Objective Potassium Dihydrogen Phosphate (KH₂PO₄) (KDP) crystal is currently the only nonlinear optical material that can be grown into a large aperture. It is widely used in inertial confinement fusion large aperture laser drivers as the terminal element of the harmonic conversion unit. The angle phase matching method is generally used to obtain high harmonic conversion efficiency so that the KDP crystal axis angle (the angle between the crystal plane normal and the crystal optical axis) is equal to the phase matching angle. From the growth to use, KDP crystals have gone through the following stages: slicing, processing, chemical coating, assembly, and adjustment. The crystal axis angle orientation accuracy is poorly controlled in the slicing stage, resulting in the crystal axis error of the milliradian. If the crystal axis error is encountered in the adjustment stage, it takes a considerable amount of time to adjust the crystal pose to achieve the best phase matching condition. This will increase the difficulty in assembly and reduce the efficiency of assembly and adjustment. It is also not conducive for batch assembly and large-scale production. Therefore, large-aperture laser devices require high-precision crystal axis angle correction during the processing stage.

Methods To solve the problems of a large correction angle and high precision requirements in the angle error correction of the crystal axis of the KDP crystal in the processing stage, a correction strategy of an in-site detection feedback combined with multiple adjustment approximations is proposed. The crystal element is clamped on an adjustable angle vacuum chuck, and the noncontact distance measuring unit is erected above the crystal surface. With the movement of the machine tool slide, the measuring unit moves at a uniform speed relative to the crystal. The distance between the crystal surface and the probe is recorded at a fixed sampling frequency. Combining this distance and the movement distance of the machine tool sliding table, the crystal surface inclination angle can be obtained by the straight-line fitting. This angle is subtracted from the crystal axis angle error detected offline as the suction cup's adjustment value. After adjusting the suction cup, the proposed method is employed to detect the crystal surface tilt angle again. The above steps are repeated until the tilt angle of the on-site inspection crystal surface gradually approaches and converges toward the crystal axis angle error. Cutting the crystal surface with a diamond tool can complete the correction of the surface crystal axis angle. The crystal axis angle on the other side is corrected by turning over and cutting. The advantage of this method is that the correction accuracy does not depend on advanced adjustment tools, small reclamping errors, and precise linear axes.

Results and Discussions The crystal axis correction on the first side of the KDP crystal is a process of an in situ detection and repeated iterative adjustments. The relevant parameters of three crystal samples during the iteration (Table 4) show that they gradually approached the crystal axis error angle after three rounds of adjustments. After the cutting is completed by the machine tool, the in situ detection result of 1#, 2#, and 3# crystal surface angles are -0.12 μrad, +0.76 μrad, and +0.82 μrad, respectively. In other words, the surface angle after cutting is controlled within 1 μrad [Fig. 8(b)]. The results show that the angle of in situ detection before cutting is equal to the change in the crystal surface angle before and after cutting. After cutting the other side of the KDP crystal, use a large-diameter interferometer to detect the crystal wedge angle. The angle of both sides is 0.2″(0.93 μrad) (Fig. 9), indicating that the crystal axis angles on both sides are the same after the second surface is cut. After completing the crystal axis angle correction of the three samples, the off-line precision crystal axis inspection equipment is used to detect the crystal axis angle error of the crystal. The results showed that the angle errors of the three crystal axes are +11.4 μrad, -9.0 μrad, and 0.59 μrad, respectively (Fig.10 and Table 5).

Conclusions The proposed crystal axis error correction strategy of KDP crystal in the processing stage is based on the on-site detection feedback and multiple adjustment convergence. The correction requirements of the milliradian angle and microradian accuracy can be achieved using the proposed method. Results suggest that the roposed method can meet large-scale laser devices' requirements for KDP crystal axis use. The verification experiment results showed that only three rounds of iterative adjustment, the proposed method can quickly converge the angle error of the crystal axis from several millimeters to 20 μrad or less. Further analysis shows that the correction accuracy of the strategy is only determined by the length of the probe movement and the test accuracy. The larger the element diameter and higher the measurement accuracy, the higher the correction accuracy, which is particularly suitable for the crystal axis angle correction of the large-diameter KDP crystal element. Although the correction accuracy is unrelated with the suction cup adjustment accuracy, the correction efficiency is proportional to it. The higher the adjustment accuracy, the fewer the number of iterations and the higher the correction efficiency.