What Is a Beam Expander?

Jo Dunaway

A light or laser beam expander is a scientific instrument that allows parallel light or laser rays to have an input beam expanded to become a larger output beam. The instrument is used in a fashion similar to using a telescope and produces straight line telescopic rays or prismatic rays, such as the rays one can see as light is reflected off the facets of a crystal. Beam expanders are used in laser physics and nearly a dozen scientific applications that use their output rays for measurements, such as laser micro-machining, the slicing of solar cells, remote sensing, and other scientific experimentation in several fields. Their beam magnification, without affecting chromatics and purposely avoiding focus, allows applications from the smallest, as in microscopes, to the largest of astronomy measurements. Developed from established telescope optics, they have high transmission and low distortion.

The lower power beam expanders are built on the Galileo telescope design.
The lower power beam expanders are built on the Galileo telescope design.

The features available in most beam expanders are for standard entrance apertures and can preserve accurate columns of light regardless of wavelength. The expanders can handle light from the ultraviolet spectrum through all of the visible regions and into infrared regions, and they can reduce the amount of length necessary in a telescope. They are designed for both variable and fixed output configurations with column adjustment controls.

For a little background, optical telescopes are either refractory or reflecting. The refracting telescopes refract light by means of lenses that bend or refract light, whereas the reflecting telescopes use large optical mirrors to reflect light. A beam expander is essentially a telescope with the principle that the beam divergence and beam expansion ratios are of the same factor. The lower power beam expanders are built on the Galileo telescope design with a negative input and positive output set of lenses. There are Kepler telescope designs available, however, which have an intermediate, pinhole, focusing lens and two positive lenses that are very long, telescoping, beam expanders.

Designs for laser beam expanders produce placements of image lenses and objective lenses that are the opposite of their placement within a Kepler telescope. The input columned beam is focused to a spot between the lenses where laser heat accumulates and heats the air leading to wavefront distortions, therefore, the Galilean design is often preferred to prevent distortion. As a laser beam expander will magnify the laser input by a set expansion power, it will decrease the divergence of the beam on output by the same power, and at a great distance, the columned beam will be smaller.

What are called hybrid extra-cavity optical designs in beam expanders follow up the standard beam expander with a convex lens, shaped like the curvature of a human eye, that produces a multiple prismatic effect. These expanded beams can be beamed to very long distances and yet appear very thin when viewed from an angle. These line illuminations are used in interferometry procedures to make measurements in optical and engineering metrology, and are also used in nuclear, particle and plasma physics.

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