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The article invites papers on the development and potential of next generation free-electron lasers to be published in the journal.

This paper proposes a multiple-input multiple-output (MIMO) terrestrial-satellite laser communication uplink system based on optical spatial modulation with medium-based modulation (OSM-MBM) joint modulation. The Málaga(M) distribution is used to describe the combined effects of near-ground turbulent light intensity scintillation, beam wander and angle of arrival fluctuation. The closed expression of the bit error rate (BER) of the OSM-MBM joint modulation MIMO terrestrial-satellite laser communication uplink is derived. The simulation analyzes the influence of transmitting radius, receiving aperture, beam divergence, zenith angle and signal-to-noise ratio on the system error performance, and compares it with traditional modulation, media-based modulation and optical space modulation. Finally, the experiment is verified by Monte Carlo. This research provides a theoretical reference for the engineering realization of the modulation technology for the MIMO terrestrial-satellite laser communication uplink system. [ABSTRACT FROM AUTHOR]

Specific forms of the travelling and trapped vector mode functions for a three-dimensional Fabry-Perot cavity are developed from the general results of the preceding paper, with parameters describing the output cavity mirror chosen for a typical high Q cavity case. Cavity and external quasi-mode functions associated with the quasi-mode theory of macroscopic canonical quantization are then obtained via an idealized choice of output mirror parameters. The coupling constants describing photon exchange processes between the single cavity quasi-mode associated with each Fabry-Perot resonance and various external quasi-modes are calculated, and their slow dependence on the external quasi-mode frequency shows that the conditions for irreversible Markovian damping of the cavity quasi-mode are satisfied. For radiative atoms placed in the cavity the coupling constants for energy exchange processes with sideways travelling external quasi-modes also vary slowely, so that Markovian spontaneous emission damping occurs for the radiative atoms. However, their coupling with the isolated cavity quasi-modes is associated with reversible photon exchanges as represented via one photon Rabi frequencies. The standard model in cavity quantum electrodynamics, in which the basic processes are described by a cavity damping rate, a radiative atom spontaneous decay rate and an atom-cavity mode coupling constant has now been justified in terms of the quasi-mode theory of macroscopic canonical quantization. [ABSTRACT FROM AUTHOR]

Existing laser scanning approaches for highly reflective surfaces generally require multiple scans to cope with specular reflection. This paper proposes a dual-camera, line-laser scanning strategy to reconstruct them efficiently, which only needs a single scanning path. To bypass complicated point cloud processing algorithms for outlier elimination, a feasible image processing scheme was designed to delete the disturbed stripe regions. By merging two sets of point clouds computed from residual stripes on a common world frame, these removed noise patches can be compensated by the opposite camera. Therefore, the data fusion result can reconstruct the 3D geometry of targets completely. Experimental results demonstrate good performance with the ability to handle highly reflective surfaces. The scanning period is significantly shortened and scanning accuracy can meet the industrial requirement. [ABSTRACT FROM AUTHOR]

In this paper, the dynamics of quantum correlation quantified by local quantum uncertainty (LQU) and local quantum Fisher information (LQFI) for two uncoupled qubits driven by classical phase noisy laser have been investigated. The results show that, both LQU and LQFI exhibit similar behaviours, either display exponential decay or oscillatory decay with revivals depending on the ratios of classical phase noisy laser rate and system-environment coupling strength. Beside, our results also confirm the amount of quantum correlation measured by LQU is strictly bounded by LQFI in an open system. Finally, we propose a scheme to protect quantum correlation against classical phase noisy laser with the technique of weak measurement and reversal. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study of the shot-to-shot intensity and spectral distribution fluctuations in the femtosecond coherent pulses from the storage ring (SR) based seeded free electron laser (FEL) at DELTA. A unique implementation of a gated ICCD camera coupled to a Czerny-Turner monochromator is utilized to reveal the details of the individual spectra and intensity of femtosecond pulses accompanied by a significant synchrotron background. This technique demonstrates the potential to study the dynamics of the electron bunches in an SR by scanning over a single electron bunch with seeded femtosecond laser pulses at a subharmonic repetition rate. To the best of our knowledge, this picture provides the first direct experimental evaluation of shot-to-shot intensity and spectral fluctuation in seeded SR-based FELs. [ABSTRACT FROM AUTHOR]

