HK1008605B - Device for reading from or writing to an optical recording - Google Patents

Description

Reading or writing device for optical recording medium

Technical Field

The invention relates to a reading and/or writing device for optical recording media having at least two information carrier layers separated from each other.

Background

A device of this type is disclosed in US-A-4,908,813. A drawback of this known device is that the optical recording medium has layers that are sensitive to different wavelengths, thus requiring the recording device to have a plurality of lasers with different wavelengths. Depending on the wavelength, these lasers are relatively complex and costly.

Disclosure of Invention

It is an object of the present invention to provide a reading and/or writing device for optical recording media having a plurality of information carrier layers separated from each other, allowing a plurality of information carrier layers to be read and/or written simultaneously using light of a single wavelength.

According to the above object of the present invention, there is provided a reading or writing device for optical recording media having at least two information carrier layers (9, 10) separated from each other, having a light source (1), an optical unit and a detection unit, characterized in that the optical unit comprises means for generating a difference in polarization directionA partial-beam generating element (4, 1 ', 1 ", 19) of two partial beams (5, 6, 2 ', 2") for propagating the two partial beams (5, 6, 2 ', 2 ") through different optical paths and focusing on the different information carrier layers (9, 10) so as to simultaneously have a focus point of a first polarized light beam (5, 2 ') on one of the at least two layers (9, 10) and a focus point of a second polarized light beam (6, 2") on the other of the at least two layers (9, 10), and for reflecting the two partial beams (5, 6, 2 ', 2 ") through different optical paths from the different information carrier layers (9, 10) onto the detection unit; and the detection unit has two sensors (S)₁、S₂) For simultaneous detection of two sub-beams reflected from said different information carrier layers (9, 10), whereby said reading or writing means provide simultaneous access to both layers (9, 10).

This object is achieved by the feature that the device according to the invention has a partial-beam generating element which emits two partial beams of different polarization directions and propagation characteristics, so that two layers have a focus point at the same time. It is advantageous to be able to read or write simultaneously or both simultaneously using a single wavelength of light. The different propagation characteristics cause the two partial beams to be focused on two different information carrier layers by means of a simple optical unit; the different polarizations result in a simple splitting of the split beams in the detection device. The device according to the invention is suitable for reading from and writing to optical recording media as well as for reading from and writing to optical recording media. The optical recording medium may be in the form of a disc, for example a CD or DVD, but a cassette-type recording medium or other forms of recording medium having two information carrier layers one above the other and separated from each other are also within the scope of the invention. The light source is used to generate a light beam for reading from or writing to the optical recording medium. Said light beam is usually reflected from the optical recording medium, but it is also within the scope of the invention to use a translucent recording medium. The optical unit is used for focusing the light beam generated by the light source on the optical recording medium and transmitting the reflected or transmitted light beam to the detection device. The latter is used for detecting the light beam from the optical recording medium and for emitting, for example, electrical signals for obtaining, on the one hand, the information stored on the information carrier layer and, on the other hand, parameters that are necessary or useful for the operation of the apparatus, such as tracking signals. Advantageously, the partial beam generating element is a birefringent element, which can be used for converging or diverging beams.

The component beam generating element provided by the invention is a birefringent lens. Since the optical unit includes a lens in any case, it is advantageous that no additional element is required in the optical unit. A further advantage is seen in the fact that the lens made of birefringent material divides an incident light beam into two sub-beams which, on the one hand, are polarized perpendicular to each other and, on the other hand, are dispersed or converged to different extents, depending on the type of lens and its arrangement.

The polarization direction rotating element provided by the invention is arranged in an upstream optical path of the sub-beam generating element. It is advantageous to achieve a variable distribution of the light intensity between the two partial beams by rotating the polarization direction of the incident light. It is advantageous if the intensity of the sub-beams needs to be matched to the possibly different reflection properties of the information carrier layer. Furthermore, the method is advantageous when recording an information item on an information carrier layer. In this case, the partial beams for effective recording generally require higher light energy, while the other beams for ineffective recording may have very low energy. Another advantageous application of the method according to the invention is the assignment of 100% of the effective light intensity to the respective sub-beams to achieve the ability to read either information carrier layer. This may be useful for various applications, for example when trying to operate with reduced power of the light source or for noise suppression purposes by shielding a corresponding further information carrier layer, or in other suitable applications. All of these different intensity distributions between the individual sub-beams enable the use of lower power, low cost light sources that do not require a change in their power output. Advantageously, the polarization direction rotating element is a rotatable pole filter. It is likewise advantageous to arrange the light source, in particular the semiconductor laser, such that it can be rotated or rotated, for example, by the polarization direction of the light emitted by the semiconductor laser by means of a λ/2 plate, a kerr cell or other suitable means.

