U.S. patent application number 10/272694 was filed with the patent office on 2004-10-21 for optical backplane array connector.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Berger, Christoph, Brooks, Cameron J., DeCusatis, Casimer M., Emma, Phillip G., Jacobowitz, Lawrence, Knickerbocker, John U..
Application Number | 20040208453 10/272694 |
Document ID | / |
Family ID | 32467704 |
Filed Date | 2004-10-21 |
United States Patent
Application |
20040208453 |
Kind Code |
A1 |
Jacobowitz, Lawrence ; et
al. |
October 21, 2004 |
OPTICAL BACKPLANE ARRAY CONNECTOR
Abstract
An apparatus and method for forming a staircase arrangement for
the connection of optical waveguides between a card and backplane.
A card having optical waveguides and electrical conductors embedded
in the card has an edge ending in a staircase arrangement with
optical fiber-ribbons protruding from the edge. A guidance
structure is connected to the edge and contains channels to guide
and align the optical fiber-ribbons. A backplane having embedded
optical waveguides and electrical conductors also has an edge
ending in a staircase arrangement with a guidance structure
connected to the edge and tapered openings which receive and guide
the optical fiber-ribbons into close proximity with the optical
waveguides and forming a staircase arrangement of connected
waveguides between a card and backplane.
Inventors: |
Jacobowitz, Lawrence;
(Wappingers Falls, NY) ; Berger, Christoph;
(Horgen, CH) ; Brooks, Cameron J.; (Elmsford,
NY) ; DeCusatis, Casimer M.; (Poughkeepsie, NY)
; Emma, Phillip G.; (Danbury, CT) ; Knickerbocker,
John U.; (Wappingers Falls, NY) |
Correspondence
Address: |
INTERNATIONAL BUSINESS MACHINES CORPORATION
DEPT. 18G
BLDG. 300-482
2070 ROUTE 52
HOPEWELL JUNCTION
NY
12533
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
32467704 |
Appl. No.: |
10/272694 |
Filed: |
October 16, 2002 |
Current U.S.
Class: |
385/50 ; 385/14;
398/164 |
Current CPC
Class: |
G02B 6/3829 20130101;
G02B 6/3817 20130101; G02B 6/30 20130101; G02B 6/3839 20130101;
G02B 6/43 20130101; G02B 6/2826 20130101 |
Class at
Publication: |
385/050 ;
385/014; 398/164 |
International
Class: |
G02B 006/26 |
Claims
What is claimed is:
1. An apparatus for providing a staircase arrangement for the
connection of optical waveguides between a card and backplane
comprising: a card having first optical waveguides, said first
optical waveguides having a low index of refraction region
surrounding a high index of refraction core region; first
electrical conductors embedded in said card, said card having a
first edge ending in a first staircase arrangement; optical
fiber-ribbons having a low index of refraction region surrounding a
high index of refraction core region and having a first end in
close proximity with said first optical waveguides, said optical
fiber-ribbons protruding from said first edge; a first guidance
structure connected to said first edge, said first guidance
structure containing channels to guide and align said optical
fiber-ribbons and having a first inner side comprising a staircase
arrangement and a first outer side having an inclination; a
backplane having second optical waveguides, said second optical
waveguides having a low index of refraction region surrounding a
high index of refraction core region; second electrical conductors
embedded in said backplane, said backplane having a second edge
ending in a second staircase arrangement; and a second guidance
structure connected to said second edge, said second guidance
structure having a second inner side comprising a staircase
arrangement and a second outer side having an inclination and
tapered openings which receive and guide a second end of said
optical fiber-ribbons into close proximity with said second optical
waveguides when said first outer side of first guidance structure
is brought into contact with said second outer side of second
guidance structure, thereby forming a staircase arrangement of
connected waveguides between a card and backplane.
2. The apparatus of claim 1 wherein said fiber-ribbons have a
D-shaped cross section such that said high index of refraction core
region is in close proximity to a flat edge of said D-shaped cross
section and said flat edge is in close proximity to said first and
second optical waveguides.
