuracy of the measurement results. In addition, this interferometer structure does not easily form a push-pull design at the sensor probe to further suppress common-mode noise. In addition, FP interferometric demodulation is generally complex and is not suitable for relatively simple modulation and demodulation schemes such as the 3×3 coupler method, which does not require the introduction of a carrier and has a larger dynamic range.

[0005] Therefore, developing a compact fiber optic accelerometer with high reliability and high sensitivity has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0006] In order to overcome the shortcomings of traditional fiber optic interferometric accelerometers and existing MEMS fiber optic accelerometers, the present invention proposes a push-pull fiber optic MEMS accelerometer based on a Michelson interferometer, which integrates fiber optic sensing technology and microelectromechanical system (MEMS) technology, adopts the Michelson interferometer principle and push-pull design, and realizes high-sensitivity measurement of external acceleration through two sensing optical fibers and optimized mechanical components. It has the characteristics of compact structure and simple manufacturing process, while reducing the size and weight of the sensor while still maintaining high sensitivity and high reliability. It is suitable for application scenarios such as aerospace, precision instrument measurement, etc. that require sensors to meet both lightweight and high performance.

[0007] The technical solutions of the present invention are as follows:

[0008] A push-pull fiber optic MEMS accelerometer based on Michelson interferometer is characterized in that it includes a mechanical component and two sensing optical fibers.

[0009] The mechanical assembly includes a fixed base, an acceleration sensor chip and a chip fixing plate;

[0010] The fixed base includes four fixing screw holes and two thick optical fiber slots. Furthermore, the fixed base has a hollowed-out portion downwardly from the middle position. The hollowed-out portion is large enough to allow the spring-mass oscillator structure of the acceleration sensor chip to be suspended in the air and to move freely along the sensitive axis (x direction) under the action of external acceleration. The fixing screw holes are used to fix and install the mechanical components; the thick optical fiber slots are used to fix the sensing optical fiber.

[0011] The acceleration sensor chip includes an external frame, a mass block, and two sets of spring cantilever beams. The external frame has an overall U-shaped structure, with a thin optical fiber groove on each side for fixing the sensing optical fiber after the coating layer is stripped. The mass block also includes two convex structures. The convex top surface serves as a reflective surface to reflect the laser emitted by the sensing optical fiber and re-couple it into the sensing optical fiber. During the processing, a layer of metal film is sputtered on the acceleration sensor chip to increase the reflectivity of the convex top surface. The spring cantilever beam adds an intermediate vertical beam to the serpentine folding beam, and adds small semicircular curved beams on both sides of the beam to increase the stiffness in the non-sensitive axis direction (non-x direction). Furthermore, the spring cantilever beam connects the external frame and the mass block to form a spring-mass block oscillator structure.

[0012] The end faces of the two sensing optical fibers are ground at an 8-degree tilt angle and coated with an anti-reflection film, and the bare fiber part and the coated part are respectively fixed to the thin optical fiber groove of the acceleration sensor chip and the thick optical fiber groove of the fixed base by structural adhesive; further, two air cavities are formed between the end faces of the two sensing optical fibers and the two raised top surfaces of the mass block, and their cavity lengths change under the drive of the relative displacement of the mass block, thereby causing the optical path to change; further, the two sensing optical fibers are of the same length to form a Michelson balanced interferometer structure, and the two sensing optical fibers need to be placed side by side and closely after being led out of the acceleration sensor chip to ensure that the two arms of the Michelson interferometer are in a noise common mode state, that is, the noise received by the two arms has the same characteristics and source, showing a high degree of consistency and synchronization. When the mass block (123) moves along the sensitive axis direction, the cavity lengths of the two air cavities will be changed at the same time, but in opposite directions, thereby realizing push-pull change.

[0013] Furthermore, it also includes a light source (21), a 3×3 coupler (22), a first photodetector (231), a second photodetector (232), a data acquisition circuit (25) and a computer (26);

[0014] After the light emitted by the light source (21) is injected into the 3×3 coupler (22), it enters the two arms of the interferometer formed by the two sensing optical fibers (20) and the top surfaces of the two convex structures (1231) to form two optical signals. The two optical signals are respectively reflected by the top surfaces of the two convex structures (1231), return along the original path, and are re-coupled back into the two sensing optical fibers (20). The 3×3 coupler (22) outputs two interference signals.

[0015] The two interference signals are converted into electrical signals by the first photodetector (241) and the second photodetector (242), respectively, and then collected by the data acquisition circuit (25) and sent to the computer (26) for demodulation processing to obtain a vibration acceleration signal.

