Ultra-low Noise Frequency Research Articles

Critical analysis of the ULF power depression as a possible Tohoku earthquake precursor

Among electromagnetic methods of short-term earthquake prediction, an approach is being actively developed based on the phenomenon of magnetic ultra-low-frequency (ULF) power depression occurring a few days before an earthquake. In particular, a nighttime geomagnetic power depression in the band 0.03–0.05 Hz was observed approximately 5 days before the catastrophic Tohoku 2011 earthquake. To verify the reliability of this method, we performed an extended analysis using data from magnetometer arrays JMA, MAGDAS, PWING, and INTERMAGNET. The selected stations included sites close to the epicenter (<300 km) and remote points (∼10000 km). The band-integrated spectral power of nighttime magnetic noise decreased significantly from March 6–9, several days before the earthquake. However, such variations occur simultaneously not only at nearby stations but also at distant stations. During this event, the ULF power depression was caused by low global geomagnetic activity, as evidenced by the planetary index SME. Thus, the depression of geomagnetic ULF noise cannot be considered a reliable short-term precursor.

Ultra-low frequency noise external cavity diode laser systems for quantum applications.

We present two distinct ultra-low frequency noise lasers at 729 nm with a fast frequency noise of 30 Hz2/Hz, corresponding to a Lorentzian linewidth of 0.1 kHz. The characteristics of both lasers, which are based on different types of laser diodes, are investigated using experimental and theoretical analysis with a focus on identifying the advantages and disadvantages of each type of system. Specifically, we study the differences and similarities in mode behavior while tuning frequency noise and linewidth reduction. Furthermore, we demonstrate the locking capability of these systems on medium-finesse cavities. The results provide insights into the unique operational characteristics of these ultra-low noise lasers and their potential applications in quantum technology that require high levels of control fidelity.

Sub-kilohertz linewidth free-running monolithic cavity VECSEL with 10−12 stability

We report the development of a compact, highly stable, monolithic-cavity, GaInP/AlGaInP-based vertical-external-cavity surface-emitting laser (VECSEL) with electronically tunable emission wavelength centered at 689.4425 nm for neutral strontium (Sr)-based applications. The output power reaches 40 mW (pump-power-limited) with ultra-low frequency and intensity noise performance resulting in a free-running linewidth of 720 Hz, reduced to 390 Hz when frequency locked to a reference cavity and verified via a heterodyne beat note measurement with 2 s averaging time. For shorter averaging times (0.1 ms), the free-running linewidth is as low as 40 Hz. We estimate a Lorentzian, or intrinsic, linewidth of 64 mHz from the frequency noise power spectral density at high frequencies, thus providing further evidence of the ultra-narrow fundamental linewidth of VECSELs. High frequency stability was measured via Allan deviation resulting in 1.05 × 10−12 at 2 s and 2.11 × 10−13 at 7 s averaging times when the 689 nm monolithic cavity VECSEL is free-running and locked, respectively, suitable for neutral Sr-based quantum technologies, such as optical clocks and atom interferometry.

Self-injection locking of a low-noise erbium-doped random fiber laser by a random fiber grating ring.

We demonstrate a self-injection locking (SIL) in an Er-doped random fiber laser by a high quality factor (high-Q) random fiber grating ring (RFGR) resonator, which enables a single-mode narrow-linewidth lasing with ultra-low intensity and frequency noise. The RFGR resonator includes a fiber ring with a random fiber grating to provide random feedback modes and noise suppression filters with self-adjusted peak frequency adaptable to small perturbations allowing single longitudinal mode over 7000 s with frequency jitter below 3.0 kHz. Single-mode operation is accomplished by carefully controlling phase delays and mode coupling of resonant modes between main ring and RFGR with a side-mode suppression ratio of 70 dB and narrow linewidth of 1.23 kHz. The relative intensity noise is -140 dB/Hz above 100 kHz and the frequency noise is 1 Hz/Hz1/2 above 10 kHz.

Low and ultra-wide frequency wave attenuation performance and tunability of a new cruciate ligament structure

This paper presents two novel cruciate ligament structures with outstanding contributions in lightweight design of metamaterials, band gap tuning, and ultra-wide and ultra-low frequency vibration and noise reduction, as well as providing a new design idea. The novelty of this paper is manifested in various aspects, combining Bloch's theorem and finite element method to calculate the band gap, and using the transfer function to verify the evaluation; using structural optimization and applying active strain to test the structural tunability; and illustrating the wave propagation and band gap opening mechanism from the vibration mode and energy perspectives. The results show that the widest band gap can reach 5061.9 Hz after structural optimization and tuning, while the lowest can be reduced to 274.3–285.6 Hz, and the vibration attenuation peak is around −620, so the structure has excellent vibration and sound insulation effect, while the band gap can be widened and reduced under active strain. The opening of the band gap is closely related to the directionality and regionality of wave propagation, and the comprehensive analysis method of wave propagation provides innovative research ideas for wave propagation, and also provides theoretical and technical support for structure and band gap optimization.

