CN106645029A - Weld type open-cavity optical fiber EFPI (extrinsic Farby-Perot interferometric) refractive index sensor - Google Patents

Abstract

The invention provides a fusion-spliced open-cavity optical fiber EFPI refractive index sensor, which belongs to the technical field of optical fiber sensing. The sensor adopts a single-mode fiber-capillary-single-mode fiber fusion structure, and the two end faces form a double-beam interference. Optical fiber fusion splicer is used for fusion splicing of single-mode optical fiber and quartz capillary. The fused silica capillary with an outer diameter of 125um is used here. The outer diameter of the capillary is the same as the outer diameter of the optical fiber, and the inside is hollow. The inner diameter of the air layer is about 50-65um. This air cavity is the F-P cavity , the cavity length is between 1mm and 1.5mm. Grind a gap in the capillary with optical fiber grinding paper, so that the liquid can flow into the cavity. By measuring the effective cavity length of the F-P interferometer, the absolute measurement of the refractive index of the liquid to be measured can be realized by using correlation demodulation. The sensor is simple in structure, easy to manufacture, low in cost, and flexible and convenient in application. Absolute measurement of the refractive index can be achieved.

Description

A fusion-spliced open-cavity optical fiber EFPI refractive index sensor

technical field

The invention belongs to the technical field of optical fiber sensing and relates to a fusion-spliced open-cavity optical fiber EFPI refractive index sensor.

Background technique

Optical fiber sensor has been developed deeply due to its advantages of compact structure, light weight and resistance to electromagnetic interference and corrosion. Among them, the phase modulation optical fiber sensor has high detection sensitivity due to the interference technology adopted, and the interference of light in the optical fiber sensor is mainly realized by various optical fiber interferometers. The fiber optic sensor with Fabry-Perot interferometer structure has been widely used due to its simple structure, small size, high reliability, high sensitivity, fast time response, and single fiber signal transmission. Most of the refraction index sensors studied now need to be calibrated and can only achieve relative measurement. This sensor can realize absolute measurement of refraction index by using signal correlation processing algorithm.

Contents of the invention

The object of the present invention is to provide a fusion-spliced optical fiber EFPI refractive index sensor with a simple structure, which can realize the absolute measurement of the liquid refractive index by using a signal correlation processing algorithm. Compared with the general traditional EFPI sensor, this sensor has the advantages of large cavity length, simple structure, simple manufacturing process, flexible and convenient application, etc. Only single-mode optical fiber and quartz capillary are needed, and two sections of single-mode optical fiber are fused to the quartz capillary with an optical fiber fusion splicer. Grinding the capillary with optical fiber polishing paper, the liquid to be tested can enter the capillary, thereby realizing the measurement of the refractive index of the liquid to be tested.

Technical scheme of the present invention:

A fusion-spliced open-cavity optical fiber EFPI refractive index sensor adopts the method of demodulating the F-P cavity length to measure the refractive index of the liquid to be measured; the open-cavity optical fiber EFPI refractive index sensor is composed of a capillary and a single-mode optical fiber. A section of single-mode fiber and capillary are welded as a whole, and two sections of single-mode fiber are respectively welded at both ends of the capillary, using the two end faces of single-mode fiber-capillary-single-mode fiber to form a double-beam interference structure, and the length of the capillary is the initial F-P cavity length ; It is bent and fixed as a whole, and the top surface of the curved side of the capillary is ground until a gap appears, so that the liquid can flow into the air cavity of the capillary.

The outer diameter of the capillary is 125um, the outer diameter of the capillary is the same as that of the optical fiber, the inner diameter of the capillary is 50-100um, and the length of the hollow F-P cavity is 50um-2mm.

A kind of preparation method of fusion-spliced open-cavity optical fiber EFPI refractive index sensor is as follows:

Using an optical fiber fusion splicer for the fusion of single-mode optical fiber and quartz capillary requires constant trial and error to set and adjust the fusion parameters, and finally obtain the complete parameters with good fusion effect, and then make the sensor head by fusion. The sensing head is fixed on the grinding tool, and the optical fiber is used to grind until a gap appears in the capillary, so that the liquid can flow into the cavity, and the absolute measurement of the refractive index of the liquid to be measured can be realized by correlation demodulation.

The reflected light intensity I of a typical F-P interferometer is mainly affected by the value of the effective cavity length nd of the interferometer. The reflected light intensity can be described by the following formula:

where I is the total intensity of the interference spectrum, A and B are constants, n is the refractive index of the cavity in the F-P interferometer, d is the actual physical cavity length of the F-P interferometer, and λ is the wavelength of free-space light.

