A low-cost and reproducible fiber-optic non-enclosed Fabry-Perot sensor

Abstract

The present invention discloses a low-cost and reproducible optical fiber non-enclosed Fabry-Perot sensor, comprising two parts: a connected transmission optical fiber and a sensing optical fiber; the transmission optical fiber comprises a cladding and a core; the sensing optical fiber comprises a cladding and a core, the cladding is provided with one or more air holes, the free end of the sensing optical fiber is a 45-degree inclined end face, the core is shorter than the air hole, the air hole in the cladding forms at least two reflection surfaces with refractive index jumps and parallel to the axial direction of the optical fiber, and constitutes at least one non-enclosed cross-axis Fabry-Perot cavity located inside the sensing optical fiber; the outer wall of the optical fiber cladding provides at least one reflection surface; the normal of the inclined end face is located in the same plane as the normal of the reflection surface formed by the sensing optical fiber axis and the air hole, and is at a 45° angle to the sensing optical fiber axis. The present invention can realize mass production, has low cost, and highly consistent sensitivity, and can be used for simultaneous measurement of the refractive index, temperature, or both of gas or liquid.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to an optical fiber non-enclosed Fabry-Perot sensor that can be repeatedly produced at low cost. Background Art

[0002] As a typical representative of optical interference structures, fiber optic Fabry-Perot sensors are the most widely used and most mature optical devices in current practical applications. They have the advantages of large dynamic range, high sensitivity, good stability, anti-electromagnetic interference, suitability for extreme environments, and long-distance multiplexing. They have broad application scenarios in the fields of electric power transportation, biochemistry, aerospace, astronomy and meteorology, and microbial detection.

[0003] There are various types of fiber optic Fabry-Perot sensors, and the preparation methods include: using a capillary to align two optical fibers, fixing a reflective film with a sleeve at a certain distance outside the end face of the optical fiber, splicing a section of special type optical fiber (such as capillary fiber or hollow core fiber, etc.) between two standard single-mode optical fibers, forming a microbubble structure on the end face of the optical fiber through special discharge, etc. Although the above method for preparing fiber optic Fabry-Perot sensors is low-cost and has a high yield, it has the following problems: the Fabry-Perot cavity is completely closed and can only sense physical quantities such as temperature and strain, but cannot measure the refractive index of ambient gas or liquid; the repeatability is poor, and special types of optical fibers need to be obtained through a cutting process. It is difficult to ensure the consistency of the size of special types of optical fibers during mass production. Therefore, it is difficult to keep the Fabry-Perot cavity prepared each time consistent, and it is difficult to mass-produce sensors with almost the same performance parameters, resulting in tedious calibration work for different sensors. Currently, the only technologies that can produce non-closed cavity and mass-produced fiber Fabry-Perot sensors are femtosecond laser processing and plasma beam etching. However, these processing technologies are generally expensive and have a long preparation cycle, which limits the practicality of fiber Fabry-Perot sensors. Summary of the invention

[0004] In order to overcome the deficiencies of the prior art, the object of the present invention is to provide a low-cost and reproducible optical fiber non-enclosed Fabry-Perot sensor.

[0005] A low-cost and reproducible optical fiber non-enclosed Fabry-Perot sensor comprises two parts: a connected transmission optical fiber and a sensing optical fiber; the transmission optical fiber comprises a cladding and a core; the sensing optical fiber comprises a cladding and a core, the cladding is provided with one or more air holes, the free end of the sensing optical fiber is a 45-degree inclined end face, so that the core is shorter than the air hole, the air holes in the cladding form at least two reflection surfaces with refractive index jumps and parallel to the optical fiber axis, forming at least one non-enclosed cross-axis Fabry-Perot cavity located inside the sensing optical fiber; the outer wall of the optical fiber cladding provides at least one reflection surface; the normal of the inclined end face is located in the same plane as the normal of the reflection surface formed by the sensing optical fiber axis and the air hole, and forms an angle of 45° with the sensing optical fiber axis, so that the light of the core is vertically emitted or the light vertically injected into the core is coupled into the core.

[0006] The inclined end surface is plated with a metal film to increase reflectivity.

[0007] The cavity length of the Fabry-Perot cavity is determined by the optical fiber drawing process.

[0008] The Fabry-Perot cavity is filled with external gas or liquid to directly measure the refractive index of the ambient gas or liquid; or after being filled with a medium, it is used to measure one or more of the physical parameters of temperature, humidity, stress, and vibration.

