Spatial filter

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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.

<swf width="500" height="400">images/7/71/Z-scanclean.swf</swf>
The result is a clean gaussian distribution of beam power across the cross section. This is desirable for many optical setups such as the Femtosecond Z-Scan Spectrometer

A gaussian power distribution

see wikipedia:Gaussian_beam