Researchers generate DWDM channels without a modelocked laser
A 40-channel DWDM transmitter based on a spectrum-sliced supercontinuum was recently reported by Luc Boivin, Chris Doerr, and others at Lucent Technologies Bell Labs (Holmdel, NJ) and Politecnico di Milano (Italy). The system is unusual because it does not use a modelocked laser as a seed source for the multiple channels.1 Instead, it uses a distributed-feedback (DFB) laser with an electroabsorption module on the same chip, and expands the spectrum using self-phase modulation in fibers.
"This approach gets away from the modelocked laser," Doerr explains. "The huge advantage is that with a modelocked laser it is hard to adjust the pulse rate to the bit rate you want—it requires stabilizing the path length. Semiconductor lasers are easier to modulate: they provide fewer timing problems." The DFB lasers are also smaller and cheaper: Doerr says that 2.5-Gbit/s electroabsorption-modulated lasers (EMLs) are commercially available from Lucent and other companies, and it is possible to get small numbers of 10-Gbit/s EMLs, such as the one used in this experiment.
Forty-channel DWDM transmitter uses an electroabsorption-modulation laser (EML) as the light source. Pulses are shortened and the spectrum broadened in two (dotted boxes) stages, each containing dispersion-shifted fiber (DSF), a filter, standard single-mode fiber (SMF), a polarization controller, and a polarizer. The signal is modulated with a signal from the pulse-pattern generator (PPG) via a lithium-niobate modulator (LN-MOD).
The 10-Gbit/s EML, with a 3-dB bandwidth of 8.3 GHz, was modulated by a 9.953-GHz (10-V amplitude) sine wave to provide a train of 21.5-ps pulses with a center wavelength of 1553.8 nm. Pulse width was narrowed in two stages (see figure).
Pulses from the EML were nonlinearly compressed to 6 ps by first amplifying them, then passing them through a dispersion-shifted fiber (DSF) and a 1.1-nm bandpass filter, and then linearly compressing them in a standard dispersion fiber. A second set of DSF, filter, and standard fiber shortened the pulses to 2.7 ps.
A lithium-niobate modulator encoded a pseudorandom pattern of 9.953-Gbit/s data onto the optical pulses. After amplifying the pulses again, they were launched into 4 km of DSF with a zero-dispersion wavelength of 1573 nm. Spectrum-broadening in the fiber, from self-phase modulation, formed the super-continuum. A WDM demultiplexer with a channel spacing of 50 GHz was used to slice 40 channels from the broad spectrum, with wavelengths ranging from 1546.2 to 1562 nm. Then delay lines were used to decorrelate the data patterns on the different channels. A WDM multiplexer recombined the channels, resulting in bandwidths of about 0.16 nm.
All the channels transmitted over 544 km without errors. Some channels, however, had better performance than others, and the signal-to-noise ratio is a problem in this system. Doerr explains that such a system isn't suited to high-performance ultralong-haul systems because in those cases a high-quality source is important. For lower-cost long-distance systems, such as metro-area networks, this approach may be attractive.
- L. Boivin et al., IEEE Phot. Tech. Lett. 12, 1695 (December 2000).
Yvonne Carts-Powell is a science and technology writer based in Belmont, MA.