ABSTRACT

In the most general sense, mode locking (ML) is a regime of laser operation that involves emitting light in several modes with a time-independent relation between them, i.e., with constant and precisely equidistant frequencies. Most mode-locked semiconductor lasers operate on fundamental interband transitions. Semiconductors thus have both a higher gain per unit length and a higher nonlinear refractive index than other gain media. The main advantages of ML over other methods of generating ultrafast pulses by laser diodes are the higher repetition rate pulses and shorter pulse durations. Purely active ML in a semiconductor laser can be achieved by direct modulation of the gain section current with a frequency very close to the pulse repetition frequency in the cavity or to a subharmonic of this frequency. In practical terms, the saturable absorber can be monolithically integrated into a semiconductor laser by electrically isolating one section of the device.