A high temperature resistant packaging structure for sapphire MEMS fiber optic accelerometer

Abstract

This invention discloses a high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers, belonging to the field of fiber optic sensing technology. This invention aims to solve the problems of structural failure caused by thermal mismatch of heterogeneous materials in existing high-temperature sensors, and creep failure of traditional adhesive bonding processes at 1200℃. The packaging structure includes a hollow, double-threaded main body made of high-purity alumina ceramic or high-temperature resistant metal. A hexagonal flange is located in the middle of the main body, with symmetrically extending external threaded sections at both ends. A precision limiting ring is placed inside the main body to support the sapphire sensing element and provide an axial limiting reference for the ceramic or quartz fiber optic ferrule. By screwing the bottom support and top locking cap to both ends of the main body, a closed axial mechanical force chain is formed internally, rigidly pressing and fixing the precision limiting ring, sapphire sensing element, and fiber optic ferrule. This invention adopts a purely mechanical modular locking design, abandoning chemical adhesives, and utilizes independent limiting units to achieve stress decoupling and precise positioning, significantly improving the long-term stability and measurement accuracy of the sensor under extreme high-temperature and strong vibration environments.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing and precision instrument manufacturing technology, specifically to a sapphire MEMS fiber optic accelerometer packaging structure based on a high-temperature resistant material system (ceramic or high-temperature resistant metal), employing a modular mechanical locking and stress isolation design, and suitable for extreme high-temperature environments up to 1200℃. Background Technology

[0002] In cutting-edge fields such as aerospace propulsion systems, hypersonic vehicle thermal protection monitoring, and deep geothermal energy exploration, accelerometers must maintain long-term stable operation under extreme high-temperature and strong vibration coupling environments ranging from 1000℃ to 1200℃. Sapphire (single-crystal α-Al₂O₃), with its excellent high-temperature mechanical strength, chemical stability, and superior optical properties, has become an ideal sensing material for manufacturing high-temperature MEMS accelerometers. However, reliably encapsulating microscale sapphire sensing chips and applying them to the surface of macroscopic high-temperature components still faces significant technical bottlenecks. Existing sapphire high-temperature sensor packaging technologies mainly suffer from the following significant drawbacks: First, the thermal expansion mismatch between heterogeneous materials leads to structural failure. Existing packaging shells mostly use common high-temperature resistant metals (such as ordinary stainless steel), whose coefficient of thermal expansion (typically >16×10⁻⁶) is... -6 The temperature (°C) was significantly higher than that of sapphire (~7.0×10⁻⁶). -6 / ℃). During the thermal cycling process from room temperature to 1200℃, the volume expansion of the outer shell far exceeds that of the internal sensitive element. This severe "bimetallic effect" induces huge axial and radial thermal stresses. Without an effective buffer or isolation structure, the thermal stress will directly act on the brittle sapphire film, causing it to catastrophically fracture, or causing relative displacement of the reflecting surface of the Fabry-Perot (FP) interferometer cavity, resulting in irreversible drift of the cavity length and thus loss of measurement accuracy. Second, the reliability of the high-temperature connection medium is insufficient. Existing packaging processes heavily rely on organic adhesives, medium- and low-temperature inorganic solders, or glass sealing. These materials are prone to carbonization, pulverization, interface peeling, or softening creep at temperatures above 300℃ to 800℃, and cannot provide sufficient mechanical holding force at 1200℃. Even when using laser welding of metal structures, the grain coarsening and creep characteristics of the metal material in ultra-high temperature environments will lead to slight deformation, thereby compromising the alignment stability of the precision optical path. Third, there is a lack of effective installation stress isolation mechanisms. Traditional packaging structures are typically monolithic, with the accelerometer screwed directly into the object being measured via threads. Lacking independent internal support and limiting units, the enormous mechanical torque and preload applied during external installation force the housing to undergo elastic or plastic deformation. This deformation is transmitted undamped to the internal fiber optic ferrule and sensing element, causing unintended geometric distortion of the FP cavity in its initial state, severely deteriorating the sensor's scaling factor consistency and output linearity. Furthermore, existing ferrule positioning methods often lack precise axial references, making the sensor prone to optical path defocusing under severe vibration. Therefore, developing a purely mechanical modular packaging structure compatible with ceramic and high-temperature resistant metal materials, utilizing independent limiting units for stress decoupling, and completely abandoning traditional chemical bonding processes is a key technological requirement for achieving high-reliability acceleration measurement at 1200℃. Summary of the Invention

[0003] This invention aims to solve the technical problem in existing sapphire high-temperature sensor packaging technology where traditional organic adhesives or low-temperature inorganic solders are prone to carbonization, pulverization, or softening creep under extreme high-temperature environments above 1000℃, leading to connection failure. Simultaneously, this invention also strives to overcome the defects of thermal expansion mismatch between heterogeneous materials and installation stress coupling in existing structures. Specifically, it addresses the problem of thermal expansion of the metal casing and installation preload causing casing deformation, which is transmitted to the internal sensitive optical path, resulting in Fabry-Perot (FP) cavity length drift or sapphire diaphragm rupture. Furthermore, it addresses the shortcomings of traditional ferrule positioning, which lacks precise axial reference and is prone to optical path defocusing under strong vibrations.

