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Suspension Systems


In order to detect gravitational waves from astronomical events, gravitational wave detectors must be capable of unambiguously measuring displacements of the order of 10^-18 m/sqrt(Hz) or less. As the residual seismic motion of the earth can be a trillion times larger than this, significant care must be taken to isolate the measurement optics from such disturbances.

The ambitious goals of the AEI 10 m Prototype impose similarly strict limits on the noisy motions of the measurement optics. As in all current (and planned) full-scale interferometric detectors, the optics will be suspended as the final stages of multiple-stage cascaded pendulums, where the natural frequency filtering effects of resonant pendulum systems are used to isolate the optics from seismic motions at frequencies above the pendulum resonances.

In addition to individual optic suspensions, a "pre-isolation" stage is formed by the optical tables themselves. These are passive systems (based on the Advanced Ligo HAM-SAS design) designed to provide attenuation of >60 dB and >70 dB in the horizontal and vertical directions respectively at frequencies above ~1 Hz. Additional stability will be provided by a Suspension Platform Interferometer (SPI) controlling the relative motions and orientations of the tables.

Suspension Systems

An interferometer is a complicated system, where the noise and suspension requirements for each optic have to be tailored to their individual purpose. Broadly, there will be three classes of suspension required for the first stage of experiments in the prototype: those forming the frequency reference cavity; the main interferometer optics; and ancillary optic suspensions.

Reference Cavity

The first stage to be implemented in the prototype will be a suspended frequency reference cavity, used to suppress laser frequency noise. This will consist of a three-mirror, high finesse triangular ring cavity, with a round-trip length of 25 m. The mass of the optics was chosen to be 850 g; balancing the reduction of radiation pressure induced noise with the desire to have a compact suspension. 

The whole suspension system will comprise three stages, of similar mass.

Reference Cavity Suspension Mock-up
Figure 1: a mock-up of a reference cavity multi-stage suspension

The final (silica) optic will be suspended from an intermediate (aluminium) mass by thin stainless steel wires: 30 µm diameter wire will be used to both lower the stage's vertical resonance frequency to 20 Hz and raise the first transverse mode to 400 Hz, keeping these features out of the desired measurement band of the main interferometer.

The intermediate stage is connected to the uppermost stage via stainless steel wires attached to cantilever springs (made of maraging steel) mounted on the upper stage. These springs help to lower the overall vertical resonance frequencies, thus reducing additional noise contributions from vertical-to-horizontal noise coupling. The upper stage is then used to provide local eddy-current damping of pendulum resonances, as well as providing a platform for the application of alignment and drift control signals through adapted Bosem style magnet and coil assemblies.

Figure 2: a Bosem coil/magnet sensor and actuator assembly

Finally, the uppermost stage is joined to a suspension cage structure by two additional cantilever blades. The combination of vertical and horizontal isolation stages, combined with the thin steel suspension elements to reduce thermal noise, and the addition of the pre-isolation optical tables gives the following residual noise contribution for the reference cavity optics; some way below the requirement for the main experiment to reach the desired noise limit.

Modelled Suspension Noise
Figure 3: modelled thermal and seismic noise contributions for a reference cavity suspension


Interferometer Optics

To reach the goal of SQL limited measurements, high laser power and low mirror masses are required. A mirror mass of 100 g should allow this goal to be reached while still allowing a practical - though challenging - suspension design to be realised. In order to sufficiently isolate the optics, a triple-stage suspension (with two stages of vertical isolation) will be needed, where the final stage comprises a monolithic fused-silica structure (with the optics suspended from 20 µm fused silica elements) to reduce thermal noise.


Ancillary Optics

Other optics are required to steer and point the laser beam into and out of the main interferometer and the vacuum system. Due to the excellent isolation and stability provided by the isolated optical tables the SPI system, such ancillary optics can be isolated with relatively simple single or double stage suspensions systems, with no additional vertical isolation stages.



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