Low detection limit refractive index sensor based on open microcavity Mach-Zehnder interferometer and fiber ring laser

Abstract

An optical fiber refractive index (RI) sensor based on open microcavity Mach-Zehnder interferometer (OMZI) and fiber ring laser (FRL) is proposed and demonstrated experimentally. The OMZI is manufactured by splicing a tiny single mode fiber (SMF) segment with multi-mode fiber (MMF) joints laterally. The large offset structure forms an open microcavity which can be filled with the liquid under test. Through inserting the OMZI into an erbium-doped FRL, the RI measurement can be achieved by discriminating the lasing wavelength, and the detection limit (DL) can be effectively improved owing to the laser sensing spectrum with narrower 3-dB bandwidth and higher optical signal-to-noise ratio (OSNR). Experimental results show that the output laser wavelength has a linear response to the RI change with a sensitivity of −2947.818 nm/RIU during the range of 1.33302~1.33402, and the DL is as low as 5.89×10−6 RIU. Compared with other optical fiber RI sensors, the proposed fiber laser RI sensor with an open microcavity has the advantages of small size, high sensitivity and low DL, making itself a competitive candidate for the microfluidic RI measurement in biochemistry.