In-Fiber Cascaded FPI Fabricated by Chemical-Assisted Femtosecond Laser Micromachining for Micro-Fluidic Sensing Applications

An ultra-compact cascaded Fabry Perot (F-P) interferometer in fiber, which exhibits a high-visibility and enhanced detection limit, has been developed by chemical-assisted femtosecond laser micromachining. The reflection spectrum of the cascaded F-P interferometer shows stronger and narrower resonance than single F-P cavity. The liquid refractive index (RI) measurement demonstrates that the wavelength sensitivity and the dip intensity sensitivity are ~888 nm/RIU and -882 dB/nm/RIU, respectively, and the obtained detection limit is 4.22 × 10 ^-5 RIU in RI range 1.325-1.330, which is higher than that of the conventional single F-P cavity in optical fibers based on wavelength modulation (10 ^-4 RIU). Moreover, the gas pressure measurement demonstrates that the wavelength sensitivity is 2.86 nm/MPa and the detection limit reaches 1.311 × 10 ^-2 MPa in pressure range 1.5-2.0 MPa. The proposed in-fiber cascaded F-P interferometer will find potential applications in chemical and biological sensors, biomedical probes, and low-cost care products.