Optically injected dual state quantum dot lasers
University College Cork
The spikes and pulses observed in biological neurons have similar characteristics to the excitable pulses seen in optically injected lasers. This thesis investigates the feasibility of optically injected dual state quantum dot lasers as artificial photonic neurons. Photonic waveguides are not as sensitive to centimetre long transmission lines as electronic circuits. This could be particularly advantageous for neural networks where one element, a neuron, must receive signals from many different other neurons. Having many lasers on a chip will inevitably lead to long spacings between neurons. With integrated photonic circuits, shorter pulses can be sent between distant neurons without having to worry about adverse RC dynamics. Therefore photonics offers a solution to the fan in and fan out bottleneck hindering the development of efficient hardware that can support an artificial neural network. Quantum dot lasers are some of the most stable semiconductor lasers in optical injection configurations due to their highly damped relaxation oscillations. This stability introduces a bistability between two phase locked states. However, we show that this bistabilty is broken by an optothermal effect and is replaced by a deterministic cycle between the remnants of the two states. Excitable trajectories around this cycle can be triggered by noise and deterministically through phase perturbations. Quantum dot lasers can lase from multiple energy states. The lower energy state, the ground state (GS), is achieved through low pump currents and the excited state (ES) is reached through higher pump currents. This dual state or two colour laser produces many unique dynamics, particularly when biased to emit from the ES and then optically injected near the GS, so as to turn off the ES and turn on the GS. A locking map is recorded for different injection strengths and injection frequencies. A hysteresis cycle is found along with optothermally induced squares waves trains on the negatively detuned boundary. On the positively detuned boundary a second optothermal effect caused by the heating of the laser facet introduces a different square wave train, with a period of a tens of milliseconds. Excitable GS dropouts with accompanying ES pulses are also observed close to the negative unlocking boundary. These events are shown to be noise induced or can be deterministically triggered. The events can display a true all or none response, analogous to a leaky integrate and fire neuron. The dual state nature of the quantum dot laser introduces two excitable thresholds, one for each direction of phase perturbation around the phasor diagram. The thresholds, delay times and refractory periods of these pulses are investigated for each perturbation direction. An excitable interval is uncovered and an integrate and inhibit mechanism is observed. Not only this but an integrate and fire mechanism is also found. A prototypical photonic neuron is realised and is used in a rudimental image edge detection technique.
Optothermal , Photonic neuron , Non-linear laser dynamics , All or none , Leaky integrate and fire , Integrate and inhibit , Refractory period , Refractory time , Dual state , Optical injection , Quantum dot lasers
Dillane, M. 2022. Optically injected dual state quantum dot lasers. PhD Thesis, University College Cork.