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In-wheel motor is recognized as the ideal form for electric vehicle drive. To reduce the mechanical shock and the electromagnetic impulsion brought by the rigid connection between the hub and the motor, a permanent magnet bistable electromagnetic clutch which flexibly connects the motor and the hub is proposed. In this paper, several power supply schemes for the clutch are discussed. Firstly, based on the dynamic simulation model, the static and dynamic performances of the clutch unit are studied, and then the advantages and disadvantages of these power supply systems are compared. Furthermore, the capacitor discharge pulsed power supply defined as “rotor side harmonic power generation + rechargeable battery + clutch wireless remote control” is intensively investigated. Based on the proposed analytical solution of the current pulse width, the optimal design is carried out, the capacitance, charging voltage, coil turns, and coil wire diameter is determined. Finally, experimental results verified the effectiveness of these power supply systems and the rationality of design.

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.

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%.

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.

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.

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.

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.

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.

We report on a mid-infrared (MIR) comb based on a Yb-fiber self-similar amplifier. The MIR comb has a tunable spectral coverage from 3.3 to 5.2 μm with a maximum average power of 90 mW.

We demonstrated a method to directly produce transform-limited few-cycle mid-infrared pulses at 90 fs duration employing optical parametric amplifier seeded by mid-infrared continuous-wave laser and the wavelength tunable ability is also proved.

We report on an 80-W, 50-fs self-similar-amplification comb and a 100-W, 150-fs chirped-pulse amplification comb. Frequency stability and noise characteristics of two locked pulse trains can confirm the generation of these high-power frequency fiber combs.

The determination of airflow parameters is essential to the research of critical information on environment monitoring, chemical kinetics, and aerodynamic and propulsion applications. During the past few decades, tunable diode laser absorption spectroscopy has become a common and efficient tool for the flow velocity measurement based on the Doppler shift of the absorption line. Dual-comb absorption spectroscopy (DCAS), as a state-of-the-art Fourier-transform broadband spectroscopic technique, not only can detect multiple trace molecules in parallel but also can extract Doppler shifts to derive the flow velocity through the analysis of dozens of molecular absorption lines simultaneously with high precision. Here, we report a proof-of-principle demonstration of the velocity measurements of acetylene at various flow velocities by means of a high-resolution and broadband DCAS. Mode-resolved Doppler-shifted rotational-vibrational lines in the P branch of acetylene molecules are obtained. A model for multiline Doppler frequency determination is investigated and experimentally verified. The flow velocity measurements with a measuring uncertainty down to the submeter per second over the range from 8.7 m/s to 44.8 m/s at an effective time resolution of 1 s and a measuring uncertainty of 1.97 m/s at 0.1 s are demonstrated. With broadband mid-infrared frequency combs covering atmospheric transmission windows, the open-path measurement for monitoring diffusion of the weak pollutant source would be realized.

We report on a high-power fiber optical frequency comb consisting of a 250-MHz mode-locked fiber laser and a three-stage cascaded fiber chirped-pulse amplification system. After power scaling, the group velocity dispersion and third-order dispersion, generated in fiber stretcher and amplifiers, are compensated by a grism compressor, outputting a 132-W, 180-fs pulse train. The repetition rate and carrier-envelope offset frequency are locked to a Rb clock with the standard deviations of 1.07 and 0.87 mHz, corresponding to the fractional instability of 8.3×10−13 and 1.35×10−19, respectively. Moreover, we investigate the noise characteristics at high average powers, presenting a low-noise property of this high-power fiber OFC.

We demonstrate an effective method to obtain high-repetition-rate femtosecond mid-infrared (mid-IR) pulses by nonlinear optical modulation of mid-IR continuous-wave (CW) quantum and interband cascade lasers (ICLs and QCLs). In the experiment, a high-repetition-rate femtosecond ytterbium-doped fiber laser with nanojoule-level pulse energy was used as the pump source of optical parametric amplifiers to modulate and amplify the mid-IR CW laser. Near transform-limited 84 fs duration (7.3 cycles) mid-IR pulses were generated with above 200 mW average power and a repetition rate of 160 MHz. Moreover, the spectral tunability was demonstrated using CW QCL at different wavelengths. The scheme offered a simple method to produce high-repetition-rate ultrashort pulses and that can be flexibly adopted in other mid-IR regions.

