HIGH-EFFICIENCY DIFFRACTIVE WAVE-GUIDE LENSES BY PARAMETRIC OPTIMIZATION

The first-order diffraction efficiency of a waveguide diffraction grat ing is maximized, for a wide range of grating periods, by optimization of the effective-index modulation profile. Three different values of effective-index modulation in the range of 0.02-0.11 are considered. T he analysis is performed with the thin-grating-decomposition method. T he results are verified by electromagnetic grating theory and applied to the construction of diffractive waveguide lenses with an improved o verall efficiency. In the neighborhood of the optical axis, the optimi zed lens structure is a close approximation of a gradient-thickness Fr esnel lens. Significant deviations from this shape appear when the loc al grating period reduces below approximately 15-25lambda, where lambd a is the wavelength of the guided mode. Near the edges of a high-numer ical-aperture lens, where the local period is approximately 3-6lambda, an approximate Bragg grating structure is obtained.