Lumeau Julien, Glebova Larissa, Glebov Leonid B
CREOL, the College of Optics and Photonics, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816-2700, USA.
Appl Opt. 2011 Oct 20;50(30):5905-11. doi: 10.1364/AO.50.005905.
Volume Bragg gratings (VBGs) in photothermorefractive (PTR) glass are widely used for laser beam control including high-power laser systems. Among them, spectral beam combining based on VBGs is one of the most promising. Achieving 100+ kW of combined laser beams requires the development of PTR glass and VBGs with an extremely low absorption coefficient and therefore methods of its measurement. This paper describes the calorimetric method that was developed for measuring a low absorption coefficient in PTR glass and VBGs. It is based on transmission monitoring of the intrinsic Fabry-Perot interferometer produced by the plane-parallel surfaces of the measured optical elements when heated by high-power laser radiation. An absorption coefficient at 1085 nm as low as 5×10(-5) cm(-1) is demonstrated in pristine PTR glass while an absorption coefficient as low as 1×10(-4) cm(-1) is measured in high-efficiency reflecting Bragg gratings with highest purity. The actual level of absorption in PTR glass allows laser beam control at the 10 kW level, while the 100 kW level would require active cooling and/or decreasing the absorption in PTR Bragg gratings to a value similar to that in virgin PTR glass.
光热折变（PTR）玻璃中的体布拉格光栅（VBG）广泛应用于激光束控制，包括高功率激光系统。其中，基于VBG的光谱合束是最有前景的方法之一。要实现100千瓦以上的合束激光束，需要开发具有极低吸收系数的PTR玻璃和VBG，因此需要测量其吸收系数的方法。本文介绍了一种用于测量PTR玻璃和VBG中低吸收系数的量热法。它基于对由被测光学元件的平行平面在高功率激光辐射加热时产生的本征法布里-珀罗干涉仪的透射监测。在原始PTR玻璃中，1085纳米处的吸收系数低至5×10⁻⁵厘米⁻¹，而在纯度最高的高效反射布拉格光栅中，吸收系数低至1×10⁻⁴厘米⁻¹。PTR玻璃中的实际吸收水平允许在10千瓦水平进行激光束控制，而100千瓦水平则需要主动冷却和/或将PTR布拉格光栅中的吸收降低到与原始PTR玻璃中相似的值。
