Compensated Fiber-optic Frequency Distribution Equipment

One-way frequency distribution of atomic clock standards has been widely studied, and several architectures have emerged that employ the use of a return path signal to compensate for the effects of temperature and phase noise. These architectures have generally relied upon temperature-controlled delay lines to compensate for fiber thermal effects, and polarization scrambling techniques to minimize the impact of polarization mode dispersion (PMD). These compensation techniques can limit the performance of the system by introducing spurious noise products onto the optical carrier. Uncompensated one-way fiber optic distribution systems are currently deployed with a demonstrated stability of 2·10 –13 (1 sec). Without temperature compensation, this equipment is suitable for intra-facility distribution of hydrogen maser references. However, operation over moderate length inter-facility links (IFL) will degrade the stability significantly, due in large part to the temperature effects of the outdoor or underground fiber cable. The stability can be improved through the use of temperature-compensated fiber cable, but this type of cable is typically an order of magnitude more costly than standard fiber. Furthermore, many locations already have installed dark fiber whose use would be more cost effective. We are developing a compensated one-way frequency distribution system that is compact, easy to install, and requires neither polarization scramblers nor large, thermally controlled delay lines. The system employs forward and reverse path microwave modulated optical carriers on a single fiber. Electronic compensation is used to correct for path length phase variations, and a novel technique to moderate the PMD without the need for polarization scramblers.