Abstract
In this paper, we report a sensitivity-controllable refractive index (RI) sensor based on a reflective θ-shaped microfiber resonator cooperated with Vernier effect. The θ-shaped microfiber resonator is a reflective all-fiber device with comb spectrum under weak coupling condition. By cascading it with a fiber Fabry-Perot interferometer, Vernier effect is generated to demodulate surrounding RI with enhanced sensitivity. Theoretical analysis reveals that RI sensitivity of the combined structure with Vernier effect is m times higher than the sensitivity of singular θ-shaped microfiber resonator. Moreover, by adjusting cavity length of the θ-shaped microfiber resonator, magnification factor M = (m + 1) can be tuned which enables the RI sensitivity to be controlled. Experimental result demonstrates that the RI sensitivity can be widely tuned from 311.77 nm/RIU (Reflective index unit) to 2460.07 nm/RIU when the cavity length of the θ-shaped microfiber resonator is adjusted from 9.4 mm to 8.7 mm. The θ-shaped microfiber resonator based all-fiber RI sensor featuring controllable sensitivity and compact size can be widely used for chemical and biological detections. The proposed scheme of generating Vernier effect also offers a universal idea to increase measurement sensitivity for optical fiber sensing structures with comb spectrum.
Highlights
Refractive index (RI) of liquid is a basic quantity that can be applied to characterize the changes of many chemical and biological measurands, such as gas concentration[1], seawater salinity[2], PH values[3], DNA4, glucose concentration in serum[5], etc
Wen Bin Jin et al utilized a non-adiabatic microfiber assisted by facet reflection of a patch cord to realize an refractive index (RI) sensor with sensitivity high to 18681.82 nm/ RIU18
Due to the features of high Q-factor, further-reduced size, and easy fabrication, much effort has been dedicated to RI sensors based on microfiber resonators[20,21,22]
Summary
Where neff[1] and Δneff[1] are the original effective RI of modes in θ-shaped microfiber resonator and its variation. This offers a universal idea of generating Vernier effect to increase measurement sensitivity for other fiber sensing structures with comb spectra. Their RI sensitivities are 311.77 nm/RIU, 514.41 nm/RIU, and 2460.07 nm/RIU separately, indicating that the RI sensitivity of the θ-shaped microfiber resonator can be controlled by adjusting the cavity length. The proposed scheme of generating Vernier effect offers a universal idea to increase measurement sensitivity for other optical fiber sensing structures with comb spectrum
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