GB1559112A - Optical data processing systems - Google Patents

Description

(54) IMPROVEMENTS RELATING TO OPTICAL DATA PROCESSING SYSTEMS (71) We, SPERRY RAND CORPORATION, a Corporation organised under the laws of the State of Delaware, United States of America, of 1290 Avenue of the Americas, New York, New York 10019, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to optical data processing systems. In particular, it relates to holographic memory systems, and to optical page composers for such systems.

An optical page composer is a matrix or array of cells, each cell acting at a light valve which passes light vel non according to the binary state of the cell. The page composer is best used in an holographic memory system. For writing into the storage medium or memory a coherent light beam from a laser impinging on the page composer emerges in a modulated pattern as an object beam carrying an optical equivalent of the information that is stored in the page composer. This ob ject beam is caused to interfere with a write reference beam at some location of the storage medium, e.g., thermoplastic film, where the information is stored in holographic form as a single page of information.

The information that is stored in each page of the storage medium can be read out from the memory system by illuminating the page in the storage medium by a read reference beam. If the direction of the read referenc beam is conjugate to that of the coherent light beam, i.e., the write reference beam, a real image will be formed on the page composer or at a symmetrical position on either side of the storage medium by reflective diffraction.

An array of photosensors, an array of memory storage cells and an array of light valves, i.e., the page composer, can be superimposed to form a latrix (light accessible transistor matrix - see the publication "Promise of Optical Memories", J.

A. Rajchman, JAP, Volume 41, No. 3, March 1, 1970, pp.1376-1383). The latrix is then a main memory whose contents may be manipulated by a central pro censor; when additional storage of data or data is required, the contents of the latrix may be "photographed" instantly into a holographic record for storage in the storage medium and new information may then be brought into the latrix. U.S.

Patent No. 3,761,155 discloses an holographic memory system incorporating an electrically alterable page composer utilizing bubbles as the light valves.

The invention provides a page composer comprising a planar layer of material capable of supporting magnetic bubbles, D digit lines parallel to each other and to the plane of the layer, W word lines parallel to the plane of the layer and perpendicular to the digit lines, the digit and word lines being rectilinear so as to define four rightangled quadrants at each crossing point, digit line and word line drivers for applying selectively a half-select write current signal of one or the other polarity, each half-select write current signal being of insufficient amplitude to substantially affect the position of a bubble at the crossing point, but both in combination being able to move a bubble between one or the other of a pair of diagonally opposite positions, and an opaque shield at each crossing point such as to expose a magnetic bubble when it is in a first quadrant but to shield it when it is in any of the other three quadrants, the word driver further being able to supply a full-select current of one or the other polarity and of such magnitude as to shift a bubble from a quadrant on one side of the word line to a neighbouring quadrant on the other side of the word line in the absence of any current in the digit line.

Coincident current selection of the memory areas by half-select write currents of suitable polarity enables a 1 or a 0 to be written into each memory area, thereby positioning the bubble in that area in either the first quadrant for 1, or the third, diagonally opposite, quadrant for a 0 without substantially affecting the bubbles in the half-selected memory areas along the half-selected digit line and word line.

A full current selection of the one word line by a full select store signal then positions or transfers the bubbles in the fully selected memory areas along the one fully selected word line from the write 1 first quadrant and the write 0 third quadrant into the store 1 second quadrant and the store 0 third quadrant, respectively.

For the read operation, full current selection of one word line by a full select read signal then positions or transfers the bubbles from the store 1 second quadrant and the store 0 third quadrant into the read 1 first quadrant and the read 0 fourth quadrant, respectively. After con-current readout of the data or information stored in the D memory areas along the one fully selected word line, that one word line is again fully selected by a full select restore signal of the same polarity as the full select store signal but of opposite polarity to the full select read signal. This two-dimensional page composer is then combined with a one-dimensional array of photosensors and a storage medium ;on- figured in a two-dimensional array of memory cells to form a latrix for an optical memory system. The use of a onedimensional array of D photosensors as compared to the prior art use of a twodimensional array of DW photosensors provides a substantial savings in photosensors and the associated electronics and controls therefor.

The page composer of the present invention is utilized in an holographic memory system to perform three functions, to: A. Compose data for subsequent stor age in the two-dimensional storage medium, i.e., a page composer.

