Narrow linewidth semiconductor laser with a multi-period-delayed feedback photonic circuit.

A multi-period-delayed feedback (MPDF) photonic circuit constructed by a Sagnac ring and two coupled rings was designed. By coupling a distributed feedback (DFB) laser diode (LD) with the MPDF, a narrow linewidth semiconductor laser was demonstrated. The linewidth of the DFB-LD with MPDF was narrowed to be around 2 kHz, which is reduced by three orders of magnitude, and the linewidth reduction capability could be maintained when the wavelength of the DFB-LD was tuned in a range wider than 3 nm. The laser frequency stability can also be improved using the proposed technique, and the frequency fluctuation was reduced for nearly 8 times in comparison with the DFB-LD.