LIGO Publication Abstract
Noise Analysis of a Suspended High Power Michelson Interferometer
Saha, P.
PhD Thesis, MIT (1997)

The Laser Interferometer Gravitational Wave Observatory (LIGO Project) will search for gravitational waves by observing shifts in the interference of a Michelson interferometer. To start detecting gravitational waves with any measure of confidence, current estimates require the interferometer to be sensitive to differences of at least 10E-9 radians in the phase of light. Ground-based LIGO will offer this sensitivity in a band around 100 Hz. The sensitivity of LIGO is limited at frequencies below 200 Hz by random (mainly seismic and thermal) forces acting on its optical elements. Around and above 200 Hz -- where this "displacement noise" is no longer significant -- the sensitivity is determined by how well the interference shift can be determined at the detector. The quantum nature of coherent laser light in the interferometer imply a power fluctuation at the detector that scales as squareroot I, where I is the light intensity at the Michelson beam-splitter. With the signal power scaling as I, a fundamental sensitivity limit is thus set for detection. To obtain the desired sensitivity given this limit, LIGO will have close to 100 watts of laser light incident on the beam-splitter. No laboratory in the world, to the best of our knowledge, has had experience with interferometry at these high power levels prior to this thesis. This thesis experimentally tests an important assumption used in the noise estimates -- that the noise above 200 Hz will be quantum limited. To investigate the noise in interference detection at high power levels, a team at MIT (to which the author belongs) has constructed a suspended Michelson interferometer. The noise in the detection of the differential phase of this interferometer was investigated at two stages. At the first stage, several hundreds of milliwatts from a frequency stabilized Ar+ gas laser was incident directly on the beam-splitter. At the second stage, the input light was constructively built (recycled) to above 30 watts at the beam-splitter using an optical cavity -- this cavity was formed by placing a partially transmitting mirror in the input light as the front (power recycling) mirror and the Michelson interferometer as the back mirror. Our experience showed that above 1 kHz, the noise indeed was quantum limited consistent with the incident power. This led to the measurement of a phase noise sensitivity of about 3x10E-10 radian / squareroot Hz in the recycled interferometer, better than any known measurement to date. Below 1 kHz, we examined the "technical" noise sources that caused the noise to be above the quantum limit. We concluded that back scattering of light, input beam jitter, and residual frequency noise need to be controlled to get down to the fundamental limit required by LIGO at these frequencies. This thesis discusses the construction of the interferometer, the noise models and experiments used to analyze the measured phase noise at the two stages, and the implications of the experimental results to LIGO.


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