Walk-off compensated nonlinear crystal stacks for efficient harmonic wavelength conversions

The e-ray spatial walk-off effect is intrinsic with all anisotropic crystals but is of special concern for nonlinear optical (NLO) crystals in critical phase matching harmonic conversions. It presents one of the fundamental limitations for obtaining optimum conversion efficiency with NLO crystals. The principle of walk-off compensation has long been recognized and various schemes for overcoming walk-off has been discussed in the literature. A key prerequisite in this process for compensation is the ability to determine the deviation from the theoretical cut angle in starting NLO crystals and its subsequent correction. Our approach is to combine phase-angle-corrected NLO crystals by Adhesive-Free Bond (AFB®) into a stack of components for WOC by periodically inverting stack components to minimize walk-off and allow the generated power to grow proportionally with the length of the stack. We have implemented a measuring and correction system that consists of a precision six axes hexapod of a 0.0003° angular scanning resolution and an in-situ detection system with feedback of the generated power level as a function of angular position. As-received nonlinear crystals are oriented within ≤±.05° accuracy. We report on critical parameters of stack formation and improvement of a BBO (BaB₂O₄) quadruplet for conversion efficiency of 532 nm to 266 nm over a single BBO crystal and considerations for bonding LBO (LiB₃O₅) stacks.