Photon drag detectors were incredibly simple; take a crystal of germanium, solder some electrodes to it, shine a powerful infrared laser beam at it, and a voltage would be generated proportional to the power of the laser beam. The initial work had all been done at Essex in CO2 at 10.6 microns and the department wished to probe the effect at different wavelengths.
The laser I built worked fine with CO2, but that was no surprise; with water vapour I got nowhere. For weeks and weeks I persevered but could extract no laser output.
I shared an office with an Egyptian student, Salah, a lovely gentle man, who was working on a five micron laser. We were chewing the fat one day, when he mused that hydrogen and oxygen combine to make water… The reaction is exothermic – releases energy – and this could help achieve a population inversion, a requirement for laser action.
It all seemed to make sense, although hydrogen and oxygen notoriously combine explosively. I decided that it would be safe at low pressure, and I fixed up a hydrogen and oxygen supply to the laser along with some helium. Gingerly, I ran up the high voltage. I got a beautiful blue – violet discharge but no laser activity. I tried increasing the pressure; nothing; I increased the pressure again; still nothing. Slowly I increased the pressure some more and there was an almighty bang. The laser tube exploded, throwing the end with a mirror assembly thirty feet down the laboratory where it shattered against the end wall. I hit the ground and crawled towards the power supply where I managed to shut off the power by the expedient of pulling the mains plug out of the socket. It was all very quiet except for the ringing in my ears…
That was the end of my water vapour laser. My supervisor was sympathetic, and somehow cobbled together an alternative project to allow me to continue with the degree work. I was able to salvage much of the hardware and repaired the ‘water vapour’ laser, using it with CO2. It did finally take six years before I was awarded the degree.