HeNe laser frequency noise

Probably.

The 5501A and 7900 both use an analog feedback loop so they should be quite clean at least over the short term. However, the 5501A HeNe laser power supply is generally crap with a lot of ripple. The MarkTech is a common brick. Neither of these really should result in anything like you're sseing since the resulting ripple is at 10s of kHz.

All the later HP/Agilent lasers (e.g., 5501B, 5517A/B/C/D etc.) would have this sort of dance built in as they compare the H and V polarization on a discrete time scale of a second or so. So, their optical frequency often drifts back and forth by a small amount. But the 5501A is well behaved.

This may be an early version. It's a similar tube and this may be before they replaced the nice 4-bar resonator with the cheezy case REO uses now. :) That would be quite old, or someone's custom creation.

You might try looking at the beat between the 5501A and Marktech to further confirm they are not at fault, or beat one of them with another stabilized laser like an SP-117/A. You can manual tune the frequency of the 5501A with the mode balance (bottom) pot on the small PCB on the left side of the laser. With some luck, you'll be able to match up their optical frequencies close enough for bandwidth of your photodiode.

With a stable power supply, the tunable should not do anything erratic like that.

However, don't overlook the mundane - it doesn't take much in the way of mechanical vibration to disturb the cavity enough to cause a noticeable frequency shift. A 1 nm change in cavity length is about 1 part in 316 of the FSR of the laser cavity - 300-400 MHz. The fan of the spectrum analyzer or air currents could even be at fault!

zed one, and seeing something like this:

laser cavity) which I can cope with, but I am not happy with the +/-5 MHz fast jumps of the beat-note.

type PSU powered off +12V there is very large intensity modulation at 20-22 kHz. I am now using a better PSU which results in very small intensity noi se.

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. this one:

c tuning of the laser.

d with the piezo grounded - but I still see the same jumpy beat-signal.

Hmm, I know very little about HeNe's. But years ago I beat two diode lasers together. I like Sam's idea of beating the two stabilized lasers as a test of the setup. I was going to suggest some crud on the piezo drive... but re-reading I see you turned that off. What's the time span for the spectrum analyzer? Have you looked at the signals directly with a 'scope? Maybe there's some clue in the time domain that is harder to see with the spectrum anlyzer. (I'm not really sure whta you might see with the 'scope.. just throwing out ideas.)

George H.

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On the mundane front I was thinking about doppler shifts from moving mirror mounts. But putting in some guesstimate numbers, I get kHz type shifts and not MHz. (motion of a few microns at maybe 100Hz?)

George H.

Yes definitely. I will try a beat-measurement between the MarkTech7900 and the HP5501A to check that they are both stable - or not.

I've seen the same frequency jumps with two different power supplies - I might still try another power supply - or I might have a possibility to try another identical 5-color tunable laser to see if it is any better.

I installed a plastic tube that encloses the beam between the back brewster window and the Littrow prism:

However this has little or no effect as I still see a jumpy beat-signal: (it take a few hours of warm-up before thermal-drift slows to this level)

Anders

HP5501A to check that they are both stable - or not.

still try another power supply - or I might have a possibility to try another identical 5-color tunable laser to see if it is any better.

window and the Littrow prism:

Are both lasers on an optical table but with the spectrum analyzer on a separate shelf or cart vibration isolated from the lasers?

Yes and yes! :) I am starting to think the frequency jumping I am seeing might be because of current ripple in the PSU. Looking at Sam's laser FAQ there are at least two options. Either the ripple reducer over here:

would a typical "brick" type HeNe PSU work with something like this on the output?:

Anders

It would have been neat to have a time-series plot of this. I have observed mode hopping before, and it has a characteristic set of repeating peaks. The laser basically hops between two VERY closely spaced modes as the laser tube expands thermally. It usually settles down after a couple hours when the laser reaches thermal equilibrium.

Jon

of current ripple in the PSU.

ple reducer over here:

Hmm I was going to ask if anyone has used a capacitor multiplier at high voltage.

circtuits here: (many 450V caps in series, with resistors in parallel over each cap)

e output?:

The resistors in parallel with the caps aren't doing any low pass filtering. I think they are there to help balance the charge on the caps. (HV is not my forte.) R2 and R19 with the caps do form low pass filters.

Sometimes if you can't figure out how to make something better, and you're not sure what's causing the problem. You can learn something by trying to make it worse.

George H.

current ripple in the PSU.

reducer over here:

circtuits here: (many 450V caps in series, with resistors in parallel over each cap)

output?:

I'm not quite sure what the circuit is supposed to be, but keep in mind that the voltage across the tube may exceed 10 kV when starting.

The ripple reducer is one option. An old linear power supply may be another. Something like an SP-247, and you can add additional filtering inside that. There are many others.