Laser Beam Focusing

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Introduction

Different manufacturers prioritize different parameters in their laser cutting heads. Some emphasize the smallest possible spot size, others put maximum effort into lens orientation and the perpendicularity of the lens axis to the laser beam. In reality, neither parameter can be singled out as more important than the other — their importance rises under different cutting conditions. Maintaining the focus position inside the material is critical for laser cutting parameters to work correctly and to deliver consistently high edge quality.

Focusing the laser beam for cutting thick metal

When working with sheets thicker than 20 mm, a larger melt zone is essential to create a bigger melt pool that must be ejected during cutting. To create this enlarged burn-through spot, the beam is focused either above or below the material surface, depending on the assist gas. As a result, focusing a small spot on the surface is usually a less successful approach for thicker material.

Focusing the laser beam for cutting thin metal

For thin sheets of 1–3 mm thickness, a spot focused on the metal surface is required. It is far more effective than a larger spot, since no wide channel is needed to evacuate 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 related to accelerating the combustion process, evacuating molten material, or both.

In laser cutting, assist gases support one of two reactions: exothermic or endothermic. Focusing rules depend on the reaction type and the gas used.

Focusing for exothermic cutting

In an exothermic reaction, the gas gives cutting accelerating properties — for example, oxygen. In this reaction the metal literally boils: the intense laser beam energy vaporizes it, and oxygen reacts efficiently with the metal in its molten state. The process runs under high pressure; with oxygen, the base material is heated to the highest temperature during cutting, producing metal vapor and further evaporation.

Cutting thick sheets implies a larger burn-through pattern — used in production to create a wide kerf that evacuates molten material during laser cutting.

Focusing rules for exothermic reactions: the focus must be above the surface for thick materials, or on the top surface for thinner ones.

When the focus is above the material, low pressure and low flow are typically used to promote liquefaction and then displace the melt. Very little material actually evaporates, because a small oxygen flow cannot sustain full evaporation.
When the focus is exactly on the material surface, high pressure and high flow are typically used. This is enough to sustain intensive material evaporation.

That is why on cutting tables that mostly cut thin material you see very little slag accumulated on the supports. Tables used for thicker materials accumulate far more material on their supports.

Focusing for endothermic cutting

Endothermic reactions occur when an inert gas is used. Nitrogen and argon fall into this category.

During this type of reaction, the gas only supports blowing molten material out through the kerf. The endothermic process depends heavily on the original energy of the focused laser beam to rapidly bring the base metal to its molten state and form a clean kerf. This lets the inert gas displace the liquefied material through the cut channel, leaving a clean cut surface without dross adhesion.

Focusing rules for endothermic reactions: the focus position must be at the bottom of the material or slightly below it. Keeping the focus below the material creates a small V-shape inside the kerf cross-section, allowing the high-pressure gas to compress the molten material and expel it through the base of the channel at high speed.

Endothermic reactions require high volume and high pressure to support rapid removal of molten material.

Compressed air

Using compressed air as assist gas actually produces both endothermic and exothermic reactions simultaneously. Since air is composed mostly of nitrogen (≈78 %), it is primarily an endothermic reaction; the small oxygen content (≈20 %) causes a simultaneous but smaller exothermic reaction. This leads to faster melting of the base material due to oxygen reactivity. The rest of the air is essentially inert and participates only in the endothermic reaction driven by nitrogen.

Compressed air cutting delivers the best results when the focus position is held at the center of the material thickness.

Controlling the focus point

Every aspect of maintaining the correct focus point projection must be controlled. The raw beam inside the optical resonator must be in good condition, and the beam must be delivered correctly to the lens.

Using a lens of the right focal length changes the material melting rate and the maximum workable thickness.

The choice of assist gas largely defines how the focus position should be set inside the material:

Laser cutting with oxygen (exothermic) — focus directly on or above the material surface. Very small focus changes are needed unless you switch between high-pressure and low-pressure cutting, because the focus always sits on or near the material surface and therefore does not depend on thickness changes.
Laser cutting with nitrogen (endothermic) — focus is strongly thickness-dependent, since the focus sits at or near the bottom of the material.

In any case, all key focus points can be served by the CNC and an autofocus device such as an adaptive mirror.

