The advent of growth techniques for the formation of high optical quality quantum dots (QDs) has allowed the testing of the predictions of enhanced device performance using QDs over those utilizing quantum wells. We have concentrated on the InAs/InP materials system where the 1.55 μm wavelength range is readily obtained, but the growth of quantum dots is more difficult than for the InAs/GaAs materials system. The initial claims for quantum dot lasers of lower threshold current densities and temperature sensitivity have not been realized on InP, but other very interesting properties have been observed that separate the behaviour of quantum dot based devices from that observed for quantum well based devices. In this talk I will describe two types of device, multwavelength lasers, and ultrahigh repetition rate modelocked lasers.
When operated with as-cleaved facets (Fabry-Perot type devices) a quantum dot device will lase on many longitudinal modes simultaneously over a wide wavelength range (e.g. 32 lines over 18 nm). Through appropriate choice of cavity length these modes can be matched to the ITU grid thus providing a single chip multiwavelength source that can replace many single wavelength lasers or a spectrally chopped amplified spontaneous emission (ASE) source. Since the non-linearities are very strong in quantum dot gain material it is possible to phase lock the longitudinal cavity modes together to create short pulse mode-locked lasers (<300 fs) at high repetition rates (92 GHz). By coupling the laser to a fibre Bragg grating we were able to force the device to lase on a well-defined comb of frequencies defined by the grating. These modes could be locked together to allow pulsed operation, where the repetition rate was set by the Bragg grating, completely independently of the cavity length. We demonstrate repetition rates of 437 GHz and 1 THz, providing a possible route to tunable THz generation