WO1985001606A1

WO1985001606A1 - A composite photo-sensitive detector array for optical disk storage systems

Info

Publication number: WO1985001606A1
Application number: PCT/US1984/001591
Authority: WO
Inventors: Brad R. Reddersen
Original assignee: Storage Technology Partners Ii
Priority date: 1983-10-03
Filing date: 1984-10-03
Publication date: 1985-04-11
Also published as: EP0156903A1

Abstract

A composite photo-sensitive diode array (14 in Fig. 5). Four individual photo-detectors (37, 38, 39, 40) and two photo-detector quad arrays (41, 42), each consisting of four photo-detectors arranged in a rectangular shape, are precisely located on a single semiconductor chip. The photo-detectors receive spots of light reflected from the surface of a rotating disk (1) in an optical disk storage system and are used to provide radial and tangential tracking for the read/write head, focus the read spots and write beam, read after writing, and to read data.

Description

A COMPOSITE PHOTO-SENSITIVE DETECTOR ARRAY FOR OPTICAL DISK STORAGE SYSTEMS

BACKGROUND OF THE INVENTION

This invention relates to a single, photo-sensitive, diode array for use in an optical disk storage system, and more particularly, to a single photo-sensitive detector unit for detecting the reflected write beam, reflected read beam, and the reflected focusing and tracking beams, in an optical disk system using a single optical path for the beams.

In operation, data is recorded on, and read from, the surface of a disk having a multiplicity of concentric tracks centered on the axis of disk rotation as the read and write beams move radially over the rotating disk.

Optical, electronic and mechanical components move the read/write head to the proper position over the desired track in order to read or write data. A plurality of light beams (herein called "spots" when they strike the disk or a detector element) are directed through the appropriate optical components to the surface of the rotating disk. One of the light beams.is a high .power beam, such as a laser beam, and is used to write data on the disk surface. A second, lower power, laser beam is used for generating the reading, tracking and focusing beams. The light spots are reflected off the surface of the disk and, through the optical components, to photo-sensitive detector elements. The photo-sensitive elements convert the reflected light spots to electrical signals which are used to read data and provide tracking and focusing information.

The prior art teaches the use. of discrete photo-sen iti e detector elements to detect the light spots reflected from the disk

BAD ORIGINAL surface. Typically, this has required separate optical components for each optical path, which adds to the expense and complexity of the read/write head. See for example, U.S. Patent No. 3,971,002 issued July 20, 1976 to Bricot et al. In U.S. Patent No. 4,290,122 issued September 15, 1983 to Bates et al., the use of multiple spots for tracking and data reading, including a photodetector array to receive the reflected beams, is used to address this problem. However, in Bates tracking can only be done on the special servo/clocking tracks, therefore requiring a separate reading spot for each data track associated with a given servo track. Also, in Bates, there is no provision for using these beams for focusing. The present invention is an improvement in that each data track, after being written, can serve as a tracking guide for its stepwise neighbor. This allows the use of single, read spot. The present invention is also an improvement in that the reflected read and write beams are used for both data gathering and beam focusing.

SUMMARY OF THE INVENTION The disclosed invention, a photo-sensitive detector array, is comprised of a single semiconductor chip which has a plurality of photo-sensitive diodes fabricated into its surface. The diodes are precisely positioned,^' relatively close together, and arranged in an array on the chip surface so as to intercept the reflected beams, ail of which are following a single path after being reflected from the disk surface. Since the diode array pattern matches the pattern of the beams, when the beams are correctly focused and properly aligned, the spots fully center upon the photo-sensitive diodes, When the system is incorrectly focused or improperly tracking, the signals generated by the diodes are processed by the appropriate servo system and a correction signal produced. It is an object of the present invention to provide a means for detecting the read, focusing and tracking beams reflected from the surface of a disk in an optical disk storage system.

It is a further objective of the invention to provide a single unit means for detecting a plurality of beams, including read, tracking and focusing beams.

It is still a further object of 'this invention to provide a means for detecting read, tracking and focusing signals, with said beams traveling in a single optical path.

BRIEF DESCRIPTION OF THE DRAWINGS The above objectives, and other objectives, features and advantages of the present invention will become readily apparent by reading the description of the preferred embodiment which is given in conjunction with the following drawings, wherein:

FIGURE 1 is a schematic representation showing the relationship between a rotating optical disk and a read/write head assembly.

FIGURE 2 is a schematic representation of a portion of the disk surface, showing two data tracks on the disk surface, with seven light spots impinging upon the two tracks.

FIGURES 3a, 3b and 3c show a reflected beam of light impinging, on a quad array of photo-sensitive detector elements, representing respectively the in-focυs condition, the out-of-focus condition in a first direction, and out-of-focus in a second direction.

FIGURE 4 is a schematic representation of the simplified optical path achievable with the disclosed invention.

FIGURE 5 is a top view of a semiconductor chip of the present" invention, depicting the layout of the photo-sensLtivc diodes.

O PI DESCRIPTION OF THE PREFERRED EMBODIMENT The following is a description of the best presently contemplated mode of carrying out the present invention. This description is given only to illustrate the general principles of the invention and is not to be taken in a limiting sense. To ascertain the true scope of the invention, refer to the appended claims.

