The present invention relates to an optical scanning device comprising a phase structure intended to be used in various modes of operation of the optical scanning device.
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 player.
Japanese patent application JP-A-2001209966 describes an optical scanning device that can operate in various modes of operation. In a first mode, the optical scanning device is intended to scan a first information carrier with a first radiation beam having a first wavelength. In a second mode, the optical scanning device is intended to scan a second information carrier with a second radiation beam having a second wavelength. In a third mode, the optical scanning device is intended to scan a third information carrier with a third radiation beam having a third wavelength. Spherical aberration is generated in this optical scanning device, due to the difference in cover layer thicknesses of the first, second and third information carriers. In order to compensate for the spherical aberration, a phase structure is used. Depending on the selected mode, the phase structure has to behave differently in order to generate different amounts of spherical aberration. To this end, the phase structure comprises a liquid crystal material which can be switched by application of an electric field, as a function of the selected mode. The design of the phase structure and the application of an electric field are chosen in such a way that the phase structure forms a diffracted radiation beam of the zeroth order for the first radiation beam and a diffracted radiation beam of a higher order for each of the second and third radiation beams.
Such an optical scanning device uses polarized light. To, this end, a polarizing beam splitter is placed between the radiation source that generates the radiation beam and the objective lens that focuses the radiation beam on the information carrier. As the phase structure generates spherical aberration, the amount of decentering between the phase structure and the objective lens that can be allowed is small. As a consequence, the phase structure has to be mounted on the actuator that moves the objective lens during tracking. This means that the phase structure has to be placed between the polarizing beam splitter and the objective lens, because the polarizing beam splitter is not mounted on the actuator. Now, a λ/4 wave plate is used in such an optical scanning device using polarized light. As the phase structure requires linear polarized light, it has to be placed before the λ/4 wave plate, i.e. the λ/4 wave plate has to be placed between the phase structure and the objective lens.
Due to this placement of the various optical elements in an optical scanning device such as described in JP-A-2001209966, the polarization of the radiation beam coming back from the information carrier towards the phase structure is orthogonal to the polarization of the radiation beam coming from the polarizing beam splitter towards the phase structure. This introduces artifacts in the detected radiation beam. For example, the second radiation beam, which is diffracted on the way towards the information carrier, because its polarization is such that the phase structure act as a diffractive grating for this polarization, will not be diffracted on the way back from the information carrier, because it has an orthogonal polarization for which the phase structure does not act as a diffractive grating anymore. This means that this second radiation beam follows a different optical path on the way towards and on the way back from the information carrier, which creates artifacts on the detector.
It is an object of the invention to provide an optical scanning device comprising a phase structure that can be used in various modes of operation of the optical scanning device, wherein no artifact is created in the detected radiation beam.
To this end, the invention proposes an optical scanning device comprising a first phase structure comprising a first birefringent material having a first extraordinary axis and a second phase structure comprising a second birefringent material having a second extraordinary axis perpendicular to said first extraordinary axis, wherein the first and second phase structures have substantially the same pattern, the optical device comprising means for modifying the extraordinary refractive index of the first and the second birefringent material such that the extraordinary refractive indices of the first and the second birefringent materials remain substantially equal.
According to the invention, the optical scanning device comprises two phase structures comprising birefringent materials which extraordinary axes are perpendicular. As will be explained in the detailed description, such a combination of two phase structures is polarization independent. This means that the behaviour of the combination of these two phase structures does not depend on the polarization of the radiation beam that passes through said combination. As a consequence, no artifact is created in the detected radiation beam. For example, the second radiation beam of the prior art, which is diffracted on the way towards the information carrier, will also be diffracted on the way back from the information carrier, because the combination of the two phase structures will act as a diffractive grating, whatever the polarization of the radiation beam that passes through said combination. The second radiation beam will thus follow the same optical path on the way towards and on the way back from the information carrier.
The optical device in accordance with the invention comprises means for modifying the extraordinary refractive index of the first and the second birefringent material. This allows using the two phase structures in various modes of operation of the optical scanning device. When the mode of operation is changed, the extraordinary refractive index of the first and second birefringent materials is modified, in order, for example, to introduce a different amount of spherical aberration in the radiation beam. The modifying means are arranged such that the extraordinary refractive indices of the first and the second birefringent materials remain substantially equal. This ensures that the combination of the two phase structures in accordance with the invention is polarization independent.
