The present invention relates to a method and a device for determining spherical aberration occurring during reading from and/or writing to optical recording media, and to an apparatus for reading from and/or writing to optical recording media using such method or device.
Spherical aberration occurs in optical pickups during reading from and/or writing to optical recording media, when the thickness of the cover-layer of the recording medium is not optimal for the objective lens of the optical pickup. In general objective lenses are only well corrected for a single cover-layer thickness. However, variations of the thickness of the cover-layer occur due to the manufacturing process of the recording medium, and, which is more important, in multi-layer optical recording media systems.
Currently proposed methods for correcting the spherical aberration are telescopes or liquid crystal elements. For correctly setting these elements a control signal has to be provided by the optical pickup, which gives information about the current amount of spherical aberration. For generating such a control signal the amount of spherical aberration has to be detected.
US patent application US 2002/0057359 discloses an aberration detection device which detects spherical aberration by separating a light beam appropriately such as to enlarge a difference in the positions of minimum spot diameter of the separated light beams. The light beams are separated by a hologram having two regions divided by a boundary corresponding to an extreme value of a curve representing a wave front when the light beam has a minimum beam diameter. The separated beams are focused onto two detection devices. One detection device is located closer to the hologram than the focus position of a first-order diffracted light of the hologram, while the second detection device is located farther from the hologram. By measuring the spot size of the separated beams the spherical aberration is detected.
It is an object of the invention to propose a further method for determining spherical aberration in a light beam.
According to the invention, this object is achieved by a method including the steps of:
The splitting of the light beam into partial light beams is performed such that the position of at least one of the partial light beams depends on the amount of spherical aberration of the light beam. Therefore, by determining the position of this partial light beam, the spherical aberration is measured. Preferably, only a small part of the light beam is directed onto the detectors for measuring the spherical aberration, while the main part is used for focusing and tracking and for reading the data from the optical recording medium.
Favourably, the step of determining the spherical aberration using the signals generated by the detectors includes determining the slope of the wavefront of a selected region of the light beam. The slope of the wavefront is a measure for the spherical aberration. Furthermore, since the direction of propagation of the partial light beam corresponding to the selected region is perpendicular to the slope of the wavefront, this slope has a direct influence on the position of the partial light beam. Measuring the position of the partial light beam, therefore, allows to determine the slope of the wavefront of the partial light beam, which in turn allows to determine the spherical aberration. Of course, other methods for determining the slope of the wavefront can also be employed.
Advantageously, a special beam splitter including two diverting elements is provided for splitting the light beam into the partial light beams. Favourably, the two diverting elements are prisms, which are glued to a glass plate such that they direct parts of the outer part of the light beam onto the respective detectors. In this way the special beam splitter is realized at low cost. Alternatively, the diverting elements are mirrors or gratings.
Preferably the diverting elements and the respective detectors are arranged symmetrically to the optical axis of the light beam. This symmetry of the arrangement allows to tolerate small lateral movements of the optical components such as the detectors, a laser diode, a focusing lens or the like. The system, therefore, becomes more reliable.
Favourably the respective detectors are position-sensitive two-quadrant detectors. These detectors allow to determine the positions of the partial light beams in a very convenient way and at low cost.
Advantageously, a normalized difference signal is generated from the signals generated by the detectors for determining the slope of the selected region of the light beam. The normalization allows to take into account the variations of the energy of the partial light beams caused by differences in the reflectivity of the recording medium, or by data stored on the recording medium. By determining the difference signal not only the presence or absence of spherical aberration is detected, but also the sign of the aberration is determined.
According to a further aspect of the invention, a hologram is provided for splitting the light beam into the partial light beams. The hologram can, for example, direct parts of the outer part of the light beam onto the respective detectors. Alternatively, a plurality of wavefront patterns are stored in the hologram. In this way the wavefront of the incoming light beam is compared with the plurality of stored wavefronts. The splitting of the light beam into a plurality of partial light beams then depends on the grade of similarity of the wavefront of the incoming light beam with one of more of the stored wavefronts.
Favourably, the partial beams are focused onto the respective detectors in dependence on the amount of spherical aberration in the light beam. For example, if the wavefront has a positive spherical aberration, a part of the light beam is directed onto a first detector. If the wavefront has no spherical aberration, a part of the light beam is directed onto a second detector. If, however, the wavefront has a negative spherical aberration, a part of the light beam is directed onto a third detector. In practice more than one detector receives a partial light beam from the hologram. The amount of spherical aberration is then determined from the amount of energy in each partial light beam.
According to another aspect of the invention, the object of the invention is achieved by a device for determining spherical aberration in a light beam, including:
According to yet another aspect of the invention, the object of the invention is achieved by a device for determining spherical aberration in a light beam, including:
Favourably, an apparatus for reading from and/or writing to optical recording media uses a method or comprises a device according to the invention for determining spherical aberration in a light beam.
For a better understanding of the invention, an exemplary embodiment is specified in the following description with reference to the figures. It is understood that the invention is not limited to this exemplary embodiment and that specified features can also expediently be combined and/or modified without departing from the scope of the present invention. In the figures:
For reading from an optical recording medium generally a light beam is focused on a data layer of the recording medium. The light beam reflected from the recording medium is then modulated by the data stored in the data layer, which allows to recover the stored data. When the thickness of the substrate of the recording medium is constant over the whole recording medium, the light reflected from the recording medium has a nearly flat phase profile. If, however, there are deviations in the thickness of the substrate, this wavefront assumes a donut shaped profile with spatially varying phase gradients. The phase distribution of such a wavefront is depicted in
SA=0.5*([(A−B)/(A+B)]+[(D−C)/(D+C)]).
In principle this measurement method corresponds to a very simple Shack-Hartmann wavefront sensor. Due to the design of the beam splitter 9 and the positioning of the detectors 12, 13 the normalized difference signal SA is equal to zero for a collimated wave without spherical aberration. When spherical aberration is present, the spots on the two-quadrant detectors 12, 13 move and the absolute value of the normalized difference signal SA is larger than zero. As the sign of normalized difference signal SA depends on the sign of the spherical aberration, the normalized difference signal SA can be used as a control signal for a spherical aberration corrector (not shown).
The special beam splitter 9 is shown in
A simulation of the normalized difference signal SA as a function of the thickness of the cover layer of the recording medium, which was obtained with the help of a ray tracing program, is shown in
In
A further approach for measuring the spherical aberration is shown in
Number | Date | Country | Kind |
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04000258.6 | Jan 2004 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP04/12593 | 11/6/2004 | WO | 00 | 7/10/2006 |