Ultrahigh sensitive liquid core fiber Mach–Zehnder interferometer using a low light absorption

Abstract

This paper presents an advanced liquid-core fiber Mach–Zehnder interferometer (LCFMZI) designed to have the ultra-low light absorption properties of the liquid core for achieving ultrasensitive spectral responses. The device structure features a micro-sized hollow-core fiber (HCF) with a core diameter of 10 μm, spliced between two single-mode fibers (SMFs) with tilted ends. This configuration minimizes Fresnel reflections and creates a miniature oblique gap for liquid filling. The interference mechanism is based on the superposition of core and cladding modes, with a 5 μm core offset strategically introduced to enhance mode coupling and achieve a high extinction ratio in the output interference spectra. When irradiated by a 980 nm laser diode (LD), the liquid core, serving as an absorber, undergoes a temperature-induced refractive index (RI) change, which alters the optical path difference in the LCFMZI. This results in a substantial wavelength shift in the interference pattern. Experimental results demonstrate remarkable spectral shifts of over 124 nm over the broadband range of 1250–1650 nm with an LD input power as low as 0.566 mW. The proposed LCFMZI achieves an exceptional sensitivity of + 219.08 nm/mW and 3.647 nm/mA with a highly linear response. A theoretical analysis was also performed, demonstrating good agreement with the experimental results. These results validate the effectiveness of the LCFMZI’s low light absorption mechanism in achieving ultrahigh sensitivity.