The transient spectral spread of directly modulated DFB LD's, which appears in the time-resolved chirping measurement, is studied experimentally and numerically. Such a phenomenon has been already reported as a side mode oscillation called "subpeak", but there has been little argument as to the physical origin. We make it clear that the subpeak is a spurious mode due to the influence of the photodetector bandwidth. The minimum photodetector bandwidth which is necessary in the time-resolved chirping measurement is examined. Furthermore the distortion of the long-distance transmitted waveform is also explained by one mode oscillation.

The mode partition noise of 1.3µm distributed feedback laser diodes (DFB LD's), which is induced by the externally reflected light, is studied experimentally and numerically. The mode partition noise is evaluated by the k-value. It is observed that the mode parition noise monotonically increases with the DC bias current when the reflected light affects DFB LD's and the DC bias current is above the threshold current. From the dependence of the k-value on the external power reflection coefficient, it is observed that the k-value dramatically increases when the external power reflection coefficient is above a value which differs from LD to LD. This is closely related to the fact that the tolerance to the externally reflected light depends on the threshold gain difference between the main mode and the dominant side mode.