Demultiplexing using fiber cross-phase modulation
An optical time-domain demultiplexer based on an unusual design demonstrated demultiplexing an 80-Gbit/s signal to 10 Gbit/s using fiber cross-phase modulation and filtering. Future high-bit-rate systems are likely to use optical time-domain multiplexing (OTDM), in addition to WDM, to increase throughput. In OTDM systems, optical demultiplexers convert the signals to lower bit rates, before conversion to electronics, to accommodate the limits at which electronic circuits work. Most devices proposed for demultiplexing OTDM data are based on a nonlinear optical-loop mirror that allows switching due to cross-phase modulation (or four-wave mixing) in either a fiber or a semiconductor optical amplifier.
The drawback of these methods is that the switch window for the demultiplexed OTDM channel usually cannot be any shorter than the width of the control pulse that initiates the nonlinear process. Therefore, a very short, high-quality optical pulse is usually required.
At the IEEE LEOS Annual Meeting in Rio Grande, Puerto Rico (Nov. 13-16, 2000), Bengt-Erik Olsson and Daniel Blumenthal of the University of California (Santa Barbara) reported a different technology that evades the requirement for short, high-quality pulses. With this technique, the control pulse can be broader than the switch window.
The researchers based the demultiplexer on a previous development, in which they used fiber cross-phase modulation to develop a new type of wavelength-converter technology. Cross-phase modulation in the fiber broadens the signal spectrum. The experimental device broadened one channel of the OTDM data and then extracted that channel using a narrow-bandpass filter to one side of the original spectrum.
Bits or packets that have not been broadened do not pass the filter, as the broadened data does; thus, only one edge of the control pulse governs the width of the demultiplexing switch window, so a control pulse broader than the actual bit-slot can be used.
The researchers used an actively modelocked fiber laser to generate an 80-Gbit/s OTDM signal, then demultiplexed one channel using cross-phase modulation in a fiber, followed by a narrow-bandpass filter. The channel was demultiplexed to 10 Gbit/s with less than 2 dB receiver penalty, using a control pulse of 14 ps. (For comparison, the bit-slot is 12.5 ps at 80 Gbit/s.)
Unlike alternative methods, the demultiplexer is not an interferometer, so it is insensitive to environmental disturbances. "We are currently looking into the possibility of simultaneously demultiplexing all channels in an OTDM data stream," Olsson said. Without such a system, each channel would need a demultiplexer. For more information, contact Bengt-Erik Olsson at firstname.lastname@example.org.