Arm-Locking in a LISA-like Hardware Model: A Status Report

The Laser Interferometer Space Antenna (LISA), is a proposed mission to detect gravitational waves in the mHz regime. The mission calls for a triangular constellation of three spacecraft, with inter‐spacecraft distances of approximately 5 Gm. Laser interferometry is used to measure fluctuations in the inter‐spacecraft distances to a level of ∼ 10 pm, sufficient for detecting gravitational waves. A major challenge in reaching this precision is addressing laser phase noise, which couples into the distance measurement at a level several orders of magnitude above the gravitational‐wave signal. Arm‐locking is a proposed stabilization method which uses the inter‐spacecraft distance(s) as a frequency reference. In this paper we describe the implementation of arm‐locking in an electro‐optic model of the LISA interferometer. The system includes input phase noise derived from cavity‐stabilized lasers and a single‐arm delay of 1.065 ms. The residual frequency noise of the locked system was measured to be less than 200 mHz/Hz from 10 mHz – 250 mHz and from 1 Hz – 100 Hz.