Precision timing control for radioastronomy: maintaining femtosecond synchronization in the Atacama Large Millimeter Array

This article discusses three important subsystems of the photonic LO reference distribution system for the ALMA radiotelescope. Each subsystem relies on specific control systems that allow ordinary components to be assembled to form a global system that is more accurate and stable than its individual components. By using appropriately designed loops, lasers with phase noise of many thousands of rad² were phase-locked to generate high-purity microwave signals with phase noise below 3.3 × 10^-5 rad² at 27 GHz, fibers with lengths fluctuating by more than a few millimeters were compensated within a fraction of a micrometer, and optical frequencies of lasers normally fluctuating by tens of megahertz were stabilized to within a few kilohertz. These examples demonstrate how control systems can be used with photonic and electronic systems to achieve remarkable results.