Optical network

An optical interconnection network in which more than one communication can ccur at any give time is disclosed. The interconnection network can form part of a parallel computer, a fiber optic switching network, a massive video or data server or an asynchronous transfer mode (ATM) network. The network includes transmission network elements, reception network elements and a holographic storage element. The holographic storage element is located equidistant form all the transmission and reception network elements and stores therein a multiplicity of holograms. Each volume hologram is responsive to a different angle of incidence of wavelength. Each transmission network element includes a light directing unit which selectively provides at least one light beam of at least one desired angle of incidence to the holographic storage element, which, in turn, redirects each light beam towards a corresponding one of the reception network elements in accordance with the one of the volume holograms responsive to the corresponding angle of incidence.

FIELD OF THE INVENTION 
The present invention relates to interconnection networks in general and to 
holographic interconnection networks in particular. The present invention 
also relates to parallel computers having such holographic interconnection 
networks therein. 
BACKGROUND OF THE INVENTION 
The features which make electronics a wonderful medium for performing 
computations also make electronics a poor choice for massive high 
performance communication networks between and within computers. Despite 
this, a large number of parallel computer architectures, based on 
electronics, have been proposed and many have been built. They are 
summarized in the book by G. Lerman and L. Rudolph, Parallel Evolution of 
Parallel Processors, Plenum Press, New York, 1993, which book is 
incorporated herein by reference. 
Currently, most of the commercial parallel supercomputers consist of a 
multiplicity of high performance processors which communicate via a 
multi-stage interconnection network. Each processor communicates with the 
interconnection network via its own specially designed processor-network 
interface. 
The individual processors execute operations in excess of 100 million 
instructions per second (MIPS), have a local memory in excess of 64 Mbytes 
and can transmit messages at a rate of tens of Mbytes/second. Modern 
parallel supercomputers, such as the CM-5 manufactured by Thinking 
Machines Inc. of Cambridge, Mass., USA, the SP-1 manufactured by 
International Business Machines Inc. of the USA, the CS-2 manufactured by 
Meiko of England, the Paragon manufactured by Intel Corporation of the USA 
and the T3D manufactured by Cray Research Corporation of Maynard, Minn., 
provide the programmer with the ability to send a message between any pair 
of processors even if a direct link does not exist between the two 
processors. Each processor is typically known as a "node". 
Since electronic interconnection networks cannot support full 
interconnectivity (i.e. each processor being directly connected to every 
other processor), they typically resort to multistaged networks, as 
described in the book by G. Almasi and A. Gottlieb, Highly Parallel 
Computing, Benjamin-Cummings, 1989, which book is incorporated herein by 
reference. Unfortunately, in many communication patterns, there are not 
enough links and therefore, a plurality of messages must use the same 
communication links. Since at most one message may traverse a link at any 
time, serious performance degradations can ensue. Moreover, the latency 
(i.e. the time needed for a message to traverse the interconnection 
network) increases with the size of the network. At the present time, 
electronic networks appear to be limited to 500 nodes. 
As a result of these drawbacks, optical interconnection networks, 
supporting thousands of nodes, have been proposed. Some mimic the 
multistage networks of electronic interconnection networks and, although 
the optical networks may be faster, they have the same limitations as the 
electronic ones. Others try to mimic a bus interconnection arrangement; 
however, this arrangement does not scale easily. Still others route the 
signals through the network via a central device which, when it is 
modifying one connection, cannot be utilized for any other task. Finally, 
there are schemes based on bulk optics which require precise alignment of 
the optics. Almost all of the designs are on paper only and none of them 
are appropriate for massively parallel processing in which there are 
10,000 or more processors. Furthermore, all of the prior art designs 
suffer from indeterminant transnission times. 
SUMMARY OF THE PRESENT INVENTION 
It is, therefore, an object of the present invention to provide an optical 
interconnection network in which more than one communication can occur at 
any given time. Accordingly, the interconnection network has a centralized 
holographic storage element in which are stored a multiplicity of volume 
holograms. The interconnection network can form part of a massively 
parallel computer. The interconnection network alternatively can form part 
of a fiber optic switching network, a massive video or data server or an 
asynchronous transfer mode (ATM) network. 
In accordance with one preferred embodiment of the present invention, the 
optical interconnection network includes a multiplicity of network 
elements arranged in a geometric arrangement, such as a sphere, and a 
holographic storage element centrally located within the geometric 
arrangement. The structure ensures that the network elements are all 
equidistant from the holographic storage element. 