Low frequency noise control in duct based on locally resonant membrane with attached resonators

In this paper, locally resonant membrane with attached resonators is introduced to duct system to achieve good noise control effect especially in low frequency range. A rigorous analytical model based on modal expansion method is developed to investigate the vibroacoustic coupling characteristic of proposed duct-membrane-resonator system. The corresponding theoretical results show a good agreement with numerical simulation. The velocity distribution, sound transmission loss curve, and the mechanism behind are discussed and explained deeply. It is found that the attached resonators on the membrane can lead to new transmission loss peak which is related to but not exactly equal to the resonance frequency of attached resonator itself. Correspondence between zero equivalent density mass and transmission loss peak is achieved at the non-antisymmetric modes. Compared to single resonator, multi-resonators system can realize more transmission loss peaks for better sound transmission effect. In principle, ultra-low frequency noise reduction and arbitrarily adjustable frequency can be achieved by changing the frequency, position, and number of exerted resonators, which provide a new approach for duct noise control.

Design and optimization of three-dimensional composite multilayer cylindrical pentamode metamaterials for controlling low frequency acoustic waves

For three-dimensional pentamode metamaterials, it is of great significance to realize underwater low frequency acoustic wave control. Therefore, in order to compare with traditional double-cone pentamode metamaterials, two multilayer composite cylindrical three-dimensional pentamode metamaterials with low frequency and broad band gaps are proposed in this paper. By using pentamode metamaterials with lattice constants on the order of centimeters, the phononic band gaps below 60 Hz and the single-mode area below 30 Hz can be obtained. In addition, compared with asymmetrical double-cone locally resonant pentamode metamaterials, the lower edge frequency, relative bandwidth and figure of merit of the first phononic band gap can be reduced by up to 61.4%, 10.3% and 40.6%, respectively. It will provide reference and guidance for the engineering application of pentamode metamaterials in controlling the ultra-low frequency broadband acoustic waves, vibration and noise reduction.

High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser

The coherent population trapping (CPT) atomic clock is very promising for use in next-generation spaceborne applications owing to its compactness and high performance. In this paper, we propose and implement a CPT atomic clock based on the direct modulation of a large-modulation-bandwidth and narrow-linewidth distributed Bragg reflector laser, which replaces the usually used external bulk modulator in high-performance CPT clocks. Our method retains the high performance while significantly reducing the clock size. Using this highly compact bichromatic light source and simplest CPT configuration, in which a circularly polarized bichromatic laser interrogates the 87Rb atom system, a CPT signal of clock transition with a narrow linewidth and high contrast is observed. We then lock the local oscillator frequency to the CPT resonance and demonstrate an encouraging short-term frequency stability of 3.6 × 10−13 τ −1/2 (4 s ⩽ τ ⩽ 200 s). We attribute it to the ultralow laser frequency and intensity noise as well as to the high-quality-factor CPT signal. This study can pave the way for the development of compact high-performance CPT clocks based on our scheme.

High bandwidth laser frequency locking for wideband noise suppression.

Ultra-low frequency noise lasers have been widely used in laser-based experiments. Most narrow-linewidth lasers are implemented by actively suppressing their frequency noise through a frequency noise servo loop (FNSL). The loop bandwidths (LBW) of FNSLs are currently below megahertz, which is gradually tricky to meet application requirements, especially for wideband quantum sensing experiments. This article has experimentally implemented an FNSL with loop-delay-limited 3.5 MHz LBW, which is an order higher than the usual FNSLs. Using this FNSL, we achieved 70 dB laser frequency noise suppression over 100 kHz Fourier frequency range. This technology has broad applications in vast fields where wideband laser frequency noise suppression is inevitable.

