Acousto optic data storage system on a stationary and high density data storage media

A miniaturized stationary optical storage system capable of reading and writing data on an optical storage media is disclosed. In the disclosed invention, the disk is held stationary and the beam is made to scan on both the axes by using an acousto optic technique. The preferred embodiment has a technique of obtaining parallel scanning beam, which is focused on to the optical storage media and the photo detector receives the reflected beam carrying the optical signal. The system has the capability of reading as well as writing on the optical storage media by using the same source or different source. The preferred embodiment also utilizes square as well as circular optical storage media of reduced size, which is achieved, by adopting smaller spot size and higher scanning resolution. The method of magnifying the scan angle of the scanning beam from the acousto optic deflector is also disclosed. The alternative embodiment utilizes phase shifting for reading the signal rather than the intensity of the signal.

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.

DETAILED DESCRIPTION OF THE INVENTION
 Referring to FIG. 1A, the optical beam 2A is emitted from the source 1,
 which is preferably a light or laser beam. The source preferably emits
 fine collimated laser beam of linear polarization. The beam is filtered on
 passing through a spatial filter 3A. The purpose of the spatial filter is
 to improve the beam quality. The filtered beam is reduced in size on
 passing through preferably a beam reducer 3B. The beam reducer reduces the
 beam size to the desired size using a combination of preferably
 collimating or focusing optics. The reduced beam 2B passes through a lens
 4, which is preferably of a collimating type. The beam 2C is thus focused
 on to the acoustic crystal in the acousto optic deflector 5 and 6. The
 scanning beam 2D obtained on passing through the acousto optic deflector
 is made to pass through the lens 7, which is preferably of collimating
 type and is of preferably the same specification as that of the lens 4.
 This process will probably generate parallel-scanning beams at all
 deflection angle from the acousto optic deflector. The parallel scan beam
 2E preferably pass through beam splitter 8, which is preferably a
 polarization beam splitter. Also a wave plate 9 is fixed on to the beam
 splitter or in front of it, the Wave plate being preferably a quarter wave
 plate in nature. The beam 2F preferably strikes the stationary disk 10 at
 an angle perpendicular to the axis of the disk at all scan angled of the
 deflector and is reflected back. The reflected beam preferably passes
 through the same optical component 9 and 8 which changes the polarization
 state of the beam deflects the beam in another direction. The deflected
 beam 2G passes through a focusing optics 11, which is preferably of
 collimating type, and is focused on to the photo detector 12, which is
 preferably of array type or wide window type. In the current system the
 disk remains stationary and the laser beam is made to rotate at a preset
 speed to read the data.
 The source 1 preferably emits fine collimated light of linear polarization
 is of preferably in the wavelength range from 550 nm to 750 nm. A diode
 laser can be also alternatively applied for the purpose. The laser source
 may preferably emit a laser beam of small or large beam diameter depending
 on the combination of optical component in the system. The beam reducer
 employed in the system may preferably made by combination of lens of
 different focal length, thus reducing the beam diameter to the
 requirement.
 Referring to FIG. 2 the lens 7 preferably be of collimating type, placed at
 a distance of twice the focal length from the lens 4, which is of the same
 specification as that of the lens 7. This process will produce collimated
 scan beams, which is of the same beam diameter as that of the input beam.
 The larger the focal length of the lens 4 and 7 the larger will be the
 scans length. But when the focal length of the lens 7 is larger than the
 focal length of the lens 4, the systems acts also as a beam expander,
 which may be essential to reduce the beam spot size using a scan lens. The
 parallel scanning beam of minimum divergence is made to strike the disk 10
 perpendicular to the wide surface of the disk so that the beam is
 reflected back from the disk 10 in the same optical path. The system can
 also act as a beam reducer when the lens 4 is of longer focal length
 compared to the lens 7. Here the lens 4 and 7, are separated by a distance
 equal to the sum of the focal length of the lens 4 and 7. The longer the
 focal length of the lens 4 and shorter the focal length of the lens 7, the
 smaller will be the beam size. In this case the need for the beam reducer
 3 is eliminated. But the system will eventually may result in the
 reduction of scan length and may preferably require a larger scan angle
 acousto optic deflector.
