WO2006030348A1

WO2006030348A1 - Optical device compatible with two disc types

Info

Publication number: WO2006030348A1
Application number: PCT/IB2005/052930
Authority: WO
Inventors: Petrus Jutte
Original assignee: Arima Devices Corporation
Priority date: 2004-09-15
Filing date: 2005-09-08
Publication date: 2006-03-23
Also published as: TW200623085A

Abstract

An optical scanning device for scanning a first information carrier (100) with a first track pitch q1 and a second information carrier with a second track pitch q2 different from q,. The optical scanning device comprises means (101) for generating a first radiation beam and a second radiation beam. It also comprises light separation means (102) for separating the first radiation beam into at least three first sub light beams and the second radiation beam into at least three second sub light beams. The light separation means are arranged in such a way that the radial distance x01 between two adjacent first sub light beams on the information carrier is such that formula (I), where n is an integer such that 1≤ n ≤ 3.

Description

Optical device compatible with two disc types

FIELD OF THE INVENTION

The present invention relates to an optical device for scanning at least two different information carriers.

The present invention is particularly relevant for an optical disc apparatus compatible with two optical storage standards, such as a CD and DVD recorder and/or player.

BACKGROUND OF THE INVENTION Different types of optical scanning device have been developed recently. In order to increase the data capacity, the wavelength of the scanning beam is more and more reduced, whereas the numerical aperture of the scanning beam is more and more increased. For example, in a CD recorder, the wavelength of the scanning beam is 790 nanometers and the numerical aperture is 0.5. hi a DVD recorder, the wavelength of the scanning beam is 660 nanometers and the numerical aperture is 0.65. In a BD recorder, the wavelength of the scanning beam is 405 nanometers and the numerical aperture is 0.85. This allows reducing the track pitch of the corresponding information carriers. For example, the track pitch of a CD is 1.6 micrometers whereas the track pitch of a DVD is 0.74 micrometer. It is important that a new optical scanning device is compatible with old information carriers, such that a user buying a new optical scanning device can still read his old information carriers. For example, a DVD player should be able to play DVDs and CDs.

An optical scanning device compatible with at least two different types of information carrier is described in Fig. 1. This optical scanning device comprises means 101 for generating a first and a second radiation beam, a grating 102 for generating three first and second sub light beams, a beam splitter 103, a collimator lens 104, a folding mirror 105, an objective lens 106, a quarter wave plate 107 and detecting means 108. This optical device is intended for scanning an information carrier 100, which can be of a first or a second type.

During a scanning operation, which may be a writing operation or a reading operation, the information carrier 100 is scanned by either the first or the second radiation beam produced by the generating means 101, depending on the type of information carrier. Three sub light beams are generated by the grating 102, and the collimator lens 104 and the objective lens 106 focus said sub light beams on an information layer of the information carrier 100. A focus error signal may be detected, corresponding to an error of positioning of the sub light beams on the information layer. This focus error signal is detected by the detecting means 108 and may be used for correcting the axial position of the objective lens 106, so as to compensate for a focus error of the sub light beams. To this end, a controller drives an actuator in order to move the objective lens 106 axially.

The information carrier comprises tracks on which data is recorded. The three sub light beams comprise a central light beam and two satellite light beams. In order to scan a given track, the central light beam has to be focused on said given track. To this end, radial tracking error detection is performed. A radial tracking error signal is measured, and a control loop is used in order to radially modify the position of the central spot on the information carrier, such that the central spot remains on the center of the track being scanned. A conventional radial tracking method is the so-called three spots push-pull or differential push- pull radial tracking method. Patent application US 2002/0185585 describes an optical scanning device comprising means for performing the three spots push-pull radial tracking method.

Fig. 2 shows the detecting means 108. They comprise a first detector array 108a, a second detector array 108b and a third detector array 108c. They also comprise a fourth detector array 108d, a fifth detector array 108e and a sixth detector array 108f. The first, second and third detector arrays 108a to 108c are used when the second information carrier is scanned, whereas the fourth, fifth and sixth detector arrays 108d to 108f are used when the first information carrier is scanned. The first detector array 108a comprises two detectors Al and A2, the second detector array 108b comprises four detectors Cl, C2, C3 and C4 and the third detector array 108c comprises two detectors Bl and B2. The first and third detector arrays 108a and 108c are called satellite detector arrays, whereas the second detector array 108b is called the central detector array. The fourth detector array 108d comprises two detectors Dl and D2, the fifth detector array 108e comprises four detectors El, E2, E3 and E4 and the sixth detector array 108f comprises two detectors Fl and F2.