Ultra-precise optical phase-locking approach for ultralow noise frequency comb generation

Optical frequency comb (OFC) with low phase noise enables more stringent metrology in classical and quantum systems. To eliminate phase noise, it is essential to extend the feedback bandwidth of the carrier-envelope phase and the repetition rate. Here, we present an approach to the construction of an ultralow-noise OFC. By exploiting different electro-optic modulators as fast actuators, this method can extend the feedback bandwidth to more than 150 kHz at the repetition rate phase locking and carrier-envelope offset phase locking We obtain a residual phase noise of 21.8 mrad (18.1 as) and 86.1 mrad (71.3 as) for the stabilised optical beat signal (fbeat) and carrier-envelope offset frequency (fceo), respectively. We validate this architecture by a measurement the relative linewidth between two combs, which reveals an in-loop beat fractional instability of less than 1.18 × 10−18 and an out-of-loop beat fractional instability of less than 2.31 × 10−15 at a 1-s averaging time. The out-of-loop beat phase noise is 145 mrad (120 as). These results indicate that the combination of Er-fiber laser technology and active feedback with a high bandwidth ensures coherent tracking of an optical reference with ultralow noise. The ultralow-noise OFC offers a universal tool for accurate, high-resolution optical spectroscopy.

Ultra-high resolution strain sensor network assisted with an LS-SVM based hysteresis model

Optical fiber sensor network has attracted considerable research interests for geoscience applications. However, the sensor capacity and ultra-low frequency noise limits the sensing performance for geoscience data acquisition. To achieve a high-resolution and lager sensing capacity, a strain sensor network is proposed based on phase-sensitive optical time domain reflectometer (φ-OTDR) technology and special packaged fiber with scatter enhanced points (SEPs) array. Specifically, an extra identical fiber with SEPs array which is free of strain is used as the reference fiber, for compensating the ultra-low frequency noise in the φ-OTDR system induced by laser source frequency shift and environment temperature change. Moreover, a hysteresis operator based least square support vector machine (LS-SVM) model is introduced to reduce the compensation residual error generated from the thermal hysteresis nonlinearity between the sensing fiber and reference fiber. In the experiment, the strain sensor network possesses a sensing capacity with 55 sensor elements. The phase bias drift with frequency below 0.1 Hz is effectively compensated by LS-SVM based hysteresis model, and the signal to noise ratio (SNR) of a strain vibration at 0.01 Hz greatly increases by 24 dB compared to that of the sensing fiber for direct compensation. The proposed strain sensor network proves a high dynamic resolution of 10.5 pε·Hz^-1/2 above 10 Hz, and ultra-low frequency sensing resolution of 166 pε at 0.001 Hz. It is the first reported a large sensing capacity strain sensor network with sub-nε sensing resolution in mHz frequency range, to the best of our knowledge.

Low noise all-fiber amplification of a coherent supercontinuum at 2 \xb5m and its limits imposed by polarization noise

We report a low noise, broadband, ultrafast Thulium/Holmium co-doped all-fiber chirped pulse amplifier, seeded by an Erbium-fiber system spectrally broadened via coherent supercontinuum generation in an all-normal dispersion photonic crystal fiber. The amplifier supports a − 20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The total relative intensity noise (RIN) integrated from 10 Hz to 20 MHz is 0.07%, which to our knowledge is the lowest reported RIN for wideband ultrafast amplifiers operating at 2 µm to date. This is achieved by eliminating noise-sensitive anomalous dispersion nonlinear dynamics from the spectral broadening stage. In addition, we identify the origin of the remaining excess RIN as polarization modulational instability (PMI), and propose a route towards complete elimination of this excess noise. Hence, our work paves the way for a next generation of ultra-low noise frequency combs and ultrashort pulse sources in the 2 µm spectral region that rival or even outperform the excellent noise characteristics of Erbium-fiber technology.

Ultralow noise DFB fiber laser with self-feedback mechanics utilizing the inherent photothermal effect

Single frequency laser sources with low frequency noise are now at the heart of precision high-end science, from the most precise optical atomic clocks to gravitational-wave detection, thanks to the rapid development of laser frequency stabilization techniques based on optical or electrical feedback from an external reference cavity. Despite the tremendous progress, these laser systems are relatively high in terms of complexity and cost, essentially suitable for the laboratory environment. Nevertheless, more and more commercial applications also demand laser sources with low noise to upgrade their performance, such as fiber optic sensing and LiDAR, which require reduced complexity and good robustness to environmental perturbations. Here, we describe an ultralow noise DFB fiber laser with self-feedback mechanics that utilizes the inherent photothermal effect through the regulation of the thermal expansion coefficient of laser cavity. Over 20 dB of frequency noise reduction below several tens of kilohertz Fourier frequency is achieved, limited by the fundamental thermal noise, which is, to date, one of the best results for a free-running DFB fiber laser. The outcome of this work offers promising prospects for versatile applications due to its ultralow frequency noise, simplicity, low cost, and environmental robustness.

