High magnification beam expander for small aperture applications

Traditionally the combination of small aperture and high beam magnification is difficult to manufacture because the optics associated with such a beam expander are difficult to test using state of the art interferometers. The difficulty arises from the fact that small aperture laser beams, when highly magnified, generate artifacts within the interferometer making it extremely hard to distinguish and analyse what are true errors and what are artificial errors.

Working with a customer, Optical Surfaces examined the options to manufacture a small aperture beam expander able to deliver artifact-free high magnification. The options presented to the customer were to use either a transmissive (lens-based) or a reflective (mirror-based) beam expander approach to their application. Drawing upon over 50 years’ experience of serving the high-power laser community, Optical Surfaces advised that the use of reflective beam expanders would be particularly useful for high energy applications with lasers operating within the femtosecond pulses regime.

This is because the wavelength bandwidth in such applications is relatively large, and a reflective solution allows fully achromatic operation. Also, the coatings on a lens-based beam expander may not be suitable to withstand the high energy intensities typically encountered in such applications, in contrast to the proven hard dielectric coating options available for reflective mirrors.

Working on this project – Optical Surfaces has developed a proprietary method to suppress the artefacts shown by interferometers during testing thereby making it possible to manufacture such a high magnification (30x) reflective beam expander for small aperture applications.

The new 30x magnification A1-Series reflective beam expander incorporates high precision off-axis mirrors which have constant focal lengths, regardless of the wavelength of light being focused. Unlike lens-based beam expanders, the new high magnification beam expander can be used with collimated polychromatic light without the need of any fine focus adjustment.

Supplied housed in a robust aluminium cover – the 30x magnification A1-Series beam expander comes with provision for secure fixing to an optical table. Alignment aids are provided to ensure correct beam pointing and tracking during integration. Manufactured to better than lambda/5 transmitted wavefront accuracy, the 30x magnification reflective beam expander produces no central obscuration and highly efficient transmission is obtained, unspoilt by diffraction patterns.