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要旨

Low frequency high precision laser interferometry is subject to excess laser
frequency noise coupling via arm-length differences which is commonly mitigated
by locking the frequency to a stable reference system. This is crucial to
achieve picometer level sensitivities in the 0.1 mHz to 1 Hz regime, where
laser frequency noise is usually high and couples into the measurement phase
via arm-length mismatches in the interferometers. Here we describe the results
achieved by frequency stabilising an external cavity diode laser to a
quasi-monolithic unequal arm-length Mach-Zehnder interferometer read out at
mid-fringe via balanced detection. This stabilisation scheme has been found to
be an elegant solution combining a minimal number of optical components, no
additional laser modulations and relatively low frequency noise levels. The
Mach-Zehnder interferometer has been designed and constructed to minimise the
influence of thermal couplings and to reduce undesired stray light using the
optical simulation tool IfoCAD. We achieve frequency noise levels corresponding
to LISA-like (laser interferometer space antenna) displacement sensitivities
below 1 pm/$\sqrt{\textrm{Hz}}$ and are able to demonstrate the LISA frequency
pre-stabilisation requirement of 300 Hz/$\sqrt{\textrm{Hz}}$ down to
frequencies of 100 mHz by beating the stabilised laser with an Iodine locked
reference.