CN101261117A - Strain sensor based on porous microstructured optical fiber - Google Patents

Abstract

The strain sensor based on the porous microstructure optical fiber is characterized by comprising a light source, a lead-in single-mode optical fiber, the porous microstructure optical fiber, a lead-out single-mode optical fiber and a photoelectric detector which are sequentially connected in series, wherein the porous microstructure optical fiber is a pure quartz fiber core surrounded by a circle of fan-shaped air holes, and the edge of the fiber core is of a ratchet-shaped structure. The invention can measure the tensile, bending and torsional strain and has the advantages of high sensitivity, high temperature resistance and electromagnetic radiation resistance.

Description

Strain sensor based on porous microstructured optical fiber

technical field

The invention belongs to the sensing technology, and is a strain sensor based on a porous microstructure optical fiber, which can be applied to static and dynamic radial, axial, torsion, bending strain and stress sensing.

Background technique

Due to its strong anti-electromagnetic interference ability, high sensitivity, good electrical insulation, safety and reliability, corrosion resistance, and many advantages such as the ability to form an optical fiber sensor network, fiber optic sensors are widely used in various fields such as industry, agriculture, biomedicine, and national defense. prospect.

There are currently several technical solutions for fiber optic strain sensors, including Fiber Bragg Grating (FBG for short), Long Period Grating (LPG for short), Mach-Zehnder Interferometer (Mach-Zehnder Interferometer, Referred to as MZ-I) and optical fiber Sagnac ring, etc., although their structures are different, they are actually based on the principle of interference correlation. In multimode fibers, since different propagation modes have different propagation constants, the mutual interference between modes can also be used for strain sensing. There are many technical routes to realize strain sensing by using mode interference in multimode fiber. One of the commonly used methods is single mode-multimode-single mode fiber (single mode-multimode-single mode fiber, hereinafter referred to as SMS). That is, the multimode fiber is connected in series between two single-mode fibers, one of which is used to lead in the light field, and the other single-mode fiber is used to lead out the light field. In the SMS fiber structure, since there can be more than one transmission mode in the multimode fiber, the mutual interference between the modes will cause the optical field to form a relatively complex spatial distribution at the exit end of the fiber, which is generally called speckle. At the same time, because the core aperture of the exported single-mode fiber is smaller than that of the multimode fiber, it can only receive part of the speckle energy, and the shape and position of the speckle will change with the wavelength of the incident light, so the entire SMS fiber structure has unevenness. transmission spectrum. The transmittance of the spectrum oscillates up and down with the wavelength, and its period is related to the length of the multimode fiber. When strain is applied to the multimode fiber, each transmission mode will change accordingly, resulting in a corresponding change in the spatial distribution of the speckle, and thus a corresponding change in the transmission spectrum of the entire structure. If a monochromatic light source or a multi-longitudinal mode laser light source is used, the change of the transmission spectrum will lead to the change of the output light power, and the detection of this light power change can realize the strain sensing.

In recent years, research on a new type of optical fiber—microstructured optical fiber has become a hot spot. There are many air holes running through the entire length of the fiber in the microstructured fiber, and the light guiding mechanism of the microstructured fiber depends on this air silica microstructure. At present, some exploratory research has been carried out on the sensing of stress and strain in microstructured optical fibers. The main technical paths include FBG and LPG in photonic crystal fibers, multi-core optical fibers, and so on.

One of the prior technologies, Byung Hyuk Park et al [Optical Fiber Communication Conference, 2005.Technical Digest.OFC/NFOEC 4:3-5] wrote LPG in photonic crystal fiber, when strain was applied to this device, the resonance peak of LPG The position of will change with strain, but the direction of change is different from that of LPG in ordinary fiber. Although this unique phenomenon may have potential applications, the advantages over existing technologies are not obvious.

In the second state of the art, P M Blanchard et al. [Smart Mater.Struct., 2000, 9: 132-140] used the relative positions of several light spots of multi-core fiber to realize the sensing of two-dimensional strain, but the position of the light spots changes The law is more complicated, which brings difficulties to demodulation.

The third prior technology, Yu Liu et al [APPLIED OPTICS, 2007, 46: 2516-2519] used graded index (Graded Index, GI) multimode fiber to realize strain and temperature sensing based on SMS fiber structure. However, the transmission spectrum of the device is relatively flat, the oscillation amplitude is only about 0.4dB, and the oscillation period is relatively large, reaching tens of nanometers, so the sensitivity is not high.

The fourth prior art, Joel Villatoro et al. [OPTIC LETTERS, 2006, 313: 305-307] used fused tapered refractive index-guided photonic crystal fiber to realize strain sensing based on SMS fiber structure. The advantage of this structure is that the amplitude of transmission spectrum oscillation is large, and the oscillation period is small, so the sensitivity is high. At the same time, the device is not sensitive to temperature changes, avoiding the influence of temperature changes. However, the price of the photonic crystal fiber used is relatively expensive, and its fusion tapering process is difficult to control.

