Fabry-Perot microcavity strain sensor based on conical hollow-core optical fiber

Abstract

The invention relates to a Fabry-Perot microcavity strain sensor based on a conical hollow-core optical fiber. The sensor not only has the advantages of being simple to manufacture, small in size and high in strain sensitivity, but also has very low strain and temperature crosstalk due to the fact that the sensor is made of a single quartz material. Interference caused by temperature can be ignored within a certain error range. Therefore, the structure can be widely applied to various fields of building structure monitoring, deep well exploration, chemical and biological sensing and the like.

Description

Technical Field

[0001] The present application belongs to the field of optical fiber sensing, and specifically relates to a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber. Background Art

[0002] Optical fiber strain sensors have broad application prospects in many professional fields that require high-precision strain measurement, such as engineering monitoring, clinical medicine, and micro-machining, due to their advantages of anti-electromagnetic interference, ultra-small size, corrosion resistance, and high sensitivity. So far, different types of sensors have been developed to meet various needs. They are mainly based on Mach-Zehnder interferometers, Fabry-Perot interferometers, Bragg gratings, and long-period gratings. However, except for the Fabry-Perot interferometer, all of the above sensors have high temperature sensitivity, which makes them a potential source of temperature error in strain measurement. In order to solve this problem, temperature compensation can be introduced, but it will increase the complexity of the system. The Fabry-Perot interferometer has a wide range of application value in work that requires precise strain measurement due to its advantages such as flexible manufacturing, simple operation, convenient combination, and insensitivity to temperature.

[0003] The mechanism of strain measurement by Fabry-Perot interferometer is to obtain the strain change trend by observing the change of FP cavity length. In the closed cavity structure, the FP cavity is not in direct contact with the external environment, and the strain can be detected by monitoring the change of the reflection spectrum caused by the change of FP cavity length. The strain sensitivity of Fabry-Perot interferometer depends on the shape and size of the microcavity. So far, many articles have reported strain sensors based on air cavity. For example, in 2014, Zhang et al. spliced ​​a sapphire derivative fiber between a single-mode fiber and a capillary to form a Fabry-Perot interferometer, but they used a chemical etching method, which increased the risk level in the preparation process; in 2018, Zhang et al. inserted a tapered fiber tip into the capillary, and then spliced ​​the other end of the capillary with a single-mode fiber. The tapered fiber end face and the splicing surface formed an FP cavity. Since the inserted cone tip has the degree of freedom along the fiber axis, when the Fabry-Perot interferometer is subjected to temperature changes, the thermal expansion of the fiber cone tip will resist the change in FP cavity length caused by the change in capillary length. This characteristic will form a large temperature crosstalk in strain measurement. In 2020, Ke et al. fabricated an FP cavity by embedding microspheres into a hollow-core fiber with a suspended three-rod. This method not only has a complicated preparation process, but also uses expensive special hollow-core fibers, which makes it difficult to promote to practical applications. In 2023, Gang et al. only achieved a strain sensitivity of 3.67 pm / µɛ by fusing two sections of single-mode fiber and a section of tapered hollow-core fiber to form an FP cavity. Therefore, it is of practical significance to explore a fiber strain sensor with high sensitivity, low temperature crosstalk, low cost, simple preparation, safety and reliability.

[0004] The designed Fabry-Perot microcavity strain sensor based on tapered hollow-core optical fiber not only has the advantages of simple fabrication, small size, and high strain sensitivity, but also has very low strain and temperature crosstalk because the sensor is made of a single quartz material. The interference caused by temperature can be ignored within a certain error range. Therefore, this structure can be widely used in various fields such as building structure monitoring, deep well exploration, chemical and biological sensing, etc. Summary of the invention

[0005] The embodiment of the present application provides a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber to overcome the above-mentioned problems.

