Photonic techniques for use on the Atacama Large Millimeter Array

Progress on a photonic local oscillator that may be suitable for the Atacama Large Millimeter Array (ALMA) is described. The proposed system is based on the coherent heterodyne of 1550 nm infrared signals generated at a central location and then distributed over optical fiber to each antenna. The length of each fiber is held constant by a servo system which uses an optical interferometer to measure the fiber path length. The difference between the two laser frequencies is converted at the receivers to a radio frequency (RF) signal by a high-speed photomixer.