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Gain/loss of a laser power

  1. Feb 7, 2007 #1
    1. The problem statement, all variables and given/known data
    A laser produces 15.6 dBm of power. The light is propogated through a 10dB neutral density filter that attenuates the signal so that it can be detected by a photodiode without saturation. The optical signal is steppeddown by 15dB when captured by teh photodiode and coverted to an electric signal. This signal is later amplified by 20.2 dB by an inverting amplifier. What is the output signal power from teh amplifier in mW?

    2. Relevant equations
    [tex] G [dB] = 10 \log_{10} \left(\frac{P_{out}}{P_{in}}\right) [/tex]
    where P is the power

    [tex] G [dBm] = 10 \log_{10} (P[mW]) [/tex]

    3. The attempt at a solution
    15.6 is in dBm while the others are in dB. How do we convert from dBm to dB??

    Since G represents the gain or loss
    then the total gain by the signal is -10 - 15 + 20.2 = -4.8

    Thus we would use hte first equation
    with G = -4.8 , Pin = 15.6 dBm and calculate Pout

    would that be correct??
  2. jcsd
  3. Feb 8, 2007 #2


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    Homework Helper

    The power of a laser is usually stated in units of watts, or more likely milliwatts. Even 15.6 mW will be a reasonably powerful laser.

    See bottom of this webpage:

    http://www.lumalaser.com/laser-power_recommendations.htm" [Broken]
    Last edited by a moderator: May 2, 2017
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