A spatial filter is an optical device which uses the principles of Fourier Optics to alter the structure of a beam of coherent light. Spatial filtering is commonly used to remove aberrations in the beam due to imperfect, dirty or damaged optics, or due to variations in the laser gain medium itself. This can be used to produce a laser beam containing only a single transverse mode of the laser’s optical resonator.
In spatial filtering, a lens is used to focus the beam. A beam that is not a perfect plane wave will not focus to a single spot, but rather will produce a pattern of light and dark regions in the focal plane. It can be shown that this two-dimensional pattern is the two-dimensional Fourier transform of the initial beam’s transverse intensity distribution. Light in the very center of the transform pattern corresponds to a perfect, wide plane wave. Other light corresponds to “structure” in the beam, with light further from the central spot corresponding to structure with higher spatial frequency. A pattern with very fine details will produce light very far from the transform plane’s central spot. This pattern is called an Airy pattern.
By altering the distribution of light in the transform plane and using another lens to reform the collimated beam, the structure of the beam can be altered. The most common way of doing this is to place an aperture in the beam that allows the desired light to pass, while blocking light that corresponds to undesired structure in the beam. In particular, a small circular aperture or “pinhole” that passes only the central bright spot can remove nearly all fine structure from the beam, producing a smooth transverse intensity profile. With good optics and precisely measured pinhole, one could even approximate a plane wave.
The diameter of an aperture is chosen based on the focal length of the lens, the diameter and quality of the input beam, and its wavelength. If the hole is too small, the beam quality is greatly improved but the power is greatly reduced. If the hole is too large, the beam quality may not be improved as much as desired.
The size of the aperture that can be used also depends on the size and quality of the optics. To use a very small pinhole, one must use a focusing lens with a low f-number, and ideally the lens should not add significant aberrations to the beam.
A commonly used spatial filter configuration is to use a microscope objective lens for focusing the beam, and an aperture made preferably by laser drilling a small, precise, hole in a piece of metal foil. Such apertures made in a variety of sizes and materials are readily available commercially from companies, such as, Lenox Laser, Inc., the leader in microhole technology.