Cavity-Engineered VCSELs with Integrated Photodetectors for Optical Entropy and Security

Overview

Vertical-cavity surface-emitting lasers (VCSELs) are widely used in photonic systems due to their compact size, low power consumption, and low cost. Moreover, their surface emission configuration naturally supports large-scale 2D arrays for parallelism. Conventional VCSEL design has primarily focused on suppressing multimode behavior and minimizing noise to achieve stable and coherent emission. This thesis adopts an alternative perspective, investigating how multimode and low-coherence lasing introduce optical chaos and fluctuations, which can be deliberately engineered and exploited as valuable physical resources for security oriented applications. The main contribution of this work is cavity geometry engineering as a fundamental control knob for tailoring the optical behavior of VCSELs. By introducing non-circular and symmetry-broken cavity geometries, the spatial mode structure, coherence properties, and polarization behavior of VCSEL emission are systematically modified. These geometric perturbations give rise to strong multimode interactions and nonlinear dynamics, resulting in broadband temporal fluctuations and intrinsic optical chaos, without relying on external modulation or complex feedback architectures.

Building on this physical foundation, the resulting chaotic emission is interpreted from an information-theoretic perspective as a source of intrinsic optical entropy. This entropy source is subsequently exploited through two parallel security-relevant pathways. In the temporal domain, intensity fluctuations are converted into high-quality random bit streams suitable for physical random number generation. In the spatial domain, device-specific far-field emission patterns emerge as robust and reproducible physical fingerprints, enabling optical physical unclonable functions for hardware authentication. The common physical origin of entropy supports multi-key operation through biasing conditions and confers robustness against environmental variations and optical perturbations. The two-dimensional nature of the emission further enables parallel entropy channels, allowing multiple independent channels to be extracted from a 2D array.

To progress beyond discrete optical demonstrations and toward chip-scale realizations, this thesis further investigates the integration of cavity-engineered VCSELs with on-chip photodetectors. The resulting VCSEL–photodetector platform reduces system complexity while preserving the intrinsic randomness and uniqueness generated at the optical level. This integration enables compact, stable, and direct electrical extraction of optical entropy, effectively closing the loop between entropy generation and readout. Overall, this thesis establishes cavity-engineered VCSELs with integrated photodetectors as a unified and scalable photonic platform for optical entropy generation and security. By bridging cavity-engineered laser physics, nonlinear dynamics, and integrated photonic systems, this work lays a physical and architectural foundation for future photonic secure technologies.