Acoustic inerter: Ultra-low frequency sound attenuation in a duct.

This letter investigates an acoustic metamaterial exhibiting a unique sound pressure amplification mechanism for ultra-low frequency sound attenuation. The system is constructed by integrating a flexible panel into the side-branch duct of a Herschel-Quincke (HQ) tube. A new peak emerges in the Sound Transmission Loss (STL) at a frequency far lower than the frequencies of the HQ tube-induced STL peaks. It cannot, after careful comparisons, be attributed to any local resonances, including structural resonances of the flexible panel or air resonances inside the side-branch cavities. To explain the underlying physics, several numerical simulations are performed. The results reveal that analog to a mechanical inerter, a "push-pull" force is created by the sound pressure difference between the sub-cavities in which a pressure amplification mechanism is generated at the interface of the embedded panel. This force is large enough to activate an out-of-plane motion of the flexible panel, trapping the incident sound power in a circular flow around the duct-branch loop. The unique phenomenon is successfully reproduced in experiment, where the flexible panel is made of carbon fiber. The proposed acoustic metamaterial can be used as silencing components for ultra-low frequency noise control in duct.

Influence of Substorm Activity on the Formation of Ultra Low Frequency Noise Emissions in the Frequency Range of 0–7 Hz

The results of the study of simultaneous observations of midlatitude ultra low frequency (ULF) noise emissions in the frequency range of 0–7 Hz and disturbances in the night sector of an auroral oval are presented. Data from observations of the magnetic field at the midlatitude Borok Observatory (L = 2.8) and 1-min data of the AL index are used for the analysis. It is shown that two kinds of ULF noise emission are observed simultaneously in the hertz range primarily in the summer season: one looks like a diffuse spot, and the other one is characterized by the occurrence of spectral resonance structures determined by the influence of the ionospheric Alfven resonator. Diffuse spots are recorded mostly in the evening sector of the magnetosphere, and they precede the observation of spectral resonance structures, which are detected primarily at about midnight. Comparison of the observation intervals for diffuse spots with the dynamics of the AL index showed that diffuse spots form during substorm activity on the night side of the Earth in 80% of cases. It is established that the interval between the onset of substorm disturbances and the onset of the observation of diffuse spots is ∼60 min in most cases. It is assumed that the formation of diffuse spots is related to characteristics of the dynamics of protons injected from the magnetospheric tail in substorms and to the appearance of plasmaspheric plumes in the evening sector.

High-Resolution Brillouin Optoelectronic Oscillator Using High-Order Sideband Injection-Locking

Microwave signals with high frequency, flexible tunability, and low phase noise are highly desired in signal processing and radar systems. In this letter, an optical injection-locking scheme using the high-order phase modulation sideband is introduced to realize a stable Brillouin optoelectronic oscillator (OEO). Pure microwave signals up to 40 GHz with low phase noise are obtained using an electrical drive signal below 5 GHz. A flexible frequency tuning with high resolution of 8 MHz is maintained by controlling the electrical drive signal and the slave laser, and the side mode suppression ratio is higher than 60 dB. The phase noise of the OEO-generated microwave signals under different master lasers is measured and compared to verify the significance of the master lasers with ultralow frequency noise. The maximum frequency drift of a 10.79-GHz signal is 1.4 kHz within 600 s, showing the stability of the proposed Brillouin OEO.