3. The apparatus of claim 1 wherein said fiber-ribbons are
multi-mode high refractive index polymer optical fibers with bend
radii of approximately 1 mm.
4. The apparatus of claim 1 wherein said card and said backplane
further comprises a third guidance structure to provide fine on
card or board alignment between said fiber-ribbons and said first
and second optical waveguides.
5. The apparatus of claim 4 wherein said third guidance structure
is funnel shaped and is fabricated simultaneously with said optical
waveguides.
6. The apparatus of claim 1 wherein said second guidance structure
has a V-groove geometry.
7. The apparatus of claim 1 wherein said fiber-ribbons are attached
to said first edge by an epoxy.
8. The apparatus of claim 1 wherein said fiber-ribbons are attached
to said first edge by fusion splicing.
9. The apparatus of claim 1 further comprising a grating structure
formed in said low index of refraction region of said first and
second optical waveguides such that said grating structure is in
close proximity to said high index of refraction core region of
said fiber-ribbons.
10. The apparatus of claim 1 further comprising a grating structure
formed in said high index of refraction core region of said first
and second optical waveguides such that said grating structure is
in close proximity to said high index of refraction core region of
said fiber-ribbons.
11. The apparatus of claim 1 further comprising a grating structure
formed in said high index of refraction core region of said
fiber-ribbons such that said grating structure is in close
proximity to said first and second optical waveguides.
12. The apparatus of claim 9 wherein said grating structure is
formed in said low index of refraction region of said first and
second optical waveguides by lithographically defining and
physically etching said grating structure.
13. The apparatus of claim 10 wherein said grating structure is
formed in said high index of refraction core region of said first
and second optical waveguides by interferometrically writing an
index variation into said core region.
14. The apparatus of claim 10 wherein said grating structure is
formed in said high index of refraction core region of said
fiber-ribbons by interferometrically writing an index variation
into said core region.
15. The apparatus of claim 10 wherein said waveguide core or
fiber-ribbon core is comprised of a UV-sensitive material.
16. The apparatus of claim 15 wherein said UV-sensitive material is
Ge-doped silica.
17. The apparatus of claim 1 wherein said high index of refraction
core region of said first and second optical waveguides have a
flared geometry to enhance alignment between said waveguide core
region and said fiber-ribbons core region.
18. The apparatus of claim 1 wherein said card and said backplane
further comprise parallel planar arrays of said first and second
optical waveguides.
19. The apparatus of claim 1 wherein said card and said backplane
further comprise multiple embedded layers of said first and second
optical waveguides and where successive layers of said first and
second optical waveguides have increased length to mate with said
first and second staircase arrangements.
20. The apparatus of claim 1 wherein the connection of said card
and said backplane is an orthogonal connection and said first outer
side and said second outer side have an inclination of
approximately 45 degrees.
21. A method for forming a connection of optical waveguides between
a card and backplane comprising the steps of: providing a card
having first optical waveguides, said first optical waveguides
having a low index of refraction region surrounding a high index of
refraction core region; forming a first edge in said card ending in
a first staircase arrangement; providing fiber-ribbons having a low
index of refraction region surrounding a high index of refraction
core region and having a first end in close proximity with said
first optical waveguides, said fiber-ribbons protruding from said
first edge; connecting a first guidance structure to said first
edge, said first guidance structure containing channels to guide
and align said fiber-ribbons and having a first inner side
comprising a staircase arrangement and a first outer side having an
inclination; providing a backplane having second optical
waveguides, said second optical waveguides having a low index of
refraction region surrounding a high index of refraction core
region; forming a second edge ending in a second staircase
arrangement; connecting a second guidance structure to said second
edge, said second guidance structure having a second inner side
comprising a staircase arrangement and a second outer side having
an inclination and tapered openings; inserting a second end of said
fiber-ribbons into said tapered openings which receive and guide
said second end of fiber-ribbons; and placing said first outer side
of first guidance structure into contact with said second outer
side of second guidance structure thereby placing said
fiber-ribbons into close proximity with said second optical
waveguides and forming a staircase arrangement of connected
waveguides between a card and backplane.
