stråldiameter

Stråldiameter, in physics and engineering, is a measure of the cross‑sectional width of a propagating beam, such as light, particles, or other radiation. It describes how large the beam is perpendicular to its direction of travel and is used to assess focusing, coupling efficiency, and interactions with matter. Because different communities adopt different reference points, the reported diameter depends on the convention chosen.

In optics, several conventions are common. The most widely used for laser beams is the diameter defined by the 1/e^2 intensity points, i.e., the 1/e^2 diameter. The full width at half maximum (FWHM) of the intensity profile is another frequently reported measure. For focused beams, the term waist diameter or D1/e^2 is often used, with the waist w0 being the minimum beam radius at the focus (and D1/e^2 = 2w0 for a Gaussian beam).

Measurement methods include the knife-edge technique, where a sharp edge scans across the beam while transmitted power is recorded; beam profilers based on cameras or scanning slits; and, for high-energy beams, imaging on scintillating screens or using wire scanners. It is important to specify the diameter definition used, since different methods yield different numerical values for the same beam.

For Gaussian beams, propagation follows w(z) = w0 sqrt(1 + (z/zR)^2), where zR = π w0^2/λ and the far-field divergence is θ ≈ λ/(π w0). Real beams may deviate from this ideal model due to aberrations, truncation, or misalignment.

Applications span laser processing, spectroscopy, optical communications, and particle or electron-beam systems. In fiber optics, the mode field diameter (MFD) is a related concept describing the effective width of the fundamental mode, critical for coupling efficiency.