Research on compensation of suction deformation error of potassium dihydrogen phosphate crystal

Abstract

Potassium dihydrogen phosphate (KDP) crystals used in high power laser systems require high figure accuracy. Deformation caused by vacuum suction is one of the most significant factors affecting the final surface figure accuracy. It is difficult to compensate the error caused by the suction deformation when using a single point diamond flycutting method. This paper presents a spiral turning method to machine KDP crystals on a common ultraprecision lathe, and the transfer rule and compensation theory of the suction deformation error are studied. The in situ measurement of deformation error is accomplished with a displacement test apparatus, which can acquire submicron precision, and then compensates for it by utilizing the three-axis servo control technology. Meanwhile, cutting parameters are strictly controlled to achieve full-aperture ductile material removal. The compensation experiments for both reflected and transmitted wavefront error are carried out on a Φ270 mm KDP crystal and the transmission wavefront error of 0.591λ (peak-to-valley, λ = 632.8 nm) and 25 nm/cm (gradient root-mean-square) are obtained after one compensation with full-aperture surface roughness of about 2 nm RMS.