The purpose of a beam collimator is essentially to transform a strongly diverging light beam into a collimated beam, i.e., a beam where light propagates essentially only in one direction, and the beam divergence is weak.
The output beam may have its focus close to the output aperture, or a mild focus somewhat away from it (see Fig. 1).
Essentially, a beam collimator contain some kind of focusing lens, or sometimes a curved mirror, and usually some opto-mechanics for stable mounting and possibly adjustment.
Typically, beam collimators are applied in conjunction with the following types of light sources:
In many cases, the collimation is done in both directions perpendicular to the beam, but there are also beam collimators working in one direction only – for example, fast axis collimators made as rod lenses, which are attached to certain laser diodes.
The requirements on a beam collimator can be very different, depending on the light source with which it is used:
Some light sources exhibit a strong beam divergence, and it is then often necessary to use aspheric optics to avoid significant beam quality deterioration due to spherical aberrations.
In some cases, the beam is strongly asymmetric, with much stronger divergence in one direction than the other.
A simple beam collimator may then produce an elliptical beam, but one may apply additional elements such as an anamorphic prism pair for obtaining a circular beam profile.
While some collimators can be used with a fixed alignment, others need to be adjustable, at least concerning the focus, which can be modified through the distance between light source and collimation lens.
The beam pointing stability of the generated beam depends on how mechanically stable the setup is.
Note that tiny thermal drifts, for example, may cause significant changes of beam direction particularly in cases where the focal length of the collimating lens is small.
Some devices need to work with very high optical powers, so that one needs to minimize propagation losses and avoid thermal effects.
For the highest power levels, purely reflective telescopes (i.e., with curved mirrors instead of lenses) must be used.
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