Hafnium diboride (HfB2) is used in hypervelocity re-entry vehicles (such as intercontinental ballistic missile (ICBM) or ICBM heat shields), nuclear reactors and aerodynamic leading edges because of its strong tensile strength and high thermal resistance. HfB2 is an ultra-high-temperature ceramic (UHTC) with a melting point of 3250°C. In this paper, we study the performance of HfB2 as a saturable absorber in Q-switched lasers. HfB2 has a non-saturable absorptance of 2.5 dB/µm, a saturable absorptance of 0.8 dB/µm and a saturation fluence of 12 µJ/cm2. In addition, the saturation lifetime is estimated as 989.3 ps, while the recovery lifetimes are estimated as 254.9 and 22.1 ps. As a saturable absorber in our Q-switched laser, we observed pulses with durations between 880 and 2000 ns. With the addition of an acousto-optic modulator, we have observed Q-switching mode-locking, with pulses as low as 250 ns. HfB2 can potentially work at very high power, since its damage fluence is no less than 361 mJ/cm2. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a thermal coherent state defined with the Lie–Trotter product formula under the formalism of the thermo field dynamics. In the definition of our thermal coherent state, we treat the thermalizing operator and the displacement operator symmetrically. We examine its uncertainty relation and quasiprobability distributions. Although this thermal coherent state is equivalent to the conventional ones except for different parameterizations and a phase factor, it is convenient for describing an experimental setup of the optical parametric oscillator laser. [ABSTRACT FROM AUTHOR]

Underwater target detection and ranging in seawater are of interest in unmanned underwater vehicles. This study presents an underwater detection system that synchronously scans a collimated laser beam and a narrow field of view to circumferentially detect an underwater target. Hybrid methods of range-gated and variable step-size least mean squares (VSS-LMS) adaptive filter are proposed to suppress water backscattering. The range-gated receiver eliminates the backscattering of near-field water. The VSS-LMS filter extracts the target echo in the remaining backscattering and the constant fraction discriminator timing method is used to improve ranging accuracy. The optimal constant fraction is selected by analysing the jitter noise and slope of the target echo. The prototype of the underwater detection system is constructed and tested in coastal seawater, then the effectiveness of backscattering suppression and high-ranging accuracy is verified through experimental results and analysis discussed in this paper. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a scheme to generate tunable attosecond coherent X-ray radiation for future light source applications. This scheme uses a short-pulse seeding laser, two bunch compressors, and a laser chirper to generate a prebunched, kilo-Ampere current electron beam from an initial 10-Ampere low-current electron beam. Such an electron beam sent into a short undulator generates attosecond coherent soft X-ray radiation. The final X-ray radiation wavelength can be tuned by adjusting the compression factor. It also allows one to control the final radiation pulse length by controlling the seeding laser pulse length or the compression factor. As an illustration, we present an example to generate 1 GW, 200 attosecond, 1 nm coherent X-ray radiation from a 28 A current, 200 nm laser modulated beam. [ABSTRACT FROM AUTHOR]

In this paper, we propose and demonstrate a dual wavelength fibre laser (DWFL) based on the use of an inhomogeneously-broadened semiconductor optical amplifier (SOA) gain medium as well as two arrayed waveguide gratings (AWGs) together with two optical channel selectors (OCSs) and a broadband fibre Bragg grating (FBG) to generate dual wavelength output at variable channel spacings. The widest spacing obtained from the DWFL is 12.21 nm, while the narrowest spacing is 1.16 nm. The DWFL has good stability with only minor power fluctuations of less than 2 dB and a side mode suppression ratio (SMSR) of approximately 38.5 dB with fluctuations of less than 0.5 dB. [ABSTRACT FROM AUTHOR]

