Modeling of supersonic diode pumped alkali lasers

3D computational fluid dynamics (CFD) modeling of supersonic diode pumped alkali lasers (DPALs), taking into account fluid dynamics and kinetic processes in the lasing medium, is reported. For a supersonic Cs DPAL with laser section geometry and resonator parameters similar to those of the 1-kW flowing-gas subsonic Cs DPAL [Quantum Electron. 42, 95 (2012)] the maximum achievable output power, ∼7 kW, is 25% higher than that achievable in the subsonic case. Comparison between semi-analytical and 3D CFD models for Cs shows that the latter predicts much higher maximum achievable output power than the former. Optimization of the laser parameters using 3D CFD modeling shows that very high power and optical-to-optical efficiency, 35 kW and 82%, respectively, can be achieved in a Cs supersonic device pumped by a collimated cylindrical (0.5 cm diameter) beam. Application of end pumping or transverse pumping by collimated rectangular (large cross section ∼2-4 cm²) beam makes it possible to obtain even higher output power, >250 kW, for ∼350 kW pumping power. The main processes limiting the power of Cs supersonic DPAL are saturation of the D2 transition and large ∼40% losses of alkali atoms due to ionization, whereas the influence of gas heating is negligibly small. For supersonic K DPAL, both gas heating and ionization effects are shown to be unimportant and the maximum achievable power, ∼40 kW and 350 kW, for pumping by a ∼100 kW cylindrical and ∼700 kW rectangular beam, respectively, are higher than those achievable in the Cs supersonic laser. The power achieved in the supersonic K DPAL is two times higher than for the subsonic version with the same resonator and K density at the gas inlet, the maximum optical-to-optical efficiency being 82%. The power of the mechanical pump required for closed-cycle operation of supersonic DPALs was estimated for constant and variable area ratio diffusers. The nucleation rate of the alkali atoms during supersonic expansion was calculated and found to be negligible.