Small-scale recombination He – Sr$^+$(Ca$^+$) lasers

A review of specific features and output characteristics of small-scale ($l \le $ 30 cm, $d \le$ 1.1 cm) gas-discharge recombination He–Sr$^+$ ($\lambda$=430.5 nm) and He–Ca$^+$ ($\lambda$=373.7 nm) lasers is provided. Such compact lasers are characterised by a relatively high reliability and durability. These lasers are easy to operate and are capable of generating laser pulses with high repetition rates $f$, a high specific mean power $P_{sp}$, and an improved quality of output radiation. A typical mean power $P$ of self-heating sealed-off He – Sr$^+$ (Ca$^+$) laser tubes with $l \approx$ 30 cm and $d \approx$ 1 cm is 0.5 W. The best specific characteristics of a He – Sr$^+$ laser with tubes of this type were achieved with $l$=20 cm and $d$=0.6 cm ($P_{sp}$=73 mW cm$^{-3}$) and $l$=9 cm and $d$=0.55 cm ($P_{sp}$=65 W cm$^{-3}$). The best specific characteristics for a He – Ca$^+$ laser were achieved with $l$=26.5 cm and $d$=0.7 cm ($P_{sp}$=50 mW cm$^{-3}$). The use of forced water cooling in a He – Sr$^+$(Ca$^+$) laser allowed the powers $P$=3.9 W and $P_{sp}$=137 mW cm$^{-3}$ to be achieved with a tube with $l$=30 cm and $d$=1.1 cm for $f$=29 kHz. A new method of inputting metal vapours based on cataphoresis is considered in detail. Using this method allowed the output characteristics of a He – Sr$^+$ laser to be considerably improved and a record specific power $P_{sp}$=277 mW cm$^{-3}$ to be achieved for a tube with $l$=26 cm and $d$=0.3 cm with $f$=30 kHz and $P$=510 mW. A record gain, 0.15 cm$^{-1}$, was also achieved under these conditions. Some possible areas of applications of small-scale He – Sr$^+$(Ca$^+$) lasers in semiconductor microtechnology, holography, ecology, and medicine are outlined.