Dual-core photonic crystal fiber sensing structure

Abstract

The invention relates to a dual-core photonic crystal optical fiber sensing structure, comprising a background material area, a hexagonal six-layer air hole arrangement structure, a first circular core structure, and a second circular core structure; polymethyl methacrylate is doped in the first circular core structure; a plurality of small-aperture air holes are arranged around the first circular core structure; photonic crystal optical fiber is introduced, a dual-core structure is arranged in PCF, dual interference arms are made in one optical fiber, and a single optical fiber realizes transmission and interference of two modes, and has a simple structure, a small size, and is easy to process and integrate; photonic crystal is introduced as a sensitive element of the sensor, and the cladding air holes of the photonic crystal itself can reduce the equivalent Young's modulus of the cladding area, increase the material elastic coefficient, and improve the deformation; in the dual-core structure designed by PCF, polymethyl methacrylate with good elastic performance is doped in the single core, and internal stress is generated under the action of pressure on the dual cores, and the refractive index difference of the effective modes of the dual cores is linearly related, thereby improving the sensitivity.

Description

Technical Field

[0001] The invention belongs to the technical field of photonic crystal optical fibers, and in particular relates to a dual-core photonic crystal optical fiber sensing structure. Background Art

[0002] Fiber optic hydrophones provide many technical approaches to solve underwater acoustic research and provide an ideal solution to many problems in shallow sea low-frequency underwater acoustic research and application. The most widely used acoustic pressure hydrophone is the mandrel-type structure of the traditional fiber optic hydrophone, which usually uses a Michelson interferometer or a Mach-Zehnder interferometer as a sensitive element. The technical principle of sensing is:

[0003] (1) One or both arms of the interferometer made of optical fiber material are differentially wound around the core shaft elastic structure;

[0004] (2) The sound pressure acts on the elastic structure, causing the refractive index of the optical fiber to change;

[0005] (3) Sound pressure sensing is achieved by detecting the phase difference caused by the change in refractive index.

[0006] This fiber optic hydrophone has the advantages of high sensitivity, light structure, easy large-scale array formation, strong anti-electromagnetic interference ability, and mature manufacturing technology.

[0007] However, in aquatic environments, traditional fiber optic hydrophones are difficult to meet the needs of weak signal detection due to material and structural limitations, and their structure is relatively complex, which hinders the development of sound pressure sensors towards miniaturization and integration.

[0008] Because traditional fiber-optic hydrophones use Mach-Zehnder interferometers as sensitive elements, the reference arm and the measurement arm cannot be integrated into the same element, that is, the two arms are separated and the structure is relatively complex, which limits the miniaturization of the sensor to a certain extent, and makes further reduction of the structural size encounter bottlenecks.

[0009] Because the two arms of the traditional hydrophone interferometer are made of quartz material, its Young's modulus and Poisson's coefficient are large, the photoelastic coefficient is small, and it does not have flexible structural design characteristics, which is directly manifested as insensitivity to pressure; the materials of the layer and the core are different, the thermal expansion coefficients are inconsistent, and temperature compensation cannot be performed; resulting in smaller optical fiber deformation under greater pressure, cross-sensitivity to temperature and pressure, and low sensitivity.

[0010] Therefore, it is necessary to propose a core photonic crystal fiber sensing structure to solve the above problems. Summary of the invention

[0011] The purpose of the present invention is to provide a dual-core photonic crystal fiber sensing structure in order to solve the above problems.

[0012] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0013] A dual-core photonic crystal fiber sensing structure includes a background material region and a hexagonal six-layer air hole arrangement structure, wherein the hexagonal six-layer air hole arrangement structure is placed in the background material region;

[0014] A first circular core structure and a second circular core structure are distributed on both sides of the center line of the hexagonal six-layer air hole arrangement structure of the core layer;

[0015] The first circular core structure is placed inside the first hexagonal air hole arrangement structure; the second circular core structure includes the second hexagonal air hole arrangement structure and a base material; the first hexagonal air hole arrangement structure and the second hexagonal air hole arrangement structure are symmetrically distributed on both sides of the center line of the core layer hexagonal six-layer air hole arrangement structure;

[0016] The first circular core structure is doped with polymethyl methacrylate;

[0017] A plurality of small-aperture air holes are arranged around the first circular core structure; the air holes formed in the hexagonal six-layer air hole arrangement structure, the first hexagonal air hole arrangement structure and the second hexagonal air hole arrangement structure are all large-aperture air holes.

