Picometer Sensitive Prototype of the Optical Truss Interferometer for LISA

TL;DR

This work presents a fiber-based optical truss interferometer (OTI) prototype to verify picometer-level optical-path stability for LISA telescope metrology. Using PDH-locking of two cavity configurations inside a vacuum-isolated bench and beat-note readout against a reference cavity, the authors demonstrate $1 \frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ sensitivity in the 0.1 mHz–1 Hz band. They dissect noise contributions, with temperature-induced effective CTE and parasitic RAM as primary limits, and achieve RAM mitigation by tuning the modulation frequency to the parasitic etalon's FSR. The results support OTI as a robust ground-testing tool and potential flight contingency, with proposed future work on offset-locking a single pre-stabilized laser to read all cavities.

Abstract

The optical truss interferometer (OTI) is a contingent subsystem proposed for the LISA telescopes to aid in the verification of a $1 \frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ optical path length stability. Each telescope would be equipped with three pairs of compact fiber-coupled units, each forming an optical cavity with a baseline proportional to the telescope length at different points around the aperture. Employing a Pound-Drever-Hall approach to maintain a modulated laser field on resonance with each cavity, the dimensional stability of the telescope can be measured and verified. We have designed and developed prototype OTI units to demonstrate the capability of measuring stable structures, such as the LISA telescope, with a $1 \frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ sensitivity using a set of freely mountable fiber-injected cavities. Aside from its initial motivation for the telescope, the OTI can also be readily integrated with other systems to aid in ground testing experiments. In this paper, we outline our experimental setup, measurement results, and analyses of the noise limitations.

Figures (10)

• Figure 1: Layout of the optical truss input stage. An adjustable fiber collimator is secured to the housing, and two lenses are used to mode match the cavity beam. Lens 2 serves as both a mode matching lens and the cavity input mirror. Adapted with permission from Jersey et al. (2023) © Optica Publishing Group jersey_optical_2023.
• Figure 2: Photograph of the assembled and aligned prototype optical truss cavities. Each ULE plate is supported by three PEEK (polyether ether ketone) legs. OTI 1 is formed with the original input and return stages, while OTI 2 has a modified return mirror mounted to a custom post anchored in a through hole in the ULE.
• Figure 3: Photo of the in-vacuum optical bench with the optical paths for each cavity labeled. The reference cavity input and reflection is coupled in free space, while the OTI cavities are fiber-injected. The cavity transmission beams (dashed lines) are routed out of the chamber through a view-port window via periscopes.
• Figure 4: Cavity readout scheme for the OTI experiment. The OTI laser path, including the cavity injection and reflection, is purely fiber-coupled while the reference laser is primarily coupled in free space apart from the fiber feed-through into the vacuum chamber. The figure shows an example of the reflected power and resulting PDH error signal as the laser frequency is scanned over a resonance. The two laser fields are combined in a fiber coupler to interfere on a high-bandwidth PD.
• Figure 5: Temperature stability as measured by thermistors attached to the cavities and optical bench in our vacuum chamber, and compared with the benchmark LISA temperature requirement.