Editing Diffraction-limited system (section)

==Laser beams==

The limits on focusing or collimating a laser beam are very similar to the limits on imaging with a microscope or telescope. The only difference is that laser beams are typically soft-edged beams. This non-uniformity in light distribution leads to a coefficient slightly different from the 1.22 value familiar in imaging. However, the scaling with wavelength and aperture is exactly the same.

The beam quality of a laser beam is characterized by how well its propagation matches an ideal [[Gaussian beam]] at the same wavelength. The beam quality factor [[M squared]] (M<sup>2</sup>) is found by measuring the size of the beam at its waist, and its divergence far from the waist, and taking the product of the two, known as the [[beam parameter product]]. The ratio of this measured beam parameter product to that of the ideal is defined as M<sup>2</sup>, so that M<sup>2</sup>=1 describes an ideal beam. The M<sup>2</sup> value of a beam is conserved when it is transformed by diffraction-limited optics.

The outputs of many low and moderately powered lasers have M<sup>2</sup> values of 1.2 or less, and are essentially diffraction-limited.