Reduction Of Thermal Noise Research Articles

Upper limit to the transverse to longitudinal motion coupling of a waveguide mirror

Waveguide mirrors (WGMs) possess nano-structured surfaces which can potentially provide a significant reduction in thermal noise over conventional dielectric mirrors. To avoid introducing additional phase noise from the motion of the mirror transverse to the reflected light, however, they must possess a mechanism to suppress the phase effects associated with the incident light translating across the nano-structured surface. It has been shown that with carefully chosen parameters this additional phase noise can be suppressed. We present an experimental measurement of the coupling of transverse to longitudinal displacements in such a WGM designed for 1064 nm light. We place an upper limit on the level of measured transverse to longitudinal coupling of one part in seventeen thousand with 95% confidence, representing a significant improvement over a previously measured grating mirror.

Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 μm

The sensitivity of second and third generations of interferometric gravitational wave (GW) detectors will be limited by the thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating that is optimized for high reflectivity for a wavelength of 1064 nm. The absorption was measured at 1530 nm, where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than ppm, which is equivalent to ppm for a coating stack that is highly reflective at 1530 nm. While this is a very promising low absorption result for alternative low-loss coating materials, further work will be necessary to reach the requirements of ppm for future GW detectors.

Publisher’s Note: Thermal noise reduction and absorption optimization via multimaterial coatings [Phys. Rev. D91, 042001 (2015)

Publisher’s Note: Thermal noise reduction and absorption optimization via multimaterial coatings [Phys. Rev. D<b>91</b>, 042001 (2015)

Thermal noise reduction and absorption optimization via multimaterial coatings

Future gravitational wave detectors (GWDs) such as Advanced LIGO upgrades and the Einstein Telescope are planned to operate at cryogenic temperatures using crystalline silicon (cSi) test-mass mirrors at an operation wavelength of 1550 nm. The reduction in temperature in principle provides a direct reduction in coating thermal noise, but the presently used coating stacks which are composed of silica (${\mathrm{SiO}}_{2}$) and tantala (${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$) show cryogenic loss peaks which results in less thermal noise improvement than might be expected. Due to low mechanical loss at low temperature amorphous silicon (aSi) is a very promising candidate material for dielectric mirror coatings and could replace ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$. Unfortunately, such an $\mathrm{aSi}/{\mathrm{SiO}}_{2}$ coating is not suitable for use in GWDs due to high optical absorption in aSi coatings. We explore the use of a three material based coating stack. In this multimaterial design the low absorbing ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ in the outermost coating layers significantly reduces the incident light power, while aSi is used only in the lower bilayers to maintain low optical absorption. Such a coating design would enable a reduction of Brownian thermal noise by 25%. We show experimentally that an optical absorption of only $(5.3\ifmmode\pm\else\textpm\fi{}0.4)\text{ }\text{ }\mathrm{ppm}$ at 1550 nm should be achievable.

Comparison of Observing Modes for Statistical Estimation of the 21 cm Signal from the Epoch of Reionisation

AbstractNoise considerations for experiments that aim to statistically estimate the 21 cm signal from high redshift neutral hydrogen during the Epoch of Reionisation (EoR) using interferometric data are typically computed assuming a tracked observation, where the telescope pointing centre and instrument phase centre are the same over the observation. Current low frequency interferometers use aperture arrays of fixed dipoles, which are steered electronically on the sky, and have different properties to mechanically-steered single apertures, such as reduced sensitivity away from zenith, and discrete pointing positions on the sky. These properties encourage the use of two additional observing modes: (1) zenith drift, where the pointing centre remains fixed at the zenith, and the phase centre tracks the sky, and (2) drift + shift, a hybrid mode where the telescope uses discrete pointing centres, and the sky drifts during each fixed pointing. These three observing modes view the sky differently, and therefore yield different uncertainties in the power spectrum according to the balance of radiometric noise and cosmic variance. The coherence of measurements made by the instrument in these modes dictates the optimal reduction in thermal noise by combination of coherent modes, and the reduction in cosmic variance by combination of incoherent modes (views of different patches of the sky). Along with calibration and instrument stability considerations, the balance between these noise components provides one measure for the utility of these three modes for measuring a statistical signature of the EoR signal. We provide a general framework for estimating the uncertainty in the power spectrum for a given observing mode, telescope beam shape, and interferometer antenna distribution. We then apply this framework to the Murchison Widefield Array (MWA) using an analysis of the two-dimensional (2D) and one-dimensional (1D) power spectra for 900 hours of observing. We demonstrate that zenith drift scans can yield marginally lower uncertainty in the signal power compared with tracked scans for the MWA EoR experiment, and that moderately higher signal-to-noise ratio (S/N) estimates of the amplitude (3%) and slope (1%) of the 1D power spectrum are accessible, translating directly into a reduction in the required observing time to reach the same estimation precision. We find that the additional sensitivity of pointing at zenith, and the reduction in cosmic variance available with a zenith drift scan, makes this an attractive observing mode for current and future arrays.

