  Laser frequency stabilization is a critical part of the interferometry
measurement system of space-based gravitational wave observatories such as the
Laser Interferometer Space Antenna (LISA). Arm locking as a proposed frequency
stabilization technique, transfers the stability of the long arm lengths to the
laser frequency. The arm locking sensor synthesizes an adequately filtered
linear combination of the inter-spacecraft phase measurements to estimate the
laser frequency noise, which can be used to control the laser frequency. At the
University of Florida we developed the hardware-based University of Florida
LISA Interferometer Simulator (UFLIS) to study and verify laser frequency noise
reduction and suppression techniques under realistic LISA-like conditions.
These conditions include the variable Doppler shifts between the spacecraft,
LISA-like signal travel times, optical transponders, realistic laser frequency
and timing noise. We review the different types of arm locking sensors and
discuss their expected performance in LISA. The presented results are supported
by results obtained during experimental studies of arm locking under relevant
LISA-like conditions. We measured the noise suppression as well as initial
transients and frequency pulling in the presence of Doppler frequency errors.
This work has demonstrated the validity and \ feasibility of arm locking in
LISA.
