Showing papers on "Laser published in 1973"

Multiple absorption of laser photons by atoms

Abstract: Applying a space translation operation, the Schrodinger equation for an atom in an electromagnetic field is solved with sufficient accuracy to obtain probabilities for multiple absorption of photons from a monochromatic laser beam of arbitrary intensity or frequency. It is shown that the derived expression for the N-photon T-matrix contains the usual single photon matrix elements given by the perturbation theory and that the perturbative result is obtained in the limit of low intensity. Other explicit examples are considered. The conditions of applicability of the method are specified.

Role of cracks, pores, and absorbing inclusions on laser induced damage threshold at surfaces of transparent dielectrics.

Abstract: The concentration of the electric field strength in the neighborhood of micropores and cracks may lower the nominal external intensity for electric avalanche breakdown by a factor 2-100 depending on the geometry of the crack and the dielectric constant. The presence of absorbing inclusions at the edge of microcracks will often be the dominant mechanism giving the lowest surface damage threshold. Inclusions and cracks with characteristic dimensions less than about 10(-6) cm will not lower the breakdown threshold appreciably.

Luminescence of Bi4 Ge3 O12 : Spectral and decay properties

Abstract: Intense broadband emission in the visible is observed from crystals of Bi4Ge3O12 under optical and x‐ray excitation. From measurements of absorption, reflection, fluorescence, and excitation spectra, the emission is assigned to 3P1 → 1S0 transitions of Bi3+. The Stokes shift is large, [inverted lazy s]14 000 cm−1. The temperature dependences of the fluorescence intensity and lifetime in the range 77–400 °K establish that nonradiative decay becomes significant at temperatures ⪞250°K. Comparison of the properties of Bi4Ge3O12 with those of Bi12GeO20 and other bismuth‐activated materials demonstrates the importance of the Stokes shift and the 1S‐3P energy difference in determining the luminescence behavior. The use of Bi4Ge3O12 as a laser host crystal for rare‐earth and iron group activator ions, and as a scintillator material is discussed briefly.

Laser-Induced Damage in Optical Materials

Abstract: : The report contains the results of damage experiments performed at 0. 694 and 1.06 micrometer using well controlled ruby and Nd:YAG lasers. Materials studied at 1.06 micrometer are proustite (Ag3AsS3) and lithium niobate (LiNbO3). Proustite studies have been directed toward improving surface quality by employing different polishing techniques. Little or no improvement has been obtained over previously measured thresholds. Chemical changes in proustite damage sites are evidenced by the use of electron excited x-ray microprobe measurements. Preliminary ion beam polishing experiments were performed on LiNbO3, but no improvement in damage threshold was observed. However, the damage in this material appears to be inclusion limited in the samples studied so far. The temporal and spatial characteristics of laser pulses were studied for transmitted, reflected, and backscattered pulses both above and below surface damage threshold.

Laser mirror damage in germanium at 10.6 μm

Abstract: Damage in the germanium output mirror of a TEA CO2 laser shows a periodicity of 10.6 μm, the laser wavelength. A mechanism is proposed in which scattered and cavity radiation interfere. An interference mechanism is confirmed by experiments outside of the laser cavity. Periodic damage is obtained in thin films irradiated at angles other than the normal where the fringe spacing is modified.

cw degradation at 300°K of GaAs double-heterostructure junction lasers. II. Electronic gain

