Fundamental limitations in the performance of chirped grating lenses on planar optical waveguides

The fundamental limitations in the performance of chirped grating lenses on planar optical waveguides caused by diffraction and by fabrication tolerances have been analyzed based on the generalized coupled mode theory (GCMT). The two most crucial lens parameters that affect the performance have been identified as the minimum grating periodicity and the maximum coupling coefficient. They have a strong effect on both the higher orders of diffraction and the phase distortions that eventually reduce the efficiency, enlarge the spot size and restirct the angular field of view. Both etched and deposited linear and curved grating lenses have been fabricated and evaluated on single-mode glass waveguides. Their measured performance is in good agreement with the predictions of the GCMT. High throughput efficiency (in excess of 80 percent) combined with very large angle of view (larger than 0.1 rad), as well as the performance limitations caused by diffraction have been demonstrated experimentally.