中文摘要：

发现高频聚焦CO2激光脉冲三束对称写入法制备的长周期光纤光栅（LPFG）具有新奇的扭曲特性：首先,当被扭曲光纤和长周期光纤光栅长度接近时,扭曲特性有明显的方向相关性,顺时针扭曲时,谐振波长和峰值损耗随扭曲率的增大而减小;逆时针扭曲时,谐振波长和峰值损耗随扭曲率的增大而增大,并与之成良好的线性关系,其平均灵敏度分别为0.133 nm/（rad·m^-1）和0.061 dBm/（rad·m^-1）;其次,当被扭曲光纤远长于长周期光纤光栅时,扭曲特性随扭曲率变化周期性起伏,而且起伏次数（10次）超过光纤被扭曲周数（6周）,但整体特性基本保持不变。利用扭转光栅导致椭圆双折射理论对扭曲特性进行了解释,并发现光栅内部固有线性双折射对扭曲特性有着重要的作用。

英文摘要：

It is found that the torsion characteristics of the long-period fiber grating（LPFG） are novel,which is written symmetrically by three-beam focused high-frequency CO2 laser pulses.Firstly,when the length of the twisted fiber is close to the length of the LPFG,the torsion characteristics have obvious relevance with the twisting direction.If the LPFG is twisted clockwise,the resonant wavelength and the peak loss will decrease linearly with the twist rate increasing.If the LPFG is twisted anticlockwise,the resonant wavelength and the peak loss will increase linearly with the twist rate increasing.The average sensitivities of resonant wavelength and peak loss are 0.133 nm/（rad·m^-1） and 0.061 dBm/（rad·m^-1） respectively.Secondly,when the twisted fiber is much longer than the LPFG,the torsion characteristics oscillate periodically;and the number of the oscillation（10 periods） overcomes the number of turns（6 turns） that the fiber has been twisted.The overall characteristics are basically unchanged if the volatilities are ignored.The torsion characteristics are analyzed theoretically by taking into the consideration of the twist-induced elliptic birefringence.Moreover it is found that the linear birefringence in the LPFG also plays a decisive role in the torsion characteristics.