Spatial filter

A laser beam is by definition coherent with respect to longitudinal wavelengths which is determined by the length of the cavity. But coherent light can also exhibit transverse modes. A cross section of laser beam might exhibit a combination of various modes creating an uneven intensity. The spatial filter is a device such as a pinhole used to eliminate transverse modes.

Cylindrical transverse mode patterns TEM(pl)

A pinhole selects for the central TEM₀ mode and blocks all the others.

Coherent wavefront also have transverse waves which are elimnated by a pinhole.

The result is a clean gaussian distribution of beam power across the cross section.

A gaussian power distribution

see [wikipedia.org:Gaussian_beam}