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Optical activity (OA) spectroscopy is a powerful tool to characterize molecular chirality, explore the stereo-specific structure and study the solution-state conformation of biomolecules, which is widely utilized in the fields of molecular chirality, pharmaceutics and analytical chemistry. Due to the considerably weak effect, OA spectral analysis has high demands on measurement speed and sensitivity, especially for organic biomolecules. Moreover, gas-phase OA measurements require higher resolution to resolve Doppler-limited profiles. Here, we show the unmatched potential of dual-comb spectroscopy (DCS) in magnetic optical activity spectroscopy (MOAS) of gas-phase molecules with the resolution of hundred-MHz level and the high-speed measurement of sub-millisecond level. As a demonstration, we achieved the rapid, high-precision and high-resolution MOAS measurement of the nitrogen dioxide $${\upsilon }_{1}$$ υ 1 +$${\upsilon }_{3}$$ υ 3 band and the nitric oxide overtone band, which can be used to analyze fine structure of molecules. Besides, the preliminary demonstration of liquid-phase chiroptical activity (as weak as 10−5) has been achieved with several seconds of sampling time, which could become a routine approach enabling ultrafast dynamics analysis of chiral structural conformations.

We demonstrate three typical mode-locking processes of a nonlinear amplifying loop mirror (NALM) fiber laser via a general nonlinear Schrödinger equation-based (GNLSE) simulation model. First, the pulse evolutions in the NALM cavity were separately simulated under asymmetric and weakly asymmetric conditions. We found that the splitting ratio and positions of the gain fiber can result in a suitable phase bias between clockwise and counter-clockwise beams, enabling the realization of a self-starting low-threshold operating condition. To assess the roles of the splitting ratio and gain in the mode-locking process, we simulated three pulse formation processes: in the soliton, stretched-pulse, and dissipative soliton mode-locking regimes. The simulation results show that the splitting ratio, gain, and dispersion directly influence the mode-locking condition and pulse characteristics, thereby providing effective quantified guidance for high-quality pulse generation. Finally, an experimental NALM oscillation operating under stretched pulse conditions was established to investigate the impact of the splitting ratio and pump power on the pulse characteristics. The experimental results prove that the splitting ratio, gain, and dispersion can be used to manipulate the mode-locking threshold, self-starting threshold, nonlinear effects, and pulse characteristics.

Dual-comb spectroscopy (DCS) is a powerful spectroscopic technique, which is developing for the detection of transient species in reaction kinetics on a short time scale. Conventionally, the simultaneous determination of multiple species is limited to the requirement of broadband spectral measurement at the cost of the measurement speed and spectral resolution owing to the inherent trade-off among these characteristics in DCS. In this study, a high-speed multi-molecular sensing is demonstrated and achieved through using a programmable spectrum-encoded DCS technique, where multiple narrow encoding spectral bands are reserved selectively and other comb lines are filtered out. As a dual-comb spectrometer with a repetition rate of 108 MHz is encoded spectrally over a spectral coverage range of 1520 to 1580 nm, the measurement speed is increased 6.15 times and single-shot absorption spectra of multiple molecules (C2H2, HCN, CO, CO2) at a time scale of 208 µs are obtained. Compared to conventional single-shot dual-comb spectra, encoded dual-comb spectra have improved short-term signal-to-noise ratios (SNRs) by factors of 3.65 with four encoding bands and 5.68 with two encoding bands. Furthermore, a fiber-Bragg-grating-based encoded DCS is demonstrated, which reaches 17.1 times higher average SNR than that of the unencoded DCS. This spectrum-encoded technique can largely improve the DCS measurement speed, and thus is promising for use in studies on multi-species reaction kinetics.

In this paper, an improved interpolating element-free Galerkin method (IIEFGM) with nonsingular weight function is introduced to calculate the magnetic field distribution of electrical machines. Since the shape function satisfies the property of the Kronecker-delta function, the IIEFGM can impose the boundary conditions directly. Meanwhile, the IIEFGM adopts a nonsingular weight function, which means the weight function of the conventional element-free Galerkin method (EFGM) can be used. In order to demonstrate the effectiveness of IIEFGM, the magnetic field distribution of a permanent magnet coreless disc machine is analyzed. The selection of weight function and the nodes distribution are discussed, the comparison with the finite element method (FEM) shows the IIEFGM has high computational accuracy.

