How Laser Cutting Works and Laser Types
CO2, diode, fiber and UV lasers, wavelength and absorption, the focused beam, spot size and depth of focus, and the machine types.
What a laser is
A laser makes light of one wavelength, traveling in one direction, in a beam that stays narrow over a long distance. That last property is what a cutter needs: a lens can focus the beam to a spot a tenth of a millimeter across, and the power in that spot is enough to melt, burn or vaporize most materials in a fraction of a second. The cut is made by heat, with no contact and no cutting force: the sheet does not need to be clamped hard, thin or fragile parts can be cut, and the cut edge is smooth, but heat has side effects (a heat-affected zone, charring, a tapered kerf) that the other pages deal with.
Light of a given wavelength carries a definite energy per photon, E = h c ÷ λ. Shorter wavelengths carry more energy per photon, which matters for how they interact with materials:
| Laser | Wavelength | Energy per photon |
|---|---|---|
| UV | 0.355 µm | 3.492 eV |
| Blue diode | 0.450 µm | 2.755 eV |
| Fiber | 1.064 µm | 1.165 eV |
| CO2 | 10.600 µm | 0.117 eV |
The types of laser
| Type | How the light is made | Strengths | Weaknesses |
|---|---|---|---|
| CO₂ | An electric discharge in a gas mixture of CO₂, nitrogen and helium, in a sealed glass tube (cooled by water) or a metal RF-excited tube | Excellent on organic materials, acrylic, glass, fabric; good beam quality; mature and cheap per watt | Long wavelength: bare metals reflect it; tube life (glass tubes about 2 000 to 10 000 hours); needs mirrors and water cooling |
| Fiber | Light from diodes pumps a rare-earth-doped optical fiber (ytterbium), which makes the beam and carries it | Efficient, compact, no mirrors to align, long life (tens of thousands of hours); cuts and marks metals | Expensive; reflective metals can send light back; clear plastics and wood are poor |
| Diode (direct) | Semiconductor laser diodes directly, often several combined | Low cost, small, no tube; the blue ones (450 nm) cut and engrave wood and dark materials | Beam quality is poor, so the spot is larger and the depth of focus short; low power; light passes through clear materials |
| Nd:YAG and disk | Pumped solid crystal | Pulsed, high peak power; welding, drilling and marking | Largely replaced by fiber lasers for cutting |
| UV (355 nm) and ultrafast | Frequency-tripled solid state; picosecond or femtosecond pulses | “Cold” marking and micro-machining with very small heat-affected zones | Costly; slow material removal |
Wavelength and absorption
A material heats only if it absorbs the light. How much it absorbs depends on its composition, surface and temperature, and strongly on the wavelength. Organic materials and most plastics absorb the far-infrared of the CO₂ laser; bare metals reflect most of it (and absorb the near-infrared of a fiber laser much better); clear materials pass the light of a diode or a fiber laser straight through.
- CO₂ cuts clear acrylic cleanly because it absorbs the light in a thin surface layer; a diode or fiber laser would pass through it, and a blue diode would cut only dark or colored acrylic.
- Bare metals reflect a CO₂ beam, so a CO₂ laser engraves coated or painted metal, or cuts it only with an assist gas and very high power (industrial CO₂ cutters do cut steel with oxygen).
- Absorption rises with temperature: a metal that reflects at the start absorbs far better once a pool of melt forms. That is why piercing a reflective metal is the hardest moment, and why a back-reflection protects the machine with an isolator.
- Surface matters: paint, oxide, anodizing and roughness raise absorption; a mirror-polished or oiled surface lowers it.
The focused beam
A lens focuses the beam to its narrowest point, the waist. For a Gaussian beam of wavelength λ and beam quality M² entering a lens of focal length f with a diameter D, the spot diameter and the depth of focus (twice the Rayleigh range zR) are:
d = 4 M² λ f ÷ (π D) zR = π w₀² ÷ (M² λ) depth of focus = 2 zR
Worked example: lenses for a CO₂ laser
An 8 mm beam of 10.6 µm light with M² = 1.1:
| Lens focal length | Spot diameter | Depth of focus |
|---|---|---|
| 38.1 mm (1.5 in) | 71 µm | 0.7 mm |
| 50.8 mm (2.0 in) | 94 µm | 1.2 mm |
| 63.5 mm (2.5 in) | 118 µm | 1.9 mm |
| 101.6 mm (4.0 in) | 189 µm | 4.8 mm |
- A shorter lens gives a smaller spot and a shorter focus. The spot is proportional to f; the depth of focus to f². A 2 in lens cuts 3 mm material finely if the sheet is perfectly flat; a 4 in lens tolerates warped sheet and thick material at a wider kerf.
