Generation of a superstable Lorentzian pulse train with a high repetition frequency based on a Fabry-Perot resonator integrated with an electro-optic phase modulator

A simple technique for converting a continuous-wave laser beam into a stable Lorentzian pulse train with a high repetition frequency is demonstrated experimentally. We generated transform-limited pulses of up to 40 GHz, which were composed of higher-order sidebands produced by a Fabry-Perot resonator integrated with an electro-optic phase modulator (EOM). The rf power supplied to the EOM determines the pulse width in the pulse train. This approach enables the pulse width to be continuously tuned from 2.1 to 7.0 ps at the same repetition frequency without any wavelength shift. Furthermore, we experimentally evaluated the stability of the pulse train's amplitude and obtained stable bit-error-free operation at 9.95 GHz.     

AM and high-harmonic FM laser mode locking

We demonstrate a new technique of active mode locking that combines amplitude-modulated (AM) mode locking at the cavity fundamental repetition rate with frequency-modulated (FM) mode locking at a high harmonic. This method combines the advantages of pulse shortening by high-harmonic mode locking while preserving the higher peak powers available at the fundamental repetition rate. We demonstrate this technique using a Nd:YAG laser that is simultaneously AM mode locked at 80 MHz and FM mode locked at the 22nd harmonic (1.76 GHz). Pulses as short as 16 ps with a peak power of 6.25 kW were measured.     

Synphase operation of nanosecond relativistic 37-GHz backward-wave oscillators without electrodynamic coupling

The possibility of in-phase excitation of two independent nanosecond-pulsed relativistic 37-GHz backward-wave oscillators (BWOs) with high-current electron beams has been studied. This regime can be achieved using BWO switching with a picosecond precision. It is shown that long-term (up to 100?C200 periods of the field) phase locking in each channel is stably reproduced from pulse to pulse, which ensures coherent summation of the output wave beams at a megawatt power.     

Simultaneous Mode Locking in a Diode-Pumped Passively Q-Switched Nd:YVO(4) Laser with a GaAs Saturable Absorber

Simultaneous mode locking and Q switching is accomplished in a diode-pumped Nd:YVO(4)/GaAs laser. The average output power is ~2.0 W at 10.6-W absorbed pump power, and the repetition rate of the Q-switched pulse is ~120 kHz. The mode-locked pulse inside the Q-switched pulse has a repetition rate of ~148 MHz, and its average duration is estimated to be ~100 ps.     

Obscuration-free pupil-plane phase locking of a coherent array of fiber collimators

Control methods and system architectures that can be used for locking in phase of multiple laser beams that are generated at the transmitter aperture plane of a coherent fiber-collimator array system (pupil-plane phase locking) are considered. In the proposed and analyzed phase-locking techniques, sensing of the piston phase differences is performed using interference of periphery (tail) sections of the laser beams prior to their clipping by the fiber-collimator transmitter apertures. This obscuration-free sensing technique eliminates the need for a beam splitter being directly located inside the optical train of the transmitted beams--one of the major drawbacks of large-aperture and/or high-power fiber-array systems. Numerical simulation results demonstrate efficiency of the proposed phase-locking methods.     

Energy and operation management of a microgrid using particle swarm optimization

This article presents an efficient algorithm based on particle swarm optimization (PSO) for energy and operation management (EOM) of a microgrid including different distributed generation units and energy storage devices. The proposed approach employs PSO to minimize the total energy and operating cost of the microgrid via optimal adjustment of the control variables of the EOM, while satisfying various operating constraints. Owing to the stochastic nature of energy produced from renewable sources, i.e. wind turbines and photovoltaic systems, as well as load uncertainties and market prices, a probabilistic approach in the EOM is introduced. The proposed method is examined and tested on a typical grid-connected microgrid including fuel cell, gas-fired microturbine, wind turbine, photovoltaic and energy storage devices. The obtained results prove the efficiency of the proposed approach to solve the EOM of the microgrids.     

Air Liquide space cryocooler systems

After successful developments these last 3 years, AL/DTA is now in position to propose two pulse tube cryocooler systems for space applications in the 40–80 K temperature range. The two pulse tube cryocoolers are yet qualified against stringent thermal and mechanical environmental constraints. AL/DTA also develops associated Cooler Drive Electronic at QM level implementing launch locking and vibrations cancellation. This paper presents these complete cryocooler systems available for space applications.     

