This invention relates to an optical scanning device for scanning optical record carriers.
In known optical scanning devices, optical heads scanning the record carrier with multiple radiation beams have been proposed to increase the speed at which the data on the record carrier can be read. The data buffering needed to output a sustained byte stream in such multi-beam optical disk drive requires substantial memory sizes. This is because most optical disks such as CD and DVD store data along a continuous spiral and they are not specifically designed for random access of the data stored. For this reason, it is necessary to store the entire quantity of information read during one disk revolution by several beams in order to reconstruct electronically the continuous spiral.
Instead of multiple radiation beams the use of multiple optical heads has been proposed. In this way more data can be read at higher speeds, thereby improving the data throughput. JP 08-180455A describes an optical scanning device having two optical heads. Only one optical head is operable at any one time, but access times can be reduced by using the heads sequentially. JP 03-192525A describes a device having two optical heads. However, the optical heads and the associated optics and detector array all move during operation of the system. This makes the unit difficult to implement, expensive and bulky.
According to the invention there is provided an optical scanning device for scanning an optical record carrier, the device including radiation source means for generating radiation and optical elements for transmitting the radiation towards the optical record carrier, the elements comprising a plurality of independently moveable optical heads, each optical head being mounted in a separate radial tracking path and each radial tracking path being angularly separated about the centre of the optical record carrier, wherein the device is adapted to supply each of said optical heads simultaneously with radiation from the radiation source means.
In this way, an improved data throughput can be achieved, and similar radial areas of the record carrier can be scanned simultaneously. Furthermore, it is possible to implement such a system to be compatible with existing record carrier formats. implementation is also relatively inexpensive, unlike multiple beam systems, which require substantial redesign of existing systems.
Further objects, advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings, in which:
a shows an optical scanning device, in accordance with the invention, incorporating two optical heads, each optical head focusing a radiation beam on the record carrier;
b is a schematic drawing of one of the optical heads of
The record carrier 2 is in the form of an optical disk comprising a trnsparent layer 3, on one side of which an information layer is arranged. The side of the information layer 4 facing away from the trasparent layer 3 is protected from environmental influences by a protection layer 5. The side of the transparent layer 3 facing the device is called the entrance face 6. The transparent layer 3 acts as a substrate for the record carrier by providing mechanical support for the information layer. Alternatively, the transparent layer may have the sole function of protecting the information layer, while the mechanical support is provided by a layer on the other side of the information layer, for instance by the protection layer 5 or by a further information layer and a transparent layer connected to the information layer 4. Information may be stored in the information layer 4 of the record carrier in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks, not indicated in
The scanning device comprises a linearly polarized radiation source in the form of a semiconductor laser 9 emitting a radiation beam 7. The radiation beam is used for scanning the information layer 4 of the optical record carrier 2. A polarizing beam splitter 13 reflects the diverging radiation beam 12 on the optical path towards a collimator lens 14, which converts the diverging beam 12 into a collimated beam 16. The beam 16 is incident on an objective system 18. The objective system may comprise one or more lenses and/or a grating. The objective system 18 in
Radiation reflected by the information layer 4 forms a diverging beam 22, transformed into a substantially collimated beam 23 by the objective system 18 and subsequently into a converging beam 24 by the collimator lens 14. The beam splitter 13 separates the forward and reflected beams by transmitting at least part of the converging beam 24 towards a detection system 25. The detection system captures the radiation and converts it into electrical output signals 26 which are processed by signal processing circuits 27 and 29 which are located in the scanning device separately from the optical head 1. A signal processor 27 converts these output signals to various other signals.
One of the signals is an information signal 28, the value of which represents information read from the information layer 4. The information signal is processed by an information processing unit for error correction 29. Other signals from the signal processor 27 are the focus error signal and radial error signal 30. The focus error signal represents the axial different in height between the spot 21 and the information layer 4. The radial error signal represents the distance in the plane of the information layer 4 between the spot 21 and the centre of a track in the information layer to be followed by the spot.