The holographic storage element has stored therein one volume hologram, 
responsive to a particular angle of incidence, per communication link 
between a transmission and a reception network element. Each transmission 
network element includes a light directing unit and each reception network 
element includes a light receiving unit. The light directing unit 
selectively provides a light beam at a desired incidence angle, where, due 
to the sensitivity of the volume holograms, the desired incidence angle 
defines the output angle towards the desired reception network element. 
The light receiving unit receives beams from the holographic storage 
elements and ensures that only one of the beams is considered at any time. 
There is therefore provided, in accordance with a preferred embodiment of 
the present invention, an optical interconnection network having a first 
multiplicity of interconnections. The network includes transmission 
network elements, reception network elements and a holographic storage 
element located between the transmission and reception network elements. 
The holographic storage element stores volume holograms, wherein each 
volume hologram corresponds to one of the interconnections between one of 
the transmission network elements and one of the reception network 
elements. The transmission network elements communicate with the reception 
network elements by illuminating the holographic storage element with 
light at desired position and angles of incidence corresponding to desired 
reception network elements thereby to activate the corresponding volume 
hologram. Additionally, in accordance with a preferred embodiment of the 
present invention, the light directing unit includes a spatial light 
modulator formed of a multiplicity of selectable modulator elements, at 
least one for each reception network element with which communication is 
desired. The modulator elements can be activated individually or a set of 
modulator elements can be activated. In the latter case, the result can be 
communication with many reception network elements or it can be with a 
single reception network element which requires many copies of the same 
message. 
Moreover, in accordance with a preferred embodiment of the present 
invention, each of the reception network elements comprises a light 
detecting unit for detecting and receiving light from the holographic 
storage element and for enabling a predetermined number of communications 
to occur. The light detecting unit can be formed of a single light 
detector which enables only a single communication at a time, or it can be 
formed of a matrix of detector elements. In the latter case, the light 
detecting unit additionally includes a pre-processor which, when a 
communication is initiated by activating at least one detector element, 
disables all non-activated detector elements. The light detecting unit can 
also be capable of receiving a set of light beams from one transmission 
network element at one time. 
Finally, the network preferably includes a calibration unit locatable at 
positions symmetrically across from a desired reception network element 
for use in implanting the volume holograms corresponding to the desired 
reception network element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Reference is now made to FIGS. 1 and 2 which illustrate a holographic 
interconnection network 10, constructed and operative in accordance with a 
preferred embodiment of the present invention. It is noted that, in the 
drawings, optical connections are indicated by dashed lines and 
non-optical connections are indicated by solid lines. 
The interconnection network 10 typically comprises a multiplicity of 
network elements 12 arranged in a geometric arrangement, such as a sphere 
16. The network elements communicate with each other via a holographic 
storage element 14 centrally located within the sphere 16. For parallel 
computers, each network element 12 would be connected to one processing 
unit of the computer. 
The holographic storage element 14 stores volume holograms therein. 
Typically, the number of volume holograms stored is equal to the number of 
interconnections between network elements which are desired. For example, 
if there are N network elements and it is desired that each network 
element speak with all of the others, then there are N.sup.2 volume 
holograms stored in storage element 14. 
As is discussed in the book by R. J. Collier et al Optical Holography, 
Academic Press, 1971, which book is incorporated herein by reference, 
volume holograms respond uniquely to light at a specific angles of 
incidence (i.e. illumination at one angle of incidence will be bent along 
only one optical path). Light at differing wavelengths will also have the 
same effect. The present invention will be described with respect to 
angles of incidence, it being appreciated that a similar network 
responding to wavelengths is also within the purview of the present 
invention. 
Due to this specificity, many uncoupled optical paths can be defined within 
the holographic storage element 14 and illumination of one path (at a 
specific angle or a specific wavelength) will not generate illumination 
along any other path. Thus, many network elements 12 can communicate 
through holographic storage element 14 at one time and the angle of 
incidence of a light beam defines the desired interconnection. 
The network elements 12 communicate with each other by transmitting 
incoming light beams at a given angle to the holographic storage element 
14. The storage element 14 in turn, redirects each light beam to its 
destination network element 12, in accordance with the volume hologram 
which responds to the angle of the incoming light beam. For example, in 
order to send a message from a first network element, labeled 12a, to a 
second network element, labeled 12b, the first network element 12a 
provides an incoming light beam 30, to be called herein a "source light 
beam", at an angle corresponding to the second network element 12b. When 
illuminated with the source light beam 30 at the selected angle of 
incidence, the volume hologram corresponding to the interconnection from 
network element 12a to network element 12b is activated. This volume 
hologram bends the source light beam 30 into a "destination" light beam 32 
directed towards the destination network element 12b. Thus, the 
destination of a message is defined by the angle of illumination of the 
holographic storage element 14. 
