Single-helix chiral long-period fiber gratings for wavelength- interrogated liquid level sensing

摘要

We develop coupled-mode formulism for multimode couplings of LP_(01) core modes to LP_(1n) cladding modes in single-helix chiral long-period fiber gratings (S-CLPG). Then mode-coupling characteristics of the structure, such as coupling coefficients and relative bandwidths of dips are studied and compared with those of the conventional long-period fiber grating (LPG). Subsequently, transmission spectral characteristics in response to the liquid level are investigated for S-CLPG partially immersing in a liquid with refractive index lower than that of the fiber cladding. Simulation results indicate that the dips corresponding to the multimode couplings in the transmission spectrum will shift continuously and monotonically as liquid level changes. Thus S-CLPG could be used for liquid level sensing based on wavelength interrogation, which is beneficial to avoid sensitivity deterioration caused by power instability in power interrogation for most conventional LPG-based liquid level sensors. In addition, we identify that the sensitivity can be enhanced with a thinner fiber cladding or by utilizing higher-order cladding mode resonance. The optimization result indicates that the wavelength shift will be over 1.03 nm per millimeter liquid level change.