How does diffraction affect the width of a laser beam?

Although laser light is highly collimated, it is not perfectly parallel; its width increases over distance due to diffraction, which scatters light at the edges of the laser's emitting aperture.1

The extent of this beam spreading is determined by the ratio of the laser's wavelength to the size of the aperture from which it is emitted. Specifically, diffraction is proportional to the wavelength divided by the aperture size. Consequently, a larger aperture results in a slower rate of beam divergence.1

While a typical laser might have a very small divergence angle—such as one milliradian for a helium-neon laser—this effect remains a fundamental physical limit on how tightly a beam can be maintained over long distances. In contrast, lasers with very small apertures, such as semiconductor lasers, experience significantly higher divergence and often require external optics to focus their beams effectively.1