  It is shown that optical synthesis of terahertz and millimeter-wave
frequencies can be achieved using two-mode and mode-locked discrete mode diode
lasers. These edge-emitting devices incorporate a spatially varying refractive
index profile which is designed according to the spectral output desired of the
laser. We first demonstrate a device which supports two primary modes
simultaneously with high spectral purity. In this case sinusoidal modulation of
the optical intensity at terahertz frequencies can be obtained. Cross
saturation of the material gain in quantum well lasers prevents simultaneous
lasing of two modes with spacings in the millimeter-wave region. We show
finally that by mode-locking of devices that are designed to support a minimal
set of four primary modes, we obtain a sinusoidal modulation of the optical
intensity in this frequency region.
