Showing papers in "Quantum Electronics in 2009"

Filamentation of high-power femtosecond laser radiation

Abstract: The state of the art of investigations on filamentation of a high-power femtosecond laser radiation in transparent media is reviewed. The physical picture of this phenomenon is presented and its relation to the fundamental concepts of nonlinear optics and practical applications is demonstrated. Experimental and theoretical methods are briefly considered and laser radiation parameters in the case of filamentation are given. The review can be of interest both for specialists and researches wanting to become familiar with a new, rapidly developing direction in laser physics.

Bismuth-doped fibre amplifier for the range 1300 — 1340 nm

Abstract: We demonstrate the first bismuth-doped fibre amplifier operating in the second transmission window of silica-based fibres. At a pump power of 460 mW and pump wavelength of 1230 nm, its gain reaches 24.5 dB at 1320 nm, with a gain bandwidth of 37 nm, saturation power near 10 mW, and noise figure of 5 dB.

Quasi-collinear tunable acousto-optic paratellurite crystal filters for wavelength division multiplexing and optical channel selection

Abstract: Quasi-collinear acousto-optic interaction is studied in acoustically and optically anisotropic paratellurite crystals. The possible applications of this interaction in acousto-optic tunable filters with a high spectral resolution are discussed. Different modifications of devices are compared and variants of devices intended for processing light beams and selection of light signals in fibreoptic communication systems with wavelength division multiplexing (WDM) at λ 1550 nm are considered.

Surface nanotexturing of tantalum by laser ablation in water

Abstract: Surface nanotexturing of tantalum by ablation with short laser pulses in water has been studied experimentally using three ablation sources: a neodymium laser with a pulse duration of 350 ps, an excimer laser (248 nm) with a pulse duration of 5 ps and a Ti:sapphire laser with a pulse duration of 180 fs. The morphology of the nanotextured surfaces has been examined using a nanoprofilometer and field emission scanning electron microscope. The results demonstrate that the average size of the hillocks produced on the target surface depends on the laser energy density and is ~200 nm at an energy density approaching the laser-melting threshold of tantalum and a pulse duration of 350 ps. Their surface density reaches 106 cm-2. At a pulse duration of 5 ps, the average hillock size is 60–70 nm. Nanotexturing is accompanied by changes in the absorption spectrum of the tantalum surface in the UV and visible spectral regions. The possible mechanisms of surface nanotexturing and potential applications of this effect are discussed.

Optical properties of fibres with aluminophosphosilicate glass cores

Abstract: The optical properties of fibres with aluminophosphosilicate (APS) glass cores have been studied at Al2O3 and P2O5 concentrations varied independently from 0 to 20%, and the ranges of dopant concentrations that ensure an optical loss level acceptable for the fabrication of active fibres have been established. The origin of the increased losses observed previously in APS fibres containing excess phosphorus pentoxide has been identified, and the contamination source responsible for such losses in our fibres has been eliminated.

Structure of modes of a smoothly irregular integrated-optical four-layer three-dimensional waveguide

Abstract: The asymptotic method and the method of coupled waves used to study an integrated-optical multilayer three-dimensional waveguide satisfying the conditions of a continuously variable effective refractive index are considered. Three-dimensional fields of smoothly deforming modes of a four-layer integrated-optical waveguide are described analytically. Explicit dependences of the contributions of the first order of smallness to the electric and magnetic field amplitudes of quasi-waveguide modes are obtained. The canonical type of quasi-wave equations describing the structure of quasi-TE and quasi-TM modes in a smoothly irregular four-layer integrated-optical three-dimensional waveguide is presented for the asymptotic method. By using the perturbation theory, shifts of complex propagation constants are obtained in an explicit form for these modes. The elaborated theory can be used to analyse structures from dielectric, magnetic and metamaterials in a rather broad wavelength range of electromagnetic waves.

Surface plasmon?polaritons with negative and zero group velocities propagating in thin metal films

Abstract: The conditions of existence of surface electro- magnetic waves with negative and zero group velocities propagating in a thin metal Olm bounded by dielectric media with different refractive indices are found. Analytic expres- sions are derived to determine the group velocities of such waves, which are of interest for calculating and optimising optical systems of insulatorometaloinsulator type.

Excited states in the active media of oxygen — iodine lasers

Abstract: A review of investigations of kinetic processes in active media oxygen — iodine lasers (OILs) performed in the last decade is presented. The mechanisms of pumping and quenching of electronically and vibrationally excited O2 and I2 molecules are considered, and dissociation mechanisms of I2 in the active medium of the OIL are analysed. The values of kinetic constants of processes proceeding in the active media of OILs are recommended.