B. Read out data stored in the two dimensional storage medium, i.e., a light valve to pass only selected light from the two-dimensional storage medium to the one-dimen sional array of photosensors.

C. Read out data stored in the page composer, i.e., an electrically alter able two-dimensional memory plane.

The invention will be further described by way of example with reference to the accompanying drawings, in which Fig. 1 is a schematic drawing of an holographic memory system incorporating the present invention.

Fig. 2 is an illustration of a layer of magnetizable material included in the page composer of the present invention.

Fig. 3 is an illustration of the Faraday effect rotation upon an incident laser beam by the second magnetization vector direction of a selectively switched single wall domain or bubble in the layer of Fig. 2.

Fig. 4 is an illustration of the Faraday effect rotation upon an incident laser beam by the first magnetization vector direction of the layer of Fig. 2.

Fig. 5 is an illustration of an holographic storage medium in which a plurality of holographs are stored as pages.

Fig. 6 is a perspective view of one memory area of the page composer of the present invention.

Fig. 7a and 7b are illustrations of the stable positions of a bubble about a stripline when affected by a magnetic field generated by a current flowing therethrough.

Fig. 8 is a graph of the stable positions of a bubble with respect to a stripline as a function of the amplitude of the current flowing through the strip-line.

Fig. 9 is a schematic illustration of a bubble memory system for positioning bubbles within the memory areas of the page composer of the present invention.

Fig. 10 is an illustration of a timing diagram employed by the bubble memory system of Fig. 9.

Fig. 111 are illustrations of the stable positions of a bubble within the memory area of Fig. 6 when subjected to the selection current signals of Fig. 10.

Fig. 12 is a conceptional schematic drawing illustrating the manner in which the one selected word line of the bubble memory system of Fig. 9 is optically read out by the one-dimensional array of photosensors of Fig. 1.

With particular reference to Fig. 1 there is presented a conceptual implementation of an holographic memory system incorporating the page composer 10 of the present invention. Prior to a detailed discussion of the operation of the holographic memory system of Fig. 1 and page composer 10 of the present invention, it may be best to review the theory of operation of an electrically alterable page composer using, e.g., a platelet or thin layer of magnetizable material of orthoferrite, hexagonal ferrites or garnets as the page composer medium, all as discussed in U.S. Patent No. 3,761,155.

With particular reference to Fig. 2 there is presented an illustration of a page composer depicting only the planar platelet or layer 12 of the magnetizable material in which a plurality of single wall domains or bubbles 14, having a magnetization vector M directed vertically, e.g., out of the paper, are established in a two-dimensional array along orthogonal X, Y axes by drive conductors and controls not illustrated - see the article "A New Approach to Memory and Logic-Cylindrical Domain Device," A. H. Bobeck, et al, Proceedings of the Fall Joint Computer Conference, 1969, pp. 489498. The magnetization vector M of the layer 12 is initially uniformly magnetically oriented in a first magnetization direction normal to its planar surface, e.g., directed downward into the paper, with the magnetization of the selectively written bubbles 14 magnetically oriented in a second magnetization directed opposite to the first magnetization direction, e.g., directed vertically upward out of the paper. Thus, selected portions 14 of the magnetizable material of layer 12 are switched in a second magnetization direction directed vertically upward out of the paper while the remaining portion 16 of the magnetizable material remains in its initial first magnetization direction directed vertically downward into the paper.

With particular reference to Fig. 3 there is presented a schematic illustration of the Faraday effect rotation by the magnetization direction 20 of the magnetization vector M or a bubble 14 of layer 12, upon an incident object beam that is plane polarized along polarization axis 22 by plane polarizer 9 of Fig. 1. The object beam is directed incident to the planar surface of a bubble 14 of layer 12 along a transmission axis 24 which is normal to the planar surface of bubble 14. As the object beam passes through bubble 14 it undergoes a Faraday effect clockwise rotation through an angle +0 being rotated clockwise from the first plane polarization axis 22 into the second plane polarization axis 26.

With particular reference to Fig. 4 there is presented a schematic illustration of the Faraday effect rotation, by the magnetization direction 21 of the magnetization vector M of the portion 16 of layer 12, which is that portion of layer 12 not including bubbles 14, upon an incident object beam that is plane polarized along polarization axis 22 by plane polarizer 9 of Fig. 1. The object beam is directed incident to the planar surface of portion 16 of layer 12 along a transmission axis 24 which is normal to the planar surface of portion 16. As the object beam passes through portion 16 it undergoes a Faraday effect counterclockwise rotation through an angle - being rotated coun- terclockwise from the first plane polarization axis 22 into the third plane polarization axis 27.