Analysis and reduction of thermal magnetic noise in liquid-He dewar for sensitive low-field nuclear magnetic resonance measurements

For sensitive measurements of micro-Tesla nuclear magnetic resonance (<TEX>${\mu}T$</TEX>-NMR) signal, a low-noise superconducting quantum interference device (SQUID) system is needed. We have fabricated a liquid He dewar for an SQUID having a large diameter for the pickup coil. The initial test of the SQUID system showed much higher low-frequency magnetic noise caused by the thermal magnetic noise of the aluminum plates used for the vapor-cooled thermal shield material. The frequency dependence of the noise spectrum showed that the noise increases with the decrease of frequency. This behavior could be explained from a two-layer model; one generating the thermal noise and the other one shielding the thermal noise by eddy-current shielding. And the eddy-current shielding effect is strongly dependent on the frequency through the skin-depth. To minimize the loop size for the fluctuating thermal noise current, we changed the thermal shield material into insulated thin Cu mesh. The magnetic noise of the SQUID system became flat down to 0.1 Hz with a white noise of 0.3 <TEX>$fT/{\surd}Hz$</TEX>, including the other noise contributions such as SQUID electronics and magnetically shielded room, etc, which is acceptable for low-noise <TEX>${\mu}T$</TEX>-NMR experiments.

A correlation force spectrometer for single molecule measurements under tensile load

The dynamical-mechanical properties of a small region of fluid can be measured using two closely spaced thermally stimulated micrometer-scale cantilevers. We call this technique correlation force spectroscopy (CFS). We describe an instrument that is designed for characterizing the extensional properties of polymer molecules that straddle the gap between the two cantilevers and use it to measure the stiffness and damping (molecular friction) of a dextran molecule. The device is based on a commercial atomic force microscope, into which we have incorporated a second antiparallel cantilever. The deflection of each cantilever is measured in the frequency range dc–1 MHz and is used to generate the cross-correlation at equilibrium. The main advantage of cross-correlation measurements is the reduction in thermal noise, which sets a fundamental noise limit to force resolution. We show that the thermal noise in our cross-correlation measurements is less than one third of the value for single-cantilever force microscopy. The dynamics of the cantilever pair is modeled using the deterministic motion of a harmonic oscillator initially displaced from equilibrium, which yields the equilibrium auto and cross-correlations in cantilever displacement via the fluctuation-dissipation theorem. Fitted parameters from the model (stiffness and damping) are used to characterize the fluid at equilibrium, including any straddling molecules.

Entropy production and fluctuation theorems under feedback control: the molecularrefrigerator model revisited

We revisit the model of a Brownian particle in a heat bath submitted to an activelycontrolled force proportional to the velocity that leads to thermal noise reduction (colddamping). We investigate the influence of the continuous feedback on the fluctuations ofthe total entropy production and show that the explicit expression of the detailedfluctuation theorem involves different dynamics and observables in the forward andbackward processes. As an illustration, we study the analytically solvable case of aharmonic oscillator and calculate the characteristic function of the entropy production in anonequilibrium steady state. We then determine the corresponding large deviationfunction which results from an unusual interplay between ‘boundary’ and ‘bulk’contributions.

