Patent ID: 6956807

Claim:
An optical head for scanning a first optical record carrier including a first information layer and a first transparent layer having a first thickness and for scanning a second optical record carrier including a second information layer and a second transparent layer having a second thickness different from the first thickness, the head including a radiation source for generating a first radiation beam having a first wavelength and a second radiation beam having a second wavelength different from the first wavelength, an optical system for converging the first radiation beam with a first numerical aperture through the first transparent layer to a focus on the first information layer and for converging the second radiation beam with a second numerical aperture through the second transparent layer to a focus on the second information layer, the first numerical aperture being larger than the second numerical aperture, the optical system including a non-periodic phase structure for compensating the difference in spherical aberration due to the first and second transparent layer, the phase structure including a plurality of concentric areas forming a pattern of optical paths, the pattern of optical paths forming a wavefront deviation that globally approximates a flat wavefront deviation for the first wavelength and the difference in spherical aberration for the second wavelength, wherein the phase structure introduces a discontinuity in the gradient of the wavefront of the second radiation beam at the second numerical aperture; and wherein a first one of the plurality of concentric areas has a slanting surface and a second and third ones of the plurality of concentric areas have level surfaces, the first concentric area being positioned radially outward from the second and third concentric areas.