CN103940455A - All-fiber high accuracy sensor based on optical fiber multi-mode interference and application thereof - Google Patents

Abstract

The invention discloses an all-fiber high-precision sensor based on optical fiber multi-mode interference and its application. The all-fiber high-precision sensor is composed of four parts: a first single-mode fiber, a multi-mode fiber, a coreless fiber, and a second single-mode fiber. The first single-mode optical fiber, multi-mode optical fiber, coreless optical fiber, and second single-mode optical fiber are sequentially combined and welded; the specifications of the first single-mode optical fiber and the second single-mode optical fiber are the same; the diameter of the core layer of the single-mode optical fiber is smaller than the specified The core diameter of the multimode fiber; the first single-mode fiber and the second single-mode fiber are respectively used as the incident end and the transmission end of the light source, and the multimode fiber is used as a mode coupler to improve the excitation of high-order modes in the coreless fiber Efficiency, the coreless fiber is the measurement area. The invention has the advantage of designing an all-fiber sensor with low cost, high precision, small volume and compact structure. Using a section of multimode fiber as the mode coupler further optimizes the measurement accuracy of the invention.

Description

An all-fiber high-precision sensor based on optical fiber multimode interference and its application

technical field

The invention relates to the fields of optical fiber sensing such as medical and health care, biological sensing, and environmental monitoring, and in particular to an all-fiber high-precision sensor based on the principle of optical fiber multimode interference (MMI) and its application. the

Background technique

All-fiber sensor is an important field of engineering research on fiber optic sensing technology. Compared with traditional sensors, it has inherent advantages such as high safety, anti-electromagnetic interference, corrosion resistance, simple structure, small size, and high sensitivity. At the same time, this sensor based on the all-fiber structure is made of different types of optical fibers, which is low in cost, simple in manufacturing process, and sensitive advanced features. In recent years, with the development of optical fiber sensing technology and the in-depth research on all-fiber structural sensors at home and abroad, optical fiber sensors based on this principle will gradually be produced and applied in the fields of medical and health, biological sensing, and environmental monitoring. . the

Contents of the invention

The technical problem to be solved by the present invention is to provide an all-fiber high-precision sensor based on optical fiber multimode interference, which overcomes the shortcomings of traditional sensors, as well as the high cost and complicated manufacturing process of fiber Bragg gratings (FBG) and long-period gratings (LPG). defect. the

In order to solve the above technical problems, the present invention provides an all-fiber high-precision sensor based on optical fiber multi-mode interference. It consists of four parts: the first single-mode optical fiber, the multi-mode optical fiber, the coreless optical fiber and the second single-mode optical fiber. The first single-mode optical fiber, the multi-mode optical fiber, the coreless optical fiber and the second single-mode optical fiber are sequentially combined and welded; The specifications of the first single-mode fiber and the second single-mode fiber are the same; the core diameter of the single-mode fiber is smaller than the core diameter of the multimode fiber; the first single-mode fiber and the second single-mode fiber are respectively used as The incident end and transmission end of the light source, the multimode fiber is used as a mode coupler to improve the excitation efficiency of the high-order mode in the coreless fiber, and the coreless fiber is the measurement area. the

Preferably, the optical fibers have the same cladding diameter. The length of the multimode optical fiber is 20-150cm. The length of the coreless optical fiber is n*(1.45-1.47) cm, wherein n is an integer of [1-10]. the

More preferably, the length of the coreless optical fiber is 4*(1.45-1.47) cm. The automatic fusion splicing mode is selected for the fusion splicing of each optical fiber. the

The invention also proposes the application of an all-optical fiber high-precision sensor based on optical fiber multi-mode interference as an all-fiber liquid level gauge. the

The invention has the advantage of designing an all-fiber sensor with low cost, high precision, small volume and compact structure. Using a section of multimode fiber as the mode coupler further optimizes the measurement accuracy of the invention. the

Description of drawings

The technical solutions of the present invention will be further specifically described below in conjunction with the accompanying drawings and specific embodiments. the

Fig. 1 is a schematic structural diagram of an all-fiber sensor of the present invention. the

Among them, 1—first single-mode fiber, 2—multimode fiber, 3—coreless fiber, 4—second single-mode fiber; the white part in the figure is the cladding layer of the fiber, and the gray part is the core layer of the fiber. the

Detailed ways

As shown in FIG. 1 , the all-fiber sensor of the present invention is combined and welded in the order of the first single-mode fiber, multi-mode fiber, coreless fiber and second single-mode fiber. The specification of the first single-mode fiber is the same as that of the second single-mode fiber, which is a common single-mode fiber; the multimode fiber is a common multimode fiber. The diameter of the core layer of the single-mode optical fiber is smaller than that of the multi-mode optical fiber; the coreless optical fiber has no core layer structure; the optical fibers have the same cladding diameter, and the automatic fusion splicing mode can be selected when splicing. The length of the multimode optical fiber is 20-150 cm. The length of the coreless optical fiber is n*(1.45-1.47) cm, wherein n is an integer of [1-10]. The length of the coreless optical fiber is 4*(1.45～1.47)cm is optimal. the

