An analytic model for the cw diode pumped alkali laser is developed by considering the longitudinally averaged number densities of the ground 2S1/2 and first excited 2P3/2, and 2P1/2 states. The pump intensity to reach threshold requires fully bleaching the pump transition and exceeding optical losses, typically about 200 Watts/cm2. Slope efficiency depends critically on the fraction of incident photons absorbed and the overlap of pump and resonator modes, approaching the quantum efficiency of 0.95 - 0.98. For marginal cavity transmission losses, peak performance is achieved for low output coupling. For efficient operation, the collisional relaxation between the two upper levels should be fast to prevent bottlenecking. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasitwo level system is achieved. For properly designed gain conditions, the quasi two level solution is usually achievable and represents ideal performance.
An analytic model for the cw diode pumped alkali laser is developed by considering the longitudinally averaged number densities of the ground 2S1/2 and first excited 2P3/2, and 2P1/2 states. The pump intensity to reach threshold requires fully bleaching the pump transition and exceeding optical losses, typically about 200 Watts/cm2. Slope efficiency depends critically on the fraction of incident photons absorbed and the overlap of pump and resonator modes, approaching the quantum efficiency of 0.95 - 0.98. For marginal cavity transmission losses, peak performance is achieved for low output coupling. For efficient operation, the collisional relaxation between the two upper levels should be fast to prevent bottlenecking. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasitwo level system is achieved. For properly designed gain conditions, the quasi two level solution is usually achievable and represents ideal performance.
A three level model for optically pumped alkali metal vapor lasers is developed by considering the steady state rate equations for the longitudinally averaged number densities of the ground 2 S1/2 and first excited 2 P3/2, and 2 P1/2 states. For efficient operation, the collisional relaxation between the two upper levels should be fast relative to stimulated emission. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasi-two level system is achieved. A second limiting solution is identified for strongly bleached conditions where the atom recycle rate, limited by spin-orbit relaxation, fully specifies the output power. Performance in the intermediate regime depends significantly on pump laser bandwidth and requires numerical simulation. The ratio of populations for the two excited, 2 P3/2,1/2 states completes an analytic solution and depends primarily on pump laser bandwidth, threshold and alkali concentration.
A model for an end-pumped double-pass alkali vapour laser is set up which has considered all the main physical features, including the fine-structure mixing rate that represents the three-level nature of this kind of laser, the spectral dependence of pump light absorption for broadband pumping, the longitudinal population variation and the distributed intra-cavity losses. To solve this model, we have proposed a searching algorithm for a single-pass configuration and developed a novel iterative algorithm for a double-pass case which has taken the single-pass solution as an initial value. The calculation process demonstrates a fast rate of convergence and high degree of accuracy. By this algorithm, some special cases, for example, the non-uniform longitudinal distribution of alkali atom concentration, can also be solved well. The model and the numerical approach described in this paper demonstrate a new method to simulate the CW end-pumped quasi-three- or three-level lasers. In this way, no significant assumptions or simplifications are needed in the model, and both the detailed intra-cavity information and pump power distribution can be obtained. To ensure the correctness of our model, a comparison between our model and Beach et al 's model is made. The result shows that when the non-uniformity of the longitudinal population distribution is less than 20% or the pump intensity is far beyond the threshold, the assumption of longitudinal population average is valid and the two models are equivalent. When the pump intensity is near the threshold, this assumption may be invalid due to the large longitudinal population variation. If this case happens or more detailed laser information is required, we could use this model to obtain a more accurate solution.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.
Abstract Diode-pumped alkali lasers (DPALs) exhibit remarkable efficiency. The modeling of these lasers has been limited by rate equations that neglect quantum coherence. In this paper, we introduce a semiclassical approach based on the density matrix formalism, which accurately accounts for relaxation processes and coherent effects. This framework enables us to reveal several previously unreported coherent effects in DPALs. Some of these effects, such as gain saturation due to power broadening, are expected. However, others are entirely unexpected and counterintuitive. Notably, we demonstrate the existence of laser gain even in the absence of population inversion, with a significantly broad lasing without inversion (LWI) regime extending beyond just the laser threshold. Even more surprising are the optimal pumping conditions, which reveal that a counterintuitive mix of coherent and incoherent pumping yields the highest efficiency. Our simulations highlight the crucial role of coherence, even in scenarios with high relaxation rates.
Based on an analysis of kinetic processes in the continuous-wave (CW) single-wavelength two-photon pumped Rb-vapor laser (Rb-TPAL), a three-dimensional computational model is established using the rate equation. Based on the application of the model to cavity-enhanced Rb-TPAL [Yuanet al.,Optics Express29, 4858 (2021)], the calculated and measured dependences of the laser power on the pump power show good agreement. We simulate the Rb-TPAL pumped by a high-power CW laser diode and analyze the effects of temperature, vapor cell length, pump waist, vapor cell transmission, and output mirror reflectance on the characteristics of the blue laser. The results suggest that the output power and efficiency of blue laser can be improved by optimizing the above parameters; however, the particles in the52D5/2level face difficulty to transition to the62P3/2level, and this limits the further improvement of blue laser efficiency; therefore, an approach is proposed to improve the efficiency of Rb-TPAL. This study can be useful for designing a two-photon pumped alkali metal laser with high-power output.
In this paper, coupled power-gain equations were derived and solved to investigate the role of different power-degradation mechanisms on the continuous-wave power performance of a laser in the absence of thermal effects. Nine different power-degradation cases were considered, including a low-gain ideal four-level operation, ground-state reabsorption, pump excited-state absorption with and without storage, laser excited-state absorption with and without storage, intermediate storage, and Auger-type energy transfer upconversion with and without storage. There were three distinct types of laser performance identified, in terms of how the output power varies as a function of the pump power. Explicit analytical expressions were derived for the threshold pump power Pth, slope efficiency η, and the product Pthη/T as a function of output coupler transmission T and other physical parameters that describe the power degradation mechanism. The analytical approach developed here provides valuable insight about the role of major power degradation mechanisms in lasers, and it should be applicable to the analysis of a wide range of low-power, optically pumped laser media, including solid-state, fiber, and ceramic lasers.