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I'm talking about laser diode for engraving/cutting cnc. As far as I know led or laser diodes are often driven by pulse signals to enhance their performances.

Please note I'm not talking about the PWM modulation used to create a smooth range of intensity. I'm referring about the low-level driver of the diode itself.

Well, also this is a sort of PWM but with a very short pulse (typically a dozen of ns) followed by a recovery time (much longer) to allow the junction to cool down, ready for the next pulse.

What I'm asking here is the advantage in a practical application like engraving or cutting. Let's consider a real case, two different products:

  • Product A: 6W laser diode driven by a constant current

  • Product B: 15W laser diode, pulsed-driven but with a mean power of 6W

Why should I prefer the B?

Here my thoughts: if I want to cut something I need to burn long enough the surface of the material to create a hole. Then I can advance a bit and burn another hole, and so on until I have a trench. If my cutting depth is less than the stock height I need to make multiple passes.

Well, let's say the Product A create a trench of 0,3 mm depth at a given feed rate. Product B would create a deeper trench (due to the more power applied) but due to the recovery time I need to lower the feed rate. Because the mean power are identical the cutting time would be the same (i.e. half passes at half speed).

I'm pretty sure my opinion is incorrect. Would you please help me to understand the advantages in such an application of pulse-driven laser diodes?

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    \$\begingroup\$ There are things where you can't get to the necessary temperature with lower wattage \$\endgroup\$ – PlasmaHH Dec 10 '17 at 20:10
  • \$\begingroup\$ the intensity of the light can be higher and more penetretive if it's pulsed, metal does let a bit of light through, perhaps it stresses the metal more to receive inconsistent light rays, a bit like the teeth of a wood saw. \$\endgroup\$ – aliential Dec 10 '17 at 21:10
  • \$\begingroup\$ I think in your example there is no significant difference, but if you have a pico-second pulsed laser with 15 kW peak power but 300 mW mean power, then there will be quite a difference. \$\endgroup\$ – Ale..chenski Dec 10 '17 at 22:25
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There are really a couple of different classes of laser that matter for this:

  • CW : Runs continuously, this is your 6W non pulsed diode.
  • QCW : Quasi CW, usually a thermal limit to the pulse length, good for tens of microsecond to tens of ms pulses, this is your 15W peak, 6W average device.
  • Q Switched : This is where the difference really appears, 6W average, but made up of 6mJ pulses 10s of ns wide at 1000 pulses per second, power during a pulse is 100s of kW, and the mechanism is as much ablation as classic bulk heat transfer.

For most things the first two are much of a muchness, with the Q switched job being the one that behaves differently. For example copper is a bear to cut with a CW laser because it is so horribly thermally conductive, A Q switched job, because the pulses are so short but with enormous power makes a far better job of it.

The bear with Q Switched lasers (Apart from the very much increased eye hazard), is that the pulses tend to burn the coatings on your optical chain.

Only some laser technology Q switches in a useful way, usually YAG and some fiber lasers.

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B is worse. Not only is efficacy lower , there is no combustion advantage if the mean power is the same.

From my experience on power Blue Lasers for cutting and burn print on wood, what matters is the average power which means averaged over the response time of the material.

Most materials are passive yet non-linear in that as they carbonize, they absorb more optical power and accelerate temperature rise at that threshold.

An active material I would consider is where the chemical reaction of photo energy triggers an immediate ionization and spontaneous molecular reaction faster than the time interval of the pulse period. THis assumes a linear absorption until blue absorption rises then triggers some hyptothetical spontaneous reaction (fusion).

So from the phenolic, epoxy, metallic, & cellulose materials that I have tried used so far, none have shown the latter fusion property and all roughly like the former property. If you do plan on making a fusion reactor, let me know. ;)

This means the peak energy is not what triggers faster temperature rise but rather the time duration. dynamic absorption rate, and average power density (optimal focus).

Not only that, the efficacy of power LEDs and Laser LEDs continually declines with rising current above lasing current from I²ESR losses and rising temperature. Although a well designed substrate for heat transfer will support fairly flat efficacy up to 50% of rated power , it measurably declines above this.

I think your only hope is to design nitrogen or liquid closed loop cooling like that used on overclocked CPU's if you wan to overdrive the Laser LEDs or invest in superior optics to focus the beam to a smaller dot, commensurate with Z axis stability.

Also Lens aberration causes the optical laser density to reduce significantly from precision expensive glass optics used on gas laser cutters.

I found that Laser printing on wood was extremely slow compared to an ink-jet printer due to the thermal response time of raising the temperature to carbonization levels. I tried changing gantry speeds, pulsed over current and various PWM levels and nothing worked any faster than continuous full rated power at a constant baseplate temperature with forced air cooling.

side notes

I recall instrumenting power absorption for Zirc-Monel Steel Diffusion Bonding during my late 70's work at Bristol Aerospace and even with 100kW available power to the surface, the best solution was continuous power, controlling the feed rate, cooling rate and power levels. Although here the plasma ionization was rather hot and probably resonating, this was not a laser rather 1~5V AC at very high currents. There were significant sparks of optical IR,UV and Xray emissions from sputtered diffusion of 3 surfaces between the giant solid copper roller contacts, so you might say it was pulsed from contact noise and high pressure gas emissions but this was an example of fusion welding or Diffusion bonding when it is done right. but the plasma reaction was an active surface of combustion to assist with generating the heat to diffuse 3 layers of material in a pipe joined together into one solid material so it could handle 1k atmospheres of pressure in a nuclear reactor without leaking.

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