Reduction Of Thermal Noise Research Articles

Perspectives on beam-shaping optimization for thermal-noise reduction in advanced gravitational-wave interferometric detectors: Bounds, profiles, and critical parameters

Suitable shaping (in particular, flattening and broadening) of the laser beam has recently been proposed as an effective device to reduce internal (mirror) thermal noise in advanced gravitational-wave interferometric detectors. Based on some recently published analytic approximations (valid in the infinite-test-mass limit) for the Brownian and thermoelastic mirror noises in the presence of arbitrary-shaped beams, this paper addresses certain preliminary issues related to the optimal beam-shaping problem. In particular, with specific reference to the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment, absolute and realistic lower bounds for the various thermal-noise constituents are obtained and compared with the current status (Gaussian beams) and trends (mesa beams), indicating fairly ample margins for further reduction. In this framework, the effective dimension of the related optimization problem, and its relationship to the critical design parameters are identified, physical-feasibility and model-consistency issues are considered, and possible additional requirements and/or prior information exploitable to drive the subsequent optimization process are highlighted.

Facility and methods for the measurement of micro and nano forces in the range below 10−5 N with a resolution of 10−12 N (development concept)

This contribution describes the concept of a new high precision micro- and nano-force measurement facility. It consists of a measuring disc-pendulum and an identical reference disc-pendulum for the reduction of thermal and seismic noise. According to the theoretical calculations in the paper, the disc-pendulums with electrostatic stiffness reduction and electrostatic deflection compensation should allow quasi-static forces to be measured in the range <10−5 N with a force resolution of 10−12 N.

Coatingless, tunable finesse interferometer for gravitational wave detection

Future generation gravitational wave interferometric detectors will require a strong reduction of thermal noise, and will be limited by quantum fluctuations of laser light, namely, shot noise and radiation pressure noise. One of the factors which currently constrain the thermal noise performances of this kind of instruments is the dissipation introduced by the optical coating of the mirrors. In a recent article Braginsky and Vyatchanin proposed to reduce the contribution of thermoelastic noise in the coating building an optical cavity with corner reflectors instead of spherical mirrors. In this work we study a solution which, avoiding completely any kind of coating, allows for injection and extraction of light. We discuss the proposed optical scheme, evaluating the expected thermal noise performances.

Measurement of vibration of the top of the suspension in a cryogenic interferometer with operating cryocoolers

In the LCGT and CLIO projects for the interferometric gravitational wave detectors of Japan, the mirrors and a part of the suspension systems are cooled by cryocoolers to reduce the thermal noise. For the CLIO, extremely small vibration cryocoolers were specially developed by improving a commercial Gifford-McMahon type pulse tube cryocooler. We measured the vibration at the top of the suspension base in the CLIO interferometer while operating these cryocoolers. Although the seismic motion of 10−9(1 Hz/f)2 m/Hz½ at the site of the CLIO and LCGT in the Kamioka mine is 100-times smaller than that around Tokyo, these cryocoolers did not seriously increase the vibration. Consequently, a reduction of thermal noise by the cooled mirrors and suspension fibers using these cryocoolers is expected to be observed without any additional fluctuation disturbance due to the cryocoolers.

R&D on thermal noise in Europe: the Strega Project

The design of cryogenic interferometers and quantum limited ultra-cryogenic resonators (3rd generation Advanced Gravitational Wave Detectors) is driving the R&D activities in the area of thermal noise reduction at low temperature. The project Strega coordinates the R&D activities of 18 labs in Europe involved in Interferometric, Resonant and Electromagnetic Detectors. The aim of the collaboration is to develop a technology that will reduce the thermal noise in the third generation detectors 10 times with respect to the second generation (advanced resonators and advanced room temperature interferometers). The work carried out in Strega, in the areas of Materials, Cryogenics and Thermal Noise specific Studies after 14 months since the project started, will be reviewed.

Enhanced phase noise modeling of fractional-N frequency synthesizers

Mathematical models for the behavior of fractional-N phase-locked-loop frequency synthesizers (Frac-N) are presented. The models are intended for calculating rms phase error and determining spurs in the output of Frac-N. The models describe noise contributions due to the charge pump (CP), the phase frequency detector (PFD), the loop filter, the voltage control oscillator, and the delta-sigma modulator. Models are presented for the effects of static CP gain mismatch, CP dynamic mismatch and PFD reset delay mismatch. A simple analytic expression shows the level of /spl Delta//spl Sigma/ sequence noise caused by static CP current mismatch. We further show that un-equal rise time and fall time constants of the CP result in dynamic mismatch noise. Reset delay mismatch in PFD is shown to also contribute significantly to close-in phase noise. The model takes into account the reduction in CP thermal and flicker noise due to the changing duty cycle of Frac-N CP. Our model is therefore useful in characterizing the noise performance of Frac-N at the system-level, simplifying the design of fractional-N synthesizers and transmitters. Analytical and simulated results are compared and show good agreement with prior published data on Frac-N realizations.