A chip-scale integrated cavity-electro-optomechanics platform

We present an integrated optomechanical and electromechanical nanocavity, in which a common mechanical degree of freedom is coupled to an ultrahigh-Q photonic crystal defect cavity and an electrical circuit. The system allows for wide-range, fast electrical tuning of the optical nanocavity resonances, and for electrical control of optical radiation pressure back-action effects such as mechanical amplification (phonon lasing), cooling, and stiffening. These sort of integrated devices offer a new means to efficiently interconvert weak microwave and optical signals, and are expected to pave the way for a new class of micro-sensors utilizing optomechanical back-action for thermal noise reduction and low-noise optical read-out.

Reducing thermal noise in future gravitational wave detectors by employing Khalili etalons

Reduction of thermal noise in dielectric mirror coatings is a key issue for the sensitivity improvement in the second and third generation interferometric gravitational wave detectors. Replacing an end mirror of the interferometer by an anti-resonant cavity (a so-called Khalili cavity) has been proposed to realize the reduction of the overall thermal noise level. In this article we show that the use of a Khalili etalon, which requires less hardware than a Khalili cavity, yields still a significant reduction of thermal noise. We identify the optimum distribution of coating layers on the front and rear surfaces of the etalon and compare the total noise budget with a conventional mirror. In addition we briefly discuss advantages and disadvantages of the Khalili etalon compared with the Khalili cavity in terms of technical aspects, such as interferometric length control and thermal lensing.

Control of Magnetic Fluctuations by Spin Current

We use microfocus Brillouin light scattering spectroscopy to study the interaction of spin current with magnetic fluctuations in a Permalloy microdisk located on top of a Pt strip carrying an electric current. We show that the fluctuations can be efficiently suppressed or enhanced by different directions of the electric current. Additionally, we find that the effect of spin current on magnetic fluctuations is strongly influenced by nonlinear magnon-magnon interactions. The observed phenomena can be used for controllable reduction of thermal noise in spintronic nanodevices.

Cryogenic Mirrors.The State-of-the-art in Interferometric Gravitational Wave Detectors

The "cryogenic mirror" is one of the most important key issues for the Large-scale Cryogenic Gravitational wave Telescope (LCGT), which is the Japanese interferometric gravitational wave detector project. In this article, three main advantages of cryogenic mirrors are introduced: reduction in thermal noise, decrease of the thermal lens effect, and suppression of the parametric instability. A future project planned in Europe, the Einstein Telescope (ET), which includes the use of cryogenic interferometers, is also introduced.

Measurement of thermal noise in multilayer coatings with optimized layer thickness

A standard quarter-wavelength multilayer optical coating will produce the highest reflectivity for a given number of coating layers, but in general it will not yield the lowest thermal noise for a prescribed reflectivity. Coatings with the layer thicknesses optimized to minimize thermal noise could be useful in future generation interferometric gravitational wave detectors where coating thermal noise is expected to limit the sensitivity of the instrument. We present the results of direct measurements of the thermal noise of a standard quarter-wavelength coating and a low noise optimized coating. The measurements indicate a reduction in thermal noise in line with modeling predictions.

Thermal-noise-limited underground interferometer CLIO

We report on the current status of CLIO (Cryogenic Laser Interferometer Observatory), which is a prototype interferometer for LCGT (large scale cryogenic gravitational-wave telescope). LCGT is a Japanese next-generation interferometric gravitational-wave detector featuring the use of cryogenic mirrors and a quiet underground site. The main purpose of CLIO is to demonstrate a reduction of the mirror thermal noise by cooling the sapphire mirrors. CLIO is located in an underground site of the Kamioka mine, 1000 m deep from the mountain top, to verify its advantages. After a few years of commissioning work, we have achieved a thermal-noise-limited sensitivity at room temperature. One of the main results of noise hunting was the elimination of thermal noise caused by a conductive coil holder coupled with a pendulum through magnets.

Breaking strength tests on silicon and sapphire bondings for gravitational wave detectors

The realization of third-generation gravitational wave interferometric detectors is under way. An important improvement in detector sensitivity can be obtained with the reduction of thermal noise by reaching a cryogenic stage. Two materials have been identified to perform efficiently at low temperatures: silicon and sapphire. In this work the breaking strength of the silicate bonding that glues cylinders of silicon (Si) and sapphire (Al2O3) has been investigated. All material combinations, flatness quality and orientations have been tested over time. Breaking strength between Si–Si and Si–Al2O3 shows a value similar to that of fused silica, while Al2O3–Al2O3 does not seem to be a promising combination.

