The self-heated 442-nm He-Cd laser: Optimizing the power output, and the origin of beam noise

Power output and beam noise parametric results are obtained for the blue transition of the self-heated He-Cd laser in tubes of 0.10-0.16-cm bore diameter. Presentation of this data in the form of performance contour maps shows that the usual scaling laws for laser discharges are accurate for this laser (<12-percent parameter variation for a 60-percent diameter variation) and is a convenient form for use in tube design. The contour maps imply the existence of a direct connection between the output and noise of this laser. A theory is proposed that the noise in the He-Cd discharge arises from the rapid increase in ionization rate due to Penning collisions as Cd is added to the He discharge. This produces a high gain in the discharge column for the growth of striation waves (which modulate the output). The theory correctly predicts the details of the laser noise behavior, showing that both the noise and output share a common origin in the Penning effect. The dependence of Cd vapor density on tube current is found, from sidelight studies, to be more pronounced in self-heated tubes (as opposed to tubes with externally heated Cd sources). A threshold dissipation value is given for puncture of the hot glass bore by the discharge (this is a common failure mode in the He-Cd laser).