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

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.