 Further modification of the preferred embodiment is the capability of the
 system to act as a read as well as write data storage system as shown in
 FIG. 1B. The system includes a writing light source 1A, preferably of the
 same wave length as that of the reading light source 1 but of higher power
 compared to light source 1 depending on the power and beam spot size
 required to write on the disk. The beam reducer 3C may preferably be of
 higher beam reducing power compared to that of beam reducer 3B. The beam
 15A from the light source 1A is orthogonal polarized compared to the beam
 from the light source 1. Otherwise a wave plate, preferably a half wave
 plate is placed in optical lay out of the write beam before the beam
 splitter 13. The write beam 15A then passes through a spatial filter 3A
 and then through the beam reducer 3B leading to reduced beam 15B, which is
 deflected by the plane mirror 14 on to the beam splitter 13, which is
 preferably a polarization beam splitter. Thereafter the beam takes the
 same optical path as that of the reading beam from the light source 1 and
 strikes the disk surface to record the information on to the disk. The
 need for separate light source can be eliminated if the light source has
 the capacity of switching the beam power to two different modes, i.e., one
 mode for writing of higher light power and another mode for reading of
 comparatively lower beam power. Also common beam reducer can be employed.
 Larger scan angle is essential for disk of larger diameter. In order to
 obtain a larger scan angle so that the light beam cover a larger area an
 additional attachment is made on the optical layout of the system. FIG. 4A
 shows the optical arrangement required for increasing the scan angle and
 also to reduce the beam spot size. The system comprises a combination of
 two lenses 20 and 21 of preferably positive focal length separated by a
 distance equal to the sum of the focal length of the two lenses. The beam
 2D passes through the lens 20 and 21 which eventually results in the
 output beam 2G preferably of larger deflection angle and also preferably
 of smaller beam diameter. The output beam 2G is then made to pass through
 the lens 7 and thereafter takes the optical path as shown in FIG. 1. The
 system acts as a Keplerian telescope.
 Another way to establish the same is as shown in FIG. 4B. This system
 consists of two lenses 22 and 23 preferably of positive and negative focal
 length. Here the two lens 22 and 23 are separated by a distance equal to
 the difference in the focal length of the two lenses. Thereby the system
 compact compared to the previous method. This system acts as a Galilean
 telescope. The system will lead to more compact structure than the
 Keplerian telescope.
 Second Embodiment of the Present Invention
 Referring to FIGS. 3A and 3C, the second embodiment of the present
 invention replaced the beam reducer 3B by a beam expander 3D. Here the
 beam 2A from the source 1 passes through the spatial filter 3A in order to
 filter the beam thereby improving its quality. The filtered beam passes
 through a beam expander, which is of fixed or variable beam expansion. The
 expanded beam then passes through the collimating optics 4 and through the
 acousto optic deflector 5 and 6. The deflected beam passes through the
 collimating lens 7, which leads to parallel scanning beam as described in
 the first embodiment. The beam 2E them passes through the polarizing beam
 splitter 8 and the wave plate, which is preferably a quarter wave plate.
 The scanning beam then strikes the scanning lens 3F normal to its optical
 axis. The purpose of the scanning lens is to focus the beam on to the disk
 to a very small spot size. The spot size and hence the specification of
 the scanning lens depends on the pitch diameter of the data written on the
 disk. The smaller the disk size, greater is the density of the disk and
 smaller is the focused beam spot size. The beam 2F emerging from the
 optics 8 and 9 passes through a scan lens 3B that focuses the beam on to
 the optical disk 10 to a very small spot size. A distance equal to the
 focal length of the scanning lens separates the scan lens from the disc.