Reduction and Possible Elimination of Coating Thermal Noise Using a Rigidly Controlled Cavity with a Quantum-Nondemolition Technique

Thermal noise of a mirror is one of the most important issues in high-precision measurements such as gravitational-wave detection or cold damping experiments. It has been pointed out that thermal noise of a mirror with multilayer coatings can be reduced by mechanical separation of the layers. In this Letter, we introduce a way to further reduce thermal noise by locking the mechanically separated mirrors. The reduction is limited by the standard quantum limit of control noise, but it can be overcome with a quantum-nondemolition technique, which finally raises a possibility of complete elimination of coating thermal noise.

Design and analysis of a CMOS passive Σ∆ ADC for low power RF transceivers

Design and analysis of a ΣΔ modulator with a passive switched capacitor loop filter is presented. Design steps for optimum loop filter design for quantization noise suppression and thermal noise reduction is outlined. Design specifications for sampling clock phase noise, reference buffer and input buffer settling is analyzed. Presented design has a 2nd-order loop filter and uses only metal-metal capacitors and thin oxide digital transistors with no additional components occupying less than 0.1 mm2 silicon area in 0.13 µm CMOS digital process. Measurement results show that the ADC achieves 80 dB peak SNR at a 100 kHz integration bandwidth with 1 pJ/sample conversion efficiency. With decimation filter power consumption of 0.22 mW at 104 MHz sampling rate, the ADC consumes only about 1 mA at 1.5 V for each channel.

On the reduction of thermal and flicker noise in ENG signal recording amplifiers

Because of the extremely low amplitude of the input signal, the design of electro-neuro-graph (ENG) amplifiers involves a special care for flicker and thermal noise reduction. The task becomes really challenging in the case of implantable electronics, because power consumption is restricted to few hundreds μW. In this work, two different circuit techniques aimed to reduce flicker and thermal noise, in ultra-low noise amplifiers for implantable medical devices, are demonstrated. The circuit design, and measurement results are presented, in both cases showing an excellent performance, and noise to power consumption trade-off. In the first circuit, a very simple low-pass Gm---C chopper amplifier is used for flicker noise cancellation. It consumes only 28 mW, with a measured input referred noise and offset of 2 $$ {{{\text{nV}}} \mathord{\left/ {\vphantom {{{\text{nV}}} {\sqrt {{\text{Hz}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {{\text{Hz}}} }} $$ , and 2.5 μV, respectively. In the second circuit, a ultra-low noise amplifier, a energy-efficient DC---DC down-converter, and low voltage design techniques are combined, for the reduction of thermal noise with a minimum power consumption. Measured input referred noise in this case was 5.5 $$ {{{\text{nV}}} \mathord{\left/ {\vphantom {{{\text{nV}}} {\sqrt {{\text{Hz}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {{\text{Hz}}} }} $$ at only 380 μW power consumption. Both circuits were fabricated in a 1.5 μm technology.

Parametric instabilities in the LCGT arm cavity

We evaluated the parametric instabilities of LCGT (Japanese interferometric gravitational wave detector project) arm cavity. The number of unstable modes of LCGT is 10-times smaller than that of Advanced LIGO (USA). Since the strength of the instabilities of LCGT depends on the mirror curvature more weakly than that of Advanced LIGO, the requirement of the mirror curvature accuracy is easier to be achieved. The difference in the parametric instabilities between LCGT and Advanced LIGO is because of the thermal noise reduction methods (LCGT, cooling sapphire mirrors; Advanced LIGO, fused silica mirrors with larger laser beams), which are the main strategies of the projects. Elastic Q reduction by the barrel surface (0.2 mm thickness Ta2O5) coating is effective to suppress instabilities in the LCGT arm cavity. Therefore, the cryogenic interferometer is a smart solution for the parametric instabilities in addition to thermal noise and thermal lensing.

1064 赤外線サーモグラフィによる温度場計測に基づく塑性域同定(J09 逆問題解新手法の開発と最新応用,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Plastic zone identification was conducted by 2f lock-in technique, in which temperature change due to plastic heat generation was separately identified from thermoelastic temperature change under variable loading. Accuracy improvement was performed using motion compensation based on modified digital image correlation, inverse thermal conduction analysis and noise reduction based on mollification method.