Stolarz, Piotr Michal
Development of a phase-sensitive pulse measurement technique for semiconductor mode-locked lasers.
PhD thesis, University of Glasgow.
Full text available as:
The ultrashort pulses emitted by passive semiconductor mode-locked lasers (PSMLLs) can be applied to a wide range of applications, including modern optical communication systems, optical sampling, security, imaging or sensing. For most of these applications, it is of critical importance to gather detailed information on the mode-locked laser (MLL) dynamics as well as on the temporal intensity and phase profiles of the pulses. The pulse formation in a PSMLL is in fact a very complex mechanism that is governed by the close interplay between a number of linear and nonlinear phenomena, influenced by various semiconductor parameters. The complete characterisation of the devices as a function of the laser driving parameters, geometry and semiconductor material structure has therefore the potential to provide a deeper understanding of the PSMLL behaviour. As the available detectors are usually incapable of resolving the temporal structures of ultrashort pulses from the high repetition rate MLLs, a number of indirect measurement solutions have been developed for full pulse characterisation. However, these methods are designed for lasers with high-energy optical pulses or require pulse synchronisation or ultrafast modulation. This obviously restricts their suitability for the unsynchronised, low energy and high repetition rate pulses as those emitted by the mode-locked laser diodes. In this work, an extensive study of various dynamical regimes, such as mode-locking, self-pulsation and continuous-wave operations of the monolithically integrated AlGaInAs/InP MLLs is reported. The devices operate around 1.55 µm and emit optical pulses with sub-40 GHz repetition frequencies. The influence of the biasing conditions, laser geometry and semiconductor material on the lasers performance is analysed in detail. The complete characterisation includes the evaluation of both the phase and time profiles of pulses, using a sonogram system developed as part of this thesis. It is based on a self-referenced method, capable of ambiguity-free measurements of low power and sub-picosecond pulses. A sensitivity as low as 5mW on the pulse peak power has been achieved through the design and fabrication of a two-photon absorption (TPA) detector, optimised for polarisation insensitivity and high nonlinear response. The travelling-wave operation enables the characterisation of high-repetition rate pulses and minimises the amount of introduced dispersion. The sonogram system has been successfully employed to study the evolution of the temporal intensity and group delay profiles as a function of the laser biasing conditions and for different device geometries. The obtained results indicate a prevailing positive chirp present in the pulses, which can be reduced by a careful adjustment of the device biasing. The minimum pulse width emitted from the investigated MLLs and measured with the sonogram technique was ~500 fs.
Actions (login required)