WO2006003571A1

WO2006003571A1 - Optical device for recording and reproducing.

Info

Publication number: WO2006003571A1
Application number: PCT/IB2005/052085
Authority: WO
Inventors: Sjoerd Stallinga; Ole Andersen; Robert Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-07-02
Filing date: 2005-06-24
Publication date: 2006-01-12
Also published as: TW200617929A

Abstract

An optical scanning device comprises a radiation source for producing a radiation beam, means for focusing said radiation beam on an information layer, and a detection branch comprising an astigmatic lens (106) and detecting means (108) located along an optical axis (109). The astigmatic lens produces a focal line between the astigmatic lens and the detecting means when the radiation beam is focused on the information layer. The detection branch further comprises filtering means (107) located at the focal line produced by the astigmatic lens. The filtering means comprise a non-transparent area (107b) surrounding a transparent area (107a) having a first dimension (dl) in a first direction parallel to the focal line and a second dimension (d2) in a second direction perpendicular to the first direction and to the optical axis. The first dimension is larger than the second dimension.

Description

Optical device for recording and reproducing

FIELD OF THE INVENTION

The present invention relates to an optical device, in particular an optical device for scanning an information carrier comprising at least two information layers.

The present invention is particularly relevant for an optical disc apparatus for recording to and reading from an optical disc, e.g. a CD, a DVD and/or a Blu-Ray Disc (BD) recorder and/or player.

BACKGROUND OF THE INVENTION

The present trend in optical storage is to increase the data capacity of the storage medium. This can be performed in that the numerical aperture of the objective lens is increased, while the wavelength of the scanning beam is decreased. The data capacity can also be increased in that the medium comprises a plurality of information layers. Optical storage mediums under development, such as BD discs, can have 2, 4, 8 or more information layers. In order to scan an information layer, the scanning beam is focused on said information layer, and the reflected beam is detected by means of a detection branch. In order to scan another information layer, the scanning beam is focused on said other information layer. However, although the scanning beam is focused on the desired layer, it also interacts with the out-of-focus layers. As a consequence, light coming from the out-of-focus layers is reflected towards the detection branch. This perturbs the detected signal corresponding to the information recorded in the scanned information layer. The amount of out-of-focus light increases with the number of information layers in the information carrier.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical scanning device in which the influence of the signal coming from the out-of focus layers is reduced.

To this end, the invention proposes an optical scanning device comprising a radiation source for producing a radiation beam, means for focusing said radiation beam on an information layer, a detection branch comprising an astigmatic lens and detecting means located along an optical axis, said astigmatic lens producing a focal line between the astigmatic lens and the detecting means when the radiation beam is focused on the information layer, the detection branch further comprising filtering means located at the focal line produced by the astigmatic lens, said filtering means comprising a non-transparent area surrounding a transparent area having a first dimension in a first direction parallel to said focal line and a second dimension in a second direction perpendicular to said first direction and to said optical axis, wherein the first dimension is larger than the second dimension. When scanning an information carrier comprising a plurality of information layers, spherical aberration compensation means are needed in the optical scanning device. Actually, the objective lens is designed for a given information layer. When the optical scanning beam is focused on another information layer, spherical aberration is created in the optical scanning beam, because of the spacer layer between two consecutive information layers. There are known means for compensating for the spherical aberration, such as moving the collimator of the optical scanning device. To this end, the spherical aberration has to be measured. A typical way of measuring the spherical aberration is the so-called differential focus-detection method. An astigmatic lens is used in combination with an eight-segment detector in order to generate two different focus error-curves from which the spherical aberration is measured. The invention makes use of this astigmatic lens. According to the invention, a filter is placed around the first focal line produced by the astigmatic lens, which is located between the astigmatic lens and the detector. This filter comprises a transparent area through which light coming from the astigmatic lens can pass. Light that impinges on the other portion of the filter is blocked by said filter. The transparent area can have different shapes, such as a rectangle slit or an ellipse. The dimensions of the transparent area are chosen in such a way that the light coming from the in- focus layer passes through the transparent area. At the first focal line produced by the astigmatic lens, light coming from the out-of- focus layers is very diffuse, such that only a relatively small part of the light coming from the out-of- focus layers will pass through the transparent area. As a consequence, the influence of the signal coming from the out-of- focus layers is reduced.

