Optical storage device having a plurality of juxtaposed memory cells

The invention relates to a read-write optical memory comprising a plurality of juxtaposed memory cells (11), each receiving a respective light beam (3). Each memory cell contains a storage medium (10), which includes a storage element (23) having stable optical states. The storage element (23) is divided into a number of memory points, and the optical state in a given memory point can be both changed and read by means of a light beam (3) directed towards the memory point. The memory can be implemented entirely without any movable mechanical parts and has a very short read-write time and an exceptionally high storage capacity. Parallel writing and reading of multibit words is possible.

FIELD OF THE INVENTION 
The present invention relates to an optical memory. More specifically, the 
invention relates to an optical memory which has a high storage capacity, 
short access time for writing and reading of information, and which can be 
implemented entirely without any movable mechanical parts. 
DESCRIPTION OF THE PRIOR ART 
Today's systems for mass storage of binary information can be roughly 
divided into three main categories: 
1. Magnetic medium (hard disc) 
2. Optical medium (CD-ROM) 
3. Magneto-optical medium 
1. A hard disc, i.e. a magnetic disc storage which is mechanically rotated 
when writing and reading information, requires mechanical positioning of a 
read-write head, entailing a relatively long average seek time for 
positioning in the order of 10-15 ms, a limited read-write speed, as well 
as a relatively limited storage capacity. In principle, the storage 
capacity of a hard disc can only be increased by using a larger area, 
which entails the disadvantage of necessitating a larger movement for 
positioning, i.e. a longer seek time. 
2. A CD-ROM has a comparatively high storage capacity, but is a slow medium 
which does not permit writing but only reading since, traditionally, the 
information is stored when making the memory. 
3. A magneto-optical medium, which is of a design closely related to that 
of a CD-ROM, acts such that the information is stored on a rotary disc 
containing magnetic particles which, when writing the information, are 
heated by a laser beam above their Curie temperature. Drawbacks of this 
type of storage medium are the relatively limited capacity and the slow 
writing speed. Writing may, for example, be up to six times slower than 
for a hard disc. 
SUMMARY OF THE INVENTION 
With the ever increasing need of information storage, there is a great 
demand for a storage medium that does not suffer from the aforementioned 
drawbacks of the prior art. A general object of the invention is to 
provide a storage medium meeting this demand. 
A first object of the invention is to provide a storage medium having a 
high storage capacity. 
A second object of the invention is to provide a storage medium having 
short access time for writing and reading of information. 
A third object of the invention is to provide a storage medium that can be 
implemented without any mechanically movable parts. 
A fourth object of the invention is to provide a storage medium having a 
compact structure and relatively small dimensions, and being relatively 
inexpensive to manufacture. 
A fifth object of the invention is to provide a storage medium enabling 
parallel writing and reading of information. 
To achieve these and other objects, there is provided according to the 
invention an optical memory, comprising: 
a plurality of juxtaposed memory cells, each of which is adapted to receive 
a respective light beam and each of which comprises a storage medium 
including: 
(a) a storage element switchable between at least two stable states having 
mutually different optical properties by applying corresponding electric 
fields between a light entry side and an opposite light exit side of the 
storage element; 
(b) an electrode matrix arranged on one of the light entry side and the 
light exit side of the storage element and exhibiting a plurality of 
mutually electrically insulated electrode points distributed over the 
storage element, corresponding to an equal plurality of memory points in 
the storage medium; 
(c) a common electrode arranged on and extended over the other of the light 
entry side and the light exit side of the storage element; and 
(d) a photoconductive layer acting as a light-controlled switch and having 
light-dependent electric resistance, said layer being arranged over and in 
electrical contact with the electrode matrix on the side thereof facing 
away from the storage element, 
whereby an electric field is selectively applicable over a given memory 
point in the storage element by selectively illuminating the corresponding 
point on the photoconductive layer while applying an electric control 
voltage between the photoconductive layer and the common electrode, and 
electrically controlled means for bringing about a simultaneous and 
mutually equally large displacement of the point of incidence of each of 
the light beams of the memory cells on the corresponding memory cell. 
In an optical memory according to the invention, each memory point of the 
storage medium of each individual memory cell can thus be efficiently and 
rapidly addressed with the aid of a light beam, preferably an accurately 
focused laser beam, directed towards the memory point. 
It should be noted that no separate electric lead wires are required to the 
individual punctiform electrodes of the electrode matrix. This enables an 
extremely high packing density of the memory points of the optical memory. 
The electrode points of the matrix electrode may, by way of example only, 
have an extent of about 1 .mu.m and a mutual spacing of about 1 .mu.m. 
This gives 5000.times.5000=25 million memory points per square centimeter 
of light receiving area in the storage medium. 
Writing 
Writing of desired information, such as a logic 1 or 0, into a specific 
memory point of the optical memory according to the invention is carried 
out in the following way. 