We demonstrate a high average power, widely tunable, dielectric-mirror-less optical parametric oscillator (OPO) based on MgO:PPLN (MgO-doped periodically poled lithium niobate), which is synchronously pumped by a 1040 nm femtosecond fiber laser. The OPO does not require any dielectric coating mirrors. By exploiting the four-prism sequence system, combined with the gold mirrors, the oscillating laser pulses could span the spectral regions in both the signal and idler, and the output pulses of OPO can be tuned across 1367–1914 nm in the signal, and 2152–4480 nm in the idler as well. This device can deliver as much as 1.2 W of average power at 1482 nm in the signal and up to 411 mW at 3487 nm in the idler, respectively. The ultrabroad-band spectra tunability, along with the high average output property, makes the dielectric-mirror-less OPO an attractive alternative to conventional OPOs.

Herein, the method of spectrum-encoded dual-comb interferometry is introduced to measure a three-dimensional (3-D) profile with absolute distance information. By combining spectral encoding for wavelength-to-space mapping, dual-comb interferometry for decoding and optical reference for calibration, this system can obtain a 3-D profile of an object at a stand-off distance of 114 mm with a depth precision of 12 μm. With the help of the reference arm, the absolute distance, reflectivity distribution, and depth information are simultaneously measured at a 5 kHz line-scan rate with free-running carrier-envelope offset frequencies. To verify the concept, experiments are conducted with multiple objects, including a resolution test chart, a three-stair structure, and a designed “ECNU” letter chain. The results show a horizontal resolution of ∼22 μm and a measurement range of 1.93 mm.

A compact and robust mode-locked fiber laser with a locked repetition rate is demonstrated in a polarization-maintain fiber ring cavity. The repetition rate is locked at 108 MHz and the standard deviation of the controlled signal is 0.997 mHz in a 10-hours test.

A simple and robust dual-comb scheme with different repetition rates is demonstrated from a passively mode-locked polarization-maintain fiber cavity. With mutual coherent properties in a single cavity, mode-resolved spectroscopy is obtained with free-running state.

We report a four-prism-pumped all-gold-mirror idler resonant optical parametric oscillator (OPO) driven by an Yb-fiber femtosecond laser. The idler is tuned from 2560–4180 nm, with the corresponding signal covering 1380–1825 nm.

A novel electromagnetic clutch system is proposed to resolve the mechanical and electromagnetic impacts in the in-wheel motor drive electric vehicles. The dynamic analysis of the system is presented for researching the engagement process. Based on the dynamic analysis, the relationship between the action forces in the engagement is studied and the range of clutch's angular deflection is given for the design optimization of the clutch.

In-wheel motor system is recognized as the ideal drive system for electric vehicles. To reduce mechanical shock and electromagnetic impulsion brought by rigid connection between hubs and motor, our group proposed a novel permanent bistable electromagnetic clutch which flexibly connecting the motor and hubs. In this paper, several power supply schemes for the clutch are proposed and compared, then, pulsed power supply as a better scheme is further studied and optimal designed. Subsequently, the capacitance, charging voltage, coil turns and coil diameter are determined. Finally, experiments verified the power supply system we proposed.

