In-fiber Michelson interferometric sensor fabricated by single CO₂ laser pulse

An in-fiber Michelson interferometric sensor was presented by fabricating a concavity on the end face of a single mode fiber using a single CO₂ laser pulse. Reflected beams from the bottom and air-cladding boundary of the concavity are coupled into the fiber core and superimpose to generate a two-beam in-fiber Michelson interferometer. Compared with other laser-machining methods where multiple scanning cycles with precise manipulation are needed, the proposed method is more straightforward because only a single laser pulse is used to construct the sensor. The concavity constructed by the CO₂ laser is very smooth, and its shape could be controlled flexibly by changing the position of the single mode fiber and the parameters of the CO₂ laser pulse, so the fringe visibilities of the proposed sensors could be more than 15 dB, which is higher than that of the most reported laser-machining in-fiber Michelson interferometers. The proposed sensor was demonstrated by measuring the temperature with a sensitivity of 11.13 pm/°C. Furthermore, the proposed device is compact (<100 µm), economical, and robust. These advantages make it a promising candidate in practical applications.