Short-pulse Injection-locking of a Mercury Chloride Laser

Abstract

We report the injection locking of a mercury chloride discharge laser. This has been achieved using an injection pulse much shorter than the slave pulse duration. Selective enhancement of HgCl laser output at wavelengths to the blue of the usual 558 nm lasing region has been observed. Mercury metastables, produced in the discharge, are found to have an absorption line coincident with an HgCl gain peak at 547 nm, and laser action at this wavelength is inhibited. This absorption has been counteracted by correctly timing the entry of the injected pulse into the slave laser cavity.     

A long cavity passive mode-locking fibre laser with the reflective non-linear optical loop mirror

In this paper, we report a long cavity passively mode-locked fibre laser. The proposed mode locker is a reflective long cavity non-linear optical loop mirror (NOLM) which consists of a 50:50 coupler and 2-km single-mode fibres. The laser achieves stable mode locking at a fundamental repetition rate of 100 kHz. The rectangular pulses operating in dissipative soliton resonance region is generated in the laser. The relationship between the pulse duration and the pump power is investigated in detail. When the pump power is 200 mW, the laser generates rectangular pulses at 1565.57 nm (central wavelength) with pulse duration of 81.5 ns. The single pulse energy as high as 33.34 nJ is obtained. The results show that the reflective NOLM is an efficient mode locker and useful for the generation of high energy pulse.     

Low-temperature-GaAs device used simultaneously as a mode-locking device and as a photoconductive switch

We present a low-temperature-grown GaAs device that combines the features of mode locking and photoconductive switching. The mode-locking mechanism is based on intensity-dependent defocusing. Additionally, the generated carriers produce an electrical signal in the biased switch geometry. This technique allows for simultaneous generation of synchronized optical and electrical pulse trains with a single device.     

Dynamic pull-in instability of a prestretched viscous dielectric elastomer under electric loading

The dynamic response of a pre-stretched elastomer membrane under electric loading was analysed. Thereby, two cases of voltage application, constant voltage and an incrementally increased voltage were regarded. The equation of motion (EOM) was derived from the Euler–Lagrange equation and the Rayleigh dissipation function. This allowed to include the influence of prestretch into the evolution equation of the viscous stretch. The critical values of pull-in instability under dynamic assumptions were determined at the initial state by an analytical model derived from the classical approaches of stability theory regarding geometric instabilities by using an energy criterion and were used to validate the numerical solution of the EOM. The results showed that both inertia and viscous effects have a notable influence on the pull-in stability behavior. In particular, at applied electric fields below the critical electric field the viscous behavior can still induce failure, which occurs time delayed. A non-monotonic dependence of the failure stretch on the magnitude of the applied electric fields below the critical value could be observed.     

Active synchronization and carrier phase locking of two separate mode-locked femtosecond lasers

Two independent mode-locked femtosecond lasers are synchronized to an unprecedented precision. The rms timing jitter between the lasers is 4.3 fs observed within a 160 Hz bandwidth over tens of seconds, or 26 fs within a 50 kHz bandwidth. Novel multi-stage phase-locked loops help to preserve this ultrahigh timing resolution while setting on arbitrary delay between the two pulse trains (0–5 ns). Under such synchronization, phase locking between the carrier frequencies of the two femtosecond lasers has been achieved. It is also demonstrated that the same level of synchronization can be achieved with two lasers at different repetition frequencies.     

Generation of periodic high-power ultrashort pulse sequences in a chain of coupled traveling-wave tubes operating in the regimes of amplification and nonlinear Kompfner suppression

We have studied the dynamics of an electron microwave oscillator with a feedback loop containing two coupled traveling-wave tubes, the first operating in the regime of amplification and the second in the regime of nonlinear Kompfner absorption. It is established that oscillations generated in this system can take the form of a periodic sequence of ultrashort pulses. The proposed mechanism of pulse generation is analogous to the well-known method of passive mode locking.     

Field-directed self-assembly with locking nanoparticles

A reversible locking mechanism is established for the generation of anisotropic nanostructures by a magnetic field pulse in liquid matrices by balancing the thermal energy, short-range attractive and long-range repulsive forces, and dipole-dipole interactions using a specially tailored polymer shell of nanoparticles. The locking mechanism is used to precisely regulate the dimensions of self-assembled magnetic nanoparticle chains and to generate and disintegrate three-dimensional (3D) nanostructured materials in solvents and polymers.     

