1.28 Tbit/s–70-km OTDM Femtosecond-Pulse Transmission Using Third- and Fourth-Order Simultaneous Dispersion Compensation with a Phase Modulator Takashi Yamamoto, 1 Kohichi R. Tamura, 1 and Masataka Nakazawa 2 1 NTT Network Innovation Laboratories, NTT Corporation, Yokosuka, 239-0847 Japan 2 Research Institute of Electrical Communication, Tohoku University, Sendai, 980-8577 Japan SUMMARY By means of the higher-order dispersion compensa- tion technique using a phase modulator, a 70-km transmis- sion experiment with 1.28 Tbit/s OTDM (Optical Time Domain Multiplexing) signals has been successfully con- ducted for the first time. The details are reported in this paper. The 1.28 Tbit/s signal is obtained by 64-fold multi- plexing of 10 Gbit/s signals with additional polarization multiplexing. For the signal pulse with a pulse width of 380 fs prior to transmission, a linear chirp is imparted and then a cosine phase modulation with an appropriate amplitude and the timing is set in such a way that the imparted chirp is opposite to the chirps caused by the third- and fourth-or- der dispersion in the transmission path. In this way, the waveform distortion of the signal due to the third- and fourth-order dispersion can be suppressed simultaneously. The pulse widening of the signal after transmission over 70 km is only 20 fs. Hence, an error rate of less than 1 × 10 –9 can be realized over the entire channel (10 Gbit/s–128 channels after multiplexing separation). © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(3): 68–79, 2003; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/ecja.10028 Key words: terabit OTDM transmission; higher- order dispersion compensation; phase modulator; femto- second pulse. 1. Introduction With the rapid spread of the Internet, the demand for further increases in the speed of optical communications networks has been increasing. Transmission technologies for ultrafast OTDM (Optical Time Domain Multiplexing) are critically important for ultrahigh-speed optical commu- nications networks. To date, ultrafast OTDM transmission experiments at 40 Gbit/s–40 km [1], 320 Gbit/s–200 km [2], 640 Gbit/s–63 km [3], and 640 Gbit/s–101 km [4] have been reported. When ultrafast OTDM transmission of more than 1 Tbit/s is realized, the spacing between the signal pulses is less than 1 ps. Hence, it is necessary to use femtosecond pulses (1 fs = 1 × 10 –15 s). However, since femtosecond pulses have a broad optical spectrum, it is important to reduce waveform distortion due not only to the second-order dispersion of the transmission fiber but also to higher-order dispersion of more than third order [5–7]. The second- and third-order dispersions of the transmission fiber can be compensated simultaneously by a combination of appropriate lengths of fibers with different dispersion characteristics such as 1.3-μm zero-dispersion SMF (Sin- gle-Mode Fiber), RDF (Reverse Dispersion Fiber) [8], and DSF (Dispersion-Shifted Fiber) [4]. However, since the fourth-order dispersions (d 2 D/dλ 2 ) of these fibers have the same signs (negative), it is not possible to simultaneously compensate the second-, third-, and fourth-order disper- sions by a combination of these fibers. In contrast, Pelusi and colleagues [6] inserted a fiber with an appropriate © 2002 Wiley Periodicals, Inc. Electronics and Communications in Japan, Part 1, Vol. 86, No. 3, 2003 Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J84-B, No. 9, September 2001, pp. 1587–1597 68