Push-pull optical fiber MEMS accelerometer based on Michelson interferometer

Abstract

A push-pull type optical fiber MEMS accelerometer based on a 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 external frame, a mass block and two sets of spring cantilever beams, the external frame is provided with a thin optical fiber groove, and the thin optical fiber groove is provided with an optical fiber. The mass block is provided with two convex structures serving as reflecting surfaces, and two air cavities are formed between the end face of the sensing optical fiber and the top faces of the convex structures of the mass block and serve as sensing arms of the Michelson interferometer. When the accelerometer is subjected to the action of external acceleration, the mass block moves relative to the external frame to drive the cavity lengths of the two air cavities to change in a push-pull manner, so that the optical path is changed and an interference signal is generated. Interference signals are acquired and demodulated through a photoelectric detector, a data acquisition circuit and a computer, and vibration acceleration signals can be obtained. The invention has the advantages of high sensitivity, strong common-mode noise suppression capability and the like, and is suitable for various dynamic vibration response measurement occasions.

Description

Technical Field

[0001] The 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 the development of optical fiber and fiber optic communication technology. Interferometric fiber optic accelerometer uses optical fiber as a medium, inheriting the advantages of optical fiber such as anti-electromagnetic interference, light weight, and easy reuse. Compared with traditional electrical accelerometers, it has extremely high sensitivity and good dynamic response characteristics. When used for dynamic vibration signal demodulation, it can achieve high resolution and large dynamic range detection. Therefore, this technology is widely used in many fields such as earthquake and volcano monitoring, tsunami warning, and building structure health monitoring.

[0003] After years of development, micro-electromechanical system (MEMS) technology has made major breakthroughs in many fields. MEMS accelerometers are manufactured using microelectronics and micromachining technology, and have the characteristics of small size, light weight, low cost, low power consumption, and high reliability. Compared with traditional accelerometers, MEMS accelerometers are easier to integrate into various systems to achieve intelligent measurement. At the same time, MEMS technology can achieve some functions that traditional mechanical accelerometers cannot accomplish at a characteristic size of microns.

[0004] In order to meet the growing requirements in the field of measurement, interferometric fiber optic MEMS accelerometers that combine traditional interferometric fiber optic sensor technology and MEMS sensor system technology have emerged. The Fabry-Perot (FP) interferometer has a simple structure and can produce obvious interference signal changes for small changes in physical quantities, which makes most of the interferometric fiber optic MEMS accelerometers currently on the market 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 will introduce additional interference signals during the measurement process, affecting the accuracy of the measurement results. And this interferometer structure is not easy to form a push-pull design at the sensor probe to further suppress common-mode noise. In addition, FP interferometric demodulation is generally more complicated 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, and can achieve relatively simple modulation and demodulation.

[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 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, and maintains high sensitivity and high reliability while reducing the size and weight of the sensor. 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 solution of the present invention is as follows:

[0008] A push-pull optical fiber 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 grooves; further, the middle position of the fixed base is hollowed out downward, and the hollowing size is required to make the spring-mass block oscillator structure of the acceleration sensor chip suspended in the air and able to move freely along the sensitive axis direction (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 grooves are used to fix the sensing optical fiber;

[0011] The acceleration sensor chip includes an external frame, a mass block, and two groups of spring cantilever beams; the external frame is in a U-shaped structure as a whole, with a thin optical fiber groove on each of the left and right sides 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 reflection surface to reflect the laser emitted by the sensing optical fiber so that it is re-coupled into the sensing optical fiber, and a layer of metal film is sputtered on the acceleration sensor chip during the processing process to increase the reflectivity of the convex top surface; the spring cantilever beam adds an intermediate vertical beam on the basis of the serpentine folding beam, and adds semicircular small curved beams on both sides of the beam to increase the stiffness in the non-sensitive axis direction (non-x direction); further, 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 angle of 8 degrees 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 mass block that undergoes relative displacement, thereby causing the optical path to change; further, the two sensing optical fibers use 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 simultaneously, but in opposite directions, thereby achieving 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 recoupled back into the two sensing optical fibers (20), and the 3×3 coupler (22) outputs two interference signals.