Experimental study on 1/f intrinsic thermal noise in optical fibers

Intrinsic thermal noise in optical fibers is an ultimate factor limiting the performances of fiber-based sensors and measurement systems. Therefore, it is important to have a thorough understanding of this kind of noise. However, the mechanism of the intrinsic thermal noise which has a 1/&lt;i&gt;f&lt;/i&gt; spectral density remains unclear so far. There are two theoretical models to explain the mechanism of this kind of noise: thermoconductive noise model and thermomechanical noise model. The thermoconductive noise model states that the noise is caused by diffusion of local entropy fluctuations associated with random spontaneous emission events, while the thermomechanical noise model says that the noise is caused by spontaneous fluctuations of fiber length induced by mechanical dissipation. Which theoretical model is correct is still an open question. In this paper, we experimentally investigate the intrinsic thermal noise in optical fibers by using a balanced fiber Michelson interferometer with heterodyne detection technique. When a fiber-stabilized laser with ultralow frequency noise is used as a laser source and other noise sources are carefully controlled, the 1/&lt;i&gt;f&lt;/i&gt; spectral intrinsic thermal noise can be observed down to infrasonic frequency. According to these measurements, in order to verify which theoretical model is the mechanism of generating the intrinsic thermal noise with 1/&lt;i&gt;f&lt;/i&gt; spectral density, we study the relationship between the level of the intrinsic thermal noise and the concentration of the dopant in fibers and the applied tension of fibers. We observe that the level of the 1/&lt;i&gt;f&lt;/i&gt; spectral intrinsic thermal noise is independent of the concentration of the dopant in fibers. This means that the thermoconductive noise model is not suitable to this case. We also observe that the level of the 1/&lt;i&gt;f&lt;/i&gt; spectral intrinsic thermal noise can be reduced by increasing the tension exerted on the optical fibers. Because the mechanical loss of a fiber can be lower than the loss of the material which the fiber is made of when the fiber is subjected to a certain tension, this observation proves the fact that the 1/&lt;i&gt;f&lt;/i&gt; spectral intrinsic thermal noise in optical fibers originates from the mechanical dissipation process inside optical fibers. This is consistent with the predictions of the thermomechanical noise model. Finally, the inconsistency between the experimental data and the theoretical results for thermomechanical noise is discussed.

Noise analysis and improvement of a micro-electro-mechanical-systems fluxgate sensor

An integrated micro solenoid fluxgate sensor with a Co-based amorphous core is presented in this work based on the Micro-Electro-Mechanical System (MEMS) technologies. As the advantage of miniaturization, the micro fluxgate sensor is found to be operated under wide excitation frequency range of 100 kHz–1 MHz. Ultra-low frequency noise (<0.1 Hz) always arises in zero field time-domain signal acquisition under full excitation frequency range. An additional broadband transformer is placed before the excitation end of the sensor to eliminate the ultra-low frequency noise. The noise power density of the sensor working at 600 kHz is analyzed. The result shows the noise is 1.15 nT/Hz1/2@1 Hz without the transformer and 0.11 nT/Hz1/2@1 Hz with the transformer, while the noise rms level is 5.38 nT and 0.23 nT in the frequency range of 50 mHz–10 Hz respectively. For the MEMS micro fluxgate sensor, more stringent electromagnetic compatibility design for the sensor signal system is needed to eliminate additional noise derived from the electromagnetic radiated interference and the voltage disturbance.

Effect of the Ionospheric Parameter Variations at Altitudes of 80–350 km on Diurnal Dynamics of the ULF Magnetic Field Spectra at Midlatitudes

We study the causes of sporadic and long-term variations in the polarization spectrum of the ultralow-frequency (ULF) magnetic background noise at midlatitudes. Data records of low-frequency horizontal magnetic components at the Novaya Zhizn midlatitude observatory of the Radiophysical Research Institute (56°N, 45.74°E) and the Japanese station Moshiri (44.37°N, 142.27°E) were used. Ionosonde data from the sites located in Russia (Vasilsursk station) and in Japan (Wakkanai station) were also analyzed. The effect of the sporadic Es layers on the frequency spectrum of noise polarization was detected. It is shown that the appearance of quite intense sporadic Es layers with the cutoff frequencies f0Es > 3–5 MHz can significantly alter the polarization spectrum parameters, which is due to variations in the optical depth and Q factor of the sub-ionospheric Alfven resonator (sub-IAR) formed by the valley between the E and F regions. Numerical simulation of the polarization parameters of the background noise using the IRI-2012 model corrected by the ionosonde data confirmed that the Es layer about 5 km thick, with the cutoff frequencies f0Es > 3–5 MHz, may lead to a sharp decrease in the boundary frequency fb between the frequency ranges of the left- and right-hand polarized noise, or even to a complete disappearance of the sub-IAR manifestation in the polarization parameter spectrum. Numerical calculations have also revealed the impact of the altitude position of the Es layer on the Q factor of the sub-IAR. The effect of variations in the altitude and cutoff frequency of the F-layer maximum, as well as the electron density at the altitudes of the E layer and the valley, on the features of the diurnal dynamics of the ULF noise polarization spectra is discussed.

Quantum frequency bridge: high-accuracy characterization of a nearly-noiseless parametric frequency converter.

We demonstrate an efficient and inherently ultra-low noise frequency conversion via a parametric sum frequency generation. Due to the wide separation between the input and pump frequencies and the low pump frequency relative to the input photons, the upconversion results in only ≈100 background photons per hour. To measure such a low rate, we introduced a dark count reduction algorithm for an optical transition edge sensor.