Device for adding and dropping optical signals

Details Device for adding and dropping optical signals Device for adding and dropping optical signals

1999-05-07

2001-09-11

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200

Reexamination Certificate

active

06288810

ABSTRACT:

DESCRIPTION
It is an object of the present invention to provide a device for adding or dropping optical signals of different wavelengths into or from an optical path and a method for inserting and/or extracting optical signals of different wavelengths into and from an optical path.
In the latest telecommunications technology, it is known to use optical fibers to send optical information-carrying signals for long-distance communication.
Optical telecommunication systems are known that use wavelength division multiplexing (WDM) transmission. In these systems several channels, i.e. a number of independent transmission signals, are sent over the same line by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
EP 668674 (Toshiba) discloses an optical WDM network system designed to permit communication between any of a plurality of nodes via a main trunk line. The nodes are interconnected by an optical fiber ring. Each of the nodes includes an add-rop multiplexer (ADM) for extracting light of a particular wavelength among the lights of a plurality of wavelengths transmitted via the main trunk line into the node and inserting the light of the preset wavelength from the node into the main trunk line. Each node further includes an optical receiver for receiving part of the light extracted by the ADM and a modulator for modulating the light extracted by the ADM with data to be transmitted and send the light back to said ADM. In an embodiment, the optical wavelength division multiplexer and demultiplexer of each of the terminal nodes can be realized by an ADM filter such as an acousto-optic filter or a waveguide type lattice filter. In this embodiment, at a node k only optical signals of wavelengths &lgr;k and &lgr;k′ are directed to an optical WDM whereby the two optical signals are demultiplexed, while the optical signals of other wavelengths are transmitted by the ADM filter to the main trunk line.
Electronics Letters, Vol. 31, No. 6, 16/03/95, pp. 476-477 (M. J. Chawki et al.) discloses an experimental optical WDM ring network using an add/drop multiplexer (ADM) based on a fibre grating filter. The ring consists of a central node connecting N secondary nodes using dedicated wavelengths &lgr;
i
(i=1. . . N) over one common fibre. The central node has four transmitters with a common erbium doped fibre amplifier (EDFA) and a 1:4 demultiplexer with four receivers. The optical wavelength ADM located in each secondary node is made of a 2:1 coupler, a fibre grating filter, a second 2:1 coupler and an EDFA. The node transmitters and receivers are connected to one of the input ports of the first and second coupler, respectively.
IEEE Photonics Technology Letters, Vol. 5, No. 7, July 1993, pp. 825-828 (K. Oda et al.) discloses that optical add/drop multiplexers (ADM's) can be constructed by optical filters and that there are many candidates for the filters which could be used for the ADM's; for example, the acousto-optic tunable filter (AOTF). The paper teaches that, however, the channel spacing of the AOTF is basically limited to a few nm, so that the number of channels is limited to approximately 10 because EDFA gain bandwidth is narrower than 30 nm. On the other hand, the channel spacing of the Fabry-Perot filter can be set to less than 1 nm because its finesse is sufficiently high. Accordingly, the paper proposes an optical ring network based on optical frequency division multiplexing and add/drop multiplexers (ADM's), wherein each ADM consists of a fiber Fabry-Perot filter and an optical circulator.
WO 96/19884 (Ericsson) discloses a method of configuring subnodes, or configuring a system of subnodes, in an optical network ring against both node and fiber failure by means of an optical add-rop multiplexer (OADM). The network comprises a working ring and a stand-by ring and each subnode Includes selective optical filter means, optical 2×2 switch means and optical amplifier means. The switching configuration of the optical 2×2 switch is controlled by the different available node states so that it is possible to select either of two measures in case of a fiber breakage, such as folding the ring or line switching. According to the inventor of WO 96/19884 the disclosed OADM structure, besides its advantages in connection with failures, offers a greater simplicity for adding and removing nodes in an existing optical network.
The Applicant has observed that known WDM communication systems are limited as concerns the number of channels, i.e. the independent wavelengths that can be used for transmission within the wavelength band available for signal transmission and amplification.
In order to combine and separate signals with different wavelengths-to combine the signals at the transmission station, for example, to drop some toward receivers located at intermediate nodes of the line or to introduce others at intermediate nodes or to send them to separate receivers at the receiving station--adjacent channels (in wavelength terms) must be separated by more than a minimum predetermined value.
Said minimum value depends on the characteristics of the components employed in the system, such as the spectral characteristics of the wavelength selective components (e.g. bandwidth, center-band attenuation, figure of merit) and the wavelength stability (thermal and temporal) of the selective components themselves and of optical signal sources.
In particular, the Applicant has observed that spectral selectivity of currently available wavelength selective components currently available may greatly limit the possibility of adding and dropping signals in multichannel transmission systems, particularly when there are signals with close wavelengths, e.g. separated by less than 2 nm.
The Applicant found it is possible to add and/or drop in an optical telecommunication system a number of independent optical channels greater than that permitted by known techniques, and closer in wavelength, by employing wavelength selective components of equivalent characteristics, if the input signals are divided into two series of staggered wavelengths. The signals of each series are independently dropped or signals of corresponding wavelength added, and the signals of the two series are then combined.
According to a first aspect, the present invention is related to a device for adding and dropping optical signals as claimed in claim
1
.
Preferred embodiments of the device are given in dependent claims
2
to
4
.
According to another aspect, the present invention relates to a multichannel optical telecommunication system for the transmission of optical signals as claimed in claim
5
.
According to a third aspect, this invention relates to a method for adding/dropping optical signals as claimed in claim
6
.
According to a fourth aspect, the present invention is related to a device for dropping optical signals as claimed in claim
7
.
According to a fifth aspect, the present invention is related to a device for adding optical signals as claimed in claim
8
.


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P.C. Hill et al., “Strain gradient chirp of fibre Bragg gratings”, Electronics Letters, vol. 30, No. 14, pp. 1172-1174, (1994).
F. Ouellette, “Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides”, Optic Letters, vol. 12, No. 10, pp. 847-849, (1987).
M.J. Chawki et al., “Wavelength reuse scheme in a WDM unidir

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