Skip to content The Concise RDWorks Learning Lab Series
Session 39 examines how a laser cut develops as the beam interacts with the material. Russ uses raw-beam burns and lenses ranging from 1 inch to 7.5 inches to explore beam diameter, irradiance, exposure time, focal geometry and the shape of the resulting cavity.
The experiments are valuable because they show that a cut is not determined by nominal focal length alone. The important refinement is that the cavity shape is produced by a coupled optical and material-removal process: the focused beam geometry, material damage threshold, heat flow, decomposition or melting, vapour and gas flow all interact.
Release Date: 08th April 2022
What You Will Learn in This Session
Russ investigates how beam diameter and exposure time affect material damage, how a converging beam changes irradiance before and after the waist, and why different focal-length lenses can produce unexpectedly different cavity shapes in acrylic.
A Burn Mark Is Not a Direct Measurement of Beam Diameter
The masking-tape test is a useful way to visualise the region of the beam that exceeds the tape’s damage threshold. It does not directly reveal the full optical beam diameter. The visible boundary depends on beam power, pulse duration and the material threshold, so a longer exposure can make the apparent burn wider even though the underlying beam has not changed.
Laser beam diameter also needs a defined convention. In Gaussian-beam work, a common definition is the 1/e² intensity diameter. A burn boundary on masking tape is therefore best treated as a comparative diagnostic rather than proof that a manufacturer’s quoted optical beam diameter is wrong.
The Damage Threshold Explains Why the Visible Burn Grows with Time
Across an approximately Gaussian intensity profile, the centre receives the highest irradiance and the outer regions progressively less. With a short pulse, only the central region may receive enough energy to damage the material. Increase the exposure time and more of the lower-intensity outer region can accumulate enough energy to cross the material’s damage threshold, so the visible burn expands.
This is the same practical mechanism discussed earlier in the series: changing speed or exposure can change the width of the visible mark without changing the fundamental optical beam geometry.
The Raw Beam Can Produce a Pointed Cavity Without a Focusing Lens
Russ’s unfocused acrylic burns are useful because they demonstrate that a pointed cavity does not prove that the beam has reached an optical focus. The central region of the raw beam can simply deliver higher irradiance and therefore remove material faster than the outer region.
The exact cavity geometry, however, cannot be read as a direct picture of the intensity graph. Heat conduction, decomposition or melting, material ejection and changing cavity geometry all influence what remains after the pulse.
Hot Gas Does Not Mechanically Straighten the Light
The video uses an intuitive model in which hot gases wander sideways and the light effectively “straightens” the developing cone. That is not a literal optical mechanism. Light does not mechanically push a wandering gas channel back onto the optical axis in the way the diagram suggests.
A developing cavity is better understood as the result of where optical energy is absorbed, how quickly material heats and decomposes or melts, how vapour and melt are expelled, how gases move through the opening, and how the changing cavity walls alter subsequent energy deposition. The visible result can still straighten or become asymmetric, but several coupled processes are responsible.
The Lens Controls Irradiance Throughout the Beam Caustic
Russ correctly observes that moving down a converging beam increases irradiance as the beam becomes smaller. The important qualification is that what happens inside the material is not independent of the lens or focal region. The lens defines the beam caustic — the changing beam radius, waist and divergence along the optical axis — and therefore affects irradiance throughout the depth of the cut.
Once a kerf or cavity forms, material effects become increasingly important as well. The useful model is therefore not “focus only controls the entry hole”, but “the optical caustic sets the available irradiance distribution, while the material-removal process determines how that energy develops into a real cut”.
Gaussian Is an Idealised Reference Model
An ideal TEM00 beam has a Gaussian transverse intensity profile, and this remains an excellent teaching model. A real glass CO₂ tube is not guaranteed to be “always Gaussian”. It may operate in higher-order or asymmetric modes, and mode structure, beam diameter and M² can change with tube condition and operating point.
Likewise, increasing commanded power does not necessarily mean that an identical fixed-width Gaussian curve simply scales upwards. The diagrams in the video are conceptual illustrations of irradiance and power, not measured beam profiles.
Shorter Focal Length Does Not Automatically Mean Better Cutting
For the same wavelength, input beam and beam quality, a shorter focal-length lens can generally form a smaller theoretical waist. The trade-off is a shorter Rayleigh range and faster divergence away from that waist. A longer focal-length lens produces a larger theoretical waist but carries a relatively narrow beam over a longer distance.
That trade-off is why thick-material cutting cannot be judged only by the smallest possible surface spot. The result also depends on beam diameter at the lens, M², actual lens focal length, spherical aberration, lens form and orientation, nozzle clipping, process focus, material thickness and assist gas.
Russ’s 7.5-Inch Lens Result Is Real for His Test — Not a Universal Ranking
In this experiment the 7.5-inch lens produces a surprisingly narrow and deep cavity compared with the much shorter lenses. That is a valid experimental result for this tube, lens set, beam diameter, alignment, focal setting and acrylic sample. It does not establish a general rule that a 7.5-inch lens will outperform a 1-inch or 1.5-inch lens on every machine.
The most useful conclusion is Russ’s own experimental mindset: nominal focal length alone is not enough. If an unexpected result appears, check the actual process focus, optical quality, lens orientation, beam clipping and source beam before drawing a universal conclusion.
Flipping a Plano-Convex Lens Changes Aberration
Russ obtains a better-looking cavity in one test after reversing the plano-convex lens. That observation should remain exactly what it is: a result from his setup. For an approximately collimated input beam, the usual plano-convex orientation is curved face toward the incoming beam and plane face toward the work because this generally reduces spherical aberration.
An apparently better stationary burn after flipping a lens can be caused by the actual focal position changing, lens quality, aberration interacting with the real input beam, or the way the material responds. It does not overturn the general optical design rule.
“Ballooning” Is a Useful Observation Label
Russ uses “ballooning” to describe a widened region below a narrow entry neck. It is a useful informal visual label. Rather than assigning it to individual high-intensity rays carving the sidewalls, it is safer to treat the shape as the combined outcome of the beam caustic, absorption on the changing cavity walls, heat transfer, vapour or melt removal and gas flow.
What the Experiments Establish
- Smaller beam area at a given optical power raises irradiance.
- Higher irradiance can cause the material to cross its damage threshold more quickly.
- Exposure time changes the apparent size and depth of a burn because the material response has a threshold.
- The lens determines a three-dimensional beam caustic, not a single mathematical point that acts independently of the rest of the beam.
- Short and long focal-length lenses trade minimum waist against useful depth.
- Stationary burn shapes are useful diagnostics but do not, by themselves, prove the underlying optical mechanism.
Safety of Stationary Burn Experiments
The long stationary burns shown here create substantial heat, vapour and flame. They are diagnostic demonstrations rather than normal production techniques. Use the minimum pulse needed for a test, maintain effective extraction and appropriate airflow, keep covers and interlocks functional, and have immediate fire control available. Never leave a stationary burn or any active laser process unattended.
Key takeaway: Russ’s central cause-and-effect lesson remains useful: concentrating the same optical power into a smaller effective beam area increases irradiance and accelerates material damage. The real cutting process then combines that optical distribution with material physics, gas flow and the evolving geometry of the kerf.
Podcast Download for How Laser Cutting Happens
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Transcript for How Laser Cutting Happens
The original transcript is preserved below as the historical record of Russ’s explanation. The lesson notes above preserve his practical experimental observations while distinguishing them from conclusions that require a more complete optical and material-process model.
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Last updated August 26, 2021
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