Low Intensity Noise Research Articles

High-resolution frequency-comb spectroscopy with electro-optic sampling and instantaneous octave-wide coverage across mid-IR to THz at a video rate

Ultrabroadband electro-optic sampling using few-optical-cycle probing pulses is a sensitive technique to detect electric field amplitudes with a high dynamic range and up to near-infrared optical frequencies. By combining this method with dual-frequency-comb spectroscopy and using a new class of ultrafast lasers, we perform high-resolution, 80 MHz/0.0027 cm−1 (10 MHz/0.0003 cm−1 with spectral interleaving), spectroscopic measurements in the frequency range 1.5–45 THz (6.6–200 µm), excluding the strongly absorbing Reststrahlen band of lattice resonances at 4.5–9 THz, with an instantaneous spectral coverage exceeding an octave (e.g., 9–22 μm). As a driving source, we use a pair of mutually coherent combs from Kerr-lens mode-locked solid-state Cr:ZnS (2.35 μm) lasers. One of the combs is frequency downconverted via intrapulse difference frequency generation to produce a longwave “sensing” comb, while the second comb is frequency doubled to produce a near-IR “probe” comb for electro-optic sampling (EOS). The low intensity and phase noise of our dual-comb system allow for capturing a large amount of spectral information (200 000 comb-mode-resolved spectral lines spaced by 80 MHz) in the mid-IR portion of the spectrum at a video rate of 69 Hz, with the signal-to-noise ratio limited by the shot noise of the near-IR EOS balanced detection system. Our dual-comb spectroscopy measurements with low-pressure gaseous ethanol, isoprene, and dimethyl sulfide reveal Doppler-limited spectroscopic signatures that have never been explored before.

Auto-pump-depleted stable single-longitudinal-mode 1.5 μm source with the assistance of SHG.

To realize a stable single-longitudinal-mode (SLM) 1550-nm light source for the generation of non-classical states, a ring auto-pump-depleted singly resonant optical parametric oscillator (SRO) with the assistance of second-harmonic-wave generation (SHG) is designed and built in this Letter. A magnesium oxide doped periodically polarized lithium niobate (MgO:PPLN) crystal and a lithium triborate (LBO) crystal are employed as the optical parametric downconversion (OPDC) and SHG crystals, respectively. Especially, the introduced SHG can firstly increase the loss difference between the lasing and non-lasing modes so that the dual-mode or multi-mode coupling in the achieved SRO can be effectively eliminated and the stable SLM operation is achieved. At the same time, the SHG will automatically adjust the output coupling efficiency of SRO, so as to achieve efficient conversion efficiency and auto-pump depletion of SRO. In addition, due to the SHG, it is easy to achieve the low-intensity noise multi-wavelength output for the stable SLM SRO. As a result, the output powers of the SLM 1550 nm and 775 nm are up to 4.05 W and 3.25 W, respectively, and the total optical conversion of the built SRO can achieve 45.58%. The presented method paves a way to develop a compact stable SLM multi-wavelength SRO, and the obtained SRO is further beneficial to develop compact continuous-variable non-classical light fields.

Noise intensity modulates the responses of mantled howler monkeys to anthropophony.

Anthropogenic noise is a major global pollutant but its effects on primates are poorly understood, limiting our ability to develop mitigation actions that favor their welfare and conservation. In this study, we used an experimental approach to determine the impact of variation in noise intensity on mantled howler monkeys (Alouatta palliata). We conducted the study at Los Tuxtlas (México), where we studied the physiological stress (proxied via fecal glucocorticoid metabolites, fGCM) and behavioral responses of 16 males. We played back chainsaw noise at two intensities (40 and 80 dB) and used days in which groups were not exposed to noise as matched controls. With increased noise intensity fGCM increased, vigilance and vocalizations were longer, and vigilance, vocalizations, and flight occurred quicker. Physiological and behavioral responses occurred even after low-intensity noise playbacks (i.e., 40 dB). Therefore, noise intensity is a significant factor explaining the responses of mantled howler monkeys to anthropogenic noise. These results imply that management actions aimed at eradicating anthropogenic noise are required for the conservation and welfare of mantled howler monkeys at Los Tuxtlas.

