CN106289600A - A kind of optical fiber stress sensor part - Google Patents

Abstract

The present invention provides an optical fiber stress sensor, comprising a first single-mode optical fiber, a few-mode optical fiber and a second single-mode optical fiber sequentially connected; wherein the normalized frequency of the few-mode optical fiber satisfies: 3.83171<V<7.01559. The present invention proposes to use a simple combination of single-mode fiber, few-mode fiber, and single-mode fiber to realize stress sensing, without using complex manufacturing processes such as gratings, and can obtain high-sensitivity sensing. Compared with other structures, the sensing structure composed of few-mode fibers with small normalized frequency value and few modes in the fiber has high sensitivity and regular frequency spectrum, and it is easy to determine parameters such as fiber length.

Description

An optical fiber stress sensor device

technical field

The invention belongs to the field of optical fiber sensing research, in particular to an optical fiber stress sensing device.

Background technique

As the modernization process continues, stress sensing is an extremely important issue in the safety monitoring of the entire construction in many monitoring systems ranging from large-scale building structures to some fine engineering structures. As we all know, the traditional method of measuring stress is usually with the continuation of the modernization process. In many monitoring systems ranging from large-scale building structures down to some fine engineering structures, stress sensing is an extremely important part of the safety monitoring of the entire construction. Important issues. As we all know, the traditional method of measuring stress usually relies on the corresponding relationship between the resistance value and stress reflected by the resistance strain gauge to detect engineering construction. Although the resistance strain gauge is cheap, its adaptability to the external environment is not good, especially it is easily affected by the electromagnetic field, and it cannot work in many corrosive environments, which is very important for stress sensors used in engineering testing. Very big flaw. And the traditional stress sensor of this kind can only measure a single point. However, since the 1870s, because the fiber optic stress sensor has the advantages of small size, light weight, high precision, no electromagnetic interference and corrosion resistance [1], in the development process of the stress sensor, various A variety of fiber optic stress sensors have emerged as the times require. And many of them are already commercialized. There are already many kinds of stress sensing structures or devices based on optical fibers. For example, sensors based on optical fiber microbend structures ^[2] and Fabry-Perot optical fiber structures have already been applied [2-4].

Among all the optical fiber sensors for measuring strain, the sensor based on FBG structure is the most widely used. This kind of sensor uses FBG as the sensitive element, and its principle is to realize the measurement based on the modulation of the Bragg central wavelength by the strain. Since then, there have been some studies on stress sensors based on SMS optical fiber structure [5]. This structure uses multimode optical fiber, and achieves the effect of mode interference by connecting with single-mode optical fiber. However, the number of modes in multimode fibers is large, and the mode interference effect is complex, which makes it difficult to achieve high-precision sensing.

references:

【1】Peng Shiyu. Research on Fiber Bragg Grating Axial Stress Sensing Model[J]. Journal of Hunan Institute of Technology, 2007,20(2):35-37.

【2】Nicholas Laoakos, Cole J, bucaro J A. Microbend fiber optic sensor [J]. Applied Optics, 1987, 26(11): 2171-2180.

【3】Heredero R L, Santos J L, Ferndndez de Caleya R, et al..Micromachinedlow-finesse Fabry-Perot interferometer for the measurement of DC and AC electrical currents[J].Sensors Journal,IEEE,2003,3(1):13 -18.

【4】Furstenau N, Schmidt M, Horack H, et al.. Extrinsic Fabry-Perotinterferometer vibration and acoustic sensorsystems for airport groundtraffic monitoring [J]. Optoelectronics, IEE Proceedings, 1997, 144(3): 134-144.

【5】Wu Q, Hatta A M, Wang P, et al.Use of a bent single SMS fiberstructure for simultaneous measurement of displacement and temperature sensing[J].IEEE Photonics Technology Letters,2011,23(2):130-132.

Contents of the invention

The object of the present invention is to provide an optical fiber stress sensing device to solve the above problems, realize stress sensing through the simple combination of the first single-mode optical fiber, few-mode optical fiber and second single-mode optical fiber sequentially connected, and improve the stress sensing sensitivity.

The technical solution of the present invention is: an optical fiber stress sensing device, including a single-mode fiber and a few-mode fiber; the single-mode fiber includes a first single-mode fiber and a second single-mode fiber; the first single-mode fiber, few-mode fiber The mode fiber and the second single mode fiber are sequentially connected;

The normalized frequency of the few-mode fiber satisfies:

$\mathrm{<span\; class="notranslate">\; V</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;a</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ,</span>$

3.83171<V<7.01559,

Wherein, n ₁ represents the refractive index of the few-mode fiber core;

n ₂ represents the refractive index of the few-mode fiber cladding;

a ₁ represents the radius of the few-mode fiber core;

λ ₀ represents the working wavelength.

