P010010-00-R - LIGO - California Institute of Technology

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The LIGO interferometers are large enough such that reaching the maximum
storage time isn’t a problem. As a matter of fact, the bandwidth could be made much
smaller, increasing the storage time. This wouldn’t be good for the integration of the
signal, but it would be good for the energy stored in the detector. Increased storage
time in the arms causes more of the light to be lost through the arm loss mechanisms,
which are much smaller than the loss mechanisms in the power recycling cavity. The
less light returning from the arms, the less efficiently light is lost due to the large
power recycling losses, and the more light energy is stored. In 1993, Mizuno realized
that the same signal mirror that Meers proposed to recycle the signal could be used
to more efficiently “extract” the signal from a high storage time arm cavity.[27] By
choosing the propagation phase defined by the signal mirror to be resonant, the band
of frequencies resonant with the signal cavity transmit more efficiently, and so the
storage time for signals in the arms is decreased. This is known as resonant sideband
extraction, or RSE.
The issue of the losses in the power recycling cavity is actually somewhat more
complicated than the simple linear losses at AR coatings and scatter and absorption
in the substrates. The absorption in the substrates will heat the test masses with a
temperature distribution similar to the Gaussian intensity profile of the beam. This
temperature gradient forms a lens in the substrate via the dependence of the index
of refraction on temperature. The lenses created by the substrates of the optics are
sources of loss due to the fact that they change the profile of the beam, causing
scattering of light out of the fundamental spatial mode into higher order modes of
the modified cavity.[28] RSE addresses this issue simply by taking more light out
of the power cavity and placing it in the arm cavities where these effects should be
smaller.[25]
Similar to dual-recycling, detuning the signal cavity phase away from resonance
will generate frequency responses which have peak sensitivities at frequencies away
from DC. Thus, RSE allows the design of a detector which can store more light in the
arms, while optimizing the shape of the transfer function with respect to shot noise
and the desired gravitational wave source.