Intensity Demodulated Refractive Index Sensor Based on Front-Tapered Single-Mode-Multimode-Single-Mode Fiber Structure

Abstract

A novel intensity demodulated refractive index (RI) sensor is theoretically and experimentally demonstrated based on the front-tapered single-mode-multimode-single-mode (FT-SMS) fiber structure. The front taper is fabricated in a section of multimode fiber by flame-heated drawing technique. The intensity feature in the taper area is analyzed through the beam propagation method and the comprehensive tests are then conducted in terms of RI and temperature. The experimental results show that, in FT-SMS, the relative sensitivity is −342.815 dB/RIU in the range of 1.33~1.37. The corresponding resolution reaches 2.92 × 10−5 RIU, which is more than four times higher than that in wavelength demodulation. The temperature sensitivity is 0.307 dB/°C and the measurement error from cross-sensitivity is less than 2 × 10−4. In addition, fabricated RI sensor presents high stability in terms of wavelength (±0.045 nm) and intensity (±0.386 dB) within 2 h of continuous operation.

Ultra-Sensitive Intensity Modulated Strain Sensor by Tapered Thin-Core Fiber Based Modal Interferometer

“…From Figure 8b, the wavelength sensitivity is 0.0736 nm/ • C with the linearity of~0.99. The corresponding intensity sensitivity is merely 0.0381 dB/ • C in the range from 25 • C to 75 • C. Therefore, the calculated cross-sensitivity caused by ambient temperature is~0.32 µε/ • C. Moreover, according to [27], the measured axial strain and temperature changes can be effectively discriminated by using the inverse matrix method shown as Equation (7).…”

“…Photonics 2021, 8, 372 2 of 11 portability of sensors without an expensive and heavy high-precision spectrometer [27]. Zhang et al proposed an intensity modulation scheme by using a piece of micro-structure hollow-core fiber (MS-HCF), but the presented sensitivity was less than 0.004 dB/με [28].…”

“…Moreover, the highest sensitivity so far reached 1.15 nm/µε in the range of 0~230 µε, through a cascaded micro-cavity structure [26]. Compared with wavelength sensitive structures, the intensity demodulation schemes can greatly improve the practicability and portability of sensors without an expensive and heavy high-precision Photonics 2021, 8, 372 2 of 11 spectrometer [27]. Zhang et al proposed an intensity modulation scheme by using a piece of micro-structure hollow-core fiber (MS-HCF), but the presented sensitivity was less than 0.004 dB/µε [28].…”

Ultra-Sensitive Fiber Refractive Index Sensor with Intensity Modulation and Self-Temperature Compensation

“…Further, the variations of RI and temperature in DP structure can be discriminated by the inversion matrix method [28], which can be described as [ΔTΔn]=1D[ kIn −kλn−kIT kλT] [ΔλΔI], where ∆ n and ∆ T are the variations of RI and temperature, respectively. ∆ λ and ∆ I are the wavelength shift and intensity change.…”

“…Moreover, based on the RI-free feature of multimode fiber (MMF), the bias-taper based structures with self-temperature compensation are respectively fabricated by arc-discharge and flame-brushing techniques. The reported sensitivities are about −340 dB/RIU with the linearity of >0.98 [28,29]. Further, Chen et al form a weak Fabry–Perot cavity by the slight RI difference of TCF and single mode fiber (SMF) and an exceeding −1100 dB/RIU sensitivity is demonstrated, but only at the point of 1.4305 [30].…”