Lossy Mode Resonance Sensors in Uncoated Optical Fiber

This work demonstrates that the use of an unconventional optical fiber allows for generating lossy mode resonances (LMR), without the need for an external high refractive index thin film. The basic idea consists of using few centimeters of an optical fiber having large core and cladding with higher refractive index, which is spliced between two multi-mode fibers. Here, the role of such kind of fiber is got by a commercially available double cladding fiber (DCF) having a W-type refractive index (RI) profile. The possibility to induce and tune the phenomenon is demonstrated, where the resonance wavelength of LMR peaks and mode order can be adjusted by varying the thickness of DCF outer cladding, e.g., through chemical etching. This novel sensing scheme becomes a valid alternative to thin-film coated optical fibers, where LMR-based sensors have been developed so far, due to advantages in terms of simplicity, cost, and stability. The response of fabricated LMR devices is characterized towards surrounding medium refractive index, demonstrating a sensitivity up to about 1700 nm/RIU in the RI range of 1.33-1.39, which makes this fiber sensor suitable for bio-chemical sensing applications. Low cross sensitivity to temperature is also found.