Frequency Noise Research Articles

Short-term stability of a microcell optical reference based on the Rb atom two-photon transition at 778 nm

We report on the development and short-term stability characterization of an optical frequency reference based on the spectroscopy of the rubidium two-photon transition at 778 nm in a microfabricated vapor cell. When compared against a 778 nm reference signal extracted from a frequency-doubled cavity-stabilized telecom laser, the short-term stability of the microcell frequency standard is 3.5×10−13τ−1/2 until 200 s, in good agreement with a phase noise level of +43dBrad2/Hz at 1 Hz offset frequency. The two main contributions to the short-term stability of the microcell reference are currently the photon shot noise and the intermodulation effect induced by the laser frequency noise. Retaining a relevant margin of progress, these results show the interest of this spectroscopic approach for the demonstration of high-stability miniaturized optical vapor cell clocks. Such clocks are poised to be highly beneficial for applications in navigation, communications, and metrology.

Effect of Vehicle Vibration on Interior Noise of Railway Vehicles Passing Through Rail Corrugation Sections

This study involved experimental research to analyze the effect of rail corrugation on interior noise levels inside railway vehicles. Measurements taken on the Incheon Line 2 light rail indicated that the vehicle’s age and maintenance condition have minimal effects on interior noise. Although wheel wear slightly reduces interior noise, it is insufficient to address the issue of abnormal noise. The analysis of the relationship between vehicle vibrations and interior noise revealed that at 80 km/h and with a 24 mm rail corrugation wavelength, vibrations at 920 Hz in the axle box and car body increase, coinciding with the dominant interior noise frequency of 926.6 Hz. Furthermore, an analysis using a car body sweep confirmed a relative increase in noise in the 920 Hz range. Therefore, abnormal noise in rail corrugation sections is caused by vibrations at 920 Hz due to the corrugation wavelength and train speed, which align with the car body resonance frequency, leading to increased car body vibrations and interior noise.

Experimental Study on the Suppression of Cavity Noise in a Locking-On State by a Slanting Inner Wall

This paper presents an experimental investigation into the noise characteristics of various slanted wall configurations. The study focuses on the noise suppression effects of cavities with slanted walls on cavity coupling noise. A total of eight configurations, with different slanting angles on the front and rear walls, were analyzed by varying the inclination of the inner wall. Noise and flow field measurements were conducted in an aeroacoustic wind tunnel, utilizing microphones for near-field and far-field noise data acquisition and hot-wire probes for flow field analysis. The results indicate that larger slant angles lead to more effective noise reduction. As the slant angle increases, the acoustic resonance frequency associated with the slanted inner wall rises, which alters the self-excited oscillation modes involved in coupling with the acoustic resonance. This reduces the impact of coupling on the sound pressure levels. The change in acoustic resonance frequency also modifies the phase delay term of the dominant mode, ultimately leading to a shift in the noise frequency.

Practical ultra-low frequency noise laser system for quantum sensors

The laser’s frequency noise is crucial to the sensitivity of quantum sensors. Two commonly used methods to suppress the laser’s frequency noise are locking the laser to an atomic transition by the lock-in technique or to an ultra-low thermal expansion (ULE) glass cavity by the PDH technique. The former cannot suppress rapidly changing frequency noise and hardly meets the needs; the latter has powerful performance but a heightened cost. The lack of high-performance and low-cost laser noise suppression methods dramatically limits the practical application of quantum sensors. This work demonstrates that, in many quantum sensing applications such as the Rydberg atomic superheterodyne receiver, by cascade locking the laser to both the atomic transition and a low-cost (LC) cavity, the same performance as locking to the ULE cavity can be achieved. This work is significant in promoting the practical application of quantum sensors.

Let long-term interests talk: An disentangled learning model for recommendation based on short-term interests generation

In e-commerce recommendation systems, users’ long-term and short-term interests jointly influence product selection. However, the behavioral conformity phenomenon tends to be more prominent in short-term sequences, and the entanglement of true preference and popularity conformity data confuses the user’s real interest needs. To address this issue, we propose a sequential recommendation model called DFRec to disentangle short-term interests from popularity bias. By leveraging long-term interest trends, the model promotes the separation of short-term interests from popularity-driven deviations, thereby reducing the impact of popularity interference in short-term sequences. Firstly, we propose a Disentangled Frequency Attention Network(DFAN) to address the entanglement between real sequence features and conformity data in users’ short-term behavioral sequences. The approach clarify the non-entangled representation of the user’s short-term interest and conformity on the basis of long-term interest trends. Secondly, in order to capture the real long-term interest characteristics of users, this paper suggests using a Learnable Filter(LF) to filter the noise frequencies in long-term sequence. The method decouples the horizontal and vertical directions of the sequence and filters out the noise in both directions. Finally, consider the importance of the two interests characteristics is dynamic, we propose a joint learning framework with dual embeddings to balance and fusion these two features of users’ interests. Experimental results on three public datasets demonstrate that our model effectively captures dynamic user interests and outperforms six baseline models.

