Impact of Cyclic Error on Absolute Distance Measurement Based on Optical Frequency Combs.

Absolute distance measurements based on optical frequency combs (OFCs) have greatly promoted advances in both science and technology, owing to the high precision, large non-ambiguity range (NAR), and a high update rate. However, cyclic error, which is extremely difficult to eliminate, reduces the linearity of measurement results. In this study, we quantitatively investigated the impact of cyclic error on absolute distance measurement using OFCs based on two types of interferometry: synthetic wavelength interferometry and single-wavelength interferometry. The numerical calculations indicate that selecting a suitable reference path length can minimize the impact of cyclic error when combining the two types of interferometry. Recommendations for selecting an appropriate synthetic wavelength to address the tradeoff between achieving a large NAR and minimizing the risk of failure when combining the two methods are provided. The results of this study are applicable not only in absolute distance measurements but also in other applications based on OFCs, such as surface profile, vibration analysis, etc.

The role of gut-lung axis in COPD: Pathogenesis, immune response, and prospective treatment.

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and healthcare burden worldwide. The progression of COPD is a combination of genetic predisposition and environmental factors, primarily cigarette smoking, and the underlying mechanisms are still unknown. Intestinal microecology impacts host immunity, metabolism, and resistance to pathogenic infections, which may be involved in pulmonary disease. Moreover, substantial interaction occurs between the intestinal and respiratory immune niches. After reviewing nearly 500 articles, we found the gut-lung axis plays an important role in the development of COPD. COPD patients often have dysbiosis of the intestinal microenvironment, which can affect host immunity through a series of mechanisms, exacerbating or protecting against COPD progression. This paper summarizes how the gut-lung axis influences COPD, including the alterations of intestinal microecology, the pathological mechanisms, and the involved immune responses. Finally, we summarize the latest research advances in COPD treatment from the perspective of regulating the gut-lung axis and intestinal immunity and evaluate the potential value of the gut-lung axis in improving COPD prognosis.

Broadband, high-power optical frequency combs covering visible to near-infrared spectral range.

Optical frequency combs (OFCs) have become essential tools in a wide range of metrological and scientific research fields. However, in the reported literature, OFCs that cover the visible spectral range have a limited bandwidth and pulse energy. These drawbacks limit their potential applications, such as high-signal-to-noise ratio spectroscopic measurements. In this work, we demonstrate a broadband, high-power optical frequency comb covering the visible to near-infrared range (550 nm to 900 nm) with a high average power of approximately 300 mW. This is accomplished by the power scaling of optical pulses from a fully stabilized Er:fiber comb, coherent spectral broadening and finally the utilization of a PPLN's χ(2) nonlinearity. The broadband, high-power, fully stabilized visible OFCs showcased in this work offer reliable laser sources for high-precision spectroscopic measurements, imaging, and comparisons of optical clocks.

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.

Lipase-catalyzed hydrazine insertion for the synthesis of N'-alkyl benzohydrazides.

N'-alkyl benzohydrazides are classic organic compounds that have been widely utilized in organic chemistry. In this study, an efficient method was developed for the synthesis of N'-alkyl benzohydrazides by hydrazine insertion catalyzed by lipase. Under the optimal conditions (Morita-Baylis-Hillman ketone [1 mmol], phenylhydrazine [1.3 mmol], N,N-dimethylformamide [2 ml], lipase [20 mg], room temperature, 12 h), satisfactory yields (71-97%) and substrate tolerance were obtained when porcine pancreatic lipase was used as biocatalyst. These findings imply the great potential for the lipase-catalyzed synthesis of N'-alkyl benzohydrazides and extend the utilization of lipase in organic chemistry.

Environment-stable sub-100 fs Er: fiber laser with a 3†dB bandwidth of 78†nm.