FIG. 2 illustrates the conversations of three network elements 12a, 12b and 
12c, all of which occur at the same time. Network element 12a sends a 
message to element 12b, network element 12b sends a message to element 12c 
and network element 12c sends a message to element 12a. The source beams 
are labeled 30a, 30b and 30c and are respectively produced by network 
elements 12a, 12b and 12c. The corresponding destination beams are labeled 
32a, 32b and 32c. Because the communications are through the holographic 
storage element which has a different volume hologram per interconnection 
and because the source beams 32a, 32b and 32c are mutually incoherent, all 
three communications can occur at the same time, as described hereinabove. 
It is noted that the path from a first network element to a second network 
element is not the same as the path from the second network element to the 
first. Thus, each interconnection is a one-way communication path. 
Since all network elements 12 are on the sphere 16, they are equidistant 
from holographic storage element 14. Since all communication is through 
the holographic storage element 14, all light beams, both source and 
destination, have optical paths of equal lengths. It is noted that the 
network elements 12 can be placed around any geometric arrangement that 
ensures that all of the light beams have optical paths of equal lengths. 
The remaining description will utilize the sphere; it being recognized 
that this is by way of example only. 
In the embodiment of FIG. 2, each network element 12 comprises an node 
controller 20, a spatial light modulator and detector unit 22, a modulated 
light source 24 of temporally modulated light, and a pre-processor 26. The 
modulated light source 24 produces light which is temporally modulated 
with data to be transmitted. 
The spatial light modulator and detector unit 22 comprises a modulator 
matrix 34 of spatial light modulator elements 23 for providing source 
light beams 30 and a detector matrix 36 of detector elements 28 for 
receiving destination light beams 32. 
The modulator matrix 34 is lit by the temporally modulated light from light 
source 24. If the interconnection network of the present invention is 
implemented in a parallel computer, the light source 24 is typically 
formed of a laser having a frequency which is suitable for the holographic 
storage element 14 and a modulating element for temporally modulating the 
light emitted by the laser. If the holographic storage element is formed 
of Lithium Niobate doped with iron (LiNbO.sub.3 :Fe), a diode pumped 
double YANG laser providing green light, is appropriate. 
As can be seen in FIG. 2, each network element 12 and, therefore, each unit 
22 is at a different location on the sphere 16 (FIG. 1). In addition, each 
spatial light modulating element 23 of each unit 22 is at a slightly 
different angle to the holographic storage element 14. Therefore, each 
element 23 corresponds to and enables communication with a different 
network element 12. 
It is noted that the elements 23 can be any appropriate type of spatial 
light modulating elements known in the art. They can be operated such that 
only one element 23 is active at once or such that a predetermined number 
can be active at once. In the latter case, the same message is provided to 
many destination network elements. 
It is further noted that each detector element 28 is located at a slightly 
different angle to the holographic storage element 14. Therefore, each 
detector element 28 defines a different angle for the destination light 
beams 32. 
The node controller 20 typically controls unit 22, activating the spatial 
light modulating element 23 which corresponds to the desired destination 
network element 12. For the example of network elements 12a and 12b 
communicating, element 23b is activated. The network element 12a utilizes 
the selected pixel 23b to emit light beam 30a. 
As mentioned hereinabove, the holographic storage element 14 bends light 
beam 30a into light beam 32b. Beam 32b impinges upon detector element 28a 
of the unit 22 corresponding to network element 12b, wherein detector 
element 28a corresponds to network element 12a. The detector element 28a 
is typically a photodetector. Each detector 28 provides a modulated signal 
to the preprocessor 26 when a light beam impinges upon it. It is noted 
that the pre-processor 26 can be separate from or integral with the 
detector matrix 34. 
Pre-processor 26 can operate in many different ways. Typically, it ensures 
that, at any one time, only one detector element 28 is active. 
Alternatively, the pre-processor 26 can enable a predetermined number of 
detector elements 28. 
If the interconnection network of the present invention is implemented into 
a parallel computer, the pre-processor 26 also converts the modulated 
signal provided by the active detector element 28 into a format, typically 
digital, which the processor connected to the network element 12 
understands. To prevent many communications from occurring at once, as 
soon as one detector element 28 becomes active, the pre-processor 26 
disables all the other detector elements 28 for the duration of the 
communication. 