Abstract: The rapid degradation at 300°K in the cw regenerative output of stripe‐geometry GaAs double‐heterostructure junction lasers is shown to be a result of the formation of a local optical absorber in the laser cavity. Gain measurements performed on diodes before and after degradation show that the optical loss within the cavity increases during degradation. By observing the (predominantly) spontaneous emission from the active region directly through the n‐GaAs substrate, it is confirmed that the increased loss is localized in a region where little or no spontaneous emission takes place at lasing energies. In such diodes, the internal radiative efficiency of the undegraded portion of the optical cavity shows a relatively small decrease compared to the external differential quantum efficiency. When the local absorber extends over a sufficient length of the cavity the electronic gain in the undegraded section is insufficient to overcome the loss and the device ceases to act as a regenerative optical oscillator. Net gain measurements on DH laser devices in which the active region is lightly (≈ 1017) n‐doped indicate that the optical gain increases linearly with current prior to degradation. At lasing threshold the medium exhibits net gain over a wavelength range of 100 A. After degradation the gain dependence on current can become superlinear due to the saturation of the optical absorber. Estimates on the attenuation constant in the local absorber at low currents give a value of ≈ 60 cm−1 at 8800 A. For pulsed currents close to lasing threshold the attenuation constant increases to nearly 160 cm−1 at 8760 A.

X‐ray emission in laser‐produced plasmas

Abstract: Numerical results are presented for the characteristics of heavy‐ion laser‐produced plasmas. A conversion ratio defined as x‐ray power/absorbed power is then computed. Collisional‐radiative equilibrium is assumed for the ionization model. In the case of low radiation emission, a relation between laser flux and plasma temperature is derived. The one‐dimensional geometrical model is then corrected in the expansion by assuming that the emission occurs, in fact, over a length L only. Results are given and compared with experimental ones. The influence of the laser wavelength is also discussed and conclusions on the possible applications of these laser‐produced plasmas are drawn.

Optical third-harmonic generation in alkali metal vapors

Abstract: This paper considers third-harmonic generation in phase-matched mixtures of alkali metal vapors and inert gases. Calculations show that the combination of near-resonant nonlinear susceptibilities, the ability to phase match, and the relatively high UV transparency of these vapors should allow high conversion efficiency for picosecond laser pulses with a peak power of 108-109W. Calculations of the nonlinear susceptibility and of the ratio of xenon atoms to metal vapor atoms which is necessary to achieve phase matching are given for each of the alkalies as a function of incident laser wavelength. Processes that limit the allowable peak power density and energy density are discussed and guides for determining the metal vapor pressure, cell length, and beam area are given.

A GaAs-AlxGa1-xAs Double Heterostructure Planar Stripe Laser

Abstract: A stripe laser structure called a planar stripe was developed. The edge blurring of the current path is improved by the fact that the current spreads only in the thin p-AlxGa1-xAs layer with relatively high resistance. The planar stripe laser has a small threshold current resulting from the small current spreading effect and a good thermal contact. It also shows finely controlled transverse modes, compared with a usual contact stripe laser, and relatively high external differential quantum efficiency. The threshold current density is comparable to that of the proton bombarded stripe laser. The transverse mode shows an approximate Hermite-Gaussian distribution.

The Pulsed Dye Laser as a Light Source for the Fluorescent Antibody Technique

Abstract: Recently, the substitution of conventional light sources for fluorescence microscopy by laser system have yielded promising results. The system reported here is capable of creating fluorescent images by excitation with defined pulses of coherent light. Photographic recordings of fluorescent objects have been produced with exposures of less than 10 μsec. No significant amount of fluorescence bleaching was noted, even after illumination for 50 μsec. The laser light source holds a promise for improvement in micro-fluorometry, and is discussed with special reference to quantification in immunofluorescence.

Neodymium‐doped silica lasers in end‐pumped fiber geometry

Abstract: Fused SiO2 has been used as a new noncrystalline host for the fabrication of two types of neodymium‐doped room‐temperature lasers, one of which operates at 1.06‐μm and the other at 1.08‐μm wavelength. The lasers have the geometry of clad optical fibers, with active cores as small as 15‐μm diameter by 1‐cm length. They are end pumped at 0.590 and 0.5145 μm with a pulsed dye laser and an argon ion laser, respectively. Thresholds as low as 1–2 mW of absorbed pump power in a 40‐μm‐diam core have been obtained, and eventual pumping with high‐radiance semiconductor optical sources appears feasible.

