Laser Power Noise Measurements on a Cataphoretic He - Cd + Laser

Power output fluctuations at frequencies in the kHz to MHz range are commonly observed in gas lasers. In case of the cataphoretic He-Cd laser violent fluctuations of the order 50% peak to peak can occur [1]. This noise appears to be different in nature from e.g. He-Ne laser noise [2]. Its origin was attributed to neutral cadmium vapor density fluctuations [3] and to the coupling of Penning electrons to helium metastables [4]. In [5, 6, 7, 8] the role of moving striations in the positive column was investigated as being responsible for the output power fluctuations. Especially in [8] direct connections between noise and plasma oscillations were proven. Several different attempts have been made to reduce He-Cd laser noise to slightly below the 1% rms level by special tube constructions [3, 4]. Unfortunately these constructions are more complex than the simple cataphoretic He-Cd+ lasers with a single externally heated Cd-source and no diffusion return pass, but which normally exhibit rather high noise levels of the order 3 — 10% rms [5, 10]. To reduce this noise to the 1% rms level or slightly below, negative feedback [9] or modulation of the discharge current [10] has been employed. On the other hand, no attempt has been made to our knowledge to investigate the possibility of noise reduction by careful temperature stabilisation in case of these laser tubes. Our He-Cd laser tubes are similar to the ones described in [5]. The positive column of the dis-