[0018] Specifically, the hexagonal six-layer air hole arrangement structure, the first hexagonal air hole arrangement structure, and the second hexagonal air hole arrangement structure are all regular hexagons.

[0019] Specifically, there are six small-aperture air holes, which are arranged in a regular hexagon around the center of the first circular core structure.

[0020] Specifically, the small-diameter air holes are placed inside the first hexagonal air hole arrangement structure.

[0021] Preferably, the distance between the centers of any two nearest large-aperture air holes is 5.5 um;

[0022] The diameter of the large-aperture air hole is 2.5um;

[0023] The distance between the centers of the first circular core structure and the second circular core structure is 11um;

[0024] The diameter of the undoped region in the second circular core structure is 5um;

[0025] The diameter of the small-aperture air hole is 1.4um;

[0026] The diameter of the region doped with polymethyl methacrylate in the first circular core structure is 2.2 um;

[0027] The distance between the center of the first circular core structure and the center of any small-aperture air hole is 2 um.

[0028] The beneficial effects of the present invention are:

[0029] The dual-core photonic crystal optical fiber sensing structure of the present invention;

[0030] 1. Miniaturization of structure: Photonic crystal fiber is introduced, a dual-core structure is set in PCF, and the dual interference arms are put into one optical fiber. A single optical fiber realizes the transmission and interference of two modes, and there is no need to subpackage and package the reference arm. The structure is simple, the size is small, and it is easy to process and integrate.

[0031] 2. Improve accuracy and sensitivity; Based on the fact that photonic crystals are insensitive to temperature, photonic crystals are introduced as sensitive elements of the sensor. The cladding air holes they have can reduce the equivalent Young's modulus of the cladding area, increase the elastic coefficient of the material, and increase the deformation.

[0032] In the dual-core structure designed by PCF, polymethyl methacrylate with good elastic properties is added to the single core. When the dual cores are under pressure, internal stress is generated, and the refractive index difference of the effective modes of the dual cores is linearly related, thereby improving sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the structure of a traditional optical fiber sensor head;

[0034] Figure 2 A schematic diagram of the cross-sectional structure of this application;

[0035] Figure 3 This is an enlarged view of the doped core region in this application;

[0036] Figure 4 It is the change diagram of transmission spectrum under different sound pressure;

[0037] Figure 5 This is a schematic diagram of the structure of the first hexagonal air hole arrangement structure and the second hexagonal air hole arrangement structure in the present application;

[0038] Description of reference numerals:

[0039] 11-fiber coupler; 12-elastic tube; 13-reference arm; 14-sensing arm; 21-background material area; 22-hexagonal six-layer air hole arrangement structure; 23-first circular fiber core structure; 24-second circular fiber core structure; 25-first hexagonal air hole arrangement structure; 26-second hexagonal air hole arrangement structure. DETAILED DESCRIPTION

[0040] Below, the exemplary embodiments according to the present application will be described in detail with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application, and it should be understood that the present application is not limited to the exemplary embodiments described here.

[0041] Application Overview

[0042] like Figure 1 As shown in the figure, it is a schematic diagram of the structure of a traditional optical fiber sensor head, which includes an optical fiber coupler 11, an elastic tube 12, a reference arm 13 and a sensor arm 14. In this structure, one sensor arm of the interferometer is wound outside the elastic tube, and the reference arm is fixed outside the sound field or subjected to sound pressure shielding. The two interference arms interfere with each other, and the interference fringes move under the action of pressure. By monitoring the size of the fringes movement, the change of the physical quantity in the measured environment can be measured.

[0043] However, this optical fiber structure is made of traditional optical fiber quartz material, and the Young's modulus is as high as 79GPa, which results in the sensor unit failing to detect information under low pressure. However, due to the limitations of materials and structures, it is difficult to improve the sensitivity and accuracy of traditional optical fiber hydrophones. Moreover, the two arms of the optical fiber are packaged separately and cannot be integrated into a single optical fiber. The structure is relatively complex and it is difficult to further achieve miniaturization. This application is based on the structural principle of the traditional optical fiber hydrophone interferometer to produce a doped dual-core photonic crystal fiber sensor. The dual-core photonic crystal fiber has two optical fiber cores, which can ideally replace the sensing optical fiber arm and the reference optical fiber arm of the traditional optical fiber hydrophone, and the dual interference arms are made into one optical fiber. A single optical fiber realizes the transmission and interference of two modes. This structure is easy to miniaturize the sensor. There is a certain difference between the two cores, that is, an effective refractive difference. After pressure acts on the photonic crystal fiber, the effective refractive index difference of the same polarization of the dual cores will change with the pressure, and the phase difference of the two cores at the output end will change, forming an interference transmission spectrum with certain characteristics at the output port. If the sound pressure further expands the difference between the two cores, the interference transmission spectrum will change accordingly, and the sensing characteristics can be obtained according to the movement of the analytical transmission peak. Introducing air holes in the cladding of photonic crystal fiber can obtain a large refractive index difference that cannot be achieved by traditional optical fibers. By changing the size and distribution of air holes, the optical properties of photonic crystal fiber can be controlled. Therefore, this application makes an innovative design for the dual-core photonic crystal fiber structure.

