Showing papers on "Laser published in 1995"

Self-channeling of high-peak-power femtosecond laser pulses in air

Abstract: The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.

Laser-induced damage in dielectrics with nanosecond-to-subpicosecond pulses

Abstract: The application of chirped-pulse amplification to short-pulse lasers has led to a dramatic increase in the number of high-power subpicosecond laser systems. Accordingly, knowing the short-pulse damage thresholds of optical components and scaling the damage thresholds with pulse width have become increasingly important.

Quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO 3

Abstract: We review progress in quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO3. Using the electric-field poling process, we can reliably fabricate 0.5-mm-thick crystals with uniform domain structures over a 15-mm length. Periodically poled material retains the low-loss and bulk power handling properties of single-domain LiNbO3, and quasi phase matching permits noncritical phase matching with d33, the highest-valued nonlinear coefficient. Optical parametric oscillators pumped by 1.064-μm pulsed Nd:YAG lasers have been operated over the wavelength range 1.4–4 μm with tuning by temperature or by quasi-phase-matched period. We have shown an oscillation threshold as low as 0.012 mJ with a Q-switched pump laser and pumping at greater than ten times threshold without damage. We have also demonstrated a cw doubly resonant oscillator near 1.96 μm pumped directly with a commercial cw diode laser at 978 nm.

Fabrication and Optical Properties of High-Performance Polycrystalline Nd:YAG Ceramics for Solid-State Lasers

Abstract: Transparent polycrystalline YAG with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders (>99.99 wt% purity). The average grain size and relative density of the 1.1 at.% ND:YAG ceramics obtained were about 50 {micro}m and 99.98%, respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28%, respectively. These values were equivalent or superior to those of the 0.9 at.% ND:YAG single crystal fabricated by the Czochralski method.

Laser‐induced holographic surface relief gratings on nonlinear optical polymer films

Abstract: We report observation of holographic surface relief gratings with relatively large amplitude on a second order nonlinear optical polymeric material. Surface relief gratings on these polymer films were created upon exposure to polarized Ar+ laser beams at 488 nm without any subsequent processing steps. The surface structure of the relief gratings was investigated by atomic force microscopy. The depth of the surface relief in a typical case was 120 nm which is approximately 20% of the original film thickness. The diffraction efficiency of gold‐coated gratings was investigated as a function of wavelength and capability of recording orthogonal gratings on the same film was demonstrated.

Laser-assisted electrosurgery system

Abstract: A laser and electrosurgical system (10) has a handpiece (12) with a proximal and distal end (13) and (14) from which laser radiation (17) and/or electrosurgical energy (18) is selectively or concurrently directed. The ends are along an axis; an electrode extends distally along the axis. Sources of laser and electrosurgical energy (18) are available and controlled. An initiation circuit (25) for the laser selectively delivers it before electrosurgery. Prior delivery to target tissue, the laser creates an ionized conductive pathway for electrosurgery. The combined concurrent application of laser and electrosurgery has a mechanism (27) to selectively distribute energy about the axis. A laser deflecting surface (28) or a beam deflector vibrate relative to the axis. A power switch (30) has levels for laser cut and ionization and/or a delay therebetween for electrosurgical coagulation. A wave guide (34) or an array thereof deliver radiation along the axis. A method has steps of directing selectively and concurrently laser and electrosurgery by aiming the axis, controlling laser and electrosurgical energy (18), delivering laser before electrosurgery, forming an ionized conductive pathway for the electrosurgery, distributing the laser about the axis in a predetermined pattern by deflecting the laser in a predetermined pattern generally along but slightly displaced from the axis to direct electrosurgical energy (18) in a predefined pattern on the patient's (11) target tissue. In another possible approach, there are additional steps of delivering the laser radition (17) as a beam and varying an ionized conductive pathway by refracting or diffracting the laser radiation (17) beam with the mechanism (27) having a beam pattern deflector.

Alignment and trapping of molecules in intense laser fields.

