An on-chip accelerometer based on integrated optical waveguides

Abstract

An on-chip accelerometer based on integrated optical waveguide, the lens optical fiber and mechanical structure assembly are compounded, and the interferometric optical fiber sensor based on spring-mass push-pull structure is formed. Wherein, the spring cantilever beam increases the intermediate vertical beam and the semicircular small bending beam on the basis of the serpentine folding beam, connects the external frame and the mass, and jointly constitutes the spring-mass oscillator structure. The application effectively reduces the cross-axis sensitivity of the accelerometer and suppresses the cross-axis crosstalk. The mass contains two convex structures, the top surface of which is used as a reflecting surface, and a layer of metal film is sputtered during processing to increase the reflectivity. The air cavity formed by the two mode spot converters and the mass forms a balanced interferometer structure, which adopts the design of Michelson interferometer combined with push-pull structure, further improves the sensitivity, can be compatible with various demodulation schemes, and effectively suppresses the common mode noise. The application directly etches the integrated optical waveguide on the chip as the sensing optical path, which effectively suppresses the polarization fading problem while reducing the package size. The highly symmetrical design of the sensing optical path makes the noise characteristics of the two arms of the sensor nearly consistent, greatly reduces the environmental noise during interference, and improves the signal-to-noise ratio. The accelerometer structure is compact, the manufacturing process is simple, and is suitable for aerospace and other scenes with requirements of light weight and high performance.

Description

TECHNICAL FIELD

[0001] The present application relates to optical fiber sensing, in particular to an integrated optical waveguide-based on-chip accelerometer applied to measure vibration acceleration signals. BACKGROUND

[0002] With the continuous maturity and rapid development of micro-electro-mechanical system (MEMS), MEMS accelerometers have shown great potential in the field of accelerometer applications due to their small size, high reliability, high sensitivity and easy integration, and have gradually become a powerful alternative to traditional mechanical sensors, occupying a core position in the micro-sensor market. However, MEMS accelerometers still have the challenges of limited resolution and dynamic range, and their thermal stability and anti-electromagnetic interference ability in harsh environments are insufficient, which seriously hinders the further improvement of sensor performance. Therefore, the micro-opto-electro-mechanical system (MOEMS) technology combining integrated optical waveguide technology and MEMS technology emerges as the times require.

[0003] The on-chip accelerometer based on MOEMS technology not only has the advantages of MEMS accelerometers, but also greatly improves the anti-electromagnetic interference, sensitivity and other characteristics with the unique advantages of integrated optical waveguide. On-chip accelerometers are usually divided into three categories according to the modulation and demodulation principle: light intensity detection type, wavelength detection type and phase detection type. Among them, the light intensity detection type accelerometer is simple in structure, but its accuracy is often limited by the power stability of the light source. The wavelength detection type accelerometer needs to use a high-precision spectrometer to determine the wavelength change, which is extremely unfavorable for its miniaturization and commercialization process. The phase detection type accelerometer detects the change of light phase in the waveguide, has extremely high sensitivity and good dynamic response characteristics, and can realize high resolution and large dynamic range detection when applied to dynamic signal demodulation, and has a broad application prospect in the field of acceleration sensing.

[0004] However, the current on-chip acceleration sensor has a complex and precise micro-nano optical structure inside, which makes the manufacturing process complicated, not only slowing down the manufacturing speed, but also having the problem of low production yield due to the fine optical micro-nano structure. In addition, the working principle of most on-chip accelerometers relies on detecting the displacement change of the suspended mass in the inertial system and converting it into an optical quantity change to measure the acceleration. The unique small structure of this on-chip accelerometer actually limits the displacement range of the mass, and thus limits the improvement of its sensitivity. Therefore, how to develop a compact on-chip accelerometer with simple structure, high reliability and high sensitivity has become a technical problem to be solved by the technical personnel in the field. SUMMARY

[0005] To overcome the shortcomings of existing MOEMS on-chip accelerometers, this invention proposes an on-chip accelerometer based on an integrated optical waveguide. This fiber optic accelerometer features a compact structure and simple fabrication process, maintaining high sensitivity and reliability while reducing sensor size and weight. It is suitable for applications such as aerospace and precision instrument measurement, where sensors must simultaneously meet lightweight and high-performance requirements.

[0006] The technical solution of the present invention is as follows:

[0007] An on-chip accelerometer based on an integrated optical waveguide is characterized by including mechanical components and two lens optical fibers.

