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Questions using photodiode circuit to measure laser intensity

  1. Jul 25, 2013 #1
    Hello all, I'm a new Mechanical engineering graduate student who needs help using a photodiode. I'd like to rig up some kind of circuit that will let me measure the intensity of laser light. This is kind of a new area for me haven't really done anything like this before so forgive me if I come off as uninformed. Anyway I guess my basic questions are:

    1) In addition to the photodiode what other components would I need?
    2) Can you recommend any resources that would aid me?
    3) Is there perhaps a simpler alternative than a photodiode circuit?

    Thanks all for the help in advance!
  2. jcsd
  3. Jul 25, 2013 #2


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    Staff: Mentor

    What wavelength and power level(s) are involved? Is this a DC measurement, or do you need to follow changes in the intensity level? If there are changes, what is the bandwidth of those changes? Are you splitting off some of the main beam to monitor intensity during an experiment, or are you meaning to make this portable, measure the main beam, and then remove the measurement device to use the laser?

    You will probably use an opamp based photodiode circuit. The photodiode is reverse biased between ground and the V- power supply, and the opamp is powered by split V+ and V- supplies. The opamp is configured as a (photo)current to voltage converter...
  4. Jul 25, 2013 #3
    Berkeman, thanks for the response! I'll do my best to answer your questions.

    Laser wavelength is 532 nm, power levels 10-100mW.
    I'm going to have to follow changes in the intensity level.
    I'm not sure how to address the bandwidth question. Could you rephrase it perhaps?
    I won't be splitting the beam and it doesn't need to be portable.

    Also I'll describe a little better of what I'm going to do, probably should have done that in the OP. What I'm doing is shining a laser through a 2-phase solid-liquid concentrated suspension. I'm trying to match the refractive index of the two phases of the suspension which renders it transparent. In order to see how well I'm doing I'll measure the intensity of the laser beam passing through it. If the laser intensity is really low then well I know that the 2-phases don't match well. As I change my solution and get the indices to match more closely the light intensity should approach the baseline (laser shining through solution with no solid phase).
  5. Jul 25, 2013 #4


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    Staff: Mentor

    By bandwidth, I just meant how quickly the measured value can change. It sounds like the changes in your setup will be slow, on the order of a second or so? That's easier to handle than if it could change at 10kHz or something.

    Here is the basic I-to-V converter opamp circuit, although I usually prefer to connect the cathode of the diode to the V- rail instead of to ground. The large negative bias improves bandwidth, and may help the linearity of the photodiode (check its datasheet):


    Attached Files:

  6. Jul 25, 2013 #5
    Are the suspended solid particles so small that when the liquid does not have the correct refractive index, the light is diffracted? If so, you might try detecting a minimum in the diffracted light off axis.
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