KDP光学零件超精密车削加工误差的频谱特性与控制

Spectral characteristics and control of machining errors of KDP optical elements in ultra-precision turning

超精密车削技术适于加工KDP(磷酸二氢钾)等频率转换类型的强光光学零件,但车削表面存在明显的加工纹理,导致抗激光损伤阈值降低。以加工表面误差幅值及其频谱分布为对象,分析了KDP光学零件超精密车削的加工特征和误差形态,采用功率谱密度(PSD)评价方法研究了工艺参数与误差频谱的内在关系,结果表明:不同进给速度及主轴转速将使螺旋形刀痕的间距发生变化,进而影响KDP表面误差的频率成分;切削深度虽然对误差频谱影响很小,但会改变PSD的幅值;当主轴转速高于500r/min、进给速度小于2mm/min、切削深度小于2μm时能够加工出rms值优于20nm的KDP面形。在此基础上,以典型KDP光学零件加工为例,通过超精密补偿车削方法将低频误差的PSD控制在300nm2·mm以内,中高频误差的PSD控制到国家点火装置(NIF)标准线以下,满足强光系统的工作要求。

Ultra-precision turning technology is suitable for machining KDP optical elements which are used as frequency conversion unit in high-power laser system, but stripes are easy to come into being in machining process, and this will result in reduction of laser induced damage threshold. Taking magnitude and spectrum distribution of machining errors as researching object, machining features and error morphology of KDP elements are analyzed after ultraprecision turning in this paper, and correlation between error spectrum and machining parameters is studied based on power spectral density （PSD）. Results show that feeding rate and spindle speed will change distance between spiral ripples and affect error spectrums of KDP machining surface, cutting depth has hardly any influence on spectrum of errors, but it will change magnitude of PSD. KDP surface can be accurately manufactured with rms value is smaller than 20nm when spindle speed is more than 500r/min, feeding rate is less than 2mm/min and cutting depth is less than 2 μm. According to a typical KDP element, PSD of low frequency errors is controlled within 300 nm2·mm by using ultra-precision compensating turning method, PSD of mid-to-high frequency errors is under “Not-to-Exceed” line applied in national ignition facility, and working requirements for high-power laser system are satisfied on the whole.