Placing the filtering means at the first focal line produced by the astigmatic lens is advantageous, because the light distribution at this location is well defined and highly concentrated. The dimensions of the transparent area can be chosen relatively small, because the dimensions of the in- focus signal beam at the location of the first focal line are relatively small. As a consequence, a relatively large part of the out-of- focus light is suppressed. This would not be the case if the filtering means were placed somewhere else on the optical path, where the dimensions of the in-focus signal beam are larger.

Advantageously, at least part of the non-transparent area can be switched to a transparent area. When the scanning beam is in-focus, the non-transparent area should remain non-transparent, so that the light coming from the out-of-focus layers is suppressed. However, during focusing, the light at the first focal line produced by the astigmatic lens is diffuse, and a relatively large part of this light is needed in order to detect a focus error signal. This large part of the light may be obtained on the photodetector in that part of the non-transparent area is switched to a transparent state. This switch only occurs during focusing, whereas the non-transparent area remains non transparent when the scanning beam is in- focus. An example of filter that can be switched in such a way is a liquid crystal filter.

These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

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 invention; - Fig. 2 shows the detection branch of the optical scanning device of Fig. 1;

- Figs. 3a and 3b show the distribution of an in- focus scanning beam at the location of the first focal line produced by the astigmatic lens of Fig. 1;

- Figs. 4a and 4b shows the distribution of an in- focus scanning beam at the location of the , detector of Fig. 1; - Fig. 5a and 5b show transmission of the light with respect to the defocus in two different optical devices in accordance with invention.

DETAILED DESCRIPTION OF THE INVENTION

An optical scanning device in accordance with the invention is depicted in Fig. 1. This optical scanning device comprises a radiation source 101, a beam splitter 102, a collimator lens 103, a folding mirror 104, an objective lens 105, an astigmatic lens 106, filtering means

107 and detecting means 108. The astigmatic lens 106 and the detecting means 108 are located along an optical axis 109. The astigmatic lens 106, the filtering means 107 and the detecting means 108 form a detection branch. This optical device is intended for scanning an information carrier 100.

During a scanning operation, which may be a writing operation or a reading operation, the information carrier 100 is scanned by the radiation beam produced by the radiation source 101. The collimator lens 103 and the objective lens 105 focus the radiation beam on an information layer of the information carrier 100. A focus error signal is detected, corresponding to an error of positioning of the radiation beam on the information layer. This focus error signal is detected by the detecting means 108 and is used for correcting the axial position of the objective lens 105, so as to compensate for a focus error of the radiation beam. To this end, a controller drives an actuator in order to move the objective lens 105 axially, i.e. with a movement noted F perpendicular to the information carrier.

The focus error signal is detected by means of the astigmatic lens 106. The astigmatic lens 106 produces two focal lines when the radiation beam is focused on an information layer, which focal lines are perpendicular to the optical axis 109 and perpendicular to each other. When the scanning beam is in- focus, the spot on the detecting means 108 is essentially circular. When the scanning beam is not in- focus, the spot on the detecting means 108 is essentially elliptically shaped. The detecting means 108 comprise a plurality of segments. The signal in each segment depends on the shape of the spot on the detecting means 108, which depends on the defocus. The defocus is thus measured by the detecting means 108, and used in order to correct the axial position of the objective lens 105. The detecting means may also measure a spherical aberration. For example, the well- known differential focus-detection method may be used. Once spherical aberration has been measured, the collimator lens 103 is moved in order to compensate for the spherical aberration.

