  In our previous research, simulation showed that a quantum locking scheme
with homodyne detection in sub-cavities is effective in surpassing the quantum
noise limit for Deci-hertz Interferometer Gravitational Wave Observatory
(DECIGO) in a limited frequency range. This time we have simulated an optical
spring effect in the sub-cavities of the quantum locking scheme. We found that
the optimized total quantum noise is reduced in a broader frequency band,
compared to the case without the optical spring effect significantly improving
the sensitivity of DECIGO to the primordial gravitational waves.
