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fringe-locker description

Discussion in 'Electronic Design' started by [email protected], Aug 30, 2006.

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  1. Guest

    Since the other thread ("replacement for 741') is dragging on, I wanted
    to post a description of a fringe-locker circuit. But first, let me
    say that this forum is incredible. I'm not used to such an active
    forum with so many knowledgeable participants. Thank you!

    Years ago, I wanted to try fringe-locking. Commercial fringe-lockers
    are in the 2k range, so I wanted to build my own. Even though I did a
    lot of reading, there wasn't a single schematic (or even a patent) for
    a fringe-locker. With the help of a friend and some guesswork, I
    managed to build a working circuit. I was really surprised and happy,
    the cost in parts was probably under $100. This circuit is shown on
    the holography forum, and as far as I know, it's the only circuit
    diagram available for a fringe-locker, on-line or off.

    When a certain kind of hologram is made (called a split-beam hologram)
    laser light in split into two beams. These beams are eventually
    re-combined at the hologram to record an interference pattern in the
    holographic emulsion. The path lengths of each beam need to remain
    exactly the same for the length of the exposure, even a variation of
    one micron will degrade the hologram. (Holographers do everything they
    can to use rigid mounts, vibration isolation tables, draft exclusion,
    temperature control, etc.) to minimize this, and these are known as
    "passive" methods to keep the path lengths exactly the same.

    But sometimes for long exposures, "active" means are needed, and this
    is where a fringe-locker comes in. Exactly how the fringe-locker is
    used is difficult to explain, but a "secondary" interferometer (with
    fringes) is included in the setup which "mimics" the actual fringes
    which form in the holographic emulsion. If this secondary
    interferometer is locked in place, then the hologram fringes will also
    be locked, to a certain extent at least.

    The photodiodes shown above are placed in the fringes of the secondary
    interferometer. These fringes are large, alternating bright and dark,
    with a width of maybe 1/4 inch for each fringe. The photodiodes
    straddle a bright fringe, or a number of fringes. The differential
    amplifier circuit drives a transducer (small speaker with a mirror on
    it) to move back and forth over small distances. This mirror is placed
    in one the beam-paths. After turning the locker on, the potentiometers
    are "tuned" and the system will lock.

    The circuit sends a singal to the transducer which increases (or
    decreases) the path of one beam, so that it matches the other exactly.
    It actually works. But sometimes my locker will "lose" its lock, and
    this may be because the speaker cannot move far enough to make the
    necessary corrections. I think commercial fringe-lockers use a piezo
    stack for the transducer, but they typically require 100 to 150 volts,
    and I don't know how to modify the above circuit for this. I have only
    had a brief opportunity to test my homemade locker, but want to build a
    couple more. I thought the 741 might be too ancient, that's why I
    asked about it.
  2. Guest

    Ooops, I forgot to mention something. When "unlocked" those fringes
    are normally in a state of motion. If the interferometer (which
    generates the fringes) is set up on the kitchen table, the fringes will
    be in a rather wild state of motion (unless the kitchen has a very
    solid table resting on a concrete-slab floor). Even if it's set up on
    a commercial vibration-isolated optics table, there will still be some
    fringe drift over the long term, due to thermal expansion and
    contraction of the steel table-top (not Invar, however, but that would
    be incredibly expensive). Fringe-locking is best at stopping slow
    drift, that is to say about 1 fringe width over 5 minutes. But it will
    also stop gross vibration effects, such as from a train passing nearby.

  3. colin

    colin Guest

    I gues the dominant pole here is the mass of the speaker,
    but maybe you should roll of the gain of the op amp with a capacitor
    feedback to reject any mains hum from mains lights etc.
    You might want to try an op amp with more output current if it isnt able to
    drive the speaker very far.
    also reverse biasing the optos might be a bit better,
    the best op amp to use would probably be a ultra low bias current bipolar
    type such
    as the op97 wich is an improved op07, nice although it doesnt seem to
    specify the output current
    this is probably over the top though.

    Colin =^.^=
  4. Guest

  5. If you use a bass speaker you get about +/- 10 mm of travel.

    You might want to add a buffer stage or use a higher current amplifier
    for fast response.
    There are op-amps that will deliver large currents.

    It could be that the speaker overshoots the target - this is maybe
    where the noise performance of the 741 comes in.
  6. Guest

    Thank you for your helpful advice. Wow, 10mm is a huge amount of
    travel, I didn't know bass speakers could handle that. I will look
    into op-amps that can deliver larger currents.
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