Jon Elson writes:

Andrew Back wrote:

Hello,

I recently picked up an old Spectra Physics 120 HeNe laser head plus 256
Exciter, and I'm not getting any laser output. However, the tube glow
looks to be around the right sort of colour and I don't think there is air
leakage.

I've yet to try cleaning the optics, but I have checked the tube voltage
and current and I'm not sure if I have a problem here, as it's 2.7kV and
6mA, whereas Sam's Laser FAQ suggests it should be 3.7kV and 7mA. Also,
after being powered up for a while the power supply seems to drop out -- I
get a buzzing sound and the plasma goes intermittent, before stopping
altogether. When it's intermittent the tube voltage spikes up to around
4-5kV (at least that's what I see it jump to on my DMM). So, perhaps a PSU
component is on its way out?
How old is it? Most glass-case HeNe lasers slowly leak the Helium
away. You still get the pinkish glow from the He-Ne mixture, but there
isn't enough He left to develop the population inversion of excited states
to produce laser action. if the tube is over 20 years old, that is the most
likely cause.

Are the mirrors separate from the HeNe tube? If so, this is an "open
cavity" laser, and the mirrors need VERY careful alignment to get laser
action. If the mirrors are glued to the end of the tube, then no alignment
is possible.

This ia an SP-120, probably from the 1970s. And, yes, those old tubes
do leak due to the Epoxy seals. Modern tubes do not leak.

The dropout current is higher than 6 mA, so he should increase the current
to 7 mA and let it run. It may recover, at least partially.

If no one has attempted to adjust alignment or clean the optics, they
are probably OK. Else, all bets are off. It could be that someone found
it didn't lase and really mucked it up.

--
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