Optical motion sensor for resonant-bar gravitational wave antennas

Area of Science:

• Experimental Physics
• Gravitational Wave Detection
• Optical Sensing

Background:

• Fabry-Perot cavities are sensitive to minute length changes.
• Gravitational wave antennas require high-sensitivity displacement sensors.
• Understanding noise sources is critical for detector performance.

Purpose of the Study:

• To measure length fluctuations in a Fabry-Perot sensor cavity using an optical method.
• To determine the sensor's sensitivity to vibration amplitudes relevant to gravitational wave detection.
• To characterize noise sources affecting the sensor's performance.

Main Methods:

• An optical method was employed to monitor mirror separation fluctuations in a Fabry-Perot cavity.
• Noise measurements were conducted to assess sensitivity in the 1.1-2.1 kHz frequency range.
• The experiment involved a cavity finesse of 117 and estimated mirror power dissipation of 1.9 μW.

Main Results:

• The root-mean-square (rms) spectral noise density for length fluctuations was measured at 3.7 x 10^-15 m/Hz^(1/2).
• This noise is attributed to a combination of electronic and environmental vibrational noise.
• A projected noise temperature of 18 μK and a strain resolution (h) of 3.7 x 10^-18 /Hz^(1/2) were calculated for a gravitational wave detector.

Conclusions:

• The developed optical sensor shows promise for gravitational wave detection applications.
• Sensitivity is limited by electronic and environmental noise, suggesting areas for improvement.
• Higher finesse mirrors are expected to enhance sensor sensitivity in quiet environments.