  Most second-generation gravitational-wave detectors employ an optical
resonator called an output mode-cleaner (OMC), which filters out junk light
from the signal and the reference light, before it reaches the detection
photodiode located at the asymmetric port of the large-scale interferometer.
The optical parameters of the OMC should be carefully chosen to satisfy the
requirements to filter out unwanted light whilst transmitting the gravitational
wave signal and reference light. The Japanese gravitational-wave detector KAGRA
plans to use a very small amount of reference light to minimize the influence
of quantum noise for gravitational waves from binary neutron stars, and hence
the requirements to the OMC are more challenging than for other advanced
detectors. In this paper, we present the result of numerical simulations, which
verify that the OMC requirements are satisfied with the current design. We use
the simulation program FINESSE and realistic mirror phase maps that have the
same surface quality as the KAGRA test masses.
