Laser Cutting Lenses

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Collimating lens (collimator)

A collimator is a separate passive optical unit that prepares the laser beam for further delivery into the focusing module. It consists of a housing (objective), a set of collimating lenses (biconvex + meniscus), upper and lower sealing rings, gaskets, and nuts.

A collimator is a key element used to collimate (gather) optical light. It plays a critical role in aligning the light in the desired direction, acting as a "straightener and gatherer" of the beam: it makes the laser beam suitable for delivery to the focusing module and ensures high beam quality along the way.

Why a collimating lens is needed in a cutting laser

A collimating lens converts a diverging laser beam into a collimated beam — a beam in which all photons travel parallel to one another. This collimated beam is then directed by the optical system into the focusing lenses, which gather it at a specified focal length into a spot. That spot is where the maximum laser energy density is concentrated — at the diameter required for efficient laser cutting.

In other words, collimating lenses narrow and straighten the beam: they either make the directions of light more parallel, or reduce the cross-section of the beam. The collimator of a laser cutting head narrows the diverging beam coming out of the fiber optic cable and is made of two lenses.

Functions and properties:

A collimating lens is a compound lens used to achieve the best optical quality and cutting performance.
High transmittance, ultra-low absorption, withstands power above 10,000 W, high enthalpy-loss threshold.
Working-distance consistency above 99.5 %.
High laser transmittance, high damage threshold — suitable for high-power laser cutting.
Good collimation, less than 1/10 of the wavelength.

Focusing lens

Mounted in the cutting head, this lens forms a laser spot of the required size on the workpiece.

Long-focus lenses produce a larger spot — this offsets "slippage" on cylindrical surfaces and gives better engraving quality on curved bodies, including cones.

But a large spot behaves like a large-diameter milling tool — fine image elements are nearly impossible to reproduce. Long-focus lenses are therefore used only for engraving text and low-detail graphics.

Otherwise, switch to a short-focus model or retune the machine (lower acceleration/deceleration, reduce head-travel speed, etc.).

The main drawback of a long-focus lens is that it nearly doubles the required cutting power. For engraving (which uses up to ~30 % of rated power) this is acceptable, but for through-cuts even 100 % of available power may be insufficient.

A high-energy laser beam must be focused on the material surface with maximum precision.

The depth of focus increases with focal length and decreases as focal length is reduced. In laser systems this is equivalent to the length of the beam waist.

That is why:

For thin, flat materials, lenses with a short focal length are preferred — for example f′ = 93.75 mm. This produces a clean cut with high speed, narrow kerf, and an extremely small heat-affected zone.
For thick or uneven materials, lenses with a longer focal length (f′ = 127–190.5 mm) are better suited.

Laser beam focusing

Manufacturers disagree about the priorities inside a laser cutting head. Some emphasise the smallest possible spot size; others focus on lens orientation and the perpendicularity of the lens axis to the laser beam. In reality, one cannot be ranked above the other — the importance of each varies with cutting conditions. Maintaining the focus position inside the material is critical for reproducible cutting parameters and consistently high edge quality.

Focusing for thick metal

When working with sheets thicker than 20 mm, a larger melt zone is essential — to obtain a deeper melt pool that must be ejected during cutting. To create this enlarged piercing spot, the beam is focused either above or below the material surface, depending on the assist gas. A small spot focused on the surface is usually less successful on thick material.

Focusing for thin metal

For sheets 1–3 mm thick, a point focused on the surface of the metal is required. It is far more effective than a larger spot, because no wide channel is needed to remove material.

Assist gas in laser cutting

A critical factor in laser cutting is the assist gas — oxygen, nitrogen, or compressed air. Each gas has specific properties tied to accelerating combustion, ejecting molten material, or both.

Assist gases support one of two reactions — exothermic or endothermic. The focusing rules depend on the reaction type and gas used.

Focusing for exothermic reactions

When the gas drives an exothermic reaction, it accelerates the cut — oxygen, for example. The metal literally boils, the intense beam energy vaporises it, and oxygen efficiently reacts with the molten metal. The process runs at high pressure; oxygen heats the base material to a very high temperature, generating metal vapour and further evaporation.

Cutting thick sheets requires a larger piercing form — this is used in production to create a wide kerf and to remove molten material during laser cutting.

Focusing rules for exothermic reactions:

For thick materials — focus above the surface.
For thinner materials — focus on the top surface.

When the focus is above the material, low pressure and low gas volume are typically used — to liquefy and then displace the molten material. Very little material is vaporised, because the small oxygen volume cannot sustain full evaporation.

When the focus is on the material surface, high pressure and high volume are used — enough to drive intense vaporisation.

That is why on cutting tables used mainly for thinner material, the supports are nearly clean; on tables used for thick material, the supports accumulate far more debris.