[0006] In a first aspect, the present application provides a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber, the device comprising a broadband light source, a circulator, a spectrum analyzer and a sensor head. The output end of the broadband light source and the input end of the spectrum analyzer are respectively connected to the port of the circulator through a single-mode optical fiber;

[0007] In a second aspect, an embodiment of the present application provides a method for manufacturing a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber, the method comprising:

[0008] A. Fabrication of a microcavity Fabry-Perot interferometer without a tapered region;

[0009] A-1. Strip off the coating of two sections of single-mode optical fiber, then use a non-woven cloth dipped in alcohol to repeatedly wipe the two sections of single-mode optical fiber with the coating stripped off in one direction, and then use a fiber cleaver to cut the end face of the single-mode optical fiber flat;

[0010] A-2. Use a flame to briefly burn the coating of a hollow-core optical fiber with an inner diameter of 50μm. When the coating turns black, use a non-woven cloth dipped in alcohol to repeatedly wipe the hollow-core optical fiber in one direction to remove the coating, and then use a fiber cleaver to cut the end face of the hollow-core optical fiber flat.

[0011] A-3. Place a section of single-mode fiber with flat end faces and a hollow-core fiber with flat end faces in the fusion splicer. Then set the pre-melting power of the fusion splicing mode to 30 bits, the pre-melting time to 150 ms, the main discharge power to 45 bits, and the main discharge time to 2200 ms. Then use the preset mode to fusion splice the single-mode fiber with flat end faces and the hollow-core fiber with flat end faces together.

[0012] A-4. Place the fused single-mode-hollow-core structure under the CCD camera, and accurately push the single-mode-hollow-core structure through the micro-displacement control platform, and measure out a 150μm hollow-core fiber between the fusion point and the fiber cleaver. Then use the fiber cleaver to cut the end face flat to obtain the ideal length of hollow-core fiber.

[0013] A-5. Repeat the operation of A-3 with a section of single-mode optical fiber with a flat end face and a single-mode hollow-core optical fiber with a flat end face to obtain a microcavity Fabry-Perot interferometer without a tapered area.

[0014] B. Preparation of a microcavity Fabry-Perot interferometer containing a conical region.

[0015] B-1. After completing A-5, align the electrode with the right half of the hollow-core fiber. Then control the left control clamp of the motor to remain stationary and stretch the right control clamp of the motor to the right. During this process, the motor continues to discharge the stretched area. The stretched part of the hollow-core fiber collapses due to the electric shock until the original cavity is completely closed. The unstretched part of the hollow-core fiber forms a micro-bubble-shaped air cavity during this process. The discharge power of the motor is 46 bits.

[0016] That is, a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber is obtained.

[0017] The present invention achieves a strain sensitivity of 8.91pm / με and a linearity of 99.2%, which greatly improves the strain sensitivity of the traditional Fabry-Perot strain sensor. The strain sensor structure of the present invention has no dislocation welding, so the sensor structure has greater mechanical strength and is more durable. Moreover, the present invention has the characteristics of high sensitivity, low cost, high mechanical strength and high repeatability of the strain sensor, and has broad application prospects in many professional fields that require high-precision strain measurement, such as engineering monitoring, clinical medicine, and micromachining.

[0018] The strain sensor of the present invention only uses single-mode optical fiber and hollow-core optical fiber, has low cost, low requirements on preparation technology, and is easier to prepare. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic structural diagram of a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber provided in an embodiment of the present application.

[0020] Figure 2 The structure of a microcavity in a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber provided in an embodiment of the present application is schematic.

[0021] Figure 3 and Figure 4 This is a relationship diagram between the spectral response and wavelength shift and strain of a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber provided in the embodiment of the present application in Example 1.

[0022] Figure 5 This is a relationship diagram between the spectral response and wavelength shift of a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber and temperature provided in Example 1 of the present application.

[0023] Figure 6 It is the result of fitting the dip point of the envelope spectrum after the temperature test of a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber provided in the embodiment of the present application in Example 1. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0025] The terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable under appropriate circumstances, so that the embodiments of the present application can be implemented in an order other than those illustrated or described here, and the objects distinguished by "first", "second", etc. are generally of one type, and the number of objects is not limited. For example, the first object can be one or more. In addition, "and / or" in the specification and claims represents at least one of the connected objects, and the character " / " generally indicates that the objects associated with each other are in an "or" relationship.

[0026] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0027] The Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber provided in the embodiment of the present application is described in detail below through specific embodiments and application scenarios in conjunction with the accompanying drawings.