The optical scanning device of the invention comprises filtering means, which are located between the astigmatic lens 106 and the detecting means 108, at the location of the first focal line produced by the astigmatic lens 106 when the radiation beam is focused on an information layer. The role of the filtering means is to filter the out-of- focus light. This is explained in detail with reference to the following Figs.

Fig. 2 is a detailed view of the detection branch in accordance with the invention.

Arranged along the optical axis 109 are the astigmatic lens 106, the filtering means 107 and the detecting means 108. The filtering means 107 comprise a non-transparent area 107a which surrounds a transparent area 107b. The filtering means 107 may be of any type comprising a transparent area and a non-transparent area. For example, the filtering means 107 may be formed by a transparent substrate on which a mask is deposited. The filtering means 107 may also be formed by two structures arranged such that a transparent area is left open. An example of such a structure is a knife blade.

In Fig. 2, the dimensions of the transparent area 107b are larger than in reality, for reasons of convenience. As will be explained with reference to the following Figs, the dimensions of the transparent area 107b are of the order of a few micrometers. The transparent area 107b has a first dimension dl in a direction parallel to the first focal line produced by the astigmatic lens 106 and a second dimension d2 in a direction perpendicular to said first direction and to the optical axis 109. The second dimension d2 is smaller than the first dimension dl. Preferably, the first dimension is at least five times larger than the second dimension. Although the transparent area 107b has been represented as a rectangle, it may have a plurality of different shape, as soon as it has the above-mentioned dimensions. For example, the transparent area 107b may have the shape of an ellipse, with its long axis having dimension dl and its short axis dimension d2.

Fig. 3a shows the light distribution of an in- focus scanning beam at the location of the first focal line produced by the astigmatic lens 106. Fig. 3b is a three-dimensional view of said distribution. The distribution is plotted in X and Y directions. Direction X corresponds to the direction of the first focal line produced by the astigmatic lens 106, whereas direction Y is perpendicular to direction X and to the optical axis 109. As can be seen from Figs. 3a and 3b, an in- focus scanning beam at the first focal line produced by the astigmatic lens 106 is mainly distributed along direction X. The unit on the axis named "lateral position" is the diffraction length λ/NA, where λ is the wavelength of the scanning beam and NA is the numerical aperture of the collimator lens 103.

Fig. 4a shows the light distribution of an in- focus scanning beam at the location of the detecting means 108. Fig. 4b is a three-dimensional view of said distribution. It can be seen that the in-focus scanning beam is equally distributed along directions X and Y. As a consequence, the section of the scanning beam at the location of the detecting means 108 is larger than the section at the first focal line produced by the astigmatic lens 106. It can be shown that the section of the in-focus scanning beam is indeed the smallest at the location of the first focal line produced by the astigmatic lens 106.

As a consequence, placing filtering means 107 at the location of the first focal line produced by the astigmatic lens 106 allows taking the smallest transparent area 107b. This leads to the smallest part of the out-of-focus light passing through the filtering means 107. The transparent area 107b is chosen to have approximately the same dimensions as the section of the in-focus scanning beam. Preferably, the first dimension dl of the transparent area 107b is between lOλ/NA and 30λ/NA and the second dimension d2 of the transparent area 107b is between 2λ/NA and 9λ/NA. With a typical scanning wavelength of 500 nanometers and a typical numerical aperture of 0.8, the first dimension dl is preferably between 6,25 and 18,75 micrometers and the second dimension d2 is preferably between 1,25 and 5,625 micrometers.

The choice of these values for the first and second dimensions dl and d2 ensures that a major part of the in-focus light passes through the transparent area 107b, while a major part of the out-of- focus light is blocked by the non-transparent area 107a.

