Mode locking of semiconductor laser with curved waveguide and passive mode expander

Active mode locking is reported for a 1.55 mum semiconductor laser with a curved waveguide and passive mode expander, placed in a wavelength tuneable external cavity. The waveguide is curved to reduce the reflectivity at only one facet, and the active region tapers down towards this facet to allow the guided mode to expand into an underlying passive waveguide. This further reduces the reflectivity due to the smaller divergence of the expanded mode, whilst also enhancing the coupling efficiency and relaxing alignment tolerances out into the external cavity. The resulting inner facet reflectivity is 8 x 10(-6), while the outer facet maintains a conventional reflectivity of approximately 0.3. These features make the device ideal for use in an external cavity setup, and to the authors' knowledge this is the first time such a design of device has been mode locked. With non-optimised pulse compression, 2.5 GHz pulses of 2.9 ps duration have been generated with a linewidth of 1.09 nm. In harmonic operation, 10 GHz pulses of 3.1 ps duration have been obtained, with a linewidth of 0.81 nm. Together with its wavelength tuning capability, these features make the setup a candidate source for a multi-wavelength optical time division multiplexed communications system.