利用严格耦合波理论分析了用于520 nm波长飞秒激光制备光纤光栅的相位掩模的衍射特性, 当相位掩模是矩形槽形时, 占宽比在0.32~0.43之间, 槽形深度在0.57~0.67 μm之间时, 能够保证零级衍射效率抑制在2%以内, 同时±1级的衍射效率大于35%。在此基础上, 利用全息光刻-离子束刻蚀技术, 制作了用于520 nm波长飞秒激光的周期为1 067 nm、有效面积大于40 mm×30 mm的相位掩模。实际制作的相位掩模是梯形槽形, 槽深是0.665 μm, 分析了梯形槽形中梯形角对衍射效率的影响。实验测量表明, 该相位掩模的零级衍射效率小于2%, ±1级衍射效率大于40%,满足飞秒激光制作光纤光栅的需要。

The diffraction characteristics of a phase mask for a 520 nm wavelength Femtosecond laser-written FBG are investigated in this study using rigorous coupled-wave theory (RCWT). The results demonstrate that when the phase mask is a rectangular profile, the groove depth and the duty cycle of phase mask must be within the range of 0.57-0.67 μm and 0.32-0.43, respectively, to achieve the desired 0-order diffraction efficiency of less than 2% and ±1-order diffraction efficiency of more than 35%. Using a fused silica phase mask in a 520 nm wavelength femtosecond laser with a period of 1 067 nm, a ruled area 40 mm×30 mm was fabricated via holographic lithography-ion beam etching. The actually produced phase mask was observed to be a trapezoidal profile with a groove depth of 0.665 μm, whose influence on the diffraction efficiency was analyzed. Experimental measurements demonstrated that the 0-order diffraction efficiency was less than 2% and ±1-order diffraction efficiency was more than 40%, which meet the requirement for fabrication of fiber Bragg grating by femtosecond laser.