Study On The Refractive Index Sensing Characteristics Of The STS Optical Fiber Stuture

Optical fiber sensing technology commonly through a variety of enhanced package structure or fiber parameters changing with the external physical quantity,which caused by changes in the parameters of optical fiber sensing technology, then get a further perception and transmission of the external signal. The optical fiber sensor is widely used because of its unique characteristics, especially in the electrical insulation, chemical stability, corrosion resistance and interference resistance.Especially in liquid refractive index detection, the optical fiber sensor has the unique advantages of high sensitivity, good flexibility, small volume, and can be made into different shapes, corrosion resistance and a large number of unique advantages. With the increasing demand for testing technology, the liquid refractive index sensor is put forward higher requirements.In this paper,we focused on the all fiber structure of the fiber liquid refractive index sensor, Compared with the existing fiber grating refractive sensor,the sensor has the characteristics of high sensitivity, simple structure,low cost and easy realization. However,a large number of studies have shown that this kind of sensor is still to be developed in low concentration liquid refractive index detection. The paper designed the fiber optic refractive index sensor that based on the single mode-thin core-single mode (STS) fiber structure for all fiber Mach-Zehnder interferometer. We studied the refractive index sensing properties, especially the improvement of the sensitivity of the sensor in small refractive index,and proposed the structure scheme to increase the sensitivity of the sensor,the main research work of this paper are as follows:1. Theoretical and experimental study of STS structure sensor system. On the basis of the principle of single mode and small diameter fiber, the relationship between the output spectrum of the sensor and the optical fiber parameters is studied.The relationship between the output wavelength and the refractive index of the sensor is studied. And we got the curves of refractive index and wavelength of different wavelength (1541nm and 1555nm) and the relationship between the sensitivity and wavelength.2. The relationship between refractive index and refractive index of STS fiber was studied experimentally. The curves of the interference wavelength shift and refractive index of the sensor with different fiber bending diameters (9.6cm, 8.3cm,7.5cm) were obtained. Measured the output spectra of refractive index of 1%-10%Nacl and 1.335-1.4384 solution, analyzed the relationship between the refractive index and the output interference wavelength of the sensor. Through bending the sensitive part of the fiber sensor, it can effectively improve the refractive index sensitivity, especially effective in low concentration liquid refractive index detection.3. Study on liquid refractive index sensing characteristics of partially clad STS fiber. In the experiment,the sensitivity of the sensor can be improved by removing the sensitive part of the optical fiber. The experimental method and process are discussed in detail. The experimental results show that the STS all fiber refractive index sensor has good sensitivity to small refractive index. For 0-1% and 1%-10%Nacl solutions, different wavelength (1539.43nm and 1545.5nm) has different sensitivity, and wavelength and refractive index changes have a higher linear fitting.