Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-10-03
2004-01-13
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S014000, C385S015000, C385S024000, C385S039000, C385S132000, C398S085000
Reexamination Certificate
active
06678446
ABSTRACT:
TECHNICAL FIELD
This invention is in the field of networks for carrying optical signals. The invention relates to apparatus for multiplexing or demultiplexing a number of optical bands. The invention relates more particularly to such apparatus which incorporates arrayed waveguide gratings.
BACKGROUND
Optical fibers provide a way to transmit large volumes of data from place to place. It is often desirable to wavelength multiplex a number of signals onto a single optical fiber. This can be accomplished by passing each signal into a different input of a multiplexer and connecting an output of the multiplexer to the optical fiber. The signals can be recovered at a destination by demultiplexing.
Arrayed waveguide grating routers (AWGRs) are commonly used as multiplexer/demultiplexers in such systems (the same device can typically be used either as a multiplexer, as a demultiplexer, or simultaneously as a multiplexer and a demultiplexer). An arrayed waveguide router uses an arrayed waveguide grating to separate signals by wavelength. Example AWGRs are described in U.S. Pat. Nos. 5,002,350 and 5,136,671, both invented by Dragone.
FIG. 1
shows the main functional parts of a simple prior art AWGR
10
. AWGR
10
comprises a pair of free propagation regions which are typically implemented as slab waveguides
20
and
30
. The free propagation regions are sometimes referred to as “star couplers”. A number, M, of input waveguides
22
couple corresponding input ports
26
to input slab waveguide
20
. A number, N, of output waveguides
32
couple slab waveguide
30
to a number of corresponding output ports
36
. In the example illustrated in
FIG. 1
, M=5 and N=5. Input waveguides
22
and ports
26
are labeled with the index p with 0≦p≦4. Output waveguides
32
and ports
36
are labeled with the index q with 0≦q≦4. The terms “input” and “output” are used herein for reference only. Light can propagate in either direction through AWGR
10
.
Waveguides
20
and
30
are coupled to one another by a plurality of grating waveguides
16
. Typically grating waveguides
16
each have a different length. The lengths of grating waveguides
16
are spaced from one another by predetermined amounts. Typically light enters AWGR
10
through at least one of input waveguides
22
and, in sequence, propagates through input slab waveguide
20
, grating waveguides
16
, output slab waveguide
30
, and at least one output waveguide
32
.
In the AWGR
10
of
FIG. 1
, light having a wavelength &lgr; which enters AWGR
10
at a certain one of input waveguides
22
is preferentially coupled into a specific one of output waveguides
32
. AWGR
10
has an optical passband associated with each pair of an input port
26
and an output port
36
. In general, a port is a location at which a waveguide of AWGR
10
couples to an optical pathway that is external to AWGR
10
. For example, a port might be a location at which an input or output waveguide couples to an optical fiber external to the PLC on which the AWGR is fabricated. Within each optical passband the optical transmission between input port p and output port q is optimized for a range of wavelengths that are centered at the nominal wavelength for the passband, &lgr;. This can be achieved by designing the AWGR to satisfy the following equation:
m&lgr;=n
s
×d
I
×sin(&thgr;
p
)+
n
s
×d
o
×sin(&agr;
q
)+
n
c
&Dgr;L
i
(1)
where:
&lgr; is the wavelength;
n
s
is the effective index of refraction of slab waveguides
20
and
30
;
n
c
is the effective index of refraction of channel waveguides
16
;
d
I
and d
o
are the center-to-center separations of grating waveguides
16
at the points where they couple to input slab waveguide
20
and output slab waveguide
30
respectively;
&Dgr;Li is the difference in length between adjacent grating waveguides
16
;
m is the diffraction order for a particular passband associated with an input port p and an output port q;
&thgr;
p
is the angle between the point at which the p
th
input waveguide
22
couples to slab waveguide
20
and an axis, A, of the focal curve on which arrayed waveguides
16
couple to slab waveguide
20
as shown in
FIG. 1
; and,
&agr;
q
is the angle between the point at which the q
th
output waveguide
32
couples to slab waveguide
30
and axis A of the focal curve on which arrayed waveguides
16
couple to slab waveguide
30
as shown in FIG.
