Optical slab waveguide for massive, high-speed interconnects

Details Optical slab waveguide for massive, high-speed interconnects Optical slab waveguide for massive, high-speed interconnects

2000-04-05

2001-12-18

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Plural

C385S001000, C385S011000, C385S015000, C385S027000, C385S028000, C385S047000, C385S129000, C385S130000, C385S131000, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06332050

ABSTRACT:

This invention pertains to optical slab waveguides that may be used as high-capacity interconnection devices, for example as a bus or other interconnection device in a massively parallel computing environment.
Interconnection devices are essential components in computer systems and other types of electronics. Existing uses for interconnection devices include connections within processors, connections on a board, and backplanes connecting boards and global shared buses in multiprocessor systems.
A bus is one type of an interconnection device. As interconnection devices, buses have several desirable features. When used as a shared resource, a bus can be used more efficiently and flexibly than dedicated connections. Bus-based systems support broadcasting easily and naturally, and have reasonable costs.
There are limits on the number of connections that may be made to electrical buses. The “loading” of a bus refers to the maximum number of connections that can be made to the bus without significantly degrading signal quality. Connections to an electrical bus cause capacitive loading that limits the rate at which the signal switches states reliably (i.e., the bus clock rate). In addition, crosstalk problems can arise from the close proximity of high frequency signals in adjacent buses. Using state-of-the-art technology, an electrical bus operating at a few hundred MHz can only support a loading of about 30 different connections. A fiber optic bus can connect to a somewhat larger number of elements (about 100 at a few hundred MHz), a loading that is still inadequate for moderately large systems.
U.S. Pat. No. 5,894,539 discloses a light pipe for illuminating a billboard or similar-type display. The light pipe was said preferably to have a rectangular, e.g., square, cross-section, to preserve the modes emitted from a point light source, i.e., the cone of emission from the light pipe was about the same as that from the light source. In one embodiment, the light pipe had a plurality of notches in the surface of the light pipe opposite the direction that light rays from a point light source were directed by the light pipe, i.e., opposite to the side of the light pipe adjacent to the display. Notches in the surface of the light pipe preferably formed an angle of about 45 degrees. The pitch, i.e., frequency, of the notches was preferably non-uniform to improve the uniformity of illumination from one end of light pipe to the other. An increasing density of notches was said to compensate for the drop in luminous flux density as light was removed from the light pipe proceeding away from the point light source. In an alternative embodiment, the light pipe was wedge-shaped and had a plurality of step-facets which extracted light from the pipe and directed it out the opposite side of the light pipe. The preferred angle of the step facets was about 45 degrees.
U.S. Pat. No. 4,786,131 discloses an M×N coupler using a planar waveguide to couple each of M input channel waveguides to each of N output channel waveguides, the input guides and the output guides being disposed on opposite sides of the planar waveguide. Uniformity of coupling was said to be improved by joining the input channel waveguides to the planar waveguide in a manner to focus the beams emitted by each input guide into the planar waveguide at a common focal point located near the center of the edge where the output guides join the planar waveguide.
We have discovered a novel interconnection device based on an optical slab waveguide. Unlike optical fibers that have a single transmission mode, or at most a small number of modes, the novel slab interconnects can support several thousands of modes simultaneously. The novel slab interconnects have greatly increased capacity as compared to existing interconnects. A single slab waveguide can admit thousands or even millions of independent channels. In a busing environment, each of these channels can be capable of supporting a loading as high as 1000 (or even more). A novel scheme that we have named “mode division multiplexing,” when used in conjunction with another multiplexing scheme such as wavelength division multiplexing, achieves channel densities, communication bandwidths, and flexibility that are unmatched by any existing technology. For example, a single slab waveguide, about 11 centimeters long and about 5 mm
2
in cross section, can accommodate around one million independent channels, each operating at 1 GHz. Moreover, each of these million channels can mix information from each of around 1000 inputs, and can be accessed from each of around 1000 outputs. These properties give the slab interconnect tremendous flexibility. The implications of this discovery are potentially huge, encompassing on-chip communications, chip-to-chip communications, multiprocessor interconnection networks, and many other applications. The communication environments that could benefit are wide-ranging as well, including one-to-one, many-to-many, and many configurations in between. Speeds even higher than 1 GHz are possible, especially on smaller slabs or with a smaller number of channels. “Mode division multiplexing,” an important feature of the new invention, is possible neither on optical fibers, nor on conventional electrical interconnects. Optical slab waveguides offer a new system that could push the limits of in-system communications within a small confine, and could motivate new architectures and applications that range from on-chip/board-level interconnects and multiprocessor interconnection networks, to network servers and communication equipment. They will also be useful in addressing holographic memories.
As used in the specification and Claims, the term “slab” denotes a waveguide for which the dimensions of the cross section are substantially larger than the wavelength of the light. In addition, unlike a fiber, a slab waveguide can transmit light in many different modes. The term “fiber” refers to waveguides that permit only a single mode (or at most a few modes).


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Feldman, M. et al., “Optical slab waveguides for massive, high-speed interconnects within a small confine,” Proposal submitted to National Science Foundation (Feb. 2000).

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