It will be appreciated that any network element 12 can, at the same time, 
send a message to one processor and receive a message from another 
processor. Alternatively, a sending processor can provide the same message 
to a plurality of processors by activating a plurality of modulating 
elements 23 at one time. 
The detector matrix 36 can be implemented in a number of ways, all of which 
must ensure that a predetermined number of communications are enabled at 
any one time. It can be formed of a single detector element 28. The 
holographic storage element 14 is designed to direct beams from any 
sending network element 12 to the single detector element 28. It will be 
appreciated that, in this embodiment, the information regarding which 
network element 12 initiated the communication must be in the message 
being sent. 
However, this embodiment requires that no two light beams impinge on the 
single detector element at the same time. Hardware arbitration circuitry 
can be used to avoid simultaneous transmissions. 
The detector matrix 36 can alternatively be implemented as a matrix of 
detector elements 28, as generally illustrated in FIG. 2 and detailed in 
FIGS. 3A and 3B to which reference is now made. In the embodiment of FIG. 
3A, the detector matrix 36 is a two-dimensional matrix of detector 
elements 28, one per interconnection, with output pins 40a per column and 
40b per row. Matrix 36 also includes column and row selection circuitry 41 
and 42, respectively, for selecting a column or row for reading. 
Initially, column selection circuitry 41 is operative and continually 
scans the column output pins 40b. Since only a few detector elements 28 
are likely to be active at any one time, most of the columns will have no 
activity on them (equivalent to a logical "0"). FIG.3A shows two active 
detector elements 28a and 28b. 
The row and column selection circuits 41 and 42 are typically implemented 
to perform a "winner-takes-all" circuit such that, as soon as it is 
determined that a detector element 28, such as 28a, is active, the output 
of the remaining detector elements 28 are disabled. In this embodiment, 
only one communication at a time is allowed. 
A further alternative embodiment is illustrated in FIG. 3B. In this 
embodiment, the detector matrix 36 is much smaller and can be implemented 
as a linear array 45 or as a matrix. In this embodiment, the sending 
network element 12 sends multiple copies, for example three, of its 
message to the destination network element 12. Each sending network 
element 12 has its own combination of detector elements 28 to which it 
sends, through the holographic storage element 14. For example, a first 
network element 12 might send to the detector elements labeled 1, 3 and 6 
(illustrated with solid lines) and a second might send to detector 
elements 6, 8 and 10 (illustrated with dashed lines). 
In the embodiment of FIG. 3B, if one of the copies of the message is 
corrupted, since another message was sent to that detector element 28 at 
the same time, one or more of the other copies of the message will not be 
corrupted at the same time. This is because no two network elements 12 
activate exactly the same detector elements 28. If the first network 
element 12 is currently sending a message, it has activated detector 
elements 1, 3 and 6. The detector array 45 disables all elements but 1, 3 
and 6. When the second network element 12 begins sending, it will attempt 
to activate detector elements 6, 8 and 10. Since detectors 8 and 11 are 
already disabled, they will have no effect. But the communication with 
detector element 6, when the first network element 12 is already 
communicating with detector element 6, will cause the output of detector 
element 6 to be corrupted. An output circuitry (not shown) which detects 
corrupted signals ignores the output of detector element 6 and only 
utilizes the output of detector elements 1 and 3. Since the output of 
detector elements 1 and 3 are identical, only one of them is really needed 
to ensure that the message is properly sent. 
In a further embodiment, a dual-rail implementation is utilized in which 
there are two detector arrays 45, one (labeled 45a) for receiving logical 
"1"s and one (labeled 45b) for receiving logical "0"s. Each network 
element 12 sends to the same set of detector elements in each detector 
array 45. Thus, the first network element 12 sends to detector elements 1, 
3 and 6 of each detector array 45, depending on the logical value of the 
data being transmitted. 
It is noted that the detector matrix 36 and spatial light modulator matrix 
34 do not have to be implemented into the single unit 22 shown in FIG. 2. 
Reference is now briefly made to FIGS. 4A and 4B which illustrate two 
embodiments of a network element 12 which separate the detector and 
spatial light modulator matrices 36 and 34, respectively. Similar elements 
are indicated with similar reference numerals. 
