Photonic array bus for data processing and communication systems

A photonic array bus provides a connectorless, computer backplane-like communication systems for advanced data processing and communications systems requiring operation rates exceeding 1 gigahertz. Components of a multiple component advanced data processing or communications system are coupled to pairs of laser transmitters and laser receivers. The laser transmitter and receiver pairs are arrayed in a coplanar circular fashion about a central disperser lens. A laser transmitter emits a laser beam incident on the central disperser lens. The central disperser lens disperses the laser beam into a coplanar array of laser beams illuminating the laser receivers. Multiple channel operation is achieved by "stacking" along the length of the disperser lens coplanar arrays of pairs of laser transmitters and receivers.

TECHNICAL FIELD 
The invention relates to ultra-high speed data processing and communication 
systems, and more particularly to data processing and communication 
systems utilizing optical communications media. 
BACKGROUND OF THE INVENTION 
Advanced data processing and communication systems are comprised of 
numerous components or subsystems. Types of advanced data processing 
systems typically include a super computer utilizing an array of 
processors, a super-memory utilizing an array of data storage devices, a 
neural network or a combination of devices, any one of which operates at 
very high data or processing rates. Advanced communication systems 
typically include communication hubs for local area networks and other 
types of networks. The emphasis in the design and development of these 
advanced systems has primarily been on increasing the operating rates of 
discrete system components and interconnecting larger numbers of 
components to achieve greater processing or networking capabilities. With 
increases in the operating rates of the discrete system components and the 
number of interconnections comes an increasing demand on the system's 
signal distribution network for ultra-high speed transfers of large 
amounts of data between the components of the system. 
Current signal distribution systems in advanced processing systems rely on 
backplanes and other types of communication busses using wires and cables 
that require very large numbers of connections between components. Because 
of the number of connections required, and because of the limited 
bandwidth or data rates of conventional cabling, optimal system 
architectures are rarely achieved. Consequently, data transfers between 
components are accomplished at data or signalling rates magnitudes slower 
than the operating rates of at least the faster system components, and 
access times for the system are significantly longer. In sum, the overall 
performance of an advanced processing system is limited by the 
conventional signal distribution systems. 
To achieve optimal system architectures or configurations of the components 
in an advanced data processing system, several problems must be overcome. 
For example, there are physical limitations in the number of input/output 
(I/O) pins available for interconnecting the components, not only limiting 
the number of components that can be interconnected, but also limiting the 
available bandwidth. The number of signal interconnections is also limited 
by loading effects where the signal is "fanned-out" from the transmitter 
of one component to the receiver of several components. The transmitter 
has limited output power. 
Increasing the bandwidth by increasing the data or signalling rates 
exacerbates several other problems. Voltage standing waves on cabling 
cause multipath "echoes", especially in a "fan-in" distribution system 
where a receiver is coupled to more than one transmitter. Cross-talk 
induced by magnetic coupling of proximate cabling increases with the 
signalling bandwidth. Thus, the task of data detection at the receivers 
become much more difficult, especially when interference is taken into 
account, as data rates increase. Voltage standing waves in the signal 
distribution system causes signal reflection problems, slow access times 
to logic devices, and limited bandwidth. Each of these problems are 
exacerbated by increasingly faster signalling and data rates. To 
compensate for some of these problems, additional data buffers, 
rate-smoothing interfaces, memories and processors are incorporated into 
the distribution system, thus increasing the processing system's design 
complexity and cost. 
At higher data speeds, the physical layout of the components of the 
advanced data processing system become critical. The number of 
connections, for example, between the processing and control equipment, 
disk and tape units and external I/O devices, requires large amounts of 
cable. Beside the logistical problems of dealing with so much cabling, 
high speed data does not travel well on long cables due to narrow 
bandwidth, significant power loss and relatively slow travel speeds. In 
present advanced data processing systems, components that require fast 
access to "remote" data are positioned in close proximity to the data 
source. Furthermore, there are time skew problems where several components 
require access to the same data at the same time. One solution to these 
problems has been to resort to densely-packed, super-cooled modules that 
are "bricked" into a computing structure. However, "bricking" leaves very 
little space for architectural modifications and requires unacceptable 
downtime for repairs. 
In the end, advanced processing and communication systems requiring large 
bandwidths and large numbers of interconnections force high-risk, 
high-cost and high-density solutions that are more difficult to design 
than the algorithms being processed, and are difficult to maintain in the 
field. What is required is an ultra-wideband signal distribution system 
for large-scale advanced data processing and communication systems. 
SUMMARY OF THE INVENTION 
Photonic array bus architecture provides a very wide bandwidth (over 1 
gigahertz per channel) photonic signalling system for a very wide variety 
of advanced systems, such as array processors, wide-band multiport 
memories, high-performance supercomputers, peripherals and network 
communications hubs for communication systems. As such, a photonic array 
bus (PAB) functions as would a computer backplane. However, it has no 
mechanical connections. 
