Beam dump
Absorbs incident rays.
Open in the canvas →In the real world
A beam dump ends a beam. Every optical setup produces light that has done its job — the unused port of a beamsplitter, the rejected polarization, the zeroth order off a grating, the beam left over when an experiment is realigned — and all of it has to stop somewhere deliberate. Left alone it lands on a wall, a colleague, or back in the laser.
Doing that well is harder than it sounds, because "absorbing" light is really converting it to heat while reflecting as little as possible. The usual design is geometric rather than material: a cone, a wedge, or a stack of angled vanes, anodised matte black, arranged so that any light not absorbed on first contact reflects deeper into the cavity rather than back out. Several bounces at a few per cent reflectivity each leave a negligible fraction escaping. The black surface does the absorbing; the geometry catches what the surface misses.
Why high-power dumps need cooling
A dump absorbs essentially the entire beam, so it receives the laser's full average power as heat in a small volume. That is a genuine thermal engineering problem, and it sets how a dump is built:
- Up to a few watts, a black-anodised aluminium cone with fins radiates and convects the heat away passively.
- From tens of watts, passive cooling stops keeping up and the dump needs forced air or a substantial heat sink.
- At hundreds of watts and above — industrial and materials-processing lasers — dumps are water-cooled, with flow interlocks that shut the laser down if circulation fails.
Exceeding a dump's rating is not a small mistake. The anodised layer can burn away, destroying the absorption it was providing and releasing particulates; absorbing glass can crack from thermal shock; and a dump that starts reflecting is worse than no dump at all, because nobody is expecting a beam to come back out of it. Ultrafast lasers add a second constraint: a femtosecond pulse train of modest average power carries enormous peak intensity, and can ablate an absorber that would handle the same average power from a CW source without complaint. Dumps are rated for both.
Safety practice around beam blocks
Beam dumps are the most basic piece of laser safety hardware on a bench, and they work only as part of a wider practice:
- Terminate every beam, including the ones you did not plan. An uncoated glass surface reflects about 4% per face at normal incidence, so every window, sample, and filter throws off stray beams. Those are what actually reach people's eyes; the main beam is usually the one everybody is watching.
- Keep every beam in one horizontal plane, well below seated eye level, and never raise your eyes to that plane. Most accidents happen when someone bends down to look at something.
- Remove watches, rings, and badges before working near an open beam. A polished surface at an unlucky angle is an unplanned mirror.
- Wear eyewear matched to both wavelength and optical density. Goggles that block 1064 nm may transmit 532 nm freely — a real hazard in multi-wavelength setups such as a two-colour Raman microscope, where the pump, Stokes, and generated signal are all different colours.
- Never look along a beam axis, even attenuated. Use a viewing card, a fluorescent target, or an IR viewer.
- Enclose the beam path where you can, and use interlocks and warning signage where you cannot.
- For Class 4 lasers, remember that even diffuse reflections can be hazardous to eyes and skin, and that the beam is a credible ignition source for paper, cloth, and solvents.
None of this is modelled by a ray tracer, and a sketch that looks tidy on screen can still describe a setup that is unsafe to build. Treat a drawing as a plan, not a risk assessment.
In OpticalSetup
The dump is drawn as a closed body whose faces are all absorbing, so any ray that reaches it from any direction stops there and is removed from the trace. Nothing is transmitted, nothing is reflected, and no ray continues past it. The only control is the clear aperture, which sets how large a target it presents.
Its practical use here is the same as on a bench: give the unused ports somewhere to end. Put one on the second output of a beamsplitter, on the rejected port of a polarizing beamsplitter, or on an unwanted diffraction order from a grating, and the figure stops showing a beam wandering off into empty space. It makes a diagram read as a deliberate design rather than an unfinished one, and it is what a reviewer of your figure will look for.
Because a dumped ray is removed rather than attenuated, a dump is also a clean way to isolate one branch of a setup while you study another — block one arm of an interferometer and the remaining path is all that is traced.
Absorption is total and perfect: there is no residual reflectivity, no wavelength dependence, and no angular limit, whereas a real dump reflects a small fraction and does so more at grazing incidence. Nothing thermal is modelled at all — no absorbed power, no temperature rise, no damage threshold, and no warning when a sketch dumps a kilowatt into a component that could not survive it. The dump's rating and its cooling requirement are entirely the designer's responsibility, and the section above is the only place this tool addresses them.