Ultra-short long-period fiber grating based on a micro air-channel structure

A new type of long-period fiber grating is proposed and demonstrated. The structure of the device consists of a few micro air-channels along a single-mode fiber, and is fabricated by using a femtosecond laser to inscribe a few groups of fiber inner waveguide arrays followed by hydrofluoric acid etching. The length of the long-period fiber grating is down to 600 µm, corresponding to only five grating periods. To the best of our knowledge, this is the shortest long-period fiber grating reported. The device has a good refractive index sensitivity of ∼587.08 nm/RIU (refractive index unit) in the refractive index range of 1.34-1.365 and a relatively small temperature sensitivity of ∼12.1 pm/°C, thus reducing the temperature cross-sensitivity. Such a device is expected to have promising photonic applications.