Laser Power vs Energy: Solving the Mystery

[DanSandberg]

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?

 

[Dale]

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).

 

[DanSandberg]

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.

 

[mgb_phys]

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

 

[DanSandberg]

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?

 

[mgb_phys]

It's normally quoted for the average power over all the pulses.
eg for http://www.coherent.com/Lasers/index.cfm?fuseaction=show.page&ID=1512&loc=765

Pulse Repetition Rate 1 to 10 kHz
Average Output Power 12W at1 kHz, 15W at 5 kHz, 15W at 10 kHz
Energy-Per-Pulse 12 mJ at 1 kHz, 3 mJ at 5 kHz, 1.5 mJ at 10 kHz

 

[DanSandberg]

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.

 

[mgb_phys]

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)

 

[DanSandberg]

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.

 

[elmashay]

I think you need to convert mJ to J in your calculation then you can come up with 300W instead of 300kW.

 

[kamyy]

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