Uvođenjem nove trajne magnetski bi-stabilne elektromagnetske spojke (PMBECU) u transmisiju direktno pogonjenog električnog vozila, ostvarena je fleksibilna veza između motora i glavine karakteristična po odvojenom pokretanju pogonskog motora i osovinskog opterećenja. Najprije se raspravlja o načinu rada i shemi dovoda energije za PMBECU. Zatim se istražuje sklop za sustav pogona kotača s uređajem za pokretanje pomoću spojke. Razmatrajući dobre dinamičke karakteristike motora kotača, daje se shema promjenljivog praznog hoda te se gradi i analizira pojednostavljeni model za brzo ukapčanje spojke. Konačno se predložena metoda za pokretanje direktno pogonjenog električnog vozila primijenjuje na uređaju za ispitivanje. Eksperimentalni rezultati pokazuju da su jačina udara i elektromagnetski impuls tijekom pokretanja efikasno smanjeni pokretanjem zupčaste spojke, što rezultira udobnošću u vožnji i manjim zagrijavanjem namotaja armature pogonskog motora., By introducing a novel permanent magnetic bi-stable electromagnetic clutch unit (PMBECU) in the driveline of directly-driven electric vehicle, a flexible connection between motor and hub is realized with characteristic of separated starting process of drive motor and wheel load. Firstly, the working principle and power supply scheme for the PMBECU is discussed. Then the assembly for the wheel-drive system with starting assistance device by the clutch is investigated. Considering the good dynamic performance of wheel motor, variable-idle speed scheme is presented and the simplified model for fast clutch engagement is built and analyzed. Finally, the proposed method for directly-driven electric vehicle launch is performed on the test rig. The experimental results indicate the shock intensity and electromagnetic impulsion during the starting process is decreased effectively by clutch-coupling startup, which results in good driving comfort and less heat in armature winding of drive motor.

We characterize, for the first time, relative intensity noise and timing jitter properties of the OPG output pulses based on the balanced optical cross-correlator technique, which is in a fairly good agreement with numerical simulation.

We demonstrate experimentally a passively relative carrier-envelope phase (CEP)-locking, broadly tunable and robust laser source which is based on narrowband cw injection seeding of a two stage femtosecond optical parametric amplifier (OPA).

We demonstrate the generation of stretched pulses from an all-polarization-maintaining (all-PM) Er-doped fiber laser by means of a nonlinear polarization evolution (NPE) method. A theoretical model is built to analyze the relationship between the nonlinear transmissivity and the angles of the fiber segments. By managing the dispersion of the oscillator to support stretched-pulse formation and exploiting an NPE section based on three PM fiber segments, a self-starting laser with a 90.5-MHz repetition rate and 90-fs pulse duration is achieved. Numerous laser systems can benefit from this approach, including frequency combs, outdoor ranging and mid-IR systems.

An ultrafast spectrum programmable femtosecond laser may enhance the performance of a wide variety of scientific applications, e.g., multi-photon imaging. In this paper, we report a digital micromirror device (DMD)-based ultrafast pulse shaper, i.e., DUPS, for femtosecond laser arbitrary amplitude shaping — the first time a programmable binary device reported to shape the amplitudes of ultrafast pulses spectrum at up to 32[Formula: see text]kHz rate over a broad wavelength range. The DUPS is highly efficient, compact, and low cost based on the use of a DMD in combination with a transmission grating. Spatial and temporal dispersion introduced by the DUPS is compensated by a quasi-4-f setup and a grating pair, respectively. Femtosecond pulses with arbitrary spectrum shapes, including rectangular, sawtooth, triangular, double-pulse, and exponential profile, have been demonstrated in our experiments. A feedback operation process is implemented in the DUPS to ensure a robust and repeatable shaping process. The total efficiency of the DUPS for amplitude shaping is measured to be 27%.

A high repetition-rate, few-cycle light pulse is of great importance due to its potential for a variety of applications, including two-dimensional infrared spectroscopy and time-resolved imaging of molecular structures, which benefit from its ultrabroadband spectrum and ultrashort pulse duration. The generation of an ultrabroadband coherent spectrum is one of the frontiers of ultrafast optics, and accessing such few-cycle pulses is presently under active exploration. Here, we demonstrate a simple yet effective pulse synthesizer. It is based on two continuous-wave (cw) injection-seeded high-repetition-rate optical parametric amplification systems and the following self-phase-modulation dominated spectra-broadening processes. The combined spectrum spans from 1250 to 1670 nm, and a near Fourier-transform-limited 3.9-cycle (19.2 fs) synthesized pulse with a central wavelength of 1470 nm is obtained accordingly.

