Theory and modelling of wavelength tunable laser transmitters with enhanced tuning range and their modulation performance

The research that is described in detail in this thesis investigates key characteristics of the operation of Tunable Laser Diodes (TLDs), such as Continuous Wave (CW) operation, discontinuous, continuous and quasicontinuous wavelength tuning and direct Intensity Modulation (IM) (small-signal analysis). Two software simulation tools were used to model the TLDs and investigate their operation, Crosslight PICS3D and VPI (Virtual Photonics Incorporated). Two different Free-Carrier (FC) contributions to the refractive index change of the TLD during FC tuning were investigated, the FC plasma effect and the band-filling effect which uses the Kramers-Kronig (KK) relations (KK effect). It was found that the band-filling effect is heavily underestimated due to the lack of its investigation in published literature as it is the main contributor to the refractive index change instead of the plasma effect. Investigation on different types of wavelength tuning also took place. It was found that with careful design of the passive sections, such as the κL product, grating composition, section length and passive waveguide thickness the discontinuous, continuous and quasicontinuous tuning range can be enhanced greatly. The issue of output power decrease during discontinuous tuning in bulk and Multiple Quantum Well (MQW) TLDs was also addressed and it was found that the power drop can be delayed at latter stages of the tuning range by carefully selecting the Lorentzian lineshape of the gain spectrum. A power stabilisation was realised with continuous tuning. A small-signal analysis of directly intensity modulated TLDs during discontinuous tuning was also made and was found that the increase of the resonance frequency depends mainly on the increase of the differential gain with the wavelength change.