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

In this paper, we explore 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, (a) peak wavelength/peak power and (b) the integrated power (intensity) variation, of the reflected spectrum are measured after passing through the edge filter. In first/second configuration, -ve/+ve slope edge filters respectively are used, coinciding with strain FBG Bragg wavelength. Further, we propose a third configuration, edge filter differential slope method, simultaneously using -ve and +ve edge filters, to achieve higher sensitivity than first and second configurations. Wavelength sensitivity of this interrogator is 1.2 pm/μϵ, and power sensitivity is 13.75 dB/nm, comparable with other configurations. 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.