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Homework Help: Sensor developement for laser power meter

  1. Jul 7, 2014 #1


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    Not really sure if this should go here or in technology, but it's a physics class sooo...

    we had to build a device to measure the power of a laser with an output of around 10mW or less.

    I went with a differential gain amplifier using 2 thermistors, one of which is kept at ambient temperature and the other has the laser shone on it.

    its built, and it works. The only problem im having is that due to the fact that I'm measuring small lasers, it's extremely sensitive to things such as air currents in the room.

    Mu question is this, is there a way for me to approximate the number of photons hitting my thermistor and a way to differentiate the ambient light from my laser ? I have a few ideas, most of which involve covering my thermistor, but I want to be able to account for the loss of photons and thus energy from my covering.

  2. jcsd
  3. Jul 7, 2014 #2
    Can you shield the therm resistor from the ambient light? For a project last year we needed to use photo sensors and had huge issues with ambient light. What worked was to put the sensor at the end of a sleeve which greatly increased their sensitivity to direction, something similar might work.
    Or what about having an photo resistor measuring ambient light and using that as a correction factor?
  4. Jul 7, 2014 #3


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    Well the second thermistor should account for the ambient light, as it's just as exposed as the other. I guess I could try rigging something out of say construction paper or something, which would eliminate the ambient light, but then I would have to do the same thing to my control. Mostly what I'm getting is air currents and if you get too close it'll pick up your body heat as well. I kinda feel like putting a tube around it would just funnel the currents in for most cases.

    I suppose I don't really have to differentiate from the ambient light and laser light, as the opaquity (?) of whatever I throw in front of it should eliminate a fairly consistant ratio of energy from both. Maybe not though. I would probably need to use something with consistent absorption rates across all visible frequencies.

    Another question, and this is more conceptual:
    Do lower frequencies (since heat is mostly IR, no?) affect my thermistor more greatly even when they have the same/less power (or energy, however you want to look at it)?
  5. Jul 7, 2014 #4


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    What about putting the laser and sensors in a box to shield it from air currents (and ambient light, if that matters). Leave the rest of the electronics outside the box, so that isn't a heat source.

    I would think 10mW won't create much convection in the air, compared with random large scale air movements in the lab caused by the aircon, people moving about, etc.
  6. Jul 7, 2014 #5


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    Well I have it in a box, but I have to be able to hit my thermistor with the laser, so those are sticking out. I'm trying to think of a way to shield those as much as possible while still having access to them.

    And you are correct Aleph, the laser itself isn't the issue, it's the environment. If I was dealing with higher powered lasers I wouldn't have to have it as sensitive and the currents and whatnot would be almost negligible. Right now, if I remember correctly, I have a gain of ~1000.
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