A versatile digital GHz phase lock for external cavity diode lasers

We present a versatile, inexpensive and simple optical phase lock for applications in atomic physics experiments. Thanks to all-digital phase detection and implementation of beat frequency pre-scaling, the apparatus requires no microwave-range reference input and permits phase locking at frequency differences ranging from sub-MHz to 7 GHz (and with minor extension to 12 GHz). The locking range thus covers ground-state hyperfine splittings of all alkali metals, which makes this system a universal tool for many experiments on coherent interaction between light and atoms.