# Pulsing a diode laser with very long pulses and very low repetition rates

1. Dec 11, 2008

### r_daniel

Greetings,

I am an electronic engineer doing some research on high power laser diodes.

The problem I try to solve needs ~2300W of optical peak power coming from a high power laser diode.

The pulses must be with very long (30ms) at very low repetition rates (2Hz).
So,the duty cycle is DC = 0.06.

The calculations I do with the above parameters yield an average power
Pav= Ppeak x DC = 2300 x 0.06 = 138W.

Now the confusing part:

I have been told that the "average optical power" is not equal with the "optical power in CW mode" for very long pulses and very low repetition rates.

Hence, I can not use a laser diode which has 140W optical power in CW mode to get the needed ~2300W peak power.

Is that true ? Why ?

Are there any formulas that I can use to do the calculations for very long pulses to drive a laser diode ?
Any suggested books to read would be greate help..

Best Regards
chris

2. Dec 11, 2008

### Manchot

The reason that average pulsed power and CW power aren't necessarily the same is that when you first turn on a laser, you get better performance out of it for a brief time. Assuming that current injection happens quickly, the population inversion is larger than the threshold value for a brief time (as the photon density hasn't yet had time to build up), and the device is also at a lower temperature than when it has been on for a long time. So, if you imagine pulsing a laser as a series of turning a laser on, turning it off again, and letting it cool off before turning it on again, then it should be clear that average pulsed power is higher than CW power.

But at the long timescales we're talking about (especially for diode lasers), any electronic effects will be washed out, and the only factor that matters is heating. What matters is whether your laser has time to cool off between pulses, and ultimately, that depends on how well heat sunk it is.

3. Dec 12, 2008