- Power density. Focused to 94 µm, a 60 W beam has an area of π ÷ 4 × 0.094² = 0.00698 mm² and a density of 8.6 kW/mm²: about 8.6 million times the intensity of full sunshine on the ground (about 0.001 W/mm²). This is why a small beam burns wood and why a stray reflection is dangerous (see Laser Safety, Fire and Fumes).
- Fiber cutting: a fiber laser images the end of its delivery fiber. A 50 µm core, a 100 mm collimating lens and a 150 mm focusing lens give a spot of 50 × 150 ÷ 100 = 75 µm; a 1 kW beam then has a density of 226 kW/mm², about 26 times that of the 60 W CO₂ spot.
Why beam quality matters
Worked example: the effect of M²
| M² (beam quality) | Spot with a 2 in lens | Depth of focus | Power density at 60 W |
|---|---|---|---|
| 1.1 | 94 µm | 1.2 mm | 8 596 W/mm² |
| 1.5 | 129 µm | 1.6 mm | 4 623 W/mm² |
| 3.0 | 257 µm | 3.3 mm | 1 156 W/mm² |
A perfect beam has M² = 1; a good CO₂ tube about 1.1 to 1.3; a diode stack or a multimode fiber can be 10 or much more. A worse beam focuses to a bigger spot with a lower power density: the same watts cut less and the kerf is wider. A cheap high-power diode can have a similar power to a CO₂ tube yet cut far more slowly for this reason.
How the beam cuts
| Mechanism | What happens | Typical materials |
|---|---|---|
| Vaporization (ablation) | The beam heats the material above its boiling or decomposition point, and the vapor leaves; little melt | Acrylic, many plastics, paper, wood (with charring) |
| Melt and blow | The material melts and a high-pressure inert gas (nitrogen) blows the melt out of the kerf; clean, oxide-free edge | Stainless steel, aluminum, thin steel |
| Reactive cutting (oxygen) | Oxygen burns the hot steel (an exothermic reaction) that adds heat and clears the kerf; an oxidized edge | Mild steel, especially thick |
| Thermal fracture and scribing | Localized heating scribes a line along which glass or ceramic cracks | Glass, ceramics (special processes) |
Worked example: how fast could 60 W cut 3 mm acrylic?
- Energy to remove a cubic millimeter. Acrylic (PMMA) has a density of 1.18 g/cm³ (0.00118 g/mm³). Heating it by about 375 °C (specific heat 1.5 J/g·K) and then vaporizing it (about 1 000 J/g, a rough effective figure for decomposition) takes 0.00118 × (1.5 × 375 + 1 000) = 1.84 J/mm³.
- Volume per millimeter of cut: a 0.2 mm kerf in 3.0 mm sheet removes 0.60 mm³ for each mm of cut length, so 1.11 J per mm.
- Speed. With a 60 W beam of which 60 % ends up doing useful work: v = 36 ÷ 1.11 = 33 mm/s. Real cuts are slower (typically 10 to 20 mm/s in this case) because heat is lost into the sheet, the kerf and the gas flow, and the edge must be clean: treat this as the upper limit, a sanity check, not a setting. See Materials, Thickness and What a Laser Can Cut for a fuller energy-balance and Power, Speed, Focus and Assist Gas for finding real settings.
