Add-drop multiplexer

Details Add-drop multiplexer Add-drop multiplexer

2001-05-11

2002-08-06

Ben, Loha (Department: 2873)

Optical: systems and elements

Single channel simultaneously to or from plural channels

C359S199200, C359S199200, C359S199200, C359S199200, C359S237000, C359S199200, C385S016000, C385S018000, C385S024000, C385S037000

Reexamination Certificate

active

06429974

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to optical networking.
BACKGROUND OF THE INVENTION
An optical add drop multiplexer (OADM) is a device used to extract a set of optical signals (also called “wavelengths” herein) from a wavelength division multiplexed (WDM) signal input to the OADM, and to subsequently reinsert the extracted set of wavelengths output by the OADM.
FIG. 1
(prior art) shows a block diagram of an OADM
100
. A WDM optical signal, (also called “wavelength band” herein) comprises a plurality of wavelengths. One of the wavelengths, &lgr;
2
, is extracted (also called “dropped” herein) from an input line side
102
via a de-multiplexer
104
, and then subsequently reinserted (also called “added” herein) onto an output line side
108
via a multiplexer
106
. The purpose of adding and dropping wavelength(s) in this manner is to obtain information encoded on the dropped wavelength (in
FIG. 1
, the dropped wavelength is &lgr;
2
); one may also transmit new information on dropped wavelengths. In most instances the carrier wavelength of the dropped wavelength is the same as carrier wavelength of the added wavelength.
It is well recognized that a preferred implementation of an OADM employs tunable de-multiplexers and multiplexers, enabling choice in the wavelength(s) to be dropped/added. However, until tunable filter technology is mature, OADMs will continue to be constructed using fixed de-multiplexers and fixed multiplexers, along with switches to select the wavelength(s) to be dropped.
There has been an evolution of OADM products in recent years. The first generation OADMs, as in
FIG. 1
, were not configurable, i.e., the determination of which wavelengths are dropped and added was set at the time of manufacture of the OADM. Second generation OADMs are configurable, but have a number of limitations.
FIG. 2
(prior art) shows a simple three-wavelength system utilizing a second generation OADM
200
. The second generation OADM
200
de-multiplexes all the wavelengths of interest via a de-multiplexer
204
, and passes each wavelength through a respective 2×2 switch
206
that is controllable by the user. The 2×2 switch
206
has two settings: in one setting the 2×2 switch
206
allows the demultiplexed wavelength to pass through with no further processing, via an optical waveguide
212
, while in the other setting the 2×2 switch
206
connects the demultiplexed wavelength, e.g., &lgr;
1
, to a drop optical waveguide
208
, and simultaneously connects an add optical waveguide
210
to the optical waveguide
212
that is attached to a multiplexer
214
, which multiplexer multiplexes all wavelengths input to the multiplexer, for example, wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, producing a multiplexed signal to an output via an egress line
216
.
The system depicted in
FIG. 2
has a number of disadvantages:
(1) Poor optical performance: Due to the filtering characteristics of each successive OADM, a series of OADMs that are concatenated typically results in channel narrowing.
(2) Poor network performance during upgrade or provisioning activities: Expanding the number of wavelengths in the system and/or expanding or changing the number of wavelengths added/dropped at a node typically requires shutting the system down for modification.
(3) High cost. For a system with a large number of wavelengths, it is costly to de-mulitplex, switch, and multiplex all wavelengths.
(4) Lack of flexibility. To avoid the high cost incurred as explained above, this type of implementation forces the user to pick a subset of wavelengths, thereby having reduced flexibility in add/drop capabilities.
(5) Inventory. Reducing the wavelength set addressed by any one OADM, as discussed in (4), requires the operator to stock a number of different types of OADM units (also called “modules” herein), leading to excessive inventory management.
What is needed is an OADM that overcomes the drawbacks enumerated above.
SUMMARY OF THE INVENTION
Methods and apparatus are contemplated for dropping and adding a user-chosen wavelength sub-band in a band of wavelengths. Methods and apparatus are contemplated for separating and dropping individual wavelengths from a wavelength sub-band, and adding the individual wavelengths back to the wavelength sub-band. An OADM is contemplated that combines a wide-band OADM stage and a narrow-band OADM stage.


REFERENCES:
patent: 6084694 (2000-07-01), Milton et al.
patent: 6154585 (2000-11-01), Copner et al.
patent: 6208441 (2001-03-01), Jones et al.
patent: 6243179 (2001-06-01), Thompson et al.
patent: 6256433 (2001-07-01), Luo et al.
patent: 6310690 (2001-10-01), Cao et al.
patent: 6348984 (2002-02-01), Mizrahi

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