You have probably wondered why the beam from a typical HeNe laser (without additional optics) is so narrow. Is it that making a tube with larger mirrors would be more costly?
No, it’s not cost. Even high quality and very expensive lab lasers still have narrow bores. The very first HeNe lasers did use something like a 1 cm bore but their efficiency was even more mediocre than modern ones. A wide bore tube would actually be cheaper to manufacture than one requiring a super straight narrow capillary. However, it wouldn’t work too well. A combination of the current density needed in the bore, optimal gas pressure, gain/unit length in the bore, the bore wall itself aiding in the depopulation of lower energy states, and the desire for a TEM00 (single transverse mode) beam (there are multimode tubes that have slightly wider bores), all interact in the selection of bore diameter. In fact, there is a mathematical relationship between bore size, gas pressure, and tube current resulting in maximum power output and long life. The optimal pressure at which stimulated emission occurs in a HeNe laser is inversely proportional to bore diameter. According the one source (Sc