Ultra-narrow linewidth and a low-noise cascading Brillouin random fiber laser with a dual pump.

A cascading Brillouin random fiber laser with a dual pump (DP-CBRFL) is proposed and demonstrated. The DP-CBRFL can improve the Brillouin gain significantly to achieve an ultra-narrow linewidth (95.5 Hz) with 200 mW pump power, due to two cascading Brillouin gain fibers with an identical Brillouin frequency shift. Compared with the general Brillouin random fiber laser, the proposed random fiber laser has a more stable Brillouin gain spectrum and a lower mode density, which makes it have a lower intensity noise and frequency noise, especially in the low-frequency range. Meanwhile, it exhibits a high slope efficiency of 28% even if the pump power has reached 1.1 W due to the strong suppression ability of the high-order Stokes light.

Observation of RIN reduction via spectral broadening in an NPR-based stretched pulse fiber laser.

We show that an optimum mode-locking state with low relative intensity noise (RIN) can be identified by continuous broadening of an optical spectrum in a stretched-pulse fiber laser based on nonlinear polarization rotation (NPR). Under the premise of keeping the overall spectral shape unchanged, either gradually increasing the pump power or unidirectionally adjusting the polarization controller (PC) can effectively reduce RIN as the optical spectral bandwidth broadens. The optimized intensity noise performance of the laser can be attributed to the increased pulse energy and reduced intra-cavity net dispersion. Moreover, the integrated RIN will further decrease as the maximum 3-dB bandwidth extends. In our experiment, the detected minimum integrated rms RIN is below 0.003% (from 100 Hz to 100 kHz). Our experimental results find that the absolute spectral width is not a necessary key condition for obtaining low RIN mode-locked laser, whereas it may help understand and design versatile low-noise ultrafast laser sources.

Exposure to low-intensity noise exacerbates nonalcoholic fatty liver disease by activating hypothalamus pituitary adrenal axis

Noise exposure induces metabolic disorders, in a latent, chronic and complex way. However, there is no direct evidence elucidating the relationship between low-intensity noise exposure and nonalcoholic fatty liver disease (NAFLD). Male mice (n = 5) on high-fat diet (HFD) were exposed to an average of 75 dB SPL noise for 3 months to reveal the effect of noise exposure on NAFLD, where the potential mechanisms were explored. In vivo (n = 5) and in vitro models challenged with dexamethasone (DEX) were used to verify the role of hypothalamus pituitary adrenal (HPA) axis activation in hepatic lipid metabolism. Typical chronic-restraint stress (CRS, n = 8) was used to explore the role of depression in modifying activity of HPA axis. Finally, animal experiment (n = 8) was repeated to validate the roles of depression and HPA axis activation in NAFLD development. Chronic low-intensity noise exposure exacerbated NAFLD in mice on HFD characterized by hepatocyte steatosis, modified lipid metabolism and inflammation level. Plasma ACTH in H + N group was 1.5-fold higher than that in HFD group. Transcription of glucocorticoid receptor target genes was increased by chronic low-intensity noise exposure in HFD-treated mice. Excessive glucocorticoids mimicking HPA axis activation induced NAFLD in vivo and in vitro. Plasma ACTH increase and lipid storage also occurred in depressive mice stressed by CRS. More interestingly, the same noise exposure simultaneously induced depression in mice, disrupted the HPA axis homeostasis and exacerbated NAFLD in a repeated experiment. Thus, three-month exposure to 75 dB SPL noise was sufficient to exacerbate NAFLD progress in mice, where activation of HPA axis played a critical role. Depression played an intermediate role and contributed to HPA axis activation up-stream of the exacerbation.

