Spectra Physics 120 HeNe / 256 Exciter problem.

Looking at the photos, this MAY be an external cavity laser. The typical HeNe has the mirrors cemented right on the end of the central glass tube. External cavity lasers have one (or rarely both) mirrors separate from the laser tube, and generally have Brewster windows (thin glass plates mounted at about 56 degrees on the end(s) of the tube.) The external mirror will have some form of adjusting screws to align the mirror.

I used to be the master mirror aligner at our lab. My technique was to shoot a working laser through the dead laser's two mirrors, and observer the spot on a card at the far end. Generally you could see a 2nd reflection on the card, and try to steer them together. if the dead laser was being excited, you would occasionally see short flashes of lasing when you passed through the proper alignment. As you get closer, you had to make extremely small adjustments and then take you hands away to let everything stablize thermally. It is a very frustrating process, but if your laser is, indeed external cavity, then you may need to do this.

Jon

The other approach is to slack one adjusting screw off all the way, jam a screwdriver in to let you rock that axis back and forth rapidly while slowly turning the other knob. Once you start seeing flashes, you're close enough to switch to using both knobs.

The auxiliary laser approach is certainly needed if you're aligning the laser for the first time after a tube change, or if somebody's been monkeying with both of the mirrors.

Cheers

Phil Hobbs

I should note that if the color of the discharge truly looks correct, it should lase despite the problems with keeping it lit.

However, it's hard to tell from digital photos whether this is the case. The proper color is bright unsaturated red-orange, sometimes refered to as "salmon" color. short of spectral analysis of the discharge glow, comparing it with a known healthy red HeNe laser tube would be best. IF it's too pink or weak, then it's probably leaked.

If the SP specs are to be believed, the tube voltage is way low, which is another symptom. That really is the only way the tube voltage can be low with the proper current. It's not a power supply fault.

I'm probably not quite motivated enough yet to measure the voltage on the one I have here that was in a stasis field for 30-40 years and works like new.

I've been looking at a low cost spectrometer solution:

Do you know which grating might be best for this sort of task, and perhaps also using with Ar lasers?

This I don't quite get -- how can the tube drag the voltage down if the current is correct? My guess was that the current would need to be too high. Regards,

Andrew

The VI relationship of a gas tube is highly non-linear and also depends critically on the gases present. Add in low-excitation energy gases like N2 and O2, and the voltage is lower at the same current. This isn't a fixed resistor!

As far as the spectrometer, I've heard that they work well but the resolution isn't that great. I use a Verity monochromator. It's more of a pain to use being manual with a micrometer. But with a set of narrow slits, it can easily small fraction of a nm.