1 Nanophotonic Devices, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Nanophotonics Theory and Signal Processing, Department of Photonics Engineering, Technical University of Denmark
This thesis deals with the fabrication and characterisation of monolithic semiconductor mode-locked lasers for use in optical communication systems. Other foreseeable applications may be as sources in microwave photonics and optical sampling. The thesis also deals with the design and fabrication of intracavity monolithically integrated filters. The common dnominator among the diffrent parts of the thesis is how to achieve and measure the lowest possible noise. Achieving low noise has been pinpointed as one of the most important and difficult challenges for semiconductor mode-locked lasers. The main result of this thesis are a fabrication process of a monolithic and deeply etched distributed Bragg reflector and a characterisation system for measurement of quantum limitid timing noise at high repetition rates. The Bragg reflector is a key component in achieving transform limited pulses with low noise. The novel fabrication process enables the realisation of Bragg reflectors without regrowth, by post-growth processing (etching), thereby enabling low-cost foundry growth of the semiconductor wafer. The characterisation system, which is the first of its kind operating at such high speeds as 40 GHz, is important when distinguishing the noise between different low-noise designs (e.g. number of quantum wells or with or without intracavity filter), each having noise below the noise floor of common characterisation systems. It has measured record low timing noise (54fs in 10kHz-20GHz) from all-active 40-GHz lasers with 1 ps-pulses. Overviews are given of selected parts of the existing noise theories for mode-locked lasers, semiconductor intracavity filters and timing noise characterisation systems.