Design and performance of an improved hollow cathode He-Cd⁺laser

Abstract

A simple hollow cathode He-Cd+ laser which emits at least 100 mW multimode power at each of 441.6 and 533.7/537.8 nm from a 4 mm bore, 60 cm active length is described. Emphasis is given to the features needed to achieve discharge stability in this as well as other inert gas-metal vapour systems. Measurements showed that the axial extent of the negative glow associated with a single anode almost doubled when cadmium was introduced into a helium discharge. Although this suggests the possibility of some design simplification, it was found that little improvement could be achieved in practive because of the onset of discharge arcing when the current carried by a single anode reached 250 mA.

Helium 23S Densities and Excitation Mechanisms in a Hollow Cathode He-Cd+ Laser

“…The hollow cathode discharge tube was the same as that described by Grace and McIntosh (1979). The cathode consisted of a long stainless steel tube (length 60 cm, bore 4 mm) with 14 separately ballasted anodes distributed along its length.…”

“…The length d 1 was thus an integral multiple of dz, the values of the latter being taken from the results of Grace and McIntosh (1979) who measured the length of discharge associated with a single anode for various discharge conditions in the same experimental tube. The large axial extent of the negative glow associated with a single anode when the discharge contained ~0·1 torr (13·3 Pa) of cadmium, together with the requirement that the multiple glows should not overlap and therefore interact, made it necessary to restrict the ratio of discharge lengths to 3 : 1 or less and this consequently limited the accuracy with which low densities of metastables could be measured.…”

“…Allowance was made for the hyperfine splitting of the triplet transition at 388·9 nm in the manner described by Browne and Dunn (1973). The calculated intensity ratio is effectively independent of gas temperature over the range of likely temperatures, but a gas temperature of 620 K, equal to the wall temperature of the discharge tube (Grace and McIntosh 1979), was used to calculate the Doppler widths of both lines.…”

“…1975;He-Hg+, Kano eta!. 1975;He-Zn+, Gill and Webb 1978;He-As + , Piper et al 1978), but, apart from reports of laser oscillation and of parametric variations in output power (Piper and Webb 1973;Fujii 1973;Fujii et al 1975;Csillag et al 1977;Hernqvist 1978;Grace and McIntosh 1979), the hollow cathode He-Cd + system has not been the subject of detailed diagnostic experiments. This omission is surprising because there are a number of reasons why the hollow cathode He-Cd + laser is an attractive candidate for further development.…”

“…McIntosh et al (1978) did not observe saturation of the He(2 3 S) density over the range of currents investigated in a pure helium hollow cathode discharge, while in the case of He-Zn Gill and Webb (1978) showed that electron impact excitation may account for up to 70 % of the upper level population of the Zn + laser transition at 589·4 nm which was previously thought to be excited almost entirely by Penning ionization. Grace and McIntosh (1979) have reported a design for a hollow cathode He-Cd + laser with ouput powers exceeding 100 m W at each of 441· 6 and 533· 7-537' 8 nm and with low noise and good long-term stability. This laser, in common with other devices of the same general type, has output at 441·6 nm which depends markedly on the cadmium concentration, the helium pressure and the gas flow rate and which ultimately saturates with increasing discharge current.…”