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Laser connected to photodiode to measure potential?

  1. Jan 30, 2013 #1
    1. The problem statement, all variables and given/known data

    I am doing a lab and need to understand how laser light from a HeNe laser goes into a Si photodiode connected to a lock in amplifier, which I believe is simply a device for measuring potential, works.

    2. Relevant equations



    3. The attempt at a solution

    This lab was extremely simple, we shot a laser through a beaker of water with food die in it, the laser went through the beaker into an Si photodiode, which was connected to an instrument to measure voltage. What I do not understand is how the light produces a potential difference in the Si photodiode.

    If anybody can provide an explanation or refer me to relevant literature, it would be of great help to me.

    Thanks
     
  2. jcsd
  3. Jan 30, 2013 #2

    Simon Bridge

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    You don't know how a photodiode works?
    Crudely - it converts electrical energy in the light to electrical energy in the circuit.
     
  4. Jan 30, 2013 #3
    What I am asking is, how does the photodiode take the incident light, which is connected to a voltmeter, and measure the potential difference of the light.

    I don't understand how the light creates a potential difference and how the photodiode, which is connected to the voltmeter, (we're using a lock in amplifier), measures it.
     
  5. Jan 30, 2013 #4

    Simon Bridge

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    The voltmeter does not measure the potential difference of the light - have a look at the photodiode circuits in the link I gave you.

    The voltage is across part of the ciruit that includes a photodiode - what it is measureing exactly depends on circuit. The designer of the circuit arranged so that the voltage on the voltmeter was proportional to the intensity of light that hits the photodiode.

    The diode itself works using a form of the photoelectric effect. Incoming photons knock electrons off the material of the diode, the freed electrons travel to one end creating a potential difference. The more photons that hit the bigger the difference.

    It's kinda the opposite of how an LED works.
     
  6. Jan 31, 2013 #5
    Alright yeah, thanks mate, the part about the diode being like a photoelectric effect kind of makes sense to me.
     
  7. Jan 31, 2013 #6

    Simon Bridge

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    No worries - it is difficult to know where to pitch the answers.
    Wikipedia has links to more detailed descriptions of how it works if you need it.
     
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