Design and construction status of CLIO

Construction of CLIO (cryogenic laser interferometer observatory) with 100 m baseline length has begun in the Kamioka mine. The tunnel for CLIO has been dug and infrastructure work is now in progress. CLIO is the final step to LCGT (large scale cryogenic gravitational wave telescope) and the first practical construction of a cryogenic interferometer in the world. The objective of CLIO is to demonstrate two of three features of LCGT, which are to utilize the quietness and stable environment of the underground site and to adopt cryogenic sapphire mirrors for thermal noise reduction. Also, it is a joint project by gravitational wave and geophysics researchers. CLIO has a locked Fabry-Perot configuration equipped with ring mode cleaners and cryocoolers to cool the sapphire mirrors to 20 K. The noise level of CLIO is designed to trace the thermoelastic noise of sapphire mirrors which varies from 10^-18 mHz^-1/2 at 300 K to 10^-19 mHz^-1/2 at 20 K around 100 Hz. A 7 m single-arm cryogenic test facility has been built at ICRR (Institute for Cosmic Ray Research), while the 20 m room temperature interferometer is in operation at Kamioka. Technical knowledge developed by these prototypes will be leveraged to realize CLIO.