CROSSOVER ARRANGEMENT FOR OPTICAL CONDUCTORS

DISPOSITIF DE CROISEMENT POUR CONDUCTEURS OPTIQUES

English Abstract

CROSSOVER ARRANGEMENT FOR OPTICAL CONDUCTORS

Abstract of the Disclosure
A crossover arrangement for optical conductors, each in the
form of a rod of light-transmissive material, includes a multifaceted,
e.g. polyhedral member having pairs of mutually parallel opposed
facets. Each pair of facets extend in a direction transverse,
preferably perpendicular, to that in which a second pair of facets
extend. The end face of each optical conductor is abutted against one
of the facets of the polyhedron. Each facet has a coating of a
material having a lower refractive index than that of the polyhedron
or the optical conductors. Light entering the polyhedron from one
conductor passes across the polyhedron and into the opposite
conductor, being constrained to stay within the polyhedron by the
coatings on the side facets. Preferably the polyhedron member is a
cube and the optical conductors are of square cross-sectional shape.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS;-

1. Apparatus comprising an assemblage of optical
conductors, each comprising a rod of light-transmissive material with
a cladding of a material having a lower refractive index than that of
said rod, and a multifaceted member of light-transmissive material
having at least two pairs of mutually parallel opposite facets, each
facet of each pair having abutted thereto an end face of a respective
one of said plurality of optical conductors, and a coating being
provided on each said facet abutted by an optical conductor, said
coating having a refractive index less than the refractive index of
the associated facet, such that light passing across the multifaceted
member from one of a pair of facets to the other facet of said pair is
constrained from emerging from said member through either facet of the
other of said pairs of facets.

2. Apparatus as defined in claim 1, wherein the
multifaceted member comprises a polyhedron.

3. Apparatus as defined in claim 2, wherein said
polyhedron comprises a cube and said optical conductors each have a
square cross-section.

4. Apparatus as defined in claim 1, wherein said coating
has a thickness of the order of one wavelength of the light to be
transmitted.



5. Apparatus as defined in claim 1 wherein said coating is
provided upon all of said facets of said polyhedron.

6. Apparatus as defined in claim 2 wherein said coating is
provided upon all of said facets of said polyhedron.

7. Apparatus as defined in claim 3 wherein said coating is
provided upon all of said facets of said polyhedron.

8. Apparatus as defined in claim 4 wherein said coating is
provided upon all of said facets of said polyhedron.

9. Apparatus as defined in claim 1 wherein said pairs of
facets comprise a first pair extending perpendicular to a second pair.

10. Apparatus as defined in claim 2 wherein said pairs of
facets comprise a first pair extending perpendicular to a second pair.

11. Apparatus as defined in claim 3 wherein said pairs of
facets comprise a first pair extending perpendicular to a second pair.

12. Apparatus as defined in claim 4 wherein said pairs of
facets comprise a first pair extending perpendicular to a second pair.

13. A multifaceted member, for apparatus as defined in
claim 17 comprising a block of light-transmissive material, at least
two pairs of mutually parallel opposite facets of the block having a



coating of a material having a refractive index that is lower than
that of said light-transmissive material.

14. A multifaceted member as defined in claim 13, wherein
the thickness of the coating on said facets is of the order of the
wavelength of the light to be transmitted.

15. A multifaceted member as defined in claim 13 or 14,
having said coating on all of its said facets.

16. Apparatus as defined in claim 1, 2 or 3 wherein said
rods are mounted upon a backplane so as to extend in a plane
substantially parallel to that of said backplane, said multifaceted
member being located in said plane at the intersection of longitudinal
axes of said rods.

17. Apparatus as defined in claim 4, 5 or 6 wherein said
rods are mounted upon a backplane so as to extend in a plane
substantially parallel to that of said backplane, said multifaceted
member being located in said plane at the intersection of longitudinal
axes of said rods.

18. Apparatus as defined in claim 7, 8 or 9 wherein said
rods are mounted upon a backplane so as to extend in a plane
substantially parallel to that of said backplane, said multifaceted
member being located in said plane at the intersection of longitudinal
axes of said rods.






19. Apparatus as defined in claim 10, 11 or 12 wherein said
rods are mounted upon a backplane so as to extend in a plane
substantially parallel to that of said backplane, said multifaceted
member being located in said plane at the intersection of longitudinal
axes of said rods.