Bi-doped fibre lasers operating in the range 1470-1550 nm

Abstract: Lasing in bismuth-doped optical fibres in the range 1470–1550 nm has been demonstrated for the first time. The gain media were Bi-doped phosphogermanosilicate and, for the first time, germanosilicate glass fibres. The gain spectrum of the phosphogermanosilicate fibres extends from 1300 to 1550 nm, the range which can be used in next-generation optical fibre communication systems.

On the motion of a charged particle in a plane monochromatic electromagnetic wave

Abstract: The motion of a charged particle in the external specified field of a plane electromagnetic wave of large amplitude, when the relativistic consideration is required, is analysed in detail. The cases of different initial conditions for the motion of the charged particle and different polarisations of the wave are studied. It is shown that the expression for the kinetic energy of an electron oscillating in the transverse field of the wave, proposed in [1], is valid only in the nonrelativistic limit.

Gerchberg—Saxton algorithm: experimental realisation and modification for the problem of formation of multimode laser beams

Abstract: An original method is proposed to calculate the formation of specified far-field intensity distributions by a bimorph mirror in the case of initial transverse-multimode beams. The method is based on the Gerchberg—Saxton algorithm with replacement of the phase function in the plane of the control element by a function that takes into account both the intensity and phase distributions of each mode. The numerical results on the formation of a beam with the third-order super-Gaussian intensity distribution from beams composed of two or four lowest transverse modes are discussed. The experimental results on using the conventional Gerchberg—Saxton algorithm to form a desired intensity distribution from single-mode laser beams using a liquid-crystal modulator are presented.

Channeling of microwave radiation in a double line containing a plasma filament produced by intense femtosecond laser pulses in air

Abstract: The channeling of microwave radiation is demonstrated experimentally in a double line in which a plasma filament produced in air by intense femtosecond laser pulses serves as one of the conductors. It is shown that during the propagation of microwave radiation in this line, ultrashort pulses are formed, their duration monotonically decreasing with increasing the propagation length (down to the value comparable with the microwave field period). These effects can be used for diagnostics of plasma in a filament.

Diode-side-pumped laser heads for solid-state lasers

Abstract: Principal configurations of diode-side-pumped laser heads for solid-state lasers are considered. A comparative analysis is performed for radiation characteristics of lasers with such laser heads.

Experimental optimisation of the gas-assisted laser cutting of thick steel sheets

Abstract: We report on the experimental optimisation of the oxygen-assisted CO2 laser cutting of low-carbon sheet steel 5 to 25 mm in thickness. It is shown that the cut edge roughness is minimal when the energy input per unit volume of the material removed and the incident beam power per unit sheet thickness remain constant at ~20 J mm-3 and ~200 W mm-1, respectively, over the entire range of sheet thicknesses examined. The corresponding Peclet number is Pe = 0.5. These results can be used to determine the optimal beam power and cutting speed for a particular sheet thickness. At sufficiently large thicknesses, the conditions that ensure the minimum roughness can be written in the form of relations between nondimensional parameters.

Self-action of a high-power 10-μm laser radiation in gases: control of the pulse duration and generation of hot electrons

Abstract: The propagation of ultrashort 10-μm laser pulses of power exceeding the critical self-focusing power in xenon and air is numerically simulated. It is shown that the pulse duration in certain regimes in xenon can be decreased by 3–4 times simultaneously with the increase in the pulse power by 2–3 times. It is found that the average energy of electrons in a filament upon filamentation of 10-μm laser pulses in air can exceed 200 eV. The features of the third harmonic and terahertz radiation generation upon filamentation are discussed.

Optical limiting and bleaching effects in a suspension of onion-like carbon

Abstract: We have studied the effect of nanosecond laser pulses (λ = 1064 nm) on the optical properties of onion-like carbon (OLC) prepared by high-temperature vacuum annealing of detonation nanodiamond and dispersed in N,N-dimethylformamide (DMF). The results demonstrate that, under low-intensity irradiation, the OLC suspension displays optical limiting behaviour. Increasing the incident intensity leads to bleaching of the suspension in the visible and near-IR spectral regions.

