Cascaded Ultra-Low Reflective Fiber Points for Distributed Sensing

Abstract

Distributed fiber sensors based on the frequency domain analysis of Rayleigh backscattered light are well established. They exhibit very good performance in both sensitivity and spatial resolution, but their application can be limited due to their cost and the complexity of the analysis. In this work we present a system based on coherent optical frequency domain reflectometry, used in Rayleigh distributed sensors, implemented with more readily available components and simplified analysis. A sensing fiber is prepared by printing uniformly spaced, ultra-low reflectivity fiber Bragg gratings of the same Bragg wavelength. When tuneable source light is introduced to the fiber the reflections from the gratings interfere with the reflection from the tip of the fiber. The gratings’ reflectivity varies randomly which produces a frequency domain trace that shares some of the properties of a Rayleigh spectral trace, but is significantly stronger. This removes the need for specialized detection equipment. These Bragg gratings act as reflectors and not as sensors per se. Use of a reference interferometer and signal processing algorithms make it possible to replace a high precision linearly tuneable laser with a standard tuneable laser as optical source.