Beam dump (beam block)
An absorber that terminates an unused laser beam, converting its power to heat while returning as little light as possible by reflection or scatter. A cone or multi-bounce trap with 5% reflectance per surface returns about 10⁻⁴ of the light after three bounces; a 10 W beam needs roughly 200 cm² of surface to shed its heat by natural convection at a 50 K rise.
A beam dump is the component that ends a beam deliberately: the unused port of a beam splitter, the zero order of a grating, the "off" state of a digital micromirror device, the residual pump after a nonlinear crystal, or the main beam when a shutter closes. Its tasks are to absorb the power, to return almost none of it toward the optics, the detector or the laser, and to survive the heat. "Beam block" usually means a small, movable plate used for alignment and zeroing measurements; "beam dump" usually means a fixed trap built for the full power. Both are the main tools for controlling stray light from known beams.
Designs
Flat absorbers. A plate of black-anodized aluminum, absorbing glass, or a painted or coated metal, tilted so that its residual reflection goes somewhere harmless. It is adequate at milliwatt powers and for alignment. Its limits are the residual reflection, a few percent for most black surfaces, and the concentration of the heat in the beam footprint.
Cone and multi-bounce traps. The beam enters an aperture and strikes a black surface at grazing incidence inside a cone or a curved cavity, so that each reflection sends the light deeper into the trap. If each surface reflects a fraction , the light escaping after bounces is about
With = 5%, three bounces give 1.25 × 10⁻⁴ and four give 6 × 10⁻⁶, which is an optical density of roughly 4–5 for the returned light. Grazing incidence also spreads the beam footprint: on a cone of 15° half-angle, a beam parallel to the axis meets the wall at 75° incidence, and its footprint is 3.9 times larger than the beam cross-section, which lowers the surface irradiance by the same factor.
Brewster and glass traps. A plate of absorbing glass set at the Brewster angle, 56.3° for an index of 1.5, reflects essentially no p-polarized light and absorbs the transmitted beam in its bulk. For s-polarized light the same surface reflects about 15%, so the geometry must match the polarization, or a second plate must catch the reflection. Stacks of razor blades, viewed edge-on, act as an array of small wedge traps and are a common improvised dump for visible and near-infrared beams. At hundreds of watts the absorbing wall is water cooled, or the beam is first spread by a diverging element.
Thermal load
A 10 W beam 2 mm in diameter has an average irradiance of 318 W/cm². All of the power ends up as heat, and in the steady state it must leave the dump by conduction to the table, convection and radiation. Natural convection from a surface removes heat at a coefficient of order 10 W/(m²·K), so shedding 10 W at a 50 K temperature rise requires about
of surface, which is why passive dumps for watt-level beams carry fins and are bolted to the table, which acts as a heat sink. In the transient, a 100 g aluminum block absorbing 10 W warms at 0.11 K/s, so a dump that is merely warm to the touch after a short test can be much hotter after an hour. The relevant quantity for steady operation is the thermal resistance from the absorbing surface to ambient.
Pulsed beams add a second limit: the peak fluence on the surface, which can ablate coatings or crack glass even when the average power is low. Spreading the beam, using grazing incidence, or using a volume absorber reduces it.
Choosing and placing a dump
- Match the absorber to the wavelength. Many dyed black anodized surfaces that look black in the visible are much more reflective in the near infrared, and paints and plastics that absorb well can outgas or char under a focused beam.
- Place dumps where the residual reflection cannot reach a detector, a fiber or the laser itself; back-reflections into a laser are the reason optical isolators exist, and a dump should not be the source of one.
- Dump every unused beam, including weak ghost reflections from uncoated surfaces (about 4% from glass, from the Fresnel equations), which from a 10 W beam carry 0.4 W.
- For power measurements, a thermopile head is itself a calibrated beam dump; its zero must be taken with the beam blocked at a point upstream.
Common questions
What is the difference between a beam dump and a beam stop?
The terms overlap. A beam stop or block is usually a simple plate used temporarily, for alignment or to blank a beam; a beam dump is designed for continuous absorption of the full power with controlled back-reflection and scatter.
Does a beam dump need to be cooled?
Below about a watt, conduction to the optical table through the mount is usually enough. Above a few watts, the steady-state temperature rise estimated from decides whether fins, a larger contact area or water cooling are needed.
References: E. Hecht, Optics, 5th ed. (Pearson, 2017); R. P. Breault, "Control of stray light," in M. Bass (ed.), Handbook of Optics, Vol. I, 2nd ed. (McGraw-Hill, 1995); J. H. Moore, C. C. Davis, M. A. Coplan and S. C. Greer, Building Scientific Apparatus, 4th ed. (Cambridge University Press, 2009).