With particular reference to Fig. 5 there is presented a schematic illustration of an holographic storage medium 30 in which there are stored a plurality of pages 32 organized along orthogonal X, Y areas.

Each of the pages 32 is an hologram of the data held in the respectively associated page of layer 12 as composed by page composer 10 of Fig. 1, which data is stored therein by the conjoint action of object beam 98 and write reference beam 96e -- see Fig. 1. Write reference beam 96c and object beam 98 are digitally controlled by light deflector 94 to electrically access, by the proper optical focusing, any one of the XY pages 32 that are stored within holographic storage medium 30.

With particular reference to Fig. 6 there is presented an isometric view of one memory area 40 in the page composer 10 of the present invention. Layer 36 is a planar layer of magnetizable material in which single wall domains or bubbles 50 may be sustained and moved about while subjected to a bias field HB that is directed normal to the plane of layer 36. Superposed layer 36 and parallel to the plane thereof are digit line 42 and word line 44 oriented perpendicular to each other for forming the memory area 40 having the four quadrants 1, 2, 3, 4 that are defined by the intersecting digit line 42 and word line 44. Oriented within the memory area 40 defined by the intersecting digit line 42 and word line 44 is an oqaque ,shield 46 that is oriented in the second, third and fourth quadrants but not in the first quadrant of the memory area 40.

With particular reference to Figs. 7a and 7b the theory of operation of the memory area 40 of Fig. 6 will now be explained. This operation is based upon the principle that a bubble 50 can be moved by the gradient of a magnetic field that is directed normal to the planar surface of the magnetizable layer, while the bubble 50 moves toward that point in the magnetizable layer where that normal field is of the greatest intensity and is in the direction of the magnetization within the bubble. Fig. 7a illustrates the condition when a current signal is coupled to a stripling 52 so as to flow in the direction denoted by vector 54 generating the associated counterclockwise magnetic field denoted by circumferential vector 56. As the field normal to the magnetizable layer is of the greatest intensity just outside the edge of the stripling 52, the bubble 50 will nestle immediately adjacent to the stripline 52. Conversely, when the current flowing in strip-line 52 is reversed, as illustrated in Fig. 7b, the stable position of the bubble 50 is on the opposite side of the stripling 52. Thus, the bubble 50 can be shifted back and forth between the two stable positions along the opposing edges of the stripling 52 by reversing the direction of current flow in the stripline 52. Since there is a coercevity associated with the bubble 50, as with all magnetic domains, the bubble 50 remains in either of these two stable positions after the termination of the current flow in the stripline 52.

With particular reference to Fig. 8 there is illustrated a graph of experimental data obtained when a bubble 50 was shifted back and forth across stripline 52 with 20 microsecond (usec) pulses. The illustrated data shows that the stable position of the bubble 50 along the stripling 52 is a function of the amplitude of the stripline current. The square hysteresis loop indicates that the bubble 50 can be used as a coincident current memory element.

When a half-select current, e.g., of 10 milliamps (mA) is supplied to a word line, e.g., word line 44 of Fig. 6, but not to the digit line, e.g., digit line 42 of Fig. 6, the bubble 50 is not moved. Also, when a halfselect current is applied to the digit line but not to the word line, bubble 50 is not moved. However, when half-select currents are simultaneously applied to both the word line 44 and the digit line 42, the bubble 50, in the memory area 40 defined by the energized intersecting word line 44 and digit line 42, is moved from the upper righthand corner, i.e., quadrant 1, to the lower lefthand corner, i.e., quadrant 3, or vice versa. In the upper lefthand corner, i.e., quadrant 2, the lower lefthand corner, i.e., quadrant 3, and the lower righthand corner, i.e., quadrant 4, bubble 50 is superposed a shield 46 of deposited opaque material such that bubble 50 is, accordingly, effectively not visible to light that is incident to the plane of layer 36. Thus, when bubble 50 is positioned in quadrant 3 and optically shielded by opaque shield 46 it is considered to define a write 0 state while, conversely, when bubble 50 is in quadrant 1 and is not covered or shielded by opaque shield 46 it is considered to define a write 1 state -see Fig. 11.

