1. Field of the Invention
The present invention relates to a tunable optical filter, to an add-drop-continue module which is produced using this filter, to an add-drop-continue device and to a circuit for bundled cross-connect functionality.
2. Description of the Prior Art
In order to ensure minimal interference during signal transmission, optical wavelength multiplex networks (WDM networks) are redundantly designed. Ring structures are often provided. At a junctions between different rings, “drop-and-continue” functions are implemented; i.e., the signal is split and is both forwarded through the original ring and transferred into the new ring. For purely optical production of a drop-and-continue function, it is possible to use wavelength demultiplexers, optical switches and wavelength multiplexers.
For the production of add-drop functions, modules from the company High Wave Technologies are known; which consist of two circulators with interposed tunable filters. In the event of tuning due to a configurational change, however, the retuning of one WDM channel generally interferes with the signals of other WDM channels. These modules are not intended for drop-and-continue functions. It is, however, conceivable to supplement this module with splitters and switches, in order to produce the drop-and-continue function.
FIG. 1 represents such an “add-drop-continue module”. It consists of a splitter SP; which divides the optical signal into two signals of roughly equal strength. One component is fed via two circulators having a tunable filter connected in between. In order to produce the drop-and-continue function, one signal component Dk is branched off via the first circulator and the other signal component Ck is forwarded via an optical switch SW (the switch position which is represented).
In the case of an add-drop function, one signal component Dk is likewise branched off, but a new signal Ak having the same wavelength is simultaneously inserted via the second circulator ZI2. Owing to the use of the optical splitter, the module has in principle an attenuation of at least 3 dB. Depending on the number of add-drop functions, the aforementioned add-drop element is multiply connected in series, so that the attenuation is further increased significantly.
The cross-connect functionality in optical multi-wavelength multiplex systems (WDM) is needed so that a specific wavelength signal of an incoming multi-wavelength signal can be distributed in any desired direction.
“WDM Gridconnect—ein transparentes faseroptisches Kommunikationsnetz mit Faser—und Wellenlängenmultiplex” [WDM grid-connect—a transparent fiber-optic communications network having fiber and wavelength multiplex] by Hubert Anton Jäger, published by Hartung-Gorre-Verlag, Constance [Germany] 1998 describes a standard optical cross-connect (OXC). Such an optical cross-connect (OXC) having optical n×n space-switching subunits with n incoming bidirectional multi-wavelength signals, each having k wavelengths, is represented in FIG. 9. In this case, the optical multi-wavelength signal is decomposed via optical wavelength demultiplexers DMUX into k single-frequency signals which are subsequently switched to any desired output of the space-switching subunit by using optical space-switching subunits of dimension n×n. The single-frequency signals coming together from the outputs of the space-switching subunits are coupled and forwarded via a multiplexer MUX.
A disadvantage with this is the large outlay on equipment which is incurred when making these optical cross-connects (OXC). A circuit having, for example, 64 wavelengths per multi-wavelength signal and 4 bidirectional conductors needs 64 space-switching subunits of dimension 4×4. Furthermore, 64 fiber-optic connections to the corresponding space-switching subunits of dimension n×n have to be installed per multiplexer MUX and demultiplexer DMUX, respectively, plus the same number again from the space-switching subunits to the demultiplexers DMUX or multiplexers MUX on the other side.
JP 1 023 479/U.S. Pat. No. 5,963,685 describes an add-drop module which contains a number of reflection filters whose frequencies can be adjusted by mechanical pressure and changing the temperature.
The patent U.S. Pat. No. 5,707,375 likewise specifies an add-drop device whose filters have different and mutually asymmetric edges. By tuning the filters in terms of wavelength, it is possible to obtain complete transmission, complete reflection or partial transmission and reflection. In this solution, it is necessary to have double the number of filters and adjustment devices. However, readjustment of the wavelength during operation leads to interference with the other signals.
The European patent application EP 0854 378 A2 describes a thermal-optical component which has a splitter and a tunable grating filter. The arrangement operates as an optical switch and can be used to produce add-drop functions. The patent U.S. Pat. No. 5,408,319 describes an optical demultiplexer in which tuning to a specific wavelength is carried out not mechanically but by changing the temperature.
The patent application WO 98/04854 describes an add-drop module which can be tuned by heating strips or magnetoresistors.
WO 99/42893 also discloses a tunable add-drop multiplexer. In order to tune the wavelength, the refractive index of the filter material is changed, for example by heating.
The known principles are unsuitable or too elaborate for producing an add-drop-continue function.
It is an object of the present invention to propose an add-drop-continue module having little attenuation, as well as a filter which is suitable for its production and which has a variable transmission characteristic. This module is also intended to permit reconfiguration of channels without causing interference.
It is another object of the present invention to propose a circuit having cross-connect functionality which allows simple allocation of dynamically assembled multi-wavelength bundles to different conductors. It is another object of the present invention to propose a circuit having cross-connect functionality, which permits reduced complexity of the system. In this case as well, reconfiguration of channels is intended to be made possible without causing interference.