Abstract: The anisotropic interaction of the electric field vector of intense laser radiation with the dipole moment induced in a polarizable molecule by the laser field creates aligned pendular states. These are directional superpositions of field-free states, corresponding to oblate spheroidal wave functions, with eigenenergies that decrease with increasing field strength. We present calculations demonstrating the utility of these states for both laser alignment and spatial trapping of molecules.

Observation of laser-induced fluorescent cooling of a solid

Abstract: THE possibility that an object might cool through its interaction with radiation was suggested as early as 1929 by Pringsheim1. After Landau2 established the basic thermodynamic consistency of such a process, certain aspects of fluorescent cooling were vigorously pursued3a¤-11. In particular, laser 'Doppler' cooling of gas-phase atoms and ions has today grown into a robust research area12a¤-15. In contrast, attempts to cool solids with light have met with limited success; non-radiative heating effects tend to dominate, and fluorescent cooling has at best resulted in a reduction in overall heating rates6. Here we report the experimental realization of net cooling of a solid with radiation. The cooling efficiencies achieved (up to 2%) are more than 104 times those observed in Doppler cooling of gases. By pumping a fluorescent cooling element with a high-efficiency diode laser, it may be possible to construct a compact, solid-state optical cryocooler, thereby allowing widespread deployment of cryogenic electronics and detectors in space and elsewhere16.

Ytterbium-doped silica fiber lasers: versatile sources for the 1-1.2 /spl mu/m region

Abstract: Ytterbium-doped silica fibers exhibit very broad absorption and emission bands, from /spl sim/800 nm to /spl sim/1064 nm for absorption and /spl sim/970 nm to /spl sim/1200 nm for emission. The simplicity of the level structure provides freedom from unwanted processes such as excited state absorption, multiphonon nonradiative decay, and concentration quenching. These fiber lasers therefore offer a very efficient and convenient means of wavelength conversion from a wide variety of pump lasers, including AlGaAs and InGaAs diodes and Nd:YAG lasers. Efficient operation with narrow linewidth at any wavelength in the emission range can be conveniently achieved using fiber gratings. A wide range of application for these sources can be anticipated. In this paper, the capabilities of this versatile source are reviewed. Analytical procedures and numerical data are presented to enable design choices to be made for the wide range of operating conditions. >

New Nonlinear Optical Crystal Cesium Lithium Borate

Abstract: A new nonlinear optical (NLO) crystal, CsLiB6O10 (CLBO) has been discovered. CLBO can quadruple and quintuple the Nd:YAG laser output. Large CLBO crystal with dimensions 13×12×10 cm3 could be grown in twelve days.

Method for controlling configuration of laser induced breakdown and ablation

Abstract: In one aspect the invention provides a method for laser induced breakdown of a material with a pulsed laser beam where the material is characterized by a relationship of fluence breakdown threshold (Fth) versus laser beam pulse width (T) that exhibits an abrupt, rapid, and distinct change or at least a clearly detectable and distinct change in slope at a predetermined laser pulse width value. The method comprises generating a beam of laser pulses in which each pulse has a pulse width equal to or less than the predetermined laser pulse width value. The beam is focused to a point at or beneath the surface of a material where laser induced breakdown is desired. The beam may be used in combination with a mask in the beam path. The beam or mask may be moved in the X, Y and Z directions to produce desired features. The technique can produce features smaller than the spot size and Rayleigh range due to enhanced damage threshold accuracy in the short pulse regime.

