LIGO Publication Abstract
Studies of Laser Interferometer Design and a Vibration Isolation System for Interferometric Graviational Wave Detectors
Giaime, J.A.
PhD Thesis, MIT (1995)

Two techniques are developed that are needed in the design of an interferometric gravitational wave (GW) detector such as the LIGO, or Long-baseline Interferometric Gravitational-wave Observatory. The detector sensitivity of a long-baseline instrument is studied. A multi-layer mechanical isolation stack to filter seismic noise from test masses is designed, modeled and tested in vacuum. This is a four-stage elastomer (spring) and stainless steel (mass) stack, consisting of a table resting on three separate legs of three layers each. The visco-elastic properties of elastomer springs are exploited to damp the stack's normal modes while providing rapid roll-off of stack transmission above these modal frequencies. The stack's transmission of base motion to top motion is measured in vacuum and compared with 3-D finite-element models. In one tested configuration, at 100 Hz, horizontal transmission is 10E-7, vertical transmission is 3x10E-6, and the cross-coupling terms are between these values. A length detection scheme using RF phase modulated light and synchronous detection is developed for Fabry-Perot arm power-recycled Michelson interferometer GW detectors. This scheme uses an external Mach-Zehnder interferometer to measure the GW signal, and a frequency-shifted subcarrier to measure ancillary interferometer degrees of freedom. Use of the Mach-Zehnder allows rejection of laser source amplitude noise from the output, as well as the ability to exploit well-balanced Fabry-Perot arms to reject frequency noise from the output. A long baseline GW detector using these techniques should meet the LIGO initial goal sensitivity to GW strain of h_{RMS} = 10E-21 at 100 Hz.


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