Future optical time-division multiplexing systems may need to reduce the data rate to slower speeds for electronic processing at 10 or 40 Gbit/s. To convert the data, researchers have typically used the nonlinear optical loop mirror that allows switching due to cross-phase modulation in a fiber amplifier, or used four-wave mixing in an amplifier. Both these schemes, however, require that the switch window for the demultiplexed channel be longer than the control pulse that initiates the nonlinear process—therefore, the control pulse must be very short and of high quality.
FIGURE 1. Data at 10 Gbit/s is modulated onto a beam from an erbium-doped fiber ring laser, and then the data is time-domain-multiplexed to provide 80 Gbit/s in 8 channels. A control pulse, generated by a tunable external-cavity laser and followed by an electroabsorption modulator, is amplified and coupled to the data. The combined signal is amplified and directed into a dispersion-shifted fiber in which nonlinear phase modulation occurs, broadening the spectrum of one channel. A narrow bandpass filter allows the broadened channel to pass, thus extracting it from the rest of the multiplexed data.
Bengt-Erik Olsson and Daniel Blumenthal of the University of California-Santa Barbara have demonstrated a new 1550-nm waveband demultiplexer in which the control pulse can be longer than the bit-slot of the incoming data.1 They demultiplexed 80-Gbit/s data to 10 Gbit/s with a bit-error rate of less than 10-12. The method they used might be useful for data rates up to or above 160 Gbit/s.
An actively modelocked fiber ring laser generated 6-ps pulses at a 1538.6-nm wavelength with a 10-GHz rep rate (see Fig. 1). An external modulator encoded PRBS data onto the output, while a passive optical interleaver multiplexer generated the 80-Gbit/s data stream. The control pulses were generated using an electroabsorption modulator that output 14-ps pulses at 1534 nm. The 80-Gbit/s data and the control pulses were combined and amplified in an erbium-doped fiber amplifier to +18-dBm average output power. A 5-km-long dispersion-shifted fiber with zero-dispersion wavelength of 1543 nm induced cross-phase modulation from the control pulses onto one of the data channels.
The demultiplexer uses the time-derivative effect of cross-phase-modulation-induced spectral broadening. The control pulse only overlaps one channel out of eight and thus only that channel experiences the high power in the control pulse. Self-phase modulation causes the spectral components associated with that particular channel to broaden (see Fig. 2). The leading edge of the control pulse generates a red shift in the spectrum of the cross-phase modulated input signal, while the trailing edge of the control pulse generates a blue shift. "Since the spectrum maintains its shape in the other channels, it is now possible to extract the desired channel by an optical bandpass filter slightly offset from the original center wavelength," explains Olsson. A narrow 0.2-nm bandpass filter extracts the desired channel.
For more information contact Bengt-Erik Olsson at firstname.lastname@example.org.
- B.-E. Olsson and D. Blumenthal, IEEE Photon. Tech. Lett. 13, 875 (August 2001).