22. The method of claim 21 wherein said fiber-ribbons have a
D-shaped cross section such that said high index of refraction core
region is in close proximity to a flat edge of said D-shaped cross
section and said flat edge is in close proximity to said first and
second optical waveguides and said backplane further comprises a
guidance structure to provide alignment between said fiber-ribbons
and said second optical waveguides.
23. The method of claim 21 further comprising the steps of forming
a grating structure in said low index of refraction region of said
first and second optical waveguides such that said grating
structure is in close proximity to said high index of refraction
core region of said fiber-ribbons.
24. The method of claim 21 further comprising the steps of forming
a grating structure in said high index of refraction core region of
said first and second optical waveguides such that said grating
structure is in close proximity to said high index of refraction
core region of said fiber-ribbons.
25. The method of claim 21 further comprising the steps of forming
a grating structure in said high index of refraction core region of
said fiber-ribbons such that said grating structure is in close
proximity to said first and second optical waveguides.
26. The method of claim 21 wherein the connection of said card and
said backplane is an orthogonal connection and said first outer
side and said second outer side have an inclination of
approximately 45 degrees.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to optical interfaces for
data communication and, more particularly, to optical interfaces
with improved alignment capability.
[0002] Optical data communications technology has a number of
advantages over wire technology, such as bandwidth, data rate and
response characteristics superior to those of conventional wire
technology. Also, optical technology is essentially immune to radio
frequency interference (RFI) and electromagnetic interference (EMI)
issues associated with wire technology. Optical data communication
is therefore desirable in a variety of applications such as
multi-chip modules (MCMs), printed circuit board (PCB)
technologies, and integrated backplanes.
[0003] In conventional optical connectors, electronic circuitry,
optical source and optical detectors are typically mounted on PCBs
which are received in card guides mounted to an equipment frame. A
backplane mounted to the rear of the frame includes board edge
connectors aligned with the card guides and electrical conductors
interconnecting the board edge connectors. The circuit boards are
provided with board edge electrical contacts which are received in
the board edge connectors when the circuit boards are slidably
inserted in the card guides to electrically connect the circuitry
to the electrical conductors on the back plane. The electrical
conductors provide the required electrical connections between
circuit boards.
[0004] The circuit boards also include optical connector parts
which are optically coupled to the optical sources and to the
optical detectors of the receivers and transmitters. The board
mounted optical connector parts must be mated with frame mounted
optical connector parts to optically connect the optical sources
and the optical detectors to optical fibers terminating on the
frame mounted optical connectors.
[0005] In the current board edge optical connector arrangements the
circuit board mounted optical connector parts are mounted at
leading edges of the circuit boards. These leading edges are
already congested with board edge electrical contacts. In addition,
in the board edge optical connector arrangements the frame mounted
optical connector parts are mounted at the back plane which is
already congested with electrical board edge connectors and
electrical conductors. In current systems, optical fibers are left
to hang loose between packs or bundles of fibers which tends to
create a "rat's nest" of fibers.
[0006] In view of these problems it is a purpose of the present
invention to provide an apparatus and method to connect large
numbers of optical fibers to an optical backplane and avoid the
`rats nest" problem associated with multiple fiber-to-fiber
connections and/or routing systems.
[0007] It is another purpose of the present invention to provide an
improved optical interface between a mother board and a daughter
card (or backplane) involving 90 degree turns and pluggable
connections.
[0008] It is another purpose of the present invention to provide
arrays of interconnects utilizing waveguides and fibers.
[0009] These and other purposes of the present invention will
become more apparent after referring to the following description
considered in conjunction with the accompanying drawings.