One of the porous microstructured optical fibers is the so-called grapefruit optical fiber, which is characterized by the fact that there is a circle of large holes surrounding the core in the cladding, and the shape of the hole can be circular or close to a fan shape. , the number of holes varies from a few to more than a dozen, because the cross-section of the fiber is shaped like a cut grapefruit, hence the name grapefruit fiber, as shown in Figure 1. The fiber core of grapefruit fiber can be single-mode fiber core doped with germanium, or multi-mode fiber core made of pure silica. Because the size of the air hole in the cladding is large, it is convenient to inject liquid and polymer into the microstructure fiber to form some special properties, such as adjustable birefringence, attenuator, etc. Inject into the hole of the micro-structured optical fiber to realize gas phase or liquid phase material sensing.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies of the above-mentioned prior art, and provide a strain sensor based on a porous microstructure optical fiber, which can measure various types of strains such as stretching, bending, torsional strain, etc., and should have The advantages of high sensitivity, high temperature resistance and resistance to electromagnetic radiation. ,

Technical solution of the present invention is as follows:

A strain sensor based on a porous microstructured optical fiber is characterized in that it is composed of a light source connected in series, an introduction single-mode optical fiber, a porous microstructured optical fiber, a deriving single-mode optical fiber and a photodetector. The porous microstructured optical fiber has A pure silica core surrounded by fan-shaped air holes, the edge of the core presents a ratchet-shaped structure.

The connecting part of the porous microstructure optical fiber and the single-mode optical fiber is welded or butted with a flat end.

The light source is a single longitudinal mode or a multi-longitudinal mode light source.

The structure of the porous microstructured optical fiber microstructured optical fiber is shown in Figure 1, which is a pure silica core surrounded by a circle of fan-shaped air holes, and the edge of the core presents a unique ratchet-like structure. The light guiding mechanism of this optical fiber is based on the total internal reflection on the quartz-air interface. Since the refractive index difference between quartz and air is very large (about 0.45), while the refractive index difference between the core and the cladding in a general optical fiber is only 0.003, so , for the same wavelength of light and the same diameter of the core, the waveguide normalized frequency of the fiber is much larger than that of ordinary fibers. In the near-infrared band, the fiber can accommodate an extremely large number of transmission modes, whose number can reach thousands. In addition, the core cross-section of this fiber has a unique ratchet-shaped edge, and this special shape of the optical waveguide boundary makes the distribution of almost every propagating mode in the cross-section very complex. The result of mutual interference of a large number of propagating modes with complex spatial distributions leads to finer speckle in microstructured fibers than in ordinary step and graded-index multimode fibers. Therefore, the transmission spectrum of the microstructured optical fiber has a large oscillation amplitude, which can reach more than 10dB, and the oscillation period is smaller, only a few tenths of the ordinary multimode optical fiber with the same length. When strain is applied to the microstructured fiber, the propagation constant of each mode and the propagation distance of light waves in the fiber, including the total propagation distance, and the distribution of the propagation distance along the radial direction will change accordingly. For a fixed frequency of injected light or a multi-longitudinal mode light source with several fixed frequencies, a small strain will cause a large change in the transmitted power of the device, and the detection of this optical power change can demodulate the strain .

Advantages and characteristics of the present invention are:

(1) The characteristics of the microstructured optical fiber in the present invention, including the large refractive index difference between the core and the cladding and the ratchet-shaped optical waveguide boundary, can accommodate an extremely large number of propagation modes and cause complex electromagnetic field distribution. Therefore, the optical fiber stress and strain sensing device constructed in this way has higher sensitivity than ordinary multimode optical fiber.

(2) The present invention adopts an all-fiber solution to realize insulation and isolation detection, and can be applied to sensing of high voltage and strong magnetic field.

(3) The microstructure optical fiber used in the present invention has a pure silica air structure, and there is no doping in the fiber core, which does not cause dopant loss at high temperatures like ordinary optical fibers, and is especially suitable for high temperature applications.

(4) Compared with the stress and strain sensing device using photonic crystal fiber, the microstructure fiber used in the present invention is relatively cheap, which is conducive to popularization and application.

Description of drawings

Figure 1 is the cross-sectional structure of the microstructured optical fiber

Fig. 2 is the structural representation of the strain sensor based on the porous microstructure optical fiber of the present invention

Fig. 3 is the strain type schematic diagram that the present invention can respond

Fig. 4 is the application embodiment 1 of the present invention -- the schematic diagram of microstructure optical fiber tensile strain sensor

Fig. 5 is the application embodiment 2 of the present invention -- the schematic diagram of microstructure optical fiber bending strain sensor

Fig. 6 is the application embodiment 3 of the present invention -- the schematic diagram of microstructure optical fiber torsional strain sensor

Detailed ways

The present invention will be further described below in conjunction with the embodiments and accompanying drawings, which should not limit the protection scope of the present invention.

