Enhancing External Cavity Laser Diode Performance Through Optimized Fano Spectrum Feedback Configuration

Laser diodes with narrow linewidth are essential for optical communication, spectral analysis, and precision measurement. This study reinterprets the linewidth narrowing mechanism in external-cavity diode lasers (ECDLs) from a perspective that the noise-induced frequency fluctuations are suppressed by the rapidly varying phase in frequency domain introduced by the external cavity. We highlight the importance of the localized high slope in the rising edge of the feedback spectrum in forming stable narrow linewidth laser modes. Based upon this understanding, we introduce an alternative to traditional Lorentzian optical feedback. We demonstrated that asymmetric lineshapes, such as Fano, can serve as the optical feedback of ECDLs with enhanced performance. Theoretical analysis and numerical simulations reveal that Fano-based external cavity lasers, benefiting from a feedback spectrum with a steeper local slope than Lorentz-based cavities, can achieve superior linewidth narrowing under the same fabrication conditions. This study offers a novel approach for the design and application of narrow linewidth laser diodes.