Study Of Single Mode-Tapered Multimode-Single Mode Fiber Liquid Refractive Index Sensor

Refractive index(RI) is one of the most important physical parameters of liquid and it also reflects the essential characteristics of liquid. It is closely related to the other parameters such as temperature, concentration, density and thermal parameters. Therefore, in physical, chemical, biological, medical, environmental, and the other fields’ scientific research and production practice, the liquid refractive index change monitor is very important. Because of optical fiber sensor has a lot of excellent features such as resistance to electromagnetic interference, electric insulation, corrosion resistance, high sensitivity, low cost, simple structure, small impact to the measured medium, so it can be used to monitor the liquid refractive index, and it has been extensively studied in recent years.Optical fiber liquid refractive index sensor utilizes the change of liquid refractive index to modulation the optical parameters of transmission light(such as intensity, phase, wavelength, etc.), by comparing the spectrum of before and after modulation, we can demodulating the change of the refractive index. In recent years, the single mode-multimode-single mode(SMS) fiber structure refractive index sensor get a lot of research. In order to improve the refractive index sensitivity, researchers generally make some micro structure on the multimode fiber of SMS, the micro structure can enhance the evanescent wave and fully accept the refractive index modulation. Based on the SMS fiber structure, this paper proposed the single mode-tapered multimode-single mode(STMS) fiber structure liquid refractive index sensor, effectively improve the refractive index sensitivity. Research work consists of the following sections:1. According to the summary of existing research results found that researchers usually use hydrofluoric acid etching, laser processing, flame heating taper to strengthen the evanescent field of MMF. But hydrofluoric acid etching experiment is complex and high risk; laser processing need expensive equipment such as 2 laser or femtosecond laser, the high cost is not easy to promote. Flame heating taper method can reduce the MMF core and cladding diameter uniformly, increase the evanescent field effect effectively, easy operation and low cost. So the experiment chooses flame heating taper method.2. Through the theoretical calculation, analyzed the multimode interference and evanescent field effect. And according to a large number of taper experiments, summarized the empirical relationship between taper length and fiber diameter, provide the theoretical basis and data reference for subsequent experiments.3. We selected the 62.5/125μm multimode fiber as raw material in the production of SMS fiber structure, and greatly reduce the cost of optical fiber. Based on a tapered multimode fiber which has 30 mm taper length and 3.5532μm waist diameter, a single mode-tapered multimode-single mode(STMS) fiber refractive index sensor structure is made, which has strong evanescent field and high sensitivity to the outside refractive index. In the liquid refractive index sensing experiment, the tapered multimode fiber is maintained straight and hung in the air at room temperature 25℃.The RI liquids are placed around the taper using a dropper. Experimental results show that this refractive index sensor structure offers RI sensitivity as 1009.9nm/RIU within the RI range of 1.3414 to 1.3586. The change of liquid refractive index can be accurately monitored by the proposed sensor.