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. In this seminar I will present a laser for which no direct or external modulation is applied, and neither is a saturable absorber required, in order to achieve the phase synchronisation. 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) domains. Radio frequency signals generated by semiconductor lasers have many applications in optical communications, such as radio-over fibre, or all-optical clock extraction where the entire signal processing at ultra-high speed is carried out in the optical domain. Terahertz signals are the focus of many research bodies due to their potential applications in areas like: industry, pharmacy, security (military), telecommunication and medicine. With continuous improvement of materials processing and technology, new ways of generating and detecting THz radiation 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.