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Means of Implementing a Computer Bus Using Optical Waveguides

IP.com Disclosure Number: IPCOM000122273D
Original Publication Date: 1991-Nov-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 3 page(s) / 97K

Publishing Venue

IBM

Related People

Li, CS: AUTHOR [+2]

Abstract

This article describes a geometry for optical waveguides that can produce the light equivalent of an electrical multidrop bus for a computer system. The current preferred means for implementing computer buses is shown in Fig. 1. The important features of Fig. 1 are: 1. A bus connects N distinct devices. 2. Each device can become a transmitter on the bus, and, at most, one transmitter can be active at a time. 3. A device that is not a transmitter is a listener. 4. The bus carries information bidirectionally.

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Means of Implementing a Computer Bus Using Optical Waveguides

      This article describes a geometry for optical waveguides
that can produce the light equivalent of an electrical multidrop bus
for a computer system.  The current preferred means for implementing
computer buses is shown in Fig. 1. The important features of Fig. 1
are:
1.   A bus connects N distinct devices.
2.   Each device can become a transmitter on the bus, and, at most,
one transmitter can be active at a time.
3.   A device that is not a transmitter is a listener.
4.   The bus carries information bidirectionally.

      It is desirable for many reasons to use optical
interconnections to replace metal interconnections.  See [*] for an
example of an optical bus for a computer system. There are physical
differences between optical waveguides and metal conductors that lead
to a different set of constraints for waveguides than for metal
conductors.  Among the key differences are:
1.   Bus loading:  An optical waveguide carries information by the
presence or absence of optical power. Unfortunately, optical power
splits among the taps.  In contrast, the information on an electric
bus is carried by voltage, which degrades with the number of taps on
the bus, but does not degrade as fast as inversely with the number of
taps.
2.   Laser illumination: It is acceptable to tie a receiver input to
a transmitter output at a tap on an electrical bus so that a
transmitter/receiver pair can share the tap for bidirectional
transmission.  It is generally not acceptable to illuminate a laser
transmitter with light from another laser, and, hence, it is not
acceptable for a laser transmitter and a receiver photo-detector to
share a waveguide tap since intended for the receiver will illuminate
the laser.

      Below is described an optical waveguide geometry that permits
the following:
1.   Bidirectional flow of information.
2.   Controlled power coupling at each tap.
3.   Isolation of laser from receiver at each tap.

      Consider Fig. 2.  In Fig....