In this paper, we report a scheme providing precise spectral analysis and surface imaging, simultaneously, based on a high-coherence dual-comb interferometer. With two tightly phase-locking frequency combs, we demonstrate a high-coherence dual-comb interferometer (DCI) covering 188 to 195 THz (1538.5 to 1595.7 nm) with comb-tooth resolution and a max spectral signal-to-noise ratio (SNR) of 159.7. The combination of the high-coherence dual-comb spectrometer and a reference arm simultaneously enables gas absorption spectroscopy and for the absolute distance information to be obtained in one measurement. As a demonstration, we measure the spectrum of CO2 and CO. From the same interferograms, we demonstrate that distance measurement, by time-of-flight (TOF), can be resolved with an rms precision of 0.53 μm after averaging 140 images and a measurement time of 1 s. Finally, we demonstrate that non-contact surface imaging, using 2D mechanical scanning, reaches lateral resolution of 40 μm. The longitudinal precision is 0.68 μm with a measurement time of 0.5 s. It verifies that DCS has the potential to be applied in standoff detection, environmental pollution monitors, and remote sensing.

To the best of our knowledge, this paper presents the first self-referenced polarization-maintaining (PM) Er: nonlinear polarization evolution-based fiber frequency comb. A short PM fiber section segment acts as an artificial saturable absorber and compensates for the group velocity mismatch. A conventional f-2f interferometer allows detecting carrier-envelope offsetting frequency (fceo) after an octave-spanning spectrum generation. A complete description of the PM Er: fiber comb combining a noise study of the repetition rate (fr) and fceo is presented. Accordingly, the in-loop residual phase noise of fceo and fr are 468 mrad and 0.336 mrad from 1 Hz to 3 MHz, respectively. Further, we verify the long-term performance of the comb system by recording the fluctuation of fr and fceo for more than 10 h. We observed that combining fast actuators for phase noise suppression would potentially enable a compact and robust frequency comb system for a range of applications such as comb-based liars, optical clock, and gas sensing.

This study presents an investigation on iron loss and start-up ability of a 1.5 kW output power with rated speed 5850 r/min 6/2 switched reluctance machine (SRM), which is used as a prime motor to directly drive centrifugal blower. Two winding connection types of the SRM are investigated, including the symmetrical magnetic topology (SyMT) and asymmetrical magnetic topology (ASyMT). The two types are proposed to settle poor self-start ability of the 6/2 SRM and high iron loss density of high-speed SRM. According to the simulation results, the SyMT has merit of self-start ability, i.e. no risk of start-up failure, whereas the ASyMT has advantage of low iron loss, i.e. low iron loss density on stator and rotor. Prototypes are manufactured to confirm and verify the predicted results and the test results prove the correctness of the analyses.

To further investigate the interaction between surface-modified nanoparticles and wormlike micelles (WLMs) and their rheological response, a newly kind of anhydride-modified silica nanoparticles (AMSNPs) was modified by succinic anhydride. A quaternary ammonium surfactant with the headgroup modified by two hydroxyethyl groups N-cetyl-N,N-di(2-hydroxyethyl)methylammonium bromide (CDHAB) was synthesized in this work. The transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS) were used to characterize AMSNPs. The formation of WLMs by CDHAB and sodium salicylate (NaSal) was observed by cryogenic transmission electron microscopy (cryo-TEM). With the addition of nanoparticles, rheological properties of WLM solution get enhancement remarkably. Furthermore, compared with the aid of normal silica nanoparticles (SNPs), WLMs with AMSNPs show higher viscosity and better viscoelasticity. This can be attributed to the high ionization degree of AMSNPs in the nanofluid and stronger electrostatic attraction and extra hydrogen bonding interaction between functional groups of AMSNPs and CDHAB, leading to more micelle-nanoparticle junctions and longer micelles. This work can give further view and knowledge of interaction between functional groups of modified nanoparticles and special surfactants of WLMs.