An advanced nanoparticle reinforced carbon fiber laminates for low frequency sound insulation

Low frequency noise is a common engineering problem in the industrial field. This study utilizes graphite particles to enhance the low frequency sound insulation of carbon fiber laminates significantly. The effects of grasphite particles with different mass and mesh on sound insulation of laminates are investigated by theory and experiment. The analytical model is proposed based on the Galerkin method for the laminates’ motion equations. The effective elastic properties of the samples are calculated using the Halpin-Tasi model and the rule of mixture. Two groups of samples are prepared for sound insulation measurement. The comparison between the theoretical and experimental results shows that the theory can well predict the sound insulation of the nanoparticle reinforced laminates. A significant improvement in low-frequency sound transmission loss of 3–8 dB is achieved without altering the laminates' thickness. However, the mesh variation of the graphite particles has little effect on the sound insulation of laminates.

Acoustically propelled winged macroparticles

Self-propelled particles harvest and harness energy from their environment, transforming it into a controlled force that propels their motion. We present a mechanism to propel active macroparticles using low frequency noise (10–200 Hz). Thin polymer plates (wings) are acoustically excited at their second natural frequency; the mass of air displaced generates a counter-force, which propels the macroparticles. We show that the magnitude and direction of the propelling force can be tweaked through the wing’s shape, dimensions, and orientation. Finally, we design a macroparticle with bidirectional rotation: its rotation direction can be inverted by changing the frequency at which it is excited.

Effect of 5 MeV proton irradiation on electrical and trap characteristics of β-Ga2O3 power diode

In this study, impact of 5 MeV proton irradiation with radiation fluence of 1013 cm−2 on β-Ga2O3 power diode is investigated by a β-Ga2O3 Schottky barrier diode (SBD). Via temperature-dependent measurements, carrier removal rate RC is determined to be 7.26 × 102 cm−1 at 300 K. Meanwhile, the threshold voltage (Von) and ideality factor (n) almost remain stable after proton irradiation. A close-to-unity n was observed for a wide temperature range indicating near-ideal Schottky characteristics. Dynamic degradation was observed at 300K, but was greatly suppressed at a low temperature of 100K. Meanwhile, two more bulk traps are discovered in proton irradiated β-Ga2O3 SBD by deep-level transient spectroscopy (DLTS). The larger corrected trap concentration (NTa) in proton irradiated β-Ga2O3 SBD was regarded as the reason behind slightly worsened dynamic on-resistance instability at 300 K. Furthermore, lower low frequency noise is revealed for proton irradiated device at room temperature and cryogenic temperature. The study demonstrates the competitive irradiation hardness of β-Ga2O3 power diodes and paves a solid path for the deployment of β-Ga2O3 in space.

Replacing diesel buses with electric buses reduced residential low frequency noise

Low frequency noise (20 - 200 Hz) from combustion engines typically dominates the indoor sound environments and is a source for annoyance and sleep disturbance. This study investigated the impact on low frequency noise when diesel operated buses were replaced by electric buses along a bus line in Gothenburg, Sweden. Noise measurements were performed indoors and outdoors for a selection of apartments directly exposed to bus traffic as well as for reference apartments, before and after the intervention. In addition, noise levels were calculated. The results show a significant reduction of the indoor low frequency noise in the range 40 - 80 Hz in the apartments directly exposed to passing buses, both during bus passages (on average 10 dB), and in one-hour measurements (on average 5 dB). The results further show that A-weighted values fail to accurately represent the reduction in noise achieved by the intervention.