A robust all polarization-maintaining (PM) passively mode-locked Er-doped fiber laser is demonstrated based on the biased nonlinear amplifier loop mirror (NALM). With a π/2 nonreciprocal free-space phase shifter, stable single pulse mode locking can be obtained at the central wavelength of 1565.7 nm with a 3 dB spectral bandwidth of 24.6 nm in the soliton regime. The repetition rate of the pulse train is 98.13 MHz. The direct output pulse duration is 109 fs, which is nearly transform-limited. After the intracavity dispersion management, the robust self-started mode-locking in the stretched-pulse regime is realized at 1564 nm, and the 3 dB spectral bandwidth reaches up to 78 nm. The repetition rate of the pulse train is 199.6 MHz. In particular, the direct output pulse width is only 77 fs with a low integrated relative intensity noise (RIN) of only 0.0044% (integrated from 1 Hz to 1 MHz). To the best of our knowledge, this is the shortest pulse width directly from the all-PM NALM laser oscillator.

Quasi-real-time dual-comb spectroscopy with 750-MHz Yb:fiber combs.

We present quasi-real-time dual-comb spectroscopy (DCS) using two Yb:fiber combs with ∼750 MHz repetition rates. A computational coherent averaging technique is employed to correct timing and phase fluctuations of the measured dual-comb interferogram (IGM). Quasi-real-time phase correction of 1-ms long acquisitions occurs every 1.5 seconds and is assisted by coarse radio frequency (RF) phase-locking of an isolated RF comb mode. After resampling and global offset phase correction, the RF comb linewidth is reduced from 200 kHz to ∼1 kHz, while the line-to-floor ratio increases 13 dB in power in 1 ms. Using simultaneous offset frequency correction in opposite phases, we correct the aliased RF spectrum spanning three Nyquist zones, which yields an optical coverage of ∼180 GHz around 1.035 µm probed on a sub-microsecond timescale. The absorption profile of gaseous acetylene is observed to validate the presented technique.

High speed time-of-flight displacement measurement based on dual-comb electronically controlled optical sampling.

We demonstrate a direct time-of-flight approach that utilizes dual-comb electronically controlled optical sampling (ECOPS) to measure small displacements. ECOPS is enabled by electrically controlling the repetition rate of one laser via an intracavity electric-optical modulator (EOM). The acquisition rate is set by the EOM modulation frequency, which is much higher than commonly used asynchronous optical sampling (ASOPS). In a proof-of-principle experiment, an 80-kHz acquisition rate is obtained with a pair of ∼105 MHz repetition rate Er-fiber lasers. At an average time of 30 ms, a measurement precision evaluated with Allan deviation reaches 26.1 nm for a 40-µm static displacement. In a dynamic measurement, a 500-Hz sinusoidal vibration with 15 µm amplitude has also been identified. The high-precision and high-speed displacement measurement technique can be potentially used in 3D surface profilometry of microelectronic step-structures and real-time monitoring of high frequency mechanical vibrations, etc.

[Acupuncture with Tiaochong Shugan method by stages for menstrual headache based on syndrome differentiation: a randomized controlled trial].

OBJECTIVE: To compare the clinical efficacy between acupuncture with Tiaochong Shugan method by stages based on syndrome differentiation and oral administration of ibuprofen sustained-release capsule in patients with menstrual headache. METHODS: A total of 90 cases with menstrual headache were randomly divided into an acupuncture group (45 cases, 1 case excluded, 3 cases dropped off) and a medication group (45 cases, 3 cases dropped off). The patients in the acupuncture group were treated with acupuncture with Tiaochong Shugan method by stages based on syndrome differentiation; during period of pain attacks, Ganshu (BL 18), Qimen (LR 14), Hegu (LI 4), Taichong (LR 3), Sizhukong (TE 23) through Shuaigu (GB 8) were selected, once a day; during period of pain relief, Qichong (ST 30), Dahe (KI 12), Guanyuan (CV 4), Taixi (KI 3) were selected, once every 1-2 days. The patients in the medication group were treated with oral administration of ibuprofen sustained-release capsule during period of pain attacks. Each menstrual cycle was taken as a course of treatment, and both groups were treated for 3 courses. The headache comprehensive score (HCS), visual analogue scale (VAS) socre, dysmenorrhea symptom score (DSS) were compared before treatment, 1, 2 and 3 courses into treatment and 1, 2, 3 menstrual cycles after treatment; the clinical efficacy was also evaluated. RESULTS: The HCS score at each time point after treatment was lower than that before treatment in the two groups (P<0.05); 2 and 3 menstrual cycles after treatment, the HCS socres in the acupuncture group were lower than those in the medication group (P<0.05). Except for the 2 and 3 menstrual cycles after treatment in the medication group, the VAS score at each time point after treatment was lower than that before treatment in the two groups (P<0.05). Except for 1 menstrual cycle into treatment, the DSS scores in the acupuncture group at each time point after treatment was lower than that before treatment (P<0.05); the DSS socres at 2 and 3 menstrual cycles into treatment and 1 menstrual cycle after treatment were lower than those before treatment in the medication group (P<0.05). Except for 1 menstrual cycle into treatment, the VAS score and DSS score in the acupuncture group were lower than those in the medication group at each time point after treatment (P<0.05). The total effective rate was 82.9% (34/41) in the acupuncture group, which was higher than 73.8% (31/42) in the medication group (P<0.05). CONCLUSION: The analgesic effect of acupuncture with Tiaochong Shugan method by stages based on syndrome differentiation is superior to oral administration of ibuprofen sustained-release capsules, which could effectively prevent the recurrence of menstrual headache, and improve irregular menstruation-related symptoms.

