<title>High-reflective multilayer coating by using the reflective control wavelength monitoring of layer thicknesses for second harmonic generation (SHG) laser</title>

Abstract

The aim of present work is the development of a method of high reflective (H/R) optical multilayer coating on KTP and BK-7 Glass with the specification: R (reflectance): greater than or equal to 99.9% at 1064 nm. T (transmittance): greater than or equal to 95% at 532 nm and T (transmittance): greater than or equal to 95% at 809 nm for a second harmonic generation (SHG) laser. The parameters that can be used to reach these goals are the number of layers in the multilayer, the layer thicknesses and refractive indices and extinction coefficients of the individual layers and of surrounding media. Clearly, the more demanding the performance specifications, the more complex is the resulting system. In our case, we have the most demanding performance specification, that is why the technology of obtaining coatings with this specification is very precise and complicated. In order to fulfill the demand of high reflectance at lambda equals 1064 nm it is necessary to deposit 30 or more alternative quarter wavelength thickness layers of ZrO<SUB>2</SUB> and SiO<SUB>2</SUB> and to fulfill the demand of high transmittance at lambda<SUB>1</SUB> equals 532 nm and lambda<SUB>2</SUB> equals 809 nm the SiO<SUB>2</SUB> layer of lambda/8 (last layer) optical thickness is used to suppress the secondary maxima. The perfect suppression of secondary maxima takes place, when we deposit multilayer coating with equal optical thicknesses of high and low refractive indexes layers. The performance of many optical multilayers depends critically on the thicknesses of individual layers. The control of the layer thicknesses during their deposition is therefore very important. The most common techniques used is optical monitoring performing indirectly on a witness glass (or the chip). R (reflectance) optical monitoring is however used to measure the quantities of each layer optical thicknesses in our system. We used different control wavelength to monitor and control each layer optical thickness with different coating materials for compensating the deposited optical thickness.