[0044] The present invention will be further described below in conjunction with the accompanying drawings:

[0045] like Figure 2 , Figure 3 , Figure 5 As shown,

[0046] The dual-core photonic crystal fiber sensing structure includes a background material region 21 and a hexagonal six-layer air hole arrangement structure 22, wherein the hexagonal six-layer air hole arrangement structure 22 is placed in the background material region 21;

[0047] A first circular core structure 23 and a second circular core structure 24 are distributed on both sides of the center line of the core layer hexagonal six-layer air hole arrangement structure 22;

[0048] The first circular core structure 23 is placed inside the first hexagonal air hole arrangement structure 25; the second circular core structure 24 includes a second hexagonal air hole arrangement structure 26 and a base material; the first hexagonal air hole arrangement structure 25 and the second hexagonal air hole arrangement structure 26 are symmetrically distributed on both sides of the center line of the core layer hexagonal six-layer air hole arrangement structure 22;

[0049] In order to obtain higher acoustic pressure sensitivity, the first circular core structure 23 is doped with a pressure-sensitive material, polymethyl methacrylate;

[0050] A plurality of small-aperture air holes are arranged around the first circular core structure 23, which reduces the effective refractive index of the first circular core structure 23 region; the air holes formed in the hexagonal six-layer air hole arrangement structure 22, the first hexagonal air hole arrangement structure 25 and the second hexagonal air hole arrangement structure 26 are all large-aperture air holes.

[0051] like Figure 2 , Figure 5 As shown,

[0052] The hexagonal six-layer air hole arrangement structure 22 , the first hexagonal air hole arrangement structure 25 , and the second hexagonal air hole arrangement structure 26 are all regular hexagons.

[0053] like Figure 3 As shown,

[0054] There are six small-aperture air holes, which are arranged in a regular hexagon around the center of the first circular core structure 23 .

[0055] like Figure 3 As shown,

[0056] The small-diameter air holes are placed inside the first hexagonal air hole arrangement structure 25 .

[0057] like Figure 2 , Figure 3 As shown,

[0058] The distance d6 between the centers of any two nearest large-aperture air holes is 5.5 um. According to the interference principle, increasing the effective refractive index difference of the dual cores can improve the acoustic pressure sensitivity. By setting the air hole spacing to d6 = 5.5 um, the effective refractive index of the undoped core (the second circular core structure 24) can be stabilized at around 1.439.

[0059] The diameter d4 of the large-aperture air hole is 2.5um;

[0060] The distance d7 between the centers of the first circular core structure 23 and the second circular core structure 24 is 11um, which is close to the 10um of the single-mode optical fiber core, and can reduce the difficulty of operation during the fusion process. The dual-core mode field area is approximately the same, which can reduce the insertion loss of the fusion process and improve the extinction ratio.

[0061] The diameter d5 of the undoped region in the second circular core structure 24 is 5 um;

[0062] The diameter d3 of the small-aperture air hole is 1.4um;

[0063] The diameter d2 of the area doped with polymethyl methacrylate in the first circular core structure 23 is 2.2 um; and enlarging the hole diameter d2 of the doped core area (first circular core structure 23) will significantly increase the effective refractive index of the doped core area (first circular core structure 23).

[0064] The distance d8 between the center of the first circular core structure 23 and the center of any small-aperture air hole is 2 um.

[0065] In some embodiments, the diameter d1 of the dual-core optical fiber is 125 um;

[0066] In some embodiments, a quartz material is used as the background material region 21 of the photonic crystal fiber to establish a basic photonic crystal fiber structure with a hexagonal six-layer air hole arrangement.

[0067] The structure of the present application was tested to see how the transmission spectrum changes under different sound pressures. Figure 4 shown.