FIG. 4A is a simplified illustration of one network element which comprises 
the node controller 20, light source 24, and pre-processor 26 as described 
hereinabove. The spatial light modulator (SLM) matrix 34 and detector 
matrix 36 are separated and the light which they respectively emit and 
receive are processed by a beam splitter 46. Beam splitter 46 passes the 
outgoing light beams from the spatial light modulator matrix 34 and bends 
the incoming light beams towards the detector matrix 36. It is noted that 
matrices 34 and 36 are located so as to ensure that the source light paths 
from the spatial light modulator matrix 34 to the holographic storage 
element 14 are of an equivalent length to the destination light paths back 
to the detector matrix 36. 
FIG. 4B is a schematic representation of an interconnection network in 
which at least some of the spatial light modulator matrices 34 of the 
different network elements 12 are combined together into a single unit 47 
and at least some of the detector matrices 36 are combined together into a 
single unit 48. Each network element 12 still comprises its own light 
source 24 and node controller (NC) 20; however, the pre-processors 26 can 
either be separate, or combined into a single pre-processor 49, as shown. 
Each light source 24 is operative only for its corresponding spatial light 
modulator matrix 34. 
Since the units 47 and 48 are large compared to the individual matrices, 
the units 47 and 48 may have to be curved so as to maintain the matrices 
34 and 36, respectively, along the surface of sphere 16 (FIG. 1). Since 
only the matrices 34 and 36 operate with light beams (noted by dashed 
lines), the remaining communication being performed via electronic 
connections (noted by solid lines), only matrices 34 and 36 have to be 
placed on the surface of the sphere 16. The remaining elements, such as 
the node controllers 20, can be placed elsewhere, as noted in FIG. 4B. It 
is noted that each node controller 20 still communicates with its 
corresponding spatial light modulator matrix 34 and detector matrix 36. 
Reference is now made to FIG. 5 which illustrates a system by which the 
volume holograms are created within the holographic storage element 14, 
which is typically formed of a crystal, such as one made of Lithium 
Niobate doped with iron (LiNbO.sub.3 :Fe). The system shown in FIG. 5 is 
typically operative with the embodiment of the network elements 12 shown 
in FIG. 4A. 
To create a hologram in a crystal, two light beams must be shone on the 
crystal, one from the input direction and one along the symmetric 
reflection of the desired output direction. Once the hologram is "fixed", 
as will be described later, light from the input direction will cause 
light to go out the output direction. 
The network elements 12 are typically placed at their locations on the 
sphere 16 (FIG. 1) and the holographic storage element 14 is placed in its 
location concentric to the center of sphere 16. To create the optical path 
from a network element, labeled 12d, to a network element, labeled 12c, 
(i.e. illumination from network element 12d will cause the light to bend 
towards network element 12c), a phase conjugate mirror 50 is placed 
directly opposite network element 12c, in order to create a light beam 56 
headed in the output direction toward network element 12c. 
Both network elements 12d and 12c emit light, through their respective 
units 22, towards the holographic storage element 14. The emitted light 
beams are labeled 52 and 54. Some of light beam 54 from network element 
12c will pass the holographic storage element 14 and impinge upon mirror 
50 which will return it, as light beam 56, towards network element 12c. 
Since the light beam 54 is of low intensity and therefore, not intense 
enough to implant the holographic information, mirror 50 is a phase 
conjugate mirror which produces beam 56 which is typically more intense 
than impinging beam 54. The beam 56, in conjunction with the less intense 
input beam 52, is intense enough to create the relevant volume hologram. 
It is noted that the reference beam 52 and beam 56 must be coherent. 
Mirror 50 remains in its location for the creation of all of the optical 
paths towards network element 12c. This involves consecutively activating 
the light sources 24 and appropriate spatial light modulator elements 23 
of each network element 12. Afterwards, mirror 50 moves to a location 
opposite another network element 12 and the process repeated. Once all of 
the volume holograms have been created, the holographic storage element 14 
is treated, in accordance with known treatments, so as to fix the 
holograms therein. 
It will be appreciated that the holographic interconnection network of the 
present invention can be utilized anywhere where optical interconnections 
are desired and where it is desired to have many communications occurring 
at the same time. Reference is now made to FIG. 6 which illustrates a 
wavelength division multiplexer (WDM) which utilizes the holographic 
interconnection network of the present invention. In effect, in this 
embodiment, the holographic interconnection network acts as a big 
switchboard for a fiber optic network. 
WDMs convert the wavelengths of light signals carried on fiber optic 
waveguides 60. Each fiber optic waveguide 60 carries on it many different 
channels of signals, each channel being defined by a different wavelength. 
