Enhanced Modulation Dynamic Performance of Optically-Injected Widely-Tunable Semiconductor Lasers

This dissertation is devoted to a comprehensive theoretical and modelling study of dynamic modulation characteristics of semiconductor wide-wavelength tunable laser diodes (TLDs). The two major goals were to investigate how modulation properties of a TLD depend on the wavelength tuning, and how the modulation performance of a TLD can be enhanced in terms of the achievable speed and improved frequency chirping behaviour under the direct modulation regimes using external light for optical injection-locking (OIL). It is demonstrated that modulation performance of free running (FR) widely tunable lasers strongly depends on the tuned lasing wavelength. The relaxation oscillation frequency (ROF) of FR TLD increases from 2.2 GHz to 5.5 GHz with tuning. The main results of investigation of modulation dynamics of OIL TLDs include demonstration of substantial (up to an order of magnitude) increase of the ROF and the modulation bandwidth in comparison with the FR regime and investigation of dependence of ROF on the wavelength tuning. The ROF increases to 24 GHz. We prove that the ROF of the OIL TLD is defined by the difference between the injected master laser’s light frequency and the cavity shifted mode frequency. The latter non-lasing mode has been identified as corresponding to the amplified spontaneous emission and was clearly reproduced in CW spectra of a steady-state OIL TLD. This finding has important practical implications as it allows to directly relating the CW lasing spectra fine features with dynamic performance of the OIL TLDs. Important results were obtained for case of large-signal modulation of the OIL TLD and for a large frequency detuning for side-mode optical injection regime when the master laser’s light is injected near the side-mode of FR TLD with large SMSR. Direct large-signal modulation of the OIL TLD using pseudo-random bit sequence shows superior performance in terms of enhanced modulation speed.