There are several different approaches to generating periodic signals using semiconductor lasers, for example: Q-switching, gain switching or mode-locking schemes. In general the active or passive mode-locking techniques require the use of a modulator or a saturable absorber in order to achieve the phase synchronisation. The laser diodes studied in this thesis, are demonstrated to operate in the mode-locked regime, while not requiring any direct or external modulation, nor the saturable absorbtion element in order to achieve the phase synchronisation. It has been demonstrated previously, that in a multimode semiconductor laser, the third order nonlinearities of a gain medium resulting in the four-wave-mixing effects, are responsible for the phase synchronisation and lead to phase locking. The repetition rate of the generated signal is fixed by the free-spectral range of the longitudinal spectrum. Therefore, with a passively mode-locked laser (PMLL) it is possible to cover a wide range of frequencies from the Radio-Frequency (RF) to the TeraHertz (THz) domain. Radio frequency signals generated by semiconductor lasers have many applications in optical communications, such as radio-over-fibre, or all-optical clock extraction. Terahertz signals are the focus of many research bodies nowadays, due to their interaction with matter. They have potential applications in areas like: industry, pharmacy, security (military), telecommunication and medicine. With continuous improvement of materials processing and technology, new ways of generation and detection of such types of signals have appeared. The key advantage of the optical RF or THz generation is that this type of device is direct current biased and operates at room temperature. In this thesis, a comprehensive study of various PMLLs, from distributed Bragg reflector bulk laser to quantum dashed Fabry-Perot lasers is given, demonstrating the origin of the phase synchronisation in these structures and some applications for these lasers such as all-optical clock recovery or THz signal generation.