Path-finding towards a cryogenic interferometer for LIGO

LIGO is exploring cryogenics as a technique of last resort to reduce the thermal noise of mirrors and suspensions in gravitational wave interferometric detectors. Some of the cryogenic, or cryogenic-related, R&D done at LIGO is reported here. Some ideas being considered to make cryogenics possible may be useful even for room temperature detectors

Interferometric gravitational wave detectors vibrational isolation

Interferometric Gravitational Wave Detectors, coming online lin late 2000, look for small space strains, leading to apparent motions of test masses of 10-19 m or less; isolation from other forces is crucial. They require a formidable vibration isolation level in a frequency range between few Hz and few kHz. The off-band residual motion must be kept below 10-12 m not to saturate the phase sensors. These exceptional requirements are met, in all degrees of freedom, with a chain of active and passive filters. The key isolation mechanism is the use of mechanical oscillators above their resonant frequencies, pendula horizontally, springs vertically. Very high quality pendular suspensions are needed at the mirror level to limit the thermal noise from fluctuations in the dissipation mechanisms. Off-band electromagnetic actuators on or near the mirror keep its magnitude of attenuation in the longitudinal direction. To provide the bulk of the attenuation, virtually all in the vertical direction, they are suspended from Seismic Noise Attenuation Systems. Attenuation filters, either active or passive, are chained, each providing 2 or 3 orders of magnitude of attenuation. Passive attenuation is obtained with springs and pendula. The vertical is the toughest direction to deal with because the oscillators also fight against gravity. The vertical attenuation requirements, although orthogonal to the beam direction, are only slightly less stringent than the vertical ones due to cross-couplings (Earth curvature is the source of one of them). High internal damping springs organized in hierarchical stacks are used in most early designs. More advanced designs increasingly rely on chains of filters equipped with high quality cantilever springs driven to low resonant frequencies by different mechanisms. The Quality Factors of each resonance are actively and/or passively spoiled at the chain suspension point. IN the latest designs, Ultra Low Frequency Oscillators filter out the microseismic and other low frequency perturbations. This paper addresses one approach to achieving the required seismic isolation level

Passive, nonlinear, mechanical structures for seismic attenuation

Gravitational wave detectors aim to detect strain perturbations of space-time on the order of 10^-21-10^-22 at frequencies between 1 Hz and a few kHz. This space-time strain, integrated over kilometer scale interferometers, will induce movements of suspended mirrors on the order of 10^-18-10^-19 m. Seismic motion in this frequency band varies between 10^-6 m and 10^-12 m. Required seismic attenuation factors, as large as 10^-12, by far exceed the performance of motion sensors, and are only obtained by means of a chain of passive attenuators. High quality springs in configurations Yielding nonlinear response are used to generate attenuation at low frequency. Similarly, nonlinear mechanisms are used in the horizontal direction, A description of I some of these systems and some of the technical challenges that they involve is presented

Monolithic folded pendulum accelerometers for seismic monitoring and active isolation systems

A new class of very low noise low-frequency force-balance accelerometers is presented. The device has been designed for advanced mirror isolation systems of interferometric gravitational wave detectors. The accelerometer consists of a small monolithic folded pendulum with 2 s of natural period and an in-vacuum mechanical quality factor of 3000. The folded pendulum geometry, combined with the monolithic design, allows a unique 0.01% cross-axis residual coupling. Equipped with a high-resolution capacitance position sensor, it is capable of a noise-equivalent inertial displacement of 1-nm root mean square integrated over all the frequencies above 0.01 Hz. The main features of this new accelerometer are here reviewed. New possible applications of monolithic folded pendula in geophysical instrumentation are discussed

Monolithic folded pendulum accelerometers for seismic monitoring and active isolation systems

A new class of very low noise low-frequency force-balance accelerometers is presented. The device has been designed for advanced mirror isolation systems of interferometric gravitational wave detectors. The accelerometer consists of a small monolithic folded pendulum with 2 s of natural period and an in-vacuum mechanical quality factor of 3000. The folded pendulum geometry, combined with the monolithic design, allows a unique 0.01% cross-axis residual coupling. Equipped with a high-resolution capacitance position sensor, it is capable of a noise-equivalent inertial displacement of 1-nm root mean square integrated over all the frequencies above 0.01 Hz. The main features of this new accelerometer are here reviewed. New possible applications of monolithic folded pendula in geophysical instrumentation are discussed

Interferometric gravitational wave detectors vibrational isolation

Interferometric Gravitational Wave Detectors, coming online lin late 2000, look for small space strains, leading to apparent motions of test masses of 10-19 m or less; isolation from other forces is crucial. They require a formidable vibration isolation level in a frequency range between few Hz and few kHz. The off-band residual motion must be kept below 10-12 m not to saturate the phase sensors. These exceptional requirements are met, in all degrees of freedom, with a chain of active and passive filters. The key isolation mechanism is the use of mechanical oscillators above their resonant frequencies, pendula horizontally, springs vertically. Very high quality pendular suspensions are needed at the mirror level to limit the thermal noise from fluctuations in the dissipation mechanisms. Off-band electromagnetic actuators on or near the mirror keep its magnitude of attenuation in the longitudinal direction. To provide the bulk of the attenuation, virtually all in the vertical direction, they are suspended from Seismic Noise Attenuation Systems. Attenuation filters, either active or passive, are chained, each providing 2 or 3 orders of magnitude of attenuation. Passive attenuation is obtained with springs and pendula. The vertical is the toughest direction to deal with because the oscillators also fight against gravity. The vertical attenuation requirements, although orthogonal to the beam direction, are only slightly less stringent than the vertical ones due to cross-couplings (Earth curvature is the source of one of them). High internal damping springs organized in hierarchical stacks are used in most early designs. More advanced designs increasingly rely on chains of filters equipped with high quality cantilever springs driven to low resonant frequencies by different mechanisms. The Quality Factors of each resonance are actively and/or passively spoiled at the chain suspension point. IN the latest designs, Ultra Low Frequency Oscillators filter out the microseismic and other low frequency perturbations. This paper addresses one approach to achieving the required seismic isolation level

Review: accelerometer development for use in gravitational wave-detection interferometers

Accelerometers were and are being developed to instrument the seismic attenuation chains of gravitational wave (GW) interferometric detectors. The main requirements of these instruments are strong directionality (>10^3 rejection of signal from orthogonal directions for feedback) and vacuum compatibility as well as high low-frequency sensitivity. Because of the directionality requirements, GW accelerometers tend to be more specialized for sensing of different degrees of freedom (horizontal, vertical, and tilt) than the classical geophysics seismometers but share several characteristics with these. The initial design of GW-dedicated accelerometers was inspired by geophysics instruments. Techniques developed for the GW field are now starting to spill over back into the geophysics field