Absorption in a self-heating He-Se laser tube

A self-heating He-Se laser tube has been developed. It was all Pyrex, 60 cm active length, 2 mm ID. The laser beam consisted of seven green lines 28 mW total power. We have examined, by perturbation spectroscopy, the existence of amplification along the laser tube axis. The experimental set-up consisted of: laser tube, two laser mirrors dielectrically deposited for full reflection on 500 ± 50 nm spectral region, a grating monochromator 0.2 nm resolution, a chopper, an EMI 9558QB photomultiplier and a lock-in amplifier. The spontaneously emitted light in the perpendicular direction to the tube axis has been monitored. The laser radiation field that exists in the active medium involves the decreasing of the upper laser level population and thus, the intensity of the corresponded spontaneously emitted line decreases too. Using the chopper, the laser beam is periodically interrupted so that, a modulation of the intensities of the spectral lines appears. The signal sign is opposite for amplification and absorption, respectively. By this technique, we have observed the presence of an absorption region, placed inside the metal reservoir, just where the metal vapors penetrate the discharge. Taking into account the energies of the involved levels, we have proposed a probable reaction.