The three spots on the three detector arrays 108a to 108c are also shown in Fig. 2. This corresponds to the scanning of the second information carrier. Fig. 2 corresponds to the situation where the central spot is focused on a track of the second information carrier. In this case, the central spot is focused on the center of the central detector array and the two satellite spots are focused on the centers of the two satellite detector arrays. The radial error signal RE2 is defined as

_RE2 _ Cl-C2 -C3 + C4-γ{Al-A2 + Bl -B2) Cl + C2 + C3 + C4 + γ{Al + A2 + Bl + B2) where Cl corresponds to the signal on the detector Cl, C2 to the signal on the detector C2, and so on. When the central spot is focused on the track being scanned, the radial error signal is null. However, when the central spot is not focused on the track being scanned, the radial error signal is not null. This property is used in order to move the objective lens 106 radially until the central spot is focused on the track being scanned.

When the first information carrier is scanned, the radial error signal REl is defined as

El-E2- E3 + E4-γ(Dl -D2 + Fl-F2) ^~ El + E2 + E3 + E4 + γ{Dl + D2 + Fl + F2)

Fig. 3 shows the three sub light beams on the information carrier 100, for either the first or the second radiation beam. The track pitch is q and the radial distance between two adjacent sub light beams is X₀. The radial error signal can also be expressed as RE=0.57m_Ppsin(2πx/c[)(l-cos(2πxo/q)), where γ is the power ratio between the central light beam and a satellite light beam, m_pp is the push-pull modulation and x is the radial position of the central spot on disc. This means that the amplitude of the radial error signal is [RE]=O.5(1 -cos(2πxo/q)).

As a consequence, the radial distance between two adjacent satellite spots is chosen equal to q/2, so that the amplitude [RE] of the radial error signal is maximum.

However, as the same grating 102 is used for generating the three first sub light beams and the three second sub light beams, the radial distance X₀I between two adjacent first sub light beams is approximately the same as the radial distance Xo2 between two adjacent second sub light beams. More precisely, if the wavelength of the first radiation beam is λi and the wavelength of the second radiation beam is λ₂, it can be shown that x_o2= x₀l.λ₂/λi.

As a consequence, if the radial distance between two adjacent first sub light beams is chosen equal to qi/2, where qi is the first track pitch of the first information carrier, the amplitude of the first radial error signal is maximum but the amplitude of the second radial error signal may not be maximum, because the second track pitch q₂ of the second information carrier differs from the first track pitch qi of the first information carrier.

In the following example, the first information carrier is a DVD with a track pitch q₁=0.74μm and the second information carrier is a CD with a track pitch q₂=1.6μm. The grating 102 is designed in such a way that the radial distance between two adjacent first sub light beams is qi/2. The first radiation beam has a first wavelength λ^όβOnm and the second radiation beam has a second wavelength It can easily be calculated that in this case, the amplitude [REl] of the first radial error signal is 1, whereas the amplitude [RE2] of the second radial error signal is only 0.58. This amplitude [RE2] is relatively low, which reduces the robustness of the radial tracking when the second information carrier is scanned.

Moreover, in a typical optical scanning device, a so-called Y-error misalignment occurs. Actually, the movement of the objective lens 106 during tracking is not always perpendicular to the tracks, because of a misalignment of the axis along which the objective lens 106 is moved with respect to a direction perpendicular to the tracks. This results in a so- called static Y-error misalignment. Moreover, a dynamic Y-error misalignment also occurs during rotation of the information carrier, due to eccentricity and ellipticity of the tracks. In this case, the radial distance between two adjacent first sub light beams is not exactly qi/2, but may vary. Variations of 10 per cent of the track pitch are often observed in conventional optical systems. For example, if the radial distance between two adjacent first sub light beams is 0.4qi instead of 0.5qi, it can be calculated that the amplitude [REl] of the first radial error signal is 0.9, which is enough for a robust radial tracking, but the amplitude [RE2] of the second radial error signal is only 0.41, which is too low for a robust radial tracking of the second information carrier.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical scanning device compatible with at least two different information carriers, in which the robustness of the radial tracking is improved.