[0028] Specific implementation method one: Figure 1 It is a schematic structural diagram of a Fabry-Perot strain microcavity sensor based on a tapered hollow-core optical fiber provided in an embodiment of the present application.

[0029] like Figure 1 As shown, the embodiment of the present application provides a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber, the device comprising: a broadband light source 1, a circulator 2, a spectrum analyzer 4 and a sensor head 3. The output end of the broadband light source 1 and the input end of the spectrum analyzer 4 are respectively connected to the port of the optical circulator 2 through a single-mode optical fiber; The micro-bubble-shaped air cavity is made by fusing two sections of single-mode optical fiber 5 with flat end faces and a section of hollow-core optical fiber, and controlling the discharge of the fusion splicer to collapse a part of the hollow-core optical fiber into a silicon wall. The length of the air cavity 6 is 75 μm, the length of the tapered hollow-core optical fiber 7 is 150 μm, and the lumbar diameter is 20 μm-50 μm.

[0030] Experimental principle of Fabry-Perot microcavity strain sensor based on tapered hollow core fiber:

[0031] The basic principle of the FPI structure is multi-beam interference. The incident light is introduced into the FP cavity through the optical fiber. Due to the different refractive indices of the media on both sides of the end face, a difference in refractive index is generated, which causes interference after the light is reflected on the two end faces. The incident light transmitted through the SMF is incident on the FP cavity, so the reflectivity of end face 1 and end face 2 can be obtained from formula (1).

[0032]

[0033] In the formula, R is the magnitude of reflectivity; n 1 and n 2 are the effective refractive indices of the media on both sides of the reflecting end face.

[0034] According to the multi-beam interference principle, the FP reflection spectrum intensity is.

[0035]

[0036] In the formula, I λ (λ) is the interference spectrum intensity; R 1 is the reflectivity of end face 1; R 2 is the reflectivity of end face 2; since the effective refractive index difference between end faces 1 and 2 is the same, we get R 1 = R 2 . L is the FP cavity length, n is the refractive index of the cavity medium. Since the FP cavity medium is air, n =1, λ is the wavelength of light. Since the refractive indexes of SMF and air are 1.445 and 1 respectively, according to formula (1), R 1 = R 2 ≈0.04.

[0037] Assume that the cavity length of the FP cavity is L, then the corresponding peak wavelength λ in the reflection spectrum is:

[0038]

[0039] When the strain applied in the external environment changes, the cavity length changes from L becomes L+ΔL , the wavelength corresponding to the same peak λ+Δλ It can be expressed as:

[0040]

[0041] Through the above formula (3) and formula (4), we can get:

[0042]

[0043] The actual axial strain applied to the sensor is proportional to the change in cavity length.

[0044] According to formula (5), when the strain applied to the sensor increases, the cavity length increases and the reflection spectrum undergoes a red shift.

[0045] Specific implementation method 2: This implementation method is based on a method for preparing a Fabry-Perot microcavity strain sensor of a tapered hollow-core optical fiber.

[0046] A. Preparation of microcavity FPI without tapered area;

[0047] A-1. Strip off the coating of two sections of single-mode optical fiber, then use a non-woven cloth dipped in alcohol to repeatedly wipe the two sections of single-mode optical fiber with the coating stripped off in one direction, and then use a fiber cleaver to cut the end face of the single-mode optical fiber flat;

[0048] A-2. Use a flame to briefly burn the coating of a hollow-core optical fiber. When the coating turns black, use a non-woven cloth dipped in alcohol to repeatedly wipe the hollow-core optical fiber in one direction to remove the coating, and then use a fiber cleaver to cut the end face of the hollow-core optical fiber flat;

[0049] A-3. Place a section of single-mode fiber with flat end faces and a hollow-core fiber with flat end faces in the fusion splicer. Then set the pre-melting power of the fusion splicing mode to 30 bits, the pre-melting time to 150 ms, the main discharge power to 45 bits, and the main discharge time to 2200 ms. Then use the preset mode to fusion splice the single-mode fiber with flat end faces and the hollow-core fiber with flat end faces together.