Optimization of passively Q-switched lasers

Abstract: In optimizing passively Q-switched lasers, there is a unique choice of output coupler and unsaturated absorber transmission which maximizes the laser output energy and efficiency for each three-way combination of laser gain medium, absorber medium, and pump intensity (i.e, inversion density). In the present paper, we generalize and solve the three coupled differential equations which describe the passively Q-switched laser to obtain closed form solutions for key laser parameters such as the output energy and pulsewidth. We then apply the Lagrange multiplier technique to determine the optimum mirror reflectivities and unsaturated absorber transmissions as a function of two dimensionless variables. The first variable, z, corresponds to the ratio of the logarithmic round-trip small signal gain to the round-trip dissipative (nonuseful) optical loss and is identical to that which was used in previous treatments to optimize the rapidly Q-switched laser. The second variable, /spl alpha/, is unique to the passively Q-switched laser and is equal to the saturation energy density of the amplifying medium divided by the saturation energy density of the absorber. It is largely determined by the ration of the absorber to stimulated emission cross sections, but also depends on the speed of relaxation mechanisms in the amplifying and absorbing media relative to the resonator photon decay time. Several design curves, valid for all four level amplifying and absorbing media, are then generated. These permit the design of an optimum passively Q-switched laser and an estimate of its key performance parameters to be obtained quickly with the aid of a simple hand calculator. In the limit of large /spl alpha/ (>10), the design curves are virtually indistinguishable from the rapidly Q-switched case. The curves can also be used to perform rapid tradeoff studies of different absorbing materials. The theory can also be applied to CW-pumped, repetitively Q-switched systems through a simple multiplicative factor for the laser gain. The theory is applied to the analysis of a passively Q-switched Nd:YAG laser previously reported in the literature and shown to give excellent agreement with the experimental results. >

Sub-picosecond UV laser ablation of metals

Abstract: Laser ablation of Nickel, Copper, Molybdenum, Indium, Tungsten and Gold by short ultraviolet laser pulses (0.5 ps, 248 nm) in vacuum is reported for the first time. For Nickel and Indium, ablation is also studied in air to demonstrate the influence of the ambient atmosphere. Metal ablation in air is significantly less efficient than in vacuum due to redeposition of ablated material. The ablation rates in vacuum are discussed using a thermal model, which also allows to estimate ablation rates for other metals from basic optical and thermal properties. A comparison of the morphology of ablation sites after nanosecond and sub-picosecond ablation shows unequivocally the advantages of short-pulse laser ablation for high-precision patterning of thermally good conducting materials in micron-scale dimensions.

Type‐II quantum‐well lasers for the mid‐wavelength infrared

Abstract: We discuss an improved mid‐wave infrared diode laser structure based on InAs‐Ga1−xInxSb‐ InAs‐Ga1−xAlxSb Type‐II multiple quantum wells. The proposed design combines strong optical coupling, 2D dispersion for both electrons and holes, suppression of the Auger recombination rate, and excellent electrical and optical confinement.

Quantitative Analysis of Aluminum Alloys by Laser-Induced Breakdown Spectroscopy and Plasma Characterization

Abstract: Laser-induced breakdown spectroscopy has been applied to perform elemental analysis of aluminum alloy targets. The plasma is generated by focusing a pulsed Nd:YAG laser on the target in air at atmospheric pressure. Such a plasma was characterized in terms of its appearance, emission spectrum, space-integrated excitation temperature, and electron density. The electron density is inferred from the Stark broadening of the profiles of ionized aluminum lines. The temperature is obtained by using Boltzmann plots of the neutral iron lines. Calibration curves for magnesium, manganese, copper, and silicon were produced. The detection limits are element-dependent but are on the order of 10 ppm.

Selectively oxidised vertical cavity surface emitting lasers with 50% power conversion efficiency

Abstract: Index-guided vertical cavity top-surface emitting laser diodes have been fabricated from an all epitaxial structure with conducting mirrors by selective lateral oxidation of AlGaAs At low voltage, a 78% slope efficiency, and a 350 mu A threshold current in a single device combine to yield a maximum power conversion efficiency of 50% at less than a 2 mA drive current The device operates in a single mode up to 15 mW >

Phase of the atomic polarization in high-order harmonic generation

Abstract: A recently formulated theory of high-order harmonic generation by low-frequency laser fields [Anne L'Huillier et al., Phys. Rev. A 48, R3433 (1993)] allows us to study the phase of the induced atomic dipole moment. We show that this phase exhibits a piecewise-linear dependence on the laser intensity. This dependence can be interpreted in quasiclassical terms, and is related to the action acquired by the electron during its motion in the laser held. The value of the phase, however, is also affected by the quantum effects of tunneling, diffusion, and interference. The phase of the dipole moment considerably influences the conversion efficiency, as well as the coherence properties, of the harmonics generated in macroscopic media.

