Diode-pumped alkali laser-bleached wave dynamics

Abstract

A three level analytic 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 ²S _1/2 and first excited ²P_3/2, and ²P_1/2 states. The threshold pump intensity includes both the requirements to fully bleach the pump transition and exceed optical losses, typically about 200 Watts/cm². Slope efficiency depends critically on the fraction of incident photons absorbed. For efficient operation, the collisional relaxation between the two upper levels should be fast to prevent bottle-necking. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasi-two level system is achieved. The highly saturated pump limit of the recently developed three-level model for Diode Pumped Alkali Lasers (DPAL) is also developed. The model is anchored to several recent laser demonstrations. A rubidium laser pumped on the 5 ²S_1/2 – 5 ²P_3/2 D₂ transition by a pulsed dye laser at pump intensities exceeding 3.5 MW/cm² (< 1000 times threshold) has been demonstrated. Output energies as high as 12 μJ/pulse are limited by the rate for collision relaxation of the pumped ²P_3/2 state to the upper laser ²P_1/2 state. More than 250 photons are available for every rubidium atom in the pumped volume during each pulse. For modest alkali atom and ethane spin-orbit relaxer concentrations, the gain medium can only process about 50 photons/atom during the 2 – 8 ns pump pulse. At 110° C and 550 Torr of ethane, the system is bottlenecked. The system efficiency based on absorbed photons approaches 36% even for these extreme pump conditions. Furthermore, at 320°C with 2500 torr of helium, a pulsed potassium laser with 1.15 MW/cm² peak intensity and 9.3% slope efficiency has been demonstrated.