BRIEF SUMMARY OF THE INVENTION
[0010] The purposes and advantages of the present invention have
been achieved by providing an apparatus for forming a staircase
arrangement for the connection of optical waveguides between a card
and backplane comprising:
[0011] a card having first optical waveguides, the first optical
waveguides having a low index of refraction region surrounding a
high index of refraction core region;
[0012] electrical conductors embedded in the card, and where the
card has a first edge ending in a staircase arrangement;
[0013] optical fiber-ribbons having a low index of refraction
region surrounding a high index of refraction core region and
having one end in close proximity with the first optical
waveguides, the optical fiber-ribbons protruding from the first
edge;
[0014] a first guidance structure connected to the first edge and
the first guidance structure containing channels to guide and align
the optical fiber-ribbons, and having a first inner side comprising
a staircase arrangement and a first outer side having an
inclination;
[0015] a backplane having second optical waveguides, the second
optical waveguides having a low index of refraction region
surrounding a high index of refraction core region;
[0016] additional electrical conductors embedded in the backplane,
and the backplane having a second edge ending in a second staircase
arrangement; and
[0017] a second guidance structure connected to the second edge,
the second guidance structure having a second inner side comprising
a staircase arrangement and a second outer side having an
inclination and tapered openings which receive and guide a second
end of the optical fiber-ribbons into close proximity with the
second optical waveguides when the outer side of the first guidance
structure is brought into contact with the outer side of the second
guidance structure, thereby forming a staircase arrangement of
connected waveguides between a card and backplane.
[0018] The fiber-ribbons may have a D-shaped cross section such
that the high index of refraction core region is in close proximity
to a flat edge of the D-shaped cross section and the flat edge is
in close proximity to the first and second optical waveguides. This
close proximity is necessary for efficient core to core coupling.
The fiber-ribbons may be a multi-mode high refractive index polymer
optical fibers with bend radii of approximately 1 mm.
[0019] The apparatus may also have grating structures formed in the
low index of refraction region of the first or second optical
waveguides or in the core regions of either the waveguide or fiber
ribbon to enable grating-assisted coupling.
[0020] It is another object of the invention to provide a method
for forming a connection of optical waveguides between a card and
backplane comprising the steps of:
[0021] providing a card having first optical waveguides, said first
optical waveguides having a low index of refraction region
surrounding a high index of refraction core region;
[0022] forming a first edge in the card ending in a first staircase
arrangement;
[0023] providing fiber-ribbons having a low index of refraction
region surrounding a high index of refraction core region and
having a first end in close proximity with the first optical
waveguides, the fiber-ribbons protruding from the first edge;
[0024] connecting a first guidance structure to the first edge, the
first guidance structure containing channels to guide and align the
fiber-ribbons and having a first inner side comprising a staircase
arrangement and a second outer side having an inclination;
[0025] providing a backplane having second optical waveguides, the
second optical waveguides having a low index of refraction region
surrounding a high index of refraction core region;
[0026] forming a second edge ending in a second staircase
arrangement;
[0027] connecting a second guidance structure to the second edge,
the second guidance structure having a second inner side comprising
a staircase arrangement and a second outer side having an
inclination and tapered openings;
[0028] inserting a second end of the fiber-ribbons into the tapered
openings which receive and guide the second end of fiber-ribbons;
and
[0029] placing the outer side of the first guidance structure into
contact with the outer side of the second guidance structure
thereby placing the fiber-ribbons into close proximity with the
second optical waveguides and forming a staircase arrangement of
connected waveguides between a card and backplane.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The features of the invention believed to be novel and the
elements characteristic of the invention are set forth with
particularity in the appended claims. The Figures are for
illustration purposes only and are not drawn to scale. The
invention itself, however, both as to organization and method of
operation, may best be understood by reference to the detailed
description which follows taken in conjunction with the
accompanying drawings in which:
[0031] FIG. 1 is an isometric view of the apparatus showing the
arrayed, orthogonal interconnection system.
[0032] FIG. 2a is a schematic view of an unmated arrangement for
connecting multiple layers of embedded waveguides.
[0033] FIG. 2b is a schematic view of a mated arrangement for
connecting multiple layers of embedded waveguides.
[0034] FIG. 3a is a cross sectional view of a structure for fiber
to waveguide alignment.
[0035] FIG. 3b is a cross sectional view of a structure for fiber
to waveguide alignment.
