  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 is the detection
of primordial gravitational waves. DECIGO plans to use a space interferometer
with optical cavities to increase its sensitivity. For evaluating its
sensitivity, diffraction of the laser light has to be adequately considered.
There are two kinds of diffraction loss: leakage loss outside the mirror and
higher-order mode loss. These effects are treated differently inside and
outside of the Fabry-Perot (FP) cavity. We estimated them under the conditions
that the FP cavity has a relatively high finesse and the higher-order modes do
not resonate. As a result, we found that the effects can be represented as a
reduction of the effective finesse of the cavity with regard to quantum noise.
This result is useful for optimization of the design of DECIGO. This method is
also applicable to all FP cavities with a relatively high finesse and
significant diffraction loss in any interferometer.
