Mode Interaction in a Structure Based on Optical Fiber with Depressed Inner Cladding

Resumo

The paper presents the results of studies of a fiber-optic structure whose principle of operation is based on the transformation of the core and cladding modes in a segment of SM630 fiber with a small core and a depressed inner cladding. The mode structure of a double-clad fiber is analyzed, and the causes of the dips in the transmission spectrum are explained. The transmission spectra of the proposed structure with a bent segment of SM630 fiber are measured. A shift of the resonances to the long-wave region with increasing curvature of the fiber is observed. A bending sensor based on this effect is developed. The dependence of the shift of the resonance wavelengths on the temperature and strain of the fiber and the refractive index of the ambient medium is measured. It is shown that, upon etching of the fiber and the reduction in the diameter of the outer cladding, the resonances shift toward shorter wavelengths. Chitosan coating sensitive to the chemical composition of the ambient medium is deposited on the fiber by layer-by-layer assembly; such coating can be used to develop an acidity sensor.