We have proposed and demonstrated a novel optical regenerator architecture employing electroabsorption modulators as wavelength converters. The employment of EA modulators is advantageous for high-speed operation and flexibility in the bit-rate for the pulse regeneration. In addition, the EA modulator-wavelength-converter acts also as a photo diode for clock extraction. Compensation of the optical SNR and Q-factor has been demonstrated, even in cascaded noise load. Furthermore, against dispersion loading, we have confirmed that waveform recovery and Q-factor improvement is obtained by midway insertion of the optical regenerator. The proposed architecture will offer a wide-band-electronics-free optical regenerator in multi-tens of gigabit per second WDM networks.

Wavelength conversion is considered to be one of the key technologies for the future advanced optical networks. So far, intense efforts have been made to realize this functionality by using semiconductor lasers/amplifiers, optical fibers, LiNbO3/semiconductor waveguides. In this paper, we propose a novel wavelength converter using an electroabsorption (EA) modulator. The operating principle is based on the cross-absorption modulation effect of an InGaAsP EA modulator. High quality conversion has been demonstrated at 20 Gbit/s over 30 nm of wavelength range. The applicability to high speed signals (>40 Gbit/s) was also demonstrated. In addition, to study the expected versatility for all optical signal processing applications, broadcasting and extinction-ratio-improvement capability were evaluated.

This paper describes the wavelength division multiplexing technology for the long-haul optical communication system, especially for the undersea cable system. At first, the present WDM technology for the undersea cable system is reviewed briefly. After that, some experiments using compensation of the dispersion slope of the transmission fiber are discussed as future technical options of undersea systems with over 100 Gbit/s capacity.

The effectiveness of Aeff enlarged positive dispersion fiber (EE-PDF) and hybrid amplification configuration with erbium-doped fiber amplifier (EDFA) and fiber Raman amplifier for reducing the fiber nonlinearity and improving the transmission performance in long distance 40 Gbit/s-based WDM transmission was investigated. We have confirmed that the use of EE-PDF in modified dispersion map for 40 Gbit/s transmission is quite effective to increase the transmissible distance and have successfully demonstrated 16 40 Gbit/s WDM transmission over 2000 km with proper dispersion management. We have also confirmed that the use of distributed Raman amplification is quite effective to extend the repeater spacing. By adding the optimum Raman amplification, almost the same transmission performance was obtained with a doubled repeater spacing in long distance 40 Gbit/s-based WDM transmission.

Feasibility of 20 Gbit/s single channel transoceanic soliton transmission systems with a simple EDFA repeaters configuration has been studied. Both a simple and versatile soliton pulse generator and a polarization insensitive optical demultiplexer, which can provide a almost square shape optical gate with duration of full bit time period, have been proposed and demonstrated by using sinusoidally modulated electroabsorption modulators. The optical time-division multiplexing/demultiplexing scheme using the optical demultiplexer results in drastic improvement of bit error rate characteristics. We have experimentally confirmed that the use of alternating-amplitude solitons is an efficient way to mitigate not only soliton-soliton interaction but also Gordon-Haus timing jitter constraints in multi-ten Gbit/s soliton transmission. Timing jitter reduction using relatively wide band optical filter bas been investigated in 20 Gbit/s loop experiments and single-carrier, single-polarization 20 Gbit/s soliton data transmission over 11500 km with bit error rate of below 10-9 has been experimentally demonstrated, using the modulator-based soliton source, the optical demultiplexer, the alternation-amplitude solitons, and wide-band optical filters. Obtained 230 Tbit/skm transmission capacity shows the feasibility of 20 Gbit/s single channel soliton transoceanic systems using fully practical technologies.

We numerically and experimentally investigated the upgradability of the longest and the typical segments of the JIH system. Through these studies, we confirmed that a 100 GHz-spaced 25 42.7 Gbit/s transmission with the total capacity of 1 Tbit/s can be attainable even by using NRZ signal and standard FEC for the typical segments. We also found that RZ signal format was desirable for the longest segment and a further wide system margin could be expected by using adjacent channel polarization control and advanced FEC technologies.

We have proposed and demonstrated a novel optical regenerator architecture employing electroabsorption modulators as wavelength converters. The employment of EA modulators is advantageous for high-speed operation and flexibility in the bit-rate for the pulse regeneration. In addition, the EA modulator-wavelength-converter acts also as a photo diode for clock extraction. Compensation of the optical SNR and Q-factor has been demonstrated, even in cascaded noise load. Furthermore, against dispersion loading, we have confirmed that waveform recovery and Q-factor improvement is obtained by midway insertion of the optical regenerator. The proposed architecture will offer a wide-band-electronics-free optical regenerator in multi-tens of gigabit per second WDM networks.

The effectiveness of periodic dispersion compensation on single-channel 40 Gbit/s soliton transmission system was experimentally investigated. This technique requires just the dispersion compensation fibers and wideband optical filters in the transmission line, which has no difficulty to be used in the practical system. By using polarization-division-multiplexing together with periodic dispersion compensation, single-channel 40 Gbit/s transmission over 4700 km was demonstrated. Single-polarization 40 Gbit/s transmission experiments, which are more suitable for system implementation and compatible with WDM were also conducted. We investigated the transmission characteristics and pulse dynamics in different dispersion maps and in the optimized dispersion map, single-channel, single-polarization 40 Gbit/s transmission over 6300 km was successfully demonstrated.