[0008] The mounting base includes four mounting screw holes and one fiber optic slot; furthermore, the mounting base has a downward-facing cutout in the middle, the cutout size being sufficient to suspend the spring-mass oscillator structure of the accelerometer chip and allow it to move freely along the sensitive axis (x-direction) under external acceleration; the mounting screw holes are used to fix and install the mechanical components; the fiber optic slot is used to fix the lens fiber.

[0009] The accelerometer chip includes an external frame, a mass block, two sets of spring cantilever beams, two lateral mode converters, two sets of transmission waveguides, and two lateral mode converters. The external frame has an overall U-shaped structure. The mass block also includes two convex structures, with the convex top surfaces acting as reflective surfaces to reflect the laser emitted from the lateral mode converters and recouple it back into the lateral mode converters. Furthermore, the lateral mode converters and the two convex top surfaces of the mass block form two air cavities, the length of which changes under the influence of the relative displacement of the mass block, thus causing changes in the optical path. Furthermore, the transmission waveguides employ a symmetrical structure of equal length to form a Michelson-balanced interferometer structure, which is simultaneously etched onto a small-volume chip, improving stability. During the processing, a metal film is sputtered onto the accelerometer chip to increase the reflectivity of the raised top surface; the spring cantilever beam is based on the serpentine folding beam with an added middle vertical beam, and semi-circular small curved beams are added 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.

[0010] The two lens fibers are fixed to the fiber groove of the fixed base with optical ultraviolet structural adhesive; furthermore, the two lens fibers must be placed side by side and closely attached in the fiber groove of the fixed base and after being led out, so as to continue to maintain the balance of the interferometer structure.

[0011] Furthermore, the lens fiber includes a first lens fiber and a second lens fiber.

[0012] Preferably, the Michelson balanced interferometer structure, as a sensing optical path, includes a 3×3 coupler, a first lens fiber, a second lens fiber, a first convex top surface, a second convex top surface, a first photodetector, and a second photodetector.

[0013] The light emitted from the light source is injected into the 3×3 coupler and split into two paths, entering the first lens fiber and the second lens fiber respectively. Then, it passes through the first lateral mode converter and the second lateral mode converter and enters the first transmission waveguide and the second transmission waveguide respectively. After that, it is emitted from the first lateral mode converter and the second lateral mode converter respectively, forming two optical signals. The two optical signals are reflected by the first convex top surface and the second convex top surface respectively, and return along the original path to be recoupled back to the first lens fiber and the second lens fiber respectively. The two interference signals output by the 3×3 coupler pass through the first photodetector and the second photodetector respectively, and are then collected by the data acquisition circuit and sent to the computer for demodulation processing.

[0014] Furthermore, the first lens fiber and the second lens fiber are closely attached to each other and side by side, with their pigtails wound into a loop and fixed together with the 3×3 coupler in the hollow space on the back of the fixed base.

[0015] Furthermore, during the dynamic vibration response of the on-chip accelerometer, the accelerometer is fixed to the surface of the object being measured and moves accordingly. The spring-mass structure moves relative to the external frame. The two air cavities formed by the first side speckle converter and the first convex top surface, and the second side speckle converter and the second convex top surface, respectively, serve as the sensing arms of the Michelson interferometer. Driven by the mass block that has undergone relative displacement, the cavity lengths of the two air gaps change in a push-pull manner, that is, when the length of one cavity increases, the length of the other cavity decreases. The phase of light propagating in the sensing optical path changes with the external vibration signal, and the total change in optical phase is twice that of the single sensing arm structure.

[0016] The features and advantages of this invention are as follows:

[0017] 1. This invention selects the serpentine folding beam with the lowest stiffness from common flexible beams in on-chip sensors, such as double-ended fixed beams, serpentine folding beams, and L-shaped beams, according to formula S. static=mk, the smaller the flexible stiffness, the greater the static sensitivity. Adding a central vertical beam to the serpentine beam creates an E-shaped folded beam structure. The central vertical beam strengthens the serpentine folded beam, increasing its stiffness in the y-direction, x-direction, and z-direction. When the same acceleration is input, the E-shaped folded beam exhibits less deformation in these directions than the serpentine folded beam, resulting in a smaller impact on the device output, thus reducing the accelerometer's cross-axis sensitivity and suppressing cross-axis crosstalk. Furthermore, semi-circular small curved beams are designed on both sides of the folded beam to increase stiffness in the z-direction, further reducing displacement in the z-direction, ultimately forming a low-resonance-frequency, large-sensitive-mass-spring oscillator structure.

