Abstract

Regeneration is a key function for next generation optical networks. However, with the use of WDM transmission technology, an all-optical regenerator would ideally process several wavelengths simultaneously. In this paper, we present the design of a 40 Gbit/s 4 channel multi-wavelength regenerator based on quasi continuous filtering (QCF). We estimate the quality (Q) factor improvement and the distribution of the receiver voltage to characterize the regenerator. The QCF regenerator is constituted by several sections of highly non-linear fibre (FfNLF), separated by filtering elements. Similar to self phase modulation (SPM) regenerators [1], optical signals are spectrally broadened in the HNLF. However, the optical filters, each of which is shifted slightly from the previous filter, reduce the spectral width at the end of each section. For the zeros where the signal intensity is low, the spectral broadening is small and the noise is progressively attenuated by the succession of filters whereas for high intensity pulses, the SPM induced spectral broadening allows the pulse to pass through the loss minima of the filters and their amplitudes are levelled [2]. In our simulations, we successively optimised the different parameters of the regenerator, measuring the estimated quality factor at the output of the regenerator. Final design is constituted by 5 sections of 200 metres of HNLF (y=13 W'.km"¹; a = 0.6 dB.km"¹) separated by WDM optical filters with Gaussian shape and 200 GHz bandwidth. Each filter is offset from the previous one by 2 GHz. In our simulations, the input signal has an OSNR of 18 dB and is made of 4 channels with 600 GHz spacing. Each signal is modulated using different 512 bit random sequences. Optical power at the input of the device was optimized to 20 dBm per channel and in this paper, we assume zero insertion loss for the filters. In order to estimate the quality factor, we considered a receiver with an 80 GHz optical filter bandwidth followed by a photodiode and a 28 GHz electrical filter bandwidth. We obtained an improvement of the signal quality between 6.8 and 8.5 dB for the four channels, due to the concatenation of the transfer functions for each section. The electrical eye diagrams in back-to-back and after regeneration for the channel with the smallest quality factor improvement are shown in Figure 1. The increased timing jitter observed after the regenerator is primarily due to the interaction of the four channels in the non-linear fibre. However, the amplitude fluctuation of the "ones" is greatly reduced. In order to confirm the amplitude restoration characteristics of the device, we estimated the electrical power density histograms for zeros and ones at the centre of the time bit slot. We considered a single channel and performed this simulation using 2¹⁵ bits for two different OSNRs. Fig. 1: 40 Gbit/s electrical eye diagram in back-to-back (left) and after the WDM regenerator (centre) of channel 3 with input OSNR of 18 dB. Probability distribution of the receiver voltage in back-to-back and after single channel regeneration with an OSNR of 12 and 18 dB. As can be seen on figure 1, the distribution of the receiver voltage is deeply modified, with a substantial reduction in the one level distribution as expected, however the zeros are not regenerated and the Gaussian fit is not conserved after regeneration, resulting in a change in the optimum decision threshold. We also considered an OSNR of 12 dB where the eye is totally closed giving an error floor, as shown by the continuous distribution of the receiver voltage in figure 1 (right). This last simulation shows that we still obtain a reduction of the distribution of the ones but the error floor, indicated by the probability for which the crossing occurs, remains the same. This means that the regenerator does not improve the bit-error-rate but only reshapes the optical signal, limiting the accumulation of errors. In conclusion, we have demonstrated the feasibility of a 4 x 40 Gbit/s multi-wavelength optical 2R regenerator using QCF. The regenerator offers a Q factor improvement of over 6.8dB, enabling pulse reshaping in an all optical WDM network. This work has been supported by the European Commission under TRIUMPH project IST-027638 STP.

© 2007 IEEE