A multi DFB laser diode for generating spontaneous pulses comprises first and
second
DFB sections each of which has a substrate including a diffraction grating, an
active layer formed on the substrate, a clad layer formed on the active layer and
including a refraction varying layer, and an electrode formed on the active layer;
and a phase tuning section including a substrate, an active layer formed on the
substrate, a clad layer formed on the active layer, and an electrode isolated from
the electrode of the first and second DFB sections. The refraction varying layer
in the active layer of the first DFB section has a refractive index different from
that of the refraction varying layer in the active layer of the second DFB section.