Method for adjusting time delays and device for synchronization of channels in a WDM system

Abstract: The invention is also related to ad evices for a 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 (10), a modulator (3) modulated by a 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 automatically adaptation.

Description: Background The invention is related to method for adjusting time delays between modulated optical signals of at least two wavelength channels in an optical WDM transmission system comprising the following steps: Step 1:Demultiplexing the signals of the wavelength channels, delaying the signals individually between wavelength channels and multiplexing the signals of the wavelength channels again for the next step (Block 1) Step 2:First remodulating the final multiplexed signal of step 1 with a clock signal of high frequency (Block 2) Step 3: monitoring the remodulated signal with at least one low frequency photodetector unit (Block 3) Step:4: 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 a device for a synchronization of modulated optical signals transmitted in wavelength channels in an optical WDM transmission system, consisting of the following parts: a demultiplexer (1) for demultiplexing the incoming optical signals of the wavelength channelsin the synchronizer, delay lines (2) for the individual wavelength channels , a multiplexer (10) for combining the optical signals of the wavelength channels before the remodulation to a signal A a modulator (3) modulating by a high frequency clock signal (5) the multiplexed signal A and at least one photodetector (4) tapped to the output signal of the multiplexer (10), where the at least one photodetector (4) is optically connected to at least a part of the multiplexed signals A and electronically connected to an electronic control circuit (6) which is connected to the individual delay lines (2) for an automatic adaptation of the individual delay lines. 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 has the problem of degradation of signals power, signals shapes and signal position in relation to the clock signal. To overcome this 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 soliton-based regenerator up to 4x40 Gb/s in the publication " Simultaneaously regenerated 4x40 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 synchronisation of the four channels of different wavelengths. Before 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 a individual wavelength channel. Optical delay lines are inserted 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 optimise the performance of the transmission system. The arrangement in the prior art document shows a laboratory solution. For a practical solution in a regeneration tool of a transmission system an automatical 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 automatic 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. The prior art document Shangsirs T: et al. "Bit synchronization using subcarriers for control signalling" IEEE Photonic Technology Letters, Vol 11, No. 5, May 1999, Pages 602-604 discloses a synchronisation stage for two indivisual signals where the signals are delayed using a control signal. The signals are synchronised on a clock frequency using a tapped part of the original signal and delaying the optical signals by a amount derived from the photodetection. The system and the method described herein is a synchronisation stage without remodulation and without handling a lot of parallel incoming wavelengh channels in a reasonable way. Description The inventive method has the advantage that the time delays are adjusted automatically. The tuning speed depends mainly on the low frequency cut-off of the photodiode and the time response of the optical time delay. The inventive method and inventive device can use any type of modulator in block 2, Mach-Zehnder or electroabsorption, InP or LiNbO 3 based. It is also an advantage of the device that the block 3 could be easily implemented when using an AWG (Arrayed Waveguide Grating) integrated with photodiodes array. The whole power consumption remains very low even when a great number of channel are simultaneously regenerated. This is due to the fact that only one RF clock recovery is required. Low cost electronic control due to the low frequency electronic response of the required components allows a cost reduced solution for the synchronization tool. It is an advantage to use a device where only a part of the signals is analysed. Then it is possible to adjust independently each wavelength without degrading too much the transmission quality when varying largely the optical delay line. Preferred embodiment of the inventional solution are shown in the figures and described in the following description. Figure 1 shows a schematic solution for a synchronization device Figure 2 shows a first embodiment of the synchronization device Figure 3 shows an advanced embodiment of synchronization device Figure 4 shows the photocurrent over the delay time Figure 1 shows the principles of the inventive device. The incoming signal is connected to a demultiplexer 1. The output lines of the wavelength demultiplexer 1 are individually connected with adjustable delay lines 2 and with a multiplexer 10. This part of the device are arranged in block 1 of the synchronizer. The output signal A is fed in a modulator 3. The modulator is connected with a clock recovery circuit 5. This clock recovery circuit is tapped to one or more of the demultiplexed and time delayed wavelength channels. After the modulator a photodetector 4 is placed. This photodetector is a low-frequency photodiode that is connected to an electronic control circuit 6. The electronic control circuit 6 is connected to the individual delay lines 2 . The principal solution is: when the RF signal applied on the modulator by the clock recovery circuit 5 and the optical pulses of one of the signals are synchronous, the photocurrent of the photodetector 4 has its maximum. On the contrary when the optical pulses are phase shifted by 180°, the attenuation is maximum and the detected current is minimum. This example is explained in the FR 2 706 710. In the first block, N WDM channels are demultiplexed to allow adjustment of the time independently and then re-multiplexed. At this stage a synchronised WDM signal can be used in output A. In