  The beamplitter in high-power interferometers is subject to significant
radiation-pressure fluctuations. As a consequence, the phase relations which
appear in the beamsplitter coupling equations oscillate and phase modulation
fields are generated which add to the reflected fields. In this paper, the
transfer function of the various input fields impinging on the beamsplitter
from all four ports onto the output field is presented including
radiation-pressure effects. We apply the general solution of the coupling
equations to evaluate the input-output relations of the dual-recycled
laser-interferometer topology of the gravitational-wave detector GEO600 and the
power-recycling, signal-extraction topology of advanced LIGO. We show that the
input-output relation exhibits a bright-port dark-port coupling. This mechanism
is responsible for bright-port contributions to the noise density of the output
field and technical laser noise is expected to decrease the interferometer's
sensitivity at low frequencies. It is shown quantitatively that the issue of
technical laser noise is unimportant in this context if the interferometer
contains arm cavities.
