Characteristics of novel ultra-long-period fiber gratings fabricated by high-frequency CO2 laser pulses

A novel ultra-long-period fiber grating with periods of up to several millimeters induced by high-frequency CO2 laser pulses is reported, for the first time to our knowledge. The refractive index change occurs mainly within the cladding area of the fiber and has an unsymmetrical distribution on the cross-section of the ULPFG. The resonant loss peaks of the ULPFG are generated from the coupling of the forward propagating core mode to the cladding modes of fundamental and harmonic orders. The characteristics of temperature, strain, torsion and refractive index are investigated by experiment, which offers greater potential for applications in measurement of multi-parameters and optical fiber communication when compared with conventional LPFGs. In addition, several new approaches for the potential applications of such an ULPFG are proposed.