Static information storage and retrieval – Read/write circuit – Optical
Reexamination Certificate
1998-12-03
2001-10-23
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Optical
C365S234000, C369S100000, C369S109010, C369S110040, C369S111000, C369S112040, C369S116000, C369S275300, C369S275100
Reexamination Certificate
active
06307799
ABSTRACT:
BACKGROUND OF THE INVENTION
Information storage and retrieval is one of the major challenge faced in the past few decades. The challenge is to reduce the size of the data storage media and to increase the access time or data retrieval time. Although technical advancement has been made for the past few years, the mechanical motion of the disk and the optical head assembly using actuator mechanism imposes limitation on the size and density of the optical storage media. The mechanical rotation of the disk results in the wobbling effect of the rotating disk, which accumulates the tolerance limit. Thus a limitation is set on the size and the density of the storage media. Also, the resolution of the actuator movement limits the size and density of the storage media. Moreover, the mechanical movement of the disk and the actuator results in additional problems such as focusing error and tracking error. Therefore, the servo system becomes quite complicated since a feed back signal is required to compensate for tracking and focusing error. Due to limitation set in the resolution of the actuator and the wobbling effect of the rotating disk, it is not possible to reduce the spot size, the distance between the tracks and pitch size in the data storage media.
U.S. Pat. No. 4,550,249 discloses a method of deflecting the beam by using a mirror rather than moving the entire head assembly. The system includes an array of lenses, and each lens focuses the beam at least to one track. This array of lenses covers a substantial portion of the recording media and the deflecting mirror directs the beam onto the desired lens. The system is quite complicated in its design of making an array of lenses and also the means of deflecting the beam using a mirror leads to poor resolution. The error in the system will increase at higher scanning rate due to the vibration on the scanning mirror.
U.S. Pat. No. 4,918,679 discloses an apparatus by which grating is employed for deflecting the beam. The wavelength of the incident beam is varied to change the deflection angle of the diffracted beam from the grating. This system shows an alternative way of positioning the beam to the desired track by producing a tracking error signal.
In order to minimize the size of the disk without compromising on the storage capacity of the optical storage media the resolution of the beam movement should be increased and the wobbling effect of the disk should be minimized. Both the above mentioned facts can be minimized to the maximum limit by making the optical storage media stationary and involving non-mechanical scanning of beam.
Since the control system works in conjunction with reading and writing the data, the simplification of the control system by eliminating or minimizing the focusing and tracking error will maximize the access rate of the data storage media.
Another limitation imposed on the present system is to read as well as record data using the same optical storage system rather than individual one. Further, the system is complicated due to the fact that the beam spot size and power of the beam is not the same for read and write system.
SUMMARY OF THE INVENTION
The present invention discloses a method of reading and writing data on a stationary optical storage medium by applying acousto optic scanning technique for scanning the beam on the surface of the stationary storage media.
The first preferred embodiment of the present invention discloses an optical layout of reading the data from the optical storage system, which is held stationary. The method of scanning the beam using two acousto optic deflector for scanning in two axes is disclosed. The beam from the source is reduced in diameter by a beam reducing technique and is scanned by the acousto optic deflector. A technique of collimating the scanned beam to make the scanning beams parallel to each other is described whereby the scanning beam strikes the stationary optical storage medium perpendicular to the reading surface. The beam is thus reflected back in the same path and is captured by the photo detector, which retrieves the stored data based on the intensity of the signal.
The first embodiment of the present invention also discloses the method of writing the data on the optical storage medium by using the same or different laser source on the same optical path. Further modification in the present embodiment involves the introduction of method for increasing the scan angle of the beam to enable its application for large area storage media. This system comprises a combination of lens of varying focal length.
The second embodiment of the present invention includes the application of scanning lens for reducing the spot size of the laser beam on the optical media. The system includes a beam expander rather than a beam reducer as in the first embodiment. The scanning beam strikes the scanning lens normal to its optical axis and the scanning lens focuses the beam onto the optical media to a small spot size nearly normal to its surface. The system can be modified for reading as well as writing by varying the combination of laser source and the beam expansion ratio or also by applying the same source and beam expander which has flexibility of varying the beam intensity and expansion ratio, respectively.
The third embodiment of the present invention includes the application of scanning lens of different operating principle compared to the second embodiment. Here the scanning beam strikes the scanning lens at an angle depending on the scanning point and the scanning lens employed deflect the beam such that the beam focuses on to the recording media and scans nearly normal to its surface. The system reduces the number of optical components employed in the optical layout there by reducing the error involved in the system. The system also involves a beam expander to expand the beam size in order to reduce the beam spot size on the optical recording media.
The system can be further modified for reading as well as writing by using a different beam source and beam expander or the same having capability of varying the intensity of the laser beam and the expansion ratio of the beam expander.
In all the embodiments of the present invention the size of the optical storage media can be reduced by employing a high-resolution beam scanning, a beam of smaller spot size and a stationary storage media. Also, the modification of the shape of the optical storage media to square and circular shape is disclosed. The nature of scanning on the storage media, linear or circular or spiral manner depending on the shape of the disk, is disclosed.
The fourth embodiment of the present invention is the employment of phase shifting technique for reading the data from the storage media rather than by reflected beam intensity. The optical layout for the modified system on employing phase shifting technique for reading data is disclosed. The need for tracking system can be eliminated since the phase shifting technique can replace the tracking technique available. Other scanning method such as PZT scanning can be employed but the resolution and the scan angle is limited.
REFERENCES:
patent: 4550249 (1985-10-01), Damen et al.
patent: 4918679 (1990-04-01), Opheij et al.
patent: 5740148 (1998-04-01), Ja
Ngoi Bryan Kok Ann
Venkatakrishnan Krishnan
Burns Doane , Swecker, Mathis LLP
Nanyang Technological University
Tran Andrew Q.
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