Advantageously, the first and second birefringent materials are liquid crystal materials and the modifying means comprise means for applying an electric field to said liquid crystal materials. Such liquid crystal materials can easily be used as birefringent materials and can easily be treated so as to give them a desired extraordinary axis.
Preferably, the first and second phase structures form part of a same and one optical element. This makes the optical scanning device relatively compact.
The invention also relates to an optical element comprising a first phase structure comprising a first birefringent material having a first extraordinary axis and a second phase structure comprising a second birefringent material having a second extraordinary axis perpendicular to said first extraordinary axis, wherein the first and second phase structures have substantially the same pattern, the optical element comprising electrodes between which a potential difference can be applied so as to modify the extraordinary refractive indices of the first and the second birefringent materials.
These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.
The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which:
a,
3
b and 3c show the optical element of
a and 4b show another optical element in accordance with the invention in two modes of operation of the optical scanning device.
An optical scanning device in accordance with the invention is depicted in
During a scanning operation, which may be a writing operation or a reading operation, the information carrier 100 is scanned by the radiation beam 102 produced by the radiation source 101. The collimator lens 103 and the objective lens 107 focus the radiation beam 102 on an information layer of the information carrier 100. A focus error signal may be detected, corresponding to an error of positioning of the radiation beam 102 on the information layer. This focus error signal may be used for correcting the axial position of the objective lens 107, so as to compensate for a focus error of the radiation beam 102. A signal is sent to the controller 111, which drives an actuator in order to move the objective lens 107 axially. The focus error signal and the data written on the information layer are detected by the detecting means 109.
In the example of
In the example of
Alternatively, a chemical or mechanical modification of the electrodes in contact with the birefringent materials may be performed, in order to induce a preferred orientation of the liquid crystal alignment.
Alternatively, additional alignment layers that enclose the birefringent materials may be used. Alignment layers may be used such as those typically used for the construction of conventional liquid crystal displays, such as rubbed polyimide alignment layers, or photoalignment layers, such as coumarin derivatives or cinnamate derivatives. Deposition of these alignment layers may be accomplished by conventional processing techniques, such as spin coating or dip coating. Depending on the type of alignment layer, subsequent rubbing is required or a brief UV-exposure, to induce the desired orientation. A benefit of the use of polyimides is their outstanding temperature stability, which is well above the typical degradation temperatures that are commonly observed for the majority of organic polymers.
In
In
In this example, the isotropic material is chosen to have a refractive index equal to no.
When returning from the information carrier, the radiation beam has a polarization that is perpendicular to its original polarization. In this example, the radiation beam has a polarization that is parallel to the extraordinary axis of the first birefringent material 203. As a consequence, the apparent refractive index of the second birefringent material 208 is no for this radiation beam. The second phase structure 106 thus acts as a transparent plate for this radiation beam, which means that the radiation beam is not diffracted. The apparent refractive index of the first birefringent material 203 is close to ne, between no and no. The first phase structure 105 accordingly acts as a diffractive grating, and the radiation beam is diffracted. Because the extraordinary refractive index of the first and second birefringent materials 203 and 208 is the same, and the pattern of the first and second phase structures 105 and 106 is the same, the angle of diffraction is the same. If, as shown in
It has been shown that this optical element, or this combination of two phase structures, is polarization independent. Whatever the polarization of the radiation beam that passes through said optical element, the optical element will behave in the same way. This has the further advantage that this combination of two phase structures can be placed anywhere on the optical path.
In
In
The optical element shown in
When the BD is scanned, the potential difference V3 is applied, as shown in
The potential differences V1 and V2 can easily be chosen in such a way that these extraordinary refractive indices are obtained. The combination of two phase structures in accordance with the invention thus can perform the same functions as the phase structure of the prior art, with the further advantage that it is polarization independent and thus does not create any artifact in the detected radiation beam.
Other orders of diffraction could be chosen, depending on the amount of spherical aberration to be compensated. For example, the potential differences can be chosen in such a way that the optical element forms a diffracted radiation beam of the zeroth order for the third radiation beam, a diffracted radiation beam of the first order for the second radiation beam and a diffracted radiation beam of the second order for the first radiation beam.
In
In
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.
Number | Date | Country | Kind |
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04300129.6 | Mar 2004 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB05/50770 | 3/2/2005 | WO | 9/5/2006 |