Laser-interferometric detectors for gravitational wave backgrounds at 100 MHz : Detector design and sensitivity
Atsushi Nishizawa, Seiji Kawamura, Tomotada Akutsu, Koji Arai, Kazuhiro Yamamoto, Daisuke Tatsumi, Erina Nishida, Masa-aki Sakagami, Takeshi Chiba, Ryuichi Takahashi, and Naoshi Sugiyama

Recently, observational searches for gravitational wave background (GWB) have been developed and given direct and indirect constraints on the energy density of GWB in a broad range of frequencies. These constraints have already rejected some theoretical models of large GWB spectra. However, at $100 \, \rm{MHz}$, there is no strict upper limit from {\it{direct}} observation, though the {\it{indirect}} limit by $^4\rm{He}$ abundance due to big-bang nucleosynthesis exists. In this paper, we propose an experiment with laser interferometers searching GWB at $100 \, \rm{MHz}$. We considered three detector designs and evaluated the GW response functions of a single detector. As a result, we found that, at $100 \, \rm{MHz}$, the most sensitive detector is the design, a so-called synchronous recycling interferometer, which has better sensitivity than an ordinary Fabry-Perot Michelson interferometer by a factor of 3.3 at $100 \, \rm{MHz}$. When we select the arm length of $0.75\,\rm{m}$ and realistic optical parameters, the best sensitivity achievable is $h \approx 7.8 \times 10^{-21}\,\rm{Hz}^{-1/2}$ at $100 \, \rm{MHz}$ with bandwidth $\sim 2 \, \rm{kHz}$.

© 2008 The American Physical Society.