The Erlangen model of tinnitus development-New perspective and treatment strategy

About one sixth of the population of western industrialized nations suffers from chronic, subjective tinnitus, causing socioeconomic treatment and follow-up costs of almost 22billion euros per year in Germany alone. According to the prevailing view, tinnitus develops as aconsequence of amaladaptive neurophysiological process in the brain triggered by hearing loss. The Erlangen model of tinnitus development presented here is intended to propose acomprehensive neurophysiological explanation for the initial occurrence of the phantom sound after hearing loss. Based on the model, anew treatment strategy will be developed. The model summarized here is based on various animal and human physiological studies conducted in recent years. The Erlangen model considers subjective tinnitus as aside effect of aphysiological mechanism that permanently optimizes information transmission into the auditory system by means of stochastic resonance (SR) even in the healthy auditory system. In fact, hearing-impaired patients with tinnitus hear better on average than those without tinnitus. This unfamiliar perspective on the phantom percept may already help affected patients to cope better with their suffering. In addition, based on the model, low intensity noise tinnitus suppression (LINTS) has been developed as anew, individually adapted treatment strategy for tonal tinnitus and has already been successfully tested in patients. Apossible limiting factor for the model and treatment strategy is the pitch of the tinnitus percept, which may require adjustments to the treatment strategy for frequencies above about 5 kHz.

VCSEL Cavity Loss and Gain Measurement for High-Speed Optical Interconnects

Over 100 Gb/s Four-level Pulse-Amplitude Modulation (PAM4) Vertical-Cavity Surface-Emitting Laser (VCSEL) based optical interconnects require high speed, damped, and low relative intensity noise (RIN) VCSELs. Measurement techniques for intrinsic parameters are essential for optimizing VCSEL designs. The proposed VCSEL cavity round-trip loss and gain coefficient measurement provides the insight of VCSEL distributed Bragg reflectors (DBRs) and quantum well (QW) design. A simple VCSEL cavity model is described and excellent matching between the model and measurement is achieved.

Improvement on stability of direct modulation laser with integrated active feedback by partial corrugated gratings.

Researchers have developed a uniform grating DFB with an integrated active optical feedback waveguide (UG-AFDFB) to enhance the modulation speed of direct modulation lasers (DMLs) while reducing costs based on identical active layer designations. However, this design has difficulties in obtaining high single mode yield (SMY) and low relative intensity noise (RIN) as a result of the strong optical feedback caused by the integrated active feedback waveguide (AFW) and the random phase of the facet phase. In this paper, a partial corrugated grating DFB with an integrated active optical feedback waveguide (PG-AFDFB) is proposed to address this issue. Comparison of SMY, S21, RIN, modulation eye pattern, and frequency chirp parameters between UG-AFDFB and PG-AFDFB based on time-domain transmission line laser mode reveals that PG-AFDFB with an optimized grating couple parameter κ performs significantly better than UG-AFDFB under the same conditions. Furthermore, the performance of PG-AFDFB is not sensitive to the random phase of the rear facet phase. Even when κ ranges from 6000 /m to 12000 /m, the current in the AFW is between 0 mA and 20 mA, and the length of the AFW ranges from 50 µm to 100 µm; the SMY of PG-AFDFB remains above 80%.

Generation of a Wideband OFC by the Correlation of Multiple Modes of a Gain-Switched Fabry Pérot Laser

We experimentally demonstrate two novel photonically integrable architectures of a wideband gain-switched optical frequency comb (OFC). The proposed method is based on correlating multiple modes of gain-switched Fabry-Pérot (GS-FP) laser. This is achieved by injection locking a GS-FP with an optimised signal generated by an source OFC and a dual-stage active demultiplexer. By employing such a configuration, we demonstrate the generation of two 6.25 GHz OFCs, spanning over 337.5 GHz and 875 GHz with expansion factors of 4 and 10, respectively. The optimisation of the OFC expansion for flatness and/or wide span, as required is also shown. Furthermore, a detailed comb line characterisation is reported, to highlight the spectral purity of the expanded comb portraying low linewidth (∼85 kHz), a low relative intensity noise (RIN better than -130 dB/Hz), and a high degree of phase correlation between lines (13 Hz beat tone width). Finally, active demultiplexing of the comb lines with various separations across the expanded OFC is reported, demonstrating its suitability for use in a superchannel-based flexible optical networks, mmW/THz generation and spectroscopy applications.