[0009] The cavity of the Fabry-Perot cavity has different sensitivities to the refractive index and temperature of the outside world, thereby achieving simultaneous demodulation of the refractive index and temperature of the target detection object.

[0010] The sensing optical fiber is a microstructure optical fiber with at least one air hole in the cladding, and can be a single-side hole optical fiber, a double-side hole optical fiber, or a single-core four-hole optical fiber.

[0011] Beneficial effects of the present invention:

[0012] The low-cost, reproducible fiber optic non-enclosed Fabry-Perot sensor provided by the present invention can realize mass production, and the parameters of the fiber optic Fabry-Perot sensor produced each time can be kept highly consistent; the Fabry-Perot cavity is non-enclosed and suitable for more measurement scenarios, especially for measuring the refractive index of environmental gas or liquid; the cost is low, the preparation process is simple, the yield is high, and the production cycle is short; because there are at least three reflection surfaces, a double Fabry-Perot interference cavity composite structure can be formed, and different cavities have different sensitivities to the refractive index and temperature of the outside world, and the dual parameters of refractive index and temperature can be simultaneously demodulated; the Fabry-Perot cavity is located in the fiber part, and the structure is more compact. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic structural diagram of an embodiment of the present invention.

[0014] Figure 2 for Figure 1 Cross-sectional view of .

[0015] Among them, 1-first reflection surface, 2-second reflection surface, 3-third reflection surface, 4-inclined end surface, 5-single-side hole optical fiber cladding, 6-air hole, 7-fiber core, 8-single-side hole optical fiber, 9-conduction optical fiber. DETAILED DESCRIPTION

[0016] 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.

[0017] It should be noted that when an element is referred to as being “connected” to another element or an element is referred to as being “connected to” another element, it may be directly connected to the other element or there may be an intervening element.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0019] See also Figure 1 , Figure 2 In this embodiment, a single-side hole optical fiber 8 is used as the sensing optical fiber. One end of the single-side hole optical fiber 8 is connected to an ordinary conducting optical fiber 9, and the other end is polished to form an inclined end face 4, on which a metal film is plated to increase the reflectivity. The inclined end face 4 has an inclination angle of 45°, which can rotate the light of the fiber core 7 by 90° and vertically shoot it into the air hole 6. The position where the light passes through the upper surface of the air hole 6 is used as the first reflection surface 1, the position where the light passes through the lower surface of the air hole 6 is used as the second reflection surface 2, and the position where the light is incident on the outer wall of the optical fiber cladding is used as the third reflection surface 3. It should be noted that in other embodiments, the single-side hole optical fiber 8 can be replaced by a microstructure optical fiber with at least one air hole in the cladding, such as a double-side hole optical fiber or a single-core four-hole optical fiber.

[0020] The claddings of the transmission optical fiber and the sensing optical fiber may be different, and the core sizes of the transmission optical fiber and the sensing optical fiber may be the same or close.

[0021] A low-cost and reproducible optical fiber non-enclosed Fabry-Perot sensor proposed in this embodiment can be made using the following preparation method:

[0022] 1) Connect one end of the single-side hole optical fiber 8 to the common transmission optical fiber 9.

[0023] 2) After the other end of the single-hole optical fiber 8 is cut flat, it is placed under a microscope for observation to determine the position of the air hole 6.

[0024] 3) Clamp the single-hole optical fiber 8 and place it on the polishing machine, so that the angle between the cut end face and the sandpaper is about 45°, and the axis of the air hole 6 and the axis of the fiber core 7 are in the same plane and perpendicular to the sandpaper plane. Polish it. The air hole 6 should be longer than the fiber core 7.

[0025] 4) Directly observe the inclined end face 4 of the single-side hole optical fiber 8 through a microscope and observe whether there is an interference spectrum in the reflection spectrum to determine whether the inclination angle of the inclined end face 4 is 45°. If not, adjust the angle between the single-side hole optical fiber 8 and the sandpaper and repeat step 3) until an interference spectrum appears.

[0026] 5) A layer of metal reflective film is coated on the inclined end surface 4.

[0027] During operation, the light from the light source is first coupled to the core of the ordinary conductive optical fiber 9, then enters the core 4 of the single-hole optical fiber 8, and finally enters the inclined end face 4 at an incident angle of 45°. The inclined end face 4 coated with a metal reflective layer reflects the light to the first reflective surface 1, the second reflective surface 2 and the third reflective surface 3. The three reflective surfaces will partially reflect the light back to the intersection of the inclined end face 4 and the core 7. The inclined end face 4 will re-couple the light into the core 7. The first reflective surface 1 and the second reflective surface 2 form a Fabry-Perot cavity with the medium therebetween. The light reflected by the two reflective surfaces will produce interference fringes. Because the Fabry-Perot cavity is connected to the outside world, external gas or liquid can flow into the Fabry-Perot cavity. The change in the refractive index of the gas or liquid will cause the optical path difference of FP interference to change, and the interference spectrum will drift.