A control voltage, which can bring the storage element to the stable 
optical state representing said desired information, is applied between 
the photoconductive layer and the common electrode. At the same time, the 
specific memory point is addressed by means of a light beam, which is 
directed towards the electrode point that coincides with the specific 
memory point. The photoconductive layer will then, in its illuminated 
point, exhibit a reduced electric resistance as compared with the 
unilluminated area of the layer. At the illuminated memory point, a major 
part of the control voltage applied will therefore be located between the 
"illuminated" electrode point of the electrode matrix and the common 
electrode, i.e. over the corresponding memory point of the storage 
element. The stable state of the illuminated memory point is thus 
determined by the control voltage applied. 
Unilluminated areas of the photoconductive layer do not exhibit any such 
reduced electric resistance. At unilluminated memory points, a major part 
of the control voltage applied will be located over the photoconductive 
layer and a minor part thereof will be located between the "unilluminated" 
electrode points and the common electrode. The stable optical states of 
unilluminated memory points, i.e. information previously stored in 
unilluminated memory points, will thus not be affected by the light beam, 
despite the presence of the control voltage between the photoconductive 
layer common to all the memory points, and the electrode common to all the 
memory points. 
For writing information into the optical memory according to the invention, 
it is thus possible, for each one of the memory cells, to address by means 
of a light beam a specific memory point among a very large number of 
memory points, and create by means of the photoconductive layer an 
electric field over the storage element, restricted precisely to that area 
of the storage element which in the direction of the light beam coincides 
with the illuminated memory point. 
Thanks to the storage element having stable states, information that has 
been written into a memory point will remain unchanged also after the 
light beam and, hence, the electric field have been removed from the 
memory point. 
Read-out 
Read-out of stored information from a specific memory point can be 
performed in the following way. 
As opposed to the writing operation as described above, no control voltage 
is applied in the read-out operation between the photoconductive layer and 
the common electrode. The specific memory point is however addressed in 
the same way as in the writing operation, with the aid of a light beam 
directed towards the electrode point coinciding with the memory point. 
Since no control voltage is applied, the decrease in resistance which 
occurs at the corresponding illuminated point of the photoconductive layer 
will not affect the information that has been stored in the addressed 
(illuminated) memory point. For read-out of the information that has been 
stored in the addressed memory point, i.e. for determining the stable 
optical state of the storage element in the illuminated memory point, use 
is made of the fact that the light beam will be affected differently 
depending on the stored optical state in the addressed memory point. By 
detecting this effect on the light beam, the optical state of the 
addressed memory point, i.e. the information stored therein, can be 
established. 
Several Memory Cells / Parallel Writing and Reading 
The optical memory according to the invention comprises a plurality of 
juxtaposed memory cells, each of which comprises a storage medium of the 
type described above for receiving a respective light beam. The 
"hierarchy" of the memory thus means that the memory has a plurality of 
memory cells, and the storage medium included in each memory cell has a 
respective array of memory points. 
Each such memory cell receives a light beam of its own for writing and 
read-out. Thus, the point of incidence of the light beam on the 
corresponding memory cell need only be displaced between memory points 
included precisely in this memory cell. Consequently, there are as many 
light beams as memory cells. 
Such a plurality of light beams may, for example, be generated with the aid 
of a laser diode field comprising a matrix of individual semiconductor 
lasers. It is however also conceivable to use a single light source 
combined with beam splitting. 
The optical memory according to the invention further comprises 
electrically controlled means for bringing about a continuous and mutually 
equally large displacement of the point of incidence of each one of the 
light beams of the memory cells on the corresponding memory cell. This 
enables a highly efficient parallel writing into and parallel read-out 
from several memory cells at a time. 
According to the invention, such a displacement of the points of incidence 
of the light beams can be brought about in some basically different ways. 
According to a first alternative, the memory cells and the light sources 
(alternatively, a single light source with beam splitting) may be 
stationary in relation to each other. In one such embodiment of the 
invention, the electrically controlled means may comprise 
light-beam-deflecting acousto- or electro-optical crystal layers which are 
located on the light entry side of the memory cells and which are common 
to several or all of the memory cells of the optical memory. Especially, 
the memory cells may be arranged beside each other in rows and columns, in 
which case such light-beam-deflecting crystal layers may comprise a first 
and a second crystal layer for deflecting the light beams parallel to said 
rows and columns, respectively. In this embodiment, the light source, such 
as a laser diode field comprising a matrix of individual semiconductor 
lasers, may form an intergral part of the optical memory. 
According to a second alternative, the displacement of the points of 
incidence of the light beams may be brought about by a displacement of the 
light source itself, while holding the memory cells stationary. In such an 
embodiment of the invention, the memory preferably includes a light-source 
matrix of individual light sources for generating a separate light beam 
for each memory cell, the electrically controlled means being connected to 
a light-source matrix to bring about a displacement thereof and, hence, a 
continuous and mutually equally large displacement of all the individual 
light sources. Preferably, the electrically controlled means may then 
comprise a piezoelectric motor device. 