 The beam reflected from the disk regains its beam size on passing through
 the scan lens 3B on its return path as shown in detail in FIG. 3C. The
 reflected beam passes through the scanning lens 3F, where it regains its
 original size and passes through the wave plate 9 which changes the
 polarization state of the beam. The beam is then deflected by the
 polarizing beam splitter 8 on to the collimating or focusing lens 11 which
 focuses the beam on to the photo detector 12.
 Referring to FIG. 3B the system can be modified for writing and reading by
 employing a different laser source 1A. The writing beam 15A from the laser
 source 1A of higher power than the reading beam passes through the spatial
 filter 3A and through the beam expander 3E of greater expansion ratio than
 the beam expander 3D. The expanded beam 15B is deflected by the defecting
 mirror 14 on to the polarizing beam splitter 13. Then the beam takes the
 path as the reading beam and strikes the optical media on which the data
 is written. The purpose of the beam expander is to increase the beam size,
 which influence the beam spot size on the optical writing media 10 on
 passing through the scanning lens 3F. The larger the beam expanded by the
 beam expander by proving a larger beam expansion ratio, the smaller will
 be the spot size. Also the wavelength of the beam affects the beam spot
 size. Smaller the wavelength of the beam smaller will be the spot size.
 The beam expansion and the wavelength of the beam are chosen depending on
 the spot size requirement. Alternatively the same source can also be
 employed for reading as well as writing when the source has the capability
 to vary the intensity of the beam. If the beam expander 3D used is of
 variable expansion capability the beam can be expanded to different ratio
 for reading and writing. For writing the expansion is higher compared to
 reading in order to achieve a smaller spot size.
 Third Embodiment of the Present Invention
 Referring to FIG. 5A the beam 2A from the source 1 passes through
 preferably a spatial filter 3A, which filters and improves the quality of
 the beam. The filtered beam passes through a beam expander 3D of fixed or
 variable beam expansion type. The expanded beam 2B passes through the
 acousto optic deflector 5 and 6, which deflects the beam on two axes. The
 deflected scanning beam passes through the beam splitter 8, which is
 preferably a polarizing beam splitter and a wave plate 9, which is
 preferably a quarter wave plate. The scanning beam strikes the scanning
 lens 3F, which deflect the beam, such that the scanning beam 2F strikes
 nearly normal to the optical recording media surface 10. The beam also
 focus on to the recording media 10 to a small spot size. The accuracy to
 which the scanning beam 2F strikes normal to the surface of the recording
 media 10 can be improved by varying the type of scanning lens used.
 Scanning lens such as F-Theta lens, Telecentric lens, Confocal microscopy
 lens etc can be applied for the current application. The beam 2F gets
 reflected from the optical storage media 10 and passes back through the
 scanning lens 3F and the wave plate 9. The beam splitter 8 then deflects
 the beam. The scanning beam 2H then arrives at the same point on the photo
 detector 12 without using a collimating optics. The collimating or
 focusing optics may also be used in order to focus the beam on to a small
 spot size depending on the optical window of the photo detector. The
 distance between the scanning lens 3F and the recording media 10 is equal
 to the focal length of the scanning lens 3F. By using the current system
 the number of optical components are considerably reduced.
 Referring to FIG. 5B the system can be modified for writing and reading by
 employing a different laser source 1A. The writing beam 15A from the laser
 source 1A of higher power than the reading beam passes through the spatial
 filter 3A and through the beam expander 3E of greater expansion ratio than
 the beam expander 3D. The expanded beam 15B is deflected by the defecting
 mirror 14 on to the polarizing beam splitter 13. Then the beam takes the
 path as the reading beam and strikes the optical media 10 on which the
 data is written. The purpose of the beam expander is to increase the beam
 size, which influence the beam spot size on the optical writing media 10
 on passing through the scanning lens 3F. FIG. 5C shows the detail
 description of the scanning technique involved in the current embodiment.