In this paper we show how it is possible to investigate the nuclear dynamics of simple molecular ions and molecules by looking at the high-order harmonic generation spectra they emit in the presence of a laser field. In particular we investigate two different effects: the presence of sidebands in the emitted spectra around the usual odd harmonics and an isotopic effect which affects the height of the plateau lines. We further study the advantages and the limitations of the semiclassical approach. [ABSTRACT FROM AUTHOR]

Reshaping of a Gaussian laser beam into a uniform or other intensity distribution is required for various applications. The laser beam shaping system with a radial birefringent filter is presented in this paper. With such a system the Gaussian beams can be transformed into uniform or annular beams. The theory and simulation of the proposed systems are described in detail. The primary advantage of such a system is that the out beam profile can be tunable with the rotation of the radial birefringent element. [ABSTRACT FROM AUTHOR]

In this paper we present a one-dimensional model describing the verso laser cleaning technique, already experimentally tested, for particle removal from a substrate. The equation for displacement is treated for a one-dimensional case, and the temporal behaviour of the rear surface deformation is determined. Taking into account the van der Waals adhesion force and elastic deformation of a spherical particle (JKR model), the ejection condition has been obtained. Numerical values for an example are consistent with experimental observation. [ABSTRACT FROM AUTHOR]

This paper describes an optical configuration for generating dual-focus from a single laser incident beam. The generated two foci have nearly equivalent spot size and both fall on the optical axis of the focusing optics, but at different focal lengths. The dual-focus optics allows for variations of the laser power of each focal point and the distance between the two focal points. The advantages of dual-focus ablation were demonstrated with a nanosecond UV laser dicing silicon substrates. The experimental results show that, compared to conventional single focus, dual-focus improves ablation rate by a factor of 2–4. [ABSTRACT FROM AUTHOR]

This paper demonstrates the use of a tapered fibre in generating a highly stable and tunable dual-wavelength fibre laser. By unique arrangement of polarization controller, adjustable spacing range between 0.94 and 3.32 nm and side mode suppression ratio (SMSR) up to 50 dB were recorded. The results were achieved at laser pump power of 94.7 mW. The inter-modal interference is achieved through the use of a non-adiabatic tapered fibre, made by a systematic flame brushing technique. The tapered fibre suppresses the mode competition in the 3-m erbium-doped fibre (EDF) gain medium. Over 60 min, the laser exhibited very high stability with acceptable peak power and SMSR. The proposed EDF laser operates from 1556.71 to 1562.13 nm range. [ABSTRACT FROM PUBLISHER]

In this paper a semi-classical model of the nonlinear operation of a laser is extended to describe a laser with a Gaussian mirror. Modified nonlinear self-consistent equations including transverse and longitudinal field dependence, gain saturation, spatial hole burning and nonlinear dispersion effects are derived. With the help of an energy theorem an approximate solution for steady-state single mode operation is presented, which relates the small signal gain to the output power and the laser system characteristic parameters. The laser output power characteristics are presented for various cavity configurations, especially, revealing an influence of the Gaussian mirror parameter on the power efficiency of the laser. [ABSTRACT FROM AUTHOR]

In this paper, we propose and demonstrate a cost-effective wavelength converter utilising a highly nonlinear fibre (HNLF) and a tunable dual-wavelength fibre laser as the pump source. The proposed system uses only the pump source and a signal probe to generate a partially degenerate four-wave mixing (FWM) effect. An FWM conversion efficiency of -12 dB is obtained at pump and signal powers at +13.3 dBm and +5 dBm, respectively, and it is predicted that a higher conversion efficiency is possible if a high power dual-wavelength fibre laser source is used. [ABSTRACT FROM AUTHOR]

