  We report on the characterization of the timing stability of passively
mode-locked discrete mode diode laser sources. These are edge-emitting devices
with a spatially varying refractive index profile for spectral filtering. Two
devices with a mode-locking frequency of 100 GHz are characterized. The first
device is designed to support a comb of six modes and generates near Fourier
limited 1.9 ps pulses. The second supports four primary modes resulting in a
sinusoidal modulation of the optical intensity. Using a cross-correlation
technique, we measured a 20 fs pulse to pulse timing jitter for the first
device, while, for the second device, a mode-beating (RF) linewidth of 1 MHz
was measured using heterodyne mixing in a semiconductor optical amplifier.
Comparison of these results with those obtained for an equivalent Fabry-Perot
laser indicates that the spectral filtering mechanism employed does not
adversely affect the timing properties of these passively mode-locked devices.