[0068] The wavelength of the trough of the transmission spectrum has been significantly displaced, and the moving wavelength interval is basically linear with the applied uniform radial sound pressure (31 under 20MPa sound pressure, 32 under 30MPa sound pressure, 33 under 40MPa sound pressure, 34 under 50MPa sound pressure, and 35 under 60MPa sound pressure), and the pressure sensitivity effect is good. This sensing structure transmits at a free space wavelength of 1.55μm, with a sensing length of 6cm. Under uniform radial pressure, the X-polarization sound pressure sensitivity can reach 0.15942nm / KPa, which is one order of magnitude higher than the existing level. Under the sound pressure of the KPa order, the free spectrum width is about 1nm; under the sound pressure of 40MPa, the free spectrum width is about 2.5465nm. Within the measurement range, the linearity is high, the insertion loss is low, and it is easy to reuse wavelengths.

[0069] Comparative analysis: The key technical parameters of the traditional hydrophone and the hydrophone designed by this patent are compared in the following table:

[0070]

[0071] Based on the above data:

[0072] (1) The design increases the difference in Young's modulus between the left and right dual cores, thereby increasing the effective refractive index difference between the dual cores;

[0073] (2) The addition of polymethyl methacrylate has a photoelastic coefficient much greater than that of traditional quartz optical fiber, which enhances pressure sensitivity.

[0074] (3) For DWDM systems, the channel width can reach 1 nm, which improves the multiplexing of hydrophones;

[0075] In this application,

[0076] 1. Six layers of air holes are introduced into the photonic crystal fiber, and a symmetrical dual-core structure is designed. One core is small and the other is large. Polymethyl methacrylate with a smaller Young's modulus is added to the small core, and a circle of small-aperture air holes is surrounded by the doped core; in this way, the effective refractive index difference is increased and the sound pressure sensitivity is improved.

[0077] 2. Photonic crystal fiber materials are used instead of traditional quartz materials. The cladding and the core have the same thermal expansion coefficient, which compensates for the temperature, thereby reducing temperature sensitivity.

[0078] 3. The dual cores are integrated into one optical fiber, and the sensor has a large numerical aperture and very low bending loss, that is, the sensor can be miniaturized, thereby reducing the structural size of the sensor unit.

[0079] 4. Low dual-core spacing and similar mode field area will help control the extinction ratio and insertion loss during the welding process, thereby improving the extinction ratio and reducing the insertion loss.

[0080] 5. The structure exhibits widely different free spectrum widths under different sound pressures. When applied to fields with different stress levels, the length can be adjusted appropriately. It is easy to use wavelength division multiplexing.

[0081] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A dual-core photonic crystal fiber sensing structure, including a background material region and a hexagonal six-layer air hole arrangement structure, wherein the hexagonal six-layer air hole arrangement structure is placed in the background material region; the background material region is made of quartz material; Features: A first circular core structure and a second circular core structure are distributed on both sides of the center line of the hexagonal six-layer air hole arrangement structure of the core layer; The first circular core structure is placed inside the first hexagonal air hole arrangement structure; the second circular core structure includes the second hexagonal air hole arrangement structure and a base material; the first hexagonal air hole arrangement structure and the second hexagonal air hole arrangement structure are symmetrically distributed on both sides of the center line of the core layer hexagonal six-layer air hole arrangement structure; The first circular core structure is doped with polymethyl methacrylate; A plurality of small-aperture air holes are arranged around the first circular core structure; the air holes formed in the hexagonal six-layer air hole arrangement structure, the first hexagonal air hole arrangement structure and the second hexagonal air hole arrangement structure are all large-aperture air holes.

2. The dual-core photonic crystal fiber sensing structure according to claim 1, characterized in that: The hexagonal six-layer air hole arrangement structure, the first hexagonal air hole arrangement structure and the second hexagonal air hole arrangement structure are all regular hexagons.

3. The dual-core photonic crystal fiber sensing structure according to claim 1, characterized in that: There are six small-aperture air holes, which are arranged in a regular hexagon around the center of the first circular core structure.

4. The dual-core photonic crystal fiber sensing structure according to claim 1, characterized in that: The small-diameter air holes are arranged inside the first hexagonal air hole arrangement structure.

5. The dual-core photonic crystal fiber sensing structure according to any one of claims 1 to 4, characterized in that: The distance between the centers of any two nearest large-aperture air holes is 5.5 μm; The diameter of the large-aperture air hole is 2.5um; The distance between the centers of the first circular core structure and the second circular core structure is 11um; The diameter of the undoped region in the second circular core structure is 5um; The diameter of the small-aperture air hole is 1.4um; The diameter of the region doped with polymethyl methacrylate in the first circular core structure is 2.2 um; The distance between the center of the first circular core structure and the center of any small-aperture air hole is 2 um.

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