Laser frequency stabilisation via quasi-monolithic, unequal arm-length Mach-Zehnder interferometer with balanced DC readout

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 0.1mHz to 1Hz 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 LISAlike (laser interferometer space antenna) displacement sensitivities below 1 pm/ √ Hz and are able to demonstrate the LISA frequency pre-stabilisation requirement of 300Hz/ √ Hz down to frequencies of 100mHz by beating the stabilised laser with an Iodine locked reference.