  DECIGO is the future Japanese gravitational wave detector in outer space. We
previously set the default design parameters to provide a good target
sensitivity to detect the primordial gravitational waves (GWs). However, the
updated upper limit of the primordial GWs by the Planck observations motivated
us for further optimization of the target sensitivity. Previously, we had not
considered optical diffraction loss due to the very long cavity length. In this
paper, we optimize various DECIGO parameters by maximizing the signal-to-noise
ratio (SNR), for the primordial GWs to quantum noise including the effects of
diffraction loss. We evaluated the power spectrum density for one cluster in
DECIGO utilizing the quantum noise of one differential Fabry-Perot
interferometer. Then we calculated the SNR by correlating two clusters in the
same position. We performed the optimization for two cases: the constant
mirror-thickness case and the constant mirror-mass case. As a result, we
obtained the SNR dependence on the mirror radius, which also determines various
DECIGO parameters. This result is the first step toward optimizing the DECIGO
design by considering the practical constraints on the mirror dimension and
implementing other noise sources.
