Femtosecond laser fabrication of long period fiber gratings and applications in refractive index sensing

An improved point-by-point inscription method is proposed to fabricate long period fiber gratings (LPFGs) by using a laser operating at 800 nm with 35 fs duration pulses. The sensitivity to misalignment between the core and the focus is reduced by scanning a rectangular part on the fiber. LPFGs with an attenuation depth of 20 dB are achieved within the wavelength range of 1465–1575 nm. Characterization of the temperature sensitivity and thermal stability of the LPFGs is presented. A 5.6 nm wavelength shift and a 1.2 dB decrease in the attenuation peak are observed following heat treatment at 600 °C for 4 h. The fabricated LPFGs are used as refractive index sensors. The effect of heat treatment on the response of the LPFGs to refractive index changes is also studied.

► An improved inscription method is proposed to fabricate LPFGs using an fs laser.
► The sensitivity to misalignment between the core and the focus is reduced.
► LPFGs with the attenuation peak of 20 dB are achieved in the wavelength of 1465–1575 nm.
► The LPFGs have high thermal stability and the background loss can be reduced by heat treatment.
► As an RI sensor, the LPFG becomes more stable after heat treatment.