Semiconductor colliding-pulse mode-locked lasers at 60 GHz subjected to optical feedback

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(1-4). 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.