Patent ID: 6928043

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 scoond 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, the second radiation beam including a central sub-beam and an outer sub-beam, an optical system for converging the first radiation beam through the first transparent layer to a focus on the first information layer and for converging the second radiation beam through the second transparent layer to a focus on the second information layer, and a detection system for receiving radiation of the first and second radiation beam from the information layer and including a photo-sensitive area arranged in a detection plane; the optical system including an optical element having a non-periodic phase structure, the phase structure including a plurality of concentric areas inducing a wavefront deviation and inducing a wavefront deviation in the central sub-beam that compensates the difference in spherical aberration due to the first and second transparent layer, wherein the phase structure introduces a defocus in the central sub-beam, wherein the optical element is transparent for the first radiation beam, the central sub-beam and the outer sub-beam, and that the wavefront deviation induced in the second radiation beam is such that, when the focus of the central sub-beam is located on the second information layer, the radiation of the central sub-beam and the outer sub-beam form a central intensity distribution and an outer intensity distribution, respectively, in the detection plane, the central intensity distribution and the outer intensity distribution being separated by a substantially dark area, and the photo-sensitive area captures radiation of substantially only the central distribution.