Thermal noise reduction for present and future gravitational wave detectors

Thermal noise in mirror suspension is and will be the most severe fundamental limit to the low-frequency sensitivity of interferometric gravitational wave detectors currently under construction. The technical solutions, adopted in the Virgo detector, optimize the current suspension scheme, but new materials and new designs are needed to further reduce the suspension thermal noise. Silicon fibers are promising candidates both for room temperature advanced detectors and for future cryogenic interferometric detectors.

Sensitivity of the Low Frequency Facility experiment around 10 Hz

The reduction of thermal noise is a fundamental issue for the improvement of future gravitational wave antennas. The main purpose of the Low Frequency Facility (LFF) is to study pendulum thermal noise in the region of 10 Hz. Data at the LFF has been taking since the beginning of 2003 and has been analyzed in order to thoroughly understand the region around 10 Hz. Above 7 Hz, the displacement noise floor is at the level of 10 −14 m/ Hz , decreasing with frequency approximately as 1/ ν. Seismic noise contamination is not observed above a few Hz.

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 m Hz−1/2 at 300 K to 10−19 m Hz−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.

Nanodots-induced pinning centers in thin films: effects on critical current density, activation energy and flux jump rate

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUID's.

Status and plans for future generations of ground-based interferometric gravitational wave antennas

Several medium- to large-scale ground-based interferometric gravitational-wave antennas have been constructed around the world. Although these antennas of the first generation could detect gravitational waves within a few years, it is necessary to improve the sensitivity of the detectors significantly with advanced technologies to ensure more frequent detection of gravitational waves. Stronger seismic isolation and reduction of thermal noise, especially using cryogenic mirrors, are among the most important technologies that can lead us to the realization of advanced detectors. Some of the advanced technologies are already implemented in some of the existing detectors and others are currently being investigated for the future-generation detectors such as advanced LIGO, LCGT, upgrade of GEO600, AIGO, and EURO. We expect that such advanced detectors will eventually open a new window to the universe and establish a new field, ‘gravitational wave astronomy’.

Strong reduction of thermally activated flux jump rate in superconducting thin films by nanodot-induced pinning centers

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices.

RESPONSE OF VIRGO DETECTORS TO PRE-BIG-BANG GRAVITONS

The sensitivity achievable by a pair of VIRGO detectors to stochastic and isotropic gravitational wave backgrounds produced in pre-big-bang models is discussed in view of the development of a second VIRGO interferometer. We describe a semi-analytical technique allowing to compute the signal-to-noise ratio for (monotonic or non-monotonic) logarithmic energy spectra of relic gravitons of arbitrary slope. We apply our results to the case of two correlated and coaligned VIRGO detectors and we compute their achievable sensitivities. We perform our calculations both for the usual case of minimal string cosmological scenario and in the case of a non-minimal scenario (originally suggested by Gasperini) where a long dilaton dominated phase is present prior to the onset of the ordinary radiation dominated phase. In this framework, we investigate possible improvements of the achievable sensitivities by selective reduction of the thermal contributions (pendulum and pendulum's internal modes) to the noise power spectra of the detectors. Since a reduction of the shot noise does not increase significantly the expected sensitivity of a VIRGO pair (in spite of the relative spatial location of the two detectors) our findings support the experimental efforts directed towards a substantial reduction of thermal noise.

Thermal noise reduction of mechanical oscillators by actively controlled external dissipative forces

We show that the thermal fluctuations of very soft mechanical oscillators, such as the cantilever in an atomic force microscope (AFM), can be reduced without changing the stiffness of the spring or having to lower the environment temperature. We derive a theoretical relationship between the thermal fluctuations of an oscillator and an actively controlled external dissipative force. This relationship is verified by experiments with an AFM cantilever where the external active force is coupled through a magnetic field. With simple instrumentation, we have reduced the thermal noise amplitude of the cantilever by a factor of 3.4, achieving an apparent temperature of 25 K with the environment at 295 K. This active noise reduction approach can significantly improve the accuracy of static position or static force measurements in a number of practical applications.

