Cladding-Mode Backward-Recoupling-Based Displacement Sensor Incorporating Fiber Up Taper and Bragg Grating

A temperature-insensitive fiber-optic displacement sensing scheme by cascading the fiber up taper and Bragg grating is experimentally demonstrated. The strength-robust up taper, which is fabricated by excessively splicing the Bragg grating to the interrogation fiber, functions as the bridge between the core mode and the cladding modes. This cascading structure realizes the cladding-mode backward recoupling, and displacement information can be directly read out by measuring the power of the highly bending sensitive recoupled cladding modes. Two sensing configurations that respectively utilize the normal fiber Bragg grating (FBG) and the uniform chirped FBG (CFBG) for the cladding-mode backward recoupling have been studied contrastively, in which the tunability of the up taper is fully validated to support a better dynamic range of the measurement. Aiming to improve the low reflection power and enhance the sensing sensitivity, the CFBG is used, and the reflection power is increased by more than 200 times and reaches 176 μW, which lowers the precision requirement for the optical power meter and potentially reduces the cost of the sensing system.