 In all the embodiments the laser beam can be pulsed for writing by
 controlling the driver of the acousto optic deflector. Here the beam is
 pulsed by varying the intensity of the scanning beam. Also the beam can be
 pulsed by applying a additional acousto optic modulator next to the beam
 source to pulse the laser beam accordingly.
 In all the embodiments the need for focusing mechanism may preferably be
 eliminated in the disclosed invention due to the stationary nature of the
 disk and the non-mechanical movement of the optical head. Thus the servo
 control mechanism involved in the system is simplified by employing the
 acousto optic deflector for scanning mechanism.
 In all the embodiments the Size of the disk that can be employed in the
 system depends on the distance between the acousto optic deflector and the
 disk surface. The larger the disk the longer the distance between the
 acousto optic deflector and the disk Surface. Since the resolution of
 scanning is high and the beam spot size is less, smaller disk can be
 employed, which reduces the distance between the acousto optic deflector
 and the disk Surface. Thus by employing a smaller disk the size of all the
 optical components involved in the system is reduced, making the system
 compact.
 In all the embodiments the disk shape can be modified to square or circular
 shape. Since the need for rotating the disk is eliminated the need for
 central slot may not be required. This eventually results in the reduction
 of the disk size. Also a square disk as shown in FIG. 6A can replace the
 circular disk. In square disk the circular scanning is replaced by linear
 scanning, which has a simpler scanning control of the beam. Here, data is
 written in a linear fashion rather than in a circular pattern as shown in
 FIG. 6B. The scanning mechanism can be of two types. Referring to FIG. 8,
 the beam scans along the path 1 and at the end of the scan length L the
 beam is stepped by a prescribed distance D which is the track pitch in the
 perpendicular direction of scan. Now the beam is scanned in the reverse
 direction along the path 2 and on reaching the end of scan, the beam is
 again stepped by the distance D. The beam is now scanned along the path 3
 in the same direction as in path 1. The process is repeated over the
 entire disk as the data is written along the paths 1, 2, 3, etc. The
 Alternative way of scanning the square disc is as shown in FIG. 9. The
 beam is scanned along the path 1 for the scan length L where it reads or
 writes data. On reaching the end of the scan length, the beam returns in
 the same path 1. On the return journey of the beam data is not read or
 written. The beam on reaching the start point is stepped by a distance D,
 the track pitch in the perpendicular direction to that of the scanning
 path 1. The beam then scans along the path 2 and the process are repeated.
 This scanning mode will employ a much more simplified control mechanism in
 comparison to the previous scanning method.
 Similarly in a circular disk the data can be written preferably in a
 circular fashion or in a spiral manner. In the modified disk of the
 present invention the need for central fixing or holding space is
 eliminated and is occupied by data storage, thereby minimizing the disk
 size. A central location pin or any other location mechanism may probably
 be the only requirement. In the method of writing data in a circular
 fashion is as shown in FIG. 7A, the beam is stepped by a distance of track
 pitch on completion of the entire circular scanning. The alternative way
 is the spiral scanning as shown in FIG. 7B where the beam is scanned
 spirally and the data is written or read accordingly.
 In all the embodiments the size of the disk can be reduced, which may
 preferably also reduce the access time. The process of reducing the disk
 size and simultaneously increase the density of the disk implies a reduced
 size for the beam and also an enhanced resolution for scanning. The
 reduction in beam size is directly related to the resolution of beam
 scanning. The smaller the beam size and higher the resolution the smaller
 a disk can be made. A beam reducer employs a combination of lenses with
 large focal length and small focal length, to reduce the size of the beam.
 To reduce the access time the scanning speed of the beam is increased.
 Referring to FIG. 10 the pit distance is given by p and distance between
 the track, the track pitch is given by t and the diameter of each spot or
 pit length is given by s. By increasing the resolution of scanning and
 reducing the beam spot size, the distance p, s, t can be reduced. Since
 the system involves no mechanical movement of the disk or the head very
 high resolution can be achieved, which can reduce the size of the disk or
 increase the data storage capacity of the disk.
