Femtosecond laser micromachining and computer modeling of ultracompact fiber inline Mach-Zehnder interferometer sensor based on a U-shape microcavity

An ultracompact fiber inline Mach-Zehnder interferometer (MZI) has been formed by side-ablating a U-shape microcavity in a single-mode optical fiber with the fiber core partially removed using a femtosecond (fs) laser, in which the two light paths are accordingly the U-shape microcavity and the remained D-type fiber core. Computer modeling of beam propagation method (BPM) analysis is utilized to illustrate the dependences of a good transmission spectrum on the ablation depth, the ablation length and the U-shape micocavity refractive index, which gives some guideline to optimize parameters for fs laser micromachining process, and estimates refractive index (RI) sensitivity and temperature sensitivity within certain range. That is, with the ablation depth of 63.5μm and the ablation length of 150μm simulated in BPM analysis, the RI sensitivities of -3243.7 ± 0.65nm/RIU for different gases and -10789.29 ± 18.91nm/RIU for different solution concentrations and the temperature sensitivity of 37.42 ± 0.07pm/°C are predicted respectively.