[0018] 2. Most commercially available phase-detection on-chip accelerometers are based on the FP interferometry principle. FP interferometry structures are prone to multi-beam interference, introducing additional interference during measurement and affecting accuracy. Furthermore, these sensors often rely on single-fiber, single-resonant-cavity structures, making push-pull designs unsuitable for further sensitivity enhancement. Additionally, 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. This invention replaces the FP interferometry structure with a Michelson interferometer structure, overcoming the aforementioned shortcomings while offering the following advantages: the two air cavities formed between the two lateral mode pattern converters and the two protruding top surfaces of the mass blocks serve as sensing arms of the Michelson interferometer. Driven by the relative displacement of the mass blocks, 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 inevitably decreases, exhibiting completely opposite trends, further improving sensitivity. This optical path structure is suitable for various demodulation schemes, such as 3×3 coupler demodulation and phase-generated carrier demodulation. Therefore, this accelerometer is directly compatible with various existing sensing systems. Furthermore, the structural design of the balanced interferometer effectively suppresses common-mode noise, compensating for the instability of the interference signal caused by the high sensitivity of the resonant cavity in the FP interferometer structure to factors such as ambient temperature.

[0019] 3. This invention integrates an optical waveguide as the sensing optical path on the accelerometer chip, replacing the conventional direct insertion coupling method of optical fiber waveguides. When light propagates in an optical fiber, its polarization state is easily affected by the environment, leading to a weakening of the interference signal intensity, i.e., polarization fading. Common methods to suppress polarization fading include using Faraday rotators, polarization-maintaining devices, and polarization diversity reception. However, Faraday rotators are not suitable for embedding in MOEMS sensors due to their large size; using polarization-maintaining devices increases system cost, and polarization diversity reception increases system complexity. In contrast, when light propagates in an integrated optical waveguide, its polarization state remains stable, effectively solving the polarization fading problem. Furthermore, the direct insertion coupling method requires that both optical fibers be inserted from both sides of the chip during packaging to match the designed push-pull structure. With an integrated optical waveguide, the two sensing arms of the push-pull structure can be led to the same location on the chip, and the two coupled optical fibers can be placed side-by-side without requiring a long length. This design reduces both the package size and the noise introduced by the external environment into the optical fiber before the light couples into the chip. This highly symmetrical structural design makes the noise characteristics of the two arms of the sensor nearly identical. When the two sensing signals interfere, noise introduced by changes in the external environment can be significantly eliminated, thus achieving a high signal-to-noise ratio.

[0020] 4. This invention can directly use commercially available short-focal-length microlens optical fibers and mature integrated optical chip end-face coupling technology and equipment for optical coupling, without the need for specially customized optical fibers and more complex coupling technology and equipment. Attached Figure Description

[0021] Figure 1 This is a 3D schematic diagram of an on-chip accelerometer based on an integrated optical waveguide;

[0022] Figure 2 This is a top view of the acceleration sensing chip of the present invention;

[0023] Figure 3 This is a schematic diagram of the spring cantilever beam of the present invention;

[0024] Figure 4 This is a top view of the fixing base of the present invention;

[0025] Figure 5 This is a schematic diagram of the optical path and circuit of an embodiment of the on-chip accelerometer based on integrated optical waveguide of the present invention.

[0026] In the figure: Mechanical component 1, fixed base 11, acceleration sensor chip 12, external frame 121, spring cantilever beam 122, mass block 123, first protruding top surface 12311, second protruding top surface 12312, lateral orientation modal converter 124, first lateral orientation modal converter 1241, second lateral orientation modal converter 1242, transmission waveguide 125, first transmission waveguide 1251, second transmission waveguide 1252, lower orientation modal converter 126, first lower orientation modal converter 1261, second lower orientation modal converter 1262;

[0027] Lens fiber 20, first lens fiber 201, second lens fiber 202;

[0028] Sensor optical path 2;

[0029] Light source 21, 3×3 coupler 22, photoelectric sensor 23, first photoelectric detector 231, second photoelectric detector 232, data acquisition circuit 24, computer 25. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0031] Example

[0032] Please see Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown in the figure, the present invention is based on an on-chip accelerometer with integrated optical waveguide, including mechanical component 1 and two lens optical fibers 20.

[0033] The mechanical component 1 includes a fixed base 11 and an acceleration sensor chip 12;

[0034] The fixed base 11 includes four fixing screw holes 111 and one optical fiber groove 112; furthermore, the fixed base has a downward cutout in the middle, the size of which is sufficient to suspend the spring-mass oscillator structure of the acceleration sensing chip 12 and allow it to move freely along the sensitive axis (x direction) under external acceleration; the fixing screw holes 111 are used to fix the mechanical component 1; the optical fiber groove 112 is used to fix the lens optical fiber 20.