According to this second variant of the invention, the memory cells may be 
arranged as a replaceable memory disc or memory diskette which in use is 
disposed stationary in the light beam paths. 
According to a third alternative, both the memory cells and the light 
sources may be movable. In such an embodiment of the invention, the memory 
cells may be provided in the form of concentric rings on a common disc or 
the like, whose opposite two main surfaces form the light entry side and 
the light exit side, respectively, of the memory cells, said disc being 
rotatable about a centre axis, and the memory points of each memory cell 
being distributed both radially and circumferentially with respect to said 
axis. This embodiment preferably includes a light-source matrix comprising 
a row of individual light sources, radially extended with respect to the 
axis of rotation of the disc, for generating a separate light beam for 
each memory cell, the electrically controlled means being connected to the 
light-source matrix to bring about a radial displacement of the light 
sources with respect to the axis of rotation of the disc. 
By the rotation of the disc and the radial displacement of the light-source 
matrix, concurrently or separately, it is thus possible to bring about 
said displacement of the points of incidence of the light beams on the 
memory cells. In this embodiment, the electrically controlled means may 
comprise a radially acting piezoelectric motor device. 
According to an embodiment of particular interest of the alternative using 
a rotary disc, the disc is provided with a photocell means which is 
adapted to receive light from a preferably stationary light source and 
generate, in response thereto, the control voltage between the 
photoconductive layer and the common electrode of the memory cells. 
Different Optical States = Different Transmittance 
According to a preferred embodiment of the invention, the different optical 
states of the storage element correspond to different states of light 
transmittance of the storage medium. In this embodiment, use may be made 
of two mutually crossed polarizers arranged on a respective side of the 
storage element in association with a storage element, the molecules of 
which can be affected by a superimposed electric field and rotate the 
light vector of an incident, polarized light beam to a different degree 
depending on the field applied. If a rotary disc is used, each of the 
polarizers may be either arranged on the disc or arranged stationary. 
Light-Sensitive Layer for Read-out 
For detecting the different states of light transmittance of the storage 
medium in connection with read-out, the storage medium of each memory cell 
may comprise a light-sensitive element of its own arranged on the light 
exit side of the storage element for receiving and detecting a light beam 
which is to be directed through the storage medium for read-out of stored 
information. These light sensitive elements of the memory cells should be 
insulated from each other to permit a simultaneous parallel read-out of 
information stored in the memory points of several memory cells. 
Choice of Material 
A storage element included in a storage medium of an optical memory 
according to the invention may, for example, comprise a ferroelectric 
liquid crystal (hereinafter termed FLC), especially in smectic C*-phase 
and advantageously of polymeric design, a PLZT ceramic or a PLZT thin 
film, or a material using electro-optic effect based on electrochromism. 
These materials are however merely given as examples. They differ in 
respect of such properties as speed, current consumption and required 
field strength for excitation. 
A conceivable variant is using for the storage element a material which is 
switchable between three or more stable optical states, for example such 
that different optical states correspond to transmittance of different 
colors. 
Several Storage Levels 
To increase the storage capacity of an area of a given size, it is possible 
to increase the resolution of the memory points, i.e. to reduce the size 
of the points and/or their mutual spacing. It is however also possible to 
modify the optical memory such that more than one bit can be stored in 
each memory point. 
To provide such a possibility of storing more than one bit in each memory 
point, the optical memory according to the invention may, in a preferred 
embodiment, comprise two or more, especially four or eight, storage levels 
arranged on each other and each having one storage medium of the 
aforementioned type, each memory point in this embodiment having a 
corresponding number of memory positions. Thus, it is possible, for 
example, especially to store four and eight bits, respectively, in each 
memory point. In addition to the increased storage capacity, the write 
time and the access time are reduced correspondingly. Such an embodiment 
having several storage levels however requires only two polarizers, one 
position-sensitive layer and one light-sensitive detecting layer of the 
aforementioned type. However, each storage level requires a 
photoconductive layer, an electrode matrix, a storage element and a common 
electrode. 
These and other features of the invention are stated in the appended claims 
.

SINGLE LEVEL--DESIGN 
A first embodiment of an optical memory according to the invention will now 
be described with reference to FIG. 1. 
The optical memory in FIG. 1 has no mechanically movable parts and is at 
least partly (bottom of FIG. 1) designed as a laminated multilayer 
structure whose dimensions are substantially dependent on the desired 
storage capacity of the memory. 
Laser Diode Field 1 
At the top of the structure in FIG. 1, there is a laser diode field, 
generally designated 1, comprising a plurality of semiconductor laser 
diodes 2 for emitting a respective laser beam 3 downwards in the 
structure. Light-emitting diodes are a possible alternative. The laser 
diodes 2 are arranged equidistantly in an N.times.M grid. FIG. 2A which is 
a schematic plan view of such a laser diode field 1 on a full scale, shows 
a grid with 4.times.4=16 laser diodes 2. In the embodiments of FIGS. 1 and 
2A, the laser diodes 2 are spaced apart about 10 mm. 
