Achromatic waveguide lenses.

An investigation of the chromatic properties of waveguide lenses is described. In general, the focal length of mode-index and Fresnel zone (diffractive) lenses will be a function of wavelength. As a result, these lenses will have high optical quality over only a relatively small wavelength range. A method is given to correct for this chromatic dispersion by forming a hybrid mode-index/diffractive doublet. Using this approach, the various lens parameters can be chosen so that the chromatic dispersion of the diffractive element will cancel the dispersion of the refractive element. In certain conditions, the power of the diffractive element in the hybrid achromatic doublet can be shown to go to zero producing an achromatic mode-index lens. It was found that, with typical waveguide materials, a 10-mm focal length, f/5 hybrid mode-index/diffractive lens can be made that has a usable wavelength range of approximately 80 nm. This is over an order of magnitude improvement compared with that obtained with conventional mode-index and diffractive lens types.