Patent number 8619534 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "Optical data storage is generally limited by the optical resolution of the read/write-system. Straightforward methods of increasing the optical resolution include using a shorter wavelength and a larger numerical aperture NA, at the costs of lens complexity. Further approaches are narrowing the allowable tilt margins for the optical storage media or reducing the wavelength of the scanning laser into the blue or near-UV range. A different approach for reducing the focus spot size in an optical data storage system is using near-field optics with a high numerical aperture (NA>1). This high numerical aperture is generally achieved by help of a solid immersion lens (SIL). While conventional systems like CD, DVD or BD operate in the optical far-field regime, which is described by classical optics, the aforementioned new systems work in the optical near-field regime, which is described by near-field optics. For conventional systems the working distance, i.e. the air gap between the surface of the optical storage medium and the first optical surface of the read/write-head, usually the objective lens, is in the scale of 100 .mu.m. In contrast, systems making use of near-field optics need a very small working distance or air gap, which is in the scale of 50 nm or less. The small air gap is necessary to ensure that evanescent waves may couple into optical storage medium.
"Usually the cover layer thickness of a near-field optical recording medium is not perfectly homogeneous due to the limitations of the spin coating process. Especially from the inner radius to the outer radius the thickness deviation is higher than at constant radius. Therefore, the thickness deviation has to be compensated by adjusting an optical element, e.g. a telescope or liquid crystal element. To this end WO 2005/104109 discloses a near-field optical data storage system using an objective including a solid immersion lens. The system includes means for adjusting an optical element in order to compensate for variations of the thickness of the cover layer of the near-field optical recording medium.
"It is especially required to re-adjust the optics if the pickup jumps from the inner area of the near-field optical recording medium to another area at a higher radius or vice versa. To enable this re-adjustment it is advantageous to detect spherical aberration introduced by changes of the substrate thickness and radial tilt. An advanced pickup with five beams, which is capable of detecting spherical aberration introduced by changes of the substrate thickness and radial tilt, has recently been proposed in
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "It is an object of the invention to propose an apparatus for reading from and/or writing to a near-field optical recording medium capable of detecting at least tilt and spherical aberration.
"According to the invention, an apparatus for reading from and/or writing to a near-field optical recording medium has a light source for generating a reading light beam, a near-field lens, an aberration compensation element, and a diffractive optical element. The diffractive optical element is switchable between a far-field mode and a near-field mode and is adapted to generate a main light beam and four or more sub-beams from the reading light beam for determining one or more error signals. The diffractive optical element makes it possible to switch the apparatus between two modes, namely the near-field mode and the far-field mode. For this purpose, 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. The inner region has a switchable part 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. The switchable part preferably includes a liquid crystal grating structure sandwiched between transparent electrodes. The near-field mode is used for data recording or readout. The far-field mode is used for radial movement. In both modes the diffractive optical element generates a main beam and four sub-beams, which are detected by a first photodetector with five four-element detectors. Advantageously, the apparatus further has a second photodetector with at least three detectors. With these two photodetectors a data signal (HF) and at least a cover layer thickness error signal (SE) are determined. The aberration compensation element is adapted to be adjusted based on the thickness error signal when the diffractive optical element is in the near-field mode. With the two photodetectors a plurality of additional error signals can be generated with reasonable complexity, namely a focusing error signal (FE), a tracking error signal (PP), a radial tilt error signal (RTE), a gap error signal (GES), and a tangential tilt error signal (TTE).
"Deviations of the cover layer thickness of an optical recording medium are detected in the far-field mode by detecting a cover layer thickness error signal using signals obtained from the main light beam and/or the sub beams before the apparatus goes into the near-field mode. Then the optics are adjusted, e.g. an aberration compensation element, which is adapted to compensate coma and/or spherical aberrations in response to one or more of the determined error signals. Subsequent to the adjustment the apparatus goes into the near-field mode. This has the advantage that contrary to known systems, where the optics are adjusted in the near-field mode, the focus system remains stable and the risk of contact between the near-field lens and the optical recording medium is reduced. Also, the tangential tilt detection is more sensitive than it is the case for known solutions based on the four quadrants used for detecting a gap error signal.
"According to a further aspect of the invention, a method for operating an apparatus for reading from a near-field optical recording medium has the steps of: operating a diffractive optical element of the apparatus in far-field mode by setting a grating pattern of an inner area of the diffractive optical element having a diameter smaller than a far-field numerical aperture of a near-field lens of the apparatus to a first grating pitch while setting a grating pattern of a ring area surrounding the inner area to a second grating pitch smaller than the first grating pitch; operating the near-field lens in far-field mode by adding a focus offset to the near-field lens; determining a cover layer thickness error signal from signals obtained from a main light beam and four or more sub-beams generated by the diffractive optical element; switching the diffractive optical element into near-field mode by setting a grating pattern of an inner region of the diffractive optical element having a diameter smaller than an effective numerical aperture of the near-field lens to the second grating pitch while setting a grating pattern of an outer region surrounding the inner region to the first grating pitch; switching the near-field lens into near-field mode; and using the thickness error signal to adjust an aberration compensation element once the diffractive optical element and the near-field lens have switched into the near-field mode.
"The invention proposes to detect deviations of the cover layer thickness already before the apparatus goes into the near-field mode. Only after the optics are adjusted the apparatus goes into the near-field mode. This has the advantage that the focus system remains stable and the risk of contact between the near-field lens and the optical recording medium is reduced. Of course, it is likewise possible to first switch the near-field lens into near-field mode before switching the diffractive optical element into near-field mode, or to perform both switching operations simultaneously.
"Preferably, during a jump the cover layer thickness between a start data zone and a target data zone is quickly scanned. This allows to obtain information about deviations of the cover layer thickness of the optical recording medium even before accessing specific areas of the optical recording medium.
"Favorably, the apparatus checks if the cover layer thickness in the target data zone is in spec before switching into the near-field mode. If this is not the case, the apparatus preferably rejects to access the optical recording medium or at least certain areas of a the optical recording medium."
URL and more information on this patent, see: Knappmann, Stephan; Przygodda, Frank. Apparatus for Reading from And/Or Writing to a Near-Field Optical Recording Medium. U.S. Patent Number 8619534, filed
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