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Laser locking (control loops with two feedback paths.)

Discussion in 'Electronic Design' started by George Herold, Jan 17, 2013.

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  1. Laser locking (control loops with two feedback paths.)

    So I finally had a user ask about side locking our diode laser.
    (That’s where you lock the frequency to the side of an absorption
    Now, you can change the laser frequency in two ways. There’s a piezo
    stack that changes the angle of a diffraction grating. And you can
    change the laser current.
    The electronics is all set up to lock the laser with the piezo.
    Signal chain looks like,

    Photodiode->low pass (1 pole, tc = 100ms)->
    DC offset->gain->modulation input of piezo control.

    With some other bits of gain adjustment sprinkled in there. (The low
    pass is working as both integrator and gain (PI), you crank up the
    overall loop gain till it oscillates and then back off a bit.)
    This works fine, up to ~3kHz the oscillation frequency.

    Now I’ve heard tell of a trick where I break the error signal into a
    low frequency and high frequency part. And then send the high
    frequency part into the laser current modulation input.
    It seems I should pick off the error signal before the lowpass (P/I
    part of signal chain).(?)
    But I'm wondering how to deal with the 'break frequency'
    What frequency for the HP?
    And do I roll off the 'DC' part at the break frequency too?
    (1 pole each)
    Or can I leave the rest of the 'DC' signal chain the same if I pick
    the right frequency?

    George H.
  2. As long as the change is small they are both approximately linear.
    Modulating the current also changes the amplitude... but I actually
    take the difference of two photodiode signals to get the error
    signal... so to first order the amplitude change caused by current
    modulation shouldn't be that much of an issue.

    (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo
    and the current.)
    Well back in the dim past I did a back of the envelope calculation and
    figured this was the self resonant frequency of the piezo stack and
    the piece of Aluminum that it is pushing around. (Ratio of mass of
    aluminum vs mass of piezo to the one half power times the unloaded SRF
    of the piezo.)
    The Piezo is part number AE0203D04F made by Tokin and a rather long
    link to a data sheet,

    SRF ~ 261 kHz. I have no idea if the simple mass scaling is correct..
    but about the right number came out the far side of the calculation.
    The aluminum and grating are part of a flexure... I sorta wondered if
    the spring constant is different too.... But I'm not sure how I get
    the spring constant for either the piezo or the flexure, and the mass
    was easy to measure. (I did try and do some measuments of the flexure
    spring constant using the piezo as the sensor, very 'squishy'
    measuments IIRC)
    Hmm... OK that's a good question. I'll have to try it!
    But for long term DC drifts it's better to change the piezo (grating
    Oh for sure closed loop control with higher bandwidth. It'd be cool to
    be able to really bang on the table and have the thing stay locked!

    I think I've got a paper describing how someone else did this...(Carl
    Weiman and Leo Hollberg?) it might be in here, (another long link...
    to a RSI paper)

    But sometimes it's more fun to 'invent' your own method and then see
    what someone else did.

    George H.
  3. Hi Tim, Thanks for that! I logged in to report that I tried locking
    with just current modulation... one peice at a time so to speak. And
    that worked fine, I could bang a bit more on the table. But the
    current loop oscillates at ~20kHz when I crank up the gain. I don't
    understand that at all! The current modulation electronics has a
    bandwdith that's near 1 MHz, so the 20kHz might be for some 'real'
    physics reason. Modulating the current changes the wavlength through
    thermal effects. I have no idea what the thermal time of the laser
    diode is. Would 50us be a reasonable time? (retorical question no
    answer expected.)
    I'm going to try measuring the current to frequency modulation
    parameter as a function of frequency. Hey I might learn someting

    If I get around to closing the 'double loop', I may have more
    It's not clear to me where I should put the integrator.

    Having friday fun,

    George H.
  4. Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode!

    George H.
  5. Joerg

    Joerg Guest

    20kHz? That's like molasses. Why so low? And it should not cause it to
  6. Joerg

    Joerg Guest

    But where does all that phase margin fall through the cracks? Unless
    everything rolls off fast, of course. 20kHz BW for the photodiode sounds
    really low, unless it is one the size of a dinner plate.
  7. Ja Ja, The photodiode design is from 10+ years ago. I hadn't heard of
    Phil H. then, let alone read his book.

    I've got at least 3 projects now that can use a faster photodiode.

    Oh for the above you have to keep the intensity low in order to not
    saturate the atomic transistion. So a fairly large PD (0.25" diam),
    at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level signal) And
    only a 1 MHz opamp (opa124... it has a bad noise gain peak.)

    George H.
  8. Grin, well not quite dinner plate size. ~6-7mm diam.

    George H.
  9. But with two feed back paths should there be an integrator in each

    Not to worry first I need a faster PD.

