Influence of the size of a micro-cavity fabricated in an optical fiber using the femtosecond laser in a form of in-line Mach-Zehnder interferometer on its refractive index sensitivity

This paper discusses refractive index (n) measurement capabilities of micro-cavity with various diameters (d = 40, 54 and 60 mu m) fabricated in optical fibers by a femtosecond laser. The bottom of the cavity intersected the fiber's core and the Mach-Zehnder interferometer effect was induced, allowing the measurement of the n of the liquid filling the cavity. After filling the cavity, a set of minima can be observed in fiber transmission spectrum which shift with change in n. Fabricated sensors exhibit high and linear sensitivity, which in the range of n=1.3333 to 1.3500 RIU barely depends on the cavity diameter in case of first observed minima. Next for different micro-cavity diameters the minima do not overlap in refractive index domain thus it is impossible to compare them in terms of the sensitivity. The highest sensitivity of up to more than 27 000 nm/RIU was obtained for the smallest cavity and the third observed minimum.