Editing Diffraction (section)

=== Propagation of a laser beam ===
The way in which the beam profile of a [[laser|laser beam]] changes as it propagates is determined by diffraction. When the entire emitted beam has a planar, spatially [[Coherence (physics)|coherent]] wave front, it approximates [[Gaussian beam]] profile and has the lowest divergence for a given diameter. The smaller the output beam, the quicker it diverges. It is possible to reduce the divergence of a laser beam by first expanding it with one [[convex lens]], and then collimating it with a second convex lens whose focal point is coincident with that of the first lens. The resulting beam has a larger diameter, and hence a lower divergence. Divergence of a laser beam may be reduced below the diffraction of a Gaussian beam or even reversed to convergence if the refractive index of the propagation media increases with the light intensity.<ref>{{cite journal | last1=Chiao|first1=R. Y. | last2=Garmire|first2=E. | last3=Townes|first3=C. H. | title=Self-Trapping of Optical Beams | journal=Physical Review Letters | date=1964 | volume=13 | issue=15 | pages=479–482 | url=http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.13.479 | bibcode = 1964PhRvL..13..479C | doi = 10.1103/PhysRevLett.13.479 | url-access=subscription }}</ref> This may result in a [[self-focusing]] effect.

When the wave front of the emitted beam has perturbations, only the transverse coherence length (where the wave front perturbation is less than 1/4 of the wavelength) should be considered as a Gaussian beam diameter when determining the divergence of the laser beam. If the transverse coherence length in the vertical direction is higher than in horizontal, the laser beam divergence will be lower in the vertical direction than in the horizontal.