    Oh all analog.
  10. Yeah, can you release the price for a PH200?
    (When I tried, your marketing people wanted my mothers maiden name
    part of my SS# :^)

    1 MHz at 1uA is that 1Meg Ohm gain?

    For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain.

    Something the about the same would work for this laser locking.

    Going from 10kHz to 1MHz is only a factor of 10^4 in Cap*GBW... :^)

    George H.
  11. That's how I've always done it.

    But now (I think, according to Phelan)

    | | |
    | | |
    | +----int----->|sum--------
    | |
    | |
    +-----------------Neg FB----->|

    Hmm Phelan's at work... I might have screwed that up.
    (Is there gain in JL's gain of one path?)

    George H.
  12. Phil Hobbs

    Phil Hobbs Guest

    What I usually do is to make both I and T loops more or less
    integrating, with the T loop dominating at

    very low freq, and make sure the I loop rails safely.
    Then just run them together. The T loop will keep the I loop
    centred, and nobody has any excess phase down at 0.1 Hz
    or wherever the low frequency cross is.


    Phil Hobbs

    (Via Google Groups, from the Carnival Miracle at Port Canaveral--
    for the beach bar. Having a daughter in the travel industry means we
    afford not to go. )
  13. Joerg

    Joerg Guest

    That should be a lot more zippy than 20kHz if connected to a somewhat
    reasonable TIA. Or did you give it a hefty dose of Ambien? :)
  14. Joerg

    Joerg Guest

    Now you'll have lots of folks banging on your door at Otis Street,
    wanting to see her :)
    Sometimes it's best not to have all the gain in the first (TIA) stage.
    Opamps are fairly cheap these days and the following ones don't have to
    be very fancy. Maybe something George could look at.
  15. Joerg

    Joerg Guest

    Tim Wescott wrote:


    Unfortunately not with many governors voted into office these days.
  16. Phil Hobbs

    Phil Hobbs Guest

    Yup. You don't want the first stage output
    to be more than about half a volt at lowest
    nominal photocurrent, because you stop gaining
    SNR and can start getting squirrelly behaviour.

    The one John's talking about uses fancy homemade
    optocouplers, with their photodiodes wired in series.
    The parlour trick is to avoid the low f_T of transistors
    running at nanoamp I_C levels. Even the late lamented
    BFG25A has an f_T of a few megahertz down there, and
    that's not even counting C-B cutoff.


    Phil Hobbs
    (Back on the ship)
  17. Joerg

    Joerg Guest

    500mV for the lowest signals? I usually try to keep it under 1Vpp for
    the highest.

    "Late" as in clanging of the last order bell? Please say it ain't so,

    Who let you back on and what's your excuse for being AWOL? :)
  18. Phil Hobbs

    Phil Hobbs Guest

    Not lowest-lowest, but lowest nominal. In a normal resistive-FB TIA,
    shot noise equals Johnson noise at 50 mV, so you lose 1dB if the
    output is 200 mV, 0.25 dB at 400 mV. Normally the other parts
    of the system are way more expensive than the TIA, so from a
    cost-benefit POV you want the TIA to be unobtrusive.
    Alas, 'tis true. You can still get BFT25As, but they're not as


    Phil Hobbs
    I had a 6-hour pass, sir. Honest.
  19. OK not that bad, I can't recall the details.

    We're not supposed to keep pricing a secret; people will find out
    1Mhz at 10M Ohm that is impressive! I only need something 100 times
    worse, so there's some chance I can make it happen.

    George H.
  20. Hi Tim, thanks again for the sage advice.

    So at the moment the gain vs freq looks like,

    L ^ |
    O |--
    G | \
    | \
    g | \
    a | \
    i | \ ~1kHz
    n | \|
    +--------\---------> gain=1
    LOG frequency

    So If I make a break point (LF to piezo and HF to current)
    at say 100 Hz do I still want the high frequency part of the gain to
    have a one pole roll-off... or maybe the gain can be flat (for a
    while.... I gotta roll off the HF eventually.)

    Not to worry, I’ll play around and maybe learn something.
    As far as the photodiode design goes, I don’t know of any ‘new’
    tricks*. But the old rule of thumb (for TIA’s) is that the max.
    frequency is the geometric mean of the RC ‘frequency’ and the opamp
    GBW. For my pedestrian circuit, with unbiased photodiode C=~700pF,
    R=1Meg, GBW =1.5MHz,(opa124) for which I get an f(rc)~240Hz and
    sqrt(f(rc)*GBW)~19kHz.. (I thought the opa124 had a 1MHz GBW, but the
    number was then a bit off.)

    George H.

    *well there are ways to reduce the capacitance.
    reverse bias the PD, bootstrapping, and then Phil H.'s cascode between
    the PD and TIA. I've never tried the cascode.
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