Low-cost and reproducible polarization-maintaining fiber end-face Michelson interferometric sensor

Abstract

The present invention discloses a polarization-maintaining fiber end-face Michelson interferometric sensor with low cost and reproducible production. Both the transmission fiber and the sensing fiber adopt polarization-maintaining fibers to maintain the polarization state of the fiber. The free end of the sensing fiber consists of a part of an inclined end-face and a part of a perpendicular end-face. The inclined end-face forms a 45° angle with the axis of the sensing fiber, vertically emitting the light in the core or coupling the light perpendicular to the core into the core. The light reflected by the perpendicular end-face part of the core will be transmitted in the reverse direction. The light reflected by the inclined end-face part of the core enters the core again after being reflected by the outer wall of the cladding, and interferes with the light reflected by the perpendicular end-face, constituting an end-face Michelson interferometer. This sensor structure can be used for the measurement of temperature or transverse stress, has reproducibility, eliminates the influence of environmental disturbances on the polarization state of the transmitted light, and improves the stability of the structure; it has low cost, simple preparation, and compact structure.

Description

Technical Field

[0001] The present invention relates to the field of fiber optic sensing technology, and particularly to a fiber optic end-face Michelson interferometer sensor with low cost and reproducible production. Background Art

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

[0003] There are various types of fiber optic interferometric sensors, including Fabry-Perot interferometer (FPI) sensors, Michelson interferometer (MI) sensors, Mach-Zehnder interferometer (MZI) sensors, and Sagnac interferometer sensors. The preparation methods include: preparing Fabry-Perot interferometer (FPI) sensors by collimating two optical fibers with a capillary or fixing a reflective diaphragm with a sleeve at a certain distance from the fiber end face, preparing Mach-Zehnder interferometer (MZI) sensors by misaligned fusion splicing of optical fibers, and preparing Sagnac interferometer sensors by the melting method. Although the above methods for preparing fiber optic interferometric sensors are low in cost and high in yield, they have the following problems: poor repeatability, large randomness in the preparation by the cutting and fusion splicing method, it is difficult to ensure the consistency of the fiber structure dimensions during mass production, it is difficult to keep the interference optical path difference of the fiber optic interferometer consistent each time, and it is difficult to mass-produce sensors with almost the same performance parameters, resulting in the need for cumbersome calibration work for different sensors. In addition, the structures in which the circular symmetry of some optical fibers is damaged have different responses to input light with different polarization states, and the stability of their output results will be greatly reduced due to the polarization sensitivity of the structure. Currently, only technologies such as femtosecond laser processing and plasma beam etching can precisely control the dimensions to produce fiber optic interferometer sensors that can be mass-produced, but such processing technologies are generally high in cost and long in preparation cycle, which limits the practicality of fiber optic interferometric 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 polarization-maintaining fiber optic end-face Michelson interferometer sensor with low cost and reproducible production.

[0005] Low-cost and reproducible polarization-maintaining fiber end-face Michelson interferometric sensor, characterized in that it comprises two connected parts: a conductive fiber and a sensing fiber; the conductive fiber and the sensing fiber are polarization-maintaining fibers, including a cladding and a core; the free end of the sensing fiber is composed of a part of a 45-degree inclined end face and a part of a perpendicular end face, and the core end face of the sensing fiber is also composed of an inclined part and a perpendicular part, and the core end face forms a beam splitter; the outer wall of the fiber cladding provides a reflecting surface; the inclined end face forms an angle of 45° with the axis of the sensing fiber, vertically emits the light in the core or couples the light incident perpendicular to the core into the core; the light reflected by the perpendicular end face part of the core will be transmitted in the reverse direction, and the light reflected by the inclined end face part of the core enters the core again after being reflected by the outer wall of the cladding, and interferes with the light reflected by the perpendicular end face, constituting an end-face Michelson interferometer.

[0006] The polarization-maintaining fiber is of a non-circular symmetric structure, has a birefringence effect, has a fast axis direction and a slow axis direction, can maintain the polarization state of the transmitted light, and can ensure stable output of polarization-related devices.

[0007] The inclined end face is parallel to the slow axis of the polarization-maintaining fiber.