[0016] Furthermore, the spring cantilever beam (122) is provided with an intermediate vertical beam (1222) on the basis of the serpentine folding beam (1221), and small semicircular curved beams (1223) are provided on both sides of the beam to increase the stiffness in the non-sensitive axis direction (non-x direction).

[0017] Furthermore, the end faces of the two sensing optical fibers (20) are ground to have an inclination angle of 8° and are coated with an anti-reflection film to improve light coupling efficiency.

[0018] Furthermore, the pigtail portions of the two sensing optical fibers (20) drawn out from the acceleration sensor chip (12) are tightly wound side by side to form a loop, and are fixed together with the 3×3 coupler (22) in the hollow space at the back of the fixed base (11).

[0019] During the processing, a 200nm layer of silver is sputtered on the acceleration sensor chip (12) to increase the reflectivity of the top surface of the convex structure (1231).

[0020] Compared with the prior art, the characteristics and advantages of the present invention are as follows:

[0021] 1. The present invention selects the serpentine folded beam with the smallest stiffness among the common flexible beams such as double-ended fixed beam, serpentine folded beam, and L-shaped beam in MEMS sensors. According to the formula S static =mk, the smaller the flexible stiffness, the greater the static sensitivity. An intermediate vertical beam is added to the serpentine beam to form an E-shaped folded beam structure. The intermediate vertical beam strengthens the structure of the serpentine folded beam and increases the stiffness in the y direction, around the x direction, and around the z direction. When the same acceleration is input, the deformation of the E-shaped folded beam in the above directions is lower than that of the serpentine folded beam, which will have a smaller impact on the device output, thereby reducing the cross-axis sensitivity of the accelerometer and suppressing cross-axis crosstalk. In addition, small semi-circular curved beams are designed on both sides of the folded beam to increase the stiffness in the z direction and further reduce the displacement in the z direction, ultimately forming a mass block-spring oscillator structure with low resonant frequency and large sensitive mass.

[0022] 2. FP interferometer structures are prone to multi-beam interference, which can introduce additional interference during the measurement process and affect measurement accuracy. Furthermore, these sensors often rely on a single fiber and single resonant cavity structure, making them unsuitable for push-pull designs to further enhance sensitivity. Furthermore, the output signals of FP optical sensors are typically complex, requiring sophisticated signal processing and data analysis to obtain accurate measurement results. This places high demands on the signal processing system, increasing system complexity and cost. The present invention replaces the FP interferometer structure with a Michelson interferometer structure. Besides overcoming these shortcomings, it also offers the following advantages: The two air cavities formed between the end faces of the two sensing fibers and the raised top surfaces of the two proof-blocks serve as the sensing arms of the Michelson interferometer. Driven by the relative displacement of the proof-blocks, the cavity lengths of the two air gaps vary in a push-pull manner. That is, as one cavity length increases, the other necessarily decreases, exhibiting opposite trends, further enhancing sensitivity. This optical path structure is inherently compatible with a variety of demodulation schemes, such as 3×3 coupler demodulation and phase-generated carrier demodulation. Therefore, the accelerometer is directly compatible with a variety of off-the-shelf sensing systems. In addition, the structural design of the balanced interferometer can effectively suppress common-mode noise, making up for the defect that the resonant cavity in the FP interferometer structure is very sensitive to factors such as ambient temperature, which leads to unstable interference signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a three-dimensional schematic diagram of a push-pull fiber optic MEMS accelerometer based on a Michelson interferometer;

[0024] Figure 2 is a top view of the acceleration sensor chip of the present invention;

[0025] Figure 3 It is a schematic diagram of the spring cantilever beam of the present invention;

[0026] Figure 4 is a top view of the fixed base of the present invention;

[0027] Figure 5 This is a schematic diagram of the optical path and circuit principle of an embodiment of a push-pull fiber optic MEMS accelerometer based on a Michelson interferometer according to the present invention when in use. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The embodiments described are part of the embodiments of this application, rather than all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments in this application without creative work are within the scope of protection of this application.

[0029] Example

[0030] See also Figure 1 、 Figure 2 、 Figure 3 and Figure 4 As can be seen from the figure, the push-pull fiber optic MEMS accelerometer based on the Michelson interferometer of the present invention includes a mechanical component 1 and two sensing optical fibers 20.

[0031] The mechanical assembly 1 includes a fixed base 11 and an acceleration sensor chip 12 .

[0032] The fixed base 11 has four screw holes 111 for securely mounting the entire mechanical assembly on the measurement platform. Two thick fiber slots 112 are also provided on the fixed base 11 for securing the sensing fiber 20, ensuring its stability during measurement. The center of the fixed base 11 is hollowed out downward to provide ample space for the spring-mass oscillator structure of the acceleration sensor chip 12, allowing it to move freely along the sensitive axis (x-direction) under external acceleration.

