Fiber structure based on a depressed inner cladding fiber for bend, refractive index and temperature sensing

A fiber-optic structure based on a section of a double-clad fiber with depressed inner cladding is investigated for bend, refractive index and temperature sensing. The structure is formed by splicing a section of SM630 fiber between two standard fibers SMF-28. The operation principle relies on the sensitivity of cladding modes that are induced at a splice of fibers having different refractive index profiles. The mode structure of the double cladding fiber and the mechanism of formation of dips in the transmission spectra are discussed. The transmission spectra of the structure are measured for different curvatures of the inserted fiber section. The shift of dips to long wavelengths with increasing curvature of the fiber is observed and its dependence on the fiber section length and the direction of bending is investigated. The sensitivities of the spectral dips to the external refractive index and temperature are also measured.