An accurate determination of the radial distribution of gain at 633 nm in small bore helium-neon discharges

Abstract

A sensitive technique for the determination of optical gain in a DC discharge is outlined. This technique has permitted an investigation of the radial distribution of optical gain at 633 nm in a 3 mm bore discharge with a spatial resolution of around 120 mu m. Results are presented which illustrate the behaviour of this distribution with gas pressure and current.

Radial distribution gain at 633 nm in a He-Ne RF-excited small bore discharge

“…We chose to investigate four different pressure of gas mixture and, for each pressure, three different power level of plasma discharge. Following the literature [14], we have chosen for each He-Ne mixture, a pressure of 0.22 mbar of Ne and then we have added He up to 1.33, 2.66, 5.00, 10.00 mbar. Before the first filling the cavity has been baked and pumped down to 10 −6 mbar, and before each refill we have restored the vacuum condition to clean the system.…”

“…Before the first filling the cavity has been baked and pumped down to 10 −6 mbar, and before each refill we have restored the vacuum condition to clean the system. Considering the size of the tube and the laser waist dimension, we chose to move the probe laser along the diameter of the plasma discharge by steps of 0.25 mm using the micrometric screws shown in Fig 1 . The difficulties of this type of study are mainly two: the handling of a RF source, and the fact that the expected gain of this kind of system is of the order of only some hundreds of ppm [12,13,14,16]. About the first issue, differently from a DC plasma discharge the power supplied by a RF source needs an initial high-voltage discharge to switch on the plasma.…”

“…Setting this level near the plasma threshold, it is possible to have a stable plasma discharge with enough low power to not activate the gain for the 633 nm transition. The second problem concerns the low expected gain around few hundreds of ppm at most [12,13,14,16]. In addition, the spontaneous emission at 633 nm wavelength makes it hard to discern between the laser gain and the offset due to the spontaneous emission.…”

“…But the differences in gas pressure and the ratio between He and Ne and also, the used discharge power make impossible a direct comparison with our results. Another possibility could be a comparison with the work of Spoor and Latimer [14]. In this case it is possible to find a couple of data-set similar in terms of gas characteristics and discharge power, but the DC supply Figure 5: Example of gain decay vs time makes, again, the analogy impossible.…”

“…In view of this, this work aims to study the optical gain of the line at 633 nm in a Radio-Frequency (RF) excited He-Ne plasma gas in function of both the radial behavior along the discharge bore and the total filling gas pressure. The studies about this topic date back to several years ago [11,12,13,14,15,16]. During years for commercial devices, the DC supply has been preferred over the RF one.…”