The focus error signal and the radial error signal are fed into a servo circuit (not shown) which converts these signals to a focus error signal and a tracking error signal for controlling mechanical focus actuators (not shown) in the optical head. The mechanical focus actuators control the position of the objective system 18 in the focus direction 33, thereby controlling the axial position of the spot 21 such that it coincides substantially with the plane of the information layer 4, and in the radial direction 34, thereby controlling the radial position of the spot 21 such that it coincides substantially with the track currently being scanned. A further mechanical actuator, such as a radially movable arm, alters the position of the optical head 1 in the radial direction 34 of the disk 2, thereby coarsely controlling the radial position of the spot 21 to lie above a track to be followed in the information layer 4. The tracks in the record carrier 2 run in a direction perpendicular to the plane of
a illustrates the general concept of the invention to be described in more detail below. As can be seen in
b illustrates optical components used in each optical head H1, H2. Each optical head includes an objective system 118, 218 mounted in mechanical actuators (not shown) for focus error and tracking error correction. A folding mirror 41, 42 reflects the beams to and from a surface in the record carrier 2.
The two separate radiation beams 35, 36 may be generated by one of several methods to be described below.
Referring first to
The radiation reflected by the information layer 4 at the two points on the record carrier 2 is transmitted back through the objective lenses 118, 218 and is incident on the mirrors, 41, 42. The reflected beams are then transmitted to the prism 43 whereupon the PBS coating reflects and merges the beams. The merged beam is then incident on the collimator lens 14 and transmitted to the beam splitter 113. The beam splitter 113 separates the forward and reflected beams by transmitting at least a part of the reflected information beam a detector system comprising a micro prism 45 incorporating a PBS coating 47. The PBS coating 47 acts to split the reflected information beam into two beams 50 and 51, the beam 50 representing the information from the region under the lens 118 on the record carrier 2 and the beam 51 representing the information from the region under the lens 218 on the record carrier 2. The beams 50, 51 are polarized at 90 degrees to each other. The radiation beams 50, 51 are then incident on two separate portions 52, 53 of a detector array 54. Respective portions of the detector array 54 produce information signals, focus error signals and radial error signals for the respective areas of the record carrier 2 currently being scanned by the different optical heads.
In
In
In operation of the scanning device, the optical heads H1, H2 maybe operated, with the assistance of a scanning control system (not shown), to move the heads independently using the coarse radial tracking units 37, 38 and each to conduct read out from separate data areas at the same time. This improves reading speed when reading out data from a number of separate data areas, as is often the case when the optical record carrier holds a large number of data files stored in such different data areas. When reading out data from a single data area, as for example in the case of audio data when a single data stream is to be produced, the heads can be used to scan different parts of the same data area. For example, a first head may be used to scan a predetermined number of tracks forming a first set of tracks whilst the second head is used, at the same time, to scan the same number of tracks forming a second set of tracks adjacent the first set of tracks, after which the tracking of the first head is jumped to a third area adjacent the second set of tracks and the tracking of the second head is jumped to a fourth area adjacent the third set of tracks, and the two heads continue to read different data at the same time in the third and fourth areas, and so on. In this way, successive parts of the same data area may be read out an increased speed whilst the final data stream is produced by combining the data streams received from the two detector systems corresponding to each head.
In the examples described above, electronic compensation of crosstalk between the channels may be provided in the signal processing stage. Adjustment of the laser for beam angle in two directions perpendicular to the optical axis may be provided after the collimator in the beam path. Furthermore, movable adjustment of the collimator lens along the optical axis may be provided. Additionally, angular adjustment of the mirrors 39, 40 may be provided, to accurately fold the beams towards the moveable optical heads and adjustment around the optical axis of the prism, polarizing plate or polarizing grating above the collimator may be provided in order to reduce crosstalk between the two channels.
The arrangement illustrated may be used to read data out from two parts of the disk simultaneously. Furthermore, the device may also include data writing capability. By using two separate optical paths and lasers, data can be written to two parts of the disk simultaneously.
Thus, the invention provides an optical scanning device that includes two independently moveable optical heads capable of reading two distinct areas of an optical record carrier simultaneously.
The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, instead of splitting radiation from a single radiation source, two separate radiation sources may be used. It is to be understood that any feature described in relation to one embodiment may also be used in other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
Number | Date | Country | Kind |
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01205087.8 | Dec 2001 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/IB02/05630 | 12/19/2002 | WO |