Pulse-forming and line-broadening in AM mode locking of the TEA-CO2laser

The present paper describes AM mode locking for homogeneously broadened systems, a procedure for measuring linewidths under laser conditions, and finally, experimental results for a 1-atm CO2laser. Working in the frequency domain, analytic solutions are given for the pulse bandwidth and pulse shape as a function of frequency detuning. We arrive at a simple expression for the maximum positional shift of the mirror distance. It turns out that the linewidth is inversely proportional to the maximum frequency shift and can be obtained independent of the modulation parameters and time constant of the pulse-signal detector. Applying this analysis to an amplitude-modulated CO2laser the pulsewidth and linewidth have been measured as a function of gas composition and modulation depth. In contrast to predictions deduced from absorption measurements under neutral conditions we find the striking result that under laser-discharge conditions the linewidth increases and the pulsewidth decreases with increasing helium percentage.