Integrated burster multiplexer duplexer device for multicore fibers

A connecting device for multicore fibers includes two superposed substrates each comprising a planar optic with multiple guides so that the guides of the two substrates are disposed in parallel planes that coincide with the cores of the fiber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns the field of multicore optical fibers. 
To be more precise, the present invention concerns an integrated 
burster-multiplexer-duplexer device for multicore fibers. 
2. Description of the Prior Art 
In the context of future deployment of FTTH (Fiber To The Home) networks, 
the economic aspect of the active component/optical infrastructure 
combination is essential. 
The design and the demonstrated feasibility of multicore fibers has shown 
that this innovation is of real benefit. 
The economic aspects of producing passive and active components suitable 
for the new fiber are now crucial to the future of dedicated or weakly 
dedicated FTTH networks. 
The concept of multicore fibers, envisaged unsuccessfully in 1978 for 
multimode fibers, resurfaced in 1991 with the application of monomode 
fibers and the use of preform technologies offering preforms of very high 
geometrical quality. 
Many documents have been published on multicore fibers. The following 
documents are cited by way of example: 
Document U.S. Pat. No. 5,519,801 concerns small, high-precision multicore 
optical guides and a method of fabricating such guides. 
Document U.S. Pat. No. 5,353,365 concerns multiguide optical conductors. 
The document "Bunched multicore monomode fiber: A new key for the future 
FTTH network" EFOC No. 94 Heidelberg, Le Noane et al, describes results of 
producing multicore optical fibers, with particular reference to 
technology, propagation theory and experimental results. 
Some documents have been particularly concerned with connection solutions 
and in particular with bursting multicore fibers for splicing them to a 
conventional monocore fiber, at one end on the subscriber premises and at 
the other end to the opto-electronic components of the head end. 
Document U.S. Pat. No. 5,608,827 concerns components for connection to a 
multicore fiber and a method of making them and proposes a pigtailed 
monobloc component after cylindrical chemical etching of the fibers and 
assembly in an elastomer mold. 
Document FR 2127913 concerns a component for connection to a multicore 
fiber and a method of producing it and proposes a connectorized component 
after conical chemical etching of the fibers and assembly in a cylindrical 
ferrule. 
The document "Distribution link components for point to point ultra low 
cost FTTH network using the bunched multicore monomode fiber design" IOOC 
95 Hong Kong, Boscher et al, publishes results of connecting multicore 
fibers. 
An aim of the present invention is now to improve the prior art multicore 
fiber connecting devices. 
SUMMARY OF THE INVENTION 
In the context of the present invention this aim is achieved by a 
connecting device for multicore fibers including two substrates adapted to 
be superposed and each comprising a planar optic with multiple guides so 
that the guides of the two substrates are disposed in parallel planes that 
coincide with the cores of the fiber. 
The present invention preferably uses silicon substrates. 
The present invention integrates on one substrate all the functions for 
providing a full duplex double multicore fiber optical link with 
advantageous overall cost. 
The present invention can couple a multicore fiber to multiplexers, 
couplers and opto-electronic components on a substrate, for example a 
silicon substrate, with no intermediate connectors, in the case of the 
system component used at the head end or can couple a fiber, preferably a 
4-core fiber, to x (for example 16) individual fibers in the case of 
application of the proposed component at the branch point. 
Other features, aims and advantages of the present invention will become 
apparent from a reading of the following detailed description given by way 
of non-limiting example with reference to the appended drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The appended FIG. 1 shows in cross-section the geometry of a conventional 
fiber 10 with four cores. A fiber 10 of this kind has four cores 11, 12, 
13 and 14 the axes of which coincide with the respective corners of a 
square centered on the longitudinal axis 15 of the fiber 10. The jacket or 
outer sheath 20 of the fiber 10 has four lobes, each lobe 21, 22, 23 and 
24 being centered on a core axis. A fiber 10 of this kind has a dimension 
M between two planes respectively tangential to the exterior surface of 
two pairs of said lobes 21, 22 and 23, 24; it also has a spacing h between 
cores. Typically, but without limiting the invention, in existing fibers 
M=125 .mu.m and h=50 .mu.m. 