Modeless Raman fiber laser

The discrete multi-longitudinal mode structure and temporal periodic fluctuation are the intrinsic features of conventional lasers. However, longitudinal mode spacing limits the maximum resolution in high-resolution laser sensing systems. In addition, temporal periodic fluctuation reduces the security of secure communication and deteriorates the randomness in fast physical random bit generation. Therefore, it still remains an open challenge to realize a laser source without discrete longitudinal mode and temporal periodic fluctuation. Here, a modeless Raman fiber laser (RFL) with high efficiency was demonstrated. The output cavity mirror adopts a fiber Bragg grating with ultralow reflectance of −27dB. Due to the modulation instability, the discrete multi-longitudinal modes in the RFL gradually broaden with the increase of intracavity Stokes wave power and eventually fully overlap. At high power levels, the RFL no longer has discrete longitudinal modes like a conventional laser but generates a quasi-continuous spectrum. Benefiting from the modeless nature, the RFL can achieve extremely low relative intensity noise, favorable temporal stability, and rather low coherence. Modeless RFLs are expected to be ushered as ideal light sources into secure communication, optical sensing, and optical imaging.

Fluoro-sulfo-phosphate fiber with long-wavelength gain for an L-band laser

In this work, a self-developed Er3+/Yb3+ co-doped fluoro-sulfo-phosphate (FSP) fiber with long-wavelength gain characteristics has been reported. The mixed-ion strategy manipulates the ligands of rare-earth ions, leading to a broad gain bandwidth (almost twice that of phosphate fiber) and excellent amplification ability at the extended wavelength (>2dB/cm at the L-band). As showcases of its features, a 1617-nm fiber laser based on a 1-cm-long FSP fiber is constructed. Furthermore, an all-fiber-integrated L-band passively mode-locked laser based on a 2.2-cm-long fiber with a fundamental repetition rate of 4.3 GHz is demonstrated. The generated pulse exhibits a high signal-to-noise ratio larger than 80 dB in the radio-frequency domain and a low relative intensity noise of 0.04%.

Optical noise characteristics of injection-locked epitaxial quantum dot lasers on silicon.

This work theoretically investigates the relative intensity noise (RIN) and spectral linewidth characteristics of epitaxial quantum dot (QD) lasers on silicon subject to optical injection. The results show that the RIN of QD lasers can be reduced by optical injection, hence a reduction of 10 dB is achieved which leads to a RIN as low as -167.5 dB/Hz in the stable injection-locked area. Furthermore, the spectral linewidth of the QD laser can be greatly improved through the optical injection locked scheme. It is reduced from 556.5 kHz to 9 kHz with injection ratio of -60 dB and can be further reduced down to 1.5 Hz with injection ratio of 0 dB. This work provides an effective method for designing low intensity noise and ultra-narrow linewidth QD laser sources for photonics integrated circuits on silicon.

Semantic similarity metrics for image registration.

Image registration aims to find geometric transformations that align images. Most algorithmic and deep learning-based methods solve the registration problem by minimizing a loss function, consisting of a similarity metric comparing the aligned images, and a regularization term ensuring smoothness of the transformation. Existing similarity metrics like Euclidean Distance or Normalized Cross-Correlation focus on aligning pixel intensity values or correlations, giving difficulties with low intensity contrast, noise, and ambiguous matching. We propose a semantic similarity metric for image registration, focusing on aligning image areas based on semantic correspondence instead. Our approach learns dataset-specific features that drive the optimization of a learning-based registration model. We train both an unsupervised approach extracting features with an auto-encoder, and a semi-supervised approach using supplemental segmentation data. We validate the semantic similarity metric using both deep-learning-based and algorithmic image registration methods. Compared to existing methods across four different image modalities and applications, the method achieves consistently high registration accuracy and smooth transformation fields.

