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2. Terabit/s OTDM transmission using femtosecond pulse train |
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Time Division Multiplexing (TDM) versus Wavelength Division Multiplexing (WDM) Ultrahigh-speed photonic networks capable of accommodating the increase in Internet data traffic will form the infrastructure of the information society of the next generation. There are two types of multiplexing schemes to accommodate such large amount of information: wavelength division multiplexing (WDM), which multiplexes signals using lightwaves with different wavelengths, and time division multiplexing (TDM), which multiplexes signals in different bit slots in the time domain. In WDM systems, transmitters and receivers in each channel work independently, and thus WDM allows signals with different format to be accommodated in one network. In this sense, WDM is an "analog" multiplexing scheme. In constrast, TDM requires sophisticated signal processing employing, for example, multiplexers, demultiplexers, clock recovery, and network synchronization. Nevertheless it supports "digital" multiplexing, where synchronized high speed signals are processed together. Optical TDM (OTDM) makes the most of these advantages in the optical domain and is another important technique for the construction of photonic networks in addition to the development of highspeed signal processing.
Fig. 2 Optical transmission systems and signal pulse interval. Figure 2 shows the improvement in the TDM transmission speed in backbone terrestial optical transmission systems in Japan. The transmission speed has increased from 400 Mbit/s to 2.4 Gbit/s and 10 Gbit/s. With the help of WDM, the capacity can be increased further. Work on a 40 Gbit/s system is currently in progress and it will be installed in the backbone system in the near future. This system benefits from the development of high speed electronic devices. The next research target is ultrahigh-speed OTDM transmission with a bit rate of 160 Gbit/s or even 1 Tbit/s, where highspeed signals are multiplexed in the optical domain alone, without the need for any electronic devices. OTDM transmission operates in a regime far beyond the capability of electronic devices. In this regime ultra short pulses are transmitted with pulse widths of pico second to a few hundred femto second order. This would be impossible without the development of advanced technologies such as the generation of femto-second pulses, higher-order dispersion compensation, and all-optical demultiplexers.
Fig. 3 Experimental setup for 1.28 Tbit/s OTDM signal transmission. Figure 3 shows our setup for a 1.28 Tbit/s-70 km OTDM transmission experiment, which was successfully achieved for the first time in the world. A 3 ps, 10 GHz regeneratively and harmonically mode-locked fiber laser operating at 1.544 mm was used as the original pulse source. The output laser pulse was intensity-modulated at 10 Gbit/s and the pulse train was coupled into a dispersion-flattened dispersion decreasing fiber. This realized adiabatic soliton compression to less than 200 fs. We incorporated a phase modulation technique that compensated for the third- and fourth-order dispersion of the transmission fiber. The pre-chirped 10 GHz pulse train was optically multiplexed to 640 Gbit/s by using a planar lightwave circuit (PLC). We obtained a 1.28 Tbit/s signal by polarization multiplexing two 640 Gbit/s pulse trains.
Fig. 4 Optical signal waveform in 1.28 Tbit/s OTDM signal transmission. Fugure 4 shows the input and output data patterns. Clean 640 Gbit/s signals were obtained in each channel. The pulse broadening after 70 km transmission was only 20 fs. We obtained a bit error rate of 10-9 was achieved for all the channels. |