In this paper, the absorption characteristics of a novel double-clad gain-guided and index-antiguided (GG-IAG) fiber are investigated with 3D ray-trace method. Additionally, the simulated correctness compared with real data was carried out by measuring the GG-IAG fiber. Analysis and experimental results show that the absorption characteristics of GG-IAG fibers are different from conventional fibers: the total absorption efficiency of a GG-IAG fiber is large, while the effective absorption length is very short, and the negative refractive index step has significant influence on the total absorption efficiency. [ABSTRACT FROM AUTHOR]

In this paper, the gain and noise of underwater laser range-gated imaging systems based on intensified CCD (ICCD) are theoretically analyzed. Combined with experimental results, the quality of underwater images at different system parameters is objectively evaluated from the aspect of modulation degree and histogram of grayscale distribution. Comprehensive selection disciplines of micro-channel plate (MCP) gain in applications is generalized according to the coherence between theory and experiment, and a scheme of auto gain control (AGC) based on image analysis is proposed for underwater range-gated imaging systems. [ABSTRACT FROM AUTHOR]

This paper presents a new method of purely optical spectroscopy of rubidium Rydberg states in a room-temperature gas cell. The atoms are excited into the Rydberg state from the ground state with a three-step diode laser cascade setup. The excitation of Rydberg states is monitored by observing the absorption of the laser performing the first excitation step on the strong rubidium D2 line. Different polarization schemes and possible excitation geometries are discussed. [ABSTRACT FROM AUTHOR]

Recent developments in laser physics have called renewed attention to Laguerre-Gaussian (LG) beams of paraxial light. In this paper we consider the corresponding LG modes for the two-dimensional harmonic oscillator, which appear in the transversal plane at the laser beam's waist. We see how they arise as Wigner transforms of Hermite-Gaussian modes, and we proceed to find a closed form for their own Wigner transforms, providing an alternative to the methods of Simon and Agarwal. Our main observation is that the Wigner transform intertwines the creation and annihilation operators for the two classes of modes. [ABSTRACT FROM AUTHOR]

In this paper, we study several designs for interferometric gravitational wave detectors, and the potential for enhancing their performance with a fast-light medium. First, we explore the effect of such a medium on designs similar to those already planned for Advanced LIGO. Then we review the zero-area Sagnac interferometer for GW detection, comparing its properties against the more conventional GW detector based on a Michelson interferometer. We next describe a modified version of such a detector where the Sagnac interferometer is replaced by a zero-area Sagnac ring resonator fed by an external laser. We then consider a GW detector based on an active, zero-area Sagnac ring resonator, where a gain medium is present inside the cavity. Finally, we show that if a medium with negative dispersion, which yields the fast-light effect, is also present inside this detector, then its sensitivity to GW strain is enhanced by the inverse of the group index of the dispersive medium. We describe conditions under which this enhancement factor could be as large as 105. [ABSTRACT FROM AUTHOR]

In this paper we present two-color GaAs/AlGaAs quantum cascade lasers with separate optical confinement of the two laser stacks. The laser stacks, each comprising cladding layers and an active region, are grown on top of each other by molecular beam epitaxy on GaAs substrates. The lasers operate at 10.4 µm and 9.1 µm wavelength and have built-in second-order intersubband nonlinearities. Due to coupling between the optical fields of the respective waveguides, not only the second-harmonic of each wavelength, but also the sum-frequency of the two lasers is measured. This concept can be extended by independently biasing the two laser stacks, whilst maintaining overlap between the two optical modes. [ABSTRACT FROM AUTHOR]

Certain crystals can split an X-ray's two linear polarization directions by reflecting them equally in two different directions. This paper gives a simple geometrical derivation for the necessary condition identified recently by Baronova and Stepanenko, namely, identical crystal planes under 120○, or three-fold symmetry. An X-ray spectropolarimeter with such polarization-splitting crystals would be particularly useful to diagnose anisotropies in hot, dense plasmas created by intense picosecond lasers. [ABSTRACT FROM AUTHOR]

