Narrow-linewidth quarter-wavelength-shifted DFB laser based on coupling-coefficient distribution engineering.

A narrow-linewidth quarter-wavelength-shifted distributed feedback (QWS-DFB) laser based on coupling-coefficient distribution (CCD) engineering is proposed and experimentally demonstrated. By tailoring the longitudinal coupling profile through symmetric apodized-coupling regions, the strong central optical-field localization in conventional QWS-DFB cavities is suppressed, leading to improved longitudinal field uniformity and reduced linewidth broadening associated with longitudinal spatial hole burning (LSHB). Numerical analysis shows that the coupling-coefficient profile can effectively regulate the intracavity field distribution and mode characteristics. The proposed CCD QWS-DFB laser exhibits a minimum Lorentzian linewidth of 75 kHz, significantly smaller than that of a reference uniform-coupling QWS-DFB laser. These results confirm that CCD engineering provides an effective and fabrication-robust route toward narrow-linewidth DFB lasers.