Description

~574~4~
CROSSOVER ARRANGEMENT FOR OPTICAL CONDUCTORS
Backqround to the Invention
The invention relates to optical waveguides or conductors
such as are used in telecommunications and like equipment and is
especially concerned with one light path crossing another light path.
Optical conductors now being proposed for use in backplanes
of computers and telecommunications equipment comprise rods of light-
transmissive material, clad with a material having a lower refractive
index. The rod may have inclined reflector surfaces spaced apart
along its length. The reflector surfaces deflect light travelling
along the optical conductor so that it emerges laterally or enters
laterally and is deflected to travel along the optical conductor.
Such an optical conductor is disclosed in copending Canadian patent
application number 503,968, filed March 12, 1986 in the name of D.
Kahn.
As the density and complexity of the optical backplane
increases, there will eventually arise a need for optical conductors
to cross, one over another, in order to satisfy the topological
requirements of the system. In the known art of metallic conductors,
this usually involves a multiple layer printed circuit card, or the
use of discrete insulated wire leads.
SummarY oF the Invention
According to the present invention a crossover arrangement
for intersecting optical conductors each in the form of a rod of
light-transmissive material with a cladding of a material having a
lower refractive index, comprises a multifaceted member of light
transmissive material, having at least two pairs of opposed mutually



i~'
. .

~ 7~

parallel facets, one pair extending transversely to the second pair of
opposed mutually parallel facets. Each facet of each pair has abutted
thereto the end face of a respective one of the intersecting optical
conductors.
The multifaceted member may have the same refractive index
as the optical conductors. Each said facet has a coating of a
material having a lower refractive index than that of the body of the
optical conductor. Conveniently, but not essentially, this coating
has the same refractive index as the cladding on the optical
conductors. Where the optical conductors and multifaceted member are
of polycarbonate, the coating may be an oxide of silicon e.g. silicon
dioxide.
Preferably, the coating thickness on the facets is of the
order of the wavelength of the light to be conveyed and very much less
than the width of the waveguide or optical conductor.
The multifaceted member may be a polyhedron, especially a
parallelepiped, and the optical conductors have a cross-sectional
shape, conveniently rectangular, corresponding to the shape of a
facet. In one preferred embodiment the multifaceted member is a cube
and the optical conductors are square rods. In other embodiments the
multifaceted member has one pair of facets inclined relative to the
other pair so that the conductors intersect at an angle other than
perpendicular.
Brief Description of the Drawing
An embodiment of the invention will now be described by way
of example only and with reference to the single accompanying drawing,




~.~
, .
... . .
~':

~ L~2 5 7 ~ 4L
which shows a cross-sectional plan view of a first embodiment of
intersecting optical conductors.
Detailed Descri~tion of a Preferred Embodiment
The drawing shows four optical conductors lO, 12, 14 and 16,
respectively, mounted upon a backplane 17 and connected together in
cruciform fashion to provide two light-transmissive paths intersecting
each other perpendicularly. The four optical conductors 10, 12, 14
and 16 lie in the same plane and have their respective end faces each
abutted against a corresponding facet of a multifaceted, specifically
cubic, coupling member 18. The optical conductors comprise rods of
light-transmissive plastics material, for example polycarbonate.
They are square in cross-sectional shape so that their end faces
conform closely to the shape of the face of the cubic coupling member
18. The longitudinal surfaces have cladding 20, 22, 24, 26,
respectively, said corresponding facets of the cube 18 having coatings
21, 23, 25 and 27, respectively. Both the claddings and the coatings
are of a material having a refractive index lower than that of the
rods and the cube 18. For example the cladding and coating might be
glass ~e.g. an oxide of silicon with a refractive index of 1.45/1.54
compared with about 1.58 for polycarbonate).
Preferably the cube should be "sealed" to prevent loss of
light. Thus, any surfaces not abutted by the end of an optical
conductor preferably having a coating, although this could, of
course, be provided by a separate blanking member. Preferably,
however, all of the faces of the cube are coated.

94
The coating thickness is preferably much less than the
spacing between the faces. Preferably the thickness is of the order
of one wavelength.
Arrow A indicates the path taken by light travelling along
optical conductor 10. It is confined within the conductor by the
cladding 20. On reaching the end of the conductor 10 the light is
substantially unimpeded by the face$ coating 21 (because of the high
angle of incidence), and enters the cube 18.
The facet coatings 23 and 27, respectively, constitute
continuations of the cladding on conductor 10 and so constrain the
light to pass across the cube 18. Having crossed the cube 18, the
light passes, substantially unimpeded by the facet 25 coating, into
conductor 14.
Light travelling along optical conductor 12 is conveyed
across the cube 18 and into the optical conductor 16 in a similar
manner.
The rods may be of polycarbonate and the cladding an oxide
of silicon, e.g. silicon monoxide. The cube may also be polycarbonate
and the coating the same oxide of silicon.
Various modifications are possible without departing from
the scope of the invention. For example, the multifaceted member
might be a parallelogram with one pair of facets inclined relative to
the other pair of facets. Where a facet of the multifaceted member is
not "used" i.e. no optical conductor is abutted against it, and it has
no coating of its own, it might be "sealed" by means of a blanking
plate of lower refractive index material.




. '

~ ~,5'7~ 91
An advantage of embodiments of the present invention is
that crossover of optical conductors in the form of rods of light-
transmissive material may be achieved in the same planes. Such a
planar arrangement will facilitate simple, and hence economical,
backplane construction.