Thus, the fiber optic waveguide 60 may carry the channels denoted by 
.lambda..sub.i, .lambda..sub.j and .lambda..sub.q. The WDM is operative to 
convert the wavelengths from one wavelength to another one, as desired. By 
changing wavelengths, the signal being carried is switched from one 
channel to another. 
In accordance with this alternative embodiment of the present invention, 
the holographic interconnection network is utilized to direct the signals 
from one channel to another one. The transmitting side comprises a 
filtering unit 62, comprising a plurality of filters each attuned to the 
wavelength of one channel, to separate the signals into their separate 
channels. For each channel, the transmitting side also comprises a 
transmission unit is 64 comprising the transmission elements of a network 
element 12. Each transmission unit 64 comprises an node controller 20 and 
a spatial light modulator matrix 34 located on the surface of the sphere 
16 at whose center is located the holographic storage element 14. On the 
receiving side are a plurality of wavelength changing apparatus 74, one 
for each outgoing channel or wavelength .lambda.. 
As in the previous embodiment, the node controller 20 activates the matrix 
element of modulator matrix 34 which corresponds to the desired 
destination. The activated matrix element emits a light beam, for example 
beam 66, having its corresponding wavelength .lambda..sub.q at an 
incidence angle corresponding with the desired output wavelength. The 
holographic storage element 14 redirects the source light beams, 
regardless of their wavelengths, in accordance with their incidence angle. 
The redirected beam is received by the appropriate wavelength changing 
apparatus 74 which, in turn, produces a corresponding output signal with 
the desired wavelength. 
Each wavelength changing apparatus 74 typically comprises a detector 76 and 
a new channel creator 78. The detector 76 is similar to the detector 
matrix 36 of the previous embodiment in that it detects the incidence of 
light upon it and ensures that only one communication occurs at any one 
time. Furthermore, detector 76 demodulates the data temporally modulated 
in the light beam and thus, produces a data signal representing the data 
carried by the light beam. Detector 76 can be formed of many detector 
elements or just one, as described hereinabove. 
Creator 78 includes therein a laser at the desired outgoing wavelength. 
Upon receipt of the data signal, creator 78 modulates the output of its 
laser in accordance with the data signal. The result is a modulated signal 
with the desired outgoing wavelength, or, in other words, on the desired 
channel. 
FIG. 6 illustrates the conversion of the three source light beams having 
wavelengths .lambda..sub.i, .lambda..sub.j and .lambda..sub.q into two 
outgoing light beams having wavelengths .lambda..sub.j and .lambda..sub.i. 
Each source light beam is processed by its own transmission unit 64 and 
illuminates the holographic storage element 14 at the angle defining the 
desired output wavelength. The beams having wavelengths .lambda..sub.i and 
.lambda..sub.q are redirected to wavelength changing apparatus 74a and the 
beam having wavelength .lambda..sub.j is redirected to wavelength changing 
apparatus 74b. Typically, if two beams fall on a detector 76, the earliest 
one will be processed. Therefore, the node controllers 20 have to be 
coordinated so as to ensure that each source beam is directed to an 
available wavelength changing apparatus 74. This is especially true if the 
WDM is to act as a switchboard. 
It will be appreciated by persons skilled in the art that the holographic 
interconnection network of the present invention has a number of features. 
A network element can transmit, at once, the same message to a desired 
number of destination network elements. This "multi-casting" occurs 
whenever the spatial light modulator matrix 34 activates more than one 
spatial light modulator element 23 at one time. 
Due to the sensitivity of the holographic storage element 14, a 
multiplicity of communications can occur at the same time between 
different network elements. These communications are independent of each 
other and therefore, do not require centralized coordination in order to 
occur. Since the communications are independent, the failure of one node 
will not cause the entire network to fail. 
Finally, it will be appreciated that, since all of the network elements are 
similar, network elements can be added or removed as desired. 
It will be appreciated that the holographic interconnection networks of the 
present invention can be implemented in any device which requires many 
interconnections between operating devices. For example, the holographic 
interconnection network can form part of a massively parallel computer. 
Alternatively, it can form part of a network of video or data servers. 
Furthermore, it can be utilized as the interconnection unit for 
asynchronous transfer mode (ATM) networks. 
It will be appreciated by persons skilled in the art that the present 
invention is not limited to what has been particularly shown and described 
hereinabove. Rather the scope of the present invention is defined by the 
claims which follow: