KAGRA is a cryogenic interferometric gravitational-wave detector being constructed at the underground
site of Kamioka mine in Gifu prefecture, Japan. We performed an optimization of the interferomter
design, to achieve the best sensitivity and a stable operation, with boundary conditions of classical
noises and under various practical constraints, such as the size of the tunnel or the mirror cooling capacity.
Length and alignment sensing schemes for the robust control of the interferometer are developed. In this
paper, we describe the detailed design of the KAGRA interferometer as well as the reasoning behind
design choices