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

[0016] Furthermore, the spring cantilever beam (122) is provided with an intermediate vertical beam (1222) based on the serpentine folding beam (1221), and semicircular small 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 the light coupling efficiency.

[0018] Furthermore, the pigtails of the two sensing optical fibers (20) drawn out from the acceleration sensor chip (12) are tightly wound side by side to form a circle, 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 layer of silver with a thickness of 200 nm 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 double-ended fixed beams, serpentine folded beams, L-shaped beams and other flexible beams in MEMS sensors, and 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-type 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 semicircular bending 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 interference structure is easy to form multi-beam interference, which will introduce additional interference in the measurement process and affect the measurement accuracy. And this type of sensor mostly relies on a single optical fiber and a single resonant cavity structure, and it is not suitable to form a push-pull design to further improve the sensitivity. In addition, the output signal of the FP optical sensor is usually complex, and complex signal processing and data analysis are required to obtain accurate measurement results, which places high requirements on the signal processing system and increases the complexity and cost of the system. The present invention adopts a Michelson interferometer structure to replace the FP interferometer structure. In addition to overcoming the above shortcomings, it also has the following advantages: the two air cavities formed between the end faces of the two sensing optical fibers and the top surfaces of the two mass block protrusions are both used as the sensing arms of the Michelson interferometer. Driven by the mass block with relative displacement, the cavity lengths of the two air gaps change in a push-pull manner, that is, when one cavity length increases, the other cavity length must decrease, showing a completely opposite change trend, which can further improve the sensitivity. This optical path structure itself is suitable for a variety of demodulation schemes, such as 3×3 coupler demodulation method, phase generation carrier demodulation method, etc. Therefore, the accelerometer can be directly compatible with various ready-made 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 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 It is a schematic diagram of the optical path and circuit when the push-pull fiber optic MEMS accelerometer embodiment based on the Michelson interferometer of the present invention is used. DETAILED DESCRIPTION

[0028] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments. The described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present 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 optical fiber 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 fixing screw holes 111 for firmly mounting the entire mechanical assembly on the measuring platform. At the same time, the fixed base 11 is provided with two thick fiber grooves 112 for fixing the sensing fiber 20 to ensure the stability of the fiber during the measurement process. The middle position of the fixed base 11 is hollowed out downward to provide sufficient space for the spring-mass block oscillator structure of the acceleration sensor chip 12, so that it can move freely along the sensitive axis direction (x direction) under the action of external acceleration.

[0033] The acceleration sensor chip 12 includes an external frame 121, a mass block 123 and two groups of spring cantilever beams 122. The external frame 121 is in a U-shaped structure, with a thin fiber groove 1211 on each of the left and right sides, which is used to fix the sensing optical fiber 20 after the coating layer is stripped. Two convex structures 1231 are provided on the mass block 123, and 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 recouple it into the sensing optical fiber 20. During the processing, a layer of 200nm of 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 semicircular small 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, and together constitutes a spring-mass block oscillator structure;

