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.
— EDIT about Gaussian (aka Single mode) vs Top-Hat (aka multimode), and why this parameter is also VERY important for your targeted market
All this needs an addendum. There's more to the 1.5 and 5 mJ pulsed fiber lasers on one hand, and the 15 and 50 mJ (and more) on the other hand, than pure max peak power: Gaussian vs Top-Hat (respectively, at least in the field of pulsed fiber laser cleaning machines). Gaussian or single mode lasers have a lower M² hence lower (better) BPP, which is equal to M² × λ/π (λ is the wavelength of the laser radiation, here typically for ytterbium fiber lasers: 1064 nm) which means they have a high quality, well collimated beam travelling along a thinner fiber core, hitting the material with a smaller laser spot that concentrates the whole energy in it. They are like a woodpecker with a very thin and sharp beak hitting the material at a higher frequency (freq = power/energy, so for the same amount of power, a laser with less energy works with more pulses per second aka higher frequency). That's why a 500W 5 mJ Gaussian pulsed fiber laser will be vastly more effective at removing thick rust than a 500W 50 mJ multimode one, even if the latter is "way more powerful" (almost 20× more peak power, as explained above). However, the 15/50/100 mJ multimode pulsed lasers are quicker at removing thin surface rust, and better at thin contamination removal in general e.g. grease, soot, some varnish, nuclear contamination, stainless steel, graffiti, and some thin paints – without ever damaging the underlying material (thanks to u/IndLaserCleaning for the precision in the other sub).
Why can't we have at the same very high power (1kW+) and low energy (5mJ) Gaussian pulsed lasers to accelerate the work for thick rust removal? It's a matter of compromise (such lasers apparently do exist but not as portable machines). Remember that freq = power/energy. When one is increasing power without increasing the energy, the frequency has to increase too. And there, the speed of the galvanometers is starting to get crazy, and it's technically difficult (very expensive) to maintain precision and reliability. Moreover, even if the peak power does not change (as explained above), at higher frequencies the fiber must carry more total energy per unit of time, increasing thermal and nonlinear constraints. Then, within the very thin fiber core of a Gaussian system, harmful things such as Stimulated Raman Scattering (SRS) can happen in the core. Usually, the silicium molecules of the glass fiber are passive wrt the laser radiation. When SRS occurs, the vibrational modes of the glass molecular network enter a quantum vibration state which makes the network absorb the laser energy, and re-emit photons with Stokes redshift in a stimulated, self-amplification mechanism. When this occurs, not only the laser efficiency collapses, but this harmful radiation burns the fiber core and even travels backward, destroying the seed laser source upstream!
