Ultra-low loss Rayleigh scattering enhancement via light recycling in fiber cladding

Abstract

Rayleigh backscattering enhancement (RSE) of optical fibers is an effective means to improve the performance of distributed optical fiber sensing. Femtosecond laser direct-writing techniques have been used to modulate the fiber core for RSE. However, in-core modulation loses more transmission light, thus limiting the sensing distance. In this work, a cladding-type RSE (cl-RSE) structure is proposed, where the femtosecond laser is focused in the fiber cladding and an array of scatterers is written parallel to the core. The refractive-index modulation structure redistributes the light in the cladding, and the backward scattered light is recovered, which enhances the Rayleigh backscattered signal with almost no effect on the core light. Experimentally, it was demonstrated that in an effectual cl-RSE structure, the insertion loss was reduced to 0.00001 dB per scatterer, corresponding to the lowest value for a point scatterer to date. The cl-RSE structure accomplished measurements up to 800°C. In particular, the temperature measurement fluctuation of the cl-RSE fiber portion is only 0.00273°C after annealing. These results show that the cl-RSE structure has effective scattering enhancement, ultra-low loss, and excellent high-temperature characteristics, and has great potential for application in Rayleigh scattering-enhanced distributed fiber sensing.