§1Light as a tool
A laser turns electrical power into a beam of light that can be focused so tightly it becomes a cutting, welding and marking tool of great precision — a tool made of light, touching nothing.
What makes this possible is that laser light, unlike ordinary light, can be brought to a focus a fraction of a millimetre across, concentrating even a modest power into an intensity that melts and vaporises metal (§2–3). Because the “tool” is a beam, it exerts no force, wears nothing, reaches where no cutter could, and is steered and switched at will. The same beam does several jobs by how it is applied: focused with an assist gas it cuts (§4); held on a spot it welds deeply; swept lightly it marks (§5). The two common industrial types — CO₂ and fibre — differ in the light they make and what it suits (§6). Across all of them the principle is one: concentrate pure light to an intensity metal cannot withstand.
Contents§2What makes laser light special
Laser light differs from ordinary light in three ways that together let it be focused to a pinpoint — it is one colour, in step, and travels as a tight parallel beam.
Ordinary light is a jumble of many wavelengths, out of phase, spreading in all directions — impossible to focus tightly. Laser light is monochromatic (a single wavelength), coherent (all its waves in step), and collimated (a nearly parallel beam that barely spreads). These properties are what a lens needs to bring light to the smallest possible spot: a single wavelength focuses to one point without the colour-blurring a mix suffers, and a parallel, in-step beam converges to a near-diffraction-limited spot. The result is that all the beam’s power can be squeezed into an area a fraction of a millimetre across — which, however ordinary the total power, makes the intensity at that spot extraordinary (§3). It is not that a laser carries huge power; it is that its special light lets whatever power it carries be focused to a point.
Contents§3Power density at focus
The laser’s cutting power comes from power density — the power divided by the tiny focused spot area — which reaches values no other tool approaches.
Focus a 2000 W beam — a common cutting laser, no more power than a couple of kettles — to a spot 0.2 mm across, and its area is only π × (0.1 mm)² ≈ 0.0314 mm², or about 3.1 × 10⁻⁸ m². The power density is then 2000 ÷ (3.1 × 10⁻⁸) ≈ 64 GW/m² — tens of gigawatts per square metre, an intensity that melts and boils steel in an instant. This is the whole secret of laser machining: the modest total power matters far less than its concentration, and the concentration is enormous because the special light (§2) focuses to so small a spot. It also shows why a smaller focused spot cuts better — halve the spot diameter and the area quarters, so the power density quadruples — and why keeping the beam in focus is critical. Power density, not power, is what does the work.
§4Laser cutting
Laser cutting melts or vaporises a narrow line through the material with the focused beam, while a coaxial jet of assist gas blows the molten metal out of the cut.
The focused beam heats a spot to melting or vaporisation, and the machine traverses it along the cut line while a assist gas, blown down the same nozzle, ejects the molten material and clears the kerf (the hero). The gas is chosen for the job: oxygen for carbon steel, where it also burns exothermically to cut faster and thicker; nitrogen (inert) for stainless and aluminium, blowing the melt clear without oxidising the cut edge, for a clean bright finish. Laser cutting’s virtues are a very narrow kerf, a small heat-affected zone, no cutting force, intricate profiles cut fast from flat sheet, and easy computer control — which is why it has largely replaced older sheet-cutting methods for precision work. Its limit is thickness (very thick plate is slow or beyond reach) and reflective, thick materials for some laser types (§6). For thin-to-medium sheet cut to a fine, complex outline, the laser is the tool of choice.
Contents§5Welding and marking
The same beam welds when it is used to melt a joint rather than cut through, and marks when its power is dropped to alter a surface without cutting.
Laser welding concentrates the beam to melt a joint, and at high power density it produces a keyhole: the beam vaporises a narrow column into the metal that the surrounding melt collapses around as it moves, giving a deep, narrow, fast weld with a small heat-affected zone and little distortion — ideal for precise, high-speed joining of thin and medium parts. It contrasts with arc welding’s broader, shallower pool (the welding page). Laser marking and engraving drop the power so the beam does not cut through but instead darkens, etches or ablates the surface, writing permanent serial numbers, codes, logos and patterns with no contact and no consumable — the origin of the fine, crisp marks on tools, components and packaging. The one beam, by its power density and how it is moved, spans cutting, deep welding and delicate marking — the same tool set to three intensities of the same effect: melting metal, more or less.
Contents§6CO₂ and fibre lasers
Two laser types dominate manufacturing, differing in the wavelength of light they make — and that difference decides what each cuts best.
The CO₂ laser generates infrared light at a long wavelength of 10.6 µm from an electrically excited gas mixture; long serving and powerful, it cuts and engraves non-metals (wood, acrylic, textiles) superbly and cuts metal well, but its long-wavelength light is poorly absorbed by shiny metals and must be steered by mirrors. The fibre laser generates light at about 1.06 µm — ten times shorter — in a doped optical fibre, and that shorter wavelength is absorbed far better by metals, especially reflective ones like aluminium and copper that trouble a CO₂ laser. Fibre lasers are also more efficient, more compact, need no gas discharge and deliver the beam down a flexible fibre, which is why they have taken over metal cutting and marking. The rule of thumb: fibre for metal, and CO₂ for many non-metals and thicker section in some cases — the choice following which material the beam’s wavelength couples into best.
Contents§7Quick reference
The working core of the page on one card rack.
Laser light
monochromatic · coherent · collimated
→ focuses to a pinpoint
Power density
power ÷ tiny spot area
2 kW @ 0.2 mm → 64 GW/m²
Cutting
beam + assist gas · narrow kerf
O₂ for steel · N₂ for stainless
Weld / mark
keyhole weld · surface marking
Types
fibre 1.06 µm → metal
CO₂ 10.6 µm → non-metals