Experimental demonstration of the combined arm- and cavity-locking system for LISA

Laser frequency noise suppression is a critical requirement for the Laser Interferometer Space Antenna (LISA) mission to detect gravitational waves. The baseline laser stabilization is achieved using cavity prestabilization and a postprocessing technique called Time-Delay Interferometry (TDI). To enhance the margins for TDI, alternate laser-locking schemes should be investigated. A novel stabilization blending the excellent stability of the arm with the existing cavity reference has been shown theoretically to meet the first-generation TDI margins. This locking system was designed to be implemented as a firmware change and have minimal or no changes to the LISA hardware. This paper experimentally verifies the hybrid laser-locking technique by utilizing two references - an optical cavity and an interferometer with delay imparted using 10 km of optical fiber. The results indicate the viability of the combination of the arm-cavity-locking system for LISA. They show the key benefits envisioned by this technique: suppression of the cavity fluctuations by the arm sensor (by 21 dB in this demonstration) and reduction of Doppler pulling of the laser frequency, a key technical challenge for arm locking.