Compact gas refractometer based on a tapered four-core fiber

A compact in-line interferometer is proposed and experimentally demonstrated for gas refractive index (GRI) measurement. The sensor comprises a tapered four-core fiber (TFCF) sandwiched between two single-mode fibers (SMFs), forming an in-line SMF–TFCF–SMF structure. The fiber taper acts as a bridge between the external GRI variation and the multimode interference within the TFCF segment. A high sensitivity of 1280.94 dB/refractive index unit is obtained in GRI measurement around 1.0. Temperature change only shifts the interference wavelength, and the cross-sensitivity of temperature can be ignored by intensity demodulation. The proposed gas refractometer, with its improved performance, can be a good candidate for chemical sensing or bio-sensing.