In the above solution, the refractive index difference Δ between the core and the cladding of the few-mode optical fiber satisfies: 0.007≥Δ≥0.002.

In the above solution, the lateral deviation d _m of the single-mode fiber and the few-mode fiber satisfies: d _m ≤ 0.8 μm.

In the above solution, the length L of the few-mode optical fiber satisfies: L≧120mm.

In the above solution, the stress only acts on the few-mode fiber.

The beneficial effects of the present invention are: compared with the prior art, the present invention proposes the simple combination of single-mode optical fiber, few-mode optical fiber, and single-mode optical fiber to realize stress sensing without using complex manufacturing processes such as gratings, and can obtain High sensitivity sensing. Compared with the sensing structure using multimode fiber, the sensing structure composed of few-mode fiber with smaller normalized frequency value and fewer modes in the fiber has high sensitivity, regular frequency spectrum, stable sensing sensitivity, and few-mode fiber The length selection range is large, and it is easy to determine parameters such as fiber length.

Description of drawings

Fig. 1 is the composition structure schematic diagram of a kind of optical fiber sensor device described in the present invention;

Fig. 2 is the change curve of the output energy of the optical fiber sensing device of the present invention with the length of the few-mode fiber, wherein (a) d _core = 25 μm, (b) d _core = 40 μm, (c) d _core = 50 μm;

Fig. 3 is the variation curve of the output energy of the optical fiber sensor device of the present invention with the length L of the few-mode fiber;

Fig. 4 is the output spectrum curve of the optical fiber sensing device of the present invention under different stresses;

Fig. 5 is a graph showing the relationship between the optical fiber sensitivity and detection limit and the length of the few-mode optical fiber in an embodiment of the optical fiber sensor device of the present invention;

In the figure, 1. first single-mode fiber; 2. few-mode fiber; 3. second single-mode fiber.

detailed description

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

Fig. 1 is the optical fiber structure schematic diagram of the present invention, and described optical fiber stress sensing device comprises single-mode optical fiber and few-mode optical fiber 2; Described single-mode optical fiber comprises first single-mode optical fiber 1 and second single-mode optical fiber 3; Said few-mode optical fiber Both ends of the optical fiber 2 are respectively connected to the first single-mode optical fiber 1 and the second single-mode optical fiber 3 to form an interference coupling mechanism, that is, the light input from the first single-mode optical fiber 1 enters the few-mode optical fiber 2 to excite the few-mode optical fiber 2 The modes in the few-mode fiber 2 are then passed through the second single-mode fiber 3 to achieve mode coherence. The length, core diameter, refractive index and other parameters of the few-mode fiber 2 will affect the number of excited modes, energy distribution and coupling energy into a single-mode fiber, thus providing a good mechanism for sensing applications.

FIG. 2 shows how the output energy varies with the multimode length when the few-mode fiber 2 takes different refractive index differences and diameters. It can be seen from Figure 2(a) that when the core and cladding refractive index difference Δ and the diameter of the few-mode fiber are small, its output energy cannot exhibit periodic coupling characteristics; as shown in Figure 2(c), when its diameter and When the refractive index difference Δ between the core and the cladding is too large, the coupling characteristics also change irregularly; as shown in Fig. (that is, the trough) is narrow, which is conducive to obtaining high sensitivity and detection effect. The reason is that when the core diameter and the refractive index difference of the few-mode fiber 2 are too small, the few-mode fiber 2 is close to single-mode transmission, so it is difficult to form effective mode coupling in the few-mode fiber 2, and when the core diameter When the refractive index difference Δ between the core and the cladding is both large, the number of modes in the few-mode fiber 2 is too large, so that the number of excited modes is too large, so the coupling curve is irregular. In fact, since the optical fiber modes themselves will be coupled due to external factors, the deviation of the refractive index distribution of the optical fiber, etc., the actual coupling characteristics will be affected by more factors, making it difficult to form the spectral curve required for sensing. It can be seen that the few-mode fiber 2 with appropriate parameters can be beneficial to mode coupling and sensing. For this reason, the normalized frequency of the few-mode fiber 2 is required to satisfy: 3.83171<V<7.01559, here Wherein, n ₁ and n ₂ represent the refractive indices of the core and cladding of the few-mode fiber 2 respectively; a ₁ represents the core radius, and λ ₀ is the working wavelength. That is, the optical fiber can at least support the transmission of LP ₀₂ and the highest order mode that can be supported is the LP ₀₃ mode. Compared with the multi-mode fiber which usually supports dozens or even hundreds of modes, the few-mode fiber of the present invention only supports 4-9 modes. At the same time, due to the characteristics of structure and parameters, it can only be excited to 2 to 3 modes, so its interference effect and law have been greatly improved, and can be used in sensing fields that require high sensitivity, such as stress sensing .

