Fig. (1): The schematic diagram of sensor structure: (A) SMS fiber structure, (B) The V shape groove with fiber structure on it.
Sensor structure fabrication:
The SCS configuration based refractive index sensor is display Figure (2). the sensor structure comprises of a broadband source (BBS Thorlabs S5FC10055) with a wavelength range of 1400 nm to 1600 nm), two segments of SMF (Coring SMF-28) and one Segment of CLS (FG125LA from Thorlabs) in between. BBS was connected to the SMF-CLF-SMF (SCS) structure from the first section of SMF and the other end of this structure connected to an optical spectrum analyzer (OSA YOKOGAWA AQ6370C) to detect the response transmission spectrum of the RI sensor. Where SMF spliced to CLF (three different lengths) by fusion splicer (Fujikura FSM-60S), the interference between the fundamental mode and the higher-order modes happens in a second spliced area between the CLF and SMF. To examine RI sensor response, The SCS configuration submerged in U groove which contains the NaCl solution with different concentrations.
RI sensor was studied for three different lengths with 20, 25 and 30 mm of CLF which is represented the sensing area. Since the CLF diameter is an essential factor in SCS fiber configuration to attain extreme sensitivity, the effect of CLF diameter on the RI sensing sensitivity was investigated. Accordingly, various tuned diameters of CLF of 100, 75 and 50 µm were achieved for each length. The CLF diameter was chemically etched using 40% hydrofluoric (HF) acid immersion. The construction method of SCS fiber configuration began with stripped-off the acrylate coating from CLF and SMF, then the CLF fusion spliced from both ends with SMF. Then, the fabricated SCS construction was set in a U-shaped groove. This groove was utilized to include the etching liquid acid (HF 40%). The 100, 75 and 50 µm CLF diameter was realized by controlling the etching time. The etching method was retained in CLF section, and the splicing zone was verified under the microscope in each single case to make certain that no etching happened due to HF liquid.
The sodium chloride solution (NaCl) was utilized as a testing solution, which was prepared by dissolving NaCl in 100 ml of deionized water utilizing the magnetic stirrer at room temperature (25oC). Under a controlled laboratory environment, a group of solutions had been made from different concentrations of NaCl powder (0, 5, 10, 15, 20 and 25 %) of NaCl dissolved in deionized water. The concentration corresponding to different RIs of 1.333-1.382 ranges. Abbe refractometer was utilized to measure the RI of the solution. In our experiment, the sensing area of the structure is totally submerged in these solutions and before each single measurement, the structure is cleaning by the deionized water and dried in air.