The first sub-picometer laser interferometer in space

Analysing the in-flight performance of LISA Pathfinder's optical metrology system

April 02, 2021

LISA Pathfinder (LPF) was an ESA test mission for LISA, the planned space-based gravitational-wave observatory. LPF successfully demonstrated key LISA technologies and exceeded expectations by far. An international team with several members from AEI Hannover has now published a first detailed analysis of the exceptional performance of the LPF Optical Metrology System (OMS). It performed the fundamental measurement of the motion of two free-falling cubic test masses and was the first laser interferometer with sub-picometer resolution in space. The team investigated the noise behaviour and its constituents in detail and developed and validated a system noise model. The interferometer performed stably and consistently much better than its requirement. This is a key milestone towards the construction of the more complex optical system to be used in LISA.

Paper abstract

We report on the first sub-picometer interferometer flown in space. It was part of ESA’s LISA Pathfinder mission and performed the fundamental measurement of the positional and angular motion of two free-falling test masses. The interferometer worked immediately, stably and reliably from switch-on until the end of the mission with exceptionally low residual noise of 32.0+2.4−1.7 fm/√Hz, significantly better than required. We present an upper limit for the sensor performance at mHz frequencies and a model for the measured sensitivity above 200 mHz.

Amplitude spectral density of the differential test mass (TM1 and TM2) displacement noise ο12 on the 1st of June 2016, showing the performance of the LISA Pathfinder (LPF) optical metrology system (OMS) compared to its noise requirement and to the individual modelled OMS noise contributions. Above 200 mHz the sum of the modelled OMS noise contributions is 29.7 fm/√Hz, agreeing well with the observed level of 32.1 fm/√Hz. Frequencies below 200 mHz are dominated by other noise sources such as Brownian force noise and Tilt-To-Length (TTL) coupling. RIN is the relative intensity noise of the laser. We show an upper limit of the OMS sensing noise arising from temperature induced path length changes in transmissive optical components.

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Amplitude spectral density of the differential test mass (TM1 and TM2) displacement noise ο12 on the 1st of June 2016, showing the performance of the LISA Pathfinder (LPF) optical metrology system (OMS) compared to its noise requirement and to the individual modelled OMS noise contributions. Above 200 mHz the sum of the modelled OMS noise contributions is 29.7 fm/√Hz, agreeing well with the observed level of 32.1 fm/√Hz. Frequencies below 200 mHz are dominated by other noise sources such as Brownian force noise and Tilt-To-Length (TTL) coupling. RIN is the relative intensity noise of the laser. We show an upper limit of the OMS sensing noise arising from temperature induced path length changes in transmissive optical components.
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