[0035] The accelerometer chip 12 includes an external frame 121, a mass block 123, two sets of spring cantilever beams 122, two lateral mode converters 124, two sets of transmission waveguides 125, and two lateral mode converters 126. The external frame 121 has an overall U-shaped structure. The mass block 123 also includes two convex structures 1231, whose convex top surfaces act as reflective surfaces to reflect the laser emitted from the lateral mode converters 124, causing it to recouple back into the lateral mode converters 124. Two air cavities are formed between the lateral mode converters 124 and the two convex top surfaces of the mass block 123. The lengths of these cavities change under the influence of the relative displacement of the mass block 123, resulting in changes in the optical path. The transmission waveguides 125 adopt a symmetrical structure of the same length to form a Michelson-balanced interferometer structure, ensuring that the noise experienced by the two arms has the same characteristics and source, exhibiting a high degree of consistency and synchronization. During the processing, a 200nm layer of silver is sputtered onto the accelerometer chip 12 to increase the reflectivity of the raised top surface; the spring cantilever beam 122 adds a middle vertical beam 1222 to the serpentine folding beam 1221, and adds semi-circular small curved beams 1223 on both sides of the beam to increase the stiffness in the non-sensitive axis direction (non-x direction); furthermore, the spring cantilever beam 122 connects the outer frame 121 and the mass block 123 to form a spring-mass block oscillator structure;

[0036] The two lens fibers 20 are fixed to the fiber groove 112 of the fixed base 11 by optical ultraviolet structural adhesive; furthermore, the two lens fibers 20 are placed side by side and closely attached in the fiber groove 112 of the fixed base 11 and after being led out, in order to continue to maintain the balance of the interferometer structure.

[0037] Please see Figure 5 , Figure 5This is a schematic diagram of the optical path and circuitry of an embodiment of a push-pull MOEMS accelerometer based on a Michelson interferometer. As shown in the figure, the Michelson balanced interferometer structure, serving as the sensing optical path 2 in this embodiment, comprises a 3×3 coupler 22, a first lens fiber 201, a second lens fiber 202, a first raised top surface 12311, a second raised top surface 12312, a first photodetector 231, and a second photodetector 232. The first lens fiber 201 and the second lens fiber 202 are closely spaced side-by-side, their pigtails wound into a loop and fixed together with the 3×3 coupler 22 in the hollow space on the back of the mounting base 11. The light emitted from the light source 21 is injected into the 3×3 coupler 22 and then enters the first lens fiber 201 and the second lens fiber 202, respectively. It then passes through the first azimuth mode converter 1261 and the second azimuth mode converter 1262, respectively, and enters the first transmission waveguide 1251 and the second transmission waveguide 1252. Afterward, it exits through the first lateral mode converter 1241 and the second lateral mode converter 1242, forming two optical signals. These two optical signals are reflected by the first convex top surface 12311 and the second convex top surface 12312, respectively, and return along their original paths, recoupled back into the first lens fiber 201 and the second lens fiber 202. The two interference signals output from the 3×3 coupler 22 are then collected by the data acquisition circuit 24 and sent to the computer 25 for demodulation processing.

[0038] During the dynamic vibration response of the on-chip accelerometer, the on-chip accelerometer of this invention is fixed to the surface of the object being measured and moves accordingly. Then, the spring-mass structure moves relative to the outer frame 121. Since the first lateral modal converter 201 and the second lens fiber 202 are both on the outer frame 121, the mass block 123 will also move relative to it. The two air cavities formed by the first lateral modal converter 1251 and the first protruding top surface 12311, and the second lateral modal converter 1252 end face and the second protruding top surface 12312, respectively, serve as the sensing arms of the Michelson interferometer. Driven by the relative displacement of the mass block 123, the cavity lengths of the two air gaps change in a push-pull manner, that is, when the length of one cavity increases, the length of the other cavity must decrease, showing completely opposite trends. Therefore, the phase of light propagating in the sensing optical path 2 will change with the external vibration signal, and the total change in optical phase is twice that of the single sensing arm structure, thereby improving sensitivity. Finally, the optical interference signal is acquired by the photoelectric sensor 23 and the data acquisition circuit 24, and the vibration acceleration signal is demodulated by the computer 25.

[0039] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the implementation methods in the above embodiments. Various modifications or variations made without departing from the spirit of the present invention are all within the scope of this patent. The present invention should not be limited thereto.

