Optical generation of a mHz linewidth microwave signal with a discriminator-aided phase-locked loop using two semiconductor lasers

Optical beam forming and steering of RF phased array antenna offers interesting advantages over standard microwave techniques, including inherently low transmission losses, large bandwidth and light weight. One important challenge of such a system is to optically generate a microwave carrier with a well controlled frequency. Offset optical phase locking of two semiconductor lasers can be an effective technique to achieve this goal. In our experiment, two external cavity semiconductor lasers were phase locked by using a discriminator-aided optical phase-locked loop with enhanced acquisition range over previous approaches.