 Referring now to FIG. 11, the two acousto optic deflectors 5 and 6 are
 positioned perpendicular to each other for x-axis and y-axis scanning.
 Also the acousto optic deflector 6 is preferably at Brag's angle to the
 acousto optic deflector 5. The control system for the acousto optic
 deflector is as shown in FIG. 11. The Frequency signal from the signal
 generator-X 32 is amplified in power by the power amplifier 30. The
 amplified signal is inputted to the acousto optic deflector 5. Similarly
 the frequency signal from the signal generator-Y 33 is amplified in power
 by the power amplifier 31. The amplified signal is inputted to the acousto
 optic deflector 6. The acousto optic deflector comprises of an acoustic
 crystal where acoustic wave is generated on supplying frequency signal to
 the transducer fixed to the crystal. The acoustic wave acts as a moving
 grating and deflects the beam to a prescribed angle depending on the
 frequency input and type of crystal involved, and the velocity of the
 acoustic wave. Therefore, by varying the frequency input to the acoustic
 crystal the deflection angle of the crystal is varied and hence the beam
 is made to scan. The input frequency to the acousto optic deflector is the
 only controlling parameter involved in the system, which simplifies the
 system.
 The detailed description of the acousto optic deflector involved in the
 system is as shown in the FIG. 12. T.sub.x is the transducer fixed to the
 acoustic crystal 5. A signal generator drives the transducer. When a
 frequency signal is given to the transducer T.sub.x, the electrical signal
 is converted to an acoustic wave A.sub.x. The acoustic wave travels away
 from the transducer and is damped at the other end to prevent back
 reflection of the acoustic wave. The acoustic wave A.sub.x has a frequency
 equal to the frequency input to the transducer. The spacing between the
 acoustic wave or the wavelength of the acoustic wave .lambda..sub.A
 depends on the frequency input to the transducer T.sub.x and the velocity
 of the acoustic wave is constant for a given material.
 The acousto optic deflector 6 is placed such that the acoustic wave in the
 acousto optic deflector 5 is perpendicular to the acoustic wave in the
 acousto optic deflector 6 as illustrated in FIG. 13. T.sub.y is the
 transducer fixed to the acoustic crystal 6. A signal generator drives the
 transducer. When a frequency signal is given to the transducer T.sub.y,
 the electrical signal is converted to acoustic wave A.sub.y. The acoustic
 wave travels away from the transducer and is damped at the other end to
 prevent back reflection of the acoustic wave. The acoustic wave A.sub.y
 has a frequency equal to the frequency input to the transducer. The
 spacing between the acoustic wave or the wavelength of the acoustic wave
 .lambda..sub.B depends on the frequency input to the transducer T.sub.y
 and the velocity of the acoustic wave is constant for a given material.
 When the beam 2C enters the acoustic crystal at Brag's angle .theta..sub.B,
 a deflected beam is obtained at an angle .theta..sub.D. Since the
 deflection angle .theta..sub.D depends on the wavelength of the acoustic
 wave, independent deflection angle is thus obtained for each of the
 frequency input as illustrated in FIG. 14. Thus by controlling the
 frequency input to the two acousto optic deflectors 5 and 6 the bean can
 be scanned in two axis to any required point. The maximum defection angle
 depends on the bandwidth of the acousto optic deflector. The resolution of
 scanning depends on the number of distinguishable frequency inputs that
 can be obtained.
 Any one of the techniques such as push-pull, phase detection, wobble,
 sample servo may detect the tracking error. The tracking error 35 is
 inputted to the servo controller, which in turn generates the tracking
 offset to the signal generator X 32 and signal generator Y 33.
