1 High resolution absolute temperature mapping of laser crystals in diode-end-pumped configuration Julien Didierjean 1 , Sébastien Forget 1 , Sebastien Chenais, Frédéric Druon 1 , François Balembois 1 , Patrick Georges 1 , Konrad Altmann 2 , Christoph Pflaum 3 1 Laboratoire Charles Fabry de l'Institut d'Optique, UMR 8501, Centre Universitaire, Bât. 503, 91403 Orsay Cedex, France. 2 LAS-CAD GmbH, Brunhildenstr, 9, 80639 Munich, Germany, Phone +49 89 173607, Fax +49 89 172594, Dr.Altmann@las-cad.com 3 Universtät Erlangen, Lehrstuhl für informatik X, Cauerstr. 6, 91508 Erlangen, Germany, pflaum@informatik.uni-erlangen.de ABSTRACT: We report on direct, absolute and spatially resolved temperature measurements in various diode-end- pumped laser crystals, using an infrared camera. Our measurement method requires careful calibrations of the camera, to take into account the emissivity of the crystals. We tested the repeatability of the calibration process, and the linearity of calibrations curves was verified to up to 100°C. We also compared our experimental results with finite elements analysis simulations done with LASCAD. We used our setup to compare different types of thermal contacts and to measure the corresponding heat transfer coefficients using an Yb:YAG crystal. Finally we compared the thermal behaviour of Nd:YVO 4 and Nd:GdVO 4 crystals under the same pumping conditions. 1. INTRODUCTION: End pumped bulk laser crystals are good candidates to high power and high beam quality lasers, but they are mainly limited by thermal effects. Thermal lensing, depolarisation losses and induced stress can lead to power losses, beam quality degradation or even fracture of the crystal [1]. A better understanding of thermal issues is therefore needed to proceed to higher laser power. The most widespread way to investigate thermal effects in laser crystals is to use thermo-optical methods, that is thermal lens measurements or depolarization measurements. However, these methods only yield quantities that are proportional to thermal gradients inside the probed area, and they depend on the values of more or less known thermo-optical coefficients. Similarly, temperature distributions obtained by finite element analysis depend on material parameters which are not always well known or cannot be directly measured for instance the heat efficiency. Therefore, a direct method leading to absolute temperature mapping with high resolution is highly desirable [2]. In this paper we present this kind of analysis for end-pumped configurations, based on an infrared camera sensitive in the 8-12 µm range. Our method enables us to measure the temperature map of the end-pumped face of the crystal. In the first section, the experimental setup will be described. In a second section, the calibration method will be detailed, and in the third part some experimental results will be compared with simulations, as a validation test. After this validation, our method will be used in the fourth and fith part to measure heat transfer coefficients with an Yb:YAG crystal and to compare Nd:YVO 4 ,and Nd:GdVO 4 thermal performances. 2. EXPERIMENTAL SETUP: The experimental setup is shown in figure 1. The thermal imaging was obtained thanks to a dichroic Zinc selenide plate, High Reflectivity (HR) coated for 800-1080 nm on one face (at 45° angle of incidence), and also coated for High Transmission (HT) in the 8-12 µm spectral range on both faces. An aberration-free germanium objective (focal length 50 mm, N.A. 0.7) was appended close to the ZnSe plate to create the intermediate thermal image. The camera was an AGEMA 570 (Flir Systems Inc.) consisting of 240x320