A PAB communicates data with a laser beam across an optical atmosphere bus. 
The PAB, when viewed from overhead, appears to look like a wagon wheel. 
Circuit cards form "spokes" in the wheel when they are inserted into a 
circular card cage or carousel. A disperser lens forms the hub of the 
wheel. One edge of each circuit card is equipped with a laser diode for 
transmitting a signal via a modulated and collimated laser beam and a 
detector for receiving a laser beam. The disperser lens splits an incident 
transmitted laser beam such that equal proportions of the incident laser 
beam reach all the detectors on the PAB substantially simultaneously. As 
in any bus operation, only one of the transmitters may signal at a time. 
Each card or module of the PAB has several optical channels analogous to a 
conventional computer bus which may have four, eight, sixteen, thirty-two 
or a higher number of bits or channels. The transmitting laser diodes and 
detectors for an optical channel are arranged in a plane perpendicular to 
the disperser lens. The dispenser lens disperses an incident collimated 
laser beam of an optical channel into an omnidirectional plane of light 
that illuminates the coplanar photodetectors of that optical channel. 
There is substantially no interference or cross-talk with adjacent 
channels 
The photonic array bus minimizes the problems associated with the standard 
computer backplanes, and presents the possibility of achieving optimal 
system architectures. Excess loading effects at high speeds and voltage 
standing waves are no longer problems. A photonic array bus has an 
ultra-wide bandwidth. The unique circular configuration of a multi-channel 
PAB provides the potential of ultra high frequency operation of an 
advanced system due to the minimal time skew between any transmitting 
laser diode and the multiple receivers on the other circuit cards. 
The use of photonic signalling and the circular configuration also provides 
a means for interconnecting the large number of signals required in an 
advanced data processing or communications system, with even greater 
numbers possible through expansion of the number of cards and channels. 
Close proximity of a large number of components is achieved without resort 
to "bricking." Furthermore, the open top and bottom of the carousel make 
it easily adaptable to methods of cooling very densely packed circuit 
cards. 
In maintenance and manufacture, the PAB has the significant advantage of 
permitting the removal and insertion of circuit cards into the carousel 
without minimum interruption or shut down of the entire system since there 
are no physical connections between the cards. The ability to insert and 
remove the cards also makes the system highly adaptable. 
Other features include the advantage of communicating analog signals with a 
laser diode transmitter with minimal detection circuitry, and the 
provision for a "pipeline" bus communication system with data flow 
channels oriented orthagonally to the card surface for laser beams 
signaling between adjacent card surfaces.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1A, a plurality of motherboards or circuit cards 101 are 
arrayed in a circular fashion, though not necessarily in a circle, about a 
disperser lens 103. Mounted on the circuit cards are circuits, functioning 
as components of advanced data processing or communication systems such as 
super memories, supercomputers, neural networks, arrayed processors, or 
communication hubs. On a front edge of each of the circuit cards 101 is 
mounted an electro-optical module 105 for interfacing the circuit cards 
with a photonic array bus. Each electro-optical module 105 may also be 
arrayed around the disperser lens 103 without being mounted to the circuit 
card. The modules would, in this case, be electrically or optically 
coupled to the circuit cards or to members of a communications network. 
Each electro-optical module 105 has twelve levels of vertically stacked 
pairs 107 of collimating lens 109 and horizontally adjacent collecting 
lens 111. Now referring to FIG. 1B, a laser diode 110 for each laser 
transmitter is located within the electro-optical module 105. The 
collimating lens 109 focuses the coherent light emitted from the laser 
diode 110 into a collimated laser beam 115 incident on disperser lens 103. 
Together, the laser diode and the collimating lens comprise a laser 
transmitter. A photodetecting diode 112 also is located within the 
electro-optical module 105. The collecting lens focuses on the diode a 
portion of resultant coplanar laser beams 117 from the disperser lens 103 
that are incident on the collecting lens. The photodetecting diode and the 
collecting lens comprise a laser receiver. 
Referring now to FIGS. 1A and 1B together, coplanar pairs of laser 
transmitters and laser receivers form optical channels. Line 113 of FIG. 
1A indicates those coplanar pairs forming a single optical channel. The 
twelve levels of vertically stacked pairs 107 of laser transmitters and 
laser receivers thus enable twelve channel operation between all the 
electro-optical modules 105 when corresponding levels in each module are 
coplanar. 
