SUMMARYTo understand the fluctuations in laser output pulses, information about the longitudinal linewidth forming the pulses and the relative intensity noise (RIN) is very important. In this paper, we conducted detailed measurements of the linewidth and RIN in the oscillating longitudinal mode of a mode-locked laser diode (MLLD) having different fabrication methods for the repetition frequencies of 10 and 40 GHz. The results are a longitudinal linewidth of several megahertz to gigahertz, a constant linewidth exhibiting the longitudinal mode independence, and a larger RIN as the distance from the center of the oscillation spectrum increases.