PAGE COMPOSER OPERATION With particular reference to Fig. 9 there is presented a schematic diagram of a bubble memory system incorporating the present invention in which, for a write operation, a selected one of the bubbles 50 at the DW intersections of the D digit lines and the W word lines is selectively affected by half-select write digit and word drive currents to be positioned in the first quadrant -- see Fig. 6 -- representa- tive of the storage of a 1 or in the third quadrant representative of the storage of a 0. Then, the associated word line is affected by a full select store signal whereby the bubble 50 if in the write 1 first quadrant is transferred into the stored 1 second quadrant while if in the write 0 third quadrant is maintained in the stored 0 third quadrant. See Figs. 10 and 11 for the related signal timing and the associated stable positions of bubble 50. Provided is a bias field HB, as represented by the circle 60, which implies a field directed upwardly normal to the plane of the layer 61 of the magnetizable material in which single wall domains or bubbles 50 may be sustained and moved about - such bias field HB may be provided by a configuration of Helmheltz coils such as illustrated in U.S. Patent No. 3,534,347.

INITIALiZE OPERATION Initially, a bubble 50 may be generated at each digit line, word line intersection by an initialize enable signal on line 63 enabling timing and control circuitry 62, via lines 68, 69, to energize the digit drivers 70-1, 70-2, 70-D and the word line drivers 72-1, 72-2, 72-W and couple the appropriate initialize current signal amplitude of approximately 1.5 a full select current signal to their associated digit lines 74-1, 74-2, 74D and as associated word lines 76-1, 76-2, 76-W, respectively.

WRITE OPERATION With a bubble 50 now positioned at each digit line, word line intersection by the initialize operation, the bubble 50 at each memory area 40 may be selectively written in the 1 state, denoted by the bubble 50 being positioned in the exposed upper righthand quadrant 1 or alternatively written into the 0 state, denoted by bubble 50 being positioned in the lower lefthand quadrant 3 and behind the opaque layer 46. This write 1, write 0 operation is typical of all coincident current, i.e., bit select, memory operation. Firstly, the X address input data is coupled to one-out-of-W word line decoder 80 which energizes one of the W output lines that are associated with word line drivers 72-1, 72-2, 72-W. Concurrently, the Y address input data is coupled to the one-out-of-D digit line decoder 82 which energizes one of the D output lines that are associated with one of the digit line drivers 70-1, 70-2, 70-D. Concurrently with the energizing of the one selected word line driver, e.g., word line driver 72-2 and the one selected digit line driver, e.g., digit line driver 70-2, a write enable signal on line 64 causes timing and control 62 to couple the appropriate polarity write 1 or write 0 half-select signal to the corresponding lines 68, 69 such that the one energized word line driver 72-2 and the one energized digit line driver 70-2 each couple their associated one-half select current signal of the proper polarity to their associated select word line 76-2 and digit line 7s2.

As an example, with the word line and digit line superposed above the layer 61 and using the well-known righthand rule, if the memory area 40a at the intersection of word line 76-2 and digit line 74-2 is to be written into a 1 state, word line driver 72-2 must couple a negative half-select current pulse to its associated word line 76-2 while, concurrently, digit line driver 70-2 must couple a positive half-select current pulse to its associated digit line 74-2.

Correspondingly, it is apparent that if such memory area 40a is to be written into a 0 state, word line driver 72-2 must couple a positive half-select current pulse to its associated word line 76-2 while digit line driver 70-2 must couple a negative half-select current pulse to its associated digit line 74-2.

Observing the same rules, it can be seen that any one of the memory areas 40 at any of the digit line 74, word line 76 intersections may be selectively affected by the concurrent effect of the coincident half-select word line and digit line drive currents. This coincident current operation is based upon the fact that each of the digit line half-select current signals and each of the word line half-select current signals are individually of insufficient amplitude to substantially affect the position of the bubbles 50 at the associated memory areas 40 but that the digit line half-select current and the word line halfselect current are collectively of sufficient amplitude to transfer the bubble 50 at the concurrently affected memory area 40 between the exposed first quadrant, representative of a stored 1, and the shielded third quadrant, representative of a stored 0.