[0034] The end faces of the two sensing optical fibers 20 are ground with an 8° inclined surface and coated with an anti-reflection film. The bare fiber part and the coated part are fixed to the thin fiber groove 1221 of the acceleration sensor chip 12 and the thick fiber groove 112 of the fixed base 11 respectively by optical ultraviolet structural adhesive. Two air cavities are formed between the end faces of the two sensing optical fibers 20 and the two raised top surfaces of the mass block 123. The cavity lengths of the two air cavities change under the drive of the mass block 123 that undergoes relative displacement, thereby causing the optical path to change. The two sensing optical fibers 20 have the same length to form a Michelson balanced interferometer structure. When the mass block is displaced, the change in the cavity length of the two air cavities causes the optical path difference of the two arms to change, thereby generating an interference signal. After the two sensing optical fibers 20 are led out from the acceleration sensor chip 12, they are placed side by side and close to ensure that the two arms of the Michelson interferometer are in a noise common mode state, that is, the noises received by the two arms have the same characteristics and sources, showing 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 also Figure 5 , Figure 5 The optical path and circuit principle diagram of the push-pull fiber MEMS accelerometer embodiment of the invention based on Michelson interferometer when in use. As can be seen from the figure, when this embodiment is used, the sensing optical path 2 is a balanced Michelson interferometer, which is composed of a 3×3 coupler 22, a first sensing optical fiber 201, a second sensing optical fiber 202, a first convex top surface 12311, a second convex top surface 12312, a first photodetector 231 and a second photodetector 232. The pigtails of the first sensing optical fiber 201 and the second sensing optical fiber 202 drawn out from the acceleration sensor chip 12 are tightly wound side by side and fixed together with the 3×3 coupler 22 in 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 optical fiber 201, the second sensing optical fiber 202, the first raised top surface 12311, and the second raised top surface 12312 respectively 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 recoupled back into the first sensing optical fiber 201 and the second sensing optical fiber 202 respectively. The two interference signals output by the 3×3 coupler 22 are respectively passed through the first photodetector 241 and the second photodetector 242, and 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 MEMS accelerometer, the fiber MEMS accelerometer of the present invention is fixed on the surface of the object to be measured and moves therewith, and then the spring-mass block structure (i.e., the complex of the sensing fiber 20 and the mechanical component 1) moves relative to the external frame 121. Since the first sensing fiber 201 and the second sensing fiber 202 are both fixed in the thin fiber groove 1211 of the external frame 121, the mass block 123 will also move relatively therewith. The two air cavities formed between the end face of the first sensing fiber 201 and the first protruding top surface 12311 and between the end face of the second sensing fiber 202 and the second protruding top surface 12312 are both used as sensing arms of the Michelson interferometer. Driven by the mass block 123 that undergoes relative displacement, the cavity lengths of the two air gaps change in a push-pull manner, that is, when one cavity length increases, the other cavity length must decrease, presenting a completely opposite change trend. Therefore, the phase of light propagating in the sensing fiber 20 will change with the external vibration signal, and the total change of the optical phase is twice that of the single sensing arm structure, thereby improving the 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] The above describes some embodiments of the present invention in detail with reference to the accompanying drawings, but the present invention is not limited to the implementation methods in the above embodiments. Various modifications or changes made without departing from the purpose of the present invention belong to this patent. The present invention should not be limited accordingly.

Claims

1. A push-pull fiber optic MEMS accelerometer based on Michelson interferometer, characterized in that: It comprises a mechanical component (1) and two sensing optical fibers (20); The mechanical component (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 direction (x direction) under the action of external acceleration; the fixing screw holes (111) are used to fix the mechanical component (1); and the thick optical fiber grooves (112) are used to fix 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, and has 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), and the top surfaces of the convex structures (1231) serve as reflection surfaces to reflect laser light emitted from the sensing optical fiber (20) and couple it back into the sensing optical fiber (20); the spring cantilever beam (122) connects 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 part and the coated part are respectively fixed to the thin optical fiber groove (1221) 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 drive of the mass block (123) that undergoes relative displacement, thereby causing the optical path to change.

2. The push-pull fiber MEMS accelerometer based on Michelson interferometer according to claim 1, characterized in that: The two sensing optical fibers (20) have the same length to form a Michelson balanced interferometer structure, and are led 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 noises received by the two arms have the same characteristics and sources, 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.

3. The push-pull fiber optic 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 recoupled back into the two sensing optical fibers (20), and the 3×3 coupler (22) outputs two interference signals. After the two interference signals are converted into electrical signals by the first photodetector (241) and the second photodetector (242), they are collected by the data collection circuit (25) and sent to the computer (26) for demodulation processing to obtain vibration acceleration signals.

4. The push-pull optical fiber MEMS accelerometer based on Michelson interferometer according to claim 1, characterized in that: 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 (non-x direction).

5. The push-pull optical fiber MEMS accelerometer based on Michelson interferometer according to claim 1, characterized in that: 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 the light coupling efficiency.

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

7. The push-pull fiber optic MEMS accelerometer based on Michelson interferometer according to any one of claims 1 to 6, 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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