According to a further advantageous development of the invention, the partial-beam generating element comprises a device with two light sources which emit polarized light and are located at different distances from the optical unit. It is advantageous in that a birefringent beam-splitting element is not required. The light source is preferably a laser, such as a semiconductor laser diode, which emits linearly polarized light. The light is paired, for example, by a half mirror or a polarizing beam splitter.

The light sources are advantageously integrated on a single carrier element. This has the advantage that the carrier element is provided in a manner adapted to the components of the device, as a result of which no adjustment is necessary during assembly of the device. It is advantageous if the carrier element is an integrated semiconductor element. This type of device can be manufactured in such a way that it has been correctly adjusted.

According to the invention, the detection unit, that is to say the detection path of the device, has a birefringent prism-type beam splitter. Since the two partial-beam sensors can be arranged in one plane, this is advantageous in that less physical space is required. The birefringent prism-type beam splitter has path lengths that are different for various polarization directions, thus making it possible to transmit two focusing points of the split beams, one behind the other in the optical axis of the beam propagation direction, the two focusing points lying one behind the other in a single focusing plane perpendicular to said optical axis.

Advantageously, the detection unit has a detector plane in which a plurality of detector elements are arranged. This has the advantage that the detector elements are all located in one plane and can thus be manufactured cost-effectively as one component. Furthermore, when they are installed in the device, no individual detector elements have to be adjusted.

The invention provides a detection unit having a half-mirror in relation to the direction of polarization. The advantage of this is that the latter enables low cost and space saving, for example as a polarizing beam splitter cube. Which has the effect of splitting the sub-beams from the optical recording medium between the various detector elements.

The invention also relates to an optical recording medium, in particular for use in the device according to the invention, which is characterized in that the optical recording medium has an information carrier layer with a pre-recorded information item, and an information carrier layer spatially separated and spaced apart from the aforementioned layer, which is suitable for recording the information item and which is free of the pre-recorded information item. Since both information carrier layers can be read and/or written simultaneously by corresponding means, it is advantageous that said pre-recorded information items, which are necessary or at least useful for reading or writing optical recording media, do not have to be pre-recorded twice, i.e. on each information carrier layer. Since the device during recording of two information layers in each case relates to information items prerecorded in one layer, the available memory space for the recording of useful data is increased, the manufacturing costs of the optical recording medium are reduced, the signal-to-noise ratio is increased, and in particular the information items recorded on the two information carrier layers are located exactly one above the other. Useful data refers to a term that includes video, audio, and other useful data as well as pure data. Of course, the optical recording medium may also have a plurality of information carrier layers, at least one of which has a pre-recorded information item that is not pre-recorded on the other layer.

The invention is not intended to be limited to the embodiments and illustrations described, but is to be construed as encompassing all possible configurations within the scope of the invention.

Drawings

Further advantageous features and improvements of the invention may be found in the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows an apparatus having a birefringent prism-type beam splitter according to the present invention;

fig. 2 shows the detection path of a device according to the invention with a half mirror in relation to the polarization direction;

FIG. 3 shows, in a three-dimensional representation, a part of an apparatus according to the invention with a polarization direction rotating element;

fig. 4 shows a part of a device according to the invention with two light sources in a first embodiment;

fig. 5 shows a part of a device according to the invention having two light sources in a second embodiment;

FIG. 6 shows a detector element of an apparatus according to the invention;

fig. 7 shows a detailed view of an optical recording medium according to the present invention.