To this end, the invention proposes an optical scanning device for scanning a first information carrier with a first track pitch ql and a second information carrier with a second track pitch q₂ different from qi, said optical scanning device comprising means for generating a first radiation beam and a second radiation beam, light separation means for separating the first radiation beam into at least three first sub light beams and the second radiation beam into at least three second sub light beams, said light separation means being arranged in such a way that the radial distance xol between two adjacent first sub light beams on the information carrier is such that ( 0.15)#, < x_ol ≤ ( h O.15)9_j , where n is an integer such

that 1 < n < 3 .

According to the invention, the radial distance between two adjacent first sub light beams is not chosen around qi/2, but is chosen around 5qi/2 or 9qi/2 or 13q_!/2. In these cases, the amplitude of the radial error signal is relatively high for the scanning of the first information carrier as well as for the scanning of the second information carrier, as will be explained in the detailed description. Hence, the tracking is robust for the first information carrier as well as for the second information carrier.

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 prior art;

Fig. 2 shows detecting means of the optical scanning device of Fig. 1;

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

Fig. 4 shows an optical scanning device in accordance with the invention; - Figs. 5a and 5b show tracks of a first and a second information carrier respectively and three spots focused on said information carriers by means of an optical scanning device in accordance with the invention;

Fig. 6 shows the radial error signals for a CD and a DVD, for different values of the radial distance between two adjacent first sub-light beams; - Figs. 7a to 7d are detailed views of the chart of Fig. 6.

DETAILED DESCRIPTION OF THE INVENTION

An optical scanning device in accordance with the invention is depicted in Fig. 4. This optical scanning device comprises the same elements as the optical device of Fig. 1. However, the grating 102 is arranged in such a way that the radial distance between two adjacent first sub light beams is around 5qi/2 or 9qi/2 or 13qi/2. The radial distance between two adjacent first sub light beams can be tuned by rotating the grating around the optical axis of the radiation beams, and perpendicularly to said optical axis. Although a grating 102 is used for separating the radiation light beam into at least three first sub light beams and the second radiation beam into at least three second sub light beams, other light separation means may be used. Moreover, the invention is not limited to the use of a single element for separating the first radiation beam into at least three first sub light beams and the second radiation beam into at least three second sub light beams. For example, two distinct gratings may be used. However, the invention is particularly advantageous when a single element is used, because in this case the radial distance between two adjacent second sub light beams is determined by the radial distance between two adjacent first sub light beams, which means that the radial distance between two adjacent second sub light beams cannot be chosen freely.

In Fig. 4, the means 101 are used for generating the first and second radiation beams. In this example, a single wavelength tunable radiation source is used, but two distinct radiation sources may be used. In other words, a first and a second radiation source are used for generating the first and the second radiation beam, wherein the first radiation source may be different from the second radiation source, or the two radiation sources may be one and the same radiation source.

In Figs. 5a and 5b, the resulting spots are schematically shown on a DVD and a CD respectively, where the shaded areas schematically represent tracks. The distance X₀I between two adjacent first sub light beams on a DVD is chosen equal to 5qi/2, where qi is equal to 0.74μm. This means that X₀I is equal to 1.85μm. As a consequence, x_o2= Xol.λ2/λi=2.19μm, where λ₂=780nm and λi=660nm. The track pith q2 is equal to 1.6μm. The respective dimensions have been kept in Figs. 5a and 5b.

In Fig. 6, the amplitudes of the radial error signal for the first and second information carriers are shown, as a function of xol/qi. In this example, the first information carrier is a DVD and the second information carrier is a CD. The amplitude of the radial error signal for DVD is : [REl]=0.5(l-cos(2πx₀l/qi))

The amplitude of the radial error signal for CD is

[RE2]=0.5(l-cos(2πxo2/q₂))= 0.5(l-cos(2π(qiλ₂/q2λi)xol/qi)) Detailed views in the shaded zones of Fig. 6 are shown in Figs. 7a to 7d.

