利用严格耦合波理论分析了线性啁啾相位掩模的衍射特性,得到只有当相位掩模的占宽比在0.37～0.50之间,槽形深度在242～270 nm之间时,才能保证零级衍射效率小于2%,同时正负一级的衍射效率大于35%。在此基础上,利用全息-离子束刻蚀和反应离子束刻蚀相结合的新方法,制作了中心周期为1000 nm,啁啾率1 nm/mm,有效面积为100 mm×10 mm的线性啁啾相位掩模。发现先用短时间Ar离子束刻蚀对光刻胶光栅掩模槽形进行修正,然后采用CHF3反应离子束刻蚀,能得到更合适的占宽比,从而确定了刻蚀新工艺。实验测量表明其零级衍射效率小于2%,正负一级衍射效率大于35%,最大非线性系数为1.6%。理论分析表明该相位掩模能够满足制作线性啁啾光纤光栅的需要。

The diffraction characteristics of the linearly chirped phase mask is investigated using rigorous coupled-wave theory. The results show that the groove depth and duty cycle of phase mask must be controlled within the range of 242～270 nm and 0.37～0.50 respectively, in order to make the zero order diffraction efficiency less than 2% and the ±1 order diffraction efficiency more than 35%. Based on the above analysis, a linearly chirped phase mask with the center period of 1000 nm, ruled area of 100 mm×10 mm and chirp rate of 1 nm/mm has been fabricated by a new technique, which combines holographic-ion beam etching and reactive ion beam etching. The experiment and simulation both indicate that the Ar ion beam etching can modify the initial photoresist grating profile and produce a suitable duty cycle under the CHF3 reactive ion beam etching. Experimental measurements show that the zero order diffraction efficiency is less than 2%, with the ±1 order diffraction efficiency more than 35% and the maximum nonlinear coefficient of 1.6%. Theoretical analysis also indicates that the phase masks can be used to fabricate the UV written chirped fiber Bragg gratings.