  Quantum optomechanical systems enable the study of fundamental questions on
quantum nature of massive objects. For that a strong coupling between light and
mechanical motion is required, which presents a challenge for massive objects.
In particular large interferometric sensors with low frequency oscillators are
difficult to bring into quantum regime. Here we propose a modification of the
Michelson-Sagnac interferometer, which allows to boost the optomechanical
coupling strength. This is done by unbalancing the central beam-splitter of the
interferometer, allowing to balance two types of optomechanical coupling
present in the system: dissipative and dispersive. We analyse two different
configurations, when the optomechanical cavity is formed by the mirror for the
laser pump field (power-recycling), and by the mirror for the signal field
(signal-recycling). We show that the imbalance of the beam splitter allows to
dramatically increase the optical cooling of the test mass motion. We also
formulate the conditions for observing quantum radiation-pressure noise and
ponderomotive squeezing. Our configuration can serve as the basis for more
complex modifications of the interferometer that would utilize the enhanced
coupling strength. This will allow to efficiently reach quantum state of large
test masses, opening the way to studying fundamental aspects of quantum
mechanics and experimental search for quantum gravity.