Parameters of an electric-discharge generator of iodine atoms for a chemical oxygen - iodine laser

Abstract: Laser-induced fluorescence is used for measuring the concentration of iodine molecules at the output of an electric-discharge generator of atomic iodine. Methyl iodide CH3I is used as the donor of atomic iodine. The fraction of iodine extracted from CH3I in the generator is ~50%. The optimal operation regimes are found in which 80%—90% of iodine contained in the output flow of the generator was in the atomic state. This fraction decreased during the iodine transport due to recombination and was 20%—30% at the place where iodine was injected into the oxygen flow. The fraction of the discharge power spent for dissociation was ~3%.

Laser ablation plume dynamics in nanoparticle synthesis

Abstract: The dynamics of the plume ejected from the surface of solid targets (YSZ, Nd:YAG and graphite) by a CO2 laser pulse with a duration of ~500 μs (at the 0.03 level), energy of 1.0–1.3 J and peak power of 6–7 kW have been studied using high-speed photography of the plume luminescence and shadow. The targets were used to produce nanopowders by laser evaporation. About 200 μs after termination of the pulse, shadowgraph images of the plumes above the YSZ and Nd:YAG targets showed dark straight tracks produced by large particles. The formation of large (~10 μm) particles is tentatively attributed to cracking of the solidified melt at the bottom of the ablation crater. This is supported by the fact that no large particles are ejected from graphite, which sublimes without melting. Further support to this hypothesis is provided by numerical 3D modelling of melt cooling in craters produced by laser pulses of different shapes.

Efficient emission at 1908 nm in a diode-pumped Tm:YLF laser

Abstract: Emission parameters at 1908 nm in a longitudinally-diode-pumped Tm:YLF laser are studied. The laser parameters are optimised to obtain the maximal power of high-quality cw radiation. The output power of ~27 W is obtained at the slope efficiency of ~50% and total optical pump conversion efficiency of ~41%.

Electronic and thermal lensing in diode end-pumped Yb:YAG laser rods and discs

Abstract: The lensing effects in diode end-pumped Yb:YAG laser rods and discs are studied. Two mechanisms of refractive-index changes are taken into account, thermal and electronic (due to the difference between the excited- and ground-state Yb polarisabilities), as well as pump-induced deformation of the laser crystal. Under pulsed pumping, the electronic lensing effect prevails over the thermal one in both rods and discs. In rods pumped by a highly focused cw beam, the dioptric power of the electronic lens exceeds that of the thermal lens, whereas in discs steady-state lensing is predominantly due to the thermal mechanism. © 2009 Kvantovaya Elektronika and Turpion Ltd.

Measurements of reflection spectra of soft X-ray multilayer mirrors using a broadband laser-plasma radiation source

Abstract: The reflection spectra of several concave (spherical and parabolic) periodic Mo/Si, Mg/Si, and Al/Zr multilayer mirrors intended for cosmic experiments were studied with the help of a laser-plasma soft X-ray radiation source. An investigation was made of the reflection spectra of a laboratory aperiodic Mo/Si multilayer mirror optimised for maximum uniform reflectivity in the 125–250-A range. 'Satellites' were observed in the neighbourhood of the principal peak in the spectrum of the periodic 132-A multilayer mirror. High-intensity second-order interference reflection peaks at wavelengths of about 160 A were experimentally revealed in the reflection spectra of the periodic Mo/Si 304-A mirrors. By contrast, the second-order reflection peak is substantially depressed in the spectra of the narrow-band mirrors based on the Mg/Si multilayer structure. The experimental data are compared with theoretical calculations. Manifestations of the NEXAFS structure of the L2,3 absorption edges of Al and Al2O3 were observed in the spectra recorded.

Spun optical fibres: A helical structure of linear birefringence or circular birefringence?

Abstract: An experiment has been proposed, theoretically substantiated and accomplished which has provided conclusive evidence in favour of one of two models for the behaviour of polarised light in optical fibres fabricated by spinning preforms with a high built-in linear birefringence (spun fibres): a helical structure of the built-in linear birefringence axes and circular birefringence. The experiment, carried out with a reflective fibreoptic dual-polarisation interferometer, has shown that the behaviour of polarisation states in spun fibres can be understood in terms of a helical structure of the built-in linear birefringence axes.

Discrete conical emission rings observed upon filamentation of a femtosecond laser pulse in quartz

Abstract: Supercontinuum emission observed upon filamentation of transform-limited collimated femtosecond laser pulses in a transparent condensed medium (fused KU-1 quartz) is studied experimentally and numerically. The splitting of diverging conical supercontinuum emission into discrete rings was observed with increasing the pulse energy.