FIGURE 1 is a schematic representation showing the relationship of a rotating optical disk 1 and a read/write head assembly 2. On the disk 1 are a multiplicity of concentric data tracks 3. This disk 1 rotates in the direction 4, moving the data tracks 3 past the read/write head assembly 2. The mechanical and electrical components necessary to radially translate the optics over the disk 1 are not a part of this invention and therefore are not described in detail. The radial movement 5 of the read/write head assembly 2 is controlled by servo systems, well known to those skilled in the art.

FIGURE 2 shows two data tracks 4a and 4b. The tracks 4a and 4b move relative to the read/write head assembly 2 in the direction shown by the arrow 6. In the preferred embodiment, data is written one track at a time, starting with the outermost track. As a track is filled with data,- adjacent inside track locations on the disk are ' used for writing. This practice has led to the use of two terms, "previous track" and "present track". The present track is the track that is being written on (or read from, as the case may be and will be described later) while the previous track is the adjacent outside track located toward the outside edge of the disk- In FIGURE 2 the track 4b is the previous track while the track •4a is the present track. FIGURE 2 shows a seven spot pattern representing six spots 7-12 of light from the read laser directed through the optical train to the disk 1 surface. The seventh spot is the write spot 13 which writes data on the disk 1. These spots reading spot 7-12 and writing spot 13 are reflected from the surface of the disk 1 to a semiconductor chip 14 the chip shown in FIGURE 5 with a set of photo-sensitive diodes arranged in the same pattern as that formed by the spots 7-12. The electrical signals produced by the diodes are used to control the operation of the reading operation including data reading, tracking and focusing and the writing operation.

The signals generated by light spots 7 and 8 provide a tracking signal and provide a signal which are averaged and used by the servo controller (not shown) to position the read/write head over the desired track. If the beam from head 2 should start to move from the desired position, one of the light spots 7 or 8 will produce a weaker signal while the other will produce a stronger signal. The fine seek servo controller, detecting this difference, will through appropriate logic circuitry, move the read/write beam from head 2 in a direction to zero out the signal difference, thus moving the read back into proper position over the track.

The signal produced by the light spot 9 is used to read information on the previous track 9. Thus, the signal from spot 9 monitors the radial position of the disk 1 surface under the read/write head 2. The- signal, as will be explained later, is also used to focus all the spots from the read beam and the write spot 13. During a write operation, the unwritten present track 4a passes under the light spot 12 before passing under the write spot 13. The signal from the spot 12 is used as a "read before write" signal that is, the present track 4a is read to check for defects in previously recorded data. If either is found, an error signal is sent to the

OMPI controller and the write operation is stopped or the unuseable area on the disk is skipped.

The signal produced by light spot 10 is a "read after write" signal that is, after the the write spot 13 writes data on the disk 1 surface, the data is read and compared to that which was written to check the validity of the writing process. The signal, is also used to check the focus and radial alignment of the write spot 13. The light spot 9 along with spot 13 can also be used to verify track spacing.

The light spots 7, 8, 10 and 12 are each reflected onto separate photo-sensitive diodes. However, the light spots 9 and 13 are reflected onto separate quad arrays of four photo-sensitive diodes. FIGURE 3 shows the quadrature array patterns for reading spots 9 and 13.

As mentioned previously, and as shown in FIG. 2, the signal produced by the light spot 9 is used to both read the previous track and to control the focus of all the light spots, including the write spot 13. To control the focus, the value (A+D)-(B+C) is calculated by a controller, where A, B, C and D represent the signals from the photo-sensitive diodes 14-17, respectively. The light spot 9 is in focus when the value (A+D)-(B+C) is within a predetermined limit. This is shown in FIGURE 3a.

In similar fashion, the write spot 13 also is reflected from the disk 1 and relayed to a second quad detector array 14-17. This array, with similar elements A, B, C and U, can be used to evaluate write focus error in a manner similar to that used for read spot. Also, by electrically calculating (A+B)-(C+D) the write radial tracking error can be determined. The "along-track", or tangential, tracking error of the write spot is calculated from (A+C)-(B+ϋ). Tf there is a radial tracking error, this error signal is used to drive a servo device in the write system which realigns the write beam with respect to the read beam. In a similar fashion, diagnostic information about tangential or focussing errors of the write beam (determined using the other error signals) can be relayed to other servo devices or to the system controller for shutdown of the write system when misalignments are too large. If, however the write spot 13 is out of focus or not tangentially aligned with respect to the read spots, a misalignment has occurred in the write spot optic system which may be addressed by appropriate servos in that system.

FIGURES 3b and 3c represent the light spot 9 being out of focus. In either case, the difference between the absolute values of the calculated differences will be out of the predefined limit so that the controller would adjust a single lens to bring the light spot back into focus. In the preferred embodiment, the six light spots and the write spot all follow a single optical path onto the disk surface. Therefore, all six spots can be focused by adjusting a single optical system.