Power, efficiency and heat
Worked example: electrical power and heat for 100 W of light
| Source | Efficiency (electrical to light) | Electrical power | Heat to remove |
|---|---|---|---|
| CO2 (sealed glass tube) | 10 % | 1 000 W | 900 W |
| CO2 (RF metal tube) | 12 % | 833 W | 733 W |
| Fiber | 35 % | 286 W | 186 W |
| Blue diode | 30 % | 333 W | 233 W |
A CO₂ tube needs about ten times the optical power in electricity, and all but a tenth of it leaves as heat in the tube, which a water circuit must carry away (see Machine Hardware: Optics, Motion and Cooling). A fiber laser makes three times less heat for the same light. This is a large part of why fiber lasers took over metal cutting: lower running cost and no tube to replace.
Types of machine
| Machine | How the beam moves | Good for | Notes |
|---|---|---|---|
| Flatbed, moving optics | The beam is steered by mirrors on a gantry that moves in X and Y; a lens on the head travels with it | Cutting and engraving sheet, up to a few meters | The common CO₂ machine; the path length changes with position (see Machine Hardware: Optics, Motion and Cooling) |
| Flatbed, flying optics with a fiber laser | A fiber carries the beam to a head on a gantry | Metal sheet cutting | Fixed path length; fast gantry drives; pallet changers |
| Galvo scanner | Two small mirrors on motors steer the beam over a small field | Fast engraving, marking, perforating | Very fast (meters per second) over a field of 100 to 300 mm; cannot cut thick material |
| Hybrid and desktop | Moving head in a small enclosure, with a camera for alignment | Makers, schools, small shops | Enclosed (Class 1) in many models: see Laser Safety, Fire and Fumes |
| Tube and 3D cutters | A robot or a rotating chuck carries the work or the head | Pipe, profiles, formed parts | Specialized (see Industrial Robotics) |
| Diode (open frame) | A small head on a belt-driven gantry, often made from a kit | Hobby engraving and thin cutting | Open-frame models are Class 4 with no enclosure; use an enclosure and goggles |
Continuous and pulsed operation
A laser can run continuously (CW), or in pulses. Most low-power CO₂ cutters are controlled by pulse-width modulation: the laser is switched on and off quickly (at 5 to 30 kHz), and the ratio of on time to period, the duty cycle, sets the average power. The control looks like a power percentage, but it is a duty cycle.
Worked example: PWM power
A tube that gives 80 W when on, run at a duty cycle of 45 % at 20 kHz, delivers an average of 36 W, in pulses of 23 µs separated by 28 µs. Material sees the average if the pulses are short compared with its thermal time, but the beam also has a minimum on time: at low percentages the tube may not light at all (the ionization threshold), so very low power settings do nothing, and a low setting gives an uneven cut. Pulses matter more for engraving and for thin materials (see Power, Speed, Focus and Assist Gas).
Related
- Laser Cutting: the overview of this guide.
- Materials, Thickness and What a Laser Can Cut: which materials each laser can process.
- Machine Hardware: Optics, Motion and Cooling: tubes, mirrors, lenses and cooling.
- Laser Safety, Fire and Fumes: the hazards of each laser type.
- CNC Machining: the machine, coordinates and G-code behind a laser cutter.
FAQ
Why does a CO₂ laser not cut bare steel well?
The 10.6 µm light is mostly reflected by a clean metal surface. Industrial CO₂ cutters overcome it with high power and oxygen, but a fiber laser at 1.06 µm is absorbed much better and has replaced CO₂ for metal cutting.
What does focal length change?
A shorter lens gives a smaller spot, a finer kerf and a shorter depth of focus; a longer lens gives a larger spot and a longer depth of focus. Choose by the thickness and flatness of the work.
What does M² mean?
The beam quality factor: 1 is a perfect beam; larger values mean the beam focuses to a larger spot and diverges faster, so it cuts less for the same power.
Can a diode laser cut clear acrylic?
No: blue diode light passes through clear acrylic. It cuts dark or colored acrylic and wood. A CO₂ laser cuts clear acrylic.
References
- Steen, W. M. and Mazumder, J., Laser Material Processing, Springer (beam focusing, absorption and cutting mechanisms).
- Siegman, A. E., Lasers, University Science Books (Gaussian beams and M²); ISO 11146 (laser beam widths, divergence and propagation ratios).
- Laser and lens makers’ application notes on focal length, spot size and depth of focus.
Last reviewed: October 2026. Absorptances, efficiencies and cutting estimates are typical and illustrative: test on your own material and machine.