Investigation of impact of photonic crystal fiber structure modified by femtosecond laser micromachining on long period gratings׳ sensing characteristics

• Sensing property of LPG in PCF can be modified by femtosecond laser micromachining.
• Sensing property of femtosecond laser-LPG is analyzed by local couple mode theory.
• The sensitivity can be enhanced or suppressed by changing the waveguide geometries.
• Forming proper waveguide modulation, we can design the desired sensitivity of LPG.

The sensing characteristics of long period gratings (LPGs) in photonic crystal fiber (PCF) can be changed by using femtosecond laser to modify the PCF waveguide structure although dispersive characteristic plays a key role in determining the sensitivity. Based on the coupled local-mode theory, the coupling behaviors and spectral characteristics of the LPGs in PCF fabricated by a femtosecond laser and a CO2 laser are analyzed which are supported by experiment results. When the distance between the central of fiber core and the peak of the drilled hole, namely the micro-hole diameter is about 3.5 μm, the temperature and strain sensitivities are changed by 27% (from 6.20 to 7.81 pm/°C) and −21% (from −2.41 to −1.91 pm/με) in comparison with the changes of the sensitivities that is induced by CO2 laser. The investigation demonstrates that the local structural changes of PCF have an impact on the sensitivity of LPGs. The investigation demonstrates the versatility of the technique in potential applications to design the desired sensitivity of fiber grating flexibly by forming proper geometrical modulations.