the second block, the modulator is driven by an RF signal synchronised with any of the WDM channels (they all operate with the same bit rate) by detection of the optical signal and clock recovery. In the third block, a small part of the modulated optical signal is extracted by a coupler and detected by a low-frequency photodiode. In a fourth block the detected signal is used by an electronic circuit to control the N time delays of block 1. By using the output B, the scheme of figure 1 is also the main part of a soliton-based regenerator which also contains amplifiers and pulse reshaping means. In a WDM context, the photocurrent variation is similar whatever the time shift is of the other channels. Each WDM channel can thus be adjusted in sequence or with various dithering frequencies. The photocurrent variation amplitude is however decreasing with the number of channels. The mean detected photocurrent is the sum of N independent contributions, each depending on the time shift between the RF signal applied on the modulator and the time arrival of the pulse in the modulator. The electronic control circuit typically varies sequentially each optical time delay τ i to maximise the mean photodetected current. Only one sequence is necessary to find the optimum value corresponding to the synchronisation of every channel. Alternatively a dithering technique can be used by attributing a control frequency to each channel. As the number of WDM channels increases, however, the current variations decrease. Let us call ρ the linear extinction ratio of the modulator, for instance ρ=0,25 for 6 dB extinction ratio. In a single channel configuration, the photocurrent variation when varying the time shift is i max /i min =1/ρ. In a WDM configuration, the photocurrent variation for a given channel depends on the synchronisation of the other channels. In the worst case, i max /i min =1-1/N+1/(ρN). Numerically for 1, 2, 4 or 16 channels, the photocurrent variations are 6 dB, 4dB, 2.4dB or 0.75 dB successively. As such small variations may be difficult to detect, a WDM demultiplexer may be added to a first embodiment of the invention to control each channel independently, as shown below in figure 2 . Figure 2 contains the same single components as described in figure 1. Additionally to the basic solution in the third block an second demultiplexer 7 is integrated. This demultiplexer demultiplexes the tapped part of the remodulated signal. In each demultiplexed channel a single photodetector is built in. This structure comprising a demultiplexer demultiplexing several different wavelengths in different lines and a photodiode device in each line can be realized in AWG structure in an optical integrated module. It might be possible that the signal power of the tapped signal is too low for a analysis of the multiplexed siganls This is due to the fact that the modulator remodulates a signal which has the problem to contain time shifted signals and than only a part of the signals is coupled into the photodiode. The modulator arises insertion losses for the signals. To alleviate the optical modulator insertion loss concatenated with the coupler withdrawing just a small part of the WDM multiplex, a second embodiment is proposed. Figure 3 shows this embodiment. In this embodiment the block 3 functions follow the block 1 functions first and than the remodulating functions of block 2 are connected. The multiplexed signal A is tapped to a second delay line 9. After this second delay line a second modulator 8 is connected to the demultiplexer and the photodetectors 4. The main signal is fed into the modulator 3. The clock recovery circuit is driving both modulators 8 and 3 with a fixed delay 11 between them. A small part of the WDM multiplex is sampled just before the modulator 3. On this derivation the second modulator 8 is inserted between an optical delay line τ x , which delays the whole multiplex, and the WDM demultiplexer. The modulator is also controlled by the same RF clock provided by λ c . The RF time delay between the RF driving of the two modulators is fixed and λ c is synchronised with modulator 3 for example by construction. In another embodiment the modulator 3 is time synchronised with the modulator 8 also by adjusting the "out of line" optical delay line τ x . If the wavelength λ c controlling the RF clock signal is synchronously modulated on both modulators 3 and 8 then the out-of-line measure of τ x will be the reference value (as a zeroing functionality). All the other wavelengths will be slaved to this initial measurement, first by adjusting for each channel the optical delay τ x maximising the per-wavelength detected photocurrent, then measuring the difference versus the reference and only after applying this difference to the in-line per-wavelength dedicated optical delay line τ i Taken into account the "out-of-line" measurement technique functionality, it is possible to adjust independently each wavelength without degrading too much the transmission quality when varying largely the optical delay line. As depicted on figure 4, the photocurrent evolution measured on a photodiode placed after an Electro-absorption modulator reaches its maximum sensitivity when the pulse centre is delayed one quarter of the centre time bit apart. The photocurrent evolution is symmetric and it seems easy to determine the optimum point electronically. In this experiment the bit-rate is 10 Gbit/s and the pulse width 32 ps which is also representative for a 40 Gbit/s regenerated transmission. The controlling function of block 4 are known from prior art. The embodiments described herein are examples for solutions of the invention. The invention is not restricted to the described embodiments. Each component can be used that a person skilled in the art can use to obtain the desired result. The device for synchronisation of parallel data in a WDM data stream is not limited to a special modulation scheme. It works with RZ or NRZ modulated signals and can also be adapted to every other modulation scheme fitting for a WDM transmission. The synchronization device is also not limited for a use in a regeneration tool . It can also be used as a stand alone device. The synchronization device is used in a transmission system for WDM transmission. The device is than be used in combination with an amplification toll and a reshaping tool.