[0004] To address the aforementioned technical problems, this invention provides a high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers, with a modular mechanical locking design at its core. The structure mainly comprises a hollow, double-threaded main body serving as the packaging skeleton. The material can be selected from high-purity alumina ceramic or high-temperature resistant metals such as nickel-based high-temperature alloys. A hexagonal flange for mounting and positioning is located in the center of the main body's outer surface, with symmetrically extending external threaded sections at both ends. A precision limiting ring is placed within the main body's inner cavity, serving as the core optical positioning reference. This ring is configured to support the sapphire sensing element on one hand and provide a precise axial stop surface for the fiber optic ferrule on the other. Furthermore, the structure includes a bottom support and a top locking cap that are threaded to both ends of the double-threaded main body, and a single-structure fiber optic ferrule, made of ceramic or quartz glass, which passes through the inner cavity of the main body. In terms of assembly, the bottom support provides upward axial support for the precision limiting ring, while the top locking cap restricts downward displacement of the fiber optic ferrule. The end face of the fiber optic ferrule directly abuts against the precision limiting ring, thus forming a pre-defined FP interference cavity with the sapphire sensing element. By tightening the bottom support and the top locking cap, a closed axial force chain is formed inside the double-threaded body, rigidly pressing and fixing the precision limiting ring, the sapphire sensing element, and the fiber optic ferrule axially.

[0005] Compared with existing technologies, this invention has significant advantages. First, the structure achieves purely mechanical, zero-adhesive encapsulation, eliminating all easily degraded chemical adhesives. Utilizing the immense axial mechanical force generated by the threaded engagement, combined with high-temperature resistant materials, the sensor can operate at 1200℃ for extended periods without structural loosening or creep. Second, by introducing an independent precision limiting ring, stress decoupling between the sensitive element's load-bearing capacity and the ferrule's limiting function is achieved. External installation stress is primarily borne by the double-threaded body, while the critical dimensions of the internal optical path are strictly defined by the protected precision limiting ring, significantly improving the sensor's scaling factor stability under strong vibrations and thermal shocks. Finally, this split-modular structure is compatible with all-ceramic or high-temperature resistant metal materials, offering strong adaptability and allowing for pre-assembly of internal components and pre-tuning of the optical path. In case of failure, it can be disassembled and maintained without damage, significantly reducing production costs and maintenance difficulty. Attached Figure Description

[0006] Figure 1 Overall axial sectional view of the packaging structure Figure 2 : A partially enlarged cross-sectional view of the FP cavity region Figure 3 : Block diagram of fiber optic accelerometer system Detailed Implementation

[0007] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Those skilled in the art, after understanding the technical solutions of this invention, can make modifications or equivalent substitutions to the technical solutions without departing from the spirit and scope of this invention.

[0008] Example 1: High-Temperature Resistant Packaging Structure and Component Details like Figures 1 to 3 As shown, this embodiment of the invention provides a high-temperature resistant packaging structure for a sapphire MEMS fiber optic accelerometer. Its core employs a purely mechanical modular locking design, abandoning traditional organic adhesives or low-temperature inorganic solders to withstand extreme high-temperature environments up to 1200℃. The packaging structure mainly includes a double-threaded body 3, a bottom support 7, a top locking cap 1, a precision limiting ring 6, a sapphire sensing element 5, a sapphire gasket 4, and a fiber optic ferrule 2 with an internally inserted fiber optic cable 8.

[0009] Specifically, the double-threaded body 3, serving as the core skeleton of the entire encapsulation structure, is a hollow tubular structure with a hexagonal flange 33 at the center of its outer surface, facilitating installation and positioning on external components or applying torque with a wrench during assembly. The double-threaded body 3 has symmetrically extending external thread sections at both axial ends (such as an upper external thread 31 and a lower external thread 32). In this embodiment, the double-threaded body 3, the bottom bearing seat 7, the top locking cap 1, and the precision limiting ring 6 are all preferably made of high-temperature resistant materials. To minimize thermal expansion mismatch, the high-temperature resistant material can be selected from high-purity alumina ceramics with a purity greater than 99%, nickel-based high-temperature alloys, titanium alloys, or stainless steel, depending on the actual application scenario. When the double-threaded body 3 is made of ceramic material, the precision limiting ring 6 is also made of a ceramic material with a matching coefficient of thermal expansion, and the two are in a clearance fit or transition fit to provide buffer space for thermal expansion.

