Optical: systems and elements – Deflection using a moving element
Reexamination Certificate
2001-10-30
2004-10-05
Cherry, Euncha (Department: 2872)
Optical: systems and elements
Deflection using a moving element
C359S205100
Reexamination Certificate
active
06801350
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improvements in optical disk drives and scanning methods.
Optical disk drives are devices that can read, and possibly also write data on, and erase data from, optical disks. The data may be analog or digital, and may contain computer data, video, audio, or other information. To be able to read data, an optical disk drive needs to focus laser light to a spot about 1 &mgr;m or less in diameter on a recording surface area of the disk, then collect the light reflected from that spot. It also needs to reach any point on the recording area of the disk quickly. To record on rewritable optical disk media, the drive must additionally be capable of delivering sufficiently high laser power into this spot, and must have means to modulate the intensity of the light.
The conventional optical disk drive has a head, typically containing one or more semiconductor lasers, lenses and other optical devices, optical detectors and electromechanical focus and tracking actuators. To reach a particular location on the disk, the head moves essentially along a radius of the disk, while the disk is rotating. Since the head is complex and contains many optical and mechanical parts, it is heavy, resulting in slow acceleration. Therefore, the time it needs to reach a particular destination, the seek time, is relatively long. This long seek time is even more disturbing when compared with that of hard disk drives in which ultra lightweight magnetic heads are employed. It is well known that the overall performance of a disk drive is strongly affected by the seek time.
Several attempts to overcome or circumvent these limitations of conventional optical disk drive technology have been described. Two of these, both using one or more laser beam scanners with an array of focusing elements, will be discussed below.
While many attempts to improve optical disk drives have been made and described in publications, they are primarily intended to improve the transfer rate of data between the optical disk media and the electronic signal processing channel, usually by applying parallelism. Transfer rate is the number of bytes per second that can be read or written after the correct physical disk location has been accessed. A well-known approach to increasing transfer rate without increasing the rotational speed of the disk is parallel access. However, the known implementations of this approach do not support decrease of seek time. In practice, since they all add complexity and weight to the moving head, they should actually increase the seek time. In addition, they are applicable only to readout and are not intended to support multiple track writing.
However, although increase in transfer rate would be desirable, it is presently believed that access time is far more significant—for most computer based applications the time spent getting to the correct location on the disk far exceeds the raw transfer time. Access time is the sum of seek time, which is the time it takes the head to reach a specific track, and latency, which is the time needed for the disk to rotate until the right part of the track (sector) is next to the head. Access time is the combination of seek time and latency—the total time from initiation of commands till actual read/write operation can start. It has been proposed to decrease the seek time in optical disk drives by decreasing the weight and size of the moving head, essentially, for example, by splitting the head into a stationary part and a lighter moving part or by adopting substrate-mode optics, or planar optics, in the internal construction of the moving head. Neither technique can approach the order of magnitude of the weight of heads already deployed in non-optical magnetic computer disk drives, or hard disk drives (HDDs).
The different characteristic of light, as opposed to electrons, makes miniaturization of the optical system of a moving-head optical disk drive to the order of magnitude of the size already employed in HDD heads inherently extremely difficult.
Glaser, U.S. Reissue Patent No. Re 36,393, entitled Two-dimensional random-access scanning for optical disks, reissued on Nov. 16, 1999, the disclosure of which is incorporated herein by reference, discloses an arrangement, as shown in FIG. 1 (taken from that patent), that uses two-dimensional (2D) scanning and a 2D array of light focusing devices, such as a 2D lenslet array. In FIG. 1, light that is generated by a laser 2 passes through beam shaping optics 5 and is then redirected by a two-dimensional optical scanner. The optical scanner is composed of two moving mirrors 7 and 8 and two electromechanical actuators, M1 and M2 toward a two dimensional array of focusing elements 1, depicted as a lenslet array. Each focusing element in the array addresses a range of tracks on the recorded, or recording surface area 3 of the disk. The specific track selected depends on the precise angle at which the laser light reaches that lenslet. For readout, light focused on the track is reflected back through the same lenslet, acting as a ‘cat-eye’ retro-reflector to send light back toward its source. This retro-reflected light is intercepted by a beam splitter 11 towards a stationary detector head 12. The detector head, with its associated electronics, derives signals for the data on the disk, as well as for servo controls such as tracking and focus.
Damen et al, U.S. Pat. No. 4,550,249, entitled Optical disk read/write apparatus, issued on Oct. 29, 1985, the disclosure of which is incorporated herein by reference, describes a different type of optical and mechanical configuration for an optical disk drive scanning system.
The basic embodiment of this configuration has two lenslet arrays (
10
,
11
and
12
in
FIGS. 2A
,
11
,
12
,
13
,
21
,
22
, and
23
in
FIG. 2B
,
FIGS. 2A and 2B
being taken from the Damen at al patent), one above the other, with a set of semi-reflecting, or slotted, mirrors between them (
41
,
42
and
43
in FIG.
2
B). A laser beam 50 from a laser
16
is directed by a series of mirrors
18
,
19
and
30
towards the gap between the two arrays. This laser beam
50
is split by a first semi-reflecting mirror
41
and the reflected beam is directed through, and focused by, a lenslet
11
toward a disk data storage and/or recording surface
14
. The beam is then reflected by the same disk surface, passes through lenslets
11
and
21
and semi-reflecting mirror
41
and is imaged onto a detector array
15
. Laser beam light that passes through mirror
41
is then split by semi-reflecting mirror
42
, where it is focused on another location on the disk recording surface
14
and imaged onto the detector array
15
, and so on. Sufficient optical power must be provided to allow this multiple parallel access. The beam steering system of moving mirrors
18
and
19
is one-dimensional in the sense that it can move the laser beam only in a single plane.
A drawback of this prior art system is that writing is essentially impractical. Since all of the illuminated tracks get the same signal, they would all be written by the same data, resulting in multiple copies and a vastly smaller effective data capacity. Laser power is distributed over many tracks, so a very powerful laser is needed to obtain sufficient writing energy. Wrong tracks will be accessed. The same patent describes an alternative configuration with slit mirrors instead of beam splitter, but a simple calculation shows that these slit mirrors will cause the focused spot to become larger, intolerably decreasing the data capacity of the disk. While that system allows parallel access, the tracks that are accessed together are not adjacent ones. As a result, tracks are read in the wrong order, at least for removable disks that are formatted for conventional drives.
This scheme seems to provide both fast access and parallel (high data rate) readout. However, since the accessed tracks are far from each other, focusing errors, due to surface irregularities etc., will probably be uncorrelated, so that central focus
Glaser Rann
Glaser-Inbari Isaia
Browdy and Neimark , P.L.L.C.
Cherry Euncha
MMRI Photonics, Ltd.
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