High strength coupling and low polarization-dependent long-period fiber gratings based on the helicoidal structure

A compact helicoidal long-period fiber grating (HLPFG) was fabricated by twisting a single mode fiber with CO2 laser beam exposure and its characteristic was experimentally investigated. The eccentricity between the core and the cladding of a fiber is introduced from the screw-type deformation. This helically induced significant periodic index change along the fiber produces strong mode coupling between the core and the cladding of −20 dB with a short grating length of ∼1 cm. The novel resonance wavelength shift of a HLPFG was analyzed with co-directional or contra-directional torsion to the helix.