After the bubble 50 in the one selected memory area has been positioned in the exposed quadrant 1 representative of a write 1 state or, alternatively, in the shielded quadrant 3 representative of a write 0 state, the bubble 50 along the one selected word line, e.g., 76-2 is unconditionally subjected to a full select current store signal via a store enable signal on line 65. The full select current store signal then positions or transfers the bubble 50 from the write 1 first quadrant to the stored 1 second quadrant while the bubble 50 in the write 0 third quadrant remains in the third quadrant, but is now defined as being in a stored 0 state. This write, store sequence completes the write operation -- see Figs. 10 and 11.

READ OPERATION For a read operation, a full select current read signal, via the one addressed word line, e,g., 76-2, and a read enable signal on line 66, positions or transfers the bubble 50 from the stored 1 second quadrant to the read 1 first quadrant and from the stored 0 third quadrant to the read 0 fourth quadrant. At this time then, only the bubbles 50 along the one selected word line 76-2 are positioned in their read states with only the read 1 state with a bubble 50 in the exposed first quadrant permitting the beam incident thereto to be passed through analyzer 11, page composer 10 and plane polarizer 9 and thence via beam splitter 128 and cylindrical lens 124 to be focused upon the one-dimensional photosensor detector array 122 see Fig. 1. After this optical read out of the data stored in the D memory areas 40 along the one selected word line 76-2 via detector 122, a full select current restore signal is, via a restore enable signal on line 67, coupled to the one selected word line 72-2 whereby the bubbles 50 in the read 1 state of quadrant 1 are transferred back into the stored 1 state of quadrant 2 or the bubbles 50 in the read 0 state of quadrant 4 are transferred back into the stored 0 state of quadrant 3. This read, restore cycle completes the read oper ation -- see Figs. 10 and 11.

OPERATION OF HOLOGRAPHIC MEMORY SYSTEM The above described operation of page composer 10 will now be utilized to perform several functions within the holographic memory system of Fig. 1.

A. Compare Data for Storage in Storage Medium.

For the use of page composer 10 as a means for writing data into storage medium 30, laser generator 90 generates a coherent monochromatic light beam, such as a laser beam 92, that is directed along transmission axis 13 uppon X, Y light deflector 94. Deflector 94 deflects beam 92 to a beam 96 along the X, Y axes for focusing the object beam 98 upon the desired one page 32a of storage medium 30 - see Fig. 5. Beam 96 next impinges upon beam splitter 100 a portion of which is deflected upon modulator 102 and another portion of which impinges upon mirror 104 and thence upon modulator 106.

For an initial write operation during which the information stored in page composer 10 is to be written into the storage medium 30 at the one selected page 32a, modulator 102 is switched in its ON state permitting beam 96a to pass therethrough while modulator 106 is switched in its OFF state preventing beam 96b from passing therethrough. Beam 96a, at beam splitter 108, is separated into a write reference beam 96c which, via mirror 110, is focused upon page 32a while the remaining portion of beam 96a passes through beam splitter 108 as beam 96d and via mirror 112 is focused upon hololens 114. Holo-lens 114 functions to generate a plurality of, e.g., XY beamlets 118 from beam 96d, each separate beamlet being associated with, and focused upon the associated one of the XY memory areas in page composer 10. The XY beamlets 118 pass through plane polarizer 9 impinging upon page composer 10 with the individual beamlets 118 that correspond to the read l's in the XY memory areas in page composer 10 passing through analyzer 11 forming object beam 98 which, via lens 120, is focused upon the one selected page 32a in storage medium 30 into which the information previously written into page composer 10 is to be recorded.

This information that has been previously written into page composer 10, preferably includes the writing of DW l's or 0's in the DW memory areas 40 of page composer 10 in accordance with the above described write operation. Alternatively, only a selected one or one's of the W word lines may be utilized for the writing of data into storage medium 30. In any of the above conditions, none, one or more or all of the memory areas along the selected word line(s) are subjected to a read operation, as described above, so that the bubbles 50 in the selected memory areas 40 are transferred into their read 1 state of quadrant 1 or read 0 state of quadrant 4 such that only the real 1 state bubbles 50 permit the associated beamlets 118 to pass therethrough and to impinge upon storage medium 30 at the one selected page 32a. This concurrent effect of the write reference beam 96c and the object beam 98, containing the information generated or composed by page composer 10, stores in the one selected page 32a the data read out of page composer 10 and carried by object beam 98.

