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fringe locking

Discussion in 'Electronic Design' started by Níkola Heímpel, Sep 6, 2004.

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  1. I have a Fabry-Perot interferometer and would like to lock the
    interferometer cavity to one point of the transmission fringe. I
    searched for basic informations on how to design the circuit for the
    piezo voltage, but so far I haven't found something like "fringe locking
    for dummies"...
    I found a few descriptions (not very detailed though) of methods using
    two photodiodes, one on a bright and one on a dark fringe, then
    basically amplifying the difference and feeding it into the piezo
    actuator. But is there also a way to do this with one diode? Or what
    exactly would I have to look at if I wanted to design a circuit for two
    diodes? I have read the basics about proportional, integral and
    derivative control, guessing I would need at least p. and i, but no idea
    how to put this in electronics.
    If you would know a webpage/book/paper or something to read on this
    topic, that would also help me a lot.
    Thanks for your help,
  2. Tim Wescott

    Tim Wescott Guest

    One diode would be subject to drift; two will drift together, and their
    difference will be much less drifty. I would be inclined to set the two
    diodes so they straddle a peak and lock the thing so the diode outputs
    are equal.

    The circuit below shows a basic PI compensator using an opamp. The
    proportional gain = R2/R1, the integral gain = 1/(R2*C), and you want to
    set R3=R1 to minimize your input offset voltage (unless you have a FET
    input opamp). If you need to know this because of a brain fart then
    proceed. If you need to know this because of a lack of op-amp theory
    then get a good book, read it, and proceed.

    | |
    === |
    GND | |\
    ___ | >--------o---o Vout
    Vin o-|___|-----o----|-/ |
    | |/ |
    R1 | |
    | |
    | ___ || |

    R2 C

    created by Andy´s ASCII-Circuit v1.24.140803 Beta
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