This paper investigates the optimal design method of switched reluctance machines (SRMs) for mitigating torque ripple and improving average torque under the low speed operation. In order to produce high torque for hill climbing or overload start-up of micro electric vehicles (EVs) and reduce the converter capacity, an SRM with four times torque overload capability and two times speed range with constant power is needed. To improve the SRM performance, first, a new method called multiphase excitation (MPE) method for calculating resultant static torque is adopted, which takes the cross-coupling and magnetic saturation into consideration to reduce the calculation error bought by the traditional method called single-phase excitation (SPE) method; second, optimal methods, including parameters sensitivity analysis, multi-objective optimization function, and winding connection types, have been taken to release the magnetic saturation, to improve average torque, and to depress torque ripple. Compared with the initial scheme, the torque ripple is minimized from 55.33% to 14.84% and the average torque is improved by 8.36%. The experimental results validate the effectiveness of the proposed methods.

We demonstrate the generation of few cycle mid-infrared (MIR) pulses from a self-compressed optical parametric oscillator (OPO), synchronously pumped by a 105 fs Yb3+ doped fiber laser system. By using a ring cavity design, 4.2-cycle (45.3 fs) MIR pulses at a central wavelength of 3250 nm (with a 10-dB bandwidth of 648 nm) are obtained in a simple, practical, and compact configuration. Detailed theoretical analysis and numerical simulation are performed to better understand the self-compression mechanism in such an OPO cavity. In addition, when pumped at 3.8 W, the OPO delivers idler output power up to 330 mW and the corresponding pump depletion is 74.7%.

This study presents the synthesis of a CO2-responsive surfactant namely 2‑alkyl‑1‑hydroxyethyl imidazoline (HEAnIBs n = 16, 18, 20), which exhibits satisfactory surface and adsorption properties. The micelle formation and adsorption of the above imidazoline based surfactant in aqueous solution were studied in this work. CMC, the effectiveness of surface tension reduction, the maximum surface excess concentration, the minimum area occupied per surfactant molecule at the air/water interface, and the micelle aggregation number were obtained by measuring surface tension. Dynamic light scattering indicates that when the surfactant concentration is 1.2 times the CMC, the hydrodynamic radii of the three surfactants studied were almost equal to 8 nm. Electrical conductivity measurements indicate the micellization of surfactants in aqueous solution. In addition, typical thermodynamic parameters, including ΔGm0, ΔGads0, ΔHm0, ΔSm0, ΔHmA, ΔSmA, and ΔCp,m0 were evaluated. The thermodynamic parameters show that micelle formation is entropy-and-enthalpy co-driven from 298 K to 318 K. We expect to elucidate the self-assembly of CO2-responsive surfactant in an aqueous solution.

Gain-parameter-dependent transfer functions and phase-noise performances in a mode-locked Yb-doped fiber laser are measured in this study. It is discovered that the corner frequency in the amplitude and phase domains is determined by the absorption coefficient of the gain fiber, when the total absorption and other cavity parameters are fixed. This shows that an oscillator using gain fiber with higher dopant concentration accumulates more phase noise. Furthermore, we present net cavity dispersion-dependent transfer functions to verify the effect of dispersion management on the frequency response. We derive a guideline for optimizing mode-locked fiber laser design to achieve low phase noise and timing jitter.

Coherent pulse synthesis in the mid-infrared (mid-IR) domain is of great interest to achieve broadband sources from parent pulses, motivated by the advantages of optical frequency properties for molecular spectroscopy and quantum dynamics. We demonstrate a simple mid-IR coherent synthesizer based on two high-repetition-rate optical parametric amplifiers (OPAs) at nJ-level pump energy. The relative carrier envelope phase between the two OPAs was passively stable for a shared continuous wave (CW) quantum cascade laser (QCL) seed. Lastly, we synthesized mid-IR pulses with a duration of 105 fs ranging from 3.4 to 4.0 µm. The scheme demonstrated the potential to obtain broader mid-IR sources by coherent synthesis from multiple CW QCL-seeded OPAs.

