Optical rigidity in signal-recycled configurations of laser gravitational-wave detectors

We analyze laser gravitational-wave interferometers with additional mirror in signal port (signal-recycled configurations) which use the optical rigidity both for the Advanced LIGO (Fabry-Perot/Michelson) and GEO-600 (pure Michelson) topologies. We found a set of parameters to obtain better than Standard Quantum Limit (SQL) sensitivity without increasing laser pumping. The gain in sensitivity is inversely proportional to the bandwidth inside which this gain is achieved. By varying parameters one can change the shape of sensitivity curve from a single minimum curve to a double minimum curve through different regimes. This gives an additional possibility to increase the bandwidth but at the sacrifice of sensitivity. We also show that our “narrow band” regimes provide the gain in signal to noise ratio even for signals with wide spectrum.