Parametric signal amplification to create a stiff optical bar

•A stiff optical spring bar is a way to detect gravitational waves at high frequencies.•A parametric amplification technique realizes the stiff optical bar without increasing the circulating laser power.•This letter introduces our theoretical work on a new interferometric configuration with the parametric amplifier.

An optical cavity consisting of optically trapped mirrors makes a resonant bar that can be stiffer than diamond. A limitation of the stiffness arises in the length of the optical bar as a consequence of the finite light speed. High laser power and light mass mirrors are essential for realization of a long and stiff optical bar that can be useful for example in the gravitational-wave detector aiming at the observation of a signal from neutron-star collisions, supernovae, etc. In this letter, we introduce a parametric signal amplification scheme that realizes the long and stiff optical bar with a non-linear crystal inside the signal-recycling cavity.