Claims: Method for adjusting time delays between modulated optical signals of at least two wavelength channels in an optical WDM transmission system comprising the following steps: Step 1 : Demultiplexing the signals of the wavelength channels, delaying the signals individually between wavelength channels and multiplexing the signals of the wavelength again to a finally multiplexed signal (A) ( Block 1 ), characterized by Step 2 : Remodulating the finally multiplexed signal (A) of step 1 with a clock of high frequency ( Block 2 ) Step 3 : Monitoring the remodulated signal with at least one low frequency photodetector unit ( Block 3 ) Step 4 : Measuring and analyzing the photocurrent of the photodetector, adjusting via an electronic circuit the time delays between the wavelength channels (Block 4) . Method for adjusting time delays between modulated optical signals of at least two wavelength channels in an optical WDM transmission system comprising the following steps: Step 1 : Demultiplexing the signals of the wavelength channels, delaying the signals individually between the wavelength channels and multiplexing the signals of the wavelength channels again to a finally multiplexed signal (Block 1) characterized by Step 2 : Tapping a part of the finally multiplexed signal (A) , modulating this tapped signal in a first modulator (8) with a clock of high frequency and monitoring the modulated tapped signal with at least one low frequency photodetector unit (Block 3) Step 3 : Remodulating the finally multiplexed signal (A) of step 1 in another modulator (3) with a clock signal of the same high frequency as of the first modulator (Block 2) Step 4 : Measuring and analyzing the photocurrent of the photodetector unit ( PD 4), adjusting via an electronic circuit ( 6 ) the time delays between the wavelength channels (Block 4). Device for synchronization of modulated optical signals transmitted in wavelength channels in an optical WDM transmission system, comprising the following parts : a demultiplexer (1) for demultiplexing the incoming optical signals of the wavelength channels, delay lines (2) for the individual wavelength channels, a multiplexer (10) for combining the optical signals of the wavelength channels providing a finally multiplexed signal (A), a modulator (3) modulating the finally multiplexed signal (A) by a high frequency clock signal (5), characterized in that at least one phototdetector ( 4 ) is tapped to the finally multiplexed signal (A) of the multiplexer (10) and is electronically connected to an electronic control circuit (6) which is connected to the individual delay lines (2) for an automatic adaption of the individual delay lines. Device according to claim 3 characterized in that a second demultiplexer (7) is tapped to the output signal of the multiplexer (10) connected to at least one photodetector (4) for each individual wavelength of the wavelength multiplex. Device according to claim 3, characterized in that a part of the wavelength multiplex signals (A) is tapped and feed in a second modulator (8) connected with at least one photodetector (4) and driven by the same clock signals as the modulator (3). Device according to claim 3, characterized in that the demultiplexer (7) and the photodetectors are integrated in an AWG optical component. Telecommunications regeneration device comprising a device for synchronization as claimed in claim 3. Telecommunication system with at least one transmitter, one receiver and transmission line with regeneration means in the line, the regeneration means comprising a device for synchronization as claimed in claim 3.

Inventors: BRINDEL PATRICK, DEVAUX FABRICE, JACQUINOT JEAN-CLAUDE, BRINDEL, PATRICK, DEVAUX, FABRICE, JACQUINOT, JEAN-CLAUDE
Applicants: CIT ALCATEL, ALCATEL
Application year: 2001
Main classification: H04L7
IPC codes: H04B10, H04J14, H04L7