To date, lightwave systems’ transmitters have used direct current modulation of the injection laser source. At gigabit/sec modulation rates, spectral control of the laser becomes more difficult and power penalties caused by dispersion become important in long haul applications1. An alternative approach, which offers the potential for efficient modulation, narrow source linewidth, and flexibility in the choice of laser structure, is the use of an external high-speed modulator2. These devices have been made low-loss with low drive voltage and wide bandwidth for the 1.3μm wavelength3 and more recently have been optimized for the 1.55μm region4. Below we describe the first use of a Ti:LiNbO3 traveling-wave directional coupler switch as an external modulator for optical fiber transmission at 4Gb/s. Repeaterless transmission at 1.53μm wavelength over 117km of conventional single-mode fiber was achieved. This result represents the largest capacity x repeater spacing product, ~0.5Tb–km/s, reported for a single-channel optical fiber span.
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