Please refer to Fig. 2 first, as can be seen from the figure, the strain sensor based on the porous microstructured optical fiber of the present invention is composed of a light source 1 connected in series, leading in a single-mode optical fiber 2, a porous microstructured optical fiber 3, leading out a single-mode optical fiber 4 and a photodetector 5, the porous microstructure optical fiber 3 is a pure silica core surrounded by a circle of fan-shaped air holes, and the edge of the core presents a ratchet-shaped structure.

The strain sensor based on the porous microstructured optical fiber shown in Fig. 2, the connection part between the porous microstructured optical fiber and the single-mode optical fiber is welded. The light source 1 is a single longitudinal mode or a multi-longitudinal mode light source.

Figure 3 is a schematic diagram of the types of strains that the present invention can respond to. The light wave emitted by the light source 1 propagates through the single-mode optical fiber 2 and enters the porous microstructured optical fiber 3. Various forms of strain are applied to the porous microstructured optical fiber 3, including tensile strain. , bending strain, twisting strain, etc., the pattern of the speckle field at the exit end of the porous microstructured optical fiber 3 will change with the strain, and it is derived that the single-mode optical fiber 4 as a spatial filter can only receive light energy in a certain fixed spatial region , can respond to the change of the speckle field, and the light intensity is detected by the photodetector 5 . The strain of the fiber can be retrieved from the measured light intensity.

FIG. 4 is a schematic diagram of application example 1 of the present invention—a microstructured optical fiber tensile strain sensor. The two ends of the porous microstructured optical fiber 3 are fixed on the component 6 by fasteners 7 and 8 or adhesives. When tensile strain is applied to the member 6 , the porous microstructured optical fiber 3 maintains the same strain as the member 6 . Tensile strain does not change the shape of the device's transmission spectrum, but shifts the transmission spectrum. For a light source with a fixed wavelength or a multi-longitudinal mode light source 1 with several fixed wavelengths, the translation of the transmission spectrum will lead to a change in the output power of the device. By measuring and deriving the change of the received optical power of the single-mode optical fiber 4 , the tensile strain of the optical fiber can be derived, and thus the tensile strain of the member 6 can be obtained.

Embodiment 2 As shown in FIG. 5 , the porous microstructure optical fiber 3 is fixed on the surface of a member 6 with an adhesive. When a force perpendicular to the direction of the fiber is applied, the member 6 will generate bending strain, while the porous microstructured optical fiber 3 maintains the same deflection as the member 6 . The strain of the porous microstructure optical fiber 3 includes both tensile strain and bending and shearing strain. The tensile strain will cause the translation of the transmission spectrum, and the bending and shearing strain will cause the shape of the transmission spectrum to change. As in Example 1, using the single longitudinal mode or multi-longitudinal mode light source 1, the deflection of the optical fiber 3 can be derived by measuring and deriving the change of the received optical power of the single-mode optical fiber 4, and thus the bending strain of the component 6 can be obtained.

Embodiment 3 As shown in FIG. 6 , the center of the cylindrical member 9 has a through hole running through the entire length, and the porous microstructure optical fiber 3 passes through the through hole and is fixed therein. When the cylindrical member 9 is subjected to torque, a torsional strain will be generated, and the porous microstructure optical fiber 3 maintains the same torsional strain as the cylindrical member 9, and the torsional strain will cause changes in the shape of the transmission spectrum of the device. As in Examples 1 and 2, using a single longitudinal mode or multi-longitudinal mode light source 1, the torsional strain of the porous microstructured optical fiber 3 can be inverted by measuring and deriving the change of the received optical power of the single-mode optical fiber 4, and the cylindrical Torsional strain of member 9.

Claims (3)

1. A strain sensor based on a porous microstructured optical fiber, characterized in that it consists of sequentially connected light sources (1), leading into single-mode optical fibers (2), porous microstructured optical fibers (3), deriving single-mode optical fibers (4) and A photodetector (5) is formed, and the porous microstructure optical fiber (3) is a pure silica fiber core surrounded by a circle of fan-shaped air holes, and the edge of the fiber core presents a ratchet-shaped structure. 2. The strain sensor based on the porous microstructured optical fiber according to claim 1, characterized in that the connection between the porous microstructured optical fiber and the single-mode optical fiber is welded or butted with a flat end. 3. The strain sensor based on porous microstructured optical fiber according to claim 1, characterized in that the light source (1) is a single longitudinal mode or multi-longitudinal mode light source.

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