Novel optical air cavity suitable for adjustable sensitivity strain sensing and application thereof

Abstract

The invention relates to the field of strain sensing, and discloses a novel optical air cavity suitable for adjustable sensitivity strain sensing and application thereof, and the core is that a long optical air cavity with a small free spectral range is manufactured through chemical etching and delayed tapering welding procedures of an optical fiber fusion splicer; the long optical air cavity serving as a sensing arm is cascaded with a simulation reference arm generated through simulation to form a vernier effect, sensitivity amplification is achieved, the free spectrum of the simulation reference arm is adjustable, and therefore the amplification factor of the sensitivity is adjustable. The structure does not need special optical fibers, is low in cost and high in repeatability, and is suitable for industrial detection, complex environment monitoring and other scenes.

Description

Technical Field

[0001] The present invention relates to the field of strain sensing, and in particular to a novel optical air cavity suitable for adjustable sensitivity strain sensing and applications thereof. Background Art

[0002] Fiber optic sensors, with their unique advantages such as miniaturization, resistance to environmental corrosion, and electromagnetic interference, have become core sensing elements for high-precision industrial testing and complex environmental monitoring. While relatively mature in fabrication, strain sensors based on grating and interference principles generally face the challenge of temperature interference caused by the combined effects of thermal expansion and thermo-optical effects. This inherent flaw restricts the sensor's measurement reliability in extreme temperature scenarios. In contrast, air cavity Fabry-Perot interferometers, by employing a gaseous medium, effectively avoid the thermal sensitivity of solid-liquid materials, exhibiting significant temperature decoupling characteristics and providing a new path for constructing strain sensors with low temperature crosstalk.

[0003] The performance improvement of existing air cavity Fabry-Perot interferometers mainly focuses on the optimization of the air cavity structure: researchers have tried to enhance the strain response from the geometric configuration dimension of the air cavity through processing methods such as multimode fiber fusion, special fiber etching, and single-mode fiber tapering. However, such solutions are still subject to two key bottlenecks: first, the traditional air cavity size design fails to fully release the physical correlation between strain and optical phase modulation, resulting in limited room for sensitivity improvement; second, the existing structure lacks systematic design in the coordinated optimization of light field coupling efficiency and strain transmission path, making it difficult to break through the theoretical threshold of sensitivity. Therefore, the development of a new optical air cavity architecture with a dynamic response enhancement mechanism to achieve simultaneous optimization of strain sensitivity and environmental interference resistance has become an important technical challenge in this field. To this end, we propose a new optical air cavity suitable for adjustable sensitivity strain sensing and its application. Summary of the Invention

[0004] (1) Technical problems solved

[0005] In view of the deficiencies in the prior art, the present invention provides a novel optical air cavity suitable for adjustable sensitivity strain sensing and its application, which solves the above-mentioned problems.

[0006] (2) Technical solution

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: a method for preparing a novel optical air cavity suitable for adjustable sensitivity strain sensing, comprising the following specific steps:

[0008] Step 1: First, strip the coating of two SMF (single-mode fibers), clean them with alcohol, and then cut the fiber ends flat with a fiber cleaver;

[0009] Step 2: Immerse one of the SMFs (single-mode fibers) in a 40% hydrofluoric acid solution for 10 minutes. Since the chemical corrosion rate of the germanium-doped quartz fiber core is faster than that of the pure quartz cladding, a concave surface is formed.

[0010] Step 3: The alcohol-cleaned SMF and the chemically etched SMF are then placed in the fiber fusion splicer. Since the etched SMF has a smaller diameter, the splicer's motor is manually adjusted so that the electrode is slightly biased toward the larger diameter SMF to ensure stress symmetry during the tapered welding process.

[0011] Step 4: A long optical air cavity is obtained by controlling the discharge intensity, discharge time, delay time and taper length of the welder in the optical fiber fusion splicer.

[0012] Preferably, the discharge power and discharge time of the welding machine in step 4 are set to "+80bit" and "1200ms;

[0013] The delay time and speed of cone welding are "700ms" and "0.9μm / ms" respectively;

[0014] The taper length is "200 μm".

