Semiconductor Lasers with Optical Feedback
A stand-alone laser diode should operate with a stable output power that is dependent on the level of injection current above the device threshold.
The laser rate equation model for this system can be expressed as a set of 3 differential equations for the amplitude and phase of the optical field, plus the carrier density. Introducing delayed optical feedback has the effect of coupling the amplitude and phase which allows infinite degrees of freedom and the development of chaotic dynamics.
The amount of optical feedback has a significant impact on the output signal. Simulations show the laser output goes through a series of bifurcations with increasing feedback before entering what is known as coherence collapse, where the output becomes chaotic.
Our experiments involve capturing the laser output power time series over a range of experimental operating conditions. Parameters varied include optical feedback level, feedback phase, injection current and temperature.
Recording highly sampled laser power fluctuations across a finely resolved operating parameter space allows extremely detailed maps of the dynamics to be produced. Using multiple maps depicting different measures provides the best overall picture of the system dynamics. These measures include basic amplitude measurements, spectral content (FFT) and quantifying complexity via geometrical (correlation dimension) or information (permutation entropy) methods.
From experiments performed here at Macquarie, and also by our collaborators in labs around the world, we have analysed several optical feedback systems:
- Fabry-Perot type semiconductor laser with bulk optic feedback system and a 4 GHz detection bandwidth.
- Fabry-Perot type semiconductor laser with bulk optic feedback system and a 16 GHz detection bandwidth.
- Fabry-Perot type semiconductor laser with bulk optic feedback system and a 16 GHz detection bandwidth, with varying temperature.
- DFB type semiconductor laser with an integrated feedback system on chip.