Peak power: an fundamental yet underrated parameter for pulsed lasers
Laser manufacturers emphasize the average power, followed by the energy deposited per pulse. For example: 500 W, 5 mJ.
But for a pulsed fiber laser, average power is not a particularly useful quantity on its own (except for roughly categorizing the product line), because it includes all the time during which the laser is doing… nothing.
Peak power, on the other hand, which is barely advertised or buried in the manufacturers' spec sheets (sometimes not even given, or worse written with a wrong value!) is the actual maximum power delivered by the laser when it fires.
I would even go so far as to say that, fundamentally, peak power is the key parameter of pulsed lasers that sets them apart from CW lasers.
But the thing is, peak power reaches completely different levels that depend much more on the pulse energy than on the advertised average power! Indeed:
P_peak = E / τ
where τ (Greek letter "tau") is the pulse width, i.e. the duration of each pulse usually expressed in nanoseconds (ns).
On a line graph, energy is the integral of the power curve across the pulse width (i.e. the area under the curve). So one does not immediately associate energy with peak power (which is the height of the curve on the Y-axis). Indeed, a very high but very narrow pulse has a high peak power yet often has relatively low energy.
Yet if you look at the waveform graphs in this JPT user manual (figures 4.1 to 4.3 on pages numbered 6-7, actual pages 10-11 in the PDF) as well as the exhaustive data at VONJAN, you can see the following max peak powers:
- A 1.5-2 mJ pulsed laser (whether 200W or 300W average!) develops a peak power of about 10 kW
- A 5 mJ pulsed laser (whether 200W or 300W) develops a peak power of about 20 kW
- A 15 mJ pulsed laser (whether 300W or 500W) develops a peak power of about 100 kW. Yes, 100,000 watts!
- A 50 mJ pulsed laser (whether 500W, 1000W or 2000W) develops a peak power of about 350 kW
- A 100 mJ pulsed laser (whether 1000W or 2000W) reaches a peak power of 1 MW!
The progression, which I added as the red bar chart (attached) appears insane and you can't unsee it afterward. Actually, it's not exponential, it's perfectly linear. It only appears exponential because the energy gap between two consecutive laser models increases dramatically in each manufacturer's lineup. Which we don't often see because the progression of a lineup for any brand gently increases linearly, at least in the beginning: 100W, 200W, 300W, 500W (not only after we switch to 1000W, 2000W).
— Special Case
You might then think: "So a 200W 5mJ laser is better than a 300W 1.5mJ laser, because it has both higher energy and higher peak power?"
Yes and no. This is where the characteristics overlap in a more complex way. Because of the cut-off frequency, the usable frequency range of lower-power models becomes limited when one wants to keep high energy per pulse. Even if a lower-average-power laser offers higher peak power and higher energy, you may have to wait longer between pulses, which slows down the work significantly.
The basic relation remains:
P_avg = E × f
So, for the same pulse energy, a higher average power (e.g. 500W vs 300W) simply means the laser can emit its pulses at peak power more frequently.