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Focal Diameter (Focal Spot)

The spot diameter is defined as the diameter in the beam waist that contains 86.5% of the total power and corresponds to the 1/e² beam diameter in the working plane for Gaussian beams.

Unit: µm

Calculation of Focal Diameter (Spot Diameter)

The focal diameter, which the below-mentioned equation approximates depends on the light source (laser wavelength and quality of the coupled laser beam) as well as the scan system’s aperture and focal length. The approximation only yields useful results for gaussian beams.

d = M2  · k(A, D) · λ · f / D

d = Focal diameter (1/e2)
λ = Wavelength (typically 193 nm – 10.6 μm)
f = Focal length (typically 30 mm – 2,000 mm)
M2 = Beam quality (laser-dependent)
k = Correction factor, depends on A and D (ideally 1.27; more typically between 1.5 and 2.0)
A = Aperture of the scan system
D = Beam diameter prior to focusing (typically 6 – 70 mm)