A bimembrane neuron for computational neuroscience

A biological neuron has endomembrane and extracellular membrane, and its capacitive effect can be described in an equiavlent neural circuit composed of two capacitors. In this paper, a smooth nonlinear resistor with cubic term for the current-voltage (i-v) is introduced into a nonlinear circuit connected with two capacitors and one inductor. The characteristic of field energy is analyzed and unknown parameters are identified with adaptive law. Furthermore, a magnetic flux-controlled memristor (MFCM) is connected to this circuit to estimate its memristive effect on dynamics and energy flow. After scale transformation, the two kinds of nonlinear circuits are replaced by equivalent neuron models, which can present similar spiking, bursting patterns as those presented in biological neurons. Energy function is obtained in theoretical way for discerning the relation between mode selection and energy level. Noisy disturbance is applied to induce coherence resonance, and the involvement of memristive current can induce coherence resonance with lower noise intensity. The results confirmed that energy flow can be controlled to regulate the electric activities in neurons, and higher energy level is obtained in periodic patterns of neuron under coherence resonance. This neural circuit and the memristive model are more suitable for identifying the neural activities in nervous system composed of biological neurons.

Can the Clocks Tick Together Despite the Noise? Stochastic Simulations and Analysis

The suprachiasmatic nucleus (SCN), also known as the circadian master clock, consists of a large population of oscillator neurons. Together, these neurons produce a coherent signal that drives the body’s circadian rhythms. What properties of the cell-to-cell communication allow the synchronization of these neurons, despite a wide range of environmental challenges such as fluctuations in photoperiods? To answer that question, we present a mean-field description of globally coupled neurons modeled as Goodwin oscillators with standard Gaussian noise. Provided that the initial conditions of all neurons are independent and identically distributed, any finite number of neurons becomes independent and has the same probability distribution in the mean-field limit, a phenomenon called propagation of chaos. This probability distribution is a solution to a Vlasov–Fokker–Planck type equation, which can be obtained from the stochastic particle model. We study, using the macroscopic description, how the interaction between external noise and intercellular coupling affects the dynamics of the collective rhythm, and we provide a numerical description of the bifurcations resulting from the noise-induced transitions. Our numerical simulations show a noise-induced rhythm generation at low noise intensities, while the SCN clock is arrhythmic in the high noise setting. Notably, coupling induces resonance-like behavior at low noise intensities, and varying coupling strength can cause period locking and variance dissipation even in the presence of noise.

Yb-Doped All-Fiber Amplifier with Low-Intensity Noise in mHz Range Oriented to Space-Borne Gravitational Wave Detection

We present a low-intensity noise single-frequency Yb-doped all-fiber amplifier oriented to space-borne gravitational wave detection. Relative intensity noise (RIN) below −70 dBc/Hz @ 1 mHz~1 Hz was achieved by virtue of feedback-loop-based intensity noise suppression. Based on systematic noise analysis and experimental investigation, we found that the pump noise and temperature-dependent noise of the fiber splitter and the photodetector contributed mainly to the RIN of the fiber amplifier. Therefore, we carefully designed a feedback-loop-based Yb-doped all-fiber amplifier, and finely stabilized the temperature of the pump diode, fiber splitters, and photodetectors. Consequently, the RIN can be suppressed down to −72.5 dBc/Hz around 1 mHz. This low-intensity all-fiber Yb-doped amplifier can be used for space-borne gravitational-wave detection.

