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First data from LIGO's Thermal Noise Interferometer

First data from LIGO's Thermal Noise Interferometer

- Contributed by Eric Black

Even as scientists are hard at work putting together the first-generation interferometers at Hanford and Livingston, other groups within LIGO are thinking over ways to improve the technology of gravitational wave detection. There is an extensive and vigorous effort to develop an advanced interferometer with an even greater sensitivity than the current instrument. One area of research involves looking for better materials for test masses.

Fundamental physics places certain limits on the sensitivity of any interferometer. LIGO is expected to be limited at low frequencies by seismic noise, at high frequencies by shot noise, and in the mid range (tens of Hertz to a few hundred Hertz) by thermal noise in the mirrors themselves. This thermal noise is due to the motions of the atoms inside the mirrors, caused by heat, and limits our sensitivity right in the frequency range where we would like to look for gravitational waves. It is clearly worthwhile for us to examine ways of reducing the thermal noise at these frequencies, and one way is to use very high-quality materials in our mirrors. Current-generation LIGO mirrors are made from high-grade, synthetic fused silica, and when the instrument is completed they are expected to exhibit some of the lowest levels of thermal noise ever observed in an interferometer. Still, current astrophysical models predict that even at this sensitivity level, a large class of interesting gravitational wave events will go unobserved, lost in the noise of the instrument. At some point, we will want to upgrade our instrument and catch these events.

One candidate material that might have better noise performance than fused silica is sapphire. There exists an established industry for growing synthetic sapphire, mostly for military applications, and a number of people are looking into the possibility of using high-grade, synthetic sapphire test masses to reduce the thermal noise in an advanced LIGO. The Thermal Noise Interferometer is a small, test-bed instrument on Caltech's campus designed to evaluate the noise performance of sapphire in an environment similar to that of a gravitational wave observatory. The instrument is currently under construction--much like the main interferometers at Hanford and Livingston--and on Thursday, July 5, 2001, we achieved first lock.

The Thermal Noise Interferometer was not very reliable initially. The first lock lasted only 15 seconds. But we have made considerable improvements since then. By August 7, the lock had been improved to the point that one arm of the suspended interferometer would remain locked for over an hour, and the sensitivity had been improved by about a factor of ten. Our current displacement sensitivity (see graph above) is approximately 3e-17m/rHz at 1kHz, and we are now focused on improving this level.

We are using fused silica mirrors in the Thermal Noise Interferometer presently while our sapphire test optics are being manufactured. We expect these sapphire masses to be ready for installation early in 2002.

Shanti Rao, Ken Libbrecht, Seiji Kawamura, and Eric Black.

Above: The Thermal Noise Interferometer team. From left to right, Shanti Rao, Ken Libbrecht, Seiji Kawamura, and Eric Black. Not shown are Luca Matone and Kevin Schulz.