Abstract

Conventional waveguide lenses suffer from a large amount of chromatic aberration, which limits their usable wavelength range to a relatively small interval around the design wavelength. If unstable or broadband light sources are used in the system, this chromatic dispersion can significantly affect the performance of an integrated optical device. It is shown that an achromatic waveguide lens can be made with a hybrid mode-index/diffractive doublet. With this approach it is possible to cancel the longitudinal chromatic aberration of the refractive (mode-index) lens by an appropriately chosen diffractive lens element. With typical waveguide materials, a 10 mm focal length, f/5 hybrid lens can be made to have a usable wavelength range of approximately 80 nm; this is a more than order-of-magnitude improvement over conventional mode-index and diffractive lens elements. The magnitude of the wavelength range for a hybrid achromatic lens is approximately proportional to the f/number of the lens, so it is possible to get even larger wavelength ranges with correspondingly larger f/numbers. Under certain circumstances, the power of the diffractive element in the hybrid achromatic doublet can be shown to go to zero, producing an achromatic mode-index singlet.

© 1990 Optical Society of America