Beam expanders are optical devices that take a collimated beam of light and expand its size (or, used in reverse, reduce its size).

In laser physics, they are used either as intracavity or extracavity elements. They can be telescopic in nature or prismatic. Generally, prismatic beam expanders use several prisms and are known as multiple-prism beam expanders.

Telescopic beam expanders include refracting and reflective telescopes. A refracting telescope commonly used is the Galilean telescope which can function as a simple beam expander for collimated light. When used as intracavity beam expanders, in laser resonators, these telescopes provide two-dimensional beam expansion in the 20–50 range.

In tunable laser resonators, intracavity beam expansion usually illuminates the whole width of a diffraction grating. Thus beam expansion reduces the beam divergence and enables the emission of very narrow linewidths which is a desired feature for many analytical applications including laser spectroscopy

Multiple-prism beam expanders

Long-pulse tunable laser oscillator utilizing a multiple-prism beam expander.

Multiple-prism beam expanders usually deploy two to five prism to yield large one-dimensional beam expansion factors. Designs applicable to tunable lasers with beam expansion factors of up to 200 have been disclosed in the literature. Initially, multiple-prism grating configurations were introduced in narrow-linewidth liquid dye lasers but eventually were also adopted in gas, solid-state, and diode laser designs. The generalized mathematical description of multiple-prism beam expanders, introduced by Duarte, is known as the multiple-prism dispersion theory.

Multiple-prism beam expanders and arrays can also be described using ray transfer matrices. The multiple-prism dispersion theory is also available in 4 X 4 matrix form. These matrix equations are applicable either to prism pulse compressors or multiple-prism beam expanders.

Extra-cavity beam shaping

Extra-cavity hybrid beam transformers: using a telescopic beam expander, followed by a convex lens, followed by a multiple-prism beam expander, a laser beam (with a circular cross-section) can be transformed into an extremely elongated beam, in the plane of propagation, while extremely thin in the orthogonal plane. The resulting plane illumination, with a near one-dimensional (or line) cross-section, eliminates the need for point-by-point scanning and has become important for applications such as N-slit interferometry, microdensitometer, and microscopy. This type of illumination can also be known in the literature as light sheet illumination or selective plane illumination.