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Pyroelectric energy detector modeling?

Discussion in 'Electronic Design' started by Chris Carlen, Jan 21, 2004.

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  1. Chris Carlen

    Chris Carlen Guest

    Greetings:

    Does anyone know how to model a pyroelectric energy detector? I mean
    the type that is used to detect the pulse energy of lasers, where a
    short deposit of energy on the surface of a pyroelectric crystal disc
    creates a temperature change which produces an output signal.

    Typically, there is a parallel RC network connected to the crystal, from
    which the signal is taken:


    +--------+--------+------>
    | | |
    | | |
    ----- \ -----
    PPP / R ----- C
    ----- \ |
    | | |
    | | |
    +--------+--------+------>

    where "PPP" is the pyroelectric crystal sandwiched between two thin
    metal plates, with one plate bonded to the grounded heatsink substrate,
    and the other plate exposed, but coated with black paint.

    I'm hoping I don't have to delve into thermal modeling here.

    Are any of these guesses as to the nature of the device's energy to
    electrical signal conversion process correct:

    1. The device can be represented as a series RC network, where the
    charge on the capacitance of the device is proportional to the energy
    delivered? (simple)

    2. The device can be represented as a series RC network, with rather
    low and probably insignificant R, but the capacitance has a charge that
    is related to the thermal gradient across the crystal? (not simple,
    since it involves modeling the spatial heat distribution on one axis in
    time, with the resulting charge being the integral of that).

    3. Something else?


    What I am ultimately trying to do is slow down a detector. They are
    typically specified with maximum pulse durations, which are about one
    tenth or less of the RC time constant of the extra network shown. This
    ensures that the peak voltage of the response will be proportional to
    the energy. However, if the pulse width of the energy source is too
    slow, then the relationship between energy and peak voltage is no longer
    linear.

    We want to adapt a sensor to slower pulse widths, by making the time
    constant of the unit longer, so longer pulses will appear as true
    impulses relative to the new time constant. But we wish to understand
    what is going on in more detail, since we may only be able to compute
    the resulting change in calibration. We don't have a variable pulse
    width energy source handy to recalibrate empirically.


    Thanks for comments.

    Good day!



    --
    ____________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
     
  2. Consider replacing the resistor with a cmos analog switch. The
    capacitor will integrate the charge out of the detector, you take a
    voltage reading, and then discharge the cap right before the next
    test.
     
  3. I found an interesting site yesterday with stuff on pvdf sensors:
    http://www.ehag.ch/HTML-Files/Components/film_sensors.htm

    But if the sensor is LiNbO, things can be more interesting =)
    http://arxiv.org/ftp/physics/papers/0209/0209079.pdf
    http://arxiv.org/ftp/physics/papers/0309/0309049.pdf
    http://www.amptek.com/coolx.html

    I mean
    It's probably much worse then mere thermal modeling =)
    I think heat transports charge.
    The first article on x-ray generation mentions that charge can persist
    for 30 hours, in a high vacuum, assuming no electons fly away.
    Tell me what you find, please =)

    --
    Scott

    **********************************

    DIY Piezo-Gyro, PCB Drill Bot & More Soon!

    http://home.comcast.net/~scottxs/

    **********************************
     
  4. Chris Carlen

    Chris Carlen Guest


    That's a dandy idea! What remains to be seen is whether the total
    charge remains linear with absorbed energy at longer pulse widths. I
    suspect that at the pulse width approaches the thermal time constant of
    about 400ms, then the thing stops responding. I hope it will still be
    linear to within 1-2% with 4ms pulses. I will test it with an AOM
    modulated argon laser.

    Good day!




    --
    ____________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
     
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