Coherence control of high-order harmonics.

Abstract: We study the conditions for optimal coherence of harmonics emitted by atoms in strong laser fields. The variation of the phase of the atomic dipole moment induces a dramatic dependence of the spatial and spectral coherence on the focusing geometry. However, these coherence properties can be controlled and optimized by moving the laser focus position relative to the nonlinear medium.

Laser diode feedback interferometer for measurement of displacements without ambiguity

Abstract: We report what, to our knowledge, is the first example of laser feedback interferometer capable of measuring displacements of arbitrary form using a single interferometric channel. With a GaAlAs laser diode we can measure 1.2-m displacements, with interferometric resolution, simply by means of the backreflection from the surface (reflective or diffusive) under test. The operation is performed at moderate (i.e., not very weak) levels of feedback, such that a two-level hysteresis is found in the amplitude modulated signal. This is shown to allow the recovery of displacement without sign ambiguity from a single interferometric signal. Experimental results are reported, which are found to be in good agreement with the underlying theory. Performances of the developed feedback interferometer are finally presented. >

Display panel with electrically-controlled waveguide-routing

Abstract: A flat panel display (1001) is based on a new switching technology for routing laser light from a laser light source (1000) through an array of optical energy routers (1008) among a set of optical waveguides (1014) and coupling light toward the viewer. The switching technology is based on poled electro-optic structures. The display technology is versatile enough to cover application areas spanning the range from miniature high resolution computer display to large screen displays for high definition television (HDTV) formats. The invention combines the high brightness and power efficiency inherent in visible semiconductor diode laser sources with a new waveguide electro-optical switching technology to form a dense two-dimensional addressable array of high brightness light emissive pixels.

Optical limiting effect in a two‐photon absorption dye doped solid matrix

Abstract: We recently reported a new lasing dye, trans‐4‐[p‐(N‐ethyl‐N‐hydroxylethylamino)styryl]‐N‐methylpyridinium tetraphenylborate (ASPT), which has also been shown to possess a strong two‐photon absorption (TPA) and subsequent frequency upconversion fluorescence behavior when excited with near infrared laser radiation. Based on the TPA mechanism, a highly efficient optical limiting performance has been demonstrated in a 2 cm long ASPT‐doped epoxy rod pumped with 1.06 μm Q‐switched laser pulses at 50–250 MW/cm2 intensity levels. The measured nonlinear absorption coefficient reached 6 cm/GW for the tested sample of dopant concentration d0=4×10−3 M/L. The molecular TPA cross section of ASPT in the epoxy matrix is estimated as σ2=2.5×10−18 cm4/GW or σ2′=4.7×10−46 cm4/photon/s, respectively. Two‐photon pumped cavity lasing is also observed in an ASPT‐doped polymer rod.

Laser-induced breakdown spectroscopy of steel samples using multiple Q-switch Nd:YAG laser pulses

Abstract: Single, double and multiple Q-switch Nd:YAG laser pulses are used to vaporize material from solid steel samples and to induce a plasma. The material ablation of different pulse bursts, emission intensities of iron lines and electron temperatures and densities are determined. Material ablation is found to increase with multiple pulses compared to single pulses of fixed total energy as well as electron temperatures and densities. Line intensities can be increased by a factor of about two using double pulses. Quantitative microchemical analysis of low-alloy steel is performed with single and double pulses. The analytical performance is improved by the double-pulse technique.

