Beam parameter product
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Definition: product of beam radius in a focus and far-field beam divergence
The beam parameter product (BPP) of a laser beam is defined as the product of beam radius (measured at the beam waist) and the beam divergence half-angle (measured in the far field). The usual units are mm mrad (millimeters times milliradians). The BPP is often used to specify the beam quality of a laser beam: the higher the beam parameter product, the lower the beam quality.
The BPP can also be defined for non-Gaussian beams. In that case, standard deviations should be used for the definitions of beam radius and divergence. The smallest possible beam parameter product is then achieved with a diffraction-limited Gaussian beam; it is λ/π. For example, the minimum beam parameter product of a 1064-nm beam is about 0.339 mm mrad.
A related, less frequently used quantity is the diameter-divergence product.