Patent ID: 8619534

Claim:
An apparatus for reading from or writing to a near-field optical recording medium, with a light source for generating a reading light beam and with a near-field lens and an aberration compensation element, wherein the apparatus comprises a diffractive optical element that is switchable between a far-field mode and a near-field mode and generates a main light beam and four or more sub-beams from the reading light beam for determining at least a cover layer thickness error signal, wherein the diffractive optical element has an outer region with a first grating period and an inner region having a diameter smaller than an effective numerical aperture of the near-field lens, which in the near-field mode has a second grating period and which has a switchable inner area having a diameter smaller than a far-field numerical aperture of the near-field lens, which in the far-field mode has the first grating period, and wherein the aberration compensation element is adjusted based on the thickness error signal when the diffractive optical element is in the near-field mode.