Dual-peak resonance-based optical fiber refractometers

The fabrication of optical fiber refractometers by means of the deposition of a thin indium-oxide coating onto an optical fiber core is presented. Indium-oxide permits the guided light in the fiber to be coupled from its core to its coating, creating resonances in the infrared and visible regions. These resonances vary as a function of the external medium's refractive index, enabling the fabrication of robust and highly reproducible wavelength-based optical fiber refractometers. Moreover, two differentiated resonances have been obtained from the same device within the 500- to 1700-nm spectrum. The central wavelength of the resonances can be adjusted by varying the thickness of the indium-oxide coating. The sensitivity of the dual-peak resonance-based refractometers is within the same order of magnitude when the resonances are situated in the same spectral region. The refractometers that we obtained showed a sensitivity of 4068-nm/refractive index unit (RIU) in the range 1.333-1.392 RIU, comparable to existing ones based on resonances and other techniques, with the advantage of permitting the realization of dual-peak reference measurements in different regions of the spectrum.