arXiv subjects

Wataru Kokuyama

Publications and source records attributed to Wataru Kokuyama.

13 recordsLinked to original sources

arXiv2026

Temperature dependence of broadband seismometer sensitivity

Broadband seismometers are widely used in global observation networks deployed for geophysics research. Recently, the calibration of their sensitivity has become an important factor for ensuring observation accuracy. One of the limiting factors of calibration uncertainty is the temperature dependence of the sensitivity because seismometers operate in a wide range of temperatures. However, systems that accurately measure the temperature coefficient in seismometers have not been established. Herein, we develop such a system using a triaxial vibration exciter combined with a thermostatic chamber. Using this system, we calibrated several broadband seismometers (Trillium Compact, Trillium Horizon 360, and CMG-3T) at various temperatures, ranging from $-15 ^\circ$C to $+45 ^\circ$C. We found that the temperature coefficient was $(0.11\pm0.01)$ %/$^\circ$C, which can be attributed to the internal magnet of the seismometers. The variation of the phase delay corresponded to a change of less than 2 ms in output time delay. Additionally, we found that the relative frequency response below 1 Hz was stable against temperature variations. These values are useful for evaluating the measurement accuracy of seismometer observation networks.

physics.geo-ph↗

arXiv2025

Accurate laboratory testing of low-frequency triaxial vibration sensors under various environmental conditions

Triaxial vibration sensor are widely used used in various application. Recently, low-cost sensors based on micro electro mechanical system (MEMS) technology are also becoming more widely adopted. However, their measurement accuracy can be affected by environmental factors such as temperature. In this study, we developed an environmental testing system integrated with a triaxial vibration exciter. The system can reproduce long-stroke, low-frequency triaxial vibrations -- such as those caused by huge earthquakes -- under temperatures ranging from $-30~^\circ\mathrm{C}$ to $+80~^\circ\mathrm{C}$. Using this system, the measurement accuracy of vibration sensors can be evaluated under different environmental conditions. The system provides highly accurate reference measurements using a laser interferometer and reference accelerometers that are primarily calibrated within the system. The overall accuracy of the reference vibration measurement is estimated to be approximately 0.23~\%. Based on these reference measurements, we investigated the accuracy of earthquake observations using a MEMS accelerometer as a demonstration. The system configuration and testing procedures are presented in this paper.

physics.ins-det↗

arXiv2025

Optical sound pressure measurement using Fabry-Pérot cavity for primary acoustic standards

Optical sound pressure measurement is a promising technology to establish primary acoustic standards without reliance on specific types of microphones. We developed a precision optical sound pressure measurement system by combining a Fabry-Pérot optical cavity, a phase-stabilized optical frequency comb, and a custom-made phasemeter. The optical cavity detects changes in the air's refractive index induced by sound waves as changes in its resonance frequency. A continuous-wave laser frequency is stabilized at the resonance, and the frequency comb detects the changes in the laser frequency. The frequency changes are measured with high sensitivity and accuracy using a phasemeter that we developed. The sound pressures measured by this system agreed with the measurement value obtained using a reference microphone within 5% at sound pressure levels of 78 dB and 84 dB, within a frequency range of 100 Hz to 1 kHz. A systematic deviation of 2.6% was observed, with the optical system yielding higher values than the microphone. To identify the cause of this deviation, we performed vibration displacement measurements of the cavity mirrors and finite element analysis, which revealed that fluctuations in the optical path length due to insufficient fixation of the mirrors were responsible.

physics.ins-det↗

arXiv2025

Measurement of the intrinsic sensitivity for a single-ended accelerometer without the influence of the mounting condition