The optical memory of FIG. 1 has a memory cell for each laser diode 2. An 
optical memory with a laser diode field 1 according to FIG. 2A thus has a 
total of sixteen memory cells. The optical memory according to the 
invention may however have considerably more memory cells than the sixteen 
cells shown in FIG. 2A. Thus, the embodiment of FIG. 1 may be considered 
to illustrate a broken-away part of a memory structure having a larger 
number of memory cells with a respective laser diode 2. In FIG. 1, each 
separate memory cell is generally designated 11. 
Collimating Lens System 4 
Below the laser diode field 1, there is a collimating lens system 4, 
comprising a collimating lens 5 for each laser beam 3. The collimating 
lenses 5 are centred to the respective laser diode 2 and serve to 
collimate the laser beam matrix 3 emitted from the laser diode field 1. 
Deflecting System 6, 7 
Below the collimating lens system 4, there is an electro-optical (EO) or an 
acousto-optical (AO) deflecting system 6, 7 for deflecting the collimated 
laser beam matrix 3. The deflecting system of the optical memory in FIG. 1 
is of acousto-optical type and comprises a first AO crystal 6 for 
deflecting the laser beams 3 in the y-direction (perpendicular to the 
drawing plane in FIG. 1) and a second AO crystal 7 for deflecting the 
laser beams 3 in the x-direction (right-left in FIG. 1). 
The purpose of the deflecting system 6, 7 is to accurately position the 
points of incidence of the laser beams 3 in the x-direction and the 
y-direction. This is achieved by applying on the crystal layer 6 and 7, 
respectively, high-frequency voltage signals, typically of 50-80 Mhz. 
Deflection can be done in an accurately linear fashion depending on the 
frequency applied, and an arbitrary centre frequency may correspond to 
zero deflection. 
As is known to a person skilled in the art, this linear relationship 
between control frequency and deflection can be made so exact that it is 
possible in an extremely accurate way to predict the x- and y-deflection 
of the laser beams 3 for given frequencies applied to the crystal layers 6 
and 7. 
In FIG. 1, the deflecting crystals 6 and 7 are extended over substantially 
the entire structure in the x- and y-directions and are thus common to all 
the memory cells 11. All the laser beams 3 are therefore deflected jointly 
and in unison, which can be compared to a field of corn where all the 
stems bend in one and the same direction when the wind changes its 
direction. This property is of great importance, since it permits parallel 
writing and reading of entire or parts of multibit words, as will be 
described in more detail below. 
In other words, there is no individual addressing in FIG. 1 for deflecting 
the laser beams 3. The deflecting crystal layers 6 and 7 are readily 
controlled by connecting voltage electrodes on peripheral sides thereof, 
as schematically shown in FIG. 1 at 6a and 7a, respectively. 
Such an AO deflecting system 6, 7 may in practice be produced from 
monocrystalline tellurium dioxide (TeO2) using anisotropic Bragg 
diffraction. The reaction speed of such a deflecting system may be some 
microseconds. This is however only one out of several possible examples of 
materials. 
As an alternative, the optical memory may instead have a separate 
deflecting system for each laser beam 3, in which case the deflecting 
systems are controlled in parallel so as to obtain the same deflecting 
function as in FIG. 1. 
Matrix of Scanning Optics 8 
Below and at a distance from the deflecting system 6, 7, there is provided 
a matrix of scanning optics generally designated 8, comprising a plurality 
of scanning lenses 9, one for each laser beam 3. The scanning lenses may, 
for example, be of the F-theta-lens type. FIG. 2B, which is a schematic 
full-scale plan view of such a matrix of scanning optics 8, shows a matrix 
having sixteen scanning lenses 9 pertaining to the sixteen laser diodes 2 
in FIG. 2A. 
Storage Medium 10 (=20-29) 
Below the matrix of scanning optics 8 is the actual storage medium 10 of 
the memory, on which the laser beams 3 are focused, as shown in FIG. 1, is 
shown especially in the enlarged detail representation of FIG. 1A (bottom 
of same drawing sheet) of part of a memory cell 11. 
The storage medium 10 is a laminated structure for each memory cell 11, 
whose height in the embodiment of FIG. 1 may, in a non-restricting 
example, be in the order of a few centimeters. 
FIG. 3, to which reference is now made, is an enlarged cross-sectional view 
showing a single memory cell 11 of the storage medium 10 in FIG. 1, e.g. 
the memory cell X6 furthest to the right in FIG. 1. 
Polarizer 20 and Substrate 21 
The focused laser beam 3 first passes a first polarizer 20 and thereafter a 
substrate 21, which by an accurately specified refractive index finally 
focuses the laser beam 3 to about 0.8 mm. 