Detailed Description

Fig. 1 shows an arrangement according to the invention with a birefringent prism-type beam splitter. A laser diode 1 emitting a linearly polarized light beam 2 is used as a light source. This beam passes through a phase grating 3, the phase grating 3 producing the +/-first order secondary beam [ sic ] required for tracking according to the well-known three-beam method. This method is discussed only to the extent required to describe the preferred embodiments. It goes without saying that the present invention can be effectively used for other tracking methods. After passing through the phase grating 3, the light beam 2 impinges on a birefringent collimating lens 4, which birefringent collimating lens 4 represents a sub-beam generating element in the preferred embodiment. The birefringent collimator lens 4 splits the light beam 2 into two sub-beams 5 and 6, the sub-beams 5 and 6 being polarized perpendicular to each other. The partial beams 5 and 6 pass through a non-polarizing beam splitter 7 and are focused by an objective lens 8 on different information carrier layers 9 and 10 of the optical recording medium, which is only schematically illustrated here. For focusing, the objective lens 8 may be placed in the direction of the optical axis, in the left-right direction in fig. 1. For tracking along a track in which information items are stored on the information carrier layers 9, 10, the objective lens 8 can be moved parallel to the information carrier layers 9 and 10. This is done by the lens actuator 11, which lens actuator 11 is here only illustrated simply by a double arrow in the respective direction of movement. To indicate the polarization direction of the partial beams 5 and 6, the polarization direction is symbolized on the information carrier layers 9 and 10. The sub-beams 5 incident on the information carrier layer 9 are thus polarized in the direction of the plane of the drawing, while the sub-beams 6 incident on the information carrier layer 10 are polarized in a direction perpendicular thereto. The partial beams 5 and 6 are reflected by the information carrier layers 9 and 10, pass through the objective lens 8 in the figure and are refracted downwards in the non-polarizing beam splitter 7 and reach the focusing lens 12. They then pass through a cylindrical lens 13, which cylindrical lens 13 is used to perform an astigmatic autofocus method, which method is not discussed in detail here. They are then illuminated onto a polarizing prism-type beam splitter 14. The latter has two reflective layers 15 and 16, each reflecting light of one polarization direction. The partial beams 5 are polarized in the direction of the plane of the drawing, the partial beams 5 passing through the layer 16 which is reflective for the other polarization direction and being reflected by the reflective layer 15 onto one of the sensors S1 of the detector arrangement 17. The sub-beams 6 are polarized perpendicular to the plane of the drawing, the sub-beams 6 being reflected by the reflective layer 16 onto the sensor S2 of the detector arrangement 17.

According to the invention the polarization of the light along the axis of the device is used to create two mutually separated focal spots on the information carrier layers 9 and 10. The longitudinally separated focal points are generated by means of a birefringent collimating lens 4. This facilitates the separation of signals originating from the different information carrier layers 9 and 10 in the detection path by using a polarization-sensitive optical element, i.e. a polarizing prism-type beam splitter 14 in the preferred embodiment. According to the invention, the objective lens 8 can also be designed as a birefringent lens. In this case, the separation in the detection path cannot be achieved as easily as in the manner shown in fig. 1. It is proposed that the birefringent material of the collimator lens 4 is quartz, for example, having a refractive index n at a wavelength of about λ ≈ 700nm_e＝1.5533，n_oThis wavelength range is commonly used in corresponding devices, let [ sic ] 1.5442]The focus spot separation, which is sufficient for the usual size of the read/write head.

The reflective layers 15 and 16 of the polarizing prism-type beam splitter 14 are arranged in parallel with each other at a distance d. By suitable choice of the distance d, the optical path length is introduced such that the two focal points impinging on the information carrier layers 9 and 10 are separated from each other and imaged on the detector means 17 in one plane. Assuming a focal length of the focusing lens 12 of 42mm, a focal length of the objective lens 8 of 3.5mm and a final magnification η of 12, the distance δ between the information carrier layers 9 and 10 is 70nm, resulting in a required thickness d of 42mm

Fig. 2 shows the detection path of the device according to the invention, i.e. the part below the cylindrical lens 13 in fig. 1. After passing through the cylindrical lens 13, the partial beams 5 and 6 from the optical recording medium impinge on a polarizing beam splitter 18, the polarizing beam splitter 18 having a reflective layer 16'. The partial beam 5 polarized in the plane of the drawing passes through this reflective layer 16 'and impinges on the sensor S1'. The sub-beam 6, which is polarized perpendicularly thereto, is reflected by the reflective layer 16 and impinges on the sensor S2'. The two sensors S1 'and S2' are adjusted relative to each other in the detection unit. In this preferred embodiment, the polarizing prism-type beam splitter with the reflective layer disposed at the distance d can be omitted.

