In this work, an experimental study of the impact of the number of longitudinal modes on the linewidth of -40 GHz signals generated by various passively mode-locked multimode semiconductor lasers is addressed. Devices under test (DUT) are multi-quantum wells InAlGaAs FP laser, bulk DBR laser and quantum-dash FP laser. It is important to stress that we analyze the total number of modes oscillating into the laser cavity, without implementing an external optical processing. In our experiment, the output of each DUT is coupled into a single mode fiber (SMF) and measured both electrically and optically. Our experimental results show that, independently from the nature of the cavity and the active media, the FWHM-linewidth of the -40 GHz signal generated by these lasers is reduced as the number of longitudinal modes selected by the laser cavity is increased.