Thermal diffusion measurements using spatially periodic temperature distributions induced by laser light

Abstract: Two light waves with different directions of propagation derived from a pulsed Nd:YAG laser are superimposed in an absorbing sample and generate an interference field. Due to absorption, a spatially periodic temperature distribution occurs, producing a spatial modulation of the refractive index which can be considered a thermal phase grating. The light of an argon laser simultaneously incident on the sample is diffracted by the thermal grating. When excitation is over, the decay time of the diffracted light is measured. From this decay time the thermal diffusivity of the sample is determined. Measurements on colored methanol and glycerin as well as on ruby compare favorably with the results of other authors. The possibility of exciting and detecting temperature waves (second sound) in solids by the method of light‐induced gratings is discussed.

CO2 Laser-Induced Dissociation of SiF4 Molecules into Electronically Excited Fragments

Abstract: Observation of the visible fluorescence of the SiF radical in SiF4 gas irradiated by the pulsed output of a CO2 laser leads to the conclusion that the molecules are dissociated into electronically excited fragments by the action of the intense laser field. A clear distinction is observed between the fluorescence produced through this process and that associated with thermalization of the incident energy.

Mode guidance parallel to the junction plane of double-heterostructure GaAs lasers

Abstract: We investigate the origins of mode confinement, parallel to the junction plane, of stripe‐geometry double‐heterostructure GaAs injection lasers. Based upon the kind of control of the mode extent by the stripe width, we establish a distinction between two types of lasing mode guidance; (i) well‐behaved, where a single mode substantially fills the entire active region, and (ii) filamentary, where the mode size is much smaller than the width of the active region and its location is random. For low‐order well‐behaved modes, it is shown, at least in principle, that gain alone (no refractive index effects are required) can explain not only the confinement but also the approximate waveguide dimensions for which leakage losses, connected with the penetration of the mode into the regions outside the active guide, become important. When only this postulated gain‐guiding prevails in the case of higher‐order modes, we find that their spatial character is markedly different from the Hermite‐Gaussian distributions which have been seen for GaAs lasers. The introduction of a positive incremental index, which may reasonably be associated with the gain and/or with a thermal mechanism, restores agreement with observation. If any incremental index within the active region is negative, then although there is a defocusing effect, confinement may still be maintained by the gain. In the case of filamentary lasing we analyze four focusing mechanisms; two are connected with local built‐in gain (loss) and refractive index variations, while two are current dependent and are related to the saturation of gain‐associated refractive index and the free carrier effect. While all four processes appear to be reasonable candidates for providing the necessary confinement in filamentary lasing, the latter two simultaneously violate a condition necessary for stability of a filament. We conclude that imperfections in lasers are likely to be related to filamentary lasing and that such behavior may not be intrinsic to such devices.

Laser injection locking

Abstract: The basic principles of laser injection locking are discussed and the status of recent experimental work is reviewed. The subject is introduced by means of appropriate lower frequency and microwave injection-locking circuits. These are used to explore the basic physical phenomena involved and to illustrate the essential role of gain saturation in laser injection locking. Oscillators and below-threshold regenerative amplifiers are discussed to provide a more complete and cohesive picture. Experimental results with a variety of CW and quasi-CW injection-locked amplifiers including unstable resonator geometries are presented. Preliminary results with a novel injection-locked passively Q-switched CO 2 amplifier are also given.

Degradation of CW GaAs double-heterojunction lasers at 300 K

Abstract: Identification of a major failure mechanism in stripe-geometry proton-bombarded double-heterojunction lasers is reported. This mechanism consists of the formation of localized "dark lines" in the optical cavity, coinciding with a decrease in light output. The dark lines are aligned in 〈100〉 crystallographic directions.

Theory for momentum transfer to a surface with a high‐power laser

Abstract: A model is developed for the momentum transferred to a solid from a laser‐supported gaseous detonation wave initiated in air above the surface. One‐dimensional gasdynamic analysis is used for the detonation wave moving away from the surface and cylindrical blast‐wave theory is employed to partially account for two dimensional effects. Momentum transfer coupling coefficients (impulse/laser energy) are calculated which are functions of