[0010] In the core sensitive optical path area (such as Figure 2 and Figure 3As shown, the precision limiting ring 6 is placed at the bottom of the inner cavity of the double-threaded body 3. This precision limiting ring 6 is designed to stably support the sapphire sensing element 5. Of particular note is that a sapphire gasket 4 is provided above the sapphire sensing element 5 and between it and the front end face of the fiber optic ferrule 2. The sapphire gasket 4 has a coaxial through-hole at its center. Its primary purpose is to act as a buffer layer, preventing the rigid end face of the fiber optic ferrule 2 from directly squeezing or scratching the fragile sensitive diaphragm on the sapphire sensing element 5. Simultaneously, the thickness of the sapphire gasket 4 defines the distance between the end face of the fiber optic 8 and the reflecting surface of the sapphire sensing element 5. Working in conjunction with the precision limiting ring 6, it provides a high-precision axial limiting reference for the fiber optic ferrule 2, thereby precisely forming a Fabry-Perot (FP) interference cavity with a preset initial cavity length L.

[0011] The fiber optic ferrule 2 is inserted into the inner cavity of the double-threaded body 3, and a single-mode fiber 8 is fixed through its center. The fiber optic ferrule 2 is preferably a monolithic structure made of ceramic or quartz glass.

[0012] In the assembled state, the bottom support 7 is screwed onto the lower external thread 32 of the double-threaded body 3, and its inner bottom surface axially supports the precision limiting ring 6 upward. The top locking cap 1 is screwed onto the upper external thread 31 of the double-threaded body 3, pressing the fiber optic ferrule 2 downward. By tightening the bottom support 7 and the top locking cap 1 respectively, a closed and rigid axial force chain is formed inside the double-threaded body 3. This purely mechanical clamping method firmly presses the precision limiting ring 6, the sapphire sensitive element 5, the sapphire gasket 4, and the fiber optic ferrule 2 axially, achieving effective decoupling of installation stress from the internal sensitive optical path.

[0013] Example 2: Assembly method of high temperature resistant packaging structure Based on the above packaging structure, this embodiment of the invention also provides a corresponding assembly method, including the following steps: 1. Preparation and cleaning: Provide pre-processed and cleaned double-threaded body 3, bottom support 7, top locking cap 1, precision limiting ring 6, sapphire sensitive element 5, sapphire gasket 4 and fiber optic ferrule 2; 2. Internal component stacking: The sapphire sensitive element 5 is placed stably on the bearing surface of the precision limiting ring 6, and then the sapphire gasket 4 is aligned and stacked on the sapphire sensitive element 5, and this assembly is installed into the inner cavity of the double-threaded body 3. 3. Bottom locking: Screw the bottom bearing seat 7 onto the bottom end of the double-threaded body 3 until its inner bottom surface axially presses against the precision limiting ring 6 to fix its position; 4. Inserting the ferrule and locking the cavity length: Insert the fiber optic ferrule 2, with the fiber optic cable 8 threaded through it, from the upper end of the double-threaded body 3 until its front end face is firmly against the upper surface of the sapphire gasket 4. At this point, the initial cavity length L of the FP interference cavity is precisely locked by the thickness of the sapphire gasket 4; 5. Top Pre-tightening for Complete Sealing: Screw the top locking cap 1 onto the top of the double-threaded body 3. In steps 3) and 5), a torque wrench is used for tightening to ensure that the axial pre-tightening force applied to the tail end of the fiber optic ferrule 2 meets the preset standard, thus completing the overall sealing.

[0014] Example 3: Fiber Optic Accelerometer Sensing System like Figure 3 As shown, this embodiment of the invention also provides an optical fiber accelerometer sensing system, comprising: Front-end sensitive detection part: namely the high-temperature resistant packaging structure in Example 1 and the sapphire MEMS fiber optic accelerometer encapsulated inside it, which can be directly installed on the surface of equipment subject to extreme high temperature and vibration. The back-end signal processing section includes an optical demodulator (preferably a white light interferometer or a spectrometer) and a signal processing unit, which are optically connected to the fiber optic accelerometer. The optical demodulator emits probe light, which passes through the central circular hole of the sapphire pad 4 via the optical fiber 8, and is reflected on the diaphragm of the sapphire sensitive element 5 to form an interference spectrum signal. When vibrational acceleration is present, the diaphragm deformation causes a change in cavity length. The optical demodulator receives the reflected signal, and the signal processing unit demodulates and calculates the real-time acceleration physical quantity.