Fig. 5a shows transmission of the light through the transparent area 107b, with respect to the defocus. In this optical scanning device, the magnification from the information carrier 100 to the detecting means 108 is 10, the numerical aperture of the collimator lens 103 is 0.1 and the second dimension d2 of the transparent area 107b is 20 micrometers. The unit on the axis of defocus is micrometers. As can be seen in Fig. 5a, one hundred per cent of the in- focus light is transmitted through the transparent area 107b. For a defocus of 20 micrometers, the transmission is reduced to ten per cent. In typical information carriers, the distance between two consecutive information layers is larger than 20 micrometers. This means than less than ten per cent of the signal coming from out-of-focus layers will reach the detecting means 108. This leads to an acceptable signal to noise ratio, which allows good detection of the information signal coming from the in-focus layer.

In Fig. 5b, the second dimension d2 of the transparent area 107b is 60 micrometers. It can be seen that for a defocus of 20 micrometers, the transmission is about twenty per cent, which is higher than in Fig. 5a. The choice of the dimension d2 depends on the acceptable transmission, which depends on the distance between two consecutive layers and on the acceptable signal to noise ratio.

Although the transparent area 107b has been represented with fixed dimensions in

Fig. 2, the filtering means 107 preferably have two modes in which the dimensions of the transparent area 107b are different. A first mode corresponds to a situation where the scanning beam is in-focus, and a second mode corresponds to a situation where the scanning beam is being jumped from one layer to another. In the first mode, the dimensions of the transparent area 107b are such that the in- focus light passes through said transparent area 107b and a major part of the out-of-focus light is blocked by the non-transparent area 107a. This corresponds to the situation of Fig. 2, and has been explained with reference to Figs. 1 to 5. In the second mode, the dimensions of the transparent area 107b are increased. This may be necessary in order to be able to focus the scanning beam when jumping from one layer to another. Actually, when jumping from one layer to another, an s-curve is used. The full width of the s-curve is needed in order to perform a jump. As a consequence, all the light coming from the information carrier is needed on the detecting means 108.

In order to increase the dimensions of the transparent area 107b, part of the non- transparent area is switched to a transparent area. For example, if the filtering means 107 comprise two structures arranged such that a transparent area is left open, this is achieved in that the gap between said two structures is increased. Preferably, the filtering means 107 comprise a liquid crystal filter which can be switched between a transparent and a non- transparent state. In this case, the dimensions of the transparent area 107b are adjusted in that a potential difference is applied or not to certain parts of the liquid crystal filter.

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 comprising a radiation source for producing a radiation beam, means for focusing said radiation beam on an information layer, a detection branch comprising an astigmatic lens (106) and detecting means (108) located along an optical axis (109), said astigmatic lens producing a focal line between the astigmatic lens and the detecting means when the radiation beam is focused on the information layer, the detection branch further comprising filtering means (107) located at the focal line produced by the astigmatic lens, said filtering means comprising a non-transparent area (107b) surrounding a transparent area (107a) having a first dimension (dl) in a first direction parallel to said focal line and a second dimension (d2) in a second direction perpendicular to said first direction and to said optical axis, wherein the first dimension is larger than the second dimension.

2 An optical scanning device as claimed in claim 1, wherein the first dimension is at least five times larger than the second dimension. 3 An optical scanning device as claimed in claim 1, wherein said radiation beam has a wavelength λ and the optical scanning device comprises a collimator lens (103) having a numerical aperture NA, wherein the first dimension is between lOλ/NA and 30λ/NA and the second dimension is between 2λ/NA and 9λ/NA.

4 An optical scanning device as claimed in claim 1, wherein said filtering means comprise a mask deposited on a transparent substrate.

5 An optical scanning device as claimed in claim 1, wherein said filtering means comprise two structures arranged such that a transparent area is left open.

6 An optical scanning device as claimed in claim 1, wherein at least part of the non- transparent area can be switched to a transparent area. 7 An optical scanning device as claimed in claim 6, wherein said filtering means comprise a liquid crystal filter which can be switched between a transparent and a non- transparent state.