Systematic Frequency Error in Laser Synchronization Circuits for Fiber-Optic Time-Transfer Systems

This article addresses the problem of a systematic frequency error occurring in semiconductor-laser frequency synchronization circuits based on counting the beat note between the two lasers in a reference time interval using a high-frequency prescaler. Such synchronization circuits are suitable for operation in ultraprecise fiber-optic time-transfer links, used, e.g., in time/frequency metrology. The error occurs when the power of the light coming from the reference laser, to which the second laser is synchronized, is below about -50 to -40 dBm, depending on the details of particular circuit implementation. The error can reach tens of megahertz if left out of consideration and does not depend on the frequency difference between the synchronized lasers. Its sign can be positive or negative, depending on the spectrum of the noise at the prescaler input and the frequency of the measured signal. In this article, we present the background of the systematic frequency error, discuss important parameters allowing for predicting the error value, and describe the simulation and theoretical models being helpful for designing and understanding operation of discussed circuits. The theoretical models presented here show good agreement with the experimental data, which demonstrates the usefulness of the proposed methods. Implementing polarization scrambling to mitigate the effect of polarization misalignment of the lights of the lasers used was considered and the resulting penalty was determined.