Sub-Hz fundamental, sub-kHz integral linewidth self-injection locked 780 nm hybrid integrated laser

Today’s precision experiments for timekeeping, inertial sensing, and fundamental science place strict requirements on the spectral distribution of laser frequency noise. Rubidium-based experiments utilize table-top 780 nm laser systems for high-performance clocks, gravity sensors, and quantum gates. Wafer-scale integration of these lasers is critical for enabling systems-on-chip. Despite progress towards chip-scale 780 nm ultra-narrow linewidth lasers, achieving sub-Hz fundamental linewidth and sub-kHz integral linewidth has remained elusive. Here we report a hybrid integrated 780 nm self-injection locked laser with 0.74 Hz fundamental and 864 Hz integral linewidths and thermorefractive-noise-limited 100 Hz2/Hz at 10 kHz. These linewidths are over an order of magnitude lower than previous photonic-integrated 780 nm implementations. The laser consists of a Fabry-Pérot diode edge-coupled to an on-chip splitter and a tunable 90 million Q resonator realized in the CMOS foundry-compatible silicon nitride platform. We achieve 2 mW output power, 36 dB side mode suppression ratio, and a 2.5 GHz mode-hop-free tuning range. To demonstrate the potential for quantum atomic applications, we analyze the laser noise influence on sensitivity limits for atomic clocks, quantum gates, and atom interferometer gravimeters. This technology can be translated to other atomic wavelengths, enabling compact, ultra-low noise lasers for quantum sensing, computing, and metrology.

Metrology-grade spectroscopy source based on an optical parametric oscillator

Continuous-wave optical parametric oscillators (OPOs) are widely tunable and powerful sources of narrow-linewidth radiation. These properties make them suitable for a wide range of spectroscopic studies - but so far not at the metrological level. Indeed, although important technical OPO developments occurred more than two decades ago, and commercial devices have been available for nearly as long, the long-hoped-for the potential of these devices, providing simultaneously ultralow linewidth, ultrahigh frequency stability, ultrahigh frequency accuracy, and wide wavelength coverage has not yet become a reality. Here, we present an OPO metrology system suitable for optical spectroscopy with ultra-high resolution and accuracy in the 2.2 - 3.9 μm range. The system relies on the second-harmonic generation of the idler wave to bridge the gap to the near-infrared regime where frequency combs are readily available. By actively controlling the pump laser frequency, the idler radiation is phase-locked to an optically stabilized frequency comb, enabling a full transfer of the frequency comb’s spectral properties to the idler radiation and measuring the idler frequency with ultra-high precision. We reach fractional line widths and Allan deviations of the idler radiation at the level of 4 × 10−14 and 1 × 10−14, respectively. We also perform a thorough characterization of the stabilized OPO via a comparison with a second, independent optically stabilized frequency comb and thereby determine an overall idler frequency systematic uncertainty of less than 1.2 × 10−14. Sources of residual frequency noise are identified. The system delivered excellent results in high-accuracy spectroscopy.

External-cavity diode laser at 2.6 µm and its frequency stabilization with a scanning Fabry-Pérot cavity

We report on the construction of an external cavity diode laser at 2.6 µm wavelength and its frequency stabilization with an external scanning Fabry-Pérot cavity using Pound-Drever-Hall (PDH) frequency stabilization technique. We discuss the frequency noise characteristics of the laser using the PDH residual error signal analysis and the locking performance. We have achieved a 2.5 x 105 suppression factor in the frequency noise power spectral density at Fourier frequencies lower than 100 Hz when the laser is stabilized. The laser is employed to observe resonant excitation on the 5s5p3P0 →5s4d3D1 transition in cold 88Sr atoms trapped in a magneto-optical trap. We have observed an enhancement in the trapped atom number by a factor of 8.3 compared to the case when a single laser at 707 nm is employed for repumping.

Signal Processing for Novel Noise Radar Based on de-chirp and Delay Matching.

Modern radar technology requires high-quality signals and detection performance. However, traditional frequency-modulated continuous wave (FMCW) radar often has poor anti-jamming capabilities, and the high sampling rates associated with large time-bandwidth product signals can lead to increased system hardware costs and reduced data processing efficiency. This paper constructed a composite radar waveform based on noise frequency modulation (NFM) and linear frequency modulation (LFM) signals, enhancing the signal's complexity and anti-jamming capability. Furthermore, a method for optimizing the processing of echo signals based on de-chirp and delay matching is proposed. The locally generated LFM signal is used to de-chirp the received echoes, resulting in a narrowband difference frequency noise signal. Subsequently, delay matching is performed in the fast time domain using the locally generated NFM signal according to the number of sampling points in the traversal processing period, allowing for the acquisition of target delay information. While reducing the analog-to-digital (A/D) sampling rate, the detection performance for wideband echo signals under high sampling rates is still maintained, with sidelobe levels and range resolution preserved. Accumulating this information in the slow time domain enables accurate target detection. The effectiveness of the proposed method is validated through simulation experiments.