In this Letter, we present an ultrafast nonmechanical axial scanning method for two-photon excitation (TPE) microscopy based on binary holography using a digital micromirror device (DMD), achieving a scanning rate of 4.2 kHz, scanning range of ∼180 μm, and scanning resolution (minimum step size) of ∼270 nm. Axial scanning is achieved by projecting the femtosecond laser to a DMD programmed with binary holograms of spherical wavefronts of increasing/decreasing radii. To guide the scanner design, we have derived the parametric relationships between the DMD parameters (i.e., aperture and pixel size), and the axial scanning characteristics, including (1) maximum optical power, (2) minimum step size, and (3) scan range. To verify the results, the DMD scanner is integrated with a custom-built TPE microscope that operates at 60 frames per second. In the experiment, we scanned a pollen sample via both the DMD scanner and a precision z-stage. The results show the DMD scanner generates images of equal quality throughout the scanning range. The overall efficiency of the TPE system was measured to be ∼3%. With the high scanning rate, the DMD scanner may find important applications in random-access imaging or high-speed volumetric imaging that enables visualization of highly dynamic biological processes in 3D with submillisecond temporal resolution.

SUMMARY This paper investigates the characteristic performance of broken bar faults in induction motors at standstill by 2D time-stepping finite-element and experiential approaches. Three identical rotors with the same stator are employed to generate one healthy-rotor, one-broken-bar and two-broken-bar fault cases in a three-phase 1.1-kW squirrel-cage induction motor. The focus is placed on how the broken bar affects air-gap flux density, stator and rotor currents, torque and magnetic force, as well as iron and rotor bar losses. Numerical results are validated by some experimental measurements obtained from a dedicated test rig. Copyright © 2012 John Wiley & Sons, Ltd.

In this Letter, we present a digital micromirror device (DMD)-based ultrafast beam shaper, i.e., DUBS. To our knowledge, the DUBS is the first binary laser beam shaper that can generate high-resolution (1140×912 pixels) arbitrary beam modes for femtosecond lasers at a rate of 4.2 kHz; the resolution and pattern rate are limited by the DMD. In the DUBS, the spectrum of the input pulsed laser is first angularly dispersed by a transmission grating and subsequently imaged to a DMD with beam modulation patterns; the transmission grating and a high-reflectivity mirror together compensate the angular dispersion introduced by the DMD. The mode of the output beam is monitored by a CCD camera. In the experiments, the DUBS is programmed to generate four different beam modes, including an Airy beam, Bessel beam, Laguerre–Gaussian (LG) beam, and a custom-designed “peace-dove” beam via the principle of binary holography. To verify the high shaping rate, the Airy beam and LG beam are generated alternately at 4.2 kHz, i.e., the maximum pattern rate of our DMD. The overall efficiency of the DUBS is measured to be 4.7%. With the high-speed and high-resolution beam-shaping capability, the DUBS may find important applications in nonlinear microscopy, optical manipulation, and microscale/nanoscale laser machining, etc.

A passively mode-locked ytterbium-doped large- mode-area photonic crystal fiber oscillator operating in the vicin- ity of zero cavity dispersion is demonstrated. The self-starting mode-locking operation is achieved by a high contrast saturable absorber mirror. Two mode-locking regimes with opposite signs of net cavity dispersion are investigated. At a net cavity disper- sion of -0.0035 ps 2 , the fiber laser directly generates 10-nJ laser pulses with an average power of 630 mW at 65.3 MHz repetition rate. The pulses can be dechirped to 78 fs by extracavity disper- sion compensation. The pulse energy is scaled up to 18 nJ, yield- ing an average power of 1.2 W, when the cavity dispersion is set at 0.0035 ps 2 . In this regime, the laser output can be extracavity- dechirped to 120 fs. Dynamics of pulse evolution in the fiber laser is illustrated by numerical simulation, which agrees well with ex- perimental results. -1.0 -0.5 0 0.5 1.0 Time, ps

We report a fiber self-similar-amplification (SSA) comb system that delivers a 250-MHz, 109-W, 42-fs pulse train with a 10-dB spectral width of 85 nm at 1056 nm. A pair of grisms is employed to compensate the group velocity dispersion and third-order dispersion of pre-amplified pulses for facilitating a self-similar evolution and a self-phase modulation (SPM). Moreover, we analyze the stabilities and noise characteristics of both the locked carrier envelope phase and the repetition rate, verifying the stability of the generated high-power comb. The demonstration of the SSA comb at such high power proves the feasibility of the SPM-based low-noise ultrashort comb.

A new structure of Entirely-integrated Spatial-bending Shape Memory Alloy Actuators (ESSMAA) is first presented. Three SMA wires, which are trained to remember "U" recovery shape, are embedded in liquid silicone rubber to be solidified upon room temperature, companied with displacement sensors and connective wires. The Spatial bending, which is accomplished through heating SMA wires by suitable current, can restore flexibly as soon as stopping heating. Thus, the compact, two-way, mechanical and electrical incorporation actuator is completed. Be provided with the merits such as high load-weight ratio, low energy dissipation and direct driving, the actuators can realize miniature configuration, simple drive and control, precise operation and flexible movement pose synthetically. On the basis of expounding the operation principle in detail, the curvature sensor, which is composed of six stain gauges, makes displacement control precise. After analyzing equilibrium of forces and moments, the numeric calculation equations are deduced. Subsequently, the approximate arithmetic making use of superpose principle is researched in contrast with numeric arithmetic. The manufacture of the prototype is discussed in details and the precise displacement control is implemented also. Finally, the experiments of the actuator proof fine qualities of the operation performances.

We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO.

A novel bistable permanent electromagnetic clutch (BPMEC) unit is proposed which can be used in electric vehicles to fulfill flexible connect the motor and hubs. The magnetic field distribution of the clutch is predicted by using finite element method and magnetic circuit method. Subsequently, the main sizes are optimized by investigating flux leakage coefficient, and micro-topology of BPMEC is optimized by investigating magnetizing current.

Induction machine with indirect vector control system has advantages of simple structure and high performance. But under the condition of parameters deviation, both steady-state and transient performance of this system are influenced. The succeeding section of this paper analyses the effect of parameters deviation for indirect vector control system. Then a slip frequency correction method is proposed in the following section to minimize error of rotor flux position and improve the system's robustness to parameters changing. The results of this paper give some reference to the design of indirect vector control system with considering parameters deviation.

By using clutch for replacing rigid connection with a flexible one between hub and wheel-motor, a novel solution for in-wheel drive is proposed with the possibilities of ‘no-load starting and variable-idle clutching’ together in this paper. It is then important to choose suitable idling speed of wheel motor for assuring drive comfort from a standing start in direct-drive electric vehicle. Based on the actual load torque, the control strategy of optimal idling speed is discussed in the aim of enhancing drive comfort and reducing electromagnetic impulsion.

A novel bistable permanent magnet electromagnetic clutch (BPMEC) unit is introduced into wheel drive so as to turn rigid connection between hub and motor into flexible form, thus remit mechanical and electromagnetic impulsion. Working principle of this idea is conceptually demonstrated. Electromagnetic relations of the BPMEC unit are presented, as well as performance calculated and verified by experiments measured on a prototype. Finally, general design principles are investigated in detail by studying leakage coefficient which is obtained from analytical solution.

Clutch is widely adopted in fuel vehicles, but it is rarely found in in-wheel motor driven electric vehicles. This paper proposes a general formula to solve the electromagnetic force and provides a clutch system for the newly designed bistable permanent magnet (PM) electromagnetic clutch in wheel hub motors based on the formula.

