Fused Fiber Coupler Demux as an Edge Filter for Optical Sensing: A Low-Cost Solution

In this article, we explore an intensity-based interrogation technique using fused fiber demux as an edge filter, for the first time, which is a simple/elegant scheme, offering advantages such as portability and cost-effectiveness without compromising sensitivity. Strain on the FBG causes the grating pitch to vary and hence the reflected Bragg wavelength to shift: 1) peak wavelength/peak power and 2) the integrated power (intensity) variation, of the reflected spectrum are measured after passing through the edge filter. In the first/second configuration, -ve/+ve slope edge filters, respectively, are used, coinciding with strain FBG Bragg wavelength. Furthermore, we propose a third configuration, the edge filter differential slope method, simultaneously using -ve and +ve edge filters, to achieve higher sensitivity than the first and second configurations. The wavelength sensitivity of this interrogator is 1.2 mu epsilon, and the power sensitivity is 13.75 dB/nm, comparable with other configurations. The slope of the edge filter (wavelength range) limits the "range" of measurement. Our proposed methods can also be used in FBG-based temperature sensing applications.