PSD Layer 22 
The finally-focused laser beam 3 thereafter reaches a transilluminable 
photolayer 22 consisting of a light-sensitive, two-dimensional lateral 
position detector, hereinafter referred to as PSD layer (positional 
sensitive device). 
The PSD layer 22 serves to detect the position, in both the x-direction and 
the y-direction, of the point of incidence of the deflected laser beam 3 
on the specific memory cell 11. In the embodiment of FIG. 1, each memory 
cell 11 has a separate PSD layer 22 of its own, as schematically 
illustrated in the plan view of FIG. 2C which illustrates how an optical 
memory having 4.times.4=16memory cells 11 correspondingly includes sixteen 
separate PSD layers 22 having lateral dimensions of about 1.times.1 cm. 
Storage Element 23 
Further down in the storage medium 10 in FIG. 3, there is provided a 
bistable storage element 23, such as a ferroelectric liquid crystal (FLC), 
a PLZT ceramic material or a material using electro-optic effect based on 
electrochromism. The storage element 23 serves to store information 
corresponding to a binary 1 or 0. The storage element 23 in FIG. 1 is 
assumed to be an FLC layer. 
Electrode Matrix 24 and Insulation 25 
Above and in contact with the storage element 23, there is provided an 
electrode matrix 24 comprising a grid of electrically mutually insulated, 
transilluminable electrode points, between which is provided an insulating 
matrix 25. There are no lead wires to the separate electrode points. 
In the illustrated embodiment, the size of, and the distance between, the 
electrode points of the electrode matrix 24 is about 1 .mu.m, each 
electrode point corresponding to one memory point of the memory cell 11. 
Each memory cell 11 therefore has 10 mm/2 .mu.m=5000 memory points in each 
direction, i.e. a total of 5000.times.5000=25 million memory points. 
Common Electrode 26 
Below and in contact with the storage element 23, there is provided a 
second transilluminable electrode 26, which is common to all the 25 
million memory points of the memory cell 11 and which is extended in the 
xy-plane over the entire memory cell 11. 
Photoconductive Layer 27 
Between said PSD layer 22 and said electrode matrix 24, there is provided, 
common to the entire memory cell 11, a transilluminable photoconductive 
layer 27 having light-intensity-dependent resistance.. The photoconductive 
layer 27 acts as a light-controlled switch, which serves to switch on and 
switch off an electric field between the electrode points of the electrode 
matrix 24 and the common electrode 26 if an electric control voltage is 
connected between the photoconductive layer 27 and the common electrode 
26. 
The function of the photoconductive layer 27 is as follows. A point P 
illuminated by the laser beam 3 on the photoconductive layer 27 will have 
low electric resistance relative to unilluminated areas of the layer 27. 
In such an illuminated memory point, a major part of the control voltage 
applied between the layers 27 and 26 will therefore be located between the 
electrode layers 24 and 26, i.e. over the storage element 23 but, as 
should be especially noted, only in the illuminated point. In 
unilluminated memory points in the same memory cell 11, a major part of 
the control voltage applied will be located over the photoconductive layer 
27, since this has relatively high resistance in unilluminated memory 
points, and so a minor part of the voltage will be located over the 
storage element 24. 
Memory Storage Function 
In the structure described above, it is thus possible by deflecting the 
laser beam 3 to address a specific memory point P in the memory cell 11 
among the 25 million memory points thereof, and create, with the aid of 
the photoconductive layer 27, an electric field over that part of the 
storage element 23 which is located below the illuminated point P, i.e. to 
store a binary 1 or 0 in this memory point. Physically, the FLC molecules 
and the light vector are rotated 90.degree. in relation to said polarizer 
20. 
Since the storage element 23 is bistable, binary information previously 
stored is held unchanged in unilluminated memory points. 
Polarizer (analyzer) 28 
For read-out of binary information stored in this manner in a memory point 
P of the memory cell 11 in FIG. 3, there is provided below the common 
electrode 26 a second polarizer (analyzer) 28, whose angle of polarization 
is rotated 90.degree. relative to the first polarizer 20 and which serves 
to let through or block a laser beam 3 which has passed through the 
bistable FLC storage element 23. 
Light-sensitive layer 29 
Below the polarizer 28, there is an amorphous light-sensitive layer 29 
which serves to detect the status of the memory point concerned, in 
connection with read-out of stored information. Below the layer 29, there 
is finally a substrate 30. 
The above function of the photosensitive layer 29 for read-out of stored 
information may also be used in connection with writing into the memory 
for checking that the information to be written in is actually stored. 
Single Level--Function 
One example of how it is possible in practice to perform writing into and 
read-out from the optical memory in FIGS. 1 and 3 will now be described in 
more detail with reference to FIG. 4. 
