Quantum Noise in a Fabry-Perot Interferometer Including the Influence of Diffraction Loss of Light

The DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is designed to detect gravitational waves at frequencies between 0.1 and 10 Hz. In this frequency band, one of the most important science targets detection primordial waves. DECIGO plans use a space interferometer with optical cavities increase its sensitivity. For evaluating sensitivity, diffraction laser light has be adequately considered. There are two kinds loss: leakage loss outside mirror higher-order mode loss. These effects treated differently inside Fabry-Perot (FP) cavity. We estimated them under conditions that FP cavity relatively high finesse modes do not resonate. As result, we found can represented as reduction effective regard quantum noise. This result useful for optimization design DECIGO. method also applicable any small beam cut sufficiently higher than 1.