Sensitivity of a three-mirror cavity to thermal and nonlinear lensing: Gaussian-beam analysis

We consider a compact three-mirror cavity consisting of a flat output coupler, a curved folding mirror, and an active medium with one facet cut at the Brewster angle and the other facet coated for unit reflectivity. We examine the sensitivity to thermal lensing and to self-focusing in the active medium of the Gaussian beam that is circulating in that cavity. We use a simple thin-lens model; the astigmatism of the beam that is circulating in the cavity and the nonlinear coupling between the field distributions along the two orthogonal axes are taken into account. We find configurations in which beam ellipticity is compensated for at either end of the cavity in the presence of thermal lensing. We have derived an analytical criterion that predicts the sensitivity of the beam size to nonlinear lensing. The ability of the cavity to favor self-mode locking is found to be sensitive to the strength of thermal lensing. In the absence of thermal lensing, cavities operated as telescopic systems (C = 0) or self-imaging systems (B = 0) are most appropriate for achieving self-mode locking, with nonlinear mode selection accomplished through saturation of the spatially varying laser gain. We identify conditions for which self-mode locking can be produced by variable-reflectivity output couplers with either maximum or minimum reflectivity at the center of the coupler. We use our model to estimate the nonlinear gain produced in laser cavities equipped with such output couplers. We identify a cavity configuration for which nonlinear lensing can simultaneously produce mode locking and correction of beam ellipticity at the output coupler.     

紧固件用锁紧材料及锁紧技术研究

叙述了紧固件的锁紧原理及方法，着重论述了紧固件用锁紧材料的种类、性能、配制及使用工艺等，提出了不同的工作条件、工作环境下紧固件所用锁紧材料的选择。     

The influence of the current-carrying electrode material on the characteristics of integral optical microwave modulators

The working frequency band of an integral electro-optical modulator (EOM) significantly depends on the specific electric conductivity of the material of electrodes. In the case of optimum matching between velocities of the light wave and modulating microwave, the frequency dependence of the EOM response is determined by losses in electrodes related to the skin effect. In this case, the passage from traditional gold to silver (having a higher specific conductivity) provides for a 1.4-fold increase in the EOM bandwidth.     

Electron–photon equation of motion with application to the Lamb shift

An equation of motion (EOM) is proposed for the electron which includes the effect of the radiation field on the electron's motion. The new EOM – the electron–photon EOM (EPEOM) – is the same as Dirac's equation with the ‘bare’ or mechanical mass replaced by a complex electromagnetic mass whose real part is interpreted as the observed mass of the electron. The Lamb shift is calculated from the difference of the EPEOM energy and the Dirac-equation energy.     

Ambiguity Resolution in Direction of Arrival Estimation with Linear Antenna Arrays Using Differential Geometry

Linear antenna arrays (LAs) can be used to accurately predict the direction of arrival (DOAs) of various targets of interest in a given area. However, under certain conditions, LA suffers from the problem of ambiguities among the angles of targets, which may result in misinterpretation of such targets. In order to cope up with such ambiguities, various techniques have been proposed. Unfortunately, none of them fully resolved such a problem because of rank deficiency and high computational cost. We aimed to resolve such a problem by proposing an algorithm using differential geometry. The proposed algorithm uses a specially designed doublet antenna array, which is made up of two individual linear arrays. Two angle observation models, ambiguous observation model (AOM) and estimated observation model (EOM), are derived for each individual array. The ambiguous set of angles is contained in the AOM, which is obtained from the corresponding array elements using differential geometry. The EOM for each array, on the other hand, contains estimated angles of all sources impinging signals on each array, as calculated by a direction-finding algorithm such as the genetic algorithm. The algorithm then contrasts the EOM of each array with its AOM, selecting the output of that array whose EOM has the minimum correlation with its corresponding AOM. In comparison to existing techniques, the proposed algorithm improves estimation accuracy and has greater precision in antenna aperture selection, resulting in improved resolution capabilities and the potential to be used more widely in practical scenarios. The simulation results using MATLAB authenticates the effectiveness of the proposed algorithm.     

Interpolation of curved shell geometries by low order finite elements—errors and modifications

Low order finite elements are of vital interest for practical applications in linear and non-linear structural analysis. The possible discretization errors, when Reissner-Mindlin shell elements with bilinear shape functions are used, and modifications to prevent these errors are presented. On some numerical examples the influence of these errors and of various modifications on the results is shown. Finally, the case of membrane locking with low order Reissner-Mindlin elements for particular assumptions concerning the base vectors and methods to prevent this locking are discussed.