The generation of short, intense, mid-infrared laser pulses allows for the exploration of atom-laser interactions deep in the tunnelling regime as well as providing the ability to explore scaled interactions. In this paper we present recent experimental and theoretical results for this largely unexplored parameter space. [ABSTRACT FROM AUTHOR]

A collective atomic recoil laser (CARL) realized with a Bose-Einstein condensate offers the possibility to investigate new effects in the coherent interaction between optical and matter waves. This paper discusses some aspects of the nonlinear evolution of scattered radiation and the matter-wave field in the high-Q cavity and superradiant CARL regimes. [ABSTRACT FROM AUTHOR]

The phenomenological equations for an amplifying medium are used for the analysis of nonlinear refraction in semiconductor lasers, this has allowed us not only to explain and to simulate experimental data, but also to estimate the nonlinear refraction parameter. [ABSTRACT FROM AUTHOR]

We report, in this paper, a narrow dip of electromagnetically induced absorption inside the optical pumping window by the interaction of two coherent laser fields with a quasi-degenerate two-level open atomic system in inhomogeneously broadened Rb vapour. The effect arises from optical pumping and laser-induced atomic multilevel Zeeman sublevels in the same ground state. The numerical simulation for a N -type four-level system shows good agreement with experimental observation. [ABSTRACT FROM AUTHOR]

This paper discusses a diode-laser-array end-pumped (Nd:YVO[SUB4]/BIBO green laser. Using a three-mirror folded cavity (V-shaped), a 1.06 W CW green output was obtained at an incident pump power of 8.5 W; the corresponding optical conversion efficiency was 12.5%. The relationship between the green output and the temperature of the BIBO crystal was investigated. The laser damage threshold of BIBO crystal was measured to be 600 MW cm[SUP-2] and 400 MW cm[SUP-2] at 1064 nm and 532 nm, respectively. [ABSTRACT FROM AUTHOR]

Dynamic beam switching of vertical-cavity surface-emitting lasers (VCSELs) has important applications for switching and routeing in optical interconnect networks. VCSEL arrays of various kinds have been quite extensively researched for tailoring and engineering near- and far-field patterns. In this paper, a new method of directional beam switching is proposed that uses two coupled VCSELs. When two VCSELs are coupled by a small separation and biased at the same steady current near threshold, then the resulting light output is dynamic at an extremely high frequency. The model equations are based on an approximation to the semiconductor Maxwell-Bloch equations. The simulations are for coupled VCSELs operating at 980nm with circular current apertures of 5.6µm diameter. Figures of the results will show far-field beam intensity patterns during a cycle of oscillation. The simulation results show directional switching at a speed of about 40GHz and between directions about 8° apart. Results of addition simulations are also presented. Simulations that use two square VCSELs show that the frequency of oscillation increases to 50GHz and that the far-field pattern remains similar. Finally, for four round VCSELs in a square pattern, two far-field circularly shaped beams moved left and right at a frequency of 50GHz. [ABSTRACT FROM AUTHOR]

The contributions of two energetically highest molecular orbitals to the harmonic emission rate are analysed for a two-component laser field. For diatomic molecules exposed to the elliptically polarised field, the emission from the highest-occupied molecular orbital (HOMO) is dominant for various molecular orientations with respect to the laser field. However, the contribution of the lower molecular orbital (HOMO-1) can become significant or even dominant for some molecular orientations. We introduce the ratio of the coherent over the incoherent sum of the HOMO and HOMO-1 contributions as a quantitative measure of the significance of the particular molecular orbital. Also, the gaseous medium response is different for the left and right elliptically polarised light and the molecular characteristics are imprinted into this difference. Moreover, for the orthogonally polarised two-colour (OTC) laser field the relative contributions of HOMO and HOMO-1 depend to a great extent on the relative phase between the field components. The importance of the HOMO-1 can be assessed by the relative error which is made if the harmonic spectra are obtained only with the HOMO contribution. Finally, we investigate the interference of the contributions of two highest molecular orbitals. We show that, for the OTC field, the destructive interference depends linearly on the intensity of the field components. Also, the interference minima shift towards the higher energies with the increase of the component wavelength. [ABSTRACT FROM AUTHOR]

