The dependence of mode partition characteristics of 1.3 µm Fabry-Perot laser diodes on the modulation frequency and the modulation code has been investigated. From measurements with the frequency domain method and a simple analysis, it is confirmed that the modulation frequency dependence of k-value is caused by a competition among longitudinal modes which occurs with a time constant of several nano seconds. The mode partition characteristics in NRZ code is more affected by this mode competition than that in RZ code. Nevertheless, the k-value in NRZ code at modulation frequency fb is about the same as that in RZ code at modulation frequency fb/2. A simple formula which gives the k-value as a function of the modulation frequency is derived.

The effect of the externally reflected light on the mode partition characteristics of 1.3 µm Fabry-Perot laser diodes is studied experimentally and numerically. It is observed that the k-value increases monotonically with the DC bias current and the external reflection coefficient. Based on these experimental results, a numerical model to study the mode partition characteristics of laser diodes in the presence of external reflections is developed. The results calculated using this model agree well with the experimental ones. It is found that the mode partition noise is mainly caused by the interference between the light in the laser diode and the reflected light, and also by the fluctuations of the induced emission and absorption. In the time domain, their contribution to the mode partition noise is almost localized in the time region within 0.1nsec at the time when the optical pulse turns on.