Ultrasensitive liquid refractometer based on a Mach-Zehnder micro-cavity in optical fibre fabricated by femtosecond laser-induced water breakdown

摘要

A Mach-Zehnder (MZ) micro-cavity with two symmetric openings in single mode fibre (SMF) was fabricated by femtosecond laser-induced water breakdown. The micro-cavity is rightly across half of the fibre core, and the opening size of the micro-cavity is 10 mu m x 100 mu m. The micro-cavity and the remaining half of the fibre core constitute two arms of the Mach-Zehnder interferometer (MZI). The MZI with micro-cavity immerged in water shows perfect interference spectrum due to the regular shape and smooth internal surface of the micro-cavity. The insertion loss is only -8 dB and the contrast of one MZI peak reaches more than 40 dB. The MZI with micro-cavity in SMF can be used as an ultrasensitive liquid refractometer as its ultrahigh refractive index (RI) sensitivity (14296.98 nm/RIU), high RI resolution (1 x 10(-5) RIU), good linearity (99.62%) and low temperature crosstalk (0.04 nm/degrees C).