Compact Long-Period Fiber Gratings With Resonance at Second-Order Diffraction

Compact 5-mm-long 100-μm-period long-period fiber gratings (LPFGs) are fabricated by femtosecond laser direct writing. Nearly symmetrical photomodified grating segments located at the center of a fiber core lead to well-defined transmission spectra. The LPFGs can couple the guided mode to high-order cladding modes of second-order diffraction, resulting in a high refractive index (RI) sensitivity of 5265 nm/RI units in the RI range of 1.433 ~ 1.443 and a low-temperature sensitivity of -15.52 pm/°C from 20°C to 500°C. The latter property is desired for crosstalk-free high-sensitivity RI sensors.