Measured Noise Spectra Research Articles

Opto-mechano-fluidic viscometer

The recent development of opto-mechano-fluidic resonators has provided—by harnessing photon radiation pressure—a microfluidics platform for the optical sensing of fluid density and bulk modulus. Here, we show that fluid viscosity can also be determined through optomechanical measurement of the vibrational noise spectrum of the resonator mechanical modes. A linear relationship between the spectral linewidth and root-viscosity is predicted and experimentally verified in the low viscosity regime. Our result is a step towards multi-frequency measurement of viscoelasticity of arbitrary fluids, without sample contamination, using highly sensitive optomechanics techniques.

Towards a sub 15-dBA optical micromachined microphone.

Micromachined microphones with grating-based optical-interferometric readout have been demonstrated previously. These microphones are similar in construction to bottom-inlet capacitive microelectromechanical-system (MEMS) microphones, with the exception that optoelectronic emitters and detectors are placed inside the microphone's front or back cavity. A potential advantage of optical microphones in designing for low noise level is the use of highly-perforated microphone backplates to enable low-damping and low thermal-mechanical noise levels. This work presents an experimental study of a microphone diaphragm and backplate designed for optical readout and low thermal-mechanical noise. The backplate is 1 mm × 1 mm and is fabricated in a 2-μm-thick epitaxial silicon layer of a silicon-on-insulator wafer and contains a diffraction grating with 4-μm pitch etched at the center. The presented system has a measured thermal-mechanical noise level equal to 22.6 dBA. Through measurement of the electrostatic frequency response and measured noise spectra, a device model for the microphone system is verified. The model is in-turn used to identify design paths towards MEMS microphones with sub 15-dBA noise floors.

Development and drift-analysis of a modular electromagnetic induction system for shallow ground conductivity measurements

Electromagnetic induction (EMI) is used for fast near surface mapping of the electrical conductivity (EC) for a wide range of geophysical applications. Recently, enhanced methods were developed to measure depth-dependent EC by inverting quantitative multi-configuration EMI data, which increases the demand for a suitable multi-channel EMI measurement system. We have designed a novel EMI system that enables the use of modular transmitter/receiver (TX/RX) units, which are connected to a central measurement system and are optimized for flexible setups with coil separations of up to 1.0 m. Each TX/RX-unit contains a coil, which is specifically adjusted for transmitting or receiving magnetic fields. All units enable impedance measurements at the coils, which are used to simulate its electrical circuit and analyze temperature-induced drift effects. A laboratory drift analysis at 8 kHz showed that 88% of the drift in the measured data is due to the change in the electrical transmitter coil resistance. The remaining 12% is due to changes in the transmitter coil inductance and capacitance, the receiver impedance and drifts in the amplification circuit. A measurement under field conditions proved that the new EMI system is able to detect a water-filled swimming pool with 50 mS m−1, using a coil separation of 0.3 m. In addition, the system allows in-field ambient noise spectra measurements in order to select optimal low-noise measurement frequencies.

Uncertainty in least-squares fits to the thermal noise spectra of nanomechanical resonators with applications to the atomic force microscope

Thermal noise spectra of nanomechanical resonators are used widely to characterize their physical properties. These spectra typically exhibit a Lorentzian response, with additional white noise due to extraneous processes. Least-squares fits of these measurements enable extraction of key parameters of the resonator, including its resonant frequency, quality factor, and stiffness. Here, we present general formulas for the uncertainties in these fit parameters due to sampling noise inherent in all thermal noise spectra. Good agreement with Monte Carlo simulation of synthetic data and measurements of an Atomic Force Microscope (AFM) cantilever is demonstrated. These formulas enable robust interpretation of thermal noise spectra measurements commonly performed in the AFM and adaptive control of fitting procedures with specified tolerances.

Nano Carbon 1D and 2D Nanomechanical Resonators

ABSTRACTWe demonstrate one-dimensional (1D) and two-dimensional (2D) resonant nanoelectromechanical systems (NEMS) derived from nano carbon materials, where the resonance frequency and the quality (Q) factor of the devices are measured experimentally using ultrasensitive optical interferometry. The 1D nano carbon resonators are formed using carbon nanofibers (CNFs) which are synthesized using a plasma-enhanced chemical vapor deposition (PECVD) process, while the 2D nanocarbon resonators are based on CVD grown graphene. The CNFs are prototyped into few-μm-long cantilever-shaped 1D resonators, where the resonance frequency and Qs are extracted from measurements of the undriven thermomechanical noise spectrum. The thermomechanical noise measurements yield resonances in the ∼3–15 MHz range, with Q of ∼200–800. Significant changes in resonance characteristics are observed due to electron beam induced amorphous carbon deposition on the CNFs, which suggests that 1D CNF resonators have strong prospects for ultrasensitive mass detection. We also present NEMS resonators based on 2D graphene nanomembranes, which exhibit robust undriven thermomechanical resonances for the extraction of ultrasmall strain levels.