The calibration technique for accelerometers has been internationally developed for up to 20 kHz to ensure the reliability of vibration measurement. However, it has been established that the calibrated sensitivity changes at over 10 kHz depending on the mounting conditions, and this makes it difficult to accurately measure the characteristics of accelerometers and degrades the accuracy of high-frequency vibration measurements. Thus, in this study, we developed a reversed-calibration method for measuring the intrinsic sensitivity of an accelerometer without the influence of the mounting conditions. Through demonstration experiment, the intrinsic resonance structure of the accelerometer at approximately 45.8 kHz was adequately determined. Furthermore, the result was independently confirmed by fitting the conventional adapter-calibration results up to 100 kHz with four different materials based on the dynamic three-body model. Concurrently, the material dependency observed during adapter calibration was quantitatively analyzed, after which its relationship with the Young's modulus was extracted. Overall, these results deepen our understanding of the performance of the accelerometer at above 10 kHz, which is essential in vibration metrology and accelerometer development.

physics.ins-det↗

arXiv2022

Primary accelerometer calibration with two-axis automatic positioning stage

In this study, we developed an automated, multipoint primary accelerometer calibration system using a two-axis positioning stage and a heterodyne laser interferometer. The proposed system offers low-cost, convenient, and automated multipoint accelerometer calibration, enabling less calibration lead time. The positioning stage also offers better positioning repeatability of 1 um, which is impossible through manual alignments. We measured the surface deformation of a laser reflection adaptor for a single-ended accelerometer by measuring more than 450 measurement positions. Visualizing the deformation of laser reflection surfaces facilitates understanding the effects of deformation or nonrectilinear motion, which are among the most significant uncertainty components in high-frequency accelerometer calibrations.

physics.ins-det↗

arXiv2022

Precise sinusoidal signal extraction from noisy waveform in vibration calibration

Precise extraction of sinusoidal vibration parameters is essential for the dynamic calibration of vibration sensors, such as accelerometers. However, several standard methods have not yet been optimized for large background noise. In this work, signal processing methods to extract small vibration signals from noisy data in the case of accelerometer calibration is discussed. The results show that spectral leakage degrades calibration accuracy. Three methods based on the use of a filter, window function, and numerical differentiation are investigated with theoretical calculations, simulations, and experiments. These methods can effectively reduce the contribution of the calibration system noise. The uncertainty of micro vibration calibration in the National Metrology Institute of Japan is reduced by two orders of magnitudes using the proposed methods. The theoretical analyses in this work can lay the foundation for the optimization of signal processing in vibration calibration, and can be applied to other dynamic calibration fields.

eess.SP↗

arXiv2020

Phase meter based on zero-crossing counting of digitized signals

We developed a compact and easy-to-use phase meter based on a zero-crossing counting algorithm for digitized signals. Owing to the algorithm, the phase meter has low-noise and wide dynamic range. Low-noise differential phase measurements can be done for square waves (-204 $\mathrm{dBrad^{2}/Hz}$ for a 1-kHz, 1-$\mathrm{V_{p-p}}$ signal, 10 Hz-1 kHz offset, with cross-correlation) as well as sinusoidal waves, with a measurement error of $<1 \times 10^{-4}$ rad. We also demonstrated a direct phase measurement of an optical-beat note from a free-running laser over 10 decades (0.25 mHz-10 MHz) with a wide dynamic range of ~280 dB at 0.25 mHz. The phase meter can be an alternative for conventional phase meters and frequency counters in wide range of experiments.

physics.ins-det↗

arXiv2016

Simple digital phase-measuring algorithm for low-noise heterodyne interferometry

We present a digital algorithm for measuring the phase difference between two sinusoidal signals that combines the modified fringe-counting method with two-sample zero crossing to enable sequential signal processing. This technique can be applied to a phase meter for measuring dynamic phase differences with high resolution, particularly for heterodyne interferometry. The floor noise obtained from a demonstration with an electrical apparatus is $5\times10^{-8} \mathrm{rad/\sqrt{Hz}}$ at frequencies above approximately 0.1 Hz. In addition, by applying this method to a commercial heterodyne interferometer, the floor-noise level is confirmed to be $7\times10^{-14} \mathrm{m/\sqrt{Hz}}$ from 4 kHz to 1 MHz. We also confirm the validity of the algorithm by comparing its results with those from a standard homodyne interferometer for measuring shock-motion peak acceleration greater than 5000 m/s^2 and a 10 mm stroke.