A method for flexibly generating dual-chirp microwave waveform (DCMW) is proposed and experimentally demonstrated. The scheme uses a monolithic integrated three-section laser, which consists of a front section laser, a phase section and a rear section laser. Under the appropriate control current, the integrated laser works in the period-one (P1) state, and the two beams of different frequencies generated by the integrated laser can produce stable beat frequency. After photoelectric conversion, the generated DCMW has the flexible tunability of the central frequency. Through theoretical analysis and experimental demonstration, the DCMW with time-bandwidth-product (TBWP) of 6 × 105 and chirp rate of ± 0.03 GHz/μs is obtained at different centre frequencies. In addition, the auto-ambiguity function of the generated DCMW is also studied and it is proved that the proposed scheme can improve the range-velocity resolution, so it has application prospects in microwave photonic radar systems. [ABSTRACT FROM AUTHOR]

We investigate in this paper the influence of intensity-dependent losses on the dynamics of an erbium-doped fibre laser. On the basis of the ion-pair model, it is theoretically demonstrated that this feedback efficiently suppresses the self-pulsing occurring in heavily doped fibre lasers. [ABSTRACT FROM AUTHOR]

Angled surfaces are significant for obtaining complex 3D glass structures to be employed in precise optical components such as light guides. However, it is challenging to fabricate angled structures in glass with high precision. In this study, femtosecond laser irradiation followed by a chemical etching process is used to create a 45-degree angled surface. The roughness values of the angled top and bottom surfaces are compared and the parameters are optimized to achieve maximum surface quality. A 45-degree angled surface with <300 nm roughness and <±0.5° deviation is obtained in a given area of 1 × 1.5 mm2. Using this approach, various complex structures have been successfully fabricated with high quality and precision. Results indicate that this method can be adapted to the high-precision glass manufacturing industry. [ABSTRACT FROM AUTHOR]

We report on a compact 808 nm diode-pumped passively Q-switched (PQS) Nd:YVO4 laser at 1064 nm with black phosphorus (BP)/ indium selenide (InSe) heterostructure as a saturable absorber (SA). When the transmission of the output coupler (OC) was 30%, the maximum continuous-wave (CW) output power of 10.3 W was obtained with a slope efficiency of 52.4% under the incident pump power of 22 W. The maximum average output power of 3.1 W was obtained at 9.29 W pump power, corresponding to the repetition rate of 598 kHz and the pulse width of 455 ns during pulse operation, further power scaling was limited by the damage threshold of the saturable absorber mirror (SAM). The results sufficiently validated that two-dimensional (2D) BP/InSe heterostructure could be used as an optical modulator for near-IR pulsed laser sources. [ABSTRACT FROM AUTHOR]

In this paper, a novel scheme is proposed to obtain the single-mode laser using parity-time symmetry breaking. Four analogue micro-rings are optically coupled in the square configuration in a way that two micro-rings on the square diameter experience gain and the other two experience loss. The coupling coefficient for horizontal and vertical coupling between micro-rings is considered to be different. According to this new scheme, the symmetry of structure is destroyed, and thus the degenerate modes that can disrupt the stability of laser, disappeared. Due to the coupling of the resonators, four super-modes, which are coupled pairwise, are generated. The Eigen-frequency and parity-time symmetry breaking of the two super-modes depend on the difference between horizontal and vertical coupling coefficient; however, these parameters depend on the summation of the coupling coefficients for the other two. Results show that the laser output is single mode in a specific condition of coupling coefficient and gains. It is concluded that the threshold of pump parameter is improved in the new scheme, relative to the two coupled micro-rings. [ABSTRACT FROM PUBLISHER]