[0015] Preferably, a novel optical air cavity is prepared by the above preparation method, which is manufactured by a delayed taper welding procedure of a chemical etching and fiber fusion machine to produce a long optical air cavity with a small free spectral range, with a length of 184.25 μm and a taper waist diameter of 62.30 μm.

[0016] Preferably, a novel optical air cavity is applied to a strain sensor, and a long optical air cavity is used as a sensing arm to be cascaded with a simulated reference arm generated by simulation to form a vernier effect, thereby achieving sensitivity amplification.

[0017] Preferably, an application of a novel optical air cavity specifically includes: the optical air cavity has two reflective surfaces, respectively labeled M1 and M2; light with an initial intensity of I0 is emitted from a supercontinuum light source and enters a single-mode optical fiber from the left. When reaching M1, the light undergoes a first reflection with a reflection intensity of I1; in the air cavity with a length of L, the light undergoes a second reflection at M2 and is then coupled into an output optical fiber with a reflection intensity of I2; since the reflectivity at the quartz / air interface is approximately 4%, higher-order reflections can be ignored, thereby simplifying the interference theory;

[0018] Therefore, the two reflected beams I1 and I2 coupled at the center of the SMF will be regarded as two-beam interference, and the interference between the two coherent lights can be expressed by the following formula:

[0019]

[0020] Where θ is the phase difference caused by the transmission of light in the air cavity. For two-beam interference, the reflection spectrum exhibits a cosine characteristic, and the free spectral range (FSR) of the interference fringes can be expressed as:

[0021]

[0022] Where m is the interference order of the fiber interferometer, λ m Corresponding to this resonant wavelength, n is the refractive index of air, and L is the length of the optical air cavity. The effect of strain on the sensor will cause a change in the length of the optical air cavity, resulting in a wavelength shift in the interference spectrum. The displacement of the interference fringes is expressed by the following formula:

[0023] Δλ=kε. (3)

[0024] Where Δλ is the wavelength change, k is the strain sensitivity of the sensor, and ε is the strain experienced by the sensor;

[0025] In order to amplify the strain sensitivity through the vernier effect, a reference arm should be cascaded. Usually, the reference arm only serves as a benchmark and should be kept as stable as possible. Therefore, a simulated reflection spectrum matching the free spectral range of the sensing arm can be generated as the reference arm. The free spectral range of the vernier envelope generated by superimposing the sensing arm and the reference arm is defined as:

[0026]

[0027] Where FSR R , FSR S is the free spectral range of the reference spectrum and the sensor arm reflection spectrum. When the wavelength change of the sensor reflection spectrum is Δλ, the wavelength change of the cursor envelope is M×Δλ, where M represents the amplification factor, which is defined as:

[0028]

[0029] By adjusting the FSR R , which can achieve an adjustable sensitivity amplification factor to adapt to various measurement scenarios;

[0030] The temperature sensitivity of the sensor is defined as:

[0031]

[0032] Where ΔT is the temperature change, α is the thermal expansion coefficient of the material, κ is the thermo-optic coefficient, and λ is the wavelength of the interference peak.

[0033] Preferably, the free spectral range (FSR) of the sensing arm is ≤8.01 nm.

[0034] Preferably, the vernier effect enables strain sensitivity to be adjustable within a range of 3.23 pm / με to 36.32 pm / με.

[0035] Preferably, the wavelength drift of the sensing arm is ≤0.04 nm at 30-100°C.

[0036] (3) Beneficial effects

[0037] Compared with the prior art, the present invention provides a novel optical air cavity suitable for adjustable sensitivity strain sensing and its application, which has the following beneficial effects:

[0038] Adjustable strain sensitivity and significant amplification: A cascaded analog reference arm creates a vernier effect, allowing for flexible adjustment of sensitivity amplification based on the measurement scenario. The measured strain sensitivity can range from 3.23pm / με to 36.32pm / με, meeting diverse requirements from high-precision microstrain monitoring to wide-range strain detection, enhancing the sensor's applicability.

[0039] Strong resistance to temperature interference: The sensor is primarily made of silicon dioxide and air, both of which have low thermal expansion and thermo-optical coefficients. Within the 30-100°C temperature range, the maximum wavelength drift of the sensing arm is only 0.04nm, effectively suppressing temperature crosstalk and maintaining measurement reliability in extreme temperature environments.

