Content area
Full text
A unique, ultra-sensitive frequency-resolved optical gating system was constructed, and the pulse shape and chirp of a quantum-dot mode-locked laser was unambiguously measured using this technique for the first time. A clear pulse asymmetry was detected, and evidence that the pulses are recompressible to sub-picosecond lengths is presented.
Introduction: Interest in quantum-dot modelocked lasers (QDMLLs) has grown substantially in recent years since their first demonstration in 2001 [1], as applications for optical-time-domain multiplexing, arbi- trary waveform generation, and optical clocking are anticipated [2, 3], Ultrafast pulses from QDMLLs have been reported as short as 390 fs using intensity autocorrelation techniques [4], but so far detailed charac- terisation examining the pulse shape, duration, chirp and degree of coherence spiking in these lasers has not been carried out. This Letter describes the first direct frequency-resolved optical gating (FROG) measurements on a QDMLL, which operates at a nominal repetition rate of 5 GHz. Even though FROG devices are available commercially, they normally lack the sensitivity required to measure the output pulses from modelocked semiconductor lasers. A unique, ultra-sensitive FROG system was assembled to realise the results presented here. With the FROG system, the pulsewidth and chirp are measured accurately, clear pulse asymmetry is observed, and evidence that pulse break-up in the QDMLLs is caused by interplay between the homogeneous linewidth of the individual quantum dots and the inhomogeneous broadening of the ensemble is described.
Device and experimental setup: The two-section passive QDMLL devices were processed using an optimised six-stack dots-in-a-well (DWELL) laser structure, which was grown by elemental source molecular beam epitaxy (MBE) on a (001) GaAs substrate [5], following standard ridge waveguide laser processing [6], The passive QDMLLs have a 1.0 mm absorber and 7.3 mm gain section. The cleaved facet near the absorber was high-reflectivity coated (R ~ 95%) and the other facet was low-reflectivity coated (R ~ 5%). The two-section laser was mounted on an AIN substrate and then on a copper heatsink. The temperature was kept at 20°C with a TEC controller. The optical output of the laser was collected with an optical head, which integrates a lens, an isolator and a short 1 m single-mode polarisation-maintaining (PM) fibre pigtail, and then was coupled into...