Conference Agenda

Conventional active mode-locking is typically described through the lens of the Haus master equation, where a modulator condenses light into an isolated Gaussian pulse. However, in gain media with ultra-fast dynamics—such as those found in many semiconductor lasers—rapid gain recovery suppresses pulse formation, necessitating a fundamental shift in our understanding of the mode-locked state.

In this talk, we present the "Quantum Walk Comb," in which active mode-locking is reinterpreted as the stabilisation of a coherent photonic quantum walk. By mapping the laser’s wave equation to a Schrödinger-like equation on a tight-binding lattice, we show that light undergoes ballistic expansion in the frequency domain and stabilises into a broadband, flat-top, continuous-wave comb state. We demonstrate this experimentally using unidirectional ring Quantum Cascade Lasers.

Moving beyond the generation of tunable lasing states, we show that this platform serves as a powerful photonic emulator for solid-state phenomena in synthetic dimensions. By applying a constant effective force to the frequency lattice via modulation detuning, we observe optical Bloch oscillations and noise-driven mobility transitions, including the shift from ballistic walks to localised states. Finally, we discuss the outlook of applying the Quantum Walk Comb in high-speed, non-interferometric spectroscopy for microsecond-scale chemical analysis.

We present an experimental and numerical study of the probability distribution of interference pulses from two InP lasers in a photonic integrated circuit (PIC) operated in gain-switching mode through a square-wave signal that turns them on and off. Without laser coupling, the lasers' phases are random due to quantum spontaneous emission noise, and their phase difference can be translated into a random intensity and exploited for quantum random number generation (QRNG). We analyze the coupling dynamics that lead to optical phase synchronization. Through a direct comparison between experiments and simulations, we demonstrate a clear correlation between laser coupling and the degradation of the randomness of the lasers' optical phases, providing a better understanding of the operation and performance of the QRNG device.

This work investigates the ability of information-theoretic measures to characterize and quantify spatial inhomogeneities in broad-area semiconductor laser intensity profiles. We focus on filamentation phenomena, where an initially smooth beam breaks into narrow, high-intensity structures. Shannon entropy (H) and Fisher information (F) are used to quantify the degree of filamentation in experimentally recorded beam intensity profiles. These information measures are systematically validated using synthetic profiles constructed as linear superpositions of Gaussian distributions. The results show that the Entropy-Fisher information (H-F) plane provides an effective tool for quantifying spatial inhomogeneities at different laser operating currents.

Under appropriate optical feedback or coupling conditions, semiconductor lasers can operate in a regime known as low-frequency fluctuations (LFFs), characterized by sudden and irregular spike-like intensity dropouts. These dropouts are interesting because of their similarity to neuronal spiking, hinting that such spiking semiconductor lasers might be used for spike-based information processing inspired by biological sensory neurons. For this analogy to hold, the spikes must be well-defined and highly responsive to weak external inputs, allowing them to encode information in an event-driven way. In this work, we present several methods to characterize and classify the LFF dropout events.