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I Understanding laser power terminology

  1. Mar 19, 2017 #1
    I see that many LIDAR systems use power in the millijoule range in atmospheric research. A typical number is 25 millijoules. 25 millijoules is equal to 25 milliwatt/seconds. This means 25 milliwatts for one second. This seems like a tiny amount of power, yet a 25 millijoule laser costs many thousands of dollars. This does not make sense as you can buy a 25 milliwatt laser diode that will easily run for one second and it costs just a few dollars.
    What is the difference?
  2. jcsd
  3. Mar 19, 2017 #2


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    do you have a link or two to that so we can see what you are referring to and to make sure you are interpreting it correctly :smile:
  4. Mar 19, 2017 #3


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    I suspect the difference is in the LI vs the DAR.
  5. Mar 19, 2017 #4
    LIDARS use pulsed lasers. The shorter the pulse, the better the spatial resolution. Since a typical pulse may last a few nanoseconds the peak power is quite large. For example, if the 25 mJ are delivered in a 2 ns pulse, the average power is about 12.5 MW. The 25 mJ aren't delivered in one second but in a very short time, that's why a simple laser diode isn't up to the task.
  6. Mar 20, 2017 #5
    Thank you Gordianus for a serious answer! How did you compute 12.5 megaWatts from 25 millijoules? What is the formula to convert joules to watts.
    For example, what is the power of a 100 Watt laser pulsed for 100 nanoseconds? 10 mJ. ?
    Last edited: Mar 20, 2017
  7. Mar 20, 2017 #6


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    For the frst sentence you mean 25 millijoules is equal to 25 milliwatt second.
    The formula to convert joules to watts is 1 Watt = 1 Joule / second

    So: a 25 mJ, 1 nanosecond wide pulse each second represents a peak power of 25 MW (big M means Mega, 1000000 -- small m means milli, 1/1000) but an average power of 25 mW.
  8. Mar 20, 2017 #7


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    Insufficient information:
    1. if it fires 1000 times per second, you would get 1000 times a much average power as when it fires once per second.
    2. you don't say whether it is average 100 W or peak 100 W
    If you call a 100 W laser a 100 W laser, its power is 100 W, isn't it :smile:

    Now, suppose it's 100 W peak and it fires one 100 ns wide pulse once per second. Average power is then
    100 W * 100 ns = ##10^2 * 100 \times 10^{-9} = 10^{-5} ## W = 0.01 mW.
  9. Mar 20, 2017 #8
    BvU, so how many Joules is that? I think it is the same answer: 10 uJ. Is that correct?
  10. Mar 20, 2017 #9


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    Per pulse you get 10 ##\mu##J, yes. And 10 ##\mu##J per second is 10 ##\mu##W
  11. Mar 20, 2017 #10
    OK, got in now. Watts X pulse length = Joules If more than one pulse per second then multiply Joules X number of pulses per second to get total energy.
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