Fig. 3 shows in three dimensions a part of a device according to the invention with a polarization direction rotating element 20. The laser diode 1 and the light beam 2 emitted by it can be identified. Polarization direction is determined by polarization vector E of polarization direction rotating element 20_LAnd (4) showing. In the birefringent collimating lens 4, the polarization direction of the ordinary beam is represented by n_oIndicating that the direction of polarization of the particular beam is n_eAnd (4) indicating. The respective partial beams 5 and 6 are indicated by a further beam path. In the illustration, the polarization directions shown have been rotated by 45 ° with respect to the preferential direction of the birefringent collimating lens 4, which are indicated by the arrow n_oAnd n_eTo represent. By passing through the crystal axis n relative to the birefringent collimating lens 4_oAnd n_eRotating the polarization vector E of the light beam 2_LThe intensity of the two collimated, polarized partial beams 5 and 6 can be adjusted in a continuously variable manner. Both intensities have the same magnitude at a 45 ° angle. In particular for writing to the writable information carrier layer 9 and/or 10, the intensity of the partial beams 5 and/or 6 for writing is actually increased compared to the respective other partial beams 6 and/or 5, for example in a ratio of 80: 20.

Fig. 4 shows a part of a device according to the invention having two light sources in a first embodiment. The two separate laser diodes 1' and 1 ″ have in this case a small lateral offset as well as an offset in the longitudinal direction, that is to say in the beam propagation direction. Which is shown in an enlarged manner in fig. 4. Furthermore, the laser diodes 1 'and 1 "are positioned such that the light beams 2' and 2" generated by them are polarized perpendicular to each other. In which case a common collimator lens 4' is used. The other beam path corresponds to the beam path described in connection with fig. 1 and 2. By means of a suitable lateral offset, a greater separation of the partial beams within the region of the detector device 17 can be achieved, as a result of which a less complex solution can be provided in this case.

Fig. 5 shows a second embodiment of the device of the invention with two light sources. In this case, too, two separate laser diodes 1 'and 1 "are used, the beams 2' and 2" of the laser diodes 1 'and 1 "being combined by the semitransparent reflective layer 19 and impinging on a common collimator lens 4'. The distance x between the laser diode 1' and the semitransparent reflective film 19 is selected₁And a distance x between the laser diode 1' and the semitransparent reflective film 19₂Different, so that a longitudinally displaced focal spot of the two light beams 2' and 2 "is generated, which corresponds to the distance between the information carrier layers 9 and 10. The advantage of this embodiment described on the basis of fig. 4 is that the beams 2 'and 2 "are not laterally displaced relative to each other, since the two laser diodes 1' and 1" can be precisely aligned in the optical axis. In this case, too, the other beam path corresponds to a junctionThe beam paths described in connection with fig. 1 and 2.

Fig. 6 shows a detector device 17 of the device according to the invention. Sensor S on which the partial beams 5 and 6 are respectively imaged₁And S₂Can be distinguished. In this preferred embodiment, the sensor S₁Is divided into four quadrants consisting of detector elements A, B, C, D. This division makes it possible to form the focus signal FE according to the astigmatic focusing method. This is shown in the right part of fig. 6. FE ═ is obtained from the difference between the sums of the respective two diagonally aligned detector elements (a + C) - (B + D). Further detector elements E and F are used to obtain the tracking signal TE according to the three-beam method. The beam 2 is split by the phase grating 3 into a zero order primary beam and two +/-first order secondary beams, which are not shown in the previous figures for the sake of simplicity. In fig. 6, the spots originating from these beams are represented as dots on the detector element in each case. Zero order primary beam strikes sensor S₁And S₂The +/-first order secondary beam of the partial beam 5 strikes the detector elements E and F. Since no detector elements are provided for the +/-first order secondary beams of the component beams 6, the corresponding spots are indicated by dashed lines. The tracking signal TE is obtained from the difference between the signals emitted by the detector elements E and F. The sub-beam 6 impinges on a sensor S consisting of a single detector element G₂. The information signal of the sub-beam 6 thus corresponds to the output signal of the detector element G and the information signal of the sub-beam 5 corresponds to the sum of the output signals of the detector elements a to D. This is also shown in the right part of fig. 6. Due to the use of a polarizing prism-type beam splitter 14, it is possible that the two focal points on the information carrier layer 9 and the information carrier layer 10 described here are imaged on a single detector device 17. It goes without saying that, for improved focusing and tracking, corresponding detector elements in the range of the sensor 1 can also be arranged in the range of the sensor 2, if appropriate. Furthermore, other focusing and tracking methods can be used, according to which the detector means 17 have to be constructed.