Communication in an optical channel takes place when one laser transmitter 
transmits a collimated laser beam 115, modulated by an information bearing 
signal, toward the disperser lens 103. The disperser lens 103 splits or 
disperses the incident laser beam 115 into a plurality of resultant 
coplanar laser beams 117 that form a plane of light coplanar with laser 
beam 115, that illuminate coplanar collecting lens 111, and that, 
consequently, illuminate the photodetectors of the laser receivers sharing 
the optical channel. Although twelve channels are shown, any number of 
independently functioning channels can be placed or "stacked" along the 
length of the disperser lens 103. Furthermore, each optical channel may be 
multiplexed by using several different frequency laser diodes 110 and 
corresponding filters between the collecting lens 111 and the photo 
detectors 112. 
Referring back to FIG. IA only, a separate data flow channel 119 between 
adjacent circuit cards 101 for "pipeline" bus communication enables 
operation of circuit cards or groups of circuit cards independently of the 
operation of the photonic array bus. A laser transmitter, comprised of a 
laser diode and collimating lens mounted on a surface of a circuit card 
101, transmits orthogonally to a surface of the circuit card on which it 
is mounted a collimated laser beam modulated by an information bearing 
signal. A photodetector mounted on an opposite surface of an adjacent 
circuit card acts as a laser receiver and detects the information bearing 
signal. No collector lens is needed for the photodetector. 
Referring now to FIG. 2, a carousel card cage 201 is designed to hold on 
edge a circular array of circuit cards 101, mounted with electro-optical 
modules 105, in a circular fashion about the disperser lens 103. The 
carousel card cage 201 is comprised of circular top and bottom sections 
203 and 205, interconnected by vertical support braces that are not shown. 
The disperser lens 103 is held in the center of the carousel by guides of 
the circular top section 203 (guide 203A) and the circular bottom section 
205. A plurality of circuit card slots 207 are formed in the top section 
203 and the bottom section 205, into which circuit cards are inserted and 
arranged in a circular fashion. Supporting the circuit cards on edge 
preserves space and enables more circuit cards to have access to the 
photonic array bus. The carousel card cage 201 is constructed of a hard 
metal alloy, such as aluminum-magnesium, which is rigid enough to maintain 
optical alignment as thermal and flexing forces occur. 
Referring now to FIG. 3, a plurality of circuit cards 101, each with 
electro-optical module 105 attached to a front edge thereof, are shown as 
being inserted into slots 207 of the 24-slot carousel card cage 201. 
Circuit cards are insertable and removable while the photonic array bus is 
functioning without interrupting the use of the photonic array bus by the 
remaining circuit cards. Not every card slot must be occupied for the 
photonic array bus to operate, nor does a particular card need to be in 
any particular slot. This "position independent" property of the circular 
array of electro-optical modules 105 is illustrated by the carousel card 
cage not being fully complemented with circuit cards and electro-optical 
modules. However, a system which also uses a data flow channel 119 must 
maintain the correct adjacency of groups of cards, but the group could 
reside at any location in the carousel card cage. An operating system 
needs only to know the logical address of the card. 
The central disperser lens 103 is a means for dispersing or splitting an 
incident collimated laser beam 115 from a laser transmitter into an 
omnidirection radial plane of resultant laser beams 117 that illuminate 
the collimating lens 111 of the laser receivers associated with that 
optical channel. In order to reduce the spacing between the optical 
channels along disperser lens 103, and thereby increase the number that 
can be stacked along the disperser lens 103, the incident laser beam 115 
has an angle of incidence to the surface of the central disperser lens 103 
close to zero degrees. A small angle of incidence minimizes refractions in 
a direction parallel to the surface of the disperser lens. Each optical 
channel thereby operates independently without interference or cross-talk 
with closely spaced adjacent optical channels. In another embodiment, the 
number of circuit cards arrayed about the disperser lens 103 may be 
increased, at the expense of increased spacing between channels, by 
switching the horizontal orientation, shown in FIG. 1A, of the laser 
receiver and laser transmitter pairs to a vertical orientation. In this 
case, the angle of incidence of the incident laser beam 115 must be 
changed so that resultant laser beams 117 form a cone of omnidirectional 
light and illuminate the photodetectors of the corresponding optical 
channel. 
For digital communication, the proportion of the incident laser beam 115 
that, after dispersion, illuminates each photodetector must be intense 
enough to trigger the threshold detector in each receiver. The disperser 
lens 103 therefore must effiCiently transmit power of the laser beam in 
order to maximize the signal to noise ratio. The disperser lens must also 
disperse the incident laser beam 115 into an onmidirectional pattern of 
light having a sufficiently uniform intensity so that all the 
photodetectors on the optical channel receive sufficient illumination, and 
so that the number of laser receivers that may occupy the photonic array 
bus is maximized. The collecting lens 111 of the laser receivers are 
positioned and focused to provide the photodetectors with a sufficient 
amount of energy to detect the information bearing signal modulating the 
resultant laser beams 117. 