[0036] FIG. 3c is a top view of a structure for fiber to waveguide
alignment including funnels to physically guide the fiber core over
the waveguide core.
[0037] FIG. 4 is a cross sectional view of fiber core to waveguide
core alignment.
[0038] FIG. 5a is a cross sectional view of a tapered structure for
fiber to waveguide alignment.
[0039] FIG. 5b is a cross sectional view of a tapered structure for
fiber to waveguide alignment.
[0040] FIG. 5c is a top view of a tapered structure for fiber to
waveguide alignment including flairs to provide
mode-conversion.
[0041] FIG. 6 is a cross section view of an optical coupler
including a grating structure.
[0042] FIG. 7 is a top view of an optical coupler including a
grating structure.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The purposes of the present invention have been achieved by
providing an apparatus and method to connect large numbers of
optical fibers to an optical backplane, preferably, forming an
orthogonal connection of optical waveguides between a card and
backplane. While the following description is directed to an
orthogonal connection, the present invention is not limited to an
orthogonal connection. The present invention could easily be
configured to connect a card and backplane at any desired
angle.
[0044] Referring to FIG. 1 there is shown a first embodiment of the
present invention comprising a structure for the orthogonal
connection of multiple layers of embedded waveguides between a card
10 and backplane 70. This embodiment allows one to extend from a
single waveguide layer on the surface of a board to multiple layers
of waveguides embedded in the board. Parallel planar arrays of
embedded waveguides 20, 25 are formed in the laminate structure or
card 10 and backplane 70 respectively. The waveguides 20, 25 are
optically coupled from the card 10 to the backplane 70 by optical
fiber-ribbons 50. As will be illustrated in more detail in the
subsequent drawings, the fiber-ribbons 50 make a 90 degree bend as
they exit the card 10 and enter the backplane 70. In the
description that follows the term "fiber-ribbon" may refer to
either an individual optical fiber or an array of optical
fibers.
[0045] In the particular example illustrated in FIG. 1 there is
shown an array of 4 parallel periodically-spaced waveguides 20
which are formed in 3 layers 1, 2, 3 on the card 10 and backplane
70 to comprise a 12-way optical connection system. It will be
apparent to one skilled in the art that this embodiment is readily
extendable more complex examples of N arrays of parallel
periodically-spaced waveguides formed in M layers to form a
N.times.M way optical connection system.
[0046] Referring to FIG. 2A there is shown a cross-sectional view
of the card 10 and backplane 70, prior to connection, with three
layers of first optical waveguides 20 and two layers of first
electrical conductors 30 embedded in the board material 40, in this
example FR4. An alternative embodiment (not shown) would be to
provide contiguous layers of optical waveguides and electrical
conductors on the surface of the card or backplane.
[0047] The card 10 has a first edge 45 which ends in a first
staircase arrangement. As shown in the figure this "staircase
arrangement" is a series of steps formed by the successive layers
of the optical waveguides 20 and electrical conductors 30 having
increased vertical length. Such a "staircase" can be formed
mechanically or chemically, or may be fabricated as part of the
board design. (ie, included in the layout of the board).
[0048] Similarly, the backplane 70 has a second edge 75 which ends
in a second staircase arrangement where a series of steps are
formed by the successive layers of the optical waveguides 25 and
electrical conductors 35 having increased horizontal length.
[0049] A first end 53 of the fiber-ribbon 50 is permanently
attached to the first optical waveguides 20. To minimize losses due
to bend radius optical leakage, it may be necessary for these fiber
pieces to be selected from the group consisting of multi-mode high
refractive index polymer optical fibers with bend radii on the
order of 1 mm. A commercially available example is provided by
Paradigm Optics' MMPOF5A. This attachment can be done using
established techniques such as thermal fusion splicing or
refractive index matched, UV-cured epoxies. Conventional active
alignment techniques can also be used during the attachment process
to maximize coupling efficiency (active alignment).