Fig. 7a shows the amplitude of the radial error signal for first and second information carriers, when the radial distance between two adjacent first sub light beams is around qi/2. In this example, the first information carrier is a DVD and the second information carrier is a CD. When the radial distance between two adjacent first sub light beams is exactly qi/2, the amplitude [REl] of the radial error signal for DVD is 1 and the amplitude [RE2] of the radial error signal for CD is 0.58. It can be considered that the tracking is robust if the amplitudes of both error signals are higher than 0.6. In this case, the tracking cannot be considered as robust. Moreover, an error may occur in the radial distance between two adjacent first sub light beams, such as a Y-error misalignment. For example, if a Y-error misalignment occurs and the radial distance between two adjacent first sub light beams becomes equal to 0.4qi, the amplitude [RE2] of the radial error signal for CD becomes 0.41, which is far too low for a robust tracking. This is because the slope of the radial error signal for CD in the zone around xol=qi/2 is relatively high, which makes that a small variation of the radial distance between two adjacent first sub light beams can lead to a large decrease of the amplitude [RE2] of the radial error signal for the second information carrier.

Fig. 7b shows the amplitude of the radial error signal for first and second information carriers, when the radial distance between two adjacent first sub light beams is around 5qi/2. When the radial distance between two adjacent first sub light beams is exactly 5qi/2, the amplitude [REl] of the radial error signal for DVD is 1 and the amplitude [RE2] of the radial error signal for CD is 0.87. In this case, the tracking can be considered as robust. Moreover, if a Y-error misalignment occurs and the radial distance between two adjacent first sub light beams becomes equal to, for instance, 2.4qi, the amplitude [RE2] of the radial error signal for CD becomes 0.74, which is still enough for a robust tracking. This is because the slope of the radial error signal for CD in the zone around

is lower than in the zone around As can be seen from Fig. 7b, the radial distance between two adjacent first sub light beams may be chosen between 2.35 and 2.65, because in this range the amplitudes of the radial error signal for both information carriers are higher than 0.6. Even if an error occurs when the distance between two adjacent first sub light beams is chosen in this range, such as a Y-error misalignment, the amplitudes of the radial error signal for both information carriers remain sufficiently high for a robust tracking, because the slopes of both radial error signals is sufficiently low in this range.

Figs. 7c and 7d show the amplitude of the radial error signal for first and second information carriers, when the radial distance between two adjacent first sub light beams is around 9qi/2 and 13qi/2 respectively. When the same analysis as in Fig. 7b is performed, it can be seen that the radial distance between two adjacent first sub light beams may be chosen between 4.35 and 4.65 in the case of Fig. 7c, and between 6.35 and 6.65 in the case of Fig. 7d.

Although examples have been described with a DVD and a CD as first and second information carriers, the invention also applies to other information carrier formats. In particular, the invention applies when the ratio qiλ₂/q2λ! is between 0.4 and 0.6, because in this range the amplitudes of the radial error signal for both information carriers are similar to the amplitudes shown in Figs. 7b to 7d. For example, the invention may apply to an optical scanning device compatible with

BD and CD that uses a single grating for generating the three sub light beams from the CD radiation beam and from the BD radiation beam. Actually, the track pitch of the BD disc is

0.32μm and the wavelength 405nm, which leads to

when the track pitch of the CD is 1.5μm.

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 a first information carrier (100) with a first track pitch ql and a second information carrier with a second track pitch q₂ different from q_1} said optical scanning device comprising means (101) for generating a first radiation beam and a second radiation beam, light separation means (102) for separating the first radiation beam into at least three first sub light beams and the second radiation beam into at least three second sub light beams, said light separation means being arranged in such a way that the radial distance xol between two adjacent first sub light beams on the information carrier is such that ( 0.15)#_! < x₀l < ( + 0.15)^ , where n is an integer such that

1 < n < 3.

2 An optical scanning device as claimed in claim 1, wherein the first radiation beam has a wavelength λi and the second radiation beam has a wavelength λ₂ and 0.4<qiλ₂/q₂λi<0.6.