FIG. 4 shows in perspective a broken-away part of a memory cell 11 
according to FIG. 3, comprising however only two memory points, and in a 
block diagram the peripheral electronics connected thereto. The means for 
generating, collimating, deflecting and focusing the laser beam 3 are not 
shown in FIG. 4. 
Incoming binary data information 40 and read-write control signals 41 and 
42 are inputted and stored in a data buffer 43. These signals are 
forwarded to a unit 44 which is a combined shift register/switch device 
serving to shift out the parallel data information to a series of pulses 
which are level-adapted to a voltage level required for excitation of the 
FLC storage element 23. A generator 45 generates a signal +V for writing a 
logic 1, and a generator 46 generates a signal -V for writing a logic 0. 
Each time the laser beam 3 is displaced by the deflecting means from one 
memory point to another, the current xy-position is computed in a unit 47 
which receives positional signals 48 from the PSD layer 22. The current 
position of the laser beam 3 is computed as follows: 
EQU X=(x1-x2)/(x1+x2) 
EQU Y=(y1-y2)/(y1+y2) 
wherein x1, x2, y1, y2 represent the voltage signals 48 from the PSD layer 
22. A differential signal 49 from the unit 47, which can thus detect 
whether the beam 3 is correctly directed to a memory point, is emitted to 
the unit 44 to block or let through a write pulse 50 from a D-output of 
the unit 44. The write pulse 50, representing +V or -V for writing a logic 
1 and a logic 0, respectively, is applied to the photoconductive layer 27 
while the common electrode 26 is grounded at 51. 
Read-out of data from a memory point in the memory cell in FIG. 4 is 
performed by deflecting the laser beam 3 towards the memory point. The 
light-sensitive layer 29 on the light exit side of the storage medium 10 
will thus generate a signal 52 which is detected by a comparator 53 and 
the size of which indicates whether the optical state of the memory point 
is a logic 1 or 0. Data read from the memory is received at the output 54 
of the comparator 53. 
Multilevel Design 
A second embodiment of an optical memory according to the invention will 
now be described with reference to FIGS. 5-7. 
FIG. 5 is a cross-sectional view similar to FIG. 3 and illustrates a memory 
cell 11 in a storage medium 10 included in an optical memory according to 
the invention. The memory cell in FIG. 5 has, in the same way as the 
memory cell in FIG. 3, a large number of memory points which are 
distributed in a matrix pattern and which can be individually addressed by 
means of a light beam 3. Components corresponding to parts 1-9 in FIG. 1 
may advantageously be used also in the embodiment of FIG. 5. 
The storage medium in FIG. 5 comprises, similarly as the storage medium in 
FIG. 3, a polarizer 20, a final-focusing substrate 21, a PSD layer 22, an 
analysing polarizer 28 and a light-sensitive photolayer 29 whose function 
is the same as in the embodiment of FIG. 3 and therefore will not be 
described in more detail here. 
The storage medium of FIG. 5 however has four storage levels N0-N3, as 
opposed to the storage medium in FIG. 3 which has only one such storage 
level. The lowermost level N0 comprises (from top to bottom) a 
photoconductive layer 27-0, and electrode matrix 24-0 with mutually 
insulated electrode points, a storage element 23-0 having at least two 
stable optical states, and a common electrode 26-0. Similarly, the other 
three levels N1-N3 each comprise a photoconductive layer 27-1, 27-2 and 
27-3, respectively; an electrode matrix 24-1, 24-2 and 24-3, respectively; 
a storage element 23-1, 23-2 and 23-3, respectively; and a common 
electrode 26-1, 26-2 and 26-3, respectively. Substrates 30 are arranged 
between the levels and below the level N0. 
The levels are substantially identical, except that they have differently 
thick storage elements 23. In FIG. 5, the level N0 has the thickest 
storage element 23-0, while the level N3 has the thinnest storage element 
23-3. 
By judiciously selecting the thicknesses, the four levels can present a 
16-degree grey scale. In other words, a 4-bit word can be stored in each 
memory point. In the illuminated memory point in FIG. 5, there are, for 
example, stored (counting from below) the optical states 
dark-light-dark-light, representing e.g. the binary information 1010. 
It obviously is of importance that the thickness of the four storage 
elements 23-0-23-3 is selected in such a manner that each light intensity 
sensed on the photolayer 29 in one memory point corresponds to one, and 
only one, out of the sixteen possible combinations of the states of the 
four storage levels in one memory point. FIG. 7 shows an example of how 
this can be achieved. The thickness of the storage elements 23 in levels 
N0-N3 is in this case selected in the following way. A dark state only in 
level N0 and a light state in each of levels N1-N3 give a relative 
transmittance reduction of 50%, as shown in the dashed box to the left in 
FIG. 7. Similarly, a dark state only in N1 gives a transmittance reduction 
of 25%, while N2 and N3 give a 15% and a 5% reduction, respectively. FIG. 