[0040] Low cost and simple process: No specialized optical fibers or complex processing techniques are required; fabrication can be performed using only single-mode optical fibers and a conventional fiber fusion splicer. Completed through chemical etching and a delayed taper welding process in a fiber fusion splicer, the fabrication process is highly repeatable, reducing material and process costs and facilitating large-scale deployment.

[0041] Good synergy between light field coupling and strain transfer: Through reasonable design, the air cavity structure has been systematically optimized in terms of light field coupling efficiency and strain transfer path, breaking through the theoretical threshold of traditional solutions, allowing the physical correlation between strain and optical phase modulation to be fully released, and enhancing the performance of the sensor.

[0042] Wide measurement range and high linearity: The strain measurement range is 0-800με, covering a variety of strain detection scenarios. When different reference arms are cascaded, the sensitivity linear fitting correlation coefficient (R 2 ) are all above 0.993, indicating that the measurement results have good linearity and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the manufacturing process of the novel optical air cavity of the present invention;

[0044] Figure 2 Schematic diagram of a micrograph of an example of the novel optical air cavity of the present invention;

[0045] Figure 3 This is a schematic diagram of the working principle of the novel optical air cavity of the present invention;

[0046] Figure 4 Schematic diagram of the reflection spectrum of the optical air cavity prepared with different fusion parameters in the present invention.

[0047] Figure 5 Schematic diagram of the linear fitting results of the cascaded different reference arm sensitivities of the present invention. DETAILED DESCRIPTION

[0048] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0049] See also Figure 1-5 A novel optical air cavity suitable for adjustable sensitivity strain sensing and its application is described. The core of the invention is the fabrication of a long optical air cavity with a small free spectral range through chemical etching and delayed taper welding using a fiber fusion splicer. This long optical air cavity serves as the sensing arm, cascaded with a simulated reference arm to create a vernier effect, achieving sensitivity amplification. The free spectrum of the simulated reference arm is adjustable, thereby enabling adjustable sensitivity amplification.

[0050] Figure 1 This is the production process of LAFPI. Figure 1 As shown in (I) and (II), the coatings of two single-mode optical fibers (SMF, YOFCG652, S8.3 / 125) were first stripped, cleaned with alcohol, and then the fiber ends were cut flat using a fiber cleaver. One of the single-mode optical fibers was immersed in a 40% hydrofluoric acid solution for 10 minutes. Since the chemical etching rate of the germanium-doped quartz fiber core is faster than that of the pure quartz cladding, a concave surface is formed. Figure 1 As shown in (III), the SMF and the chemically etched SMF were cleaned with alcohol and placed in a fiber fusion splicer (Fujikura FSM100P). Since the etched SMF had a smaller diameter, the motor of the splicer was manually adjusted so that the electrode was slightly biased toward the SMF with a larger diameter to ensure stress symmetry during the tapered welding process. The length of the air cavity is regulated by the discharge intensity, discharge time, delay time, and taper length. In order to obtain the longest possible air cavity, the discharge power and discharge time of the welder were set to "+80bit" and "1200ms", and the delay time and speed of the tapered welding were "700ms" and "0.9μm / ms" respectively, and the taper length was "200μm". The structure formed after electrode discharge taper welding is shown in Figure 2. Figure 1(IV) As shown in the figure. The length of the air microcavity produced by the above process is 184.25 μm, and the waist diameter is 62.30 μm. Figure 2 shown.

