Sub-megahertz narrow linewidth VCSELs with monolithic extended cavity

We demonstrate a monolithic extended-cavity vertical-cavity surface-emitting laser (VCSEL) achieving a linewidth of 890 kHz, which, to our knowledge, ranks among the narrowest reported for this class of lasers. The device incorporates an extended cavity and an intermediate reflector monolithically integrated into the epitaxial structure, forming a coupled configuration that significantly increases photon circulation path and enhances mode selectivity. The device has an emitting wavelength of 795 nm, with a side-mode suppression ratio of 40 dB. This work validates the feasibility of using monolithic extended cavities to achieve sub-megahertz linewidths in VCSELs without external feedback or complex packaging, offering a promising pathway for applications in coherent optical communications, laser spectroscopy, and quantum sensing.