Low-Frequency Noise Analysis of Electrostatic Discharge Tolerance of InGaN Light-Emitting Diodes

In recent years, with an extensive use of InGaN light-emitting diode (LED), how to assess the LED quality and further improve the LED reliability are very important. In this paper, the noise spectrum measurement techniques are used to assess the electrostatic discharge tolerance and quality of InGaN LED devices. Experimental results show that the noise spectrum measurement distinguishing the LED device reliability is more effective than the current-voltage curve measurement. In the evidence, emission microscope, scanning electron microscope, and transmission electron microscopy images show that the noise source and the cause of failure of the LED device are attributed by the poor quality of the SiO2 and Indium Tin Oxide (ITO) interface.

Resources of polarimetric sensitivity in spin noise spectroscopy

We attract attention to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity is achieved by replacing the 45-deg polarization geometry commonly used in conventional schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude is demonstrated by measurements of the spin noise spectra of bulk n:GaAs in the spectral range 835-918 nm. It is shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles.

Dynamics of Majorana States in a Topological Josephson Junction

Topological Josephson junctions carry 4π-periodic bound states. A finite bias applied to the junction limits the lifetime of the bound state by dynamically coupling it to the continuum. Another characteristic time scale, the phase adjustment time, is determined by the resistance of the circuit "seen" by the junction. We show that the 4π periodicity manifests itself by an even-odd effect in Shapiro steps only if the phase adjustment time is shorter than the lifetime of the bound state. The presence of a peak in the current noise spectrum at half the Josephson frequency is a more robust manifestation of the 4π periodicity, as it persists for an arbitrarily long phase adjustment time. We specify, in terms of the circuit parameters, the conditions necessary for observing the manifestations of 4π periodicity in the noise spectrum and Shapiro step measurements.

Epitaxial Graphene Sensors for Detection of Small Magnetic Moments

Micron-sized Hall sensors made of epitaxial graphene grown on 4H-SiC(0001) have been fabricated and studied at room temperature using transport and noise spectrum measurements. We report detection of 1-μm Dynal bead with the magnetic moment of ~4×108 μB, using a 2-μm epitaxial graphene device. A phase-sensitive AC-DC Hall magnetometry method was used to reliably detect the bead with a relatively large response of VxAC ~ 7 μV. Bead positioning was performed using a nanomanipulator inside of the FIB system. Long-term effect of the small dose of e-beam irradiation (~4 × 1014 e/cm2) on the performance of graphene Hall devices is reported.

Low-frequency ambient noise characteristics and budget in the South China Sea basin

A sound record measured by a moored hydrophone in the South China Sea basin was analyzed. Sampled at a rate of 1.6 kHz and with a duty cycle of approximately 1-min-on and 14-min-off, the measured time series captures the spectral characteristics and variability of the ambient noise in the less-than-800-Hz band over an annual cycle. Using a combination of automated and manual screening methods, the dominant regular and transient noise sources were identified and categorized, which include shipping, wind waves, seismic air-gun surveys, shots/explosives and sonar. Intermittent self noise (squeaking sounds) that prevailed at times during the passage of the very large-amplitude internal waves was also identified. In addition to the noise budget, the variability in the daily and monthly means and variances of the measured noise spectrum and band levels were examined. In order to gain insights into the predictability of the ambient noise field in this marginal sea, the interpretation of the data was facilitated with temperature records measured with moored instruments, wind and precipitation time series from the US Naval Operational Global Atmospheric Prediction System (NOGAPS), and vessel motion simulation based on historical shipping density and lane structure. [Research sponsored by the Office of Naval Research.]