The substrate 100 shown in the appended FIG. 2 is advantageously made from 
silicon and its top surface 102 is etched. 
The etching defines a groove 110 of constant rectangular cross-section 
between two rectilinear walls 111, 112. The width of the groove 110, which 
corresponds to the distance between the walls 111 and 112, is equal to M+1 
.mu.m or 2 .mu.m, M designating the aforementioned dimension of the fiber 
10 to be connected. The depth of the groove 110 is equal to M/2+0 .mu.m or 
-1 .mu.m. The plane bottom 113 of the groove 110 is parallel to the plane 
of the top surface of the substrate 100. The groove 110 is therefore 
adapted to receive half of a multicore fiber 10 and the aligned grooves 
110 of two superposed substrates 100 are adapted to receive all of the 
fiber 10. 
The groove 110 is preferably at the center of the width of the substrate 
100. The groove 110 is preferably parallel to the longitudinal direction 
of the substrate 100. Accordingly the walls 111 and 112 are parallel to 
the longitudinal flanks 105 and 106 of the substrate 100. The groove 110 
opens onto a lateral face 104 of the substrate 100. The groove 110 and the 
walls 111 and 112 that delimit it extend only a limited part of the length 
of the substrate 100. 
The top surface of the substrate has an etching plane 120 adapted to 
receive a planar silica optic 200 at least over a part of the area around 
the groove 110 and the walls 111 and 112. 
The etching depth of the plane 120 can coincide with that of the groove 
110. 
The planar optic 200 preferably comprises a primary silica layer 210 
deposited on the plane 120, an array of doped silica guides 220 deposited 
on the primary silica 210 and a covering silica layer 240. In practise 
guides 220 are preferably square in section. 
The top level of the primary silica 210 on which the guides 220 are 
deposited must be at a distance (M-h-a)/2 from the bottom 113 of the 
groove 110 so that the ends of the guides 220 that open opposite the 
groove 110 are centered on respective cores of the fiber 10. 
The surface of the substrate 100 adjacent the groove 110 not covered with 
the planar optic 200 is preferably also etched and set back from the tops 
of the walls 111 and 112 to form a clearance plane 130. The depth of the 
plane 130 can be identical to that of the plane 120. 
The transverse edge of the planes 120 and 130 opposite the lateral entry 
face 104 is preferably orthogonal to the longitudinal direction of the 
substrate 100. Beyond the planes 120, 130, i.e. beyond this transverse 
edge, the substrate 100 has at least one other etching plane 140, 150 that 
is preferably deeper than the previously mentioned etching planes 120 and 
130. 
To be even more precise, in the preferred embodiment shown in the appended 
drawings two additional etching planes are provided beyond the transverse 
edge of the etching planes 120 and 130. A first etching plane 140 is 
adapted to receive either a strip of optical components 300 
emitting/receiving at 1.3 .mu.m and 1.5 .mu.m and the associated 
integrated circuits (see FIG. 2) or a support 400 with Vees receiving 
monocore fibers (see FIG. 3), for example. A second etching plane 150 that 
is preferably deeper than the plane 140 forms a clearance plane adapted to 
receive the components 300 or 400 of a complementary, preferably 
identical, second substrate inverted and facing it as shown in FIG. 4, for 
example. 
Finally note that the substrate 100 preferably has longitudinal shoulders 
160 and 165 on the longitudinal edges of the etching planes 120 and 130 
having coplanar tops that preferably coincide with the tops of the walls 
111 and 112. Thus when the two relatively inverted and facing substrates 
100 are superposed as shown in FIG. 4 they are in contact with each other 
through the walls 111, 112 and the shoulders 160, 165. 
The array of doped silica guides 220 can be implemented in many ways. 
