HeNe laser frequency noise

[Anders Wallin]

Hi all,
I am measuring the beat-signal between a tunable HeNe laser and a stabilized one, and seeing something like this:
http://www.youtube.com/watch?v=3arRbVKr9jQ

There is an overall slow drift (probably thermal expansion of the tunable laser cavity) which I can cope with, but I am not happy with the +/-5 MHz fast jumps of the beat-note.
I have tried this with two reference lasers, a HP5501A and a MarkTech 7900 and the results are similar - so I believe the tunable laser it at fault, not the reference laser.

Any ideas on what could cause this kind of rapid frequency shifting?
I have tried two powersupplies for the tunable laser. With a small brick-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 noise.

A few photos of the tunable laser:
https://www.dropbox.com/sh/q4xruaiwlbhru1o/HtTelFz1j_
I believe this is an early version of the 5-color HeNe still on sale, e.g. this one:
http://www.edmundoptics.com/lasers/gas-lasers/5-color-tunable-helium-neon-laser/2550
The Littrow prism has been mounted on a piezo tube which allows electronic tuning of the laser.
I have tried looking at the beat-note with the piezo amplifier on/off, and with the piezo grounded - but I still see the same jumpy beat-signal.

Thanks in advance for any ideas & suggestions!

Anders

[Samuel M. Goldwasser]

Anders Wallin <anders.e...@gmail.com> writes:

> I am measuring the beat-signal between a tunable HeNe laser and a
> stabilized one, and seeing something like this:
> http://www.youtube.com/watch?v=3arRbVKr9jQ
>
> There is an overall slow drift (probably thermal expansion of the tunable
> laser cavity) which I can cope with, but I am not happy with the +/-5 MHz
> fast jumps of the beat-note.
> I have tried this with two reference lasers, a HP5501A and a MarkTech 7900
> and the results are similar - so I believe the tunable laser it at fault,
> not the reference laser.

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.

> Any ideas on what could cause this kind of rapid frequency shifting?
> I have tried two powersupplies for the tunable laser. With a small brick-
> 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
> noise.
>
> A few photos of the tunable laser:

> https://www.dropbox.com/sh/q4xruaiwlbhru1o/HtTelFz1j_
> I believe this is an early version of the 5-color HeNe still on sale, e.g.
> this one:

> http://www.edmundoptics.com/lasers/gas-lasers/5-color-tunable-helium-neon-
> laser/2550

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.

> The Littrow prism has been mounted on a piezo tube which allows electronic
> tuning of the laser.

> I have tried looking at the beat-note with the piezo amplifier on/off, and
> with the piezo grounded - but I still see the same jumpy beat-signal.

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!

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[George Herold]

On Jan 15, 12:49 pm, Anders Wallin <anders.e.e.wal...@gmail.com>
wrote:

> Hi all,
> I am measuring the beat-signal between a tunable HeNe laser and a stabilized one, and seeing something like this:http://www.youtube.com/watch?v=3arRbVKr9jQ
>
> There is an overall slow drift (probably thermal expansion of the tunable laser cavity) which I can cope with, but I am not happy with the +/-5 MHz fast jumps of the beat-note.
> I have tried this with two reference lasers, a HP5501A and a MarkTech 7900 and the results are similar - so I believe the tunable laser it at fault, not the reference laser.
>
> Any ideas on what could cause this kind of rapid frequency shifting?
> I have tried two powersupplies for the tunable laser. With a small brick-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 noise.
>
> A few photos of the tunable laser:https://www.dropbox.com/sh/q4xruaiwlbhru1o/HtTelFz1j_

> I believe this is an early version of the 5-color HeNe still on sale, e.g. this one:http://www.edmundoptics.com/lasers/gas-lasers/5-color-tunable-helium-...

> The Littrow prism has been mounted on a piezo tube which allows electronic tuning of the laser.
> I have tried looking at the beat-note with the piezo amplifier on/off, and with the piezo grounded - but I still see the same jumpy beat-signal.
>
> Thanks in advance for any ideas & suggestions!
>
> Anders

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.

[George Herold]

On Jan 15, 1:47 pm, s...@repairfaq.org (Samuel M. Goldwasser) wrote:

> >http://www.edmundoptics.com/lasers/gas-lasers/5-color-tunable-helium-...

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.

[Anders Wallin]

> 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.

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

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

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.

> 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!

I installed a plastic tube that encloses the beam between the back brewster window and the Littrow prism:
https://www.dropbox.com/s/kg4dzozre5bh4sy/before_after.jpg

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)
http://www.youtube.com/watch?v=uvG8z9-PkaM


Anders

[Samuel M. Goldwasser]

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

[Anders Wallin]

>
> 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:
http://repairfaq.ece.drexel.edu/sam/smhpsrr1.gif
or perhaps just more filtering capacitors on the output like many of the circtuits here: (many 450V caps in series, with resistors in parallel over each cap)
http://repairfaq.ece.drexel.edu/sam/laserhsc.htm

would a typical "brick" type HeNe PSU work with something like this on the output?:
https://www.dropbox.com/s/epbwgox0tm9lrzf/hene_hv_filter2.png


Anders

[Jon Elson]

Anders Wallin wrote:

> http://www.youtube.com/watch?v=3arRbVKr9jQ

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

[George Herold]

On Jan 17, 1:51 pm, Anders Wallin <anders.e.e.wal...@gmail.com> wrote:
> > 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:http://repairfaq.ece.drexel.edu/sam/smhpsrr1.gif

Hmm I was going to ask if anyone has used a capacitor multiplier at
high voltage.
http://en.wikipedia.org/wiki/Capacitance_multiplier
I have no idea if it can survive the turn-on transient of a HeNe.
Or if it would have to be 'bypassed' during the turn on stage.


> or perhaps just more filtering capacitors on the output like many of the circtuits here: (many 450V caps in series, with resistors in parallel over each cap)http://repairfaq.ece.drexel.edu/sam/laserhsc.htm

>
> would a typical "brick" type HeNe PSU work with something like this on the output?:https://www.dropbox.com/s/epbwgox0tm9lrzf/hene_hv_filter2.png

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.
>
> Anders

[Samuel M. Goldwasser]

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.