3.2 Long baseline detectors on Earth

An interferometric design of gravitational-wave detector offers the possibility of very high sensitivities over a wide range of frequency. It uses test masses, which are widely separated and freely suspended as pendulums to isolate against seismic noise and reduce the effects of thermal noise; laser interferometry provides a means of sensing the motion of these masses produced as they interact with a gravitational wave (Figure 3View Image).
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Figure 3: Schematic of gravitational-wave detector using laser interferometry.

This technique is based on the Michelson interferometer and is particularly suited to the detection of gravitational waves as they have a quadrupole nature. Waves propagating perpendicular to the plane of the interferometer will result in one arm of the interferometer being increased in length while the other arm is decreased and vice versa. The induced change in the length of the interferometer arms results in a small change in the intensity of the light observed at the interferometer output.

As will be explained in detail in the next Section 4, the sensitivity of an interferometric gravitational-wave detector is limited by noise from various sources. Taking this frequency-dependent noise floor into account, a design goal can be estimated for a particular detector design. For example, the design sensitivity for initial LIGO is show in Figure 4View Image plotted alongside the achieved sensitivities of the three individual interferometers during the fifth science run (see Section 6.1). Such strain sensitivities are expected to allow a reasonable probability for detecting gravitational wave sources. However, in order to guarantee the observation of a full range of sources and to initiate gravitational-wave astronomy, a sensitivity or noise performance approximately ten times better in the mid-frequency range and several orders of magnitude better at 10 Hz, is desired. Therefore, initial detectors will be upgraded to an advanced configuration, such as Advanced LIGO, which will be ready for operation around 2015.

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Figure 4: Measured sensitivity of the initial LIGO interferometers during the S5 science run (see Section 6.1.2). Reproduced with permission from [213Jump To The Next Citation Point].

For the initial interferometric detectors, a noise floor in strain close to 2 × 10–23 Hz–1/2 was achieved. Detecting a strain in space at this level implies measuring a residual motion of each of the test masses of about 8 × 10–20 m/Hz–1/2 over part of the operating range of the detector, which may be from ≃ 10 Hz to a few kHz. Advanced detectors will push this target down further by another factor of 10 – 15. This sets a formidable goal for the optical detection system at the output of the interferometer.

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