Torsion characteristics of long-period fiber gratings induced by high-frequency CO2 laser pulses

It is demonstrated, for the first time to our knowledge, that torsion characteristics of resonant wavelength of long-period fiber gratings (LPFGs) induced by high-frequency ${\mathrm{CO}}_2$ laser pulses depend strongly on the twist directions. That is, the resonant wavelength shifts linearly toward the longer wavelength as the LPFG is twisted clockwise, whereas it shifts linearly toward the shorter wavelength as the LPFG is twisted anticlockwise. On the other hand, the loss peak amplitude decreases gradually whether the LPFG is twisted clockwise or anticlockwise. If the twisted fiber is much longer than the twisted LPFG, the resonant wavelength shifts wavelike toward the longer and shorter wavelength as the LPFG is twisted clockwise and anticlockwise, respectively, and the loss peak amplitude decreases wavelike with the twist rate applied. An asymmetric refractive-index distribution exists within the cross section of the LPFG fabricated by high-frequency ${\mathrm{CO}}_2$ laser pulses, which results in obvious linear birefringence in the LPFG. The right- and left-rotatory elliptical birefringence are induced when the LPFG with a linear birefringence is twisted clockwise and anticlockwise, respectively. So the twist-induced right- and left-rotatory elliptical birefringence in the LPFG show that the shift of resonant wavelength is dependent on the twist directions. The twist-induced circular birefringence in the fiber shows that the resonant wavelength and amplitude of the LPFG change wavelike.