Improved Allan variance real-time processing system to measure frequency tracking error of heterodyne optical phase-locked loops

The performance of an Allan variance measuring system was drastically improved by employing time interval analysis incorporating a beat frequency method. It was used to evaluate the performance of a heterodyne optical phase-locked loop with a very low optical frequency tracking error of 0.4mHz at the integration time of 70s. Advantages of the system are precise measurement for highly stable frequency sources with good reproducibility and simple structure.