Seismically isolated tables
The optical tables are the mechanical interface between the vacuum system and the multiple stage pendulum suspension systems used to suspend main and ancillary optics of the interferometer. Each table is supported by a Seismic Attenuation System (SAS). The SAS is a six degrees-of-freedom vibration isolator providing the necessary shielding from the ground microseismic noise which is the main environmental source of disturbance for the experiments in the frequency range from 0.1 to 100 Hz. Attenuation factors larger than 70 dB at 10 Hz are expected in vertical and the horizontal direction along the main axis of the interferometer.
3-D model of the 10-m prototype isolation tables.
Making our lab the quietest place on Earth
The Earth crust moves randomly and continuously with amplitudes often exceeding the micron level with a broadband spectrum dominated at low frequencies (below 0.3 Hz) by the “sound” of the ocean’s swell (the so-called microseismic peak). A large amount of noise characterizes the frequency band 1-10 Hz in highly urbanized areas as the result of the human activities (mainly traffic). A typical example is our experimental hall, located in downtown Hannover surrounded by roads and tramways and few hundred meters from the Hannover-Hainholz railway junction. Vertical and horizontal displacements up to 300 nm root-mean-square above 1 Hz can be measured with a seismometer on the hall floor during daytime, about three orders of magnitude more than at the known quietest place on Earth, the QSPA Seismic Station (Antarctica, 5-miles from the South Pole, 300-m deep in the ice).
The optics suspensions have their resonances in the same 1-10 Hz band and therefore it is very important to strongly reduce the amplitude of the motion of the optical tables at those frequencies to avoid overloading the optics control systems causing noise injection and limiting the sensitivity achievable in the experiments. In the SAS the seismic attenuation is obtained passively using the properties of the mechanical oscillators which attenuate, as second-order low pass filters, above their natural frequency.
The tables operate with very low natural frequencies, 0.05 Hz in horizontal and 0.1 Hz in vertical, allowing large vibration suppression ratios to be achieved. The aimed isolation of 60 dB at 3 Hz in both directions will make our optical tables as quiet as the deep ice at the South Pole.
Anatomy of the Seismic Attenuation System
The SAS is a completely custom device capable to provide in a single stage vibration isolation factors not achievable, to our knowledge, by any existing commercial or laboratory developed system. Its design is derived from a prototype device (HAM-SAS: Horizontal Access Module Seismic Attenuation System) built by Caltech in 2006 in the framework of the R&D for the gravitational wave detector Advanced LIGO . Our optical table, consisting of a stainless steel honeycomb welded structure 1750x1750x400 mm, sits on three vertical vibration isolators (named filters) providing mechanical compliance along the vertical, pitch and roll degrees of freedom.
Full view of the Seismic Attenuation System.
Each filter is a tunable spring made by a crown of curved cantilever blades compressed each against the other: the constrained radial stress creates an anti-spring effect (geometric anti-spring)  that allows a very low effective stiffness to be achieved in the vertical direction at the nominal load. Natural frequencies down to 0.2 Hz can be obtained by mechanical adjustment of the compression rate; longer natural periods can be achieved by applying positive feedback (so-called electronic anti-spring). The inertia of the blades limits the minimum vibration transmissibility to -60 dB, which can be improved up to -80 dB by using the built-in compensators (so-called magic wands).
Geometric anti-spring vertical isolator.
The three filters are mounted on a rigid plate supported by three inverted pendulum (IP) legs: very low natural frequencies (around 0.05 Hz) are achieved for the horizontal (X,Y) translational modes and for the yaw mode, by suitably sizing the IP bottom flexures to compensate for the load gravitational torque which leads the system to the instability . The IP top short flexure hinges allow the filter plate to wobble in the horizontal plane preventing its pitch/roll movement. The inertia of the legs limits the IP minimum vibration transmissibility to -70 dB; up to -90 dB can be achieved by tuning the built-in counterweights.
Horizontal isolation stage.
The amplification of the seismic noise at the SAS resonances would make the motion of the tables uncomfortably large at low frequencies and then the attenuators are instrumented with a collection of sensors (Linear variable differential transformers and horizontal accelerometers) and voice coil actuators to perform electronic modal damping. The SAS control is made using very elementary feedback filters and is limited to a few Hz bandwidth. The natural completion to the local SAS action is the global inter-table distance and attitude stabilization using the signals from the Suspension Platform Interferometer (SPI).
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