Beam expander

Beam expander Definition

It is a combination of optical devices used to enhance the diameter of a laser beam or other light beams. In laser physics, the equipment functions either as an intracavity or extracavity element. It is more often denoted as “laser beam expander”. The low-cost optical instrument provides a high degree of accuracy.

Beam expander Types

The optical device is generally of the following types:

Prismatic beam expander

Several prisms are combined to form a multiple-prism beam expander to provide an expansion factor greater than 200. Simply put, the instrument expands a light beam without focusing it. In most cases, 2 to 5 prisms are placed together to give large one-dimensional beam expansion factors. Tunable lasers normally have multiple-prism laser arrangements to allow continuous tuning over a significant wavelength range. The expansion of the intracavity beam illuminates the whole width of a diffraction grating, an optical component that breaks up an electromagnetic wave into its different frequencies by scattering them at various angles. In this way, the optical instrument decreases the beam divergence and allows emission of narrow line widths. In fact, most tunable lasers have a beam expansion factor of 200. All narrow-line width liquid dye, solid-state, gas and diode lasers are installed with multiple-prism grating oscillators. The principle as well as mathematical description of multiple-prism arrays and dispersion has been stated in the multiple-prism dispersion theory. A 4 X 4 matrix form of the theory is also available. Multiple beam expansion can also be described well by the ray transfer matrix analysis, a type of ray tracing technique used in the design of some optical systems, such as lasers. Prism compressors that enable separation of the rays and shorten the duration of laser pulses follow the matrix equations. Devices equipped with multiple optical elements are highly useful in astronomy, beam condensing, remote sensing, interferometry and microscopy.

Telescopic beam expander

The optical instrument is designed in such a way that it produces telescopic rays. High transmission and low distortion are its prime features. Telescopic devices can either be refractory or reflecting. Galilean telescope, comprising of a concave and convex lens, is a common example of a refracting telescope. The concave lens serves as the eyepiece, while the convex lens becomes the objective for image formation. The lenses are situated on either side of a tube such that the focal point of the ocular lens is the same as that of the objective lens. The telescope expands the diameter of a collimated (parallel) input ray to a larger output beam and gives an erect image with a restricted field of view. As an intracavity light expander, the instrument provides two-dimensional beam expansion in laser resonators. On the other hand, a reflecting telescope uses a single or combination of convex and concave mirrors to reflect light, and form an image. The mirror-based expanders are used with a collimated polychromatic light without the need of any fine focus adjustment.

Variable zoom beam expanders

The special laser expander allows continuous expansion selection from 1X to 4X or 2X to 8X. It is an ideal instrument to compensate for varying optical path lengths. Here, fused silica lenses are used to minimize damage.

Extra-cavity beam expanders

This is a hybrid, elongated beam transformer that comprises of the following parts:

  • Telescopic beam expander
  • Convex lens
  • Multiple-prism beam expander
  • Laser beam

The resultant beam is able to travel long distances with a near one-dimensional or line cross section. The line illuminations are highly applicable in microdensitometry, N-slit interferometry, metrology, and nuclear physics.