Metal-organic frameworks/polydopamine synergistic interface enhancement of carbon fiber/phenolic composites for promoting mechanical and tribological performances.

Carbon fiber/phenolic composites have wide application prospects in the transmission of vehicles, where the combination of prominent mechanical and tribological properties is required. Multiscale metal-organic frameworks (MOFs) and polydopamine (PDA) as binary reinforcements were employed to construct a rigid-flexible hierarchical structure for improving the interfacial performances of friction materials. This unique rigid-flexible (MOFs/PDA) reinforcement could act as an effective interfacial linker, significantly facilitating the integration of fibers into the matrix and establishing a strong mechanical interlocking and chemical bonding onto the fiber/matrix interphase, thus boosting the mechanical and tribological properties of the composites. Benefiting from the MOF/PDA synergistic enhancement effects, the interlaminar shear strength of ZIF-8-composites (P1), MOF-5-composites (P2) and UiO-66-(COOH)2-composites (P3) was improved by 70.80%, 43.80% and 53.28%, respectively. In addition, the wear rate of P1 decreased from 3.55 × 10-8 cm3 J-1 to 2.45 × 10-8 cm3 J-1. This work provides a feasible approach for establishing rigid-flexible reinforced structures and opens up a double-component synergistic enhancement strategy to efficiently promote mechanical and tribological properties for fabricating high-performance carbon fiber/phenolic composites.

Dual-mode and two-signal-wavelength femtosecond optical parametric oscillator based on LiB3O5.

We demonstrate a tunable femtosecond dual-beam-mode (cylindrical vector beam [CVB] and Gaussian beam [GB]), dual-signal-wavelength optical parametric oscillator based on a temperature-tuned lithium triborate crystal, synchronously pumped by a frequency-doubled mode-locked Yb-doped fiber laser. When fixing the CVB wavelength at 780 nm, the central wavelength of the GB signal could be continuously tuned from 664 to 722 nm. The maximum total signal output power is 515 mW at a 4 W pump with dual-wavelength operation (664 and 780 nm). All the measured signal pulse durations are around 150 fs. Moreover, sum-frequency-generation with Gaussian mode tuning from 548 to 588 nm is obtained, with the maximum power of 52 mW at 548 nm. Thanks to the dual-channel configuration, the wavelengths of a CVB and GB can be tuned independently. Such a flexible and versatile configuration makes it a practical tool for many applications such as high-resolution microscopy and high-capacity optical communication.

Optical frequency comb noise spectra analysis using an asymmetric fiber delay line interferometer.

A simple and practical apparatus enabling repetition rate (frep) noise, carrier-envelope frequency (fceo) noise and nth optical comb mode (νn) noise spectra measurements with high precision is established. The frep and νn noise spectra are measured by a fiber delay line interferometer, while fceo noise spectrum is measured by an f-2f interferometer. We utilize this apparatus to characterize the noise performance of an Er-fiber optical frequency comb (OFC) and analyze the origin of dominant noise sources. Moreover, this apparatus provides a powerful tool for diagnosing noise dynamics intrinsic in mode-locked lasers and OFCs. To this end, we uncover the anti-correlation between frep and fceo noise as well as the impact of servo loops on noise characteristics in the stabilized OFC.

