Reflective Fiber-Optic Refractometer Using Broadband Cladding Mode Coupling Mediated by a Tilted Fiber Bragg Grating and an In-Fiber Mirror

A reflective fiber-optic refractometer based on a broadband cladding-to-core recoupling for a wide range of refractive index measurement is proposed and experimentally demonstrated. The sensing probe contains a tilted fiber Bragg grating (TFBG) together with an in-fiber “cladding” mirror upstream. The “cladding” mirror is made by a 0.3-μm-thick gold film with a transparent aperture to permit all the core-guided light go through but reflect the light in the cladding. The downstream TFBG couples the forward propagating core mode to backward propagating cladding modes and such cladding modes are reflected to the TFBG again by the in-fiber “cladding” mirror and recoupled to the fiber core through the same TFBG. With this special design, all the transmitted cladding modes now can be effectively monitored in reflection. Compared with the reported “in-fiber” and “fiber-to-fiber” coupling TFBG refractometer, our sensor demonstrates a much broader surrounding refractive index operating range with a very linear sensitivity of 528 nm/RIU. A tunable laser is used to improve the measured accuracy and a sensitivity of 7973 dB/RIU was obtained, with a limit of detection of 10 ⁻⁶ RIU. The sensor works with very controllable cross-sensitivities because the core mode is temperature dependent but insensitive to surrounding refractive index.