Linewidths below 100 kHz with external cavity diode lasers.

The linewidth of external cavity diode lasers (ECDLs) is an increasingly important characteristic for experiments in coherent optical communications and atomic physics. The Schawlow-Townes and time-averaged linewidths depend on free parameters of the design, such as cavity length, power, and grating characteristics. We show that the linewidth is also sensitive to the focus, set by the distance between the laser and the collimating lens, due to the effect on the external cavity backcoupling efficiency. By considering these factors, a simple ECDL can readily achieve linewidths below 100 kHz.