Timing jitter reduction through relative intensity noise suppression in high-repetition-rate mode-locked fiber lasers.

We reported the timing jitter reduction of an 882 MHz mode-locked NPE Yb:fiber lasers through active relative intensity noise suppression. The timing jitter spectra measurements based on balanced optical cross-correlation (BOC) technique show a reduction of ~10 dB in the Fourier frequency range from ~3 kHz to ~30 kHz with a unity-gain crossing point of 80 kHz. The results verify the theoretical prediction that the relative intensity noise (RIN) induced timing jitter by self-steepening effect dominates the jitter performance below ~100 kHz. Further comparison with the analytic model shows that the effect of RIN decays below ~3 kHz. Thus, the timing jitter reduction is not obvious at low frequency. To the best of our knowledge, this is the first experimental report on the timing jitter reduction through active RIN suppression in high-repetition-rate mode-locked fiber lasers.

[Unconstrained detection of ballistocardiogram and heart rate based on vibration acceleration].

The requirement for unconstrained monitoring of heartbeat during sleep is increasing, but the current detection devices can not meet the requirements of convenience and accuracy. This study designed an unconstrained ballistocardiogram (BCG) detection system using acceleration sensor and developed a heart rate extraction algorithm. BCG is a directional signal which is stronger and less affected by respiratory movements along spine direction than in other directions. In order to measure the BCG signal along spine direction during sleep, a 3-axis acceleration sensor was fixed on the bed to collect the vibration signals caused by heartbeat. An approximate frequency range was firstly assumed by frequency analysis to the BCG signals and segmental filtering was conducted to the original vibration signals within the frequency range. Secondly, to identify the true BCG waveform, the accurate frequency band was obtained by comparison with the theoretical waveform. The J waves were detected by BCG energy waveform and an adaptive threshold method was proposed to extract heart rates by using the information of both amplitude and period. The accuracy and robustness of the BCG detection system proposed and the algorithm developed in this study were confirmed by comparison with electrocardiogram (ECG). The test results of 30 subjects showed a high average accuracy of 99.21% to demonstrate the feasibility of the unconstrained BCG detection method based on vibration acceleration.

Active f-to-2f interferometer for record-low jitter carrier-envelope phase locking.

The f-to-2f interferometer plays a key role for carrier-envelope phase (CEP) measurement and subsequent stabilization. The CEP measurement typically relies on the application of two optical nonlinearities, namely supercontinuum generation and second-harmonic generation. Then the cascadation of these nonlinearities often leads to signal levels on the order of a few photons per pulse. We experimentally demonstrate that the introduction of optical gain into the infrared arm of an f-to-2f interferometer can mitigate this detection bottleneck and improve signal-to-noise ratios by 20 dB compared to purely passive schemes. We further show that this measure allows for residual phase jitters between the carrier and envelope of about 10 mrad, corresponding to record-breaking timing jitters in the single attosecond regime. Moreover, the method appears generally applicable to a wide range of oscillators in the near-infrared and may enable stable CEP locking of mode-locked oscillators that so far have resisted stabilization. Finally, we propose a parametric variant of the active f-to-2f interferometer that can be used for laser amplifiers with kilohertz repetition rates and below.

All-polarization-maintaining dual-wavelength mode-locked fiber laser based on Sagnac loop filter.

We demonstrate an all polarization-maintaining (PM) fiber based dual-wavelength mode-locked Er-fiber laser. A nonlinear amplifying loop mirror (NALM) with an intracavity nonreciprocal phase shifter is used for self-started mode-locking. A short segment of PM fiber is angle-spliced to the NALM, functioning as a PM Sagnac loop filter, thus enabling dual-wavelength mode-locking. The wavelength separation is solely determined by the angle-spliced PM fiber length. Stable dual-wavelength mode-locking operation is switchable between 1570/1581 nm and 1581/1594 nm. The two-color pulse trains oscillating in the same cavity have an inherent offset repetition rate of ~1 kHz owing to cavity dispersion, allowing future high precision dual-comb applications with a simple and robust configuration.

