Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-06-26
2002-12-31
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06501575
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to an optical transmission system using wavelength multiplexing, especially to the routing used therein.
BACKGROUND OF THE INVENTION
In Wavelength Division Multiplexing WDM, several independent transceiver couples use the same fibre, and each couple uses its own wavelength, which is different from the others
FIG. 1
illustrates the principle of wavelength multiplexing. Such a system is used as an example, wherein four channels are used, wherein the wavelengths used are &lgr;
1
, &lgr;
2
, &lgr;
3
and &lgr;
4
respectively. The transmission and reception channels use here their own optical fibres. At each end of the optical transmission line there are four transceiver units, of which the transmitter is marked generally as Tx and the receiver as Rx. Transmitter TX
1
transmits on wavelength &lgr;
1
and receiver RX
1
receives on the same wavelength, but from a fibre different from the one on which the transmitter transmits. The other couples use their own wavelengths in a corresponding manner.
The wavelengths produced by the transmitters located at the left end of fibre
8
are combined in an optical multiplexer
1
and they are then conducted to the same optical fibre
8
. In a similar manner, the wavelengths produced by the transmitters located at the right end of fibre
9
are combined in an optical multiplexer
3
and they are then conducted to the same optical fibre
9
.
The WDM demultiplexers
2
and
4
located at opposite ends of the fibres separate the different components of the combined signal from each other. Each one of these signals is expressed by its own receiver RX
1
, . . . RX
4
. Each optical channel may include hundreds and even thousands of logical channels.
Thus, a narrow wavelength window in a certain wavelength range is made available to the signal of each source. The International Telecommunication Union ITU-T has standardised the frequencies to be used on the band in such a way that the band begins from a frequency of 191,5 THz (1565,50 nm) and continues in steps of 100 GHz up to a frequency of 195,9 THz (1530,33 nm).
In order to achieve a maximum benefit relating to the optical transmission system, cross-connection functions are needed in the nodes of the network to route signals with different wavelengths to different paths. Components performing such a function are the multiplexerd/demultiplexer, the add/drop multiplexer, the wavelength switch and the wavelength converter. The original purpose of the multiplexer was, just as shown in
FIG. 1
, to increase the capacity of the existing fibre without adding any new fibres. However, they can work as an add/drop multiplexer, that is, as an access point for the optical layer and as a separation point for the optical layer as well as a part of the cross-connection arrangement.
The optical cross-connect is a small basic part of the optical coupler matrix. Its task is to route the arriving wavelength into one or more physical output ports. The cross-connect may be implemented with a directional coupler. By combining a great number of such 2×2 cross-connects one another by waveguides, it is possible to build a n×n coupler matrix. The cross-connect may also be implemented by using a Semiconductor Optical Amplifier SOA.
The wavelength converter converts the wavelength of the arriving signal into another wavelength, and the converter is needed when such systems are fitted together, wherein different wavelengths are used. Available converters are opto-electronic ones, wherein the incoming optical signal is converted into electrical form, it is regenerated and transmitted further by a transmitter using a fixed wavelength.
FIG. 2
shows the principal structure of a certain multiplexer/demultiplexer. It contains port-specific lenses and an interference filter marked F. The input fibre is connected to port named “common port”, and four wavelength channels )&lgr;
1
, . . . &lgr;
4
use that fibre in this example. Wavelength &lgr;
1
(channel
1
) goes through lens
21
, through interference filter
27
and through lens
22
directly to the output port. The remaining wavelength channels are reflected back from filter
27
and they go forward to filter
28
. This filter transmits wavelength &lgr;
2
which is conducted to lens
23
. Thus the channel
2
is separated. In this way, the incoming light signal is reflected back and forth within the multiplexer, and at every reflection point one exactly defined wavelength is admitted through the filter.
In the nodes of the optical network various cross-connections must be performed associated with routing, just like in electric transmission networks. In cross-connection, all wavelengths of the input fibre can be connected to the same output fibre, or certain wavelengths of the input fibre can be connected to certain output fibres. The perfect cross-connection device is an exchange, wherein all connecting functions are optical. At the present time, optical cross-connection devices are still scanty available, so in present systems the signal arriving from the optical link is in fact converted into electrical form. The necessary connections are made in electrical form, and only when the information is transferred to the output link it is converted into optical form.
FIG. 3
shows some nodes of an optical transmission network. There the node
31
multiplexes four channels to an optical link by using a coupler
35
. It is important to notice that the coupler is not a wavelength-selective component, but each entry of it can be input any wavelength, and the wavelength is then summed with other wavelengths to the same fibre.
The sum signal arrives at an optical add/drop multiplexer
33
located at the middle of the link. It is a device with which one or more of the wavelengths transmitting in the fibre can be separated aside and with which one or more wavelengths can be brought to the fibre. In the arrangement shown in the figure, four optical channels are transmitted to the fibre in transmission node
31
: channel
1
, wherein the information is transmitted on wavelength &lgr;
1
, channel
2
, wherein wavelength &lgr;
2
is used, channel
3
, wherein wavelength &lgr;
3
is used, and channel
4
, wherein the information is transmitted on wavelength &lgr;
4
. Using the add/drop multiplexer
33
, channel
1
, that is, wavelength &lgr;
1
is separated and the signal content to be transported on this wavelength is conducted further in fibre
33
towards node
34
. The other wavelengths continue further to node
32
, wherein the wavelength components are demultiplexed and they are conducted for further processing. Connections to other nodes also start out from the node (not shown).
In the arrangement outlined in the figure, all components except coupler
35
of node
31
are wavelength-sensitive. This means that e.g. in the case of the multiplexer of node
32
an exact predetermined wavelength is obtained from each output port. FIG.
2
and the related text clarify why this is so. Likewise, in the case of the add/drop multiplexer, an exact predetermined wavelength is obtained from its output port, wavelength &lgr;
1
in FIG.
3
. When it is known that the information content of channel
1
is transmitted on wavelength &lgr;
1
, it is thus known that channel
1
is conducted to node
34
. If the mode of transmission is e.g. SDH, the channel may contain hundreds or thousands of sub-channels.
The use of entirely optical network components causes a difficulty. Due to wavelength-sensitivity the routing of channels is difficult in the network. If, for example, it would be desirable in the add/drop multiplexer shown in
FIG. 3
to separate channel
3
to conduct it further to node
34
, then the necessary cross-connections must be made in the multiplexer. If the multiplexer is entirely optical, quite extensive changes must be made, the entire component must be exchanged in practice. Correspondingly, when wishing to replace the channel obtainable from a certain output port of demultiplexer
36
, in other words, when wishing to ro
Hiironen Olli-Pekka
Salomaa Ari
Altera Law Group LLC
Negash Kinfe-Michael
Nokia Networks Oy
LandOfFree
Routing in a WDM system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Routing in a WDM system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Routing in a WDM system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2943635