Effect of electromagnetically induced transparency on the spectrum of defect modes in a one-dimensional photonic crystal

Abstract: We studied the transmission spectrum of a one-dimensional photonic crystal containing a defect layer in which electromagnetically induced transparency is possible. The analysis is performed taking into account the spatial inhomogeneity of interacting fields in the photonic crystal. It is found that the transmission spectrum of such a photonic crystal depends on the spatial overlap of defect modes excited by probe and control radiations. It is shown that electromagnetically induced transparency can result in a considerable narrowing of the defect mode spectrum.

Continuous-wave solid-state two-Stokes Raman laser

Abstract: We report an experimental study of the first cw solid-state Raman laser operating simultaneously at the frequencies of the first and second Stokes components. Simultaneous generation is ensured by a cavity with an enhanced finesse at both Stokes frequencies. The threshold pump powers for the first (3.4 W) and second (3.67 W) Stokes components suggest that the second Stokes generation follows a cascade mechanism. We demonstrate for the first time Raman conversion with intensity stability exceeding the pump radiation stability and show that this approach may find application in Raman spectroscopy.

Nonlinear increase in the interaction efficiency of a second pulse with a target upon excitation of a plasma by a train of pulses from a Nd:YAG laser

Abstract: The efficiency of hole drilling in an aluminium plate was studied experimentally upon excitation of a plasma on its surface in air by a train of pulses from a Nd:YAG laser, the interval between pulses being 15–20 μs. It was found that the crater depth increases nonmonotonically with each successive pulse of the train. A nonlinear, more than by a factor of six, increase in the depth was detected upon interaction of the second pulse with the target. The mechanism explaining this increase in the interaction efficiency of the second pulse in the train with the target is proposed.

Integral criterion for selecting nonlinear crystals for frequency conversion

Abstract: An integral criterion, which takes into account all parameters determining the conversion efficiency, is offered for selecting nonlinear crystals for frequency conversion. The angular phase-matching width is shown to be related to the beam walk-off angle.

Control of filamentation of femtosecond laser pulses in a turbulent atmosphere

Abstract: Various methods for controlling the onset of filamentation of a high-power laser pulse in extended vertical atmospheric paths are analysed. It is shown that the increase in the structure constant of the atmospheric turbulence on average leads to the earlier formation of 'hot' spots when the initial pulse power exceeds the critical self-focusing power by an order of magnitude and more. It is found in the numerical experiments that the use of broad focused beams is preferable for achieving the minimal standard deviation of a distance for the filament onset.

Optimisation of the proton-exchange technology for fabricating channel waveguides in lithium niobate crystals

Abstract: We have studied the effect of process parameters on the mode profile in lithium niobate channel waveguides produced by low-temperature proton exchange. A model has been proposed for the processes underlying waveguide formation, and has been used to numerically calculate the mode profile for various fabrication parameters. We have identified low-temperature proton exchange conditions which ensure high electro-optical performance of the waveguides and stability towards fabrication errors. The obtained results may be helpful in optimising integrated optical devices and matching them to various types of optical fibres.

Generation of polarisation-nonuniform modes in a high-power CO2 laser

Abstract: A method describing radially and azimuthally polarised laser modes is presented, which is devoid of intrinsic contradictions and unjustified restrictions. The solutions of the wave equation found in the paper satisfy Maxwell's equation E = 0. This allowed the calculation of all the components of the fields of such modes, including the longitudinal one. Expressions for tight focusing of radially and azimuthally polarised modes are obtained in the Debye approximation. The methods of intracavity generation of such modes in a high-power industrial CO2 laser are experimentally realised and methods of mutual transformation of these modes outside the cavity are developed. The intracavity generation of azimuthally polarised modes was achieved by using as a rear highly reflecting mirror either V-shaped axicons or metal diffraction mirrors with the relief period comparable to the wavelength. Radially polarised radiation was generated by using diffraction mirrors with the relief period of the order of two wavelengths. The polarisation-nonuniform TEM01* and TEM11* modes of power up to 1.8 kW are generated for the first time. In this case, the degree of polarisation was close to 100% within the entire radiation power range both in the near- and far-field zones. By using two half-wave phase shifters, the azimuthally polarised radiation is transformed to radially polarised radiation and vice versa, the energy efficiency of this transformation being 92%. These results open up the possibility for industrial applications of such radiation.