EAM-based all-optical converter runs at 40 Gbit/s

02 Mar 2006

Researchers in France have developed an EAM-based converter that operates at 40 Gbit/s.

All-optical wavelength conversion - the transfer of data streams from one wavelength carrier to another - could play a central role in the management and routing of signals in next-generation optical networks. Importantly, say its advocates, it can help to decrease the number of optical-electrical-optical conversions.

Electroabsorption modulators (EAMs) operating in cross-absorption-modulation configuration perform wavelength conversion across a wide spectral range. Previously, however, they only worked efficiently at lower bit rates as their slow absorption recovery time led to intersymbol interference (ISI). Now, researchers in France have developed an EAM-based converter that operates at 40 Gbit/s with low power penalty.

The key development is red-shifting the filter that's applied to the converted signal, which reduces the impact of the limited EAM response time and improves conversion quality. The scientists, from Alcatel Thalès III-V Laboratory, France Telecom R&D and Centre National de la Recherche Scientifique, say that the red-shifted bandpass filter reduces the bit-error rate (BER) penalty at 40 Gbit/s by 4.7 dB compared with a centred filter.

"It is possible to remove the ISI by using a red-shifted filter," said France Telecom's Benoît Charbonnier. "After travelling through the modulator, the converted signal bits overlap with their nearest neighbours, leading to ISI. Light in the regions where this overlap occurs is blue-shifted by nonlinear effects occurring in the modulator. Shifting the filter towards lower frequencies attenuates the regions where the light has been blue-shifted, hence reducing the ISI."

He claims that the maximum bit rate that can be converted this way has been "pushed further towards 160 Gbit/s". The researchers demonstrated conversion of a 40 Gbit/s data channel with a 1.8 dB power penalty at a BER of 10^-9 and low switching energy.

"France Telecom's goal is to understand the implications at the network level of using wavelength converters, which have the capacity to regenerate the signals as well." said Charbonnier "This has direct impact on the architecture and design of future networks."