The development and maturation of laser technology have made laser inertial confinement an important method for controlled nuclear fusion. The final optics assembly (FOA), as a critical component of inertial confinement fusion (ICF) devices, primarily facilitates laser transmission and frequency conversion. Potassium dihydrogen phosphate (KDP) crystals are widely used in FOA due to their excellent nonlinear optical properties and resistance to laser damage. Ideal crystals are influenced by their own physical and chemical properties and are prone to deformation under external forces, leading to self-modulated wavefront distortions. Additionally, the focusing lens itself may introduce aberrations during the manufacturing process, causing the final output wavefront of the FOA system to distort and reduce energy conversion efficiency. Current measurement methods, such as laser interferometry, Hartmann detection, Shack-Hartmann wavefront sensors, Moiré deflection methods, and star point testing, have certain drawbacks in wavefront measurement applications, such as in performing in-situ measurements. As a significant indicator of the rationality of optical system design, wavefront aberration measurements can provide feedback for the design, processing, and calibration of optical systems. Therefore, it is especially important to adopt precise wavefront measurement methods to obtain in-situ wavefront aberration data for FOA systems.

To obtain the wavefront aberration of an FOA system, this paper proposes a transmitted wavefront aberration measurement method based on the transmission-phase measurement deflectometry for a simplified FOA system. The proposed method first employs a vector version Snell laws for ray-tracing through the frequency-conversion crystal and then utilizes a self-built backward ray-tracing model of the KDP crystal and focusing lens system to acquire the deflection angle of the output beam, from which the wavefront aberration is calculated. To validate the feasibility of the proposed method, MATLAB is used for numerical simulation of the forward and reverse system wavefront aberrations. Finally, a transmission deflectometry measurement system is constructed experimentally to measure the wavefront aberration of the o/e light from the KDP crystal and focusing lens system, demonstrating the feasibility of the proposed method for wavefront aberration system measurement including a KDP crystal and focusing lens system.

The proposed transmitted wavefront measurement method based on transmission-phase deflectometry is well validated through numerical simulations and experiments. The parameters used in numerical simulations are consistent with experimental parameters, with simulation results shown in Fig. 5. For further verification, an additional surface is introduced, as illustrated in Figs. 6(a) and (b); the coordinate distribution of the crystal output surface is shown in Figs. 6(c) and (d). The simulated wavefront aberration of the system with the additional surface is depicted in Fig. 7, verifying the changes in system wavefront aberration due to the introduced additional surface while also indicating the consistency between forward and reverse wavefront aberrations. The experiments measure a pupil diameter of 58 mm, yielding the wavefront aberration results shown in Fig. 11. The wavefront aberration measurement results for the o light give root mean square (RMS) of 128.4 nm and peak to valley (PV) of 653.0 nm, while the measurement results for e light give RMS of 143.5 nm and PV of 697.3 nm. These results are consistent with numerical simulation results, indicating the feasibility of the proposed method for in-situ measurement of distortion wavefronts of the simplified FOA system for o/e light.

This paper presents a wavefront aberration measurement method based on transmission-phase measurement deflectometry for a simplified FOA system. It first introduces the principles of crystal birefringence ray-tracing and transmission-phase measurement deflectometry, followed by a detailed derivation of the calculations for crystal output surface coordinates and output beam deflection angles, ultimately reconstructing the system wavefront aberration. Additionally, numerical simulations of the measurement system are conducted to obtain the changes in system wavefront aberration before and after introducing the surface of the crystal, validating the feasibility of the proposed method. An experimental transmission deflection measurement system is constructed, whose results closely agree with those of numerical simulations, confirming the effectiveness of the proposed method.