A compact actively damped vibration isolation platform for optical experiments in ultra-high vacuum

TL;DR

The paper presents a compact, UHV-compatible tabletop six-axis vibration isolation platform with integrated active damping designed for LIGO auxiliary optics. It combines vertical blade cantilevers and horizontal wires for passive isolation with six AOSEM sensor-actuator pairs for active damping, controlled by a physics-informed $P_s$/$P_f$-based loop to achieve high-frequency isolation while managing control-noise. Experimental results from a MIT prototype and two deployed units show good agreement with the mechanical model: isolation of approximately $25 dB$ at $10 Hz$ and about $65 dB$ at $100 Hz$, with structural modes damped via constrained-layer dampers and active control reducing rigid-body Q to around $20$. The platform is compact, adaptable to different optical-table geometries, vacuum-compatible, and easily deployed in LIGO chambers, offering a practical solution for precision experiments in vacuum. It enables reliable, scalable vibration isolation for optical experiments in ultra-high vacuum with tunable high-frequency performance.

Abstract

We present a tabletop six-axis vibration isolation system, compatible with Ultra-High Vacuum (UHV), which is actively damped and provides 25 dB of isolation at 10 Hz and 65 dB at 100 Hz. While this isolation platform has been primarily designed to support optics in the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, it is suitable for a variety of applications. The system has been engineered to facilitate the construction and assembly process, while minimizing cost. The platform provides passive isolation for six degrees of freedom using a combination of vertical springs and horizontal pendula. It is instrumented with voice-coil actuators and optical shadow sensors to damp the resonances. All materials are compatible with stringent vacuum requirements. Thanks to its architecture, the system's footprint can be adapted to meet spatial requirements, while maximizing the dimensions of the optical table. Three units are currently operating for LIGO. We present the design of the system, controls principle, and experimental results.

Figures (15)

• Figure 1: UHV COMPATIBILITY. The isolation platform at the LIGO Livingston Observatory. All the materials fulfill stringent UHV requirements vacuum.
• Figure 2: COMPACTNESS. Schematics of the isolation platform. The optical table is suspended from three blade assemblies, 38 mm (1.5 in) above the reference plane. The footprint is designed to match the space in the LIGO chambers.
• Figure 3: SHAPE ADAPTABILITY. Blade assembly (a-left), including the riser clamped to the base, the blade clamped at a specific launch angle, and the pendulum wire clamped to the tip of the blade. The blade assembly includes built-in hard stops that limit the range of motion of the optical table. Using independent blade assemblies to suspend the optical table makes the design adaptable to different requirements. As an illustration, two optical table shapes that can be implemented using this concept are presented (a-right). Our design was adapted to match the space available in the aLIGO chambers (b).
• Figure 4: EFFECTIVE FOOTPRINT. Most of the footprint corresponds to the optical table. The isolation platform is instrumented with six sensor-actuators positioned in vertical-horizontal pairs. The blade guards provide safety during assembly. The optical table features a stiffener underneath to increase the stiffness to mass ratio.
• Figure 5: TABLE STIFFNESS. Results of the table stiffness optimization. Using iterative finite element analysis on the optical table for different stiffener shapes and sizes. The selected dimension (indicated with a star) was chosen to have the highest stiffness to mass ratio while meeting the requirements on total mass and center of mass position.