Absolute and Spectral Sensitivities in Dark- and Light-Adapted Pagothenia Borchgrevinki, an Antarctic Nototheniid Fish

Morita, Y., V. B. Meyer-Rochow and K. Uchida. Absolute and spectral sensitivities in dark- and light-adapted Pagothenia borchgrevinki, an Antarctic nototheniid fish. Physiol Behav 61(2) 159–163, 1997.—Functional properties of the retina of Pagothenia borchgrevinki, an Antarctic nototheniid fish that lives beneath the 2.5–3 m thick sea-ice in water of −1.8°C temperature, were analyzed electrophysiologically at Scott Base (77° 50′S; 166° 45′E). The waveform of the ERG was monophasic in the dark-adapted state and showed an off-response of opposite polarity in the light-adapted condition. Responses of the light-adapted retina were smaller than those of the dark-adapted eye, although both photopic and scotopic components were observed. Spectral sensitivity measured by monochromatic photostimulation at 14 different wavelengths across the 400–700-nm range showed a single maximum at 490 nm. The spectral sensitivity curve is consistent with a rhodopsin photopigment. The dark-adapted retina exhibited a photon flux density threshold of approximately 2 × 109 photons cm−2 s−1) when monochromatic flashes of 500 nm wavelength and 250 ms duration were used. When the stimulus consisted of 1 s white light, a minimum energy flux density of approx. 2 × 10−4μW/cm2 was necessary to elicit a detectable response. It was concluded that the visual system of P. borchgrevinki was in tune with the dominant downwelling spectral irradiance and that, due to retinal thermal noise reduction in the cold environment, no great need for particular anatomical adaptations to further enhance sensitivity existed.

Multiple glazed windows: Design charts

Thermal resistance, noise reduction, daylight and solar energy admittance are some of the expected duties of a window. A window manufacturer or a building services engineer employed to select windows for optimum performance does not necessarily need to know the intricate details of the thermal, sound, daylight and solar transmission characteristics of multiple glazed windows. Rather, they require broadly based information on critical design parameters and their influence on window performance. This paper presens charts which enable appropriate design of windows for any given duty and set of constraints.

A 1.9 GHz low-voltage silicon bipolar receiver front-end for wireless personal communications systems

A 1.9 GHz wireless receiver front-end (low-noise preamplifier and mixer) is described that incorporates monolithic microstrip transformers for significant improvements in performance compared to silicon broadband designs. Reactive feedback and coupling elements are used in place of resistors to lower the front-end noise figure through the reduction of resistor thermal noise, and this also allows both circuits to operate at supply voltages below 2 V. These circuits have been fabricated in a production 0.8 /spl mu/m BiCMOS process that has a peak npn transistor transit frequency (f/sub T/) of 11 GHz. At a supply voltage of 1.9 V, the measured mixer input third-order intercept point is +2.3 dBm with a 10.9 dB single-sideband noise figure. Power dissipated by the mixer is less than 5 mW. The low-noise amplifier input intercept is -3 dBm with a 2.8 dB noise figure and 9.5 dB gain. Power dissipation of the preamplifier is less than 4 mW, again from a 1.9 V supply.

Infrared photodetector with electromagnetic carrier depletion

A method is presented for thermal noise reduction in a near room-temperature intrinsic IR photodetector. The method is based on suppression of the Auger generation-recombination processes using the electro-magnetic carrier depletion (EMCD) of a narrow gap semiconductor. The device is a lightly doped narrow gap semiconductor flake with a high backside surface recombination velocity, supplied with electrical contact band and placed in a magnetic field. Due to the action of the Lorentz force most of the device depletes charge carriers, which results in suppression of the Auger generation and recombination processes. As a result, the I-V characteristic becomes nonlinear, exhibiting regions of high positive and negative resistance. Thermal noise can be dramatically reduced, leading to a substantial improvement in performance. The ultimate detection may be determined either by background radiation or by Shockley-Read generation, depending on the ratio of the background photon flux to the recombination center concentration. Near-BLIP performance is predicted for 10.6-μm (Hg, Cd) Te devices, prepared from high-quality materials and operated at 225-250 K.

Improved design of tuned optical receivers

We present new design principles for improved heterodyne tuned optical receivers where several tuning inductances reduce the influence of thermal receiver noise over a broad frequency range. A theoretical example for a 600 MHz tuning bandwidth shows a reduction of thermal receiver noise (and thus in required local oscillator power) of up to 13dB. The example is tested experimentally for the so-called mixed tuning configuration. We obtain good agreement with the theoretical predictions. The experimental RMS noise current is <5pA/√(Hz) over a 580 MHz bandwidth with the lowest value of 3-5 pA/√(Hz) at 950MHz.

Classical limit of mechanical thermal noise reduction by feedback.

The classical theory of thermal noise of mechanical systems and the effects of feedback is reinvestigated. Previous developments which set a limit to the noise reduction by feedback when a given measuring time is assumed have ignored the situation where the system is acting as an unchanging mechanical reference. A precision rotor clock is one such system. The present work uses the galvanometer as the general model and includes this situation. It explicates the nature of the limit when it exists by treating it without assuming any particular measuring optimization method.