7 shows in the box to the right all the sixteen possible combinations of 
light and dark states of the four levels N0-N1, and hence it appears that 
by detecting the intensity of transmitted light it is possible to 
unambiguously decide which combination the illuminated memory point has, 
i.e. which 4-bit word is stored. 
The advantage of several storage levels is both an increased memory 
capacity per area and an increased read-write speed, since more bits are 
written and read simultaneously in one memory point. 
In FIG. 5, 4 bits are stored in one memory point, but it is realized that 
this number can be increased to 8 or more according to the same 
principles, corresponding to e.g. a 256-degree grey scale. 
Multilevel--Function 
One example of the writing into and the read-out from an optical memory 
having a storage medium with several storage levels according to FIG. 5 
will now be described in more detail with reference to FIG. 6. 
FIG. 6 shows in perspective a broken-away part of a memory cell 11 
according to FIG. 5, comprising however only two memory points, and in a 
block diagram peripheral electronics connected thereto. The means for 
generating, collimating, deflecting and focusing the laser beam 3 are not 
shown in FIG. 6. 
Like parts in FIGS. 4 and 6 bear like reference numerals and function in 
substantially the same way. 
Eight binary data bits D0-D7 (signals 40) are stored in a data buffer 43 
together with read-write control signals 41 and 42, respectively. D0-D7 
are divided into two 4-bit words which are stored in a buffer 55. In the 
buffer 55, the read-write control signals 41 and 42 are derived and 
forwarded to a converter 56. 
Data bits D0-D3 from the buffer 55 are also supplied to the converter 56, 
which serves to adapt these four data bits to four voltages which are to 
represent a 16-degree analog grey scale. Output signals D0-D3 from the 
converter 56 thus correspond to the digital value of grey scale 
information stored in analog form. 
The converter 56 operates as a level switch for converting information to 
write pulses (+V for a logic 1 and -V for a logic 0) with the aid of 
generators 45 and 46. 
The write control signal 42 permits the presence of write pulses D0-D3 on 
the outputs of the level switch 56. The read control signals 41 block the 
outputs of the level switch 56 for D0-D3 when read-out takes place from 
the memory. 
Writing into the storage medium 10 in FIG. 6 is performed in the following 
way. Assuming that the information to be stored from the buffer 55 has the 
following value and is to be written into the right-hand one of the two 
memory cells in FIG. 6, then: 
EQU D0=1, D1=0, D2=1, D3=0 
For this information, the outputs D0-D3 of the level converter 56 will 
assume the following voltage values: 
EQU D0=+V, D1=-V, D2=+V, D3=-V 
The voltage D0=+V is applied between the photoconductive layer 27-0 and the 
common electrode 26-0 of the lowermost level N0. In the illuminated memory 
cell, the storage element 23-0 (see FIG. 5) of the storage level N0 will 
therefore be excited to a dark state corresponding to a logic 1. 
The voltage D1=-V is applied between the photoconductive layer 27-1 and the 
common electrode 26-1 of the lowermost but one level N1. If the storage 
element 23-1 of the storage level N1 was previously excited to a logic 1 
(dark state), it will now be excited to a light state corresponding to a 
written-in binary logic 0. If the storage element 23-1 was previously 
excited to a logic 0 (light state), D1=-V is without effect and the 
storage element 23-1 remains in the light state, i.e. a logic 0. 
The same applies to voltages D2 and D3 for the two uppermost storage levels 
N2 and N3, respectively. 
The binary information D0-D3 (1010) from the buffer 55 is thus stored in 
the following optical states in the right-hand memory cell in FIG. 6, 
counting from below: 
EQU dark - light - dark - light 
Read-out of such stored information from the storage medium in FIG. 6 is 
done by deflecting the laser beam 3 towards the memory point concerned. 
The light-sensitive layer 29 on the light exit side of the storage medium 
10 will thus generate a signal 52, which represents the resulting 
transmittance from the four storage levels N0-N3 of the memory point. The 
analog detecting signal 52 is converted in an analog-to-digital converter 
57 operating according to the principle illustrated in FIG. 7, to 4-bit 
digital binary information on the outputs D0-3, which then emit the logic 
signals 1010, i.e. the same information as has previously been read from 
the outputs D0-D3 of the buffer 55. 
A third embodiment of an optical memory according to the invention will now 
be briefly described with reference to FIG. 8. The difference between the 
embodiment of FIG. 8 and the embodiment of FIG. 1 is that that deflecting 
system 6, 7 has been removed and replaced with means for displacing the 
light source itself. Thus, in FIG. 8, electrically controlled means 60 are 
connected to the upper side of the laser diode field 1 to displace this in 
the x- and y-directions. The means 60 may preferably comprise an 
xy-piezoelectric motor device of a type known to a person skilled in the 
art. 