We report on a compact and robust self-referenced optical frequency comb with a tunable repetition rate, generated by an all-polarization-maintaining (PM) mode-locked Er-doped fiber laser. The spacing between comb teeth can be tuned above 300 kHz at a repetition rate of 101 MHz. The repetition rate and the carrier–envelope offset of the laser are stabilized separately, and the relative residual phase noises are determined to be $336~\unicode[STIX]{x03BC}\text{rad}$ and 713 mrad (1 Hz–1 MHz). The accurate frequency characteristics and the stable structure show great potential for the use of such a comb in applications of precision measurements.

We report on the generation of a mid-infrared (mid-IR) frequency comb with a maximum average output power of 250 mW and tunability in the 2.7–4.0 μm region. The approach is based on a single-stage difference frequency generation (DFG) starting from a compact Yb-doped fiber laser system. The repetition rate of the near-infrared (NIR) comb is locked at 75 MHz. The phase noise of the repetition rate in the offset-free mid-IR comb system is measured and analyzed. Except for the intrinsic of NIR comb, environmental noise at low frequency and quantum noise at high frequency from the amplifier chain and nonlinear spectral broadening are the main noise sources of broadening the linewidth of comb teeth, which limits the precision of mid-IR dual-comb spectroscopy.

We present a parallel depth resolved laser fabrication technique for micro- and nano-machining metal substrates based on temporal focusing. In this system, the spectrum of a femtosecond laser pulse is first spatially separated by a digital micromirror device, which simultaneously serves as a diffraction grating and a programmable binary mask. After collimation and beam flattening, an objective lens recombines the spectrum to the focal region, forming a high-intensity, depth resolved light sheet for laser micromachining. The light sheet technology enables parallel fabrication of highly uniform micro-structures of close to diffraction-limited resolution. Experimental results demonstrate high-resolution (~800 nm) direct area patterning on various metal substrates, e.g., nickel and copper, over an area of ~100 × 60 µm 2 within tens of laser pulses. The relationship among material removal rate, surface flatness, laser power, and number of pulses have been experimentally studied; the results suggest the application of higher power with fewer number of laser pulses produce microstructures of better surface quality. The light sheet technology substantially improves the throughput of ultrafast laser machining, enabling direct area patterning without compromising the resolution.

Utilizing the Joint Typhoon Warning Center (JTWC) TC best-track data covering the period of 1951-2015, spatial and temporal variation characteristics of Northwest Pacific tropical cyclone activity in global warming scenario is discussed. The annual frequency trend of TC in the Northwest Pacific is as follows: from the late 1950s to the early 1960s, there was an obvious upward trend, the late 1960s to the early 1980s showed a downward trend, the late 1980s to the mid 1990s showed an upward trend, and the mid-1990s to the late 2010s showed a downward trend. The frequency of super TY in the Northwest Pacific showed a rapid downward trend from the late 1960s to the early 1980s, and then changed to a slow upward trend. The region where the linear change of different stages of TC life cycle stage (genesis, development, mature, decay) frequency is approaching to central Asian coastline. Northwest Pacific TC activity decreases, but from the perspective of spatial distribution of linear changes, the threat of TC towards eastern Asia is enhanced. That is probably because the global warming scenario results in the increasing temperature gradient between Northwest Pacific and central and eastern Pacific, which leads to the strengthening of walker circulation. The strengthening walk circulation could increase the magnitude of the vertical wind sheer and the relative vorticity on the tropical Northwest Pacific, which affects the spatial variation of TC activity in Northwest Pacific.

Mid-infrared dual-comb spectroscopy is of great interest owing to the strong spectroscopic features of trace gases, biological molecules, and solid matter with higher resolution, accuracy, and acquisition speed. However, the prerequisite of achieving high coherence of optical sources with the use of bulk sophisticated control systems prevents their widespread use in field applications. Here we generate a highly mutually coherent dual mid-infrared comb spectrometer based on the optical–optical modulation of a continuous-wave (CW) interband or quantum cascade laser. Mutual coherence was passively achieved without post-data processes or active carrier envelope phase-locking processes. The center wavelength of the generated mid-infrared frequency combs can be flexibly tuned by adjusting the wavelength of the CW seeds. The parallel detection of multiple molecular species, including C 2 H 2 , CH 4 , H 2 CO , H 2 S , COS, and H 2 O , was achieved. This technique provides a stable and robust dual-comb spectrometer that will find nonlaboratory applications including open-path atmospheric gas sensing, industrial process monitoring, and combustion.

