Simplifying LISA interferometry using three master lasers

The Laser Interferometric Space Antenna (LISA) will use heterodyne laser interferometry with digital phasemeters to track microcycle/√{Hz} variations in 2.5 Gigameter arms between spacecraft. To achieve this precision, one of the 6 lasers in the LISA constellation, designated Master, is frequency stabilized by locking its frequency to an ultra-stable optical reference cavity. The other 5 lasers in the constellation are frequency stabilized using a chain of offset phase locks between spacecraft, ultimately referencing all lasers in the LISA constellation to the one reference cavity. For redundancy, LISA has three cavities, one per spacecraft. In the proposed three master LISA, one laser on each spacecraft is locked to the local cavity. With three local masters there are a number of advantages: no phase locking between spacecraft is needed; there are smaller variations in beatnote frequencies; no laser frequency scan is needed during link acquisition; and the chance of the phasemeters cycle slipping is reduced. Best of all, no hardware changes are needed. The only requirement is the three optical cavities need overlapping resonant frequencies. This is achievable however with current cavity manufacturing techniques and can be optimized post-launch by tuning relative cavity temperatures on each spacecraft.