Dynamics of H2+ in intense laser fields

Abstract: A number of unexpected features of small molecules subjected to intense laser fields, with wavelengths ranging from infrared to ultraviolet, have been observed or predicted in the past few years: above-threshold dissociation, molecular bond softening, vibrational population trapping. We review these processes for the case of the molecular ion H2+ and discuss the experimental and theoretical tools that are used to study this system. Both electron and proton energy distributions are used to interpret the experimental results. Theoretically, the fragmentation dynamics can be described equivalently as a laser-assisted half-collision process, using solutions of the time-independent Floquet theory, or as the evolution of a wavepacket subjected to a classical radiation held with a given pulse shape, using solutions of the time-dependent Schrodinger equation. A broad range of laser intensity and pulsewidth has been explored, with the short-pulse results (analysed in terms of 'dressed' potential curves) offering the best interface between theory and experiment. We finally report on a promising new avenue for coherent control of fragmentation dynamics, through the use of two-colour phase-locked radiation.

Molecules in Laser Fields

Abstract: Current laser technology is capable of generating laser fields from the IR to visible wavelength regions in the form of well-tailored sequences of pulses with controllable phase and envelopes (pulse shape) [1–2]. Such pulses can be used for the efficient preparation of ensembles of atoms or molecules in specific states. This is of considerable interest not only in spectroscopy but also in studies of chemical dynamics [2–4]. Furthermore short pulses allow one to attain electric field strengths e (V cm-1) or equivalently laser intensities I (W/cm2) = ce 2 /8π (c = velocity of light) which are comparable or greater than atomic fields. An important consequence of the progress in this area is that one can greatly enhance radiative transition rates and even ionize molecules. Clearly strong field science and short pulse science are two fields which will become increasingly intertwined. Efficient rapid excitation requires increasingly higher intensities as can be seen from the simple example of a resonantly driven two-level system [6–7].

Laser range camera

Abstract: A laser range camera provides both imaging and range information using laser radar techniques. A laser transmitter having a fan shaped beam that creates a column shaped beam in a far field is scanned along a single axis within a defined field of view using an acoustical optical device. A staring focal array of multiple line receivers having a long narrow field of view by using a linear array of fiber optics receives reflected energy from the scanned scene and transfers the received information to an avalanche photo diode by bundling the linear array into a circular array for interfacing with the diode. The range and intensity of each pixel within a scene is measured using a CCD measurement system which communicates with the receiver through the electrical signals. Range is determined for each pixel of the scene using the exposure characteristics of a CCD with analog type time of flight measurements. Range and intensity data are stored on odd and even CCD elements and used to display a three dimensional range image.

Polarization-dependent high-order two-color mixing.

Abstract: High-order frequency mixing experiments using the radiation of a high-power Ti:sapphire laser and its second harmonic are described and discussed. Linearly and circularly polarized light fields with comparable intensities have been used. For the theoretical description a three-dimensional quantum-mechanical calculation with a \ensuremath{\delta}-function potential has been applied, showing quite good agreement with the experiments.

Efficient low-intensity optical phase conjugation based on coherent population trapping in sodium.

Abstract: We have observed optical phase-conjugate gain (>50) in sodium vapor, using low-intensity pump lasers (1 W/cm2), with a response time of the order of 1 μs. Coherent population trapping is experimentally identified as the phase-conjugate mechanism. A theoretical model is presented that supports these observations by showing that coherent population trapping can write large-amplitude nonlinear-optical gratings at laser intensities well below those needed to saturate the optical transitions.

Semiconductor lasers with optical injection and feedback

Abstract: In this review we discuss the theoretical framework needed for studying the dynamical behaviour of semiconductor lasers exposed to three kinds of optical modulation. We start by a derivation of the single-mode rate-equations for the slowly varying complex electric field and the inversion, and the necessary extensions for monochromatic optical injection and normal external optical feedback. The basic operating characteristics of the solitary semiconductor laser are analysed, including light-current curves and their dependence on the spontaneous emission level, as well as the optical spectrum. The effect of monochromatic injection is discussed in terms of locking and non-locking dynamics, including a thermodynamic potential for phase jumps. The basic ingredients for studying external optical feedback are given, including a derivation of the thermodynamic potential for phase-diffusion. After an introduction on optical phase conjugation, the field rate-equation for feedback from a phase-conjugate mirror is derived.