A study of active mode-locking of external cavity semiconductor lasers

This thesis describes an experimental and theoretical investigation of active mode-locking of semiconductor lasers. The lasers used in this project belong to the class of long wavelength, buried heterostructure devices. Integrated optical modulators used in this project were designed and fabricated at UCL. The waveguides were formed by titanium indiffusion into a lithium niobate substrate. The travelling-wave electrodes were devised as coplanar waveguides and asymmetric coplanar lines. Short optical pulse generation using composite cavity InGaAsP - LiNbO3 lasers was demonstrated. Three different configurations were tested and mode-locking by laser current modulation, AM and FM mode-locking were demonstrated. These structures were compared to bulk external cavities incorporating a plane mirror or a grating. The best performance was achieved from the external grating laser followed by a semiconductor optical amplifier. Optical pulses shorter than 10ps and having a peak power of 5mW in the fibre were generated. The theoretical work was based on the frequency-domain formulation of mode-locking of monolithic external cavity semiconductor lasers. The equations which describe mode-locking by current modulation, intracavity phase and amplitude modulation were derived. Numerical simulations were performed for gain-switching of a solitary laser and the equivalent monolithic external cavity laser. The frequency domain formulation of active mode-locking was tested numerically utilising twenty five modes and a parabolic loss profile. Mode-locking by current modulation was systematically investigated and AM, FM mode-locking and FM laser operation have been demonstrated.