An experimental analysis on the quality of 40-GHz radio-frequency signals generated by various passively mode-locked semiconductor lasers is addressed. The analysis is performed considering the frequency linewidth of 40-GHz optically generated signals and the number of longitudinal modes selected by the cavity of each laser under study. Four of these devices are multi-quantum wells InAlGaAs Fabry-Pérot lasers. They have been engineered to exhibit a specific number of longitudinal modes: 4, 5, 10, and 22 modes, respectively. Another device under test is a bulk distributed Bragg reflector laser exhibiting 3 lasing modes. The last device under study is a quantum-dash Fabry-Pérot laser characterized by 40 lasing modes. From our experimental results it appears that, regardless the nature of the device cavity and active media, the strength of the passively mode-locked mechanism might be enhanced with the number of longitudinal modes oscillating into the laser cavity, allowing a reduction in the frequency linewidth.