Computational fluid dynamics modeling of subsonic flowing-gas diode-pumped alkali lasers: comparison with semi-analytical model calculations and with experimental results

Abstract

Comprehensive analysis of kinetic and fluid dynamic processes in flowing-gas diode-pumped alkali lasers (DPALs) using two-and three-dimensional computational fluid dynamics (2D and 3D CFD) models is reported. The 2D model is applied to a Cs DPAL with optical resonator-flow field coaxial configuration and the 3D model is applied to an optical axis transverse to the flow configuration. The models take into account effects of temperature rise and losses of Cs atoms due to ionization. The 2D CFD model is applied to 1 kW flowing-gas Cs DPAL [Quantum Electron. 42, 95 (2012)] and the calculated results are in good agreement with the measurements. Comparison of the 2D CFD to semi-analytical model [J. Opt. Soc. Am. B 30, 1118] shows that for low pump power both models predict very close values of the laser power; however, at higher pump power, corresponding to saturation of the absorption of the pump transition, the laser power calculated using the 2D CFD model is much higher than that obtained using the semi-analytical model. At high pump power, the heat convection out of the laser resonator is more efficient for the transverse case than the coaxial case, the temperature in the resonator is lower, and consequently the calculated laser power is higher. Optimization of the Cs DPAL parameters, using 3D CFD modeling, shows that applying high flow velocity and narrowband pumping, maximum lasing power as high as 40 kW can be obtained at pump power of 80 kW for transverse flow configuration in a pumped volume of ∼0.7 cm 3 .

Theoretical analysis of the semi-ring and trapezoid LD side-pumped alkali vapor lasers

“…Up to now, numerous experimental studies on DPALs with different pumped configurations were performed, including single-end pumped configuration [1,3,4,8], double-end pumped configuration [2,7], single-side pumped configuration with stable resonator [5] and unstable resonator [6]. Their corresponding models were set up by [9][10][11][12][13][14][15] and agreed well with the experimental results. Additionally, in order to further improve the power of DPALs, researches on master oscillator power amplifier (MOPA) have also been conducted [16][17][18][19][20][21][22], demonstrating the high efficiency of DPALs and the potential for power scaling.…”

“…In each of the divided semi-annuli or squares, the rate equations for the population densities of the alkali atomic energy levels are described as follows [21]: , while + n X and + n X 2 denote the one of the + X and + X 2 ions, respectively. The rates of laser emission W 21 , relaxation W 32 , spontaneous emission S 31 , S 21 and S km , quenching Q m1 , photoexcitation I mk , energy pooling Po mk , photoionization Ph k , penning ionization Pn and recombination + R , + R 2 are given by [14,21]. The rates of three-body recombination, two-photon ionization and chemical reactions, as estimated in [13], are negligible and hence not taken into account.…”

“…The experimental [5,6] and simulated parameters including rate constants and cross sections from [14] are listed in Table 1, where the numerical symbols ①②③④ indicate the experiment of a Cs vapor laser with a stable resonator [5], the experiment of a Cs vapor laser with an unstable resonator [6], the simulations of semi-ring and trapezoid LD side-pumped Cs vapor lasers, respectively.…”

“…Therefore, according to the parameters of ③④ listed in Table 1 and a larger vapor cell radius of 3.5 mm and a flowed velocity of = u 20 m/s chosen by [13,14] to fit the high-power pump source with larger amount of laser diodes, dependence of output laser power on pump power for the semi-ring, trapezoid, single and double sidepumped configurations are presented in Fig. 6.…”

Flowing-gas diode pumped alkali lasers: theoretical analysis of transonic vs supersonic and subsonic devices

“…Demonstration of subsonic DPALs [3,4] and modeling of both supersonic and subsonic devices [5][6][7][8][9][10][11][12][13][14][15] taking into account fluid dynamics and kinetic processes in the lasing medium, show the positive influence of the gas flow on the laser performance, where the highest lasing power and optical-to-optical efficiency were predicted for supersonic DPALs. In the present paper we examine transonic DPALs as a simpler alternative to supersonic devices, where complex hardware, including supersonic nozzle, diffuser and high power mechanical pump (needed for recovery of the gas total pressure which strongly drops in the diffuser), is required for continuous closed cycle operation [6] (the term "transonic" is chosen merely to imply high subsonic velocity and does not impose a new flow regime).…”