Adaptive mirror

An adaptive mirror works by changing the shape of its surface through pressure applied to its back side. In its normal state, with no applied pressure, the mirror surface is concave. When pressure is applied, the surface goes from concave to flat and then to convex. Changing the mirror shape changes the beam wavefront and therefore the beam size on the lens and the focus projection location inside the material.

Lens focal length

For cutting, optical systems with 125 mm and 150 mm focal lengths are typically used.

125 mm optics are suitable only for thin thicknesses of 1–3 mm. The 125 mm optic produces a narrower kerf compared to the 150 mm optic, giving higher energy density at the same laser power. Therefore, cutting speeds for the 125 mm optic are slightly higher at the same material thickness and laser power. If you mostly cut thin materials, the 125 mm optic is recommended for cost reasons.
150 mm optics offer greater cutting depth. They can be used universally for a wide thickness range but are mainly used for thicker materials.

Focus position

Precise focus location is a key requirement for good cutting results.

For carbon steel laser cutting:

For sheets up to roughly 6 mm, the optimum focus position is on the sheet surface (endothermic).
For sheets 8 mm and thicker, the focus point must be above the sheet surface (exothermic).
High-pressure cutting of stainless steel or aluminum: focus on the sheet.

In practice, the focus position can be set roughly at 2/3 of the sheet thickness inside the sheet.

Consequently, every change in plate thickness usually means a change of focus position.

Nozzle centering

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

Even with good cutting quality, an off-center beam can make cut quality direction-dependent. In the worst case, the cut is satisfactory in one direction while in others the material is not cut cleanly or even not separated at all.

When cutting carbon steel with gas, sparks may appear on the sheet surface in the direction opposite to the eccentricity.

Lens contamination

Important: heavy contamination can damage the lenses and the entire cutting head.

Effects:

As the cut length grows, burrs begin to form; kerf and surface roughness increase.
In carbon steel there is a tendency for crater formation.
In the worst case, the part will not separate from the sheet after processing.

Maintenance and replacement of the focusing lens

Removing and installing the focusing lens:

Remove the cutting head and move it to a clean place. Clean all dust from the surface of the cutting head.
Position the cutting head horizontally. Remove the locking screws from bottom to top (see Fig. 6.5 — disassembly of the protective glass and nozzle assembly).
Remove the spring retaining ring and the focusing lens using the lens removal wrench.
Replace or clean the focusing lens.
In the direction shown in Fig. 6.7, carefully insert the focusing lens and the spring retaining ring into the lens holder and properly tighten the retaining ring.

Practical part: setting focus on a fiber laser

Before starting work on a fiber laser machine, you must set the correct focal distance between the cutting head and the material being cut. Kerf width, cut quality, dross formation, and cutting speed all depend on correct focus setup.

Focusing the laser beam is a key aspect of operating a laser machine. Quality metal cutting requires concentrating the energy at a specific point — this increases the laser beam intensity.

Spot diameter and depth of focus depend on the focal length.

Focus can be set: above the metal, on the metal, or below the metal.

Positive focus — the photon flux is above the workpiece plane. Used when cutting carbon steel. Dross formation is prevented, and oxygen promotes metal oxidation from the cut edge down to the bottom surface of the sheet. As positive focus increases, the spot diameter on the workpiece surface grows, leading to more heat input and a smoother steel edge along the cut.
Negative focus — the concentration peak is inside the workpiece, the radiated energy density rises, and the kerf widens. This mode suits stainless steel. The kerf is wider at the top and narrower at the bottom; the larger top amplitude improves melt flow. However, the narrower bottom requires higher gas flow. Negative defocus is typically used with air or nitrogen cutting.
Zero focus — focus on the plate surface gives the smallest possible spot. This produces a relatively narrow melt range and a smaller kerf, making it suitable for high-precision cutting of thin materials.

Understanding and controlling the focus position is critical for optimizing laser cutting operations, as it directly affects beam intensity in the kerf zone, kerf width, and overall cut quality.

On the plot of focus position (Z) versus the upper kerf width (W): when focus sits on the plate surface, kerf width is at its narrowest. As focus moves (positive or negative defocus), kerf width grows.

The amount of kerf widening depends on the cutting head lens focal length and the depth of focus. In general, the shorter the focal length and the smaller the depth of focus, the more the kerf width varies with focus position.

The thicker the metal, the higher the focus must be set.