[0050] A first guidance structure 60 is permanently attached to the
first edge 45. The guidance structure 60 contains channels 65 to
guide and align the optical fiber-ribbons 50. The first guidance
structure 60 has a first inner side 61 forming a staircase
arrangement and a first outer side 62 having an inclination of
approximately 45 degrees. In a more general case either inclination
could be more or less than 45 degrees.
[0051] The backplane 70 is prepared accordingly, but without the
fiber-ribbons 50 and with a second connector 80 attached to the
staircase. The second connector 80 has a second inner side 81
forming a staircase arrangement and a second outer side 82 having
an inclination of approximately 45 degrees. Again, in a more
general case either inclination could be more or less than 45
degrees.
[0052] The second connector 80 may also have funnel-shaped openings
90 to facilitate insertion and guidance of a second end of the
fiber-ribbon 54 during mating. Alternatively, the protruding
fiber-ribbons 50 can extend from the second connector 80 such that
the protruding fiber-ribbons 50, as well as a conventional dust and
eye protection complex, are on the backplane 70. In addition to
this possible exchange of male and female, other variations like
single-layer to multi-layer, etc., are possible.
[0053] Referring now to FIG. 2B the alignment of the fiber-ribbons
50 coming from the card 10 with respect to the waveguides 20 in the
backplane 70 is done in three stages. The first stage (not shown in
the figure) is the coarse mechanical guidance of the card 10 with
respect to the backplane 70. This guidance consists of the
mechanics that holds the card 10 in the shelf and the electrical
connections (power, ground, etc). The second stage utilizes the
funnels 90 in the female optical connector 80. They guide the
fiber-ribbons 50 very close to the optical waveguides 25. The final
stage, shown in FIG. 2B and discussed with reference to the figures
that follow, consist of a guidance structure on the board which is
defined during board fabrication (e.g. in the waveguide layer) and
which is therefore positioned with high precision.
[0054] Referring to FIG. 3A, in another aspect of the present
invention, the same process that is used to define the optical
waveguides 20, 25 or, alternatively, the electrical connections 30,
35, is used to define a third guidance structure 100 directly on
the backplane 70. This third guidance structure 100 facilitates the
fine alignment of the fiber-ribbon 50 that is to be coupled to the
waveguide 25. FIG. 3C is a top view showing the third guidance
structure 100 and optical waveguide 25 prior to the insertion of
the fiber-ribbon 50. The advantage of this approach is that the
position of the third guidance structure 100 relative to the
optical waveguide 25 is controlled by the precision of the board
manufacturing process which fabricated the optical waveguide
25.
[0055] As illustrated in FIG. 3A the second optical waveguides 25
consist of a low index of refraction region 27 surrounding a high
index of refraction core region 26. Similarly, referring to FIG. 4
the first (card) optical waveguide 20 consists of a low index of
refraction region 22 surrounding a high index of refraction core
region 21. The fiber-ribbons 50 also consist of a low index of
refraction region 52 surrounding a high index of refraction core
region 51. The core region 51 of the fiber must be accessible and
in close alignment with the core region 26 to optimize optical
coupling.
[0056] Referring again to FIG. 3C there is shown a cross section at
two positions, AA and BB, of one waveguide 25 with third guidance
structure 100 fabricated in a funnel shape. A fiber-ribbon 50, is
inserted from below into the alignment funnel 100. In a preferred
embodiment the alignment funnel is formed by the same material that
also forms the optical waveguide thereby achieving the discussed
manufacturing efficiency.
[0057] Referring again to FIG. 3A, in a preferred embodiment the
fiber-ribbon 50 is fabricated in a "D-shaped" geometry such that
the high index of refraction core region 51 is in close proximity
to a flat edge of the D-shaped cross-section to provide close
alignment proximity between the fiber-ribbon core 51 and the
optical waveguide core 26. This close alignment proximity is
necessary for efficient core to core coupling.
[0058] The coarse guidance is provided by the second connector 80.
