External cavity diode laser with kilohertz linewidth by a monolithic folded Fabry-Perot cavity optical feedback. 
We present a extended-cavity diode laser (ECDL) with kilohertz linewidth by optical feedback from a monolithic folded Fabry-Perot cavity (MFC).  In our experiments, an MFC replaces the retroreflecting mirror in the traditional ECDL configuration.  Beat-note measurements between this MFC-ECDL and a narrow-linewidth reference laser are performed and demonstrate that the linewidth of this MFC-ECDL is about 6.8 kHz.  Phase locking of this MFC-ECDL to the reference laser is achieved with a unity gain as small as 10.2 kHz.