Preparation, Characterization and Application of Ultrathick True Zero-Order KDP Crystal Waveplates

In crystal space, the birefringence varies continuously with respect to the propagating direction of the light wave. By utilizing this property, we design and fabricate a series of ultrathick true zero-order (TZ) waveplates based on KH₂PO₄ crystals with different orientations for the first time. When the cutting angle θ is 5°, the thickness d of the 1,064-nm TZ half-wave plate (HWP) is 2.0 mm, which is 33 times larger than that of the traditional quartz TZ device (0.06 mm). At a temperature of 30 ° C, its extinction ratio for the Nd:YAG pulse laser reaches 1,970:1. When the temperature increases to 80 ° C, the extinction ratio remains above 1,000:1. When the extinction ratio decreases to 200:1, the angular acceptance bandwidth and wavelength acceptance bandwidth are determined to be 0.3° and 45 nm, respectively. With the ultrathick TZ HWP and a quarter-wave plate (QWP), efficient type-II frequency doubling and electro-optic Q-switching, respectively, are achieved. As a famous optical crystal, KDP has been popularly used as a frequency converter, a laser Q-switcher, an electro-optic modulator, and a solid-state light valve display. This paper disclosure a new application of this traditional material, i.e. large size, high mechanical strength, low-cost, and high-quality TZ crystal waveplates. Such components have wide application prospects in various optical and laser systems which need controlling the polarization of light.