[0008] The interference optical path difference of the Michelson interferometer is determined by the fiber drawing process of the optical fiber and is not affected by the sensor preparation process.

[0009] The sensor uses wavelength demodulation and can be used to measure temperature and lateral pressure.

[0010] The side of the core of the polarization-maintaining fiber can be chiseled into a hollow hole, and the light reflected by the inclined end face of the core will pass through the hollow hole, and the sensor is used to measure the refractive index of the medium in the hole.

[0011] Advantages of the present invention:

[0012] The low-cost and reproducible polarization-maintaining fiber end-face Michelson interferometric sensor provided by the present invention can be mass-produced, and the parameters of the fiber Michelson sensor produced each time can be kept highly consistent; considering the polarization sensitivity of the structure, the influence of environmental disturbances on the polarization state of the transmitted light is eliminated, and the stability of the structure is greatly improved; the cost is low, the preparation process is simple, the yield is high, and the production cycle is short; the all-fiber structure is more compact. Description of the drawings

[0013] Figure 1 Schematic diagram of a polarization-maintaining fiber end-face Michelson interferometric sensor according to an embodiment of the present invention, (a) Schematic diagram of the end face of the polarization-maintaining fiber, (b) Schematic diagram of the cross-sectional optical path.

[0014] Among them, 1 - polarization - maintaining optical fiber, 2 - optical fiber core, 3 - optical fiber cladding, 4 - slow axis, 5 - fast axis, 5 - refractive index modulation region of polarization - maintaining optical fiber, 6 - inclined end face part, 7 - vertical end face part, 8 - cladding side wall reflecting surface. Specific embodiments

[0015] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0016] 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 can be directly connected to the other element or there may also be an intermediate element.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention in this specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0018] Please refer to Figure 1 , in this embodiment, the polarization - maintaining optical fiber 1 is used as the sensing optical fiber. One end of the polarization - maintaining optical fiber 1 is polished to form an inclined end face 6 and a vertical end face 7. The inclination angle of the inclined end face 6 is 45°, which can rotate a part of the light in the optical fiber core 2 by 90° and vertically inject it into the optical fiber cladding 3. The light is vertically reflected back to the inclined end face when it is incident on the cladding side wall reflecting surface 8 of the optical fiber, and then returns to the core 2 and transmits back; the vertical end face 7 directly reflects a part of the light in the optical fiber core 2 and transmits it back, thus forming a Michelson interferometer at the optical fiber end face.

[0019] Specifically, the light reflected by the vertical end face 7 of the optical fiber directly returns to the core 2 and transmits back, and the light reflected by the inclined end face 6 will be vertically incident on the cladding side wall reflecting surface 8, and is reflected back to the inclined end face 6 again, and is re - coupled into the core 2 and interferes. The interference phase difference between the two beams of light can be expressed as:

[0020]

[0021] Where is the interference optical path difference between the two beams of light, is directly determined by the self - size of the optical fiber and is not affected by the processing technology; $\varphi$ is the initial phase difference of the interfering light, and lights with different polarization directions have different initial phase differences.

[0022] Upon further analysis, assuming that a non-polarization-maintaining fiber is used to prepare the sensing structure, the s-light and p-light after being reflected by the inclined end face 6 have different phase differences when interfering. When transmitting in a non-polarization-maintaining fiber (such as a common standard single-mode fiber), the s-light and p-light will be coupled with each other, and the coupling situation is random. Affected by the defects of the fiber itself and external environmental disturbances such as stress, bending, torsion, and vibration to which the fiber is subjected, the interference spectrum thereof is also unstable.

[0023] The polarization-maintaining fiber 1 has a high birefringence effect. Specifically, the fiber has different refractive indexes for the light with the polarization direction along the fast axis 5 and the light with the polarization direction along the slow axis 4. When the light transmits in the polarization-maintaining fiber 1, the polarized lights along the fast axis 5 and the slow axis 4 will not be coupled with each other.

