The ALMA Photonic Local Oscillator system

The Atacama Large Millimeter Array (ALMA) Central Local Oscillator (LO) system generates and distributes all of the LO and timing references for ALMA. These are used by the Front End and Back End Systems at the antenna, and by the Correlator in the central building. Due to the unprecedented combination of high sky frequencies (up to 950 GHz) and long baseline lengths of up to 15 kilometers, the ALMA 1^st LO requirement is particularly stringent, with extremely precise timing and synchronization needed down to ~10 femtosecond level. The unique requirements, design, and implementation of the ALMA LO system is reviewed and presented. Further, detailed design and performance data are given on the 1^st LO reference, also called the “ALMA Photonic LO System.” The challenging phase coherence and drift requirements of the Photonic LO affected the design of all of its modules and subassemblies. The final system thus includes many novel and state-of-the-art optoelectronics and photonics modules. The system has been installed and is now in routine use supporting the ALMA Commissioning Science and Verification phase of the project.