Optical interferometry enables us to obtain displacement information of an object through a phase shift of reflected electromagnetic waves. An optomechanical coupling is a naturally existing feedback system in the interferometry, which has been applied to a variety of precise measurements including quantum ground state cooling of a macroscopic object, gravitational-wave detection, and nuclear magnetic resonance. The optomechanical coupling can be tuned through an initial offset to a resonant cavity mode and it is hitherto the only way to control the feedback system. Here we propose an active feedback system using the optomechanical coupling and a quantum filter that can be made of either a non-linear crystal or a cryogenic micro-resonator. The feedback system creates a resonator called "optical spring." An additional quantum feedback loop with a non-linear crystal increases the real part of the spring (signal gain enhancement), while in the foreseen conditions, a feedback loop with a cryogenic micro-resonator decreases the imaginary part of the spring (signal bandwidth enhancement). Our proposal is two-sided. First we establish proof-of-principle experiments for the two different types of quantum feedback system. In parallel, we start new experiments or rapidly promote on-going experiments to explore an innovative application of these state-of-the-art techniques. (i) Test of macroscopic quantum mechanics: The existence of a fundamental length at Planck scale leads to a modification of Heisenberg's uncertainty principle. An extremely high precision measurement of a macroscopic object is required to observe a possible deviation from conventional quantum mechanics. We propose to perform three experiments with different resonators: a cryogenic micro-pillar (30 µg), optically-levitated mirrors (1 mg), and a torsion pendulum (10 mg). As possible deviations from standard quantum mechanics are expected to depend on the probed mass, a comparison of the results in our three state-of-art experiments might open a window to the quantum-classical border. (ii) Gravitational-wave detection: Gravitational waves (GW) are ripples of spacetime generated by massive astronomical events. A gravitational-wave detector is a km-scale Michelson interferometer with an optical resonator in each baseline. Both the signal gain and signal bandwidth enhancement can be used to improve the sensitivity of a gravitational-wave detector. A significant improvement can be expected at frequencies higher than a few kilo-Hertz where a number of valuable astrophysics sources are yet to be observed by currently operating detectors (a) The signal gain enhancement enables us to create a 3-km optical spring with 40-kg mirrors resonating at 3 kHz, and a gravitational-wave signal is parametrically amplified at the resonant frequencies of the spring. We propose to design a next-generation gravitational-wave detector based on this scheme after demonstrating the enhancement in the prototype experiment. (b) The signal bandwidth enhancement enables us to expand the observation band from a few hundred Hertz to a few ten kilohertz. (iii) Measurement of nuclear magnetic resonance: A simple electric LC circuit can play a role of the quantum feedback filter. Although classical thermal noise in the coil will overwrite the quantum property of our optomechanical oscillator, the change of the dynamics provides us with information of the coil. We call it Electro-Mechano-Optical (EMO) system. This transition can be applied to nuclear magnetic resonance (NMR). Up-conversion of NMR signals from radio to optical frequencies with a metal-coated, high-Q membrane oscillator is a promising technique, with signal-to-noise ratio (SNR) currently limited by Brownian noise of the membrane. Using a state-of-the-art phononic- and photonic-crystal embedded SiN membrane, we are aiming at improving both mechanical and optical Qs of the EMO system to reduce the Brownian noise and thus to boost the SNR.