[0033] The acceleration sensor chip 12 includes an external frame 121, a mass block 123 and two sets of spring cantilever beams 122. The external frame 121 is in a U-shaped structure, with a thin optical fiber groove 1211 on each side for fixing the sensing optical fiber 20 after the coating layer is stripped. Two convex structures 1231 are provided on the mass block 123. The top surface of the convex structure 1231 serves as a reflection surface to reflect the laser emitted by the sensing optical fiber 20 and re-couple it into the sensing optical fiber 20. During the processing, a layer of 200nm silver is sputtered on the acceleration sensor chip 12 to increase the reflectivity of the convex top surface. The spring cantilever beam 122 adds an intermediate vertical beam 1222 on the basis of the serpentine folding beam 1221, and adds small semicircular curved beams 1223 on both sides of the beam to increase the stiffness in the non-sensitive axis direction (non-x direction). The spring cantilever beam 122 connects the external frame 121 and the mass block 123, together forming a spring-mass block oscillator structure;

[0034] The two sensing fibers 20 have their end faces polished at an 8° angle and coated with an antireflection coating. Their bare fiber portions and coated portions are respectively secured to the thin fiber slot 1221 of the acceleration sensor chip 12 and the thick fiber slot 112 of the fixed base 11 using optical UV structural adhesive. Two air cavities are formed between the end faces of the two sensing fibers 20 and the two raised top surfaces of the mass block 123. These cavity lengths vary with the relative displacement of the mass block 123, causing changes in the optical path length. The two sensing fibers 20 are of equal length, forming a Michelson balanced interferometer structure. When the mass block shifts, the changes in the cavity lengths of the two air cavities cause a change in the optical path difference between the two arms, generating an interference signal. After exiting the acceleration sensor chip 12, the two sensing fibers 20 are placed side by side, closely adjacent to each other, ensuring that the two arms of the Michelson interferometer are in a common noise mode. This means that the noise experienced by the two arms has the same characteristics and source, resulting in a high degree of consistency and synchronization. When the mass block moves along the sensitive axis, the lengths of the two air cavities will change simultaneously, but in opposite directions.

[0035] See Figure 5 , Figure 5 This is a schematic diagram of the optical path and circuitry of an embodiment of the push-pull fiber-optic MEMS accelerometer based on a Michelson interferometer. As shown, in this embodiment, the sensing optical path 2 is a balanced Michelson interferometer, consisting of a 3×3 coupler 22, a first sensing fiber 201, a second sensing fiber 202, a first raised top surface 12311, a second raised top surface 12312, a first photodetector 231, and a second photodetector 232. The pigtails of the first and second sensing fibers 201, 202, exiting the acceleration sensor chip 12, are coiled side-by-side and secured to the 3×3 coupler 22 within the hollow space at the back of the fixed base 11. After the light emitted by the light source 21 is injected into the 3×3 coupler 22, it enters the two arms of the interferometer composed of the first sensing fiber 201, the second sensing fiber 202, the first raised top surface 12311, and the second raised top surface 12312 to form two optical signals. The two optical signals are respectively reflected by the first raised top surface 12311 and the second raised top surface 12312, return along the original path, and are re-coupled back into the first sensing fiber 201 and the second sensing fiber 202. The two interference signals output by the 3×3 coupler 22 pass through the first photodetector 241 and the second photodetector 242 respectively, and are then collected by the data acquisition circuit 25 and sent to the computer 26 for demodulation processing.

[0036] During the dynamic vibration response of the fiber-optic MEMS accelerometer, the present invention's fiber-optic MEMS accelerometer is fixed to the surface of the measured object and moves accordingly. The spring-mass structure (i.e., the complex of the sensing fiber 20 and the mechanical assembly 1) then moves relative to the external frame 121. Since both the first sensing fiber 201 and the second sensing fiber 202 are fixed in the thin fiber groove 1211 of the external frame 121, the mass 123 also undergoes relative motion. The two air cavities formed between the end face of the first sensing fiber 201 and the first raised top surface 12311, and between the end face of the second sensing fiber 202 and the second raised top surface 12312, respectively, serve as the sensing arms of the Michelson interferometer. Driven by the relative displacement of the mass 123, the cavity lengths of the two air gaps vary in a push-pull manner. That is, as one cavity length increases, the other necessarily decreases, exhibiting completely opposite change trends. Consequently, the phase of light propagating in the sensing fiber 20 changes with the external vibration signal, and the total optical phase change is twice that of a single-arm structure, thereby improving sensitivity. Finally, the optical interference signal is collected by the photoelectric sensor 23 and the data acquisition circuit 24, and the vibration acceleration signal is demodulated by the computer 25.

