Femtosecond Laser Internal Processing for an Optical Fiber Sensor Inducing Interference of Optical Waveguide

A new optical fiber sensing structure was proposed by using femtosecond laser internal processing. The proposed sensor was composed of cavity regions (voids) fabricated by focused ultra-short pulse laser beam in line at the interface of optical fiber between core and cladding. Cladding-modes was induced by scattering propagation light at voids, and caused interference of core and cladding modes after interacting with external materials around the optical fiber. Induced interference spectrum was shifted due to refractive index of surrounding materials. Through some experiments interference spectrum was observed with having amplitude more than 0.20 dB. The sensitivity of proposed sensor has been achieved to reach 31.8 nm/RIU for a RI range of 1.000-1.333. The interference spectrum was ascertainable when the sensor portion was in air and immersed into pure water, but not gained when sensor portion was immersed into mediums having relatively higher RI.