Impact of non-radiative recombination and optical feedback strength on field fluctuations, noise, and spectral line shape in laser diodes

Abstract

In this article, we present findings on the impacts of external optical feedback (OFB), non-radiative recombination (NRR), and injection current on the lasing field fluctuations and the spectral characteristics of laser diodes (LDs). Utilizing an advanced simulation model, we explore OFB as a series of round trip time delays in the external-cavity. Our research categorizes laser dynamics through bifurcation diagrams of photon numbers and analyzes noise characteristics across three operational regions: continuous-wave (CW) operation under weak OFB, chaotic behavior under moderate OFB, and stable CW operation under strong OFB. Notably, lower NRR stabilize laser output, facilitating periodic oscillation (PO) or CW modes essential for high performance. Reducing the NRR in solitary lasers narrows the line shape, enhancing optical performance. In CW operation under strong OFB conditions, low-frequency components of relative intensity noise (RIN) and frequency noise (FN) are substantially suppressed. However, noise levels increase during coherence collapse and at higher NRR. Our findings indicate that while moderate OFB can induce coherence collapse leading to broadened spectral peaks, very strong OFB enhances coherence, yielding sharp central peaks and allowing for CW or PO. Overall, our research highlights the critical role of a low NRR in enhancing the stability of laser diodes while revealing that a higher NRR can improve coherence in specific contexts. These insights pave the way for future advancements in laser technology, particularly for applications requiring precision and reliability.