Interferometer design of the KAGRA gravitational wave detector

TL;DR

This paper presents a comprehensive design optimization for the KAGRA interferometer, balancing classical noises (seismic and thermal) with quantum-noise shaping to maximize the inspiral-range $IR$ while enabling stable operation. It integrates a dual-recycling configuration (BRSE/DRSE) with quantum-non-demolition techniques, including DC readout and an optical-spring detuned SRC, to beat the standard quantum limit within practical constraints. The authors detail RF sensing schemes, macroscopic-length tuning, and a folded Gouy-phase approach to suppress higher-order modes, along with a rigorous spatial-mode and alignment analysis using Optickle-based simulations. The work establishes a fully specified interferometer design, including mirror reflectivities, Gouy-phase choices, and control-signals, and discusses remaining alignment-margin challenges and deployment timelines for 2014 onward. The outcomes enable KAGRA to operate with enhanced sensitivity in an underground, cryogenic environment, contributing to a broader global network of gravitational-wave observatories.

Abstract

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.

Figures (18)

• Figure 1: Schematic of the KAGRA interferometer. Names of the mirrors as well as the signal detection ports are shown.
• Figure 2: Estimated noises of KAGRA. The total noise is the quadratic sum of all the noises.
• Figure 3: Inspiral range with different finesse and signal recycling mirror reflectivity.
• Figure 4: Resonant conditions of the carrier and the RF sidebands. Each field is represented by lines of a distinct style. A field is resonant in the parts of the interferometer the corresponding lines are drawn.
• Figure 5: Ratio of the RF sideband reflection phases by the arm cavities. We want to set it to 8/3, which is indicated by the green line.