Collimator and focusing lenses

The focus point is formed by the collimator and focusing lenses. The collimator lens captures the rapidly diverging beam coming out of the fiber and straightens it; the focusing lens then focuses that radiation onto the work surface.

Lens problems

For fiber lasers, the collimator and focusing lens are very rarely the source of focus problems. The materials for so-called "transmissive" laser optics have been known for a long time, and their manufacturing technologies are well developed. Still, one occasional defect of focusing lenses is the thermal lens effect — focus drift due to optics heating, caused by the temperature dependence of the lens material absorption coefficient. The effect can be observed with lens contamination or damage; always buy lenses from trusted optics manufacturers. Before drawing conclusions about the optics, perform nozzle centering. The protective window is the first to suffer at pierce.
Equally important is the perpendicularity of the beam hitting the lens. A non-perpendicular beam can destroy the nozzle and produce edge defects.
The focal plane is, by definition, the plane on which the focused laser spot has its minimum size.
What happens if the focal plane position on the sheet is sub-optimal? In the worst case — dross and a cutting failure. With a small error, the focus position affects the deviation of the kerf walls from vertical. The check is simple: just measure the part size at the bottom edge and at the top edge. Naturally, this must be the same location on the part.
What do operators typically do when faced with edge quality degradation? Usually they reduce the cutting speed until edge quality is acceptable. The result is not always desirable: quality usually improves but remains sub-optimal, while machine productivity drops due to the lower speed.
The focus position is most often the primary cause of quality degradation — and it is exactly what operators forget. When they do check, it is often with too coarse a refocusing step, missing the optimum, or non-systematically — moving the lens too high then too low, losing track of where the cut sheet sits within the focused beam.

Algorithm for finding the correct focus position

Finding the correct focus position should not take long. Follow a simple algorithm:

Pick a refocusing step. Not too big, so you don't miss the optimum, and not too small, to avoid wasting time.
Cut 12 test parts. The optimum shape is a small square. You can also just make straight cuts on the sheet.
Number the parts or lines and write next to each the focus position used.
Inspect the cut from the laser-incidence side and from below and pick the run with the best line or edge appearance — minimum heat-affected zone, no droplets, no dross.
If quality has improved but is still not optimal, lock the focus position you found and run similar procedures for each parameter separately: nozzle-to-sheet distance, gas pressure, cutting speed, laser power. Each time you change a parameter, move from one extreme to the other in single-step increments. This keeps you from getting lost among the many parameters you are optimizing for your specific process.

The technology parameter set you find will not stay the same forever. If any one parameter changes, you will need to revisit the others.

Gas and material influence on focus

Focus position can change both when changing material and when changing gas. Keep this in mind, for example, when switching to oxygen of a different purity — and especially when switching between oxygen and nitrogen.

When cutting with inert nitrogen, the focus must be lowered into the material.
When cutting with chemically active oxygen, the focus must be on or above the sheet surface.

Edge diagnostics

Careful inspection of the edge says a lot about focus position:

Dross with sharp tips — either insufficient nitrogen flow or focus too high.
Dross with droplets — focus too low.

Sometimes shifting the focus by 100–150 microns rescues the situation.

Types of focusing lenses

Long focal length — suitable for processing thick sheets and workpieces with varying surface curvature.
Medium focal length — best for medium and thin materials.
Short focal length — best for engraving, producing sharp and clean images.

Focusing options on the cutting head

1. Manual focusing. The cutting head has a focusing mechanism, typically a rotating ring that can be turned to raise and lower the focusing lens. On the first setup or after a change in thickness and metal type, the approximate focus position must be set manually.

2. Autofocus. Premium laser cutting equipment can be equipped with an autofocus system that automatically adjusts focus position based on preset values. This significantly improves productivity and reduces human error.

Beam waist

From the focusing diagram of lenses with different focal lengths, two key facts are evident:

The longer the focal length, the larger the spot at which the radiation is focused.
The longer the focal length, the longer the beam waist. The beam waist is a certain distance from the focal plane along the beam propagation axis.

Additional recommendations

Keep lenses and mirrors clean: regularly clean and inspect the windows, since contamination affects focus.
Monitor equipment wear and condition.
Use cutting parameter tables for your laser source (ask your supplier to provide them), and verify the focal length of the lenses in your cutting head.
Verify whether your real zero matches the program zero — use the beam waist location method.