Strain characteristics of CO2-laser-carved long period fiber gratings

Unique strain characteristics of long period fiber gratings (LPFG) fabricated by a focused CO2 laser beam carving periodic grooves on the fiber are investigated for the first time to our knowledge. Resonant wavelength, transmission attenuation, and polarization dependent loss (PDL) of the CO2-laser-carved LPFG are found to depend strongly on the tensile strain applied, and their strain sensitivities are dependent on the depth of grooves and/or the initial refractive index modulation'. The average strain sensitivity of resonant wavelength for LPFG is increased by 229 times and is up to -102.89 nm/m epsilon by means of carving periodic grooves on the fiber. When a stretching force is applied to the LPFG, the resonant wavelength can "blue" shift by -11.84 nm, the absolute value of peak transmission attenuation and the maximum PDL can be increased by 25.913 and 26.535 dB, respectively. The CO2-laser-carved LPFG combines the features of the three types of LPFGs reported previously, Le, the CO2-laser-induced LPFGs without physical deformation, the corrugated LPFGs fabricated by hydrofluoric acid etching, and the microbend-induced LPFGs. The mechanisms of refractive index modulation in the CO2-laser-carved LPFGs under tensile strain are rather complicated and may be regarded as a combination of the stress-relaxation-, the groove-, the strain-, and the microbend-induced refractive index perturbations.