We demonstrate 25-fs laser pulse generation from a photonic crystal fiber (PCF) laser system, which consists of a high energy all normal dispersion mode-locked oscillator based on an ytterbium-doped large mode area PCF and a passive PCF-based grating-pair compressor. The oscillator directly delivers 314-nJ highly-chirped pulses with a duration of 1.32 ps at a repetition rate of 15.58 MHz, which can be dechirped to 75 fs, corresponding to a peak power of 3 MW. The output laser pulses from the oscillator are directly injected to a piece of passive PCF for spectral broadening and dechirped by a transmission grating pair compressor, which results in a short pulse duration of 25 fs with 1-W average power.

We study, both numerically and experimentally, the relative intensity noise (RIN) and timing jitter characteristics of optical parametric generation (OPG) process in MgO-doped periodically poled LiNbO3 (MgO:PPLN) pumped by fiber femtosecond laser. We directly characterize the RIN, and measure timing jitter spectral density of the OPG process based on the balanced optical cross-correlator (BOC) technique for the first time as well, which are both in a fairly good agreement with numerical simulation. Both the numerical and experimental study reveals that OPG can suffer from a smaller intensity fluctuation but a lager temporal jitter when it is driven into saturation. Furthermore, we demonstrate that with a 30 mW CW diode laser injection seeding the OPG output results in superior noise performance compared to the vacuum fluctuations seeded OPG.

High-speed 3D microscopic imaging methods have led to numerous biological discoveries. In this Letter, we present a sync-free light sheet microscope (LSM) based on a static phase mask that eliminates the need for coupling the detection plane, thereby enabling high-speed volumetric imaging that is only limited by the speed of cameras, e.g., ∼1000 cross sections/s. In the sync-free design, the emission signals are first guided to the back of a galvanometric mirror, which laterally scans the emissions across a 2D phase mask, converting it to axial scanning that automatically compensates the focal shifts in the detection optics. Parametric models are developed to guide the phase mask design as well as to relate the axial scanning depth and magnification to design parameters. To quickly evaluate the different mask designs, a liquid-crystal-based spatial light modulator (LC-SLM) is used in the system. In the experiments, we scanned pollen and tissue samples via both the 2D phase mask and a piezoelectric objective scanner. The results show that the new method can generate clear images with comparable quality throughout the scanning range. The overall efficiency of the LSM is ∼10%. It is worthwhile to note that the efficiency will be significantly improved by replacing the LC-SLM with a custom-made lens. The new method realizes a compact LSM for high-speed 3D imaging that may find important applications in in vivo biological imaging.

Fast 3D microscopic imaging methods have been playing a crucial role in many biological studies. In this Letter, we present a revolutionary way to design and build a two-photon excitation (TPE) microscope for 3D and random-access imaging based on a digital micromirror device (DMD), achieving a scanning speed of 22.7 kHz. When pairing with a 40× objective lens, the maximum scanning range in the x, y, z axes are 103, 206, 524 μm, respectively. The axial and lateral scanning resolution (i.e., minimum step size) are 270 nm and 130 nm, respectively. In the system, the focal point of the femtosecond laser can be arbitrarily positioned to any random point in space by switching the binary holograms stored in the DMD. Parametric models are derived to deterministically link the DMD parameters (i.e., pixel size and aperture) with the scanner characteristics, i.e., scan range and minimum step size, in each axis. In the experiments, we demonstrate conventional raster scanning, scanning along arbitrarily programmed surfaces, and random-access scanning on a pollen grain sample via the DMD-based TPE system. We also perform experiments to demonstrate the unique capability of selective optical stimulation, where selected locations within the specimen are photobleached by extending the laser dwell time. With its versatility and high scanning rate, the TPE microscope may find important applications in brain research, realizing in vivo random-access imaging and optical stimulation with tens of microseconds temporal resolution.