[0050] A-4. Place the fused single-mode-hollow-core structure under the CCD camera, and push the single-mode-hollow-core structure accurately through the micro-displacement control platform, and measure a 150μm hollow-core fiber between the fusion point and the fiber cleaver. Then use the fiber cleaver to cut the end face flat to obtain the ideal length of hollow-core fiber;

[0051] A-5. Repeat the operation of A-3 with a section of single-mode optical fiber 5 with a flat end face and a single-mode hollow core optical fiber with a flat end face to obtain a microcavity Fabry-Perot interferometer without a tapered area.

[0052] B. Fabrication of a microcavity Fabry-Perot interferometer containing a tapered region;

[0053] B-1. After completing A-5, align the electrode with the right half of the hollow-core fiber. Then control the left control clamp of the motor to remain stationary and stretch the right control clamp of the motor to the right. During this process, the motor continues to discharge the stretched area. The stretched part of the hollow-core fiber collapses due to the electric shock until the original cavity is completely closed. The unstretched part of the hollow-core fiber forms a micro-bubble-shaped air cavity during this process. The discharge power of the motor is 46 bits.

[0054] That is, a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber is obtained.

[0055] Specific implementation method 3: The difference between this implementation method and specific implementation method 2 is that a hollow core optical fiber with an inner diameter of 50 μm is used in step A-2. The other steps and parameters are the same as those in the specific implementation method.

[0056] Specific embodiment 4: This embodiment differs from specific embodiment 2 or 3 in that: in step A-3, the automatic fusion mode is adopted, the pre-fusion time is 50 ms, the fiber pre-fusion power is 30 bits, the main discharge time is 3000 ms, and the main discharge power is 150 bits. The other steps and parameters are the same as those of specific embodiments 3 to 6.

[0057] Strain detection

[0058] Combination Figure 1~2 Example 1 is described. The light source of the Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber in this embodiment is a broadband light source 1 (BBS, CONNET VENUS, with a range of 1525-1610 nm) and a spectrum analyzer 4 (OSA, Agilent 86142B, with a resolution of 0.06nm / 0.01 dB). First, the spectrum analyzer 4 is connected to the port of the circulator 2, and the optical signal is transmitted to the proposed FPI structure through the optical fiber. Then, the prepared FPI structure is placed horizontally on the micro-displacement control platform and quickly fixed with UV glue. The distance between the two fixed points is 10.1cm. The stretching interval is 50 μm each time. Finally, the light signal that interferes inside the microcavity is reflected back to the circulator 2 and enters the spectrum analyzer 4 (OSA, Agilent 86142B, with a resolution of 0.06nm / 0.01 dB) to record the reflection spectrum.

[0059] Figure 3~4 The strain response results of the Fabry-Perot microcavity strain sensor based on the tapered hollow core fiber of this embodiment are given. Figure 5 The superposition spectrum and envelope curve of strain increasing from 0 to 2000 με are given. The spectrum is recorded every 200 με. It can be seen that the envelope spectrum red-shifts with the increase of strain. The strain sensitivity of the Fabry-Perot microcavity strain sensor based on tapered hollow-core fiber in this embodiment is 8.91 pm / με, and the linearity is 99.2%.

[0060] Temperature detection

[0061] The Fabry-Perot microcavity strain sensor based on tapered hollow core fiber is placed in a temperature control box and the temperature is increased from 30°C to 150°C. The temperature test results are shown in Figure 2. Figure 5 As shown. Figure 5 It can be seen that with the increase of temperature, the envelope spectrum undergoes a regular red shift, and the dip points of the envelope spectrum are fitted, as shown in Figure 6 As shown in the figure, the temperature sensitivity is 0.7pm / ℃ and the linearity is 99.0%. The experiment proves that the Fabry-Perot microcavity strain sensor based on tapered hollow core fiber has the characteristics of high strain sensitivity and low temperature crosstalk.

[0062] It should be noted that, in this article, the terms "comprise", "include" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise one..." do not exclude the presence of other identical elements in the process, method, article or device including the element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in reverse order according to the functions involved, for example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0063] Through the description of the above implementation methods, those skilled in the art can clearly understand that the above embodiment methods can be implemented by means of software plus a necessary general hardware platform, and of course by hardware, but in many cases the former is a better implementation method.

[0064] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the invention disclosed herein. The present application is intended to cover any variation, use or adaptation of the present application, which follows the general principles of the present application and includes common knowledge or customary techniques in the art that are not disclosed in the present application. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present application are indicated by the claims.

