Effects of nonlinear resonant absorption on sodium laser guide stars

Saturation effects may severely limit the brightness of guide stars created in the mesospheric sodium (Na) layer. Saturation arises when the laser energy density is large enough to significantly alter the population densities of the atomic states within the layer. These altered state populations lead to nonlinear absorption of the laser energy thereby producing in a reduced rate of fluorescence and an increased rate of stimulated emission. The level of saturation is determined by the laser’s pulse length, pulse energy, beamwidth and linewidth. We have quantified the saturation effects in terms of these laser parameters and have formulated design equations which will allow us to design a laser capable of achieving a specified guide star brightness while at the same time minimizing the power and pulse length requirements. Our calculations show that a Na laser having a pulse energy and pulse length on the order of 106 mJ and 69 μs respectively and an linewidth of 600 MHz will produce a guide star at zenith which is bright enough to drive an adaptive optics system with a seeing cell size of r_o = 18.5 cm. This energy corresponds to a laser power of 21 watts at 200 pulses per second.