In a preferred embodiment the second connector 80 has the shape of
a guidance groove. In this example, the coarse guidance is provided
by a V-groove like guidance, which can be part of the connector
structure, as already mentioned above. A fair amount of pressure is
required to get good coupling so mechanical force is applied to
press the fiber-ribbon 50 to the optical waveguide 25. The
mechanical force is applied via the connector after the
fiber-ribbon 50 is in place. If the mechanical force were applied
with the second connector guidance groove 80 alone it would
probably cause damage to the waveguide and produce scratching
debris that would eventually inhibit good coupling.
[0059] In addition, polymers can be molded to form "self-aligning"
structures, the key point being that polymer cores can range from 8
um and higher. Referring now to FIG. 5A there is shown a preferred
embodiment where the high index of refraction core region 21, 26 of
the first and second optical waveguides 20, 25 are used to provide
mode-conversion when coupling significantly dissimilar fiber to
waveguide core diameters and indices, for example, polymer core to
silica core. Referring to FIG. 5C, in this case the high index of
refraction core regions 21, 26 have a flared geometry 140 to
enhance alignment between the waveguide core region 21, 26 and the
fiber-ribbon core region 51.
[0060] The method for optical coupling between the optical
waveguides and fiber cores in this invention is through close
proximity or "evanescent" mode transfer. This transfer is dependent
on both the optical waveguide core separation and the critical
"coupling length". As illustrated in FIGS. 2A and 3A the two cores
26, 51 need to be brought into close proximity and co-linear to
enable efficient optical coupling. The co-linear constraints are
provided for by the second guidance structure 80 and third guidance
structure 100. The D-shaped cross section of the fiber-ribbon 50
enables the two cores to be brought into close proximity.
[0061] The joining of the fiber-ribbon 50 with the optical
waveguide 20 on the card 10 is formed by any of the attachment
techniques such as epoxy, fusion-splicing or other. This is a
"permanent" connection. The connection between the backplane 70 and
the fiber-ribbon 50 is not permanent but rather pluggable with the
alignment structures described above.
[0062] The efficiency of close proximity or evanescent optical
coupling between two dissimilar waveguides is increased by a
periodic refractive index perturbation in the coupling region (see
reference: Optical Integrated Circuits by H. Nishihara, M. Haruna,
T. Suhara, 1989, McGraw-Hill Optical and Electro-Optical
Engineering Series, p. 63). This perturbation is created using a
Bragg grating structure that can be realized through a modulation
of the refractive index along the characteristic coupling length.
The physical theory of grating assisted couplers can be found in
the reference (Nishihara). There are various possible alternative
configurations to realize this "grating coupler".
[0063] Referring to FIG. 6 there is shown a preferred embodiment
which consists of a grating structure 150 formed on the second
optical waveguide 25. Specifically, the grating structure 150 is
formed in the low index of refraction region 27 surrounding the
waveguide core 26. A top view of the grating structure 150 is
illustrated in FIG. 7. This grating structure 150 is a periodic
refractive index perturbation.
[0064] In an alternative embodiment (not shown) the grating
structure 150 may also be formed in either the waveguide core 26 or
fiber-ribbon core 51 and thereby modulating the refractive index of
either waveguide core 26 or fiber-ribbon core 51. The grating
structure 150 is preferably formed in the low index of refraction
region 27 by lithographically defining and physically etching the
grating. The grating structure 150 is preferably formed in the
waveguide core 26 or fiber-ribbon core 51 by interferometrically
writing an index variation into the waveguide or fiber core which
is preferably comprised of UV-sensitive material such as Ge-doped
silica.
[0065] This grating structure 150 may also be formed in a similar
arrangement to enhance coupling between the first optical waveguide
20 and the fiber-ribbon 50. In this embodiment the apparatus has a
grating structures 150 formed in the low index of refraction region
22 of the first optical waveguides 20 to enable grating-assisted
coupling.
[0066] It will be apparent to those skilled in the art having
regard to this disclosure that other modifications of this
invention beyond those embodiments specifically described here may
be made without departing from the spirit of the invention.
Accordingly, such modifications are considered within the scope of
the invention as limited solely by the appended claims.
* * * * *