Sideband Locking for the LISA Optical Truss Interferometer Readout

In LISA, the optical truss interferometer (OTI) is a subsystem proposed to track pathlength variations in the telescope and ensure the telescope structure meets the required picometer stability. An OTI would be located at each of the six LISA telescopes, with each OTI comprising three separate linear cavities in a triangular configuration. If a telescope is not picometer stable, on-mission cavity length measurements serve as a potential witness channel for spurious length fluctuations. A simple method of cavity length measurements would involve three separate lasers, each locked to a respective OTI cavity. However, one alternative which uses a pick-off from a single, pre-existing laser source at each telescope is sideband locking, which would lock pairs of sidebands to each of the three cavities. Via this sideband locking, a voltage-controlled oscillator (VCO) signal, applied to a broadband electro-optic modulator (EOM), is used to tune sideband frequencies, thus tracking cavity length without the requirement of additional laserheads. We report demonstrations of this scheme and plans for its development and optimization for use in the OTI.