[0051] Figure 3 The working principle of the new optical air cavity is described. The optical air cavity has two reflecting surfaces, marked as M1 and M2. Light with an initial intensity of I0 is emitted from a supercontinuum light source and enters a single-mode optical fiber from the left. When it reaches M1, the light undergoes a first reflection with a reflection intensity of I1. In the air cavity with a length of L, the light undergoes a second reflection at M2 and is then coupled into the output optical fiber with a reflection intensity of I2. Since the reflectivity at the quartz / air interface is about 4%, higher-order reflections can be ignored, simplifying the interference theory. Therefore, the two reflected beams I1 and I2 coupled at the center of the SMF will be regarded as two-beam interference. The interference between two coherent lights can be expressed by the following formula:

[0052]

[0053] Where θ is the phase difference caused by the transmission of light in the air cavity. For two-beam interference, the reflection spectrum shows a cosine characteristic. The free spectral range (FSR) of the interference fringes can be expressed as:

[0054]

[0055] Where m is the interference order of the fiber interferometer. m Corresponding to this resonant wavelength, n is the refractive index of air, and L is the length of the optical air cavity. The effect of strain on the sensor causes a change in the optical air cavity length, resulting in a wavelength shift in the interference spectrum. The displacement of the interference fringes is expressed by the following formula:

[0056] Δλ=kε. (3)

[0057] Where Δλ is the wavelength change, k is the strain sensitivity of the sensor, and ε is the strain experienced by the sensor.

[0058] To amplify strain sensitivity through the vernier effect, a reference arm should be cascaded. Typically, the reference arm serves only as a benchmark and should be kept as stable as possible. Therefore, a simulated reflection spectrum matching the free spectral range of the sensing arm can be generated as the reference arm. The free spectral range of the vernier envelope generated by superimposing the sensing and reference arms is defined as:

[0059]

[0060] Where FSR R , FSR Sis the free spectral range of the reference spectrum and the sensor arm reflection spectrum. When the wavelength change of the sensor's reflection spectrum is Δλ, the wavelength change of the cursor envelope is M×Δλ. M represents the amplification factor, which is defined as:

[0061]

[0062] By adjusting the FSR R , an adjustable sensitivity amplification factor can be achieved to adapt to various measurement scenarios.

[0063] The temperature sensitivity of the sensor is defined as:

[0064]

[0065] Where ΔT is the temperature change, α is the material's thermal expansion coefficient, κ is the thermo-optic coefficient, and λ is the wavelength of the interference peak. The sensor's temperature sensitivity is primarily determined by the material's thermal expansion coefficient, thermo-optic coefficient, and the wavelength of the interference peak. The sensor's primary materials are silicon dioxide and air, which have low thermo-optic and thermal expansion coefficients and are less susceptible to deformation under temperature changes. Therefore, the sensor has good resistance to temperature interference.

[0066] Figure 4 The spectra of the air microcavity prepared under different welding parameters: (a) is without cascade reference arm; (b) is after cascade matching reference arm. Figure 4 As shown in (a), the discharge intensity range is 0-80dB, and the minimum free spectrum range of the air microcavity can reach about 8.01nm. When the discharge intensity is +80dB, the delayed cone welding is turned on (red curve), and the free spectrum range of the prepared sensor arm is only 6.52nm. Figure 4 As shown in (b), the reflection spectra of different free spectral ranges are superimposed on the simulated reference arm signal that matches them. Obviously, for the sensor arm with a smaller free spectral range, its envelope free spectral range is also smaller, the contrast is larger, and the vernier effect is more obvious. Therefore, this optical air cavity design can make the sensor arm have a smaller free spectral range, thereby improving the fitting accuracy of the vernier envelope. Figure 5 As shown in the figure, a simulated reference arm was designed based on the formula and simulation, and then cascaded with the sensing arm. Measurements show that the strain sensitivity of the sensor can be amplified from 3.23 pm / με to 36.32 pm / με with the cascaded virtual reference arm, with a strain measurement range of 0-800 με. In temperature testing, the maximum wavelength drift of the sensing arm was only 0.04 nm at 30-100°C, demonstrating excellent resistance to temperature interference.

[0067] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A novel method for preparing an optical air cavity suitable for adjustable sensitivity strain sensing, characterized in that: The specific steps include: Step 1: First, strip the coating of two SMF (single-mode fibers), clean them with alcohol, and then cut the fiber ends flat with a fiber cleaver; Step 2: Immerse one of the SMFs (single-mode fibers) in a 40% hydrofluoric acid solution for 10 minutes. Since the chemical corrosion rate of the germanium-doped quartz fiber core is faster than that of the pure quartz cladding, a concave surface is formed. Step 3: The alcohol-cleaned SMF and the chemically etched SMF are then placed in the fiber fusion splicer. Since the etched SMF has a smaller diameter, the splicer's motor is manually adjusted so that the electrode is slightly biased toward the larger diameter SMF to ensure stress symmetry during the tapered welding process. Step 4: A long optical air cavity is obtained by controlling the discharge intensity, discharge time, delay time and taper length of the welder in the optical fiber fusion splicer.