In underwater turbulent medium, field correlations are found when the incidence is a laser beam array. Variations of the field correlations against the variations in the ring radius of laser array beam, number of beamlets composing the laser array, source size, underwater turbulence parameters, i.e. the ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of mean-squared temperature and rate of dissipation of kinetic energy per unit mass of fluid, are investigated. Field correlations of laser arrays are found to be larger than the field correlations of the single beams. The effect of underwater turbulence is to reduce the field correlation of laser arrays. [ABSTRACT FROM AUTHOR]

Provides historical comments on the pumping of masers and lasers. Research in the field of molecular microwave spectroscopy in the decade after World War II; Independent research efforts on the ammonia beam maser by Basov and Prokhorov; Masers and quantum electronics; Methods to obtain an inverted nuclear spin system; Optical pumping; Relevance of the multilevel pumping scheme.

It is not always possible to diagonalize the optical ABCD matrix, but it can be brought into one of the four Wigner matrices by a similarity transformation. It is shown that the four Wigner matrices can be combined into one matrix with four branches. This result is illustrated in terms of optical activities, laser cavities, and multilayer optics. [ABSTRACT FROM AUTHOR]

In this article we present a theoretical model for the light-matter interaction of a two-level atom and a femtosecond laser pulse, spectrally cut in the vicinity of the atomic resonance. The model utilizes a transformation into an interaction picture from a bare atomic state basis and is applicable to two-level systems. This transformation simplifies some of the dynamics of the interaction. A complementary intuitive picture is also presented and describes the interaction of the induced dipole with the pulse's transients. The calculations are in excellent agreement with experimental measurements performed on an ultracold rubidium gas. [ABSTRACT FROM AUTHOR]

We experimentally and theoretically study Raman scattering in rubidium (Rb) dimers. We observe the optical lines resulting from Raman scattering and demonstrate Raman amplification in the backward direction. A theoretical model has been developed to explain experimental the results. [ABSTRACT FROM AUTHOR]

The S -matrix element for non-sequential double ionization of an atom irradiated by a high-intensity near-infrared laser field via the recollision–impact-ionization scenario is investigated with particular emphasis on a certain classical limit where classical kinematics govern throughout and wave-packet spreading is ignored. The resulting classical model is applied to the calculation of the one-electron energy spectrum in coincidence with double ionization. Various assumptions are made for the interaction by which the returning electron frees the second, up to this time inactive, electron and its initial wave function. The resulting momentum correlations of the two emitted electrons are investigated just above and below the threshold intensity, for which the maximal energy of the recolliding electron equals the ionization energy of the singly charged ion, and the quantum-mechanical amplitude and the classical approximation are compared. The classical model is extended to intensities below this threshold by making allowance for the escape threshold being lowered by the laser field. [ABSTRACT FROM AUTHOR]

Discusses the early history of quantum electronics. Ideas essential to making a laser before; Impediment to the laser invention; Microwave amplification using a beam of molecules; Indicator that even in the 1950s physicists and engineers did not think amplification by stimulated emission; Obtaining stimulated emission from a molecular beam; Direction of masers or lasers; Scientists who made significant contributions to the field; Field of masers and 'optical masers' or lasers.

