Cylindrical lenses are optical lenses which are curved only in one direction. Therefore, they focus or defocus light only in one direction, for example in the horizontal direction but not in the vertical direction. As for ordinary lenses, their focusing or defocusing behavior can be characterized with a focal length or its inverse, the dioptric power.
Usually, the surface curvature of a cylindrical lens is of spherical type, but there are also aspheric cylindrical lenses. Further, there are cylindrical gradient-index lenses, not exhibiting any surface curvature.
Cylindrical lenses can be used to obtain a beam focus of elliptical form. That can be required, for example, for feeding light through the entrance slit of a monochromator or into an acousto-optic deflector, or for conditioning pump light for a slab laser.
There are fast axis collimators for diode bars, which are essentially cylindrical lenses – often with an aspheric shape.
Cylindrical lenses cause astigmatism of a laser beam: a mismatch of focus position for both directions. Conversely, they can also used for compensating astigmatism of a beam or an optical system. For example, they may be required for collimating the output of a laser diode such that one obtains a circular non-astigmatic beam.
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