An investigation of airfoil tonal noise at different Reynolds numbers and angles of attack

This paper presents the results of an experimental study into the aeroacoustics of airfoil instability noise and a linear stability analysis of the Tollmien–Schlichting (T–S) waves which develop on the pressure surface. The far field noise produced by a NACA0012 airfoil was measured in a low-noise and low-turbulence open jet wind tunnel. Noise measurements were made at the three angles of attack: 0°, 1.4° and 4.2°, and covered a range of Reynolds numbers between 1.0×105 and 6.0×105. An improved understanding of the causal link between the radiated tonal noise and the hydrodynamic instabilities which develop on the pressure surface of an airfoil for a wide range of Reynolds numbers and angles of attack is reported in this paper. The tonal noise produced by the boundary layer which develop on the suction surface was found to be negligible. It was also observed that the total T–S wave amplification on the pressure surface was generally in good agreement with the measured noise spectra. Based on the experimental and predicted data presented in this paper, it was found that effective tone noise generation requires the incoming T–S waves to be amplified by a separated boundary layer first. As the angle of attack increases, the change in pressure gradient on the pressure surface causes the instability noise to change from a broadband hump of moderate intensity level to become more tonal of higher noise intensity.

Spectroscopy of low-frequency noise and its temperature dependence in a superconducting qubit

We report a direct measurement of the low-frequency noise spectrum in a superconducting flux qubit. Our method uses the noise sensitivity of a free-induction Ramsey interference experiment, comprising free evolution in the presence of noise for a fixed period of time followed by single-shot qubit-state measurement. Repeating this procedure enables Fourier-transform noise spectroscopy with access to frequencies up to the achievable repetition rate, a regime relevant to dephasing in ensemble-averaged time-domain measurements such as Ramsey interferometry. Rotating the qubit's quantization axis allows us to measure two types of noise: effective flux noise and effective critical-current or charge noise. For both noise sources, we observe that the very same $1/f$-type power laws measured at considerably higher frequencies ($0.2\ensuremath{-}20$ MHz) are consistent with the noise in the $0.01\ensuremath{-}100$-Hz range measured here. We find no evidence of temperature dependence of the noises over $65\ensuremath{-}200$ mK, and also no evidence of time-domain correlations between the two noises. These methods and results are pertinent to the dephasing of all superconducting qubits.

Small epitaxial graphene devices for magnetosensing applications

Hall sensors with the width range from 0.5 to 20.0 μm have been fabricated out of a monolayer graphene epitaxially grown on SiC. The sensors have been studied at room temperature using transport and noise spectrum measurements. The minimum detectable field of a typical 10-μm graphene sensor is ≈2.5 μT/√Hz, making them comparable with state of the art semiconductor devices of the same size and carrier concentration and superior to devices made of CVD graphene. Relatively high resistance significantly restricts performance of the smallest 500-nm devices. Carrier mobility is strongly size dependent, signifying importance of both intrinsic and extrinsic factors in the optimization of the device performance.

정지비행 조건에서의 축소 로터 실험을 통한 소음 예측 기법 검증

In this paper, a series of experiment is performed for a scaled hovering rotor in a semi-anechoic chamber and the results are compared to the noise spectra predicted by using Lowson's loading noise equation and FW-H equation. It was founded that the sound directivity pattern for both experiments and predictions are similar in their trend. Meanwhile the FW-H equation showed better agreement with experiments in the near-field noise spectra, but at the far-field the Lowson's equation performed better. The discrete noise are known to be proportional to the loading on the blades, which can be controlled by collective pitch angle of the blades. It was founded that the predicted spectra with FW-H equation come close to the measured noise spectra in low collective pitch, but in high collective pitch angles the Lowson's equation be more reliable.

Scanning Noise Microscopy on Graphene Devices

We developed a scanning noise microscopy (SNM) method and demonstrated the nanoscale noise analysis of a graphene strip-based device. Here, a Pt tip made a direct contact on the surface of a nanodevice to measure the current noise spectrum through it. Then, the measured noise spectrum was analyzed by an empirical model to extract the noise characteristics only from the device channel. As a proof of concept, we demonstrated the scaling behavior analysis of the noise in graphene strips. Furthermore, we performed the nanoscale noise mapping on a graphene channel, allowing us to study the effect of structural defects on the noise of the graphene channel. The SNM method is a powerful tool for nanoscale noise analysis and should play a significant role in basic research on nanoscale devices.

