Dual-parameter Sensor Based On Tapered FBG Combined With Microfiber Cavity

Optical fiber sensor technology was suggested at 80 s of last century. Compared with traditional electric sensors, optical fiber sensor has many irreplaceable advantages, such as, immunity of electromagnetic interference, excellent confidentiality, small signal crosstalk and unification of sensing and transmission. So far, optical fiber sensor can be used for detecting up to 70 physical quantities.But for most of multi-parameter fiber sensors, the cross sensitivity is inevitable and difficult to solve. In this paper, a dual-parameter microfiber FP sensor named MFPI(Microfiber Fabry-Perot interferometer) is presented. According to study, there is cross sensitivity When MFPI is used to detect temperature and RI at the same time. To solve this problem, we propose a new kind of sensor named TFMC(Tapered FBG combined with microfiber cavity) by structural improvement. Compared with MFPI, TFMC can overcome cross-sensitivity between temperature and RI sensing under some particular conditions.In this paper, we discuss MFPI’s sensing theory and experiments firstly. That study shows that MFPI’s temperature and RI(Refractive index) sensing principles are both based on the measurement of wavelength shift. In the MFPI sensing experiments, the temperature sensitivity is 9.9pm/ ℃ and acetone gas sensitivity is 22pm/%VOL. Although MFPI is capable for temperature and RI sensing individually under some conditions, to detect these two parameters simultaneously is impossible since they are both based on wavelength shift measurement. That problem is called cross sensitivity.Then to solve this problem TFMC is suggested and studied from the following aspects:Firstly, according to F-P interference theory, we get the expression of TFMC reflectivity and analyze the influence of different parameters in the expression using Matlab. From that analysis we get its temperature and RI sensing principles preliminarily. Secondly the two fabrication methods of this new kind sensor are introduced and to each method the key skills are discussed. Lastly, its sensing principle is further studied and the temperature and RI sensing experiments are finished. We obtain the two necessary conditions TFMC must meet to detect temperature and RI at the same time from that studied. Sensing principle analysis and experiments indicate that temperature and RI variation can be interpreted as the wavelength shift of the reflection band and interference pattern depth change respectively and its temperature and RI sensitivities are 13pm/℃ and 15dBm/RIU separately. These experiments further confirm the intrinsic immunity of TFMC to cross-sensitivity between temperature sensing and RI sensing.The sensitivity of TFMC could be further improved by reducing the reflectivity of FBG and drawing microfiber with uniform diameter and smooth surface. Since its cost-effectiveness, simplicity and intrinsic immunity of cross-sensitivity, TFMC shows a great potential for practical application as a miniature temperature and RI dual-parameter sensor. Moreover, by adopting this novel sensor other dual-parameter such as strain and gas concentration can be detected as well.