In the preferred implementation shown in the appended drawings the array of 
guides 220 comprises two main guides 222 and 224 extending on the primary 
silica 210 between a first end facing the groove 110 and the 
aforementioned transverse edge of the etching plane 120. To be more 
precise, in the vicinity of this transverse edge, each main guide 222 and 
224 has a bifurcation 223, 225 allowing a bidirectional connection to and 
from a 1.3 .mu.m component 300 or 400. 
The array of guides 220 further includes two shorter auxiliary guides 226 
and 230 which each have a curved portion 227 and 231 near the main guides 
222 and 224, respectively, to enabling coupling with the latter of 
wavelength-selective evanescent waves at a wavelength of 1.5 .mu.m. Each 
of the auxiliary guides 226 and 230 terminates at the transverse edge of 
the etching plane 120 in a bifurcation 228 and 232 allowing a 
bidirectional connection to and from a component 300 or 400. 
The other ends of the auxiliary guides 226 and 230 are preferably 
mechanically treated to avoid excessive reflection here. 
A filter cutting off above 1.4 .mu.m can be placed on the 1.3 .mu.m receive 
guides 222 and 224 and a filter cutting off below 1.4 .mu.m can be placed 
on the 1.4 .mu.m receive guides. 
The array of doped silica guides 220 therefore enables the burster to be 
adapted to the offset of the position of the cores 11, 12, 13 and 14 of 
the four-core fiber, to assure 1.3 .mu.m/1.5 .mu.m 
multiplexing/demultiplexing and transmit/receive coupler functions. 
Accordingly the array of guides 220 performs the 1.3 .mu.m/1.5 .mu.m 
multiplexing function by wavelength-selective coupling and then the 
conventional 1 to 2 splitter functions at a fixed and precise distance, 
for example 250 .mu.m. 
An exit plane sawn or polished at the ends of the guides 222, 224, 226 and 
230 adjacent the components 300 or 400 and not perpendicular to the axis 
of the guides limits reflection problems. 
The component 300 can comprise, for example, a strip of opto-electronic 
components comprising 1.5 .mu.m emitters, 1.5 .mu.m photodiodes, 1.3 .mu.m 
emitters and 1.3 .mu.m photodiodes with accurate spacing, for example 250 
.mu.m, corresponding to the pitch of the ends of the guides 222, 224, 226 
and 230 at the transverse edge of the etching planes 120 and 130. A strip 
300 of this kind can be associated with an electronic module 310 
comprising driver, pre-amplifier and processor means, for example. 
The component 400 comprises a strip provided on its top surface with 
V-grooves at a pitch identical to that of the eight ends of the guides 
222, 224, 226, and 230 at the aforementioned transverse edge to receive 
monocore optical fibers and their epoxy coating. 
The monocore fibers can be aligned with the guides 222, 224, 226 and 230 in 
a collective dynamic manner or in a static manner. These standard 
alignment techniques are not described here. 
As previously indicated in the context of the present invention two 
substrates 100, which are preferably identical, are superposed in a 
face-to-face arrangement so that the aligned grooves 110 assure precise 
self-centering of the multicore fiber 10 or of a gauge rod of equivalent 
precision. 
After the multicore fiber 10 is installed the two substrates 100 are glued 
or welded together. 
The multicore fiber 10 can be positioned either directly at assembly time 
by gluing it into the two aligned U-grooves 110 or by means of a connector 
in which the multicore fiber 10 is free at the end over a length of a few 
millimeters, cleared with highly accurate positioning relative a reference 
that bears on the edge of the two substrates 100. In this case it is 
preferable when etching the central grooves 110 to provide clearance 
angles to facilitate the insertion of the multicore fiber into the 
component without precise pre-guidance. 
Of course the present invention is not limited to the particular 
embodiments that have just been described but encompasses any variant 
thereof within the spirit of the invention. 
The present invention exploits the low cost of multicore fibers. It has 
been shown that the cost per optical core of a multicore fiber is around 
half that of a conventional fiber. 
The invention proposes, for example, an integrated opto-electronic 
component of small overall size and low cost capable of transmitting eight 
optically separated bidirectional STM1 streams (standardized bit rate 155 
Mbit/s) on the same four-core fiber.