Heat Removal
The optical pump process in a solid-state laser material is associated with the generation of heat for a number of reasons: a) The energy difference of the photons between the pump band and the upper laser level is lost as heat to the host lattice and cau
- PDF / 6,364,751 Bytes
- 54 Pages / 439.37 x 666.142 pts Page_size
- 107 Downloads / 191 Views
The optical pump process in a solid-state laser material is associated with the generation of heat for a number of reasons: a) The energy difference of the photons between the pump band and the upper laser level is lost as heat to the host lattice and causes the so-called quantum defect heating; b) similarly, the energy difference between the lower laser level and the ground state is thermalized; c) since the quantum efficiency of the fluorescence processes involved in the laser transition is less than unity, heating due to quenching mechanisms takes place; d) the broad spectral distribution of arc lamps or flashlamps is such that there is considerable absorption by the host material, mainly in the ultraviolet and infrared bands. Efficient heat removal and reduction of the thermal effects which are caused by the temperature gradients across the active area of the laser medium usually dominate design considerations for high-average-power systems. One of the advantages of diode-laser pumping is that the waste heat dissipated in the laser rod is greatly reduced by the high efficiency of the pumping process. Quantum-defect heating is reduced because the pump wavelength is closer to the laser-emission wavelength, and heating of the host material by pump radiation located outside the absorption bands of the active ions is completely eliminated. For example, in Nd: YAG the thermal load of the crystal is only 1/3 that of a flashlamp-pumped system for the same laser output.
7.1 Thermal Phenomena in Laser Rods The combination of volumetric heating of the laser material by the absorbed pump radiation and surface cooling required for heat extraction leads to a nonuniform temperature distribution in the rod, which results in a distortion of the laser beam due to a temperature- and stress-dependent variation of the index of refraction. The thermal effects which occur in the laser material are thermal lensing and thermal stress-induced birefringence. An additional issue associated with thermal loading is stress fracture of the laser material. Stress fracture occurs when the stress induced by temperature gradients in the laser material exceeds the tensile strength of the material. The stress fracture limit is given in terms of the maximum power per unit length dissipated as heat in the laser medium.
W. Koechner, Solid-State Laser Engineering © Springer-Verlag Berlin Heidelberg 1992
381
The particular temperature profile which exists in the laser material depends to a large degree on the mode of operation, i.e., cw pumped, single shot, or repetitively pulse pumped. In the case of cw operation, a long cylindrical laser rod with uniform internal heat generation and constant surface temperature assumes a quadratic radial temperature dependence. This leads to a similar dependence in both the index of refraction and the thermal strain distribution. In a pulse-pumped system, laser action occurs only during the pump pulse or shortly thereafter in the case of Q-switching, therefore the main interest is centered around the time interva
Data Loading...
