Optimization of single-layer sol-gel antireflection coatings on potassium dihydrogen phosphate crystals in quadrature geometry for second-harmonic conversion of high-power Nd:glass lasers

Abstract

Quadrature geometry of two potassium dihydrogen phosphate crystals is a standard configuration to achieve efficient second-harmonic conversion of Nd:glass high-power lasers. Antireflection coatings on the entrance face of the first crystal and on the exit face of the second one are easily designed using quarterwave optical thicknesses adjusted separately for minimum reflectivity at the fundamental wavelength and at its second harmonic. It is however not possible to simultaneously obtain the lowest reflectivity for both the wavelengths on the inner faces. We describe a theoretical optimization of a single-layer sol-gel silica coating that allows maximum transmission of energy in the fundamental and second harmonic, taken together, across the inner faces of the two crystals. The optimum thickness is a function of the conversion efficiency to the second harmonic, and hence of the incident intensity of the fundamental wave. We have deposited sol-gel silica single-layer coatings of this optimum thickness, and the total reflectivity at fundamental and second-harmonic wavelengths taken together was measured and compared with the theoretically calculated values.