Laser Power vs Energy: Solving the Mystery

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Discussion Overview

The discussion revolves around understanding the relationship between laser power ratings and energy output, specifically focusing on a laser that can deliver a single 3mJ, 10nS pulse. Participants explore the implications of power ratings, pulse duration, and repetition rates in the context of laser specifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates that delivering 3mJ in 10nS results in a power of 300kW, questioning the feasibility of such a high power rating for practical lasers.
  • Another participant confirms the calculation but raises the question of whether the power ratings refer to power consumed or delivered.
  • Discussion includes the distinction between average power and peak power, with a focus on how pulse repetition rates affect energy calculations.
  • Participants mention that pulse lasers typically specify energy per pulse and repetition rate rather than average power, complicating the interpretation of specifications.
  • There is a suggestion that for a laser rated at 100W with a 10nS pulse, the energy per pulse could be very low if the repetition rate is not known.
  • One participant expresses confusion regarding the ambiguity in power ratings and pulse specifications, particularly when comparing continuous wave (CW) and pulsed power ratings.
  • Another participant notes the importance of knowing pulse width and repetition rate to accurately determine energy output.
  • A later reply suggests converting mJ to J to adjust the initial calculation from 300kW to 300W.

Areas of Agreement / Disagreement

Participants generally agree on the need for clarity regarding the relationship between pulse energy, power ratings, and repetition rates. However, there remains uncertainty and differing interpretations about how to apply these concepts to specific laser specifications.

Contextual Notes

Limitations include the lack of information on pulse repetition rates in some specifications, which affects the ability to determine energy per pulse accurately. The discussion also highlights the dependence on definitions of power ratings and the potential for confusion between average and peak power.

Who May Find This Useful

This discussion may be useful for individuals involved in laser technology, particularly those working with pulsed lasers in applications such as material processing or scientific research, as well as those needing to interpret laser specifications for practical use.

DanSandberg
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I feel like a moron for not knowing the answer to this.

I'm looking for a laser that can deliver a single 3mJ, 10nS pulse. Most of the laser specs I've found report the laser power in watts rather than the energy in mJ. So I did the following calculation:

3mJ/10nS=3x105J/S=300kW

But there is no possible way I need a 300kW laser! The lasers that typically deliver these sort of 3mJ, 10nS pulses are rated as 10W, 30W, or maybe 200W. Definitely not over 1000W! So where has my calculation gone astray?
 
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Your calculation is correct. 3 mJ in 10 nS is indeed a power of 300 kW.

Are the power ratings for the lasers ratings for power consumed or power delivered? If you have a 300 W power supply and run it at a duty cycle of less than 1/1000 then you could potentially get a pulse like what you describe (neglecting losses).
 
There are separate ratings for power delivered and power consumed. I'm looking at the power delivered. Hmm... when I get to the bottom of this I'll post again.

I know, for example, that we have a 10Hz laser that produces 3mJ per 10nS pulse. So, over a 1 second period a total of 30mJ has been delivered, thus 30mW of power. However, if you look at each pulse by itself, for those 10nS the laser sustained a power rating of 300kW. Problem is, what about CW lasers (versus pulsed)? So rep rate plays an a role to some extent but I'm not sure what the convention is when I'm looking at these laser spec sheets.
 
Pulse lasers will normally have a spec in terms of energy per pulse and rep rate rather than average power.
Remember in your calculation you have 300KW for 10ns. But if you only get 1 pulse per second then that is 300,000 * 10/1,000,000,000 = 3mW

You normally use something like a Q-switched laser which can build up laser energy in the cavity and then emit a pulse when it reaches a certain level - with these systems essentailly all the average power goes into the pulses.
So a 300KW peak with a 10ns pulse every microsec = duty cycle of 1% = 3KW laser power and probably around 6-8KW wall power
 
mgb_phys said:
Pulse lasers will normally have a spec in terms of energy per pulse and rep rate rather than average power.
Remember in your calculation you have 300KW for 10ns. But if you only get 1 pulse per second then that is 300,000 * 10/1,000,000,000 = 3mW

You normally use something like a Q-switched laser which can build up laser energy in the cavity and then emit a pulse when it reaches a certain level - with these systems essentailly all the average power goes into the pulses.
So a 300KW peak with a 10ns pulse every microsec = duty cycle of 1% = 3KW laser power and probably around 6-8KW wall power

Right - agreed. I understand all that and I understand the concept of a Q-Switched laser. My question is when a spec sheet for a laser says 100W of power and, let's say, indicates one 10nS pulse is triggerable per second - does that mean that I'm getting 1 microjoule?
 
Ok that is what I figured. So... without knowing the rep rate you can't possibly figure out the energy per pulse. So for the attached spec sheet from a laser company, they report a CW power and a pulsed power but no rep rate. Now the CW power is 20W and the pulsed power is 16W. I doubt that the pulsed power is lower than the CW power, right?

You see what I am saying? It is really ambiguous IMO to see power ratings, rep rates, and pulse durations and try to figure out if this laser can meet me needs, i.e. a 3mJ 10nS pulse.
 

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I'm guessing it can run either 20W continuous or 16W q-switched (you waste a bit of energy)
Then you need to know the pulse width and rep-rate to get the energy, or the pulse width and pulse energy to get rep-rate.

Normally they quote the pulse width and pulse energy because that's what the measurement is concerned with - the rep rate is just a matter of more power (and money)
 
mgb_phys said:
I'm guessing it can run either 20W continuous or 16W q-switched (you waste a bit of energy)
Then you need to know the pulse width and rep-rate to get the energy, or the pulse width and pulse energy to get rep-rate.

Normally they quote the pulse width and pulse energy because that's what the measurement is concerned with - the rep rate is just a matter of more power (and money)

Right and we only need a single shot (so not need to concern ourselves with a max rep rate anyway). Ok, so like I thought I didn't have enough information to solve the problem.
 
  • #10
I think you need to convert mJ to J in your calculation then you can come up with 300W instead of 300kW.
 
  • #11
any buddy tell me the formula to calculate a laser diode power ratings. . .i am doing a project on pipeline leak detection using fiber optics. . .need ur urgent help
 

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