Fig. 7 shows a detailed view of an optical recording medium according to the invention in the area of the information carrier layers 9 and 10. The arrangement of the two information carrier layers makes it possible to accommodate approximately one time the data capacity on the optical recording medium. The storage capacity is further increased if the other information carrier layers are arranged accordingly. With the multifocal optical unit according to the invention, two such information carrier layers can be read simultaneously, since the distance between the focal points of the sub-beams 5 and 6 is such that Δ Z corresponds to the distance [ sic ] between the two information carrier layers 9 and 10. In this case, the information tracks of the information carrier layers 9 and 10 are situated one above the other, with the result that not only are the two partial beams 5 and 6 simultaneously focused on the respective information carrier layer, but they can also simultaneously read the useful information items there. The information carrier layer 10 is transparent and reflective, so that the partial beams 5 can pass through it. An important application of the invention is when the optical recording medium is a so-called ROM/RAM disc. In this case, the information carrier layer 9 is a ROM layer, for example. That is to say it only comprises reading the information items in the pre-embossed information tracks. The information carrier layer 10 is designed as a storage layer. This may be a phase change layer, a photorefractive layer, a magneto-optical layer or any other layer suitable for recording. The information carrier layer 10, however, has no guide tracks which are typical of recording layers. This type of optical recording medium is read and written using a double focusing system, i.e. the apparatus of the present invention as described above. This is advantageous in that the information tracks and/or the guide tracks of the information carrier layer 9 can be used for focusing, tracking, disc control and area addressing of the information carrier layers 9 and 10. During a writing operation of the information carrier layer 10, data tracks are automatically written accurately on the tracks of the information carrier layer 9. Since the information carrier layer is a planar, uniform area without guide tracks or additional addressing information items, a higher signal-to-noise ratio can be achieved, since no additional noise signal [ sic ] is generated due to any unevenness of the guide track edges or addressing bits. Furthermore, since no address or servo information items, for example for tracking, have to be located on the information carrier layer 10, the full storage capacity of the information carrier layer can be used for data storage. Using the method described in connection with fig. 3, the intensity of the light written on the information carrier layer 10 can be adjusted accordingly by rotating the polarization direction in correspondence with the crystal axis of the birefringent collimating lens 4. The intensity of the sub-beams 5 read on the information carrier layer 9 may be, for example, approximately 10% of the total power of the laser diode 1, the intensity of the sub-beams 6 for writing on the information carrier layer 10 correspondingly being approximately 90% of said power.

Claims (9)

1. Reading or writing device for optical recording media with at least two information carrier layers (9, 10) separated from one another, with a light source (1), an optical unit and a detection unit, characterized in that the optical unit comprises a partial-beam generating element (4, 1 ', 1 ", 19) for generating two partial beams (5, 6, 2', 2") with different directions of polarization, the optical unit being designed to propagate the two partial beams (5, 6, 2 ', 2 ") through different optical paths and to be focused on the different information carrier layers (9, 10) so as to simultaneously have a first polarized beam (5, 2') on one of the at least two layers (9, 10)And a second polarized light beam (6, 2 ') on the other of the at least two layers (9, 10) and for reflecting the two partial light beams (5, 6, 2') from the different information carrier layers (9, 10) via different light paths onto the detection unit; and the detection unit has two sensors (S)₁、S₂) For simultaneous detection of two sub-beams reflected from said different information carrier layers (9, 10), whereby said reading or writing means provide simultaneous access to both layers (9, 10).

2. The apparatus according to claim 1, characterized in that the partial beam generating element is a birefringent collimating lens (4).

3. The apparatus according to claim 2, characterized in that the polarization direction rotating element (20) is arranged in the beam path upstream of the partial beam generating element (4).

4. The apparatus according to claim 1, characterized in that the polarization direction rotating element (20) is arranged in the beam path upstream of the partial beam generating element (4).

5. The apparatus according to claim 1, characterized in that the component-beam generating element is a device having two light sources (1', 1 ") which emit polarized light and which are at different distances from the optical unit.

6. The apparatus according to claim 5, characterized in that the two light sources (1', 1 ") of the partial-beam generating element are integrated on a single carrier element.

7. An arrangement according to any one of claims 1 to 6, characterized in that the detection unit has a birefringent prism-type beam splitter (14).

8. An arrangement according to claim 7, characterized in that the detection unit has a detector arrangement (17), in which detector arrangement (17) a plurality of detector elements (A, B, C, D, E, F, G) are arranged in a plane.

9. A device according to any one of claims 1 to 6, characterized in that the detection unit has a semitransparent mirror (16') dependent on the direction of polarization.