Ultrasensitive Refractive-Index Sensors Based on Tapered Fiber Coupler With Sagnac Loop

A kind of compact and highly sensitive refractive-index sensor based on reflective tapered fiber coupler is proposed. The sensing structure is fabricated by fusing and tapering a twisted optical fiber. The achieved maximum sensitivity is 3617 nm/RIU for measuring refractive index ranging from 1.33 to 1.41. Comparing with other similar reflective devices, the transmission loss is relatively low (similar to 2 dB), which is assigned to the employed Sagnac loop mirror to enhance the reflection of the system. Besides the intrinsic advantages of the reflective configuration, the proposed sensing structure has the additional advantages of compactness, low-cost, and ultrahigh sensitivity. It is also promising and cost effective for other all-fiber device applications.