[0065] It should be understood that the present application is not limited to the precise structures described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present application is limited only by the appended claims. Technical Field The present application belongs to the field of optical fiber sensing, and specifically relates to a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber.

Claims

1. A Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber, characterized in that: Including broadband light source, optical circulator, spectrometer, sensor head; The Fabry-Perot microcavity strain sensor based on tapered hollow core fiber is characterized in that the output end of the broadband light source and the input end of the spectrum analyzer are respectively connected to the optical circulator port through single-mode optical fiber; the Fabry-Perot microcavity strain sensor based on tapered hollow core fiber is connected to the optical circulator through a single-mode optical fiber.

2. The device according to claim 1, characterized in that The tapered hollow-core optical fiber Fabry-Perot microcavity strain sensor further comprises: two sections of single-mode optical fiber and one section of tapered hollow-core optical fiber, wherein the single-mode optical fibers have the same length and the length of the tapered hollow-core optical fiber is a first length; the tapered hollow-core optical fiber further comprises a microbubble-shaped air cavity, wherein the length of the microbubble-shaped air cavity is a second length 。 3. The device according to claim 1, characterized in that The first length is 150 μm, and the second length is 75 μm.

4. The device according to claim 1, characterized in that The inner diameter of the hollow core fiber is 50 μm.

5. A method for preparing a Fabry-Perot microcavity strain sensor based on a tapered hollow core optical fiber, applied to the electronic device according to any one of claims 1 to 4, characterized in that: The method comprises: Fabrication of a microcavity Fabry-Perot interferometer without a tapered region; Fabrication of a microcavity Fabry-Perot interferometer containing a tapered region.

6. The method according to claim 5, characterized in that A microcavity Fabry-Perot interferometer without a tapered region is prepared, comprising: A. Fabrication of a microcavity Fabry-Perot interferometer without a tapered region; A-1. Strip off the coating of two sections of single-mode optical fiber, then use a non-woven cloth dipped in alcohol to repeatedly wipe the two sections of single-mode optical fiber with the coating stripped off in one direction, and then use a fiber cleaver to cut the end face of the single-mode optical fiber flat; A-2. Use a flame to briefly burn the coating of a hollow-core optical fiber with an inner diameter of 50μm. When the coating turns black, use a non-woven cloth dipped in alcohol to repeatedly wipe the hollow-core optical fiber in one direction to remove the coating, and then use a fiber cleaver to cut the end face of the hollow-core optical fiber flat; A-3. Place a section of single-mode fiber with flat end faces and a hollow-core fiber with flat end faces in the fusion splicer. Then set the pre-melting power of the fusion splicing mode to 30 bits, the pre-melting time to 150 ms, the main discharge power to 45 bits, and the main discharge time to 2200 ms. Then use the preset mode to fusion splice the single-mode fiber with flat end faces and the hollow-core fiber with flat end faces together. A-4. Place the fused single-mode-hollow-core structure under the CCD camera, and push the single-mode-hollow-core structure accurately through the micro-displacement control platform, and measure a 150μm hollow-core fiber between the fusion point and the fiber cleaver. Then use the fiber cleaver to cut the end face flat to obtain the ideal length of hollow-core fiber; A-5. Repeat the operation of A-3 with a section of single-mode optical fiber 5 with a flat end face and a single-mode hollow core optical fiber with a flat end face to obtain a microcavity Fabry-Perot interferometer without a tapered area.

7. The method according to claim 5, characterized in that Preparation of a microcavity FPI having a tapered region, comprising: B. Fabrication of a microcavity Fabry-Perot interferometer containing a tapered region; B-1. After completing A-5, align the electrode with the right half of the hollow-core fiber. Then control the left control clamp of the motor to remain stationary, and stretch the right control clamp of the motor to the right. During this process, the motor continues to discharge the stretched area. The stretched part of the hollow-core fiber collapses due to the electric shock until the original cavity is completely closed. The unstretched part of the hollow-core fiber forms a micro-bubble-shaped air cavity in this process. The discharge power of the motor is 46bit; That is, a Fabry-Perot microcavity strain sensor based on a tapered hollow-core optical fiber is obtained.

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