The refractive index difference Δ between the core and the cladding of the few-mode fiber 2 satisfies: 0.007≥Δ≥0.002, and its effect is to ensure that the refractive index of the few-mode fiber is similar to that of the single-mode fiber to reduce the connection loss.

Fig. 3 is a graph showing the variation of output energy with the length of the few-mode fiber. It can be seen from Fig. 3 that there are corresponding valleys at different lengths of the few-mode fiber, so when the length of the few-mode fiber is at these valley positions, the output spectrum will have corresponding minimum values. Since its output energy exhibits periodic coupling characteristics with the length of the few-mode fiber, if the few-mode fiber is stretched or compressed by stress, its output spectrum will also shift, thereby achieving the purpose of stress sensing.

Fig. 4 shows the change of the output spectrum of the optical fiber device under different stress conditions. It can be seen from Figure 4 that as the stress increases with the mass of the added object, the characteristic wavelength of the output spectrum shifts to the right. Therefore, the stress value applied on the optical fiber can be measured by detecting the movement of the valley value of its spectrum.

Fig. 5 is a variation curve of detection sensitivity and detection limit of an embodiment of the optical fiber sensor device of the present invention. It can be seen from Figure 5 that when the length of the few-mode fiber changes, its sensitivity remains stable only in a small range of fluctuations. At the same time, it can also be found that the detection limit gradually decreases with the increase of the few-mode fiber length, and gradually tends to be stable. Therefore, the length of the few-mode fiber should be larger to ensure a smaller detection limit. For the results in Figure 5, the average sensitivity is 0.3942nm/g, which is about 4 times higher than that of the fiber optic stress sensor based on the FBG structure. It can be seen from FIG. 5 that the length L of the few-mode fiber 2 should satisfy: L≧120mm, that is, a longer few-mode fiber is used to obtain a stable detection effect and high-sensitivity detection. When the single-mode fiber is connected to the few-mode fiber, the deviation of the connection will cause the energy of the excited mode to be different, and the final result will affect the detection limit of the fiber. In order to ensure its sensing effect, it is required that the single-mode fiber and the few-mode fiber The lateral deviation d _m of 2 satisfies: d _m ≤ 0.8 μm.

Example:

The core diameter of the few-mode optical fiber 2 is 25 μm, and the refractive index difference between the core and the cladding is Δ=0.0057, and its output spectrum is shown in FIG. 4 . The sensing sensitivity and detection limit under different fiber lengths are shown in Fig. 5. When the length of the few-mode fiber is L=120mm, the average sensitivity is 0.3942nm/g, and the detection limit is 16.5×10 ^-6 με.

It should be understood that although this description is described according to various embodiments, not each embodiment only includes an independent technical solution, and this description of the description is only for clarity, and those skilled in the art should take the description as a whole , the technical solutions in the various embodiments can also be properly combined to form other implementations that can be understood by those skilled in the art.

The series of detailed descriptions listed above are only specific descriptions for feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiment or All changes should be included within the protection scope of the present invention.

Claims (5)

1. an optical fiber stress sensor part, it is characterised in that include single-mode fiber and less fundamental mode optical fibre (2)；Described single-mode fiber Including the first single-mode fiber (1) and the second single-mode fiber (3)；Described first single-mode fiber (1), less fundamental mode optical fibre (2) and second are single Mode fiber (3) is sequentially connected with；

The normalized frequency of described less fundamental mode optical fibre (2) meets:

$V=\frac{2{\mathrm{\&pi;a}}_1}{{\mathrm{\&lambda;}}_0}{(n_1^2-n_2^2)}^{1/2},$

3.83171 < V < 7.01559,

Wherein, n₁Represent the refractive index of less fundamental mode optical fibre (2) fibre core；

n₂Represent the refractive index of less fundamental mode optical fibre (2) covering；

a₁Represent the radius of less fundamental mode optical fibre (2) fibre core；

λ₀Represent operation wavelength.

A kind of optical fiber stress sensor part the most according to claim 1, it is characterised in that the fibre of described less fundamental mode optical fibre (2) Core meets with clad refractive rate variance Δ: 0.007 >=Δ >=0.002.

A kind of optical fiber stress sensor part the most according to claim 1, it is characterised in that described single-mode fiber and few mould light The lateral deviation d of fine (2)_mMeet: d_m≤0.8μm。

A kind of optical fiber stress sensor part the most according to claim 1, it is characterised in that the length of described less fundamental mode optical fibre (2) Degree L meets: L >=120mm.

5. according to a kind of optical fiber stress sensor part described in any one in claim 1,2,3 or 4, it is characterised in that Stress acts only on less fundamental mode optical fibre (2).