Claims

1. A high-temperature resistant packaging structure for a sapphire MEMS fiber optic accelerometer, comprising a sapphire sensing element (5) and an optical fiber (8), characterized in that: The optical fiber (8) is fixed in the optical fiber ferrule (2). The sapphire sensing element (5) and the optical fiber ferrule (2) are encapsulated in the double-threaded body (3). The double-threaded body (3) has a through hole in the center to accommodate the optical fiber ferrule (2). The lower end of the through hole is connected to the recess. The sapphire sensing element (5) and the sapphire gasket (4) are placed in the recess. The sapphire gasket (4) is located on the upper surface of the sapphire sensing element (5). The opening in the center of the sapphire gasket (4) serves as the Fabry-Perot interference cavity between the optical fiber ferrule (2) and the sapphire sensing element (5).

2. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 1, characterized in that: A precision limiting ring (6), a sapphire sensitive element (5), and a sapphire gasket (4) are arranged sequentially from the outside to the inside in the settling tank.

3. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 2, characterized in that: The lower end face of the double-threaded body (3) is encapsulated with a bottom bearing seat (7), and the upper end face is encapsulated with a top locking cap (1). Both the bottom bearing seat (7) and the top locking cap (1) are cap-shaped and connected to the cylindrical double-threaded body (3) by threads. The top locking cap (1) has a through hole in the center for the optical fiber (8) to pass through.

4. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 3, characterized in that: The double-threaded body (3) has a hexagonal flange in the middle of its outer surface, and external thread sections extend symmetrically from both ends of the hexagonal flange.

5. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 3, characterized in that: The materials of the double-threaded body (3), bottom bearing seat (7), top locking cover (1) and precision limiting ring (6) are all high-temperature resistant materials; the high-temperature resistant materials are selected from one of high-purity alumina ceramics with a purity greater than 99%, nickel-based high-temperature alloys, titanium alloys or stainless steel; when the double-threaded body (3) is made of ceramic material, the precision limiting ring (6) is also made of ceramic material with a matching coefficient of thermal expansion; the fiber optic ferrule is made of ceramic material or quartz glass material; the fiber optic ferrule is a single structure with a through fiber micro-hole in its center.

6. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 4, characterized in that: The external thread sections at both ends of the double-threaded body (3) are used to mechanically engage with the bottom bearing seat (7) and the top locking cover (1); by tightening the bottom bearing seat (7) and the top locking cover (1), a closed axial force chain is formed inside the double-threaded body (3), which presses and fixes the precision limiting ring (6), the sapphire sensitive element (5) and the fiber optic ferrule (2).

7. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 2, characterized in that: The precision limiting ring (6) is a flat ring structure with a coaxial through positioning hole in its center for aligning the fiber optic ferrule and determining the Fabry-Perot cavity length; the outer circumferential edge of the precision limiting ring (6) extends radially outward to form at least one positioning lug, which is used for circumferential positioning and fixing.

8. The high-temperature resistant packaging structure for sapphire MEMS fiber optic accelerometers according to claim 4, characterized in that... The assembly method includes the following steps: 1) Provide the pre-processed double-threaded body (3), bottom bearing seat (7), top locking cover (1), sapphire gasket (4), sapphire sensitive element (5), precision limiting ring (6) and fiber optic ferrule (2). 2) Place the sapphire gasket (4), sapphire sensitive element (5), and precision limiting ring (6) into the groove on the lower end face of the double-threaded body (3) in sequence; 3) Screw the bottom bearing seat (7) onto the lower end of the double-threaded body (3) until its inner bottom surface axially presses against the precision limiting ring (6) to fix its position; 4) Insert the fiber optic ferrule (2) into the through hole of the double-threaded body (3) until its front end face abuts against the limiting surface of the sapphire gasket (4), thereby locking the initial cavity length; 5) Screw the top locking cap (1) onto the upper end of the double-threaded body (3) and apply axial preload to the tail end of the fiber optic ferrule (2) to complete the overall encapsulation.

9. The high-temperature resistant packaging structure for a sapphire MEMS fiber optic accelerometer according to claim 8, characterized in that, The engagement process in steps 3) and 5) is performed using a constant torque wrench to ensure that the axial clamping force applied to the internal components meets the preset standard.

10. The high-temperature resistant packaging structure for a sapphire MEMS fiber optic accelerometer according to any one of claims 1-7, characterized in that: An optical demodulator is used to transmit probe light to the optical fiber (8) and receive the interference spectrum signal reflected back through the Fabry-Perot interferometer. The acceleration physical quantity is calculated by the cavity length change of the demodulated spectrum signal.

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