1
.
For simplicity, in the following discussion it is assumed that d
I
=d
o
=d. In general, d
I
and d
o
can be different. Also for simplicity, the length difference &Dgr;L
i
between adjacent waveguides
16
of the arrayed waveguide grating is assumed to have a constant value &Dgr;L. By applying small angle approximations to the sine functions of Equation (1) the relationship of Equation (1) can be recast as:
θ
p
+
α
q
=
(
m
′
n
s
×
d
)
(
λ
-
λ
c
)
(
2
)
where:
&lgr;
c
is the wavelength of light diffraction order m that will propagate from the center (or “pole”) of the input focal curve to the center (or “pole”) of the output focal curve (i.e. from &thgr;
p
=0 to &agr;
q
=0); and,
m′ is given by:
m
′
=
m
(
1
+
(
λ
c
n
c0
)
(
ⅆ
n
c
ⅆ
λ
)
)
(
3
)
where:
n
c0
is the value of n
c
for light of wavelength &lgr;
c
.
Equation (3) in volves the value
(
ⅆ
n
c
ⅆ
λ
)
which is a function of &lgr;. In general, however,
(
ⅆ
n
c
ⅆ
λ
)
varies slowly with wavelength and so, for most wavelengths of interest, the value of
(
ⅆ
n
c
ⅆ
λ
)
can be approximated by its value for &lgr;=&lgr;
c
.
The basic construction of
FIG. 1
can be customized for specific applications by altering: the locations at which input waveguides
22
and AWG waveguides
16
couple to input slab waveguide
20
; the locations at which output waveguides
32
and AWG waveguides
16
couple to output slab waveguide
30
; the dimensions of slab waveguides
20
and
30
; and, the relative lengths of AWG waveguides
18
. For example, it is known to provide a 1×N demultiplexer by providing an input waveguide located so that &thgr;
p
=0 and output waveguides located at angular positions which satisfy the relationship:
α
q
=
(
m
′
n
s
d
)
(
λ
0
-
λ
c
+
q
Δ
λ
)
(
4
)
where:
q=0, 1, 2, 3, . . . N−1;
&Dgr;&lgr; is a constant; and,
&lgr;
0
is the nominal wavelength of the passband for which p=0 and q=0.
In such implementations, the passbands lie on a wavelength grid. That is, the wavelengths of the passbands are centered at wavelengths given by:
&lgr;=&lgr;
0
+q&Dgr;&lgr;
(5)
FIG. 2A
is a block diagram of a 1×6 demultiplexer. In FIG.
2
A and the other block diagrams referred to herein the “input” waveguides are the lines entering the block from the left, the “output” waveguides are the lines leaving the block on the right, the sequence of output waveguides is the same as would be present in a physical device, and the sequence of input waveguides is reversed from that of the physical device (in the example of
FIG. 2A
there is no sequence of input waveguides because there is only one input waveguide). Each of the arrows within the block indicate light of a particular wavelength &lgr; being coupled from one of the input waveguides to one of the output waveguides. The slopes of the arrows within the block are proportional to the value of &lgr;−&lgr;
0
. So, for example, an arrow representing optical coupling of a signal with wavelength &lgr;
0
extends, straight across the box.
A problem encountered in manufacturing such demultiplexers is that variations in manufacturing processes may cause &lgr;
0
to depart, from its intended value. This results in reduced yields because manufactured demultiplexers having values for &lgr;
0
falling outside of an acceptable range cannot be used.
Some prior art demult
Lam Jane
McGreer Kenneth
Amin & Turcoy, LLP
Healy Brian
Lightwave Microsystems Corporation
Petkovsek Daniel
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