Laser frequency noise is a significant noise source which couples into the main science measurement of the Laser Interferometer Space Antenna via the mismatch between the interferometer arm lengths. In this paper we discuss the application of an unequal pathlength heterodyne Mach-Zehnder interferometer to measure and actively stablize the master laser frequency as used in LISA Pathfinder. In comparison with an optical cavity or atomic reference the technique has a wide operating range and does

more »

... ot require a complex lock acquisition procedure. Frequency tuning can be provided by purely electronic means and does not require physically changing the pathlength (or resonance frequency) of the frequency reference and can therefore be combined with arm locking in a straightforward manner.