The invention is based on a priority application DE 01440015 that is hereby incorporated by reference.
The invention is related to a method for adjusting time delays between at least two channels in an optical WDM (wavelength division multiplex) transmission system comprising the following steps:    Demultiplexing the signals, delaying the signals individually between channels and multiplexing the signals again for the next step (Block 1)    remodulating the multiplexed signal with a clock signal of high frequency (Block 2)    monitoring the remodulated signal with a low frequency photodetector unit (Block 3)    measuring, analyzing the photocurrent of the photodetector, adjusting via an electronic circuit the time delays between the channels (Block 4).
The invention is also related to devices for the synchronization of data in an optical WDM transmission system, consisting of the following parts: A wavelength demultiplexer (1) for demultiplexing the incoming data stream in the synchronizer, delay lines (2) for the individual wavelength channels, a multiplexer (100, a modulator (3) modulated by high frequency clock signal (5) and at least one photodetector (4) tapped to output line (B) where the photodetector (4) is connected to an electronic control circuit (6) which is connected to the individual delay lines (2) for an automatic adaption.
Wavelength division multiplex transmission systems allows to transmit a high data rate at several wavelengths of a definite wavelength grid. The transmission over large transmission distances occurs the problem of degradation of signals power, signals shapes and signal position in relation to the clock signal.
To overcome these problems the signals must be regenerated during the transmission. The invention is related to the regeneration issue of signals and especially for the position recovery of the signals in the digital bit stream.
Regeneration of WDM signals has been demonstrated in a solution-based regenerator up to 4×40 Gb/s in the publication “Simultaneously regenerated 4×40 Gbit/s dense WDM transmission over 10,000 km using a single 40 GHz InP Mach Zehnder modulator”, O. Leclerc et al, Elect. Letters; Vol. 36, n° 18. This prior art shows a synchronization of the four channels of different wavelengths. For the remodulation of the signals the signals are multiplexed and fed in a Mach Zehnder modulator driven by a clock signal that was extracted from an individual wavelength channel. Optical delay lines are inserts for the individual wavelength to insure that the pulse signals of the four channels remain time coincident in the modulator. The delay lines are adapted manually to optimize the performance of the transmission system.
The arrangement in the prior art document shows a laboratory solution. For a practically solution in a regeneration tool of a transmission system an automatically synchronization in the time domain between several wavelength channels is necessary.
It is also known from prior art to synchronize a bit stream signals with help of a clock signal. In the French Patent 2,706,710 a method is described for an automatically synchronization of a single signal by using an electrooptical absorption modulator and a signal derived from the modulator itself. This method can only be applied using an electro-optical absorption modulator and only for one optical channel.