2. The novel optical air cavity preparation method for adjustable sensitivity strain sensing according to claim 1, characterized in that: In step 4, the discharge power and discharge time of the welding machine are set to "+80bit" and "1200ms; The delay time and speed of cone welding are "700ms" and "0.9μm / ms" respectively; The taper length is "200μm".

3. A novel optical air cavity, characterized in that: It is prepared by the preparation method described in any one of claims 1 or 2, which is produced by a delayed taper welding procedure of a chemical etching and fiber fusion machine to produce a long optical air cavity with a small free spectral range, with a length of 184.25 μm and a taper waist diameter of 62.30 μm.

4. Application of the novel optical air cavity according to claim 3, characterized in that: The novel optical air cavity is applied to a strain sensor, and the long optical air cavity is used as a sensing arm and cascaded with an analog reference arm generated by simulation to form a vernier effect, thereby achieving sensitivity amplification.

5. The application of the novel optical air cavity according to claim 4, characterized in that: Specifically include: The optical air cavity has two reflective surfaces, labeled M1 and M2. Light with an initial intensity of I0 is emitted from a supercontinuum light source and enters a single-mode optical fiber from the left. When it reaches M1, the light undergoes a first reflection with a reflection intensity of I1. In the air cavity with a length of L, the light undergoes a second reflection at M2 and is then coupled into the output optical fiber with a reflection intensity of I2. Since the reflectivity at the quartz / air interface is approximately 4%, higher-order reflections can be ignored, thereby simplifying the interference theory. Therefore, the two reflected beams I1 and I2 coupled at the center of the SMF will be regarded as two-beam interference, and the interference between the two coherent lights can be expressed by the following formula: Where θ is the phase difference caused by the transmission of light in the air cavity. For two-beam interference, the reflection spectrum exhibits a cosine characteristic, and the free spectral range (FSR) of the interference fringes can be expressed as: Where m is the interference order of the fiber interferometer, λ m Corresponding to this resonant wavelength, n is the refractive index of air, and L is the length of the optical air cavity. The effect of strain on the sensor will cause a change in the length of the optical air cavity, resulting in a wavelength shift in the interference spectrum. The displacement of the interference fringes is expressed by the following formula: Δλ=kε. (3) Where Δλ is the wavelength change, k is the strain sensitivity of the sensor, and ε is the strain experienced by the sensor; In order to amplify the strain sensitivity through the vernier effect, a reference arm should be cascaded. Usually, the reference arm only serves as a benchmark and should be kept as stable as possible. Therefore, a simulated reflection spectrum matching the free spectral range of the sensing arm can be generated as the reference arm. The free spectral range of the vernier envelope generated by superimposing the sensing arm and the reference arm is defined as: Where FSR R , FSR S is the free spectral range of the reference spectrum and the sensor arm reflection spectrum. When the wavelength change of the sensor reflection spectrum is Δλ, the wavelength change of the cursor envelope is M×Δλ, where M represents the amplification factor, which is defined as: By adjusting the FSR R , which can achieve an adjustable sensitivity amplification factor to adapt to various measurement scenarios; The temperature sensitivity of the sensor is defined as: Where ΔT is the temperature change, α is the thermal expansion coefficient of the material, κ is the thermo-optic coefficient, and λ is the wavelength of the interference peak.

6. Application of the novel optical air cavity according to claim 5, characterized in that: The free spectral range (FSR) of the sensing arm is ≤8.01 nm.

7. Application of the novel optical air cavity according to claim 5, characterized in that: The vernier effect enables the strain sensitivity to be adjusted within a range of 3.23 pm / με to 36.32 pm / με.

8. The application of the novel optical air cavity according to claim 5, characterized in that: The wavelength drift of the sensing arm is ≤0.04nm at 30-100℃.

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