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Beam Collimators

Author: the photonics expert Dr. Rüdiger Paschotta

Definition: devices for collimating optical beams

Categories: article belongs to category general optics general optics, article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/az7 Cite the article: BibTex plain text HTML Link to this page LinkedIn

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:

optical fibers – see the article on fiber collimators
laser diodes of different kinds, e.g. single-mode emitters, broad-area laser diodes and devices such as diode bars (diode arrays) with multiple emitters – see the article on laser diode collimators
other waveguide devices, for example in integrated optics
free-space laser sources, e.g. diode-pumped solid-state lasers

collimated beam — Figure 1: Beam radius versus position in a set up where a tightly focused beam is collimated with a lens, generating a mild focus somewhat behind it.

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.

Typical Requirements

The requirements on a beam collimator can be very different, depending on the light source with which it is used:

fiber collimation — Figure 2: Fiber collimator with an aspheric doublet lens, not including the mechanical mount.

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.

More to Learn

Encyclopedia articles:

Suppliers

The RP Photonics Buyer's Guide contains 60 suppliers for beam collimators. Among them:

DPM Photonics

The Model 02-M010 is a three-element, air-spaced anastigmat designed specifically for collimating the output of large diameter silica fibers used in high power medical and industrial applications. It is equally suitable for collimating the output of Large Mode Area (LMA) or Photonic Crystal (PC) fibers with smaller numerical apertures. The mechanical assembly allows a precise translation of the lens (without rotation) relative to the fiber face.

The unique design of the Model 02-M010 prevents retroreflections near the fiber face or within the core material. All elements are fused silica (the exception being the 1800–2000 nm collimator optics that are Infrasil) with either V-type or broadband coatings, depending on the operating wavelength range. When used for imaging purposes, the three-element design ensures the output mode from the fiber is preserved, without distortion, even at high throughput powers.

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