Select a broad-spectrum light source (C+L band) as the incident light source. When the light enters the multimode fiber from the first single-mode fiber at the left end, the mode field mismatch effect occurs due to the difference in the core diameters of the two fibers, so that the fundamental mode in the first single-mode fiber is excited in the multimode fiber Higher-order mode; after the length of the multimode fiber is long enough, and the transmission mode field is stable, the mode field mismatch occurs again at the fusion joint between the multimode fiber and the coreless fiber, and the high-order mode in the coreless fiber is excited again, and the excitation of the two modes The coupling efficiency of energy from the fundamental mode to the high-order mode is greatly improved. At the fusion joint between the coreless fiber and the second single-mode fiber, part of the higher-order modes in the coreless fiber will be recoupled into the core layer of the second single-mode fiber, and multimode interference will occur; this is due to the high-order The modes have different longitudinal transmission constants, which are caused by the existence of optical path differences. the

The following takes the all-fiber optic liquid level gauge as an example for further explanation. For liquid level measurement, the external liquid or air acts as its cladding, which in turn changes the effective refractive index of the different modes in the coreless fiber. When the external liquid level changes, the average effective refractive index of each high-order mode in the measurement area, that is, the coreless fiber part, changes, and the effective refractive index changes of different order modes are different, which makes the effective refractive index difference between the modes change. The change of the optical path difference is caused, which in turn causes the interference spectrum to drift. the

The analysis method of the all-fiber liquid level gauge is to use the multi-mode interference principle of the optical fiber to realize the measurement of the liquid level outside the coreless optical fiber. The above structure sensing mechanism is given by the following formula. the

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 10</span>}\mathrm{<span\; class="notranslate">\; lg</span>}<span\; class="notranslate">\; \{</span>{<span\; class="notranslate">\; |</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; j</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; )</span>_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, L is the length of the coreless fiber in the liquid level measurement area, m is the excitation coefficient of the LP _0m mode in the coreless fiber, m is the longitudinal transmission constant of the LP _0m mode in the coreless fiber, and m is the label of the mode order.

$\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; \&infin;</span>}\mathrm{<span\; class="notranslate">\; E.</span>}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; rdr</span>}}{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; \&infin;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; rdr</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Among them, E(r, 0) is the optical field input into the coreless fiber, and m(r) is the field distribution of the m-order LP _0m mode. In the structure mentioned above, a longer section of multimode fiber is used as the mode coupler. After two high-order mode excitations, m is improved, so that the fundamental mode energy in the single-mode fiber can be more coupled to the coreless In the high-order LP _0m mode of the fiber, the multi-mode interference of the fiber that occurs at the fusion point of the coreless fiber and the single-mode fiber is strengthened.

$\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}^{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

where k ₀ is the wave number in vacuum, is the effective refractive index of the m-order LP _0m mode. A change in the effective refractive index will affect the transmission spectrum of the above structure. It should be pointed out that the above-mentioned structure measures the liquid level using the change of the effective refractive index with the liquid level. When the liquid level measurement area, that is, the coreless optical fiber is immersed in the liquid, the effective refractive index of the corresponding mode of the part immersed in the liquid will change, that is, it is different from the effective refractive index of the part placed in the air; with the immersion length of the measurement area The change is reflected as the shift of the wavelength of the transmission spectrum, so as to measure the liquid level.

It is worth noting that changes in the external liquid's refractive index, temperature, and axial stress will also change its effective refractive index. Therefore, the present invention can also be used to measure the liquid's refractive index, temperature, and stress. The measurement of the refractive index and temperature is also based on the change of the optical path difference caused by the change of the effective refractive index difference of each high-order mode. The measurement of axial stress is due to the deformation caused by the stress, which makes the interference spectrum drift, which can also be understood as the change of the corresponding effective refractive index difference. Therefore, the present invention has great application value in sensing fields such as medical and health care, biological sensing, and environmental monitoring. the

Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the present invention can be Modifications or equivalent replacements of the technical solutions without departing from the spirit and scope of the technical solutions of the present invention shall fall within the scope of the claims of the present invention. the

Claims (7)

1. An all-fiber high-precision sensor based on optical fiber multimode interference, characterized in that it is composed of four parts: the first single-mode optical fiber, multimode optical fiber, coreless optical fiber, and the second single-mode optical fiber. Optical fiber, multimode optical fiber, coreless optical fiber, and second single-mode optical fiber are sequentially combined and welded; the specifications of the first single-mode optical fiber and the second single-mode optical fiber are the same; the diameter of the core layer of the single-mode optical fiber is smaller than that of the multi-mode optical fiber core layer diameter; the first single-mode fiber and the second single-mode fiber are respectively used as the incident end and the transmission end of the light source, and the multimode fiber is used as a mode coupler to improve the excitation efficiency of the high-order mode in the coreless fiber, and the coreless fiber for the measurement area. 2. The all-fiber high-precision sensor based on optical fiber multimode interference according to claim 1, characterized in that, each optical fiber has the same cladding diameter. 3. The all-fiber high-precision sensor based on optical fiber multi-mode interference according to claim 2, wherein the length of the multi-mode optical fiber is 20-150 cm. 4. The all-fiber high-precision sensor based on optical fiber multimode interference according to claim 3, wherein the length of the coreless optical fiber is n*(1.45～1.47) cm, wherein n is [1～10] integer. 5. The all-fiber high-precision sensor based on optical fiber multimode interference according to claim 4, wherein the length of the coreless optical fiber is 4*(1.45-1.47) cm. 6. The all-fiber high-precision sensor based on optical fiber multi-mode interference according to any one of claims 1 to 5, characterized in that the automatic fusion mode is selected for the fusion of each optical fiber. 7. An application of an all-fiber high-precision sensor based on optical fiber multi-mode interference according to claim 6, wherein the all-fiber high-precision sensor based on optical fiber multi-mode interference is used as an all-fiber liquid level gauge.