[0024] Based on the Fresnel reflection law, the phase jumps of lights with different polarization directions after being reflected by the inclined end face 6 are different. Therefore, when preparing, the inclined end face 6 is parallel to the slow axis 4 of the polarization-maintaining fiber 1, that is, the polarization direction of the p-light component (the polarization direction is parallel to the reflection surface) when the light is reflected by the inclined end face 6 is parallel to the slow axis 4, and the polarization direction of the s-light component (the polarization direction is perpendicular to the reflection surface) is parallel to the fast axis 5. In this way, when interfering, the interference between the p-light and the s-light is independent of each other and will not be coupled with each other, and the transmission of its polarization state will not be affected by environmental disturbances. Therefore, this structure has a relatively stable output response.

[0025] Since the optical path difference of the interfering light is only affected by the size and refractive index of the fiber itself, it is possible to ensure that each prepared sensor has the same parameters and has the advantage of reproducible production.

[0026] This sensor can be used to measure temperature or transverse stress. Specifically, the change in temperature or transverse stress will change the transverse size of the fiber and the refractive index of the fiber cladding 3. Therefore, the change in temperature or stress can be judged by the drift of the interference spectrum.

[0027] The low-cost and reproducible polarization-maintaining fiber end-face Michelson interferometric sensor proposed in this embodiment can be fabricated by the following fabrication method:

[0028] 1) After the polarization-maintaining fiber 1 is cut flat, it is placed under a microscope for observation to determine the directions of the fast axis 4 and the slow axis 5.

[0029] 2) After the polarization-maintaining fiber 1 is clamped, it is placed on a polishing machine, so that the angle between the cut flat end face and the sandpaper is about 45°, and the slow axis 5 is parallel to the polishing plane, and then the polishing machine is started.

[0030] )Directly observe the inclined end face 6 of the polarization-maintaining optical fiber 1 through a microscope and determine whether the inclination angle of the inclined end face 6 is 45° and whether the boundary line between the inclined end face 6 and the vertical end face 7 just passes through the fiber core 2 by observing whether there is an interference spectrum in the reflection spectrum. If not, adjust the included angle between the polarization-maintaining optical fiber 1 and the sandpaper and repeat the above steps until an interference spectrum appears.

[0031] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, 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, it should be considered to be within the scope described in this specification.

[0032] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the appended claims.

Claims

1. A low-cost and reproducible polarization-maintaining fiber end-face Michelson interferometer sensor, characterized in that, It includes a connected conductive optical fiber and a sensing optical fiber; The conductive optical fiber and the sensing optical fiber are polarization-maintaining optical fibers, including a cladding and a core; The free end of the sensing optical fiber is composed of a part of a 45-degree inclined end face and a part of a perpendicular end face. The core end face of the sensing optical fiber is also composed of an inclined part and a perpendicular part, and the core end face forms a beam splitter; the outer wall of the fiber cladding provides a reflecting surface; the inclined end face forms a 45° angle with the axis of the sensing optical fiber, vertically emits the light in the core or couples the light perpendicular to the core into the core; the light reflected by the perpendicular end face part of the core will be transmitted in the reverse direction, and the light reflected by the inclined end face part of the core enters the core again after being reflected by the outer wall of the cladding, and interferes with the light reflected by the perpendicular end face, constituting an end-face Michelson interferometer.

2. The fiber end-face Michelson interferometer sensor according to claim 1, characterized in that, The polarization-maintaining optical fiber is a non-circular symmetric structure, has a birefringence effect, has a fast axis direction and a slow axis direction, can maintain the polarization state of the transmitted light, and can ensure stable output of polarization-related devices.

3. The fiber end-face Michelson interferometer sensor according to claim 1, characterized in that, The inclined end face is parallel to the slow axis of the polarization-maintaining optical fiber.

4. The fiber end face Michelson interferometer sensor according to claim 1, characterized in that, The interference optical path difference of the Michelson interferometer is determined by the fiber drawing process of the optical fiber and is not affected by the sensor preparation process.

5. The fiber end-face Michelson interferometer sensor according to claim 1, characterized in that, The sensor uses wavelength demodulation and is used to measure temperature and lateral pressure.

6. The fiber optic end-face Michelson interferometer sensor according to claim 1, wherein, The side of the core of the polarization-maintaining optical fiber is chiseled into a hollow hole, and the light reflected by the inclined end face of the core will pass through the hollow hole, and the sensor is used to measure the refractive index of the medium in the hole.

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