Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
2002-01-28
2004-01-27
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C359S279000, C385S002000
Reexamination Certificate
active
06683721
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to interferometers for use in optical communication networks, and more specifically to optical signal interleavers/deinterleavers designed to produce greatly reduced amounts of chromatic dispersion.
In multiplexed optical communication networks, a single optical fiber typically carries multiple independent data channels with each data channel assigned to a different optical wavelength. Such networks are referred to as wavelength division multiplexed (WDM) networks. As signals propagate through the network, data for different channels may be separated or combined using an optical frequency filter, in particular, an interleaver/deinterleaver (hereafter “interleaver”).
An interleaver is a type of optical multiplexer which, when operating as an interleaver, combines subsets of channels from different fibers into a single optical beam. When operating as a deinterleaver, the interleaver separates a single optical beam having a series of channels into two or more subset series of channels. Typically, an interleaver is used to separate or combine even and odd International Telecommunications Union (ITU) channels.
FIG. 1
conceptually illustrates the function of an interleaver. When operating as an interleaver, the interleaver receives a first optical beam
100
, which comprises a number of even channels at frequencies f
2
, f
4
, f
6
. The frequencies of each channel are such that each of these channels is separated by the same amount, e.g. 100 GHz. The interleaver also receives a second optical beam
102
, which comprises a number of odd channels at frequencies f
1
, f
3
, f
5
. Similar to beam
100
, the frequencies of each of these channels are such that these channels are separated by the same amount, e.g. 100 GHz. The even and odd channels, however, are offset from each other, normally an amount equal to half their separation distances, e.g. 50 GHz. The interleaver then interleaves the beams
100
and
102
to generate a beam
104
with the channels f
1
, f
2
, f
3
, f
4
, f
5
, f
6
, which are separated by 50 GHz. When operated as a deinterleaver, beam
104
is received and divided into beams
100
and
102
.
Optical frequency interleavers are widely recognized as key components enabling the rapid expansion of WDM networks to higher channel counts and narrower channel spacing, while preserving inter-channel cross-talk performance, in combination with existing demultiplexer technologies. Because of the periodic frequency nature of the International Telecommunications Union (ITU) grid, interleavers tend to be constructed from combinations of one or more interferometric structures, such as etalons and Mach-Zehnder interferometers. The desirable features of interleaver pass bands include a flattop and high isolation in the stop-band.
A Michelson interferometer uses a beamsplitter and two reflecting mirrors to separate wavelengths of a light signal into different optical paths. This type of interferometer provides a linear phase ramp dependent on the optical path difference between the two arms of the interferometer. The linear phase ramp generates a non-flat top response with no chromatic dispersion.
Another type of interferometer, invented by Dingel, is a Michelson interferometer in which the mirror of one arm is replaced by a Gires-Tournois (GT) etalon. As shown in
FIG. 2
, an interferometer
200
comprises a beam splitter
202
(typically an approximately 50/50 splitter), a plate
204
with a highly reflective (near 100%) coating
206
placed in one arm with spacers
207
a
and
207
b
preferably made from ultra low expansion material (ULE). A GT etalon
220
is placed in the other arm. The GT etalon
220
comprises a front plate
208
with a partially reflective (e.g., 15% reflectivity) coating
210
, spacers
211
a
and
211
b
preferably made from ultra low expansion material (ULE) and a back plate
214
with a highly reflective (near 100%) coating
212
. As shown, a gap of distance d separates front plate
208
and back plate
214
of the GT etalon
220
. Further, the GT etalon
220
is placed a distance L
2
from the beam splitter
202
, and the plate
206
is placed a distance L
1
from the beam splitter
202
.
When this set-up is used in an interleaver for deinterleaving channels, an incident beam B
1
comprising, for example, ITU even and odd channels is directed towards beam splitter
202
. Beam B
1
is split at splitter interface
222
into a beam B
3
and beam B
2
. Beam B
3
is directed towards plate
204
with highly reflective coating
206
, while beam B
2
is directed towards GT etalon
220
. Because of the near 100% reflectivity of reflective coating
206
, beam B
3
is reflected back to splitter
202
. Beam B
3
experiences a linear phase change per wavelength based upon the distance traveled from the splitter interface to plate
204
and back. An exemplary linear phase ramp of beam B
3
at splitter interface
222
is illustrated in
FIG. 2
c
as line
242
.
Likewise, because of the near 100% reflectivity of reflective coating
212
, beam B
2
is reflected back to splitter
202
. However, in addition to experiencing a linear phase change per wavelength based upon the distance traveled, beam B
2
also experiences a non-linear phase change from GT etalon
220
of,
Φ
=
-
2
tan
-
1
[
1
-
R
1
+
R
tan
(
2
πη
d
λ
)
]
where R is the power of reflectance of coating
210
, &lgr; is the vacuum wavelength and &eegr; is the refractive index of the material inside GT etalon
220
. Typically, the material inside GT etalon
220
is air, resulting in a refractive index &eegr; equal to approximately 1. An exemplary non-linear phase ramp of beam B
2
at splitter interface
222
is illustrated in
FIG. 2
c
as line
240
for a 15% reflectivity of coating
210
.
Therefore, when beams B
2
and B
3
meet at splitter interface
222
, there is a resulting phase difference of,
ΔΦ
=
4
πΔ
L
λ
+
2
tan
-
1
[
1
-
R
1
+
R
tan
(
2
πη
d
λ
)
]
where the optical path difference &Dgr;L is the difference between the distance L
1
and L
2
(i.e., L
1
−L
2
).
The phase graphs illustrated in
FIG. 2
c
result when MGTI
200
is designed such that the optical path difference, &Dgr;L, is one half, or multiples of one half, the GT air gap, d. As described, GT etalon
220
perturbs the linear phase ramp of the interferometer
200
and produces a non-linear phase ramp. When the optical path difference, &Dgr;L, is one half, or multiples of one half, the GT air gap, d, this non-linear phase ramp generates a flat top response function that is desired in telecommunication systems. For the case that &Dgr;L is one half the GT air gap, the phase difference between beam B
2
and B
3
when they meet at splitter interface
222
is,
ΔΦ
=
2
π
d
λ
+
2
tan
-
1
[
1
-
R
1
+
R
tan
(
2
πη
d
λ
)
]
When beams B
2
and B
3
meet at the splitter interface, part of beam B
2
is reflected, while part of beam B
3
is passed through, thereby forming beam B
4
. Referring to
FIG. 2
c
, at the frequencies where these two portions are substantially 180° (i.e. &pgr;) out of phase, destructive interference occurs, while constructive interference occurs at the frequencies where these two portions are substantially in phase. The interference between these portions of beams B
2
and B
3
result in beam B
4
having a standard intensity pattern of,
I
(
t
)
=
I
o
sin
2
(
ΔΦ
2
)
This spectral response is illustrated in
FIG. 2
b
as line
230
. This spectral response results in beam B
4
carrying a first sub-set of channels (e.g., the even channels).
Also, when beams B
2
and B
3
meet at splitter interface
222
, part of beam B
3
is reflected with a phase change of &pgr;, while part of beam B
2
is passed therethrough, thereby forming beam B
5
. Because the portion of B
3
that forms B
5
is reflected wit
Abraham Christopher John
Copner Nigel
Ducellier Thomas
Picard Marie Josee
Tan Kim Leong
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
JDS Uniphase Corporation
Mack Ricky
LandOfFree
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