Focusing for endothermic reactions

Endothermic reactions arise with inert gases — nitrogen and argon.

Here the gas only blows molten material out through the cut. The endothermic process depends heavily on the initial energy of the focused beam — that energy must rapidly bring the metal to molten state and form the cut. The inert gas at high pressure then expels the liquefied material through the kerf, leaving a cleanly cut surface free of slag adhesion.

Focusing rules for endothermic reactions:

Focus at the bottom of the material or slightly below it.
Keeping the focus below the material creates a small V-shape in the cross-section of the kerf, allowing the high-pressure gas to compress the molten material and push it through the base of the kerf at high speed.
High gas volume and high pressure are required to support rapid removal of the melt.

Compressed air — hybrid case

Using compressed air actually triggers both endothermic and exothermic reactions simultaneously. Because air is mostly nitrogen (≈78 %), the reaction is primarily endothermic; the small oxygen content (≈20 %) drives a parallel but smaller exothermic reaction. Thanks to oxygen, the base material melts faster. The remainder of the air is largely inert and participates only in the endothermic reaction driven by nitrogen.

Compressed-air cutting gives the best results when the focus is held in the centre of the material thickness.

Real-world applications

It is essential to control every factor that affects the projection of the focus point. The raw beam in the optical resonator must be in good condition and properly delivered to the lens. A lens with the right focal length affects melting speed and the maximum workable thickness.

Assist gas dictates focus position:

Oxygen (exothermic) — focus right on the surface or above it. Switching between high- and low-pressure cutting requires only minor focus adjustments — the focus is always at or near the surface and largely independent of material thickness.
Nitrogen (endothermic) — focus position depends strongly on material thickness, because the focus sits near the bottom.

Precise focus position can be maintained via CNC and an autofocus device — for example, an adaptive mirror.

An adaptive mirror changes the shape of its surface by applying pressure to the back of the mirror. In its normal state (no pressure applied), the mirror surface is concave. As pressure rises, the surface becomes flat, then convex. Changing the mirror shape changes the wavefront of the beam, the beam size on the lens, and the focus projection inside the material.

A key benefit of autofocus is the ability to dynamically change focus position during piercing — making maximum use of energy through the material thickness and reducing overall piercing time.

Advances in laser cutting continue, but the basics remain: deliver the raw beam to the lens correctly and maintain the correct focus position for the application. If focus position and bead geometry are maintained inside the material, all other requirements for a stable, high-quality cut are minimised. This saves setup time while keeping throughput stable.

Why a focusing lens is needed in a cutting laser

The focusing optics gather the laser beam into a single spot through the nozzle. They can be either a fused-silica lens or a parabolic mirror.

The focusing (gathering) lens must be installed correctly: the outward-curving surface must always face the apex of the converging beam.

A contaminated focusing lens absorbs more laser radiation, heats up, and deforms — shifting the focal position upward.

Important: heavy contamination can damage both the focusing lens and the entire cutting head.

Effects of contamination:

As cut length grows, burrs start to form; kerf width and surface roughness increase.
On carbon steel, a tendency to form craters.
In extreme cases, the part will not separate from the sheet after processing.

Focal length

Cutting optics are typically supplied with focal lengths of 125 mm and 150 mm.

125 mm optics suit only thin sheets, 1–3 mm.
For thicker material, 150 mm optics are used.

125 mm optics produce a narrower kerf than 150 mm optics, giving higher energy density at the same laser power. So cutting speeds with 125 mm optics are slightly higher at the same thickness and power. If you mostly cut thin material, 125 mm optics are the more economical choice.

The advantage of 150 mm optics is greater cutting depth. They can be used across a wide range of thicknesses but are primarily applied to thicker material.

Focus position

Precise focus position is a prerequisite for good cutting results.

For carbon steel laser cutting:

Sheets up to ~6 mm — optimal focus on the sheet surface (endothermic).
Sheets 8 mm and above — focus above the sheet surface (exothermic).
High-pressure cutting of stainless steel or aluminum — focus on the sheet.
Rule of thumb: focus can be set at roughly 2/3 of the sheet thickness within the sheet.

So every change in sheet thickness usually means a change in focus position.

Nozzle centring

The focusing lens must be installed so that the focused laser beam is in the centre of the nozzle bore. The focused beam may be no more than ±0.05 mm off-centre relative to the nozzle.

If the beam is off-centre, cut quality becomes direction-dependent — even with good overall quality. In the worst case, the cut is satisfactory in one direction but in others the material is not cleanly severed, or not severed at all. During gas-assisted carbon-steel cutting, sparks may appear on the sheet surface when the cut moves in a direction opposite to the eccentricity.