A High Sensitivity Heterodyne Interferometer as Optical Readout for the LISA Inertial Sensor

In the current LISA baseline design the distance between two proof masses on two distant spacecraft is measured interferometrically by three individual measurements: between both optical benches (with a distance of ∼ 5 mio km) and between each optical bench and its associated proof mass. All three measurements require the same sensitivity, i. e. below 5 pm/Hz for the translation measurement (for frequencies above 2.8⋅10−3 Hz with an f−2 relaxation down to 3 ⋅ 10−5 Hz) and below 20 nrad/Hz for the tilt measurement (for frequencies above 10−4 Hz with an f−1 relaxation down to 3 ⋅ 10−5 Hz). Here, we present a compact setup of a heterodyne interferometer which serves as a demonstrator for an optical readout for the LISA proof mass position and tilt versus the optical bench. We measured initial noise levels below 100 pm/Hz for frequencies > 10−1 Hz and below 100 nrad/Hz for frequencies > 10−2 Hz, respectively.