B. Read Out Data Stored in Storage Medium -- Light Valve.

For the use of page composer 10 as a means for reading out the data stored in storage medium 30, modulator 102 is switched OFF preventing beam 96a from passing therethrough while, conversely, modulator 106 is switched ON permitting read reference beam 96b, as deflected from mirror 104, to pass therethrough, and via mirrors 124 and 126, to cause beam 96b to be focused upon the one selected page 32a of storage medium 30. Read reference beam 96b upon impinging upon page 32a causes the light that is reflected therefrom to be focused upon lens 120, to be passed through analyzer 11, page composer 10 and plane polarizer 9, to be reflected by beam splitter 128 and thence by means of cylindrical lens 123 to be focused upon one-dimensional photosensor array of detector 122.

Detector 122 is, as discussed above, a one-dimensional array of D photosensors, each of the D photosensors being associated with an associated one of the D memory areas 40 that lay along the one selected word line of page composer 10- see Fig. 12. In this operation in which page composer 10 functions only as a light valve to pass only that data that is stored in page 32a that is associated with only a selected one of the W word line of page composer 10, only the D memory areas 40 along the selected one of the W word lines of page composer 10 are written with all bubbles 50 in the 1 state of quadrant 1 while all other memory areas 40, i.e., W(D-t), have their bubbles 50 in the stored 1 state of quadrant 2 or in the stored 0 state of quadrant 3 such that all memory areas 40 except those memory areas 40 that lay along the one selected word line block the passage of light therethrough. Thus, by using only a selected one of the W word lines, the D exposed bubbles 50 in the D associated memory areas 40 of page composer 10 are utilized to read out, by detector 122 the associated data previously stored in page 32a of storage medium 30.

C. Read Out Data Stored in Page Composer -- Memory.

For the read out of the information previously stored in page composer 10, X, Y light deflector 92 is caused to deflect beam 96 in an X, Y direction such that beam 130 will impinge upon mirror 134, e.g., positioned upon storage medium 30.

For this operation, modulator 102 is switched OFF preventing beam 130 from pas thence by means of cylindrical lens 123 to be focused upon detector 122.

Detector 122, is, as discussed above, a one-dimensional array of D photosensors, each of the D photosensors being associated one of the D memory areas 40 that lay along the one selected word line of page composer 10 -- see Fig. 12. In this operation, page composer 10 functions as a memory to pass only that data that is stored in page composer 10 and that is associated with only a selected one of the W word lines of page composer 10. Only the D memory areas 40 along the selected one of the W word lines of page composer 10 have their bubbles 50 positioned in the read 1 state of quadrant 1 or the read 0 state of quadrant 3, while all other memory areas 40, i.e., W (D-l), have their bubbles 50 in the stored 1 state of quadrant 2 or in the stored 0 state of quadrant 3 such that all memory areas 40 except the read 1 state of the memory areas 40 that lay along the one selected word line block the passage of light therethrough.

Thus, by using only a selected one of the W word lines, only the exposed bubbles 50 in the D associated memory areas 40 along the one selected word line of page composer 10 are utilized to read out the associated data previously stored therein in the manner as described above.

WHAT WE CLAIM IS:- 1. A page composer comprising a planar layer of material capable of supporting magnetic bubbles, D digit lines parallel to each other and to the plane of the layer, W word lines parallel to the plane of the layer and perpendicular to the digit lines, the digit and word lines being rectilinear so as to define four rightangled quadrants at each crossing point, digit line and word line drivers for applying selectively a half-select write current signal of one or the other polarity, each half-select write current signal being of insufficient amplitude to substantially affect the position of a bubble at the crossing point, but both in combination being able to move a bubble between one or the other of a pair of diagonally opposite positions, and an opaque shield at each crossing point such as to expose a magnetic bubble when it is in a first quadrant but to shield it when it is in any of the other three quadrants, the word driver further being able to supply a full-select current of one or the other polarity and of such magnitude as to shift a bubble from a quadrant on one side of the word line to a neighbouring quadrant on the other side of the word line in the absence of any current in the digit line.

2. A page composer according to claim 1 further including initialising means for causing the digit line drvers and word line drivers to supply current pulses substantially in excess of the full-select current so as to initiate a magnetic bubble at each word line/digit line intersection.

3. A page composer constructed and arranged to operate substantially as shown in and as herein described with reference to Figures 2 to 11 of the accompanying drawings.

4. A holographic memory system comprising a page composer according to any preceding claim, a storage medium capable of storing XY pages of data, optical means for storing a page of data composed in the page composer in the storage medium at a selected one of the XY pages, one-dimensional lens means, one-dimensional detector means comprised of a linear array of D photosensor means, the control means coupling the digit line halfselect write current signal to the D digit lines and the word line half-select write current signal to a selected one of the W word lines for positioning the bubbles in the memory areas along the selected word line into the exposed quadrant, the optical means including means for reading out data from a selected one of the XY pages stored in the storage means and corresponding to the selected word line of the page composer and optically transmitting the read-out data through the bubbles along the selected word line that are in the exposed quadrant on to the one-dimensional detector means.

5. A holographic memory system, organised and arranged to operate substantially as shown in and as herein described with reference to Figure 1 taken in conjunction with Figures 2 to 11 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. thence by means of cylindrical lens 123 to be focused upon detector 122. Detector 122, is, as discussed above, a one-dimensional array of D photosensors, each of the D photosensors being associated one of the D memory areas 40 that lay along the one selected word line of page composer 10 -- see Fig. 12. In this operation, page composer 10 functions as a memory to pass only that data that is stored in page composer 10 and that is associated with only a selected one of the W word lines of page composer 10. Only the D memory areas 40 along the selected one of the W word lines of page composer 10 have their bubbles 50 positioned in the read 1 state of quadrant 1 or the read 0 state of quadrant 3, while all other memory areas 40, i.e., W (D-l), have their bubbles 50 in the stored 1 state of quadrant 2 or in the stored 0 state of quadrant 3 such that all memory areas 40 except the read 1 state of the memory areas 40 that lay along the one selected word line block the passage of light therethrough. Thus, by using only a selected one of the W word lines, only the exposed bubbles 50 in the D associated memory areas 40 along the one selected word line of page composer 10 are utilized to read out the associated data previously stored therein in the manner as described above. WHAT WE CLAIM IS:-

1. A page composer comprising a planar layer of material capable of supporting magnetic bubbles, D digit lines parallel to each other and to the plane of the layer, W word lines parallel to the plane of the layer and perpendicular to the digit lines, the digit and word lines being rectilinear so as to define four rightangled quadrants at each crossing point, digit line and word line drivers for applying selectively a half-select write current signal of one or the other polarity, each half-select write current signal being of insufficient amplitude to substantially affect the position of a bubble at the crossing point, but both in combination being able to move a bubble between one or the other of a pair of diagonally opposite positions, and an opaque shield at each crossing point such as to expose a magnetic bubble when it is in a first quadrant but to shield it when it is in any of the other three quadrants, the word driver further being able to supply a full-select current of one or the other polarity and of such magnitude as to shift a bubble from a quadrant on one side of the word line to a neighbouring quadrant on the other side of the word line in the absence of any current in the digit line.

2. A page composer according to claim 1 further including initialising means for causing the digit line drvers and word line drivers to supply current pulses substantially in excess of the full-select current so as to initiate a magnetic bubble at each word line/digit line intersection.

3. A page composer constructed and arranged to operate substantially as shown in and as herein described with reference to Figures 2 to 11 of the accompanying drawings.

4. A holographic memory system comprising a page composer according to any preceding claim, a storage medium capable of storing XY pages of data, optical means for storing a page of data composed in the page composer in the storage medium at a selected one of the XY pages, one-dimensional lens means, one-dimensional detector means comprised of a linear array of D photosensor means, the control means coupling the digit line halfselect write current signal to the D digit lines and the word line half-select write current signal to a selected one of the W word lines for positioning the bubbles in the memory areas along the selected word line into the exposed quadrant, the optical means including means for reading out data from a selected one of the XY pages stored in the storage means and corresponding to the selected word line of the page composer and optically transmitting the read-out data through the bubbles along the selected word line that are in the exposed quadrant on to the one-dimensional detector means.

5. A holographic memory system, organised and arranged to operate substantially as shown in and as herein described with reference to Figure 1 taken in conjunction with Figures 2 to 11 of the accompanying drawings.