 Fourth Embodiment of the Present Invention
 The fourth embodiment of the present invention is the employment of a phase
 shifting technique, which measures the shift in phase of the beam to
 identify the signal instead of measuring the intensity of the beam. Once
 the phase shifting technique is employed the need for detecting the
 tracking error is eliminated. The modification of the system to phase
 shifting technique is as shown in FIG. 15. The beam 2E is circularly
 polarized rather than linearly polarized. The beam splits in to two
 orthogonal polarized beams 2H and 2F. The beam 2F as in the first
 embodiment pass through the wave plate 9, preferably a quarter wave plate
 and strikes the disk 10 and reflects back. The beam 2F on passing through
 the wave plate 9 on the return path, thereby changing the polarization of
 the beam and hence the beam is reflected by the polarizing beam splitter 8
 towards the photo detector 12. The beam 2H passes through a wave plate 19,
 preferably a quarter wave plate and strikes a plane mirror 18. The beam 2H
 is then reflected by the mirror 18 and passes through the wave plate 19 on
 its return path. Thereby the polarization state of the beam is changed and
 hence the beam passes through the polarizing beam splitter 8. Thus, the
 two beams 2H and 2F overlaps to form the beam 2G. The beam 2G passes
 through a focusing lens 11 on to the photo detector 12. A polarizer 17 is
 placed in front of the photo detector 12 so that the beam 2G comprising of
 the two orthogonal-polarized beams interferes. Based on the phase shift of
 the interference signal the data can be read. Also the need for tracking
 system can be eliminated since the phase shifting technique can replace
 the tracking technique available.
 Other scanning technique such as piezo driven scanning mirror etc can be
 employed replacing the acousto optic deflector but the control of the
 system is more complicated. Also it is comparatively difficult to reduce
 the disc size due to the vibration if the scanning mirror which will
 degrade the resolution of scanning. Moreover, the scanning speed is also
 limited by the increase in the vibration error with scanning speed and
 hence results in limited data access rate.
 For all the embodiments for writing either the beam source 1 can be pulsed
 or a continuous beam from the laser source can be pulsed by having an
 external acousto optic modulator. The acousto optic modulator is placed in
 front of the beam source 1 and is pulsed accordingly by varying the
 intensity of the first order beam or higher order beam depending on the
 beam employed for writing. Also, the pulsing can be carried out by
 controlling the intensity of the acousto optic modulator 5 or 6 which
 eliminates the need for addition pulsing modulator in front of the beam
 source 1.
 Advantages of the System over the Current Techniques
 The present invention has several advantages over the prior art for reading
 and writing on a optical disk are. Since all the mechanical movement
 including the rotation of the disk is eliminated due to the scanning
 mechanism disclosed in the invention. By making the disk stationary the
 tolerance in the optical storage system is reduced to a large extent.
 Since the tolerance in the system is very less, the data can be written on
 the storage media in a very small area compared to the ordinary storage
 system. This will eventually reduce the size of the disk. The employment
 of acousto optic device for beam scanning simplifies the control system
 employed for scanning the beam, which is simple, compared to the control
 system employed for the actuator movement. The restriction imposed on the
 actuator such as reduction in size of the optical head assembly, micro
 lens fabrication and other micro feature fabrication is eliminated. The
 need for employing a circular disk is eliminated since there is no
 rotation involved in the system. The disk shape can thus be modified to
 square or even other shapes depending on the ease of manufacturing, cost
 and the requirement.
 The resolution of the beam movement is very high compared to that obtained
 from the actuator movement. This will be an added advantage in reducing
 the size of the disk in addition to the reduction in tolerance, due to the
 elimination of wobbling of disk on rotation. Moreover the density of the
 disk can be increased for the same disk size. Both the density and the
 disk size depend on the smallest beam spot size and the highest resolution
 achievable in the system. The sweeping rate of the beam can be increased
 which will result in increase of the rate of data retrieval. Thus the time
 taken for the data retrieval is reduced. The problem of focusing is
 eliminated due to the employment of stationary disk. Thus the complexity
 of the system is reduced due to the removal of the focusing technique
 employed in current system working on actuator principle. Although the
 tracking technique may be employed in the current technique but the
 tracking error is not so complex as in the system working on actuators.