An InGaAs/InAlAs MQW modulator with the low voltage of 1.5V for 10dB extinction ratio and 16GHz bandwidth has been developed. This ultrahigh-speed modulator enables the modulator driver to be eliminated from the transmitter. 100 km transmission experiments have been carried out using either a 1V peak to peak output monolithic-IC-driven modulator at 15Gbit/s or a 2V peak to peak output multiplexer-driven modulator at 20Gbit/s. This is the first report on multigigabit operation of MQW modulators to the authors' knowledge.