Timing jitter of high-repetition-rate mode-locked fiber lasers.

We characterize the timing jitter of the pulse trains from 880 MHz Yb-doped nonlinear polarization rotation mode-locked fiber lasers based on a balanced optical cross-correlation method. Jitter spectral density at different net-cavity dispersions has been characterized, and the near-zero dispersion shows the lowest rms timing jitter (10 fs rms, integrated from 30 kHz to 5 MHz). The measurements have been compared with analytical models. The comparison shows that the RIN-coupled timing jitter by nonlinearity is the dominated origin of the measured timing jitter below ∼100 kHz, while amplified spontaneous emission noise makes a major contribution in the high frequency range above hundreds of kilohertz. To the best of our knowledge, this is the first high-precision timing jitter characterization for the ∼gigahertz level repetition rate mode-locked fiber lasers. The results will be of great importance for further improving the laser performance for many applications.

High-detectivity optical heterodyne method for wideband carrier-envelope phase noise analysis of laser oscillators.

Broadband characterization of the carrier-envelope phase (CEP) noise spectral density of free-running mode-locked lasers is essential for advanced low-noise optical frequency comb designs. Here we present a direct method that utilizes an optical heterodyne beat between a pair of repetition-rate-locked mode-locked lasers for CEP noise characterization, without requiring an f-2f interferometer or nonlinear optical conversion steps. A proof-of-principle experiment in a femtosecond Yb-fiber laser achieves CEP noise spectral density characterization with >270 dB dynamic range over a Fourier frequency range from 5 mHz to 8 MHz. The measurement noise floor is well below 1 µrad/√Hz, enabling dependable detection down to a quantum-limited noise floor. The method can resolve various noise mechanisms that cause specific CEP noise spectral shapes. The underlying mechanisms are further analyzed in terms of spurious temporal correlation to distinguish between technical and stochastic noise signatures. Moreover, a Hadamard deviation analysis reveals a varying degree of frequency stability in the measured CEP time series.

Excess carrier-envelope phase noise generation in saturable absorbers.

Attosecond spectroscopy and precision frequency metrology depend on the stabilization of the carrier-envelope phase (CEP) of mode-locked lasers. Unfortunately, the phase of only a few types of lasers can be stabilized to jitters in the few-hundred millirad range. In a comparative experimental study, we analyze a femtosecond Ti:sapphire laser and three mode-locked fiber lasers. We numerically demodulate recorded time series of the free-running carrier-envelope beat note. Our analysis indicates a correlation between amplitude and frequency fluctuations at low Fourier frequencies for essentially all lasers investigated. While this correlation typically rolls off at frequencies beyond 100 kHz, we see clear indications for a broadband coupling mechanism in one of the fiber lasers. We suspect that the observed coupling mechanism acts to transfer intracavity power fluctuations into excess phase noise. This coupling mechanism is related to the mode-locking mechanism employed and not to the gain medium itself. We further verify this hypothesis by numerical simulations, which identify resonances of the saturable absorber mirror as a possible explanation for the coupling mechanism. Finally, we discuss how to avoid a detrimental influence of such resonances.

Long-term stable coherent beam combination of independent femtosecond Yb-fiber lasers.

We demonstrate coherent beam combination between independent femtosecond Yb-fiber lasers by using the active phase locking of relative pulse timing and the carrier envelope phase based on a balanced optical cross-correlator and extracavity acoustic optical frequency shifter, respectively. The broadband quantum noise of femtosecond fiber lasers is suppressed via precise cavity dispersion control, instead of complicated high-bandwidth phase-locked loop design. Because of reduced quantum noise and a simplified phase-locked loop, stable phase locking that lasts for 1 hour has been obtained, as verified via both spectral interferometry and far-field beam interferometry. The approach can be applied to coherent pulse synthesis, as well as to remote frequency comb connection, allowing a practical all-fiber configuration.