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HeNe laser frequency noise

Discussion in 'Electronic Repair' started by Anders Wallin, Jan 15, 2013.

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  1. Hi all,
    I am measuring the beat-signal between a tunable HeNe laser and a stabilized one, and seeing something like this:


    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
     
  2. 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!

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  3. 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.
     
  4. 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.
     
  5. You might try looking at the beat between the 5501A and Marktech to further
    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:
    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)



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

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  7. 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
     
  8. 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.

    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.
     
  9. 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.

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
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