# I Understanding laser power terminology

Tags:
1. Mar 19, 2017

### pyroartist

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. Mar 19, 2017

### davenn

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

3. Mar 19, 2017

### Staff: Mentor

I suspect the difference is in the LI vs the DAR.

4. Mar 19, 2017

### Gordianus

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.

5. Mar 20, 2017

### pyroartist

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
6. Mar 20, 2017

### BvU

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.

7. Mar 20, 2017

### BvU

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

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.

8. Mar 20, 2017

### pyroartist

BvU, so how many Joules is that? I think it is the same answer: 10 uJ. Is that correct?

9. Mar 20, 2017

### BvU

Per pulse you get 10 $\mu$J, yes. And 10 $\mu$J per second is 10 $\mu$W

10. Mar 20, 2017

### pyroartist

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.
Thanks..