In-Fiber Grating Fabricated by Femtosecond Laser Direct Writing for Strain Sensing

In this letter, we reported long period fiber gratings (LPFGs) fabricated by femtosecond laser pulses direct writing in grapefruit microstructured fiber. The mode field distribution and dispersion property of this fiber was analyzed by finite-element method. Two LPFGs were fabricated with different periods of 230 and
$250~\mu \text{m}$
, the resonant dips appeared at the wavelengths of 1325.8 and 1449.4 nm, respectively. In addition, sensing characteristics of the LPFG with the period of
$250~\mu \text{m}$
were investigated in theory and experiment. The experimental results show that the strain and temperature sensitivities are 1.88 pm/
$\mu \varepsilon $
and 0.267 pm/°C, respectively. It indicates that the temperature cross sensitivity is as low as
$0.142~\mu \varepsilon $
/°C.