Kinetic analysis of p53 gene network with time delays and PIDD

p53 kinetics plays a key role in regulating cell fate. Based on the p53 gene regulatory network composed by the core regulatory factors ATM, Mdm2, Wip1, and PIDD, the effect of the delays in the process of transcription and translation of Mdm2 and Wip1 on the dynamics of p53 is studied theoretically and numerically. The results show that these two time delays can affect the stability of the positive equilibrium. With the increase of delays, the dynamics of p53 presents an oscillating state. Further, we also study the effects of PIDD and chemotherapeutic drug etoposide on the kinetics of p53. The model indicates that (i) PIDD low-level expression does not significantly affect p53 oscillatory behavior, but high-level expression could induce two-phase kinetics of p53; (ii) Too high and too low concentration of etoposide is not conducive to p53 oscillation. These results are in good agreement with experimental findings. Finally, we consider the influence of internal noise on the system through Binomial [Formula: see text]-leap algorithm. Stochastic simulations reveal that high-intensity noise completely destroys p53 dynamics in the deterministic model, whereas low-intensity noise does not alter p53 dynamics. Interestingly, for the stable focus, the internal noise with appropriate intensity can induce quasi-limit cycle oscillations of the system. Our work may provide the useful insights for the development of anticancer therapy.

Characterizing the information transmission of inverse stochastic resonance and noise-induced activity amplification in neuronal systems.

Purkinje cells exhibit a reduction of the mean firing rate at intermediate-noise intensities, which is somewhat reminiscent of the response enhancement known as "stochastic resonance" (SR). Although the comparison with the stochastic resonance ends here, the current phenomenon has been given the name "inverse stochastic resonance" (ISR). Recent research has demonstrated that the ISR effect, like its close relative "nonstandard SR" [or, more correctly, noise-induced activity amplification (NIAA)], has been shown to stem from the weak-noise quenching of the initial distribution, in bistable regimes where the metastable state has a larger attraction basin than the global minimum. To understand the underlying mechanism of the ISR and NIAA phenomena, we study the probability distribution function of a one-dimensional system subjected to a bistable potential that has the property of symmetry, i.e., if we change the sign of one of its parameters, we can obtain both phenomena with the same properties in the depth of the wells and the width of their basins of attraction subjected to Gaussian white noise with variable intensity. Previous work has shown that one can theoretically determine the probability distribution function using the convex sum between the behavior at small and high noise intensities. To determine the probability distribution function more precisely, we resort to the "weighted ensemble Brownian dynamics simulation" model, which provides an accurate estimate of the probability distribution function for both low and high noise intensities and, most importantly, for the transition of both behaviors. In this way, on the one hand, we show that both phenomena emerge from a metastable system where, in the case of ISR, the global minimum of the system is in a state of lower activity, while in the case of NIAA, the global minimum is in a state of increased activity, the importance of which does not depend on the width of the basins of attraction. On the other hand, we see that quantifiers such as Fisher information, statistical complexity, and especially Shannon entropy fail to distinguish them, but they show the existence of the mentioned phenomena. Thus, noise management may well be a mechanism by which Purkinje cells find an efficient way to transmit information in the cerebral cortex.

Characterization of optical phase-locked two distributed-feedback fiber lasers for 87Rb atom interferometry

We report the optical phase-locking of two 1560 nm distributed-feedback (DFB) fiber lasers converted to 780 nm with a frequency difference of 6.834 GHz for 87Rb. The fiber laser exhibits a narrow linewidth and low-intensity noise, which are advantageous for a Raman pulse laser in an atomic gravimeter. However, the piezoelectric actuator of the DFB laser causes a low feedback rate in an optical phase-locking loop (OPLL). To overcome this limitation, we used an acousto-optic modulator as an additional actuator to increase the modulation bandwidth of the OPLL and achieved a residual phase noise of approximately -127 dBrad2/Hz in the offset frequency range from 10 Hz to 1 kHz. The sensitivity limit estimated using the sensitivity function and experimental parameters of 87Rb atomic gravimeter shows that our OPLL system can be used in state-of-the-art gravimeters.