Laser and Laser Technology: How the Beam Actually Cuts Metal

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  Laser and Laser Technology: How a Beam Is Generated
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  Main Laser Types and What Each Does Well
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  What the Beam Can and Cannot Hold
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  How Lasers Fit Beside CNC Machining
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  Laser Type Comparison by Job

A laser is not a brighter lamp. It is a light source built so that most of the photons in the beam share the same wavelength, phase and direction. That condition comes from stimulated emission, first described by Einstein in 1917. An atom or molecule holds energy in discrete levels. If a photon of the right energy passes an already excited atom, the atom drops to a lower level and emits a second photon identical to the first. One photon becomes two, two become four.

For that multiplication to beat absorption, the medium must be inverted: more particles in the upper energy state than the lower one. A pump source does the work. In a fiber laser the pump is a set of laser diodes feeding a rare-earth-doped glass fiber. In a CO2 laser it is a high-voltage discharge through a gas mix. Either way, the inverted medium sits inside an optical cavity, and the cavity selects which wavelengths survive.

The cavity matters more than people expect. Two mirrors, one fully reflective and one partly transmissive, send light back and forth through the gain medium. Only modes that fit the mirror spacing reinforce themselves. Everything else leaks out as heat. The output coupler passes a fraction of the circulating power as the working beam.

What comes out is not one clean color. Real beams carry several longitudinal modes and a small divergence angle. Those details set how tightly the beam can be focused, and focus spot size sets cut width, kerf taper and achievable edge finish. Beam quality, quoted as M² or BPP, is the number that tells you how much of the ideal you actually get.

Three properties make the beam useful on a shop floor. Monochromaticity means the wavelength is narrow, so a lens focuses it to a small spot instead of a smear of colors. Coherence means the wavefronts stay in step, so the beam travels far without spreading. Directionality means it can be steered with mirrors and delivered through a fiber over meters of machine travel.

Those three properties are why a 1 kW beam can melt steel while a 1 kW arc spreads heat over centimeters. It is about energy density, not total energy. Focus 1 kW into a 0.1 mm spot and you reach power densities in the megawatts per square centimeter range. That is the entire trick behind laser cutting.

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  WavelengthSets absorption, spot size and which materials couple well
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  Beam quality (M², BPP)Sets the smallest focus spot you can reach
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  Power densitySets whether the material melts, vaporizes or just heats

Lasers are usually classified by the active medium. That choice drives wavelength, efficiency, cooling and cost. Fiber and CO2 lasers cover most sheet metal work today. Solid-state rod and disk lasers handle welding and cladding. Diode lasers are showing up in brazing and surface treatment because wall-plug efficiency is high and maintenance is low.

CO2 lasers run at 10.6 μm. Mild steel and stainless absorb that wavelength well, and the beam leaves a clean, nearly vertical edge on thick plate. The trade-off is delivery. You cannot send 10.6 μm light down a flexible glass fiber without heavy loss, so the beam travels through mirrors. That limits machine layout and adds alignment work.

Fiber lasers run near 1 μm. The beam moves through a flexible fiber, so the cutting head can travel on a gantry with almost no optical drift. Aluminum, copper and brass absorb 1 μm far better than 10.6 μm, which is why fiber took over those jobs. On thin gauge, fiber cuts several times faster than CO2 at the same power.

The catch is edge quality on thick mild steel. At 20 mm and above, a CO2 beam often leaves a smoother cut face. Fiber lasers answer with higher power and better gas control, but the oxygen-assisted cut on thick plate still needs tuning. If your part is 25 mm mild steel with a visible edge, test both before you commit.

Nd:YAG and disk lasers sit in the same 1 μm family. Rod lasers lose beam quality as power rises because the rod heats unevenly. Disk and fiber designs spread the gain over a large area or a long thin core, so thermal lensing stays small and beam quality holds at high power. That is why multi-kilowatt welding cells use disk or fiber sources.

Pulsed versus continuous matters as much as the medium. A continuous wave beam is best for cutting and deep welding. A pulsed beam delivers short, high-peak-power bursts that vaporize a tiny volume with little heat input, which is what you want for drilling thin foils, trimming medical stents, or marking anodized aluminum without cutting through it.