physics.ins-det↗

arXiv2016

Higher order test of Lorentz invariance with an optical ring cavity

We have developed an apparatus to search for the higher-order Lorentz violation in photons by measuring the resonant frequency difference between two counterpropagating directions of an asymmetric optical ring cavity. From the year-long data taken between 2012 and 2013, we found no evidence for the light speed anisotropy at the level of $δc/c \lesssim 10^{-15}$. Limits on the dipole components of the anisotropy are improved by more than an order of magnitude, and limits on the hexapole components are obtained for the first time. An overview of our apparatus and the data analysis in the framework of the spherical harmonics decomposition of anisotropy are presented. We also present the status of the recent upgrade of the apparatus.

gr-qc↗

arXiv2014

Search for a Stochastic Gravitational-wave Background using a pair of Torsion-bar Antennas

We have set a new upper limit on the stochastic gravitational wave background (SGWB) using two prototype Torsion-bar Antennas (TOBAs). TOBA is a low-frequency gravitational-wave detector with bar-shaped test masses rotated by the tidal force of gravitational waves. As a result of simultaneous 7-hour observations with TOBAs in Tokyo and Kyoto in Japan, our upper limit with a confidence level of 95% is $Ω_{\rm gw}h_0^2 < 1.9 \times 10^{17}$ at 0.035 - 0.830 Hz, where $h_{0}$ is the Hubble constant in units of 100 km/s/Mpc and $Ω_{\rm gw}$ is the gravitational wave energy density per logarithmic frequency interval in units of the closure density. We successfully updated the upper limit and extended the explored frequency band.

gr-qc↗

arXiv2013

Testing Lorentz Invariance with a Double-Pass Optical Ring Cavity

We have developed an apparatus to test Lorentz invariance in the photon sector by measuring the resonant frequency difference between two counterpropagating directions of an asymmetric optical ring cavity using a double-pass configuration. No significant evidence for the violation was found at the level of $δc /c \lesssim 10^{-14}$. Details of our apparatus and recent results are presented.

hep-ph↗

arXiv2013

New Limit on Lorentz Violation Using a Double-Pass Optical Ring Cavity

A search for Lorentz violation in electrodynamics was performed by measuring the resonant frequency difference between two counterpropagating directions of an optical ring cavity. Our cavity contains a dielectric element, which makes our cavity sensitive to the violation. The laser frequency is stabilized to the counterclockwise resonance of the cavity, and the transmitted light is reflected back into the cavity for resonant frequency comparison with the clockwise resonance. This double-pass configuration enables a null experiment and gives high common mode rejection of environmental disturbances. We found no evidence for odd-parity anisotropy at the level of $δc /c \lesssim 10^{-14}$. Within the framework of the Standard Model Extension, our result put more than 5 times better limits on three odd-parity parameters $\tildeκ^{JK}_{o+}$ and a 12 times better limit on the scalar parameter $\tildeκ_{\tr}$ compared with the previous best limits.

gr-qc↗

arXiv2011

Upper Limit on Gravitational Wave Backgrounds at 0.2 Hz with Torsion-bar Antenna

We present the first upper limit on gravitational wave (GW) backgrounds at an unexplored frequency of 0.2 Hz using a torsion-bar antenna (TOBA). A TOBA was proposed to search for low-frequency GWs. We have developed a small-scaled TOBA and successfully found Ωgw(f) < 4.3 \times 1017 at 0.2 Hz as demonstration of the TOBA's capabilities, where Ωgw (f) is the GW energy density per logarithmic frequency interval in units of the closure density. Our result is the first nonintegrated limit to bridge the gap between the LIGO band (around 100 Hz) and the Cassini band (10-6 - 10-4 Hz).

gr-qc↗