Claims

1. An on-chip accelerometer based on an integrated optical waveguide, comprising a mechanical component (1) and two lens optical fibers (20), characterized in that: The mechanical component (1) includes a fixed base (11) and an acceleration sensing chip (12) fixed on the fixed base (11). The fixed base (11) has four fixing screw holes (111) for fixing the mechanical component (1) and an optical fiber slot (112) for fixing the lens optical fiber (20). The middle position of the fixed base (11) is hollowed out downwards. The size of the hollowing out allows the spring-mass oscillator structure of the acceleration sensing chip (12) to be suspended and to move freely along the sensitive axis under the action of external acceleration. The fixing screw holes (111) are used to fix and install the mechanical component (1). The acceleration sensing chip (12) includes an overall U-shaped external frame (121), a mass block (123), two sets of spring cantilever beams (122), two side azimuth mode converters (124), two sets of transmission waveguides (125) of the same length, and two bottom azimuth mode converters (126). The mass block (123) has two convex structures (1231) symmetrically distributed around its periphery. The top surface of these convex structures (1231) serves as a reflecting surface, reflecting and recoupling the laser emitted from the side azimuth mode converters (124) back into the side azimuth mode converters (124). The top surface of the convex structure (1231) of the bit mode converter (124) and the mass block (123) forms two air cavities. The cavity length of the air cavity changes under the action of the mass block (123) which undergoes relative displacement, thereby causing the optical path to change. The transmission waveguide (125) adopts a symmetrical structure of the same length to form a Michelson balanced interferometer structure, ensuring that the noise received by the two arms has the same characteristics and source, exhibiting a high degree of consistency and synchronization. The spring cantilever beam (122) connects the external frame (121) and the mass block (123) to form a spring-mass block oscillator structure. The two lens optical fibers (20) are tightly fixed inside the optical fiber groove (112).

2. The on-chip accelerometer based on integrated optical waveguide according to claim 1, characterized in that, During the processing, a metal film is sputtered onto the accelerometer chip (12) to increase the reflectivity of the raised top surface.

3. The on-chip accelerometer based on integrated optical waveguide according to claim 1, characterized in that, The lens fiber (20) includes a first lens fiber (201) and a second lens fiber (202).

4. The on-chip accelerometer based on integrated optical waveguide according to claim 3, characterized in that, The Michelson balanced interferometer structure, serving as the sensing optical path (2), includes a 3×3 coupler (22), a first lens fiber (201), a second lens fiber (202), a first convex top surface (12311), a second convex top surface (12312), a first photodetector (231), and a second photodetector (232). The light emitted by the light source (21) is injected into the 3×3 coupler (22) and split into two paths, which enter the first lens fiber (201) and the second lens fiber (202) respectively. Then, they enter the first transmission waveguide (1251) and the second transmission waveguide (1252) through the first lateral mode converter (1261) and the second lateral mode converter (1262) respectively. After that, they are emitted by the first lateral mode converter (1241) and the second lateral mode converter (1242) respectively, forming two optical signals. The two optical signals are reflected by the first convex top surface (12311) and the second convex top surface (12312) respectively, and return along the original path to be recoupled back to the first lens fiber (201) and the second lens fiber (202) respectively. The two interference signals output by the 3×3 coupler (22) are then collected by the data acquisition circuit (24) and sent to the computer (25) for demodulation processing after passing through the first photodetector (231) and the second photodetector (232) respectively.

5. The on-chip accelerometer based on integrated optical waveguide according to claim 3, characterized in that, The first lens fiber (201) and the second lens fiber (202) are closely attached to each other and their pigtails are wound into a loop and fixed together with the 3×3 coupler (22) in the hollow space on the back of the fixed base (11).

6. The on-chip accelerometer based on integrated optical waveguide according to any one of claims 1-5, characterized in that, During the dynamic vibration response of the on-chip accelerometer, the accelerometer is fixed to the surface of the object being measured and moves accordingly. The spring-mass structure moves relative to the external frame (121). The two air cavities formed by the first side orientation modal converter (1241) and the first convex top surface (12311) and the second side orientation modal converter (1242) and the second convex top surface (12312) respectively serve as the sensing arms of the Michelson interferometer. Driven by the mass block (123) which has a relative displacement, the cavity lengths of the two air gaps change in a push-pull manner, that is, when the length of one cavity increases, the length of the other cavity decreases. The phase of light propagating in the sensing optical path changes with the external vibration signal, and the total change of optical phase is twice that of the single sensing arm structure.

Images