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  CO2, 10.6 μmBest edge on thick mild steel; mirror delivery, more upkeep
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  Fiber, ~1 μmFast on thin gauge, strong on aluminum and copper
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  Disk / Nd:YAG, ~1 μmHolds beam quality at high power for welding
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  PulsedLow heat input for drilling, trimming and marking

A laser cut is a thermal process, so it has a heat-affected zone. On a 3 mm mild steel sheet with a nitrogen assist, that zone is typically a few tens of micrometers wide. On 12 mm stainless it widens and the cut face picks up a slight taper. If your drawing calls for a sharp corner with no recast layer, laser cutting alone will not get you there.

Kerf width tracks focus spot size and power density. A 1 kW fiber source on 1 mm stainless gives a kerf near 0.1–0.2 mm. Push to 6 kW and the kerf can approach 0.5 mm on the same material because the beam is wider and the melt pool is larger. That matters when you nest parts: the kerf is material you lose, and it sets the smallest gap you can leave between parts.

Hole diameter has a practical floor. Below roughly one material thickness, the melt cannot clear the hole fast enough and dross builds on the underside. A 3 mm hole in 3 mm stainless is routine. A 0.5 mm hole in 3 mm stainless is a different process. For those, expect to drill, punch or sink the feature with a CNC mill instead.

Taper is the quiet problem. The beam converges to a waist and diverges again, so the top of the cut is wider than the bottom on thick sections. On 1 mm sheet the difference is negligible. On 10 mm plate it can exceed 0.1 mm per side. If both faces are functional, you either accept the taper or plan a secondary machining pass.

Thermal distortion shows up on thin, long parts. A 0.8 mm aluminum panel cut at high speed can bow because the heat input is uneven across the profile. Cutting with a nitrogen assist, using a lead-in away from the finished edge, and letting the part cool before handling all reduce the effect. Sometimes the answer is not the laser at all.

Reflective metals need attention. Copper and brass reflect most of a 10.6 μm beam back into the optics, which is why CO2 cutting of copper is rare. At 1 μm the absorption is high enough that fiber lasers cut copper and brass cleanly, provided the machine has back-reflection protection and the assist gas is right.

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  Kerf 0.1–0.2 mm1 kW fiber on 1 mm stainless
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  Kerf near 0.5 mm6 kW on the same 1 mm stainless
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  Minimum holeRoughly one material thickness without dross
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  TaperNegligible at 1 mm, can exceed 0.1 mm per side at 10 mm

Laser cutting and CNC machining solve different halves of the same problem. The laser is a 2D tool. It separates flat stock fast, with no tool wear and no fixturing for the profile itself. A 5-axis mill is a 3D tool. It creates pockets, threads, bores, faces and surfaces that a beam cannot reach.

The usual sequence is laser first, machining second. Blank the profile on the laser, then machine the critical features. That splits the work by tolerance. The laser holds ±0.1 mm on a clean profile. The mill holds ±0.005 mm on a bore. Trying to hit a ±0.005 mm bore with a laser is a losing fight, because the melt pool does not hold a sharp edge.

Laser marking is the exception that lives fully inside the machining world. We mark part numbers, lot codes and logos after finishing, with a minimum character height of 1.5 mm. On anodized aluminum the mark is a color change in the oxide layer. On stainless it is a shallow anneal or a light engrave. Neither removes material, so it does not affect fit.

Welding is where the same beam changes role. A fiber or disk laser welds with a small heat-affected zone and low distortion, which suits thin-wall enclosures, battery tabs and sensor housings. The joint still needs design work: a lap joint on 1 mm stainless wants a small gap and a clean surface, or the weld will porosity out.

Deciding between laser and machining comes down to three questions. Is the feature through-cutting on flat stock, or is it a 3D form? Is the tolerance looser than ±0.1 mm or tighter? Is the annual volume high enough that a blanking die would pay back? If the answer is through, loose and high, the laser wins. If it is 3D, tight and low, the mill wins.

At GreatLight we run both. Parts come off the laser as flat blanks and go onto 3-axis, 4-axis or 5-axis machines for the features that carry tolerance. We quote the routing as one job, so you are not coordinating two shops and two sets of inspection reports.

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  Laser holds ±0.1 mmProfile, holes above one thickness, marking
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  CNC holds ±0.005 mmBores, threads, pockets, faces, 3D form
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  Typical routingLaser blank, then mill the critical features