Critical Kerr nonlinear optical cavity in the presence of internal loss and driving noise

We theoretically analyze the noise transformation of a high-power continuous-wave light field that is reflected off a critical Kerr nonlinear cavity (KNLC). Our investigations are based on a rigorous treatment in the time domain. Thereby, realistic conditions of a specific experimental environment including optical intracavity loss and strong classical driving noise can be modeled for any KNLC. We show that, even in the presence of optical loss and driving noise, considerable squeezing levels can be achieved. We find that the achievable squeezing levels are not limited by the driving noise but solely by the amount of optical loss. Amplitude-quadrature squeezing of the reflected mean field is obtained if the KNLC's operating point is chosen properly. Consistently, a KNLC can provide a passive purely optical reduction of laser-power noise as experimentally demonstrated in Khalaidovski et al. [Phys. Rev. A 80, 053801 (2009)]. We apply our model to this experiment and find good agreement with measured noise spectra.

Front-end receiver electronics for high-frequency monolithic CMUT-on-CMOS imaging arrays

This paper describes the design of CMOS receiver electronics for monolithic integration with capacitive micromachined ultrasonic transducer (CMUT) arrays for highfrequency intravascular ultrasound imaging. A custom 8-inch (20-cm) wafer is fabricated in a 0.35-μm two-poly, four-metal CMOS process and then CMUT arrays are built on top of the application specific integrated circuits (ASICs) on the wafer. We discuss advantages of the single-chip CMUT-on-CMOS approach in terms of receive sensitivity and SNR. Low-noise and high-gain design of a transimpedance amplifier (TIA) optimized for a forward-looking volumetric-imaging CMUT array element is discussed as a challenging design example. Amplifier gain, bandwidth, dynamic range, and power consumption trade-offs are discussed in detail. With minimized parasitics provided by the CMUT-on-CMOS approach, the optimized TIA design achieves a 90 fA/√Hz input-referred current noise, which is less than the thermal-mechanical noise of the CMUT element. We show successful system operation with a pulseecho measurement. Transducer-noise-dominated detection in immersion is also demonstrated through output noise spectrum measurement of the integrated system at different CMUT bias voltages. A noise figure of 1.8 dB is obtained in the designed CMUT bandwidth of 10 to 20 MHz.

Noise properties of thick-film resistors in extended temperature range

Results of thorough experimental studies on electrical properties of thick-film resistors (TFRs) have been overviewed and summarized. Experiments covered resistance and noise measurements in wide temperature range. Low-frequency noise spectroscopy has been used for the investigations of fluctuating phenomena. Sample resistors were prepared of various combinations of commercial and laboratory-made conducting and resistive pastes and printed on alumina substrates. Resistive pastes were made of materials: (i) Pb-containing RuO 2- and Bi 2Ru 2O 7-based, (ii) Pb/Cd-free RuO 2-, CaRuO 3-, and RuO 2/CaRuO 3-based. Conducting pastes included Ag, Ag–Pd and Ag–AgPt–Pd, Au, PtAu as a main ingredients. Electrical properties of various TFRs made of different materials were compared. Cross-correlation technique of noise spectra measurements in conjunction with the multiterminal configuration of TFRs was used in investigations of noise vs. resistor volume, resulting in extraction of noise components originated in different parts of the resistor. Furthermore, noise of the resistor was split up into bulk and interface noise. Resistance noise, observed in all studied TFRs, has been found to consist of background 1/ f noise and Lorentzian noise induced by thermally activated noise sources (TANSs). Properties of TANSs have been described and their relation to TFRs performance parameters have been pointed out. Noise properties of various Pb-containing and Pb/Cd-free resistive materials have been compared with the use of bulk noise intensity parameter, C bulk . Conclusions concerning compatibility of resistive and conductive pastes have been formulated. They might be useful in further improvement of materials systems for thick-film technology in order to fabricate low-noise and stable passives.

Broadband phase noise suppression in a Yb-fiber frequency comb

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7 dB out to Fourier frequencies of 100 kHz with a unity-gain crossing of ∼700 kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ∼100 kHz is due to amplified spontaneous emission or other quantum noise sources.

Mechanism of optical degradation in microstructured InGaN light-emitting diodes

While the enhancement of light extraction efficiency from microstructured InGaN light-emitting diodes (μLED) has been firmly established, there is concern over the effect of microstructuring on the device lifetimes. A study on the electrical characteristics and reliability of μLED arrays has been carried out. Despite improved optical performance, expanded device sidewalls served to accelerate the rate of optical degradation, adversely affect the lifetimes of devices. Through current-voltage plots and noise spectrum measurements, vertical current conduction along the plasma-damaged sidewalls was identified as the key degradation mechanism.