Experimental end-to-end demonstration of intersatellite absolute ranging for the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) is a gravitational wave detector in space. It relies on a postprocessing technique named time-delay interferometry to suppress the overwhelming laser frequency noise by several orders of magnitude. This algorithm requires intersatellite-ranging monitors to provide information on spacecraft separations. To fulfill this requirement, we use on-ground observatories, optical sideband-sideband beatnotes, pseudorandom noise ranging (PRNR), and time-delay interferometric ranging (TDIR). This article reports on the experimental end-to-end demonstration of a hexagonal optical testbed used to extract absolute ranges via the optical sidebands, PRNR, and TDIR. These were applied for clock synchronization of optical beatnote signals sampled at independent phasemeters. We set up two possible PRNR processing schemes: Scheme 1 extracts pseudoranges from PRNR via a calibration relying on TDIR; Scheme 2 synchronizes all beatnote signals without TDIR calibration. The schemes rely on newly implemented monitors of local-PRNR biases. After the necessary PRNR treatments (unwrapping, ambiguity resolution, bias correction, in-band jitter reduction, and/or calibration), Schemes 1 and 2 achieved ranging accuracies of 2.0–8.1 cm and 5.8–41.1 cm, respectively, below the classical 1-m mark with margins. Published by the American Physical Society 2024

Animal-borne sensors reveal high human impact on soundscapes near a critical sea turtle nesting beach

Anthropogenic noise (anthrophony) is pervasive in natural soundscapes and has become an important aspect of conservation. While moored sound recorders have aided marine soundscape research, they do not capture the dynamic experiences of animals as they move through underwater soundscapes.This study used animal-borne acoustic recording tags to capture the marine soundscape near leatherback turtle nesting grounds in Gabon, Central Africa. Propeller noise was heard in 75 ± 14.7 % (mean ± SD) of recordings and peaks in sound intensity up to 146 dB re 1 μPa were detected in shipping noise frequency bands. Loud noise events (> 141 dB), detected in 10 % of recordings, were distributed throughout the turtle interesting habitat. An anthrophony map was created, identifying peaks in noise corresponding with the Komo estuary —gateway to the nation's main international port and near a key nesting area. The pervasive, loud anthrophony recorded in the study may have negative impacts on nesting leatherback turtles and other species of conservation concern found in the area, and warrants further monitoring and management action.This study offers one of the first spatio-temporal analyses of sound experienced by an endangered marine vertebrate through animal borne, multi-individual acoustic monitoring. It highlights the utility of animal borne acoustic tags in delineating underwater soundscapes and their applicability to studying concurrent biological phenomena and threats, while supporting the need for similar monitoring efforts in other critical sea turtle habitats.

Active noise control of refrigerator based on cascaded notch feedback algorithm

Refrigerators bring convenience to people, but the noise they produce can also affect people’s lives. Refrigerator noise is dominated by low-frequency noise, and the active noise control method is more effective for this kind of noise. However, the existing active noise control methods for refrigerators do not take into account factors such as the limited space and complex structure of the refrigerator compressor room, and external interference signals will be introduced to affect the noise reduction performance during the error signal collection. Given that, this paper proposes the cascade notch feedback algorithm, which can well reduce the influence of external interference signals to better achieve the refrigerator noise reduction. The algorithm consists of the cascaded notch filtering algorithm and the feedback algorithm. The cascade notch filtering algorithm is formed by cascading the adaptive notch filtering algorithm, which is used to deal with the main frequency and harmonic noise generated by the refrigerator. The feedback algorithm consists of a robust algorithm for dealing with external interference signals introduced during error signal collection. The simulation experiment proves that the algorithm has advantages in terms of noise reduction and computational cost compared with the adaptive notch filtering algorithm and the improved algorithm. The experimental test platform is set up to carry out the actual refrigerator noise reduction experiments under different conditions. The algorithm has the effect of noise reduction under different robust algorithms.

