Mode-Locked Lasers

Mode-locking of lasers is a means of generating very short pulses, typically on the order of picoseconds or femtoseconds, by enforcing a fixed phase relationship between the many modes of the laser cavity. The laser then only emits when all these modes interfere constructively, resulting in a regular train of ultra-short pulses. Applications for mode-locked lasers include telecommunications (signal encoding), exploration of fast chemical and physical processes, optical data storage, production of THz radiation, and the generation and extraction of clock signals.

There are several different methods for controlling the phase relation between the modes, using either active elements such as acousto-optic or electro-optic modulators, or passive elements such as saturable absorbers. The arrangement studied here (see Fig. 6.1) consists of a ridge waveguide laser with the ridge divided into two sections. This allows one section to be forward biased as normal and thus act as the gain section, while the second section can be reverse biased so that it behaves as an absorber. Careful control of the bias voltages determines the saturation level of the absorber section, and can induce mode-locking. This approach has the advantage that it is very compact, relatively inexpensive, and easily integrated with existing technologies. Fig. 6.2 shows a typical map of the behaviour of the devices as the gain and absorber section voltages are varied. Above threshold, different regions of mode-locking are observed, with different pulse repetition rates in each region.

Two Section MLL

Fig. 6.1: Two-section monolithic mode-locked laser with InAs/GaAs quantum dot active layer.
ML map

Fig. 6.2: Map of the mode-locking behaviour as the gain and absorber section voltages are varied, showing the different mode-locking regions.