CN103364370A - Annular core optical fiber sensor based on annular chamber decline - Google Patents

Abstract

The invention provides a ring core optical fiber sensor based on ring cavity fading. The two ends of a section of large-diameter ring-core fiber are connected to the ends of two 1×2 fiber couplers with high splitting ratio, and the two 1×2 fiber couplers are connected to each other on one side with one port. Then a ring cavity is formed, and the pulse light source and the detector are respectively connected to the ends of the two 1×2 fiber couplers with a lower splitting ratio. The core of the sensing part of the ring-core optical fiber is very close to the outside world and has a strong evanescent field. Changes in the environment such as the refractive index of the external liquid or gas can be measured by measuring the ring-down time. The sensor can minimize the adverse effects caused by the fluctuation of the light source. The measuring device has a simple structure, light weight, high sensitivity and strong anti-interference ability. It will be widely used in the sensing of solution or gas concentration and temperature.

Description

Ring core optical fiber sensor based on ring cavity fading

technical field

The invention relates to an optical fiber sensor, in particular to a ring-core optical fiber sensor mainly used for sensing and measuring external environment refractive index, gas concentration and the like.

Background technique

Cavity Ringdown (CRD, Cavity Ringdown) spectroscopy technology is an ultra-high sensitivity detection absorption spectrum technology that emerged in the late 1980s. It is not affected by fluctuations in light intensity and responds quickly. Great potential in pressure sensors and sensing systems. However, in order to maintain the collimation characteristics of the optical path, CRD technology has high requirements on the reflectivity of the mirror and the accuracy of position adjustment, so it is difficult to put it into practical use.

Fiber Loop Ring Down technology (FLRD, Fiber Loop Ring Down) is the development of traditional CRD technology, which was originally proposed by Stewart et al. in 2001, and was originally used for the measurement of gas concentration. It uses optical fiber and coupler to form an equivalent optical mirror, the transmitted laser is confined in the optical fiber, and the influence of external effects on transmission loss is detected by measuring the time when the light intensity decays to the threshold value. Since the physical quantity measured by FLRD sensing technology is time, no optical amplification equipment is required, and ASE noise will not be introduced, so it has high sensitivity, fast response speed, high accuracy, low light source power, and low requirements for light source power stability. advantage. It can also realize various sensing such as pressure, temperature, tension, refractive index, chemical composition, etc. by assembling sensing heads with different structures, and has greater flexibility in application. These properties of FLRD make it possible to fabricate a new generation of fiber optic sensors.

Contents of the invention

The object of the present invention is to provide a ring core optical fiber sensor based on ring cavity fading with simple structure, light weight, high sensitivity and strong anti-interference ability.

The purpose of the present invention is achieved like this:

The two ends of a section of large-diameter ring-core fiber are respectively connected to the ends of two 1×2 fiber couplers with high splitting ratio, and the two 1×2 fiber couplers are connected to each other on one side with one port. Then a ring cavity is formed, and the pulse light source and the detector are respectively connected to the ends of the two 1×2 fiber couplers with a lower splitting ratio.

The described optical fiber with large-diameter annular core is an optical fiber whose diameter is 125 microns, the shape of the optical fiber core is annular, the wall thickness of the core is 4-8 microns, and the outer diameter of the annular core is an optical fiber of 80-125 microns. The refractive index n _core is greater than the cladding refractive index n _clad .

The length of the said ring-core optical fiber with large diameter used for the sensing part is 2-5 centimeters.

The ring-core waveguide with the large-diameter ring-core fiber used for the sensing part is less than 1 micron away from the outside air.

The optical fiber with a large-diameter annular core is an optical fiber with an outer diameter of 80-123 microns, and the cladding of the optical fiber is thinned by chemical etching, so that the core is exposed to the outside world.

The high split ratio 1×2 fiber coupler has a split ratio higher than 99:1.

The connection between the large-diameter ring-core fiber and the fiber coupler is directly welded by aligning the two ends of the large-diameter ring-core fiber with the axis of the high splitting ratio 1×2 fiber coupler, and then at the welding point Fused taper to achieve.

The connection between the large-diameter ring-core fiber and the fiber coupler is achieved by aligning and welding a point at the entrance ring waveguide of the large-diameter ring-core fiber with a fiber core of a high splitting ratio 1×2 fiber coupler, The output end of the fiber with a large-diameter ring core is aligned with the axis of another high-splitting ratio 1×2 fiber coupler and directly welded, and then melted and tapered at the welding point to achieve.

A layer of sensitive film is added to the sensing part of the optical fiber with a large-diameter annular core. Improve system sensitivity.

The core of the sensing part of the ring-core optical fiber is very close to the outside world and has a strong evanescent field. Changes in the environment such as the refractive index of the external liquid or gas can be measured by measuring the ring-down time. The sensor can minimize the adverse effects caused by the fluctuation of the light source. The measuring device has a simple structure, light weight, high sensitivity and strong anti-interference ability. It will be widely used in the sensing of solution or gas concentration and temperature.

Compared with prior art, the advantage of the present invention is:

1. Using the surface of the ring-core optical fiber core as the sensing probe part, because the energy emitted by the evanescent field is more uniform and the sensing area is larger, the sensitivity of the sensor is higher;

2. The use of optical fiber ring cavity ring down technology makes it more convenient to measure and obtain data, which can be realized by using two couplers, and the price is cheaper and cost-effective;

3. When the core of the large-diameter ring-core fiber is exposed to the outside world, it is sensitive to changes in the outside temperature, bending, pressure, and refractive index. It is also convenient to do chemical or physical modifications on the surface, making this ring-based cavity The fading ring-core fiber optic sensor can be varied and has a wide range of applications.

Description of drawings

Figure 1 is a structural diagram of a ring-core optical fiber sensor based on ring cavity fading;

Figure 2(a) is a schematic cross-sectional view of a ring-shaped surface core fiber; Figure 2(b) is a schematic cross-sectional view of a ring-core fiber when the outer diameter of the ring core is less than 125 microns;

Fig. 3 is a schematic diagram of axial alignment welding of the annular surface core optical fiber and the end face of the single-mode optical fiber;

Fig. 4 is a schematic diagram of tapering after alignment of the axis of the annular surface core fiber and the single-mode fiber after welding;

Fig. 5 is a schematic diagram of the corrosion procedure of the annular core optical fiber when the outer diameter of the annular core is less than 125 microns;

Fig. 6 is a schematic diagram of the ring core fiber used as a sensing unit and its coupling with a single-mode fiber when the outer diameter of the ring core is less than 125 microns;

Fig. 7 is a schematic diagram of the structure of adding a layer of sensitive film outside the core of the annular surface core fiber.

Detailed ways

The basic scheme of the present invention is: the annular core optical fiber sensor based on the ring cavity fading effect is composed of two ends 1-1 and 1-2 with a large diameter annular core optical fiber 1 and two 1 × 2 optical fibers with high splitting ratio respectively. The ends 2-2 and 3-2 of the coupler 2 and 3 with high splitting ratio are connected, and the sides 2-3 and 3-3 of two 1×2 fiber couplers with one port are interconnected with each other to form a ring cavity, and the pulse The light source 4 and the detector 5 are respectively connected to the ends 2-1 and 3-1 of two 1×2 fiber couplers with a low splitting ratio. The splitting ratio of the high splitting ratio 1×2 fiber coupler should be at least higher than 99:1. The optical fiber with large-diameter annular core is a special optical fiber with an optical fiber diameter of 125 microns, an optical fiber core shaped as a ring, a wall thickness of the core 6 of 4-8 microns, and an outer diameter of the annular core 6 of 80-125 microns. The length used for the sensing part is 2-5 cm, and the distance between the annular core waveguide 6 of the sensing part and the outside air is less than 1 micron. If the outer diameter of the annular core 6 is an optical fiber of 80-123 microns, the cladding 7 of the optical fiber can be thinned by chemical etching such as HF acid, so that the distance between the core waveguide 6 and the outside air is less than 1 micron and exposed as much as possible In order to increase the evanescent field of the optical fiber as much as possible, and then improve the sensitivity of the system. The working principle of the ring-core optical fiber sensor based on the ring-down cavity is that when the narrow-pulse laser light source 4 is injected from the 2-1 end, and the pulse width is less than the time required for the light to move in the cavity for a week, the incident light passes through the ring core once The light intensity at the output end of the optical fiber will attenuate due to the strong evanescent field of the ring core fiber, and the light intensity will decrease exponentially with time, and then the attenuation time of the system can be measured, and the attenuation time will change due to the change of the material around the sensing probe , and then obtain the corresponding change to be measured.

The present invention is described in more detail below in conjunction with accompanying drawing example:

Example 1:

In conjunction with Fig. 1-Fig. 4, a kind of surface ring core optical fiber sensor sensing unit based on optical fiber ring cavity ring down is a ring core 6 with a length of 3 cm and an outer diameter of 125 micron ring surface core optical fiber 1, and its two ends 1- 1 and 1-2 are respectively connected with two 1×2 fiber couplers 2 and 3 with a split ratio of 99.0:1.0 and one end 2-2 and 3-2 with a split ratio of 99.0, and the two 1×2 fiber couplers have 1 One side 2-3 and 3-3 of each port are interconnected with each other, thereby forming an annular cavity. The pulse light source 4 and the detector 5 are respectively connected to one end 2-1 and 3-1 of two 1×2 fiber couplers with a splitting ratio of 1.0. The connection between the ring-core fiber 1 and the fiber couplers 2 and 3 is directly welded by aligning the two ends 1-1 and 1-2 of the ring-core fiber 1 with the axes of the coupler fibers 3-2 and 2-2 (Fig. 3), and then perform fusion tapering at the solder joint (Figure 4), and realize energy coupling through the tapered transition zone 8. When the light source 4 is injected from one end, the other end is detected by the photodetector 5 and the oscilloscope and displays the change of the emitted light intensity with time, and then measures its decay time. If the sensing unit is placed in saline solution, when the concentration of saline solution changes , the refractive index of the liquid will change. At this time, the light attenuation value observed by the detector and the measured attenuation time will change with the change of the refractive index around the sensing unit. According to the measured attenuation time, it is easy to get Refractive index of salt water of unknown concentration and its concentration.

Example 2:

With reference to Fig. 1, Fig. 5 and Fig. 6, a ring-core optical fiber sensor sensing unit based on optical fiber ring cavity ringdown is a ring-core optical fiber 1 with a ring-core 6 outer diameter of 100 microns, and its two ends 1-1 and 1 -2 is respectively connected with two 1×2 fiber couplers 2 and 3 with a split ratio of 99.0:1.0 and one end 2-2 and 3-2 with a split ratio of 99.0, and the two 1×2 fiber couplers have 1 output port One side 2-3 and 3-3 are interconnected with each other, thereby forming an annular cavity. The pulse light source 4 and the detector 5 are respectively connected to one end 2-1 and 3-1 of two 1×2 fiber couplers with a splitting ratio of 1.0. The output end 1-2 of the ring-core optical fiber 1 and the optical fiber 2-2 end of the coupler 2 are directly welded through axial alignment, and then melted and tapered at the welding point, and the energy coupling is realized by using the tapered transition zone 8 ; And the incident end 1-1 of ring-core optical fiber 1 and the optical fiber 3-2 end of coupler 3 are to utilize the fiber core of optical fiber 3-2 to be aimed at a certain point 10 at the ring-core optical fiber ring waveguide place, then carry out welding and realize energy coupling of (Figure 6). The ring-core optical fiber 1 with an outer diameter of 100 microns etched its outer cladding 7 to 100 microns with hydrofluoric acid, and the corroded length was 2 cm. When the light source 4 is injected from one end, the other end is detected by the photodetector 5 and the oscilloscope and displays the change of the emitted light intensity with time, and then measures its decay time. If the sensing unit is placed in the gas chamber, when the gas concentration changes, The refractive index of the gas will also change. At this time, the light attenuation value observed by the detector and the measured attenuation time will change with the change of the gas concentration around the sensing unit. According to the measured attenuation time, it is easy to get the unknown concentration of the gas concentration.

Example 3:

In conjunction with Fig. 1 and Fig. 7, a ring-core optical fiber sensor based on the ring-down of the fiber-optic ring cavity is different from Embodiment 1 in that a thin metal film or dielectric film is coated on the surface of the sensing part of the ring-shaped surface core 9 to increase the sensitivity of the sensor.

Claims (9)

1. A ring-core optical fiber sensor based on ring cavity fading, characterized in that: a section has two ends of a large-diameter ring-core fiber connected to two high-splitting 1 × 2 optical fiber coupler ends with a high splitting ratio, One side of the two 1×2 fiber couplers with one port is connected to each other to form a ring cavity, and the pulse light source and the detector are respectively connected to the ends of the two 1×2 fiber couplers with a lower splitting ratio. 2. the ring core optical fiber sensor based on ring cavity fading according to claim 1 is characterized in that: the optical fiber with the large diameter ring core is that the fiber diameter is 125 microns, the fiber core shape is ring, and the core wall For an optical fiber with a thickness of 4-8 microns and an outer diameter of the annular core of 80-125 microns, the refractive index n _core of the core is greater than the refractive index n _{clad of the cladding} . 3. The ring-core optical fiber sensor based on ring cavity fading according to claim 1, characterized in that: the length of the sensing portion of the ring-core optical fiber with a large diameter is 2-5 cm. 4. The ring-core optical fiber sensor based on ring cavity fading according to claim 1, characterized in that: the ring-core waveguide with a large-diameter ring-core fiber used for the sensing part is less than 1 micron away from the outside air. 5. The ring-core optical fiber sensor based on ring cavity fading according to claim 2, characterized in that: said ring-core fiber with a large diameter is an optical fiber with an outer diameter of 80-123 microns, which is removed by chemical etching Thin the cladding of the optical fiber to expose the core to the outside world. 6. The ring core optical fiber sensor based on ring cavity fading according to claim 1, characterized in that: the light splitting ratio of the high splitting ratio 1×2 fiber coupler is higher than 99:1. 7. The ring-core optical fiber sensor based on ring cavity fading according to claim 1, characterized in that: the connection between the large-diameter ring-core fiber and the fiber coupler is by combining the two ends of the large-diameter ring-core fiber with high-splitting It is achieved by aligning the axis of the 1×2 fiber optic coupler for direct welding, and then performing fusion tapering at the welding point. 8. The ring-core optical fiber sensor based on ring cavity fading according to claim 1, characterized in that: the connection between the large-diameter ring-core fiber and the fiber coupler is by placing a large-diameter ring-core fiber at the incident end ring waveguide A certain point is aligned and welded with a fiber core of a high-splitting ratio 1×2 fiber coupler, and the output end of a fiber with a large diameter ring core is directly welded with the axis of another high-splitting ratio 1×2 fiber coupler. It is then achieved by melting and tapering at the solder joints. 9. The ring-core fiber sensor based on ring cavity fading according to claim 1, characterized in that: the sensing part of the ring-core fiber with a large diameter is coated with a sensitive film.

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