Identification of incipient cavitation in control valve

Control valves as important elements in hydraulic systems are used to transport liquid and gaseous media, for example, water, water vapor, and hydrogen. They are mainly used in the power engineering, chemical industry, and so forth. Due to their large dimensions, the verification of hydraulic parameters is problematic. In the case of liquid flow, cavitation can occur, which is mainly characterized by noise, vibrations, or changes of hydraulic parameters. During the incipient cavitation in the valve, there are no noticeable changes in the hydraulic characteristics. The article, therefore, deals with the methodology of determining the cavitation by measuring and evaluating the characteristics of the valve, spectral analysis of noise, and vibrations during water flow. Mathematical modeling is also a suitable tool for determining the extent of cavitation in valve design. Newly created modified cavitation model, where the flowing medium is defined as a mixture of incompressible water and compressible gases (vapor and air), better corresponds to physical principles than the two-phase approach (water and vapor) with experimentally modified density and viscosity. Measured noise and vibration frequency characteristics and mathematically modeled pressure frequency characteristics identify cavitation in the (1 to 10) kHz range. The article proves that when identifying cavitation, the method of measuring hydraulic characteristics fails, but the method of measuring noise and vibrations is suitable in practice, i.e., in real industrial equipment, and the modified cavitation model is suitable for identifying cavitation regions in structural designs of the hydraulic elements.

Performance evaluation of 3D printed acoustic metamaterial using soft computing approach

This paper introduces a novel methodology for predicting the acoustic absorption coefficient of DENORMS cell based acoustic metamaterial. The samples were printed from resin using Digital Light Processing based 3D printing technique. The manufactured samples were tested in an Impedance tube using the two Microphone method. A virtual simulation test rig was used to generate data sets for geometrically distinct DENORMS cell based metamaterial. Four distinct soft computing techniques specifically the “Neural Networks (NN), Random Forests (RF), Decision Trees (Rpart) and Generalized Linear Model (GLM)”, were employed and compared to develop an accurate prediction model for forecasting the absorption coefficient of the developed metamaterial. The machine learning techniques were used due to their higher speed and lower computational power requirement compared to numerical simulations to determine the absorption coefficient. The input variables consist of the Spherical Diameter, Cylindrical Diameter, Cylinder Length of the DENORMS cell and Frequency of Incident noise. The performance of the four prediction models was evaluated based on criteria such as Root mean square error, Coefficient of determination, Correlation and Accuracy. Ten-Fold cross validation is performed to test the robustness of the model.

A User - Friendly Signal-Processing App For Harmonics

Harmonics, especially the 3^rd,5^th, and 7^th components, are significant disturbances in power systems, occurring simultaneously and at varying time intervals. Measurements of these components depend on factors such as fundamental frequency deviation, event duration, amplitude values, and noise levels. Accurate detection and measurement are crucial for effective harmonic mitigation and preventive strategies. This study introduces a harmonic signal analysis application utilizing methods like FFT, STFT, CWT, EMD, and HHT, with individual user interfaces for in-phase and time-variant harmonic signals. The application provides results for amplitude, frequency, event time intervals, and noise effects simultaneously. Test results at a 60 dB signal-to-noise ratio (SNR) reveal that FFT achieves precise frequency and amplitude results due to its high resolution, whereas other methods offering time-frequency domain results exhibit lower resolution. EMD, in particular, demonstrates high errors in frequency and amplitude responses, reducing four frequency components to three. HHT, utilizing EMD results, yields higher accuracy with minimal errors compared to other methods. This application, combined with test results, facilitates signal synthesis and comparative analysis in time and time-frequency domains.

GHZ protocols enhance frequency metrology despite spontaneous decay.

The use of correlated states and measurements promises improvements in the accuracy of frequency metrology and the stability of atomic clocks. However, developing strategies robust against dominant noise processes remains challenging. We address the issue of decoherence due to spontaneous decay and show that Greenberger-Horne-Zeilinger (GHZ) states, in conjunction with a correlated measurement and nonlinear estimation strategy, achieve gains of up to 2.25 decibel, comparable to fundamental bounds for up to about 80 atoms in the presence of decoherence. This result is unexpected because GHZ states do not provide any enhancement under dephasing due to white frequency noise compared to the standard quantum limit of uncorrelated states. The gain arises from a veto signal, which allows for the detection and mitigation of errors caused by spontaneous emission events. Through comprehensive Monte Carlo simulations of atomic clocks, we demonstrate the robustness of the GHZ protocol.