Recent progress toward multiterabit/s optical transmission systems employing all-optical ultrafast signal-processing technologies is described. Focus is placed on optical time-division multiplexing (OTDM), as well as optical time-and wavelength-division multiplexing (OTDM/WDM) technologies leading to multiterabit/s transmission capacity. The key technologies, including ultrafast pulse generation, all-optical multiplexing/demultiplexing, and optical timing extraction techniques, are also described, together with state-of-the-art performances and future prospects.Index Terms-All-optical multiplexing/demultiplexing (MUX/ DEMUX), all-optical signal processing, optical pulse generation, optical time-division multiplexing (OTDM), optical timing extraction, supercontinuum generation, terabit transmission. Mode-locking of LDs[4]-[10] is another way of producing high-repetition-rate ultrashort optical pulses. So far, a new type of LD capable of colliding-pulse mode-locking (CPM) has been demonstrated [4]. This can output ultra-high-speed, purely TL pulses with less than 1-ps duration at 40 and 350 GHz repetition frequency through active and passive CPM operation, respectively. At present, however, there is no way to utilize this extremely high operation speed. In order to mode-lock at moderate speed, a long-cavity LD integrated with a passive waveguide and a Bragg reflector was developed [5]. The cavity length is monolithically extended to 5.5 mm, leading to 8-GHz modelocking operation. This was used for 8-Gb/s 4000-km solitontransmission experiments. The main drawback of this method is no tunability of repetition frequency and relatively larger spectral width as compared with the TL condition.To improve the operation characteristics, various mode-locked LDs have been reported, including pulse-width tunable subpicosecond pulse generation from an active mode-locked monolithic multiquantum well (MQW) LDs integrated with MQW-EA modulators [6], repetition-rate tunable lasers using passive mode-locking of micromechanically-tunable LDs [7], or active mode-locking of external cavity LDs [8], and very high-speed or femtosecond optical-pulse generation from an active mode-locked LD [9], [10].Harmonic mode-locking of EDF lasers [11]-[17] is promising because it can generate a purely TL-pulse train in the 10-GHz region without requiring any chirp compensation or pulse compression. However, since EDF lasers are normally tens of meters long, they have to be mode-locked at very high harmonics of the cavity mode spacing, for example, harmonic orders of the order of 1000. This leads to the issue of how to stabilize such laser systems in which the competition between many sets of supermodes is apt to occur. Moreover, mode-competition is enhanced by fluctuations in cavity lengths and polarization states. To stabilize mode-locked EDF lasers, several techniques have been developed. The first method is inserting a high-finesse Fabry-Perot etalon in the cavity to eliminate all the unwanted laser-cavity modes [12]. However, this straightforward...

Promising results of various coupling experiments between laser diodes and single-mode fibers to determine the optimum coupling method are reported. The cylindrical lens method is shown to be most efficient, with a coupling loss of 1.8 dB under optimum alignment conditions. More than -2 dB coupling efficiency is attained with a cylindrical lens whose radius is less than 8 microm. The laser to fiber coupling characteristics are estimated by Gaussian beam approximation. The optimum radius of a cylindrical lens, which gives maximum coupling efficiency, is derived by theoretical calculation, using the ray matrix method. The cylindrical lens alignment tolerance is also shown theoretically and experimentally.

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