The effect of betatron oscillations on the angular gain spectrum of a free-electron laser is examined in the case of an imperfect beam trajectory. The use of an axial magnetic field is suggested to improve the beam–mode coupling and gain of the laser. [ABSTRACT FROM AUTHOR]

Broadband amplified spontaneous emission (ASK) at a peak wavelength of 1.85 μm from Tm:Ho-doped silica fibre sources is reported, using in-band pumping from a Yb:Er co-doped fibre laser operating at 1.61 μm. A maximum total output power from both ends of greater than 80mW with slope efficiency >14% has been achieved. An ASF. bandwidth at FWHM of near 70 nm was measured from the forward signal for the output power from 2–40 mW. The total fluorescence spectral width is >200 nm. This broadband ASE source has potential applications in optical metrology, fibre sensors, loss and dispersion tests on optical fibres, spectroscopy and medical imaging. including optical coherence tomography. A review of different pumping schemes in generating ASE around 1.9 μm in the Tm-doped fibre is given. Also the nature of three-level ASE and in-band pumping of Tm:Ho-doped silica fibre are also discussed. The results in this paper confirm that in-band pumped Tm-silica fibre is a route to achieve high power broadband output around 1.8 mu;m. [ABSTRACT FROM AUTHOR]

A multi-wavelength switchable random fibre laser based on a double Sagnac-loop filter is proposed. The threshold of laser is 27 mW and the slope efficiency of the total power output to pump power is about 0.04%. In the experiment, stable and switchable laser outputs of single-, dual-, tri- and quad-wavelength channels are obtained by adjusting the polarization controller at room temperature. The wavelength positions are tuneable within the range of 1556–1564 nm and intervals are switchable between 2.5, 5 and 7.5 nm. Under 300 mW pump power, average power intensity of −24dBm is achieved. Furthermore, the output of fibre laser is relatively stable with less than 0.2 nm wavelength drift and less than 1.5 dBm power fluctuation within one hour. The proposed random fibre laser has promising application prospects for distributed sensing systems and biomedical imaging. [ABSTRACT FROM AUTHOR]

The introduction of collisions and of a thermal distribution for the atomic momentum in the model for the Collective Atomic Recoil Laser (CARL) is at the origin of important modifications in the interpretation of the mechanisms that give rise to the amplification of the backreflected wave. It is shown that the atomic density grating, considered to be the cause of gain in CARL, disappears in the presence of collisions, while other gratings-in population and polarization phase-survive. While the population grating appears to be merely a consequence of the collective interaction, the latter is the likely cause for the instability. Finally, simulations show that models that make use of an exponential relaxation mechanism for the atomic momentum, rather than accounting for collisions explicitly, largely overestimate the strength of the interaction. [ABSTRACT FROM AUTHOR]

With the Hermite-Gaussian mode used to describe radial laser arrays and the M²-factor and power in the bucket (PIB) chosen as criteria for characterizing beam quality, a detailed study of beam quality of coherent and incoherent radial laser arrays is made. The closed-form expression for the M²- factor of incoherent radial arrays, which depends upon the Hermite-Gaussian order m , inverse radial fill factor r' and aspect ratio w[sub 0x]/w[sub 0y], is derived. Numerical examples are given to illustrate and compare beam quality of coherent and incoherent radial arrays. [ABSTRACT FROM AUTHOR]

We demonstrate significant temporal contrast improvement for a petawatt-class Ti:sapphire femtosecond laser by introducing a temporal pulse filter based on a self-diffraction process. A double chirped-pulse-amplification laser system was set up to stretch, amplify and recompress the generated first-order self-diffraction pulse. The maximum energy of 130 μJ has been achieved for one first-order self-diffraction seed pulse. By optimizing the initial chirp value, the spectral bandwidth was slightly broadened for the first-order self-diffraction pulse and the cleaned pulse did not exhibit visible spectral distortions in the air environment. The measurement results for the incident pulse and the recompressed first-order self-diffraction pulse showed that the temporal contrast was improved by seven orders of magnitude, i.e. from ∼105 to ∼1012 at −25 ps before the main pulse. Such a temporal contrast is beneficial for laser-matter interaction experiments with high focused intensity up to 1022 W/cm2. [ABSTRACT FROM AUTHOR]