A monolithically-integrated semiconductor optical transmitter that can index tune to any transmission wavelength in a given range, wherein the range is larger than that achievable by the maximum refractive index tuning allowed by the semiconductor material itself (i.e. .DELTA..lamda./.lamda.>.DELTA.n/n). In practice, this tuning range is >15 nm. The transmitter includes a Mach-Zehnder (MZ) modulator monolithically integrated with a widely tunable laser and a semiconductor optical amplifier (SOA). By using an interferometric modulation, the transmitter can dynamically control the chirp in the resulting modulated signal over the wide tuning range of the laser.