This work reports on the fabrication, optimisation and characterisation of monolithic mode-locked lasers (MLLs) and colliding-pulse mode-locked (CPM) lasers with repetition rate in the range 10-60 GHz. The devices consist of double section split-contact ridge waveguide lasers fabricated in GaAs/AlGaAs double quantum well (DQW) material. For CPM devices, the saturable absorber section was fabricated with a coplanar ground-signal-ground (G-S-G) pad structure. The optimum saturable absorber size for efficient mode-locking is found experimentally. The fabricated devices are characterised in terms of operating regimes (Continuous Wave, Self-Pulsation, Mode-Locking) and the mode-locking signals were observed in the frequency domain using an external fast photodiode. In the case of CPM devices a mm-wave signal could also be extracted directly from the saturable absorber section of the laser using a microwave probe.
Semiconductor mode-locked lasers and colliding-pulse mode-locked lasers have proven to be effective sources of pulsed optical signals with repetition rate of several tens of GHz and above. Examples of application can be found in OTDM systems, radio-over-fiber networks, and millimeter-wave generation. This work reports on the characterisation of monolithic colliding-pulse passive mode-locked (CPM) lasers at 60 GHz in GaAs/AlGaAs double quantum well material, subjected to optical feedback. The characteristics of the optical-to-electrical converted signals are investigated both by means of an external fast commercial photodiode and by using the saturable absorber section of the device as an intra-cavity photodetector. The power of the electrical RF signal, linewidth, and central frequency stability are measured in unperturbed condition and under the effect of optical feedback. Measurements demonstrate the deterioration of the electrical properties of the signal as the optical feedback level is increased. The reduction of the stability region for mode-locking operation is also reported.
KEYWORDS: Signal attenuation, Mirrors, Interferometry, Signal detection, Semiconductor lasers, Autocollimators, Reflectors, Signal to noise ratio, Ferroelectric materials, Photodiodes
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