The preferred embodiment of the central disperser lens 103 is a sheaf of 
thirty-seven clear capillary tubes, e.g., capillary pipettes 301, closely 
packed and arrayed in a hexagonal shape. More pipettes can be used, but 
increasing the number of pipettes increases the diameter of the disperser 
lens 103, which increase causes decreased bandwidth. Fewer pipettes can be 
used, but at the expense of a less uniform distribution of resultant laser 
beams 117. For a bandwidth of 1.8 gigahertz, the capillary pipettes have a 
diameter of two millimeters. Larger bandwidths may be achieved by reducing 
the diameter of the disperser lens 103 by, in turn, reducing the diameter 
of the pipettes 301. The diameter of the incident laser beam 115 may also 
be reduced, but to provide a uniform distribution the incident laser beam 
should be large enough so as to be incident on at least two to three 
pipettes. To reduce losses due to fluorescence common in silica and Pyrex 
glass, the capillary pipettes are made from BK-7 glass. With an incident 
laser beam 115 having a diameter of approximately five millimeters, the 
sheaf of capillary pipettes provides a omnidirectional planar distribution 
of laser resultant laser beams 117. The omnidirectional, planar 
distribution of resultant laser beams 117 has a sufficiently uniform power 
distribution to illuminate the collector lens 111 of a circular photonic 
array bus for communication approaching a bandwidth of 1.8 gigahertz. 
An array of capillary tubes provide a large number of reflective and 
refracting surfaces so that an incident laser beam is "fanned" into a 
relatively uniform omnidirectional distribution of planar resultant laser 
beams 117. Furthermore, the hexagonal shape allows for the closest 
packing, and there is no need to resort to more sophisticated methods of 
holding the pipettes together as would be required in other shaped arrays. 
However, non-hexagonal shapes may be used as a means for dispersing. 
Similarly, optical fibers, having an inner tube of glass with an exterior 
glass coating, also provides an equivalent means for dispersing. Greater 
attenuation due to dispersion and absorption results when materials other 
than air fill the pipettes. 
If the planar array of resultant laser beams does not need to be 
omnidirectional, or if the power distribution need not be completely 
uniform, other means for dispersing the incident laser beam 115 into the 
desired pattern for illumination of the photodetectors of the laser 
receivers may be used. For example, a reflective or mirrored surface 
instead of a lens, may be used with a semicircular or a quarter-circle 
array of pairs of laser transmitters and laser receivers to provide 
illumination of photodetectors of the laser receivers. Various types of 
clear lens may also be used, but at the expense of increased cost, 
complexity, inefficiency and uniformity of distribution. 
By maintaining equal distances between the disperser lens 103 and the 
photodetectors of the laser receivers, time skew between circuits coupled 
to the photonic array bus is minimized or eliminated. Use of the disperser 
lens also produces negligible loading effects on the circuit cards 101 on 
the photonic array bus, and allows the circuit cards 101 on the photonic 
array bus to be removed and inserted without disabling the bus or 
interfering with the use of the bus by the remaining circuit cards. 
Referring now to FIG. 4, in the preferred embodiment, a digital laser 
transmitter comprises a laser diode modulator which converts an ECL-level 
logic signal on data input 401, the data signal being provided by an 
information source not shown, to an amplitude modulated laser beam 403 
that is collimated by the collimating lens 109 shown in FIG. 1. The 
ECL-level logic signal is coupled to a field effect transistor (FET) 
current gate 405. Laser diode 407 is biased to a "just-lasing" point and 
held at that point by an ECL logic level "0" at the gate of the FET 405. 
An ECL logical "1" signal on input 401 causes the gate of the FET 405 to 
conduct full current through the laser diode 407. The laser diode is 
chosen based on the application demand for modulation bandwith and output 
power. 
The photonic array bus is equally capable of communicating analog 
information, or both digital and analog information. In this case, a laser 
diode is current modulated with an input signal bearing analog information 
from the information source. 
Referring now to FIG. 5, the collector lens 111, shown in FIG. 1 focuses a 
laser beam 501 onto a photodetector, preferably an avalanche photo diode 
(APD) 503. The APD 503 is biased at an optimal point for sensitivity and 
signal to noise reception. The APD 503 is AC-coupled with capacitors 505 
to RF amplifiers 507. The output of the RF amplifiers 507 is coupled to a 
logic threshold comparator circuit 509, the output of which represents the 
information signal transmitted on the photonic array bus. If analog 
information is being received, the threshold comparator circuit is 
bypassed. 
While the invention has been described in connection with a preferred 
embodiment, it is not intended to limit the scope of the particular form 
set forth, but, on the contrary, is intended to cover such alternatives, 
modifications and equivalents as may be included within the spirit and 
scope of the invention as defined in the appended claims.