Another difference in FIG. 8, as compared with the embodiment of FIG. 1, is 
that the storage medium 10 for all the memory cells 11 is at a distance 
from both the laser diode field 1 and the light-sensitive layer 29. This 
embodiment makes it possible to arrange the memory cells on a common, 
replaceable disc or diskette 61 which in use is disposed stationary in the 
beam paths. 
Finally, it should be noted that in FIG. 8 the light-sensitive layer or 
element 29 actually consists of a plurality of mutually insulated layers, 
one for each beam or memory cell, to enable parallel read-out from several 
memory cells. 
A fourth embodiment of an optical memory according to the invention will 
now be described with reference to FIGS. 9, 9A and 9B. The components 
included in this embodiment have the same reference numerals as above for 
identical functions. 
FIG. 9 shows a circular disc 62 which is rotatable about an axis of 
rotation 63 by means of an accurately controlled motor device (not shown). 
A laser diode field 1 is arranged on one side of the disc 62 (light entry 
side) and comprises a radially extended matrix or a row of individual 
semiconductor lasers. A piezoelectric motor means 64 enables radial 
displacement of the laser diode field 1. On the opposite side (light exit 
side) of the disc 62 opposite to the laser diode field 1, there is 
provided a photodetector matrix 29 comprising a separate light-sensitive 
layer for each light beam 3 from the laser diode field 1. 
In this embodiment, the memory cells 11 are designed as concentric rings on 
the disc 62. The number of rings or memory cells corresponds to the number 
of light beams 3. Each light beam 3 need therefore only be displaceable 
radially a distance corresponding to the width of the ring. This is in 
contradistinction to e.g. a conventional CD player where the read optics 
must be moved over the entire disc. An extremely quick access is thus 
obtained also in this embodiment of the invention. The memory points of 
each memory cell are distributed both radially and circumferentially with 
respect to the axis of rotation 63. 
FIG. 9A shows broken-away cross-sections of three radially distributed 
memory points of a memory cell. From this Figure appears that the storage 
medium 10 is of the same design as earlier and comprises a substrate 21, a 
storage element 23, an electrode matrix 24, a common electrode 26, a 
photoconductive layer 27 and a substrate 30. Further, there are provided 
two polarizers 20 and 28 which have the same function as in the previous 
embodiments and which optionally are stationary or accompany the disc 62. 
The two part views of FIG. 9A illustrate two different storage states of 
the three memory points illustrated. The dimensions and the spacings of 
the memory points are basically optional, but may, for example, be in 
agreement with FIG. 1. 
FIG. 9 further shows an external, stationary light source 65 emitting a 
light beam 66 towards the center of the disc 62. The light beam 66 first 
passes an electronic polarizer 67 and then a collimator 68, 69 and 
thereafter is incident as polarized light 66' on a central photocell area 
70 on the disc. 
The light 66' which reaches the disc 62 is thus polarized by the polarizer 
67. The photocell area 70 is intended, in response to the light received, 
to generate the control voltage between the photoconductive layer 27 and 
the common electrode 26 of the memory cells. To this end, the photocell 
area 70 (see FIG. 9B) comprises a first photocell 70a which generates a 
voltage when receiving light polarized in a first direction (left-hand 
part view in FIG. 9B) and a second photocell 70b which generates a voltage 
when receiving light polarized in an orthogonal second direction 
(right-hand part view in FIG. 9B). As shown in FIG. 9B, the output 
terminals are connected in parallel with opposite polarity, such that the 
control voltage to the memory cells becomes positive or negative depending 
on the direction of polarization of the incident light. The positive and 
the negative control voltage, respectively, correspond to the generators 
45 and 46 in FIG. 4. To make the two control voltages available, a 
suitable switching frequency can be used for controlling the electric 
polarizer 67. 
If required, the photocell area 70 may actually consist of a plurality of 
series-connected photocells 70a and 70b for achieving the required voltage 
level. 
Performance 
The storage capacity of an optical memory according to the invention can be 
very high. Table I shows the storage capacity and the read-write speed for 
different embodiments of an optical memory according to the invention, 
designed as a replaceable diskette according to FIG. 8. The active storage 
area is assumed to be 60 mm.times.60 mm, and the piezoelectric motor 
device 60 is assumed to operate at a speed of 15 cm/s. 
In Table I, the upper part shows values without data compression, while the 
lower part shows corresponding values with a 10:1 data compression. In the 
Table, different cases are indicated both in respect of the size of the 
memory points and their mutual spacing, and in respect of the number of 
layers. 
For a rotary medium according to FIG. 9, it may be assumed, by way of 
example, that the PZT motor 64 operates at a speed of 15 cm/s, and that 
each memory point can be reached in 7 .mu.s, which with a data compression 
of 5:1 and 40 light sources enables reading or writing 200 bits and 25 
bytes per 7 .mu.s. 
In an embodiment according to FIG. 1 using deflection, one may have, by way 
of example, a deflecting time of 1 .mu.s per memory point, the 
corresponding values of storage capacity and read-write speed being 
readily computed.