Physics in Plain English

Five concepts the rest of the course assumes you know. Five minutes of physics that turns settings recipes into something you can reason about.

1. Wavelength = what materials absorb the beam

Light at different wavelengths is absorbed by different materials. This is the single biggest reason fiber lasers and CO2 lasers do different jobs.

Laser	Wavelength	Absorbs strongly in	Reflects off
Fiber	1064 nm (near-IR)	Metals (most), some plastics, some coatings	Wood, paper, glass, untreated polymer
CO2	10,600 nm (far-IR)	Wood, acrylic, leather, paper, fabric, glass	Bare metal (highly reflective)
Diode (blue)	405-455 nm (visible)	Dark surfaces (visible-light absorbers)	Reflective metals, light/clear materials

Why metals love 1064nm: free electrons in metals absorb infrared photons efficiently. Why wood loves 10,600nm: organic C-H and O-H bonds resonate at far-IR. The wavelength chooses the material category, not the other way around.

2. Average power vs peak power

A laser's wattage rating is its average power, the energy delivered per second on average. But a pulsed fiber or RF-tube CO2 laser doesn't fire continuously; it fires pulses. The energy per pulse, and the peak power within each pulse, can be MUCH higher than the rated average.

• Average power: total watts over time. Drives long-term heat input.
• Peak power: wattage during the brief moment a pulse is firing. Drives the actual ablation work.
• For a 60W MOPA at 50 kHz with 200 ns pulses: peak power can reach 6,000 W per pulse (100× the average). That's why fiber lasers cut metal that average-power-equivalent CO2 wouldn't touch.

Practical implication: lower frequency = fewer pulses per second = same average power concentrated into fewer, more powerful pulses. That's why deep engraving uses low frequencies.

3. Beam quality (M²): why some lasers cut cleaner

M² ("M-squared") measures how close your beam is to a perfect Gaussian focus. M² = 1.0 is perfect (theoretical limit). Real-world fiber MOPA sources are M² = 1.1 to 1.5. CO2 metal-tube sources are M² = 1.2 to 1.8. Cheaper glass-tube CO2 can run M² = 3 or higher.

• Lower M² = smaller focused spot = higher power density at focus = cleaner cuts.
• This is why a "30W" RF metal-tube CO2 cuts better than a "60W" glass-tube CO2, the metal tube has half the M².
• Your JPT 60W MOPA is M² ~1.3, premium beam quality, why it color-anneals so well.

4. Power density (fluence): the actual work metric

Power density = power delivered per unit area, measured in watts per cm² or J/cm² (joules per cm²). This is what determines whether the surface ablates, melts, or just heats.

Spot size matters more than total power: a 30W laser focused to a 20 µm spot delivers higher power density than a 60W laser focused to a 60 µm spot, even though one has half the total power. Spot area scales with the SQUARE of spot diameter, so halving spot diameter quadruples power density.

Practical implications:

• Switching from 150mm to 70mm lens roughly quadruples power density at focus → drop power 30-40%.
• Defocusing spreads the beam → drops power density → you can use higher Lightburn power % without ablating. This is the secret behind black-on-stainless.
• "Why won't 60W cut metal?", average power is fine, but at the spot size of a 70mm lens you don't reach the fluence threshold for cutting steel. You'd need a kilowatt fiber.

5. The four ablation regimes

What happens to the metal under the beam depends entirely on power density × dwell time. Four distinct regimes:

Regime	Power density × dwell	Effect	Example
Surface heating	low × long	Oxide layer growth, no material removal	Color anneal on stainless
Surface ablation	moderate × short	Thin top layer evaporated, sub-surface untouched	Anodize dye removal, photo on metal
Bulk ablation	high × moderate	Material vaporized, real depth removed	Deep engraving
Melting	high × long	Material liquefies and flows; messy walls	What you DON'T want, slag, splatter

Frequency, pulse width, power, and speed are all just knobs that move you between these four regimes. Once you see them this way, settings stop feeling arbitrary.