Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1999-11-08
2001-07-10
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S016000, C385S024000, C385S040000, C385S045000
Reexamination Certificate
active
06259834
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to optical switch networks and, more particularly, to a strictly nonblocking tree network architecture with low crosstalk and efficient use of space.
Tree networks are reviewed in Andrzej Jajszczyk and H. T. Mouftah, “Tree-type photonic switching networks”,
IEEE Network
, vol. 9 no. 1 pp. 10-16 (1995), which is incorporated by reference for all purposes as if fully set forth herein.
FIG. 1
shows the high-level layout of a generic tree network for connecting P input waveguides
12
to Q output waveguides
14
. Input waveguides
12
enter a branching region
16
, where an array of 1×2 splitters connects input waveguides
12
to PQ branching region output waveguides
22
. Output waveguides
14
emerge from a combining region
20
where an array of 2×1 combiners connects output waveguides
14
to PQ combining region input waveguides
24
. Waveguides
22
and
24
are connected in an interconnection region
18
in a manner that allows any input waveguide
12
to be connected to any combination of output waveguides
14
.
FIG. 2
shows a classical 4×4 tree network architecture, for connecting four input waveguides
12
to four output waveguides
14
. The four input waveguides
12
are indexed serially by binary indices 00, 01, 10 and 11. Similarly, the four output waveguides
14
are indexed serially by binary indices 00, 01, 10 and 11.
Input waveguides
12
enter a branching region
16
that includes two branching cascades
30
of 1×2 splitters
26
. Input waveguides
12
are the input waveguides of the first branching cascade
30
. The eight output waveguides of the first branching cascade
30
are indexed, relative to input waveguides
12
, in a manner that is referred to herein as “least significant inserted bit order”. For each 1×2 splitter of the first branching cascade
30
, the index of the upper output waveguide is obtained by appending a zero to the index of the input waveguide, and the index of the lower waveguide is obtained by appending a one to the index of the input waveguide. The eight output waveguides of the first branching cascade
30
are the eight input waveguides of the second branching cascade
30
, and the sixteen output waveguides of the second branching cascade
30
are indexed relative to the eight input waveguides of the second branching cascade
30
in least significant inserted bit order.
Similarly, output waveguides
14
emerge from a combining region
20
that includes two combining cascades
32
of 2×1 combiners
28
. Output waveguides
14
are the output waveguides of the second combining cascade
32
. The eight input waveguides of the second combining cascade
32
are indexed, relative to output waveguides
14
, in least significant inserted bit order. The eight input waveguides of the second combining cascade
32
are the eight output waveguides of the first combining cascade
32
, and the sixteen input waveguides of the first combining cascade
32
are indexed relative to the eight output waveguides of the first combining cascade
32
in least significant inserted bit order.
Each of the sixteen output waveguides of branching region
16
connects to one of the sixteen input waveguides of combining region
20
via interconnection region
18
. Which input waveguide of combining region
20
a particular output waveguide of branching region
16
connects to is determined by interchanging the first and second halves of the output waveguide's index, as shown in the following table:
output waveguide
connects to input waveguide
0000
0000
0001
0100
0010
1000
0011
1100
0100
0001
0101
0101
0110
1001
0111
1101
1000
0010
1001
0110
1010
1010
1011
1110
1100
0011
1101
0111
1110
1011
1111
1111
For clarity, these connections are not shown explicitly in FIG.
2
.
FIG. 7
is a schematic diagram of a 1×2 splitter
26
implemented as a directional coupler. An input waveguide
36
leads into a coupling waveguide
38
, which in turn leads into an output waveguide
42
. Coupling waveguide
38
is close and parallel to another coupling waveguide
40
, which leads into another output waveguide
44
. Coupling waveguides
38
and
40
both are of length L. Coupling waveguides
38
and
40
are covered by respective electrodes
46
and
48
.
Coupling waveguides
38
and
40
are sufficiently close that the evanescent field of light propagating in coupling waveguide
38
overlaps with and is coupled into coupling waveguide
40
. The strength of the coupling is characterized by a coupling coefficient &kgr;, such that in a distance l=&pgr;/(2&kgr;), all of the optical energy entering waveguide
38
is transferred by this coupling to waveguide
40
. The distance l is called the transfer length. The ratio of l to L is defined herein as the “normalized coupling length” of 1×2 splitter
26
.
In one type of 1×2 directional coupler splitter
26
, L is chosen to be equal to l, so that the normalized coupling length of this type of 1×2 splitter
26
is equal to 1. With no voltage applied to electrodes
46
and
48
, this type of 1×2 directional coupler splitter
26
is in a “crossover” state, as described above, in which all of the optical energy entering directional coupler splitter
26
in input waveguide
36
is transferred to output waveguide
44
via coupling waveguide
40
. To switch this type of directional coupler splitter
26
into a “straight-through” state, in which all of the optical energy entering directional coupler splitter
26
in input waveguide
36
leaves directional coupler splitter
26
via output waveguide
42
, opposite voltages are applied to electrodes
46
and
48
to alter the refractive indices of coupling waveguides
38
and
40
sufficiently in opposite directions, thereby altering the coupling coefficient &kgr;, so that the transfer length l of directional coupler splitter
26
becomes L/2, and all of the optical energy, that is transferred from coupling waveguide
38
to coupling waveguide
40
after propagating for a distance L/2, is transferred back to coupling waveguide
38
after propagating a distance L.
In another type of 1×2 directional coupler splitter
26
, the normalized coupling length is equal to ½. With no voltages applied to electrodes
46
and
48
, this type of 1×2 directional coupler splitter
26
is in an “all-pass” state: only half of the optical energy entering this type of 1×2 directional coupler splitter
26
via input waveguide
36
is transferred to output waveguide
44
, and the remaining optical energy leaves this type of 1×2 directional coupler splitter
26
via output waveguide
42
. This type of 1×2 directional coupler splitter
26
is placed in either the crossover state or the straight-through state by the application of appropriate voltages to electrodes
46
and
48
.
FIG. 8
is a schematic diagram of a 1×2 splitter
26
implemented as a Mach-Zehnder interferometer. Input waveguide
36
is coupled, by a splitting mechanism
52
, to an upper branch waveguide
54
and a lower branch waveguide
56
. Splitting mechanism
52
may be a y-branch coupler, as drawn, or may be an active 1×2 splitter such as a directional coupler splitter. Upper branch waveguide
54
leads into a coupling waveguide
38
′, which in turn leads into output waveguide
42
. Lower branch waveguide
56
leads into another coupling waveguide
40
′ that is close and parallel to coupling waveguide
38
′ and that leads into output waveguide
44
. Coupling waveguides
38
′ and
40
′ both are of length L. Upper and lower branch waveguides
54
and
56
are partially covered by respective electrodes
58
and
60
.
Like coupling waveguides
38
and
40
of
FIG. 7
, coupling waveguides
38
′ and
40
′ of
FIG. 8
are sufficiently close that the evanescent field of light propagating in coupling waveguide
38
′ overlaps with and is coupled into coupling waveguide
40
′. Here, too, the strength of the coupling is charact
Friedman Mark M.
Lynx Photonic Networks Inc.
Sanghavi Hemang
LandOfFree
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