[0037] While some embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the implementations described in the aforementioned embodiments. Any modifications or variations made without departing from the spirit of the present invention are encompassed by this patent and should not be relied upon to limit the present invention.

Claims

1. A push-pull fiber optic MEMS accelerometer based on a Michelson interferometer, characterized by: It comprises a mechanical component (1) and two sensing optical fibers (20); The mechanical assembly (1) comprises a fixed base (11) and an acceleration sensor chip (12), wherein the fixed base (11) has four fixing screw holes (111) and two thick optical fiber grooves (112), and the middle position of the fixed base (11) is hollowed out downward to provide space for the spring-mass block oscillator structure of the acceleration sensor chip (12), so that the structure can move freely along the sensitive axis under the action of external acceleration; the fixing screw holes (111) are used for fixing and installing the mechanical assembly (1); and the thick optical fiber grooves (112) are used for fixing the sensing optical fiber (20); The acceleration sensor chip (12) comprises an external frame (121), a mass block (123) and two groups of spring cantilever beams (122), wherein the external frame (121) is in a U-shaped structure, with a thin optical fiber groove (1211) on each of the left and right sides for fixing the sensing optical fiber (20) after the coating layer is stripped; the mass block (123) is provided with two convex structures (1231), the top surfaces of the convex structures (1231) serving as reflection surfaces to reflect laser light emitted from the sensing optical fiber (20) and re-couple it into the sensing optical fiber (20); the spring cantilever beams (122) connect the external frame (121) and the mass block (123) to form a spring-mass block oscillator structure; The end faces of the two sensing optical fibers (20) are ground with an inclined angle and coated with an anti-reflection film, and the bare fiber portion and the coated portion are respectively fixed to the thin optical fiber groove (1211) of the acceleration sensor chip (12) and the thick optical fiber groove (112) of the fixed base (11), and two air cavities are formed between the end faces of the two sensing optical fibers (20) and the top surfaces of the two convex structures (1231) of the mass block (123), and the cavity lengths of the two air cavities change under the influence of the relative displacement of the mass block (123), thereby causing the optical path to change; The two sensing optical fibers (20) are of the same length to form a Michelson balanced interferometer structure, and are drawn out from the acceleration sensor chip (12) and placed side by side in close proximity to ensure that the two arms of the Michelson interferometer are in a noise common mode state, that is, the noise received by the two arms has the same characteristics and source, showing a high degree of consistency and synchronization; when the mass block (123) moves along the sensitive axis direction, the cavity lengths of the two air cavities are changed simultaneously, but in opposite directions, thereby achieving a push-pull change; The spring cantilever beam (122) is provided with an intermediate vertical beam (1222) on the basis of the serpentine folding beam (1221), and semicircular small curved beams (1223) are provided on both sides of the beam to increase the rigidity in the non-sensitive axis direction; the end faces of the two sensing optical fibers (20) are ground with an 8° tilt angle and coated with an anti-reflection film to improve the light coupling efficiency.

2. The push-pull fiber MEMS accelerometer based on Michelson interferometer according to claim 1, characterized in that: It also includes a light source (21), a 3×3 coupler (22), a first photodetector (231), a second photodetector (232), a data acquisition circuit (25) and a computer (26); After the light emitted by the light source (21) is injected into the 3×3 coupler (22), it enters the two arms of the interferometer formed by the two sensing optical fibers (20) and the top surfaces of the two convex structures (1231) to form two optical signals. The two optical signals are respectively reflected by the top surfaces of the two convex structures (1231), return along the original path, and are re-coupled back into the two sensing optical fibers (20). The 3×3 coupler (22) outputs two interference signals. The two interference signals are converted into electrical signals by the first photodetector (231) and the second photodetector (232), respectively, and then collected by the data acquisition circuit (25) and sent to the computer (26) for demodulation processing to obtain a vibration acceleration signal.

3. The push-pull fiber optic MEMS accelerometer based on the Michelson interferometer according to claim 1, characterized in that: The pigtail portions of the two sensing optical fibers (20) drawn out from the acceleration sensor chip (12) are tightly wound side by side to form a loop, and are fixed together with a 3×3 coupler (22) in the hollow space at the back of the fixed base (11).

4. The push-pull fiber MEMS accelerometer based on the Michelson interferometer according to any one of claims 1 to 3, characterized in that: During the processing, a layer of metal film is sputtered on the acceleration sensor chip (12) to increase the reflectivity of the top surface of the convex structure (1231).

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