[0028] There are two demodulation methods for low-cost and reproducible fiber-optic non-enclosed Fabry-Perot sensors: wavelength demodulation and power demodulation:

[0029] 1) Wavelength demodulation is to directly demodulate the refractive index of the medium in the Fabry-Perot cavity through the reflection spectrum of the Fabry-Perot sensor. The required light source is a broadband light source and the detector is a spectrometer. In addition, although the reflected light of the third reflection surface 3 is relatively small, it will also participate in the interference. The interference spectrum of the three-beam interference can be demodulated on the spectrometer. The medium between the third reflection surface 3 and the second reflection surface 2 is quartz, so the dual parameters of refractive index and temperature can be demodulated simultaneously, such as the simultaneous demodulation of temperature and medium refractive index. The demodulation precision and accuracy of the method are relatively high.

[0030] 2) Power demodulation is based on the principle that when the spectrum drifts, the optical power of the wavelength at the hypotenuse of the curve will also change. The required light source is a monochromatic light source and the detector is a photodetector. Compared with wavelength demodulation, this method has a smaller dynamic range, but it has a simple structure, a small detector size, low cost and can perform high-speed detection.

[0031] The size consistency of the optical fiber Fabry-Perot interferometer cavity proposed in this embodiment depends on the size consistency of the air holes in the sensing optical fiber cladding. Since the air holes in the sensing optical fiber cladding are usually produced by a mature precision wire drawing process, the diameter of the air holes can be guaranteed to be consistent along the optical fiber height. Therefore, the height of the Fabry-Perot cavity prepared each time can be guaranteed to be consistent during mass production.

[0032] The mass-producible optical fiber Fabry-Perot sensor described in this implementation case can ensure that the Fabry-Perot cavity length produced each time is highly consistent, so the free spectral range of the interference spectrum of each sensor is also highly consistent, which makes all sensor performance index parameters also highly consistent, including sensitivity, resolution, etc.

[0033] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0034] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the attached claims.

Claims

1. A low-cost and reproducible optical fiber non-enclosed Fabry-Perot sensor, characterized in that: It includes two parts: a connected transmission optical fiber and a sensing optical fiber; The transmission optical fiber comprises a cladding and a core; The sensing optical fiber comprises a cladding and a core, wherein the cladding is provided with one or more air holes, and the free end of the sensing optical fiber is a 45-degree inclined end face, so that the core is shorter than the air hole, and the air holes in the cladding form at least two reflection surfaces with refractive index jumps and parallel to the axial direction of the optical fiber, constituting at least one non-closed cross-axis Fabry-Perot cavity located inside the sensing optical fiber; the outer wall of the optical fiber cladding provides at least one reflection surface; the normal of the inclined end face is located in the same plane as the normal of the reflection surface formed by the sensing optical fiber axis and the air hole, and forms an angle of 45° with the sensing optical fiber axis, so that the light of the core is vertically emitted or the light vertically injected into the core is coupled into the core.

2. The optical fiber non-enclosed Fabry-Perot sensor according to claim 1, characterized in that: The inclined end surface is plated with a metal film to increase reflectivity.

3. The optical fiber non-enclosed Fabry-Perot sensor according to claim 1, characterized in that: The cavity length of the Fabry-Perot cavity is determined by the optical fiber drawing process.

4. The optical fiber non-enclosed Fabry-Perot sensor according to claim 1, characterized in that: The Fabry-Perot cavity is filled with external gas or liquid to directly measure the refractive index of the ambient gas or liquid; or after being filled with a medium, it is used to measure one or more of the physical parameters of temperature, humidity, stress, and vibration.

5. The optical fiber non-enclosed Fabry-Perot sensor according to claim 1 or 4, characterized in that: The cavity of the Fabry-Perot cavity has different sensitivities to the refractive index and temperature of the outside world, thereby achieving simultaneous demodulation of the refractive index and temperature of the target detection object.

6. The optical fiber non-enclosed Fabry-Perot sensor according to claim 1, characterized in that: The sensing optical fiber is a single-side hole optical fiber, a double-side hole optical fiber or a single-core four-hole optical fiber.

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