On thermal effects in solid-state lasers: The case of ytterbium-doped materials

Abstract

A review of theoretical and experimental studies of thermal effects in solid-state lasers is presented, with a special focus on diode-pumped ytterbium-doped materials. A large part of this review provides however general information applicable to any kind of solid-state laser. Our aim here is not to make a list of the techniques that have been used to minimize thermal effects, but instead to give an overview of the theoretical aspects underneath, and give a state-of-the-art of the tools at the disposal of the laser scientist to measure thermal effects. After a presentation of some general properties of Yb-doped materials, we address the issue of evaluating the temperature map in Yb-doped laser crystals, both theoretically and experimentally. This is the first step before studying the complex problem of thermal lensing (part III). We will focus on some newly discussed aspects, like the definition of the thermo-optic coefficient: we will highlight some misleading interpretations of thermal lensing experiments due to the use of the dn/dT parameter in a context where it is not relevant. Part IV will be devoted to a state-of-the-art of experimental techniques used to measure thermal lensing. Eventually, in part V, we will give some concrete examples in Yb-doped materials, where their peculiarities will be pointed out.

All-space existence and dispersion of athermal directions in monoclinic KY(WO4)2

“…It is recognized to be an important limitation of power scaling capabilities for solid-state lasers. Thermal lens can lead to the drop of the output power, distortions in the profile of the output beam, depolarization and even laser ceasing [6][7][8]. Thus, a lot of attention is paid for elimination of thermal lensing.…”

“…Thus, a lot of attention is paid for elimination of thermal lensing. The first route here is the variation of pumping and cooling conditions (the heat removal) [8] and appropriate thermal-lens-insensitive cavity design [9][10][11]. The second idea is to achieve near-zero change of the optical path length, or athermal behavior [12].…”

“…The reason for athermal behavior of the laser materials is related with the fact that OPL is changed due to several effects of different nature [6,8]. They are temperature dependence of the refractive index (expressed by the thermo-optic coefficient, dn/dT), photoelastic effect and crystal bulging due to thermal expansion.…”

“…The latter term is positive for majority of laser materials, while remaining ones can be either positive or negative [8,13]. Thus, partial or even complete compensation of OPL can be achieved.…”

Thermal conductivity of Nd3+ and Yb3+ doped laser materials measured by using the thermal lens technique

“…Several important laser materials have been investigated in the past to derive reliable thermal diffusivity, heat capacity and thermal-conductivity values, some of them being even reported as a function of temperature and/or dopant concentration [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, most of these measurements have been performed by using heat propagation methods [1][2][3][4][5][6][7][8][9][10] in which heat is provided on one side of the sample by using either an electric heater, a lamp or a laser, and the heat transmitted or the temperature on the other side of the sample is measured and analyzed.…”