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Direct observations of gravitational waves at frequencies below 10 Hz will play a crucial role in fully exploiting the potential of gravitational wave astronomy. One approach to pursue this direction is the utilization of laser interferometers equipped with Fabry–Pérot optical cavities in space. However, there are a number of practical challenges in following this path. In particular, the implementation of precision control for cavity lengths and the suppression of laser phase noises may prevent a practical detector design. To circumvent such difficulties, we propose a new interferometer topology, called a back-linked Fabry–Pérot interferometer, where precision length controls are not required and an offline subtraction scheme for laser phase noises is readily applicable. This article presents the principle idea and the associated sensitivity analyses. Despite additional noises, a strain sensitivity of $$7\times10^{-23}\,\textrm{Hz}^{-1/2}$$ may be attainable in the decihertz band. Several technological developments must occur and studies must be carried out to pave the way for the implementation. [ABSTRACT FROM AUTHOR]

LISA is a space gravitational-wave mission that will open the unexplored gravitational-wave frequency window at around millihertz, shedding light on the study of supermassive black holes and the nature of gravity. The LISA project has been propelled by international collaboration in order to maximize the scientific outcome. With the aim of making scientifically important contributions to LISA, instrument and science groups were newly formed in Japan. This article summarizes the current status of the contribution activities conducted by each group to date, highlighting a few selected topics including the development of photoreceivers and theoretical studies on compact binaries and extreme mass ratio inspirals. [ABSTRACT FROM AUTHOR]

We report the mirror suspension design for large-scale cryogenic gravitational wave telescope, KAGRA, during bKAGRA phase 1. Mirror thermal noise is one of the fundamental noises for room-temperature gravitational-wave detectors such as Advanced LIGO and Advanced Virgo. Thus, reduction of thermal noise is required for further improvement of their sensitivity. One effective approach for reducing thermal noise is to cool the mirrors. There are many technical challenges that must be overcome to cool the mirrors, such as cryocooler induced vibrations, thermal drift in suspensions, and reduction in duty cycling due to the increased number of potential failure mechanisms. Our mirror suspension has a black coating that makes radiative cooling more efficient. For conduction cooling, we developed ultra high purity aluminum heat links, which yield high thermal conductivity while keeping the spring constant sufficiently small. A unique inclination adjustment system, called moving mass, is used for aligning the mirror orientation in pitch. Photo-reflective displacement sensors, which have a large range, are installed for damping control on marionette recoil mass and intermediate recoil mass. Samarium cobalt magnets are used for coil-magnet actuators to prevent significant change of magnetism between room temperature and cryogenic temperature. In this paper, the design of our first cryogenic payload and its performance during bKAGRA phase 1 are discussed. [ABSTRACT FROM AUTHOR]

Optical phase-insensitive heterodyne (beat-note) detection, which measures the relative phase of two beams at different frequencies through their interference, is a key sensing technology for various spatial/temporal measurements, such as frequency measurements in optical frequency combs. However, its sensitivity is limited not only by shot noise from the signal frequency band but also by the extra shot noise from an image band, known as the 3-dB noise penalty. Here, we propose a method to remove shot noise from all these bands using squeezed light. We also demonstrate beyond-3-dB noise reduction experimentally, confirming that our method actually reduces shot noise from both the signal and extra bands simultaneously. Our work should boost the sensitivity of various spatial/temporal measurements beyond the current limitations.

Lock acquisition of a suspended optical cavity can be a highly stochastic process and is therefore nontrivial. Guided lock is a method to make lock acquisition less stochastic by decelerating the motion of the cavity length based on an extrapolation of the motion from an instantaneous velocity measurement. We propose an improved scheme that is less susceptible to seismic disturbances by incorporating the acceleration as a higher-order correction in the extrapolation. We implemented the new scheme in a 300-m suspended Fabry-Perot cavity and improved the success rate of lock acquisition by a factor of 30.

Parametric instabilities have long been studied as a potentially limiting effect in high-power interferometric gravitational wave detectors. Until now, however, these instabilities have never been observed in a kilometer-scale interferometer. In this Letter, we describe the first observation of parametric instability in a gravitational wave detector, and the means by which it has been removed as a barrier to progress.

The effects of residual amplitude modulation (RAM) in laser interferometers using heterodyne sensing can be substantial and difficult to mitigate. In this work, we analyze the effects of RAM on a complex laser interferometer used for gravitational wave detection. The RAM introduces unwanted offsets in the cavity length signals and thereby shifts the operating point of the optical cavities from the nominal point via feedback control. This shift causes variations in the sensing matrix, and leads to degradation in the performance of the precision noise subtraction scheme of the multiple-degree-of-freedom control system. In addition, such detuned optical cavities produce an optomechanical spring, which also perturbs the sensing matrix. We use our simulations to derive requirements on RAM for the Advanced LIGO (aLIGO) detectors, and show that the RAM expected in aLIGO will not limit its sensitivity.