Optical Disc Device for Recording and Reproducing

Abstract

An optical scanning device for scanning an information carrier comprising tracks with a track pitch q, the closest track to the center of the information carrier having a radius r. The optical scanning device comprises a radiation source for generating a radiation beam and means for generating three spots on the information carrier from said radiation beam. The means for generating three spots are arranged in such a way that the distance s between two consecutive spots on he information carrier is such that equation (I) where s and q are in micrometers and r in millimeters, r is inferior to 10 millimeters and α is superior to 0.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which:

FIG. 1 shows an optical scanning device in accordance with the prior art;

FIG. 2 shows the three-spots detector module of the optical scanning device of FIG. 1;

FIG. 3 shows the first tracks of an information carrier and three spots focused on said information carrier by means of an optical scanning device in accordance with the invention;

FIG. 4 shows a focus s-curve measured by means of an optical scanning device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows the first tracks of an information carrier intended to be scanned by an optical device in accordance with the invention. The information carrier comprises a center C, and a first track having a radius r. The first track, which is the closest track to the center C, corresponds to the first track where information is recorded or can be recorded. The information carrier comprises other tracks, which radiuses are noted R, R varying from r to the outer radius of the information carrier.

In FIG. 3, the direction of the objective lens 106 during tracking is represented by a dotted arrow. As can be seen, this direction does not pass through the center C, which means that it is not perpendicular to the tracks of the information carrier. This leads to a static Y-error misalignment Y, which is shown in FIG. 3. The Y-error misalignment also comprises a dynamic Y-error misalignment, which mainly depends on the information carrier being scanned. The Y-error misalignment is the sum of the static and dynamic Y-error misalignments. A typical value for the Y-error misalignment is 100 micrometers. In the following, the Y-error misalignment is taken equal to 100 micrometers, which is a mean value of the Y-error misalignments that can be measured in a plurality of optical scanning devices. However, the invention is not limited to optical scanning devices where the Y-error misalignment is 100 micrometers, because the Y-error misalignment varies from one optical device to another, and also from one information carrier being scanned to another.

The distance between two consecutive spots on the information carrier is s. The object of this invention is to reduce the distance s between two consecutive spots as compared with

conventional optical scanning devices. If s is chosen in such a way that

$s\le \frac{10\sqrt{1-\alpha }}{\pi }\mathrm{rq},\mathrm{then}$ $1-{\left(\frac{\pi Y·s}{\mathrm{rq}}\right)}^2>\alpha ,$

where Y is chosen equal to 100 micrometers. This leads to

$\frac{1+\left[1-\frac{1}{2}{\left(2\pi \frac{Y·s}{\mathrm{rq}}\right)}^2\right]}{2}>\alpha ,$

which, with a Taylor expansion, leads to

$\frac{2}{(1+\cos \left(2\pi \frac{\mathrm{sY}}{\mathrm{Rq}}\right)}<\frac{1}{\alpha }.$

As a consequence, the reduction of the radial error signal in an optical scanning device in accordance with the invention is less than 1/α. This means that when a is superior to 0.2, the reduction of the radial error signal is less than 5, which is enough for ensuring a robust radial tracking.

Typical values for a small form factor optical disc are r=6 mm and q=0.5 μm. In order to have a reduction of the radial error signal inferior to 2, the distance s between two consecutive spots on the information carrier should be inferior to 9 micrometers.

It should be noted that the invention also provides a relative small variation of the slope of the radial error signal. Reducing the distance between two consecutive spots on the information carrier reduces the variation of the slope of the radial error signal. This is particularly advantageous, because a small variation of the slope of the radial error signal improves the radial tracking servo control loop.

FIG. 4 shows a focus s-curve measured by means of an optical scanning device in accordance with the invention. The focus s-curve measures a focus error signal FE as a function of the distance d between the objective lens 106 and the information carrier 100. A parameter that can be measured is the focus s-curve length z. It has been shown that the relation between the focus s-curve length z and the distance s between two consecutive spots on the information carrier is

$s=2\sqrt{2}·z·\mathrm{NA}·\left(1+\frac{\Delta s}{\Delta d}\right).$

As a consequence, choosing the distance s between two consecutive spots on the information carrier in such a way that

$s\le \frac{10\sqrt{1-\alpha }}{\pi }\mathrm{rq}$

is equivalent to designing the optical scanning device in such a way that

$z\le \frac{5\sqrt{\frac{1-\alpha }{2}}}{\pi \mathrm{NA}\left(1+\frac{\Delta s}{\Delta d}\right)}\mathrm{rq}.$

Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb “to comprise” and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.

Claims

1. An optical scanning device for scanning an information carrier comprising tracks with a track pitch q, the closest track to the center of the information carrier having a radius r, the optical scanning device comprising a radiation source for generating a radiation beam, means for generating three spots on the information carrier from said radiation beam, said means for generating three spots being arranged in such a way that the distance s between two consecutive spots on the information carrier is such that

2. An optical scanning device as claimed in claim 1, wherein a is superior to 0.5.

3. An optical scanning device for scanning an information carrier comprising tracks with a track pitch q, the closest track to the center of the information carrier having a radius r, the optical scanning device comprising a radiation source for generating a radiation beam, an objective lens having a numerical aperture NA, three detectors for measuring a focus s-curve, the detectors having a width Δd and being separated by a distance Δs, said focus s-curve having a focus s-curve z such that

4. An optical scanning device as claimed in claim 3, wherein a is superior to 0.5.