A relocking scheme for optical phase locking using a digital circuit with an electrical delay line.

We present a simple scheme that enables us to automatically relock optical phase locking. Our servo system consists of an analog loop filter for the fast feedback control of a high-speed controllable actuator and a digital circuit for the slow control of another actuator with a broad dynamic range. The digital circuit detects an out-of-lock status and relocks the phase locking using a signal generated by an electrical delay line. We demonstrate to relock the phase locking of a fiber-based frequency comb to a Nd:YAG laser at 1064 nm and that of an external cavity diode laser at 1156 nm to the fiber comb. The relocking scheme is attractive for various applications that employ phase locking and is especially useful for the unattended operation of optical clocks. [ABSTRACT FROM AUTHOR]