WDM utilizing grating-based channel separators

Details WDM utilizing grating-based channel separators WDM utilizing grating-based channel separators

2000-05-25

2004-08-17

Pascal, Leslie (Department: 2633)

Optical communications

Multiplex

Wavelength division or frequency division

C398S068000, C398S082000, C398S087000, C398S088000, C398S095000

Reexamination Certificate

active

06778780

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to diffraction gratings in optical communications networks and systems, and more particularly to wavelength division multiplexers utilizing diffraction gratings.
BACKGROUND OF THE INVENTION
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this document, these individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.”
The term “wavelength,” denoted by the Greek letter &lgr; (lambda) is used herein in two senses. In the first usage, this term is used according to its common meaning to refer to the actual physical length comprising one full period of electromagnetic oscillation of a light ray or light beam. In its second usage, the term “wavelength” is used synonymously with the terms “signal” or “channel.” Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, &lgr;, and a plurality of n such channels are referred to as “n wavelengths” denoted &lgr;
1
-&lgr;
n
. Used in this sense, the term “wavelength” may be understood to refer to “the channel nominally comprised of light of a range of physical wavelengths centered at the particular wavelength, &lgr;.”
A crucial feature of fiber optic networks is the separation of the composite optical signal into its component wavelengths or channels, typically by a wavelength division multiplexer. This separation must occur to allow for the exchange of signals between loops within optical communications networks. The exchange typically occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
FIG. 1
a
schematically illustrates one form of an add/drop system, which typically exists at connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each composite optical signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
A wavelength division multiplexer (WDM) typically performs separation of a composite optical signal into component channels in an add/drop system. Used in its reverse sense, the same WDM can combine different channels, of different wavelengths, into a single composite optical signal. In the first instance, this WDM is strictly utilized as a de-multiplexer and, in the second instance, it is utilized as a multiplexer. However, the term “multiplexer” is typically used to refer to such an apparatus, regardless of the “direction” in which it is utilized.
FIG. 1
a
illustrates add/drop systems
218
and
219
utilizing wavelength division multiplexers
220
and
230
. A composite optical signal from Loop
110
(&lgr;
1
-&lgr;
n
) enters its add/drop system
218
-at node A (
240
). The composite optical signal is separated into its component channels by the WDM
220
. Each channel is then outputted to its own path
250
-
1
through
250
-n. For example, &lgr;
1
would travel along path
250
-
1
, &lgr;
2
would travel along path
250
-
2
, etc. In the same manner, the composite optical signal from Loop
150
(&lgr;
1
′-&lgr;
n
′) enters its add/drop system
219
via node C (
270
). The signal is separated into its component channels by the WDM
230
. Each channel is then outputted via its own path
280
-
1
through
280
-n. For example, &lgr;
1
′ would travel along path
280
-
1
, &lgr;
2
′ would travel along path
280
-
2
, etc.
In the performance of an add/drop function, for example, &lgr;
1
is transferred from path
250
-
1
to path
280
-
1
. It is combined with the others of Loop
150
's channels into a single new composite optical signal by the WDM
230
. The new signal is then returned to Loop
150
via node D
290
. At the same time, &lgr;
1
′ is transferred from path-
280
-
1
to path
250
-
1
. It is combined with the others of Loop
110
's channels into a single new composite optical signal by the WDM
220
. This new signal is then returned to Loop
110
via node B (
260
). In this manner, from Loop
110
's frame of reference, channel &lgr;
1
of its own signal is dropped to Loop
150
while channel &lgr;
1
′ of the signal from Loop
150
is added to form part of its new signal. This is the add/drop function.
FIG. 1
b
illustrates a second form by which add/drop systems
218
and
219
may be configured. In
FIG. 1
b,
each WDM is optically coupled to a first plurality of paths through which channels are outputted and to a second plurality of paths through which signals are inputted. For instance, the paths
250
-
1
,
250
-
2
, . . . ,
250
-n are utilized to output signals comprising wavelengths &lgr;
1
, &lgr;
2
, . . . , &lgr;
n
, respectively, from the WDM
220
and the paths
251
-
1
,
251
-
2
, . . . ,
251
-n are utilized to input signals comprising such wavelengths to the WDM
220
. Likewise, as shown in
FIG. 1
b,
the paths
280
-
1
,
280
-
2
, . . . ,
280
-n are utilized to output signals &lgr;
1
′, &lgr;
2
′, . . . , &lgr;′
n
(comprising the physical wavelengths &lgr;
1
, &lgr;
2
, . . . , &lgr;
n
) respectively, from the WDM
230
and the paths
281
-
1
,
281
-
2
, . . . ,
281
-n are utilized to input signals comprising such wavelengths to the WDM
230
.
A “channel separator” or, simply, “separator,” as the term is used in this specification, is an integrated collection of optical components functioning as a unit, which separates one or more channels of a composite optical signal from one another. One example of a channel separator is disclosed in U.S. Pat. No. 6,130,971, assigned to the assignee of the present application. This U.S. Patent is incorporated herein by reference. The channel separator disclosed in the above-referenced U.S. Patent permits fabrication of dense wavelength division multiplexers (DWDM's) having greater ease in alignment and higher tolerance to drift due to increased width of the pass bands and having greater passive stability against temperature variations. If a composite optical signal comprises more than two channels, then more than one stage of separation may be required to effect full or complete separation of each channel from every other channel. An efficient method of full or complete channel separation is disclosed in another U.S. Pat. No. 6,263,126, assigned to the assignee of the present application. This U.S. Patent is incorporated herein by reference.
A schematic illustration of the Multi-Stage Parallel Cascade Method is illustrated in
FIG. 1
c.
In
FIG. 1
c,
a composite optical signal comprising channels &lgr;
1
-&lgr;
n
enters the DWDM
100
through port A (
240
). The signal passes through a first interleaved channel separator
112
a
which divides the composite optical signal into two separate signal subsets, one containing the odd channels (&lgr;
1
, &lgr;
3
, &lgr;
5
, . . . ) (
130
) and the other containing the even channels (&lgr;
2
, &lgr;
4
, &lgr;
6
, . . . ) (
140
). These odd and even channels are each passed through another interleaved channel separator
112
b
-
112
c
which further divides them by every other channel. This division continues until only one channel is outputted to each output optical fiber
160
-
1
through
160
-n.
F

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