FIGURE 4 is a simplified drawing of the optics system on the read/write head assembly 2, illustrating the single optical path astigmatic lens focusing system made possible by the present invention. The write beam 18, generated by a first laser source 19 remote from the read/write assembly 2, passing through the focusing lenses 20-21 to a first beam splitter 22. A read beam 23, generated by a second laser source 24 is reflected off the mirror 25 passing through gratings 26-27. The first grating 26 splits the read beam 23 into two separate beams, with the second grating 27 splitting each of these two beams into three beams, creating a total of six beams, all sharing the same optical path and forming spots 7-12 on the disk 1 surface. The lenses 28-29 relay the six spot beam 23 to second beam splitter 30.

The second polarizing beamsplitter 30 transmits the read spots to the first beamsplitter 22. The second beamsplitter 22 passes the write beam 18 and reflects the six spot read beam 23 combining the transmitted write beam 18 with the read beam 23. The beams 18 and 23 then pass through a quarter-wave plate 31 which changes the polarization of the beams 18 and 23 from linearly polarized to circularly polarized, and through a single lens 32, which is used to focus the beams 18 and 23 onto the disk surface 1.

The six read beam spots 7-12 and the write beam spot 13 are reflected off the disk 1, back through the focussing lens 32 and through the quarter-wave plate 31 which again changes the polarization of the beams 18 and 23, back to linearly polarized, but at an angle of 90 degrees to the original incoming polarizations. The reflected write 13 and read spots 7-12 are thus reflected by the first beamsplitter, to the second polarizing beamsplitter 30 which reflects all spots 9-13 to focusing lenses 33 and 34 and onto photodetector array 35. All the light beams 7-13 used are therefore combined into a single optical path 36, greatly simplifying the optical components required on the read/write head assembly 2.

FIGURE 5 is a top view of the semiconductor chip 14 of the present invention with the photo-sensitive diodes disposed thereon, having four circular diodes 37-40 and two quad detectors 41 and 42, forming the same array pattern as the light spots 7, 8, 9, 10, 12 and 13 of FIGURE 2. The only difference between the two arrays is in the relative dimensions. In FIGURE 2, the dimensions of the spots 7-13, the tracks 4a and 4b and the spacing between tracks 4a and 4b is on the order of microns, while the photo-sensitive detector elements 37-41 have dimensions on the order of tens and hundreds of microns. Referring now to FIGURE 4, the lenses 33 and 34 expand the dimensions of the spot patterns 7-13 i-eflected from the disk 1 to match the spacing of the detector array shown in FIGURE 5.

The advantages of the present invention can now be suπmarized. The single semiconductor chip has an array of photo-sensitive diodes, relatively close together, and has the same pattern as the light spots reflected from the disk surface. This allows all the light beams that are reflected from the disk surface to be passed through the same optical components, i.e., the same optical path onto the disk and off of the disk. The write beam is also added to the single path allowing all the light beams to the disk surface to be focused with a single lens. The use of a single light path greatly simplifies the optical system of the read/write assembly and the circuitry necessary for detection of data, tracking and focusing functions. Also, all the functions necessary to control the reading or writing on the desired track are provided by the single semiconductor chip from the reflected light beams.

Claims

CLAIMS What is claimed is:

1. A means for detecting a plurality of light beams reflected from an optical information recording disk in an optical recording device, said device having a read and a write focused beam light source, said means comprised of a semiconductor having a plurality of light sensitive diodes disposed thereon.

2. A light beam detecting means as recited in claim 1 wherein a first set of the diodes comprise a means for detecting focus errors in the device read beam.

3. A light beam detecting means as recited in claim 2 wherein, a system having an astigmatic focusing system, said means for detecting read beam focus errors is comprised of a set of diodes arranged in a quad detector pattern.

4. A light beam detecting means as recited in claim 3 wherein the quad detector pattern further comprises a means for reading data recording on said information disk.

5. A light beam detecting means as recited in claim 4 further comprising a means for simultaneously reading data from the present track as it is being written and reading the data from immediately previously written track.

6. A light beam detecting means as recited in claim 5 wherein means for simultaneous reading from two tracks is comprised of a pair of light sensitive diodes arranged to receive the reflected

read beams from the present writing track and the previous track.

7. A light beam detecting means as recited in claim 1, wherein a second set of diodes comprise a means for detecting focus errors in the device write beam.

8. A light beam detecting means as recited in claim 7, wherein in a system having an astigmatic focusing system, said write beam focus error detecting means is comprised of a second set of diodes arranged in a quad detector pattern.

9. A light beam detecting means as recited in claim 8, wherein the write beam focus detector pattern further comprises a means to provide write beam two-dimensional track error information.

10. A light beam detecting means as recited in claim 1 wherein a third set of diodes comprises a means for detecting read beam tracking errors.

11. A light beam detecting means as recited in claim 10 wherein read beam tracking errors means is comprised of a pair of diodes, each diode separately receiving a pair of tracking spot, the spots focused on opposite edges of the track, such that the signal developed by each diode in response to illumination by each dio e's respective tracking spot is compared, and when said read beam is off track a tracking correction signal is generated.