Based on the tropical cyclone (TC) data from the United States Joint Typhoon Center, this paper analyzes the spatial variation characteristics and possible mechanism of the TC that landed in China during the 65 years from 1951 to 2015. The conclusions are as follows. (1) The TC transformation location moves to southeast in El Niño years, and shifts to northeast in La Niña years. (2) In El Niño years, the frequency of TC affecting the coast of Mainland China is relatively low, while the frequency of TCs affecting Japan is increasing. In La Niña years, the frequency of TCs affecting Japan and the surrounding waters, the Yellow Sea, the Bohai Sea and the South China Sea increased, while that of Taiwan province decreased. (3) The changes in the local circulation in the Northwest Pacific increased the sea temperature, increased convection, increased relative humidity in the middle layer, weakened vertical wind shear, increased low-level vorticity, and increased high-level divergence in the southeastern part of the Northwest Pacific during the El Niño year. The large-scale circulation adjustments provided favorable background conditions for the southeast movement of the TC generating position. The southeast movement of the spawning location makes the landing TC activities have a longer moving distance and thus increases the life history. In La Niña years, the subtropical high weakened and moved eastward. This environment is conducive to generating more TCs to land in China in the La Niña year.

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.

Originalna kruta veza između motora i glavine zamijenjena je fleksibilnom uporabom spojke za povezivanje pogonskog i gonjenog vratila sustava prijenosa. U radu se zatim predstavlja odvojeni proces pokretanja i opterećenja motora na osnovu kojega se analizira shema reguliranja rada motora u praznom hodu. Fokusiranjem na probleme koordinacije povećanog kapaciteta zakretnog momenta i smanjenja trzaja automobila samo regulacijom rada motora u praznom hodu, predlaže se poboljšano rješenje kompenzacije zakretnog momenta podešavanjem napona rotora dok se spojka aktivira gonjenom stranom. Nadalje, ispituje se i optimizira metoda za označavanje napona zakretnog momenta s ciljem stvaranja balansa između traženog prijenosnog mehanizma zakretnog momenta i jačine trzaja tijekom pokretanja vozila. Konačno, eksperimentima je demonstriran učinkovito poboljšani učinak zakretnog momenta pogonskog motora kod pokretanja vozila u svrhu povećane udobnosti., The original rigid connection between the motor and the hub is replaced with a flexible one using the clutch to connect the driving and driven sides of the driveline system. Then the paper proposes a separable process of motor start-up and load operation, based on which the scheme of idle speed control is studied. Focusing on the contradiction between the extended torque capacity and the reduced vehicle jerk merely by tuning the idle speeds, an improved solution is proposed for toque compensation by throttle adjustment during the clutch engagement. Furthermore, the method for tracking the armature voltage of the driving motor is investigated and optimized, aiming at the balance between the demanded transmitted torque and the jerk level during the starting process. Finally, effectively enhanced torque capacity of the driving motor for the vehicle start-up on the premise of the improved comfort level has been demonstrated by experiments.

In-wheel motor drive electric vehicles mainly adopt fastening connection between hub and motor, so the electromagnetic impact and the mechanical impact are inevitable. The electromagnetic impact endangers the battery and the control system and the mechanical impact brings bad driving experience. Newly designed bistable electromagnetic clutches are embedded into the hub motor to realise flexible connection and avoid the impact. The goal of this study is to analyse the dynamic movement process of the clutch in order to achieve reliable torque transmission. The dynamic differential equations of the clutch coupled with the pulse power supply are presented. By using the dynamic analysis, the critical situation of the engagement is discussed and the engagement speed requirement equation is provided. The displacement curves and the acceleration curves validate the dynamic analysis and the engagement experiment verifies the reliability of the whole clutch system.

We demonstrate a dual-comb spectroscopy with a Michelson interferometer which optically suppresses original spectra of the combs. It yields an absorption signal with absolute intensity enhancement ratio of 51.8 compared with conventional dual-comb spectroscopy.