The actual distance of the manufactured F-P interferometer is fixed, that is, the actual physical cavity length, and the optical distance cavity length of the interferometer directly depends on the refractive index of the measured substance. The optical distance can be calculated to realize the absolute measurement of the refractive index of the measured material. The incident fiber is connected to a wavelength-scanning optical fiber sensing signal demodulator such as the SM125 fiber grating demodulator. Using the cross-correlation signal processing method, the absolute effective cavity length nd of the F-P interferometer can be obtained. In the case of air, the refractive index n is about 1. What is measured is the physical cavity length of the capillary air cavity. Change, the physical cavity length d remains unchanged, the refractive index of the liquid to be measured can be obtained by dividing the two, and the absolute measurement of the refractive index can be realized.

Beneficial effects of the invention: only single-mode optical fiber and quartz capillary are needed as materials, and two sections of single-mode optical fiber can be welded to the quartz capillary by using an optical fiber fusion splicer. The sensor is simple in structure, easy to manufacture, low in cost, and flexible and convenient in application. The absolute measurement of the refractive index of the liquid to be measured can be realized without calibration, and the calculated value is the true value of the refractive index.

Description of drawings

Figure 1 is a schematic diagram of a fusion-spliced open-cavity optical fiber EFPI refractive index sensor.

Fig. 2 is a flow chart of making a fusion-spliced open-cavity optical fiber EFPI refractive index sensor.

In the figure: 1 single-mode fiber; 2 capillary.

detailed description

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

In the invention, a double-beam interference structure is formed by two end surfaces of single-mode optical fiber-capillary-single-mode optical fiber, and an all-silica optical fiber EFPI refractive index sensor is made. The fused silica capillary with an outer diameter of 125um is used here. The outer diameter of the capillary is the same as the outer diameter of the optical fiber, and the inside is hollow. The inner diameter of the air layer is about 70um. This layer of air cavity is the F-P cavity, and the cavity length is at Between 50um-2mm. The capillary is partially ground to form an open F-P cavity, so that the liquid to be measured can flow into the cavity. At this time, the refractive index of the liquid may be obtained by measuring the effective cavity length of the F-P interferometer.

(1) Put the cut flat capillary and single-mode fiber on the fusion splicer. The upper left picture in Figure 2 is the pattern after the fiber core is aligned (the left end is the single-mode fiber, and the right end is the capillary). (2) During the discharge welding process, the capillary and the single-mode optical fiber are joined together, and the two contact end faces are flush without protrusions and shrinkage. (3) Cut the capillary under a microscope. This step requires taking the part that has been fused in the previous step from the fusion splicer to the microscope. Since the capillary is very easy to break, try to hold one end of the optical fiber during operation, and hold the capillary part with your hands. According to the outer diameter of the optical fiber, find the place where the capillary length is about 1mm as the incision, and cut it off slowly with a cutting knife. (4) Carry out fusion splicing of the single-mode optical fiber and the capillary again, so far two sections of single-mode optical fiber have been welded to both ends of a capillary of about 1 mm, and the sensor head is completed. (5) Fix the welded fiber capillary structure to the glass plate, and bend and fix the structure for convenient grinding. (6) Use optical fiber polishing paper to grind the capillary in the middle until a gap is formed so that the liquid can enter the capillary.

Claims (3)

1. an open-cavity optical fiber EFPI refractive index sensor of a fusion type is characterized in that, adopts the demodulation F-P cavity length mode, realizes the refractive index measurement of liquid to be measured; This open-cavity optical fiber EFPI refractive index sensor is made of capillary and single-mode optical fiber, Weld two sections of single-mode optical fiber and capillary that are horizontally cut and flat as a whole, and weld the two sections of single-mode optical fiber to the two ends of the capillary respectively, and use the two end faces of single-mode optical fiber-capillary-single-mode optical fiber to form a double-beam interference structure, the length of the capillary It is the length of the initial F-P cavity; it is bent and fixed as a whole, and the top surface of the curved side of the capillary is ground until a gap appears, so that the liquid can flow into the air cavity of the capillary. 2. The open-cavity optical fiber EFPI refractive index sensor according to claim 1, wherein the outer diameter of the capillary is 125um, the outer diameter of the capillary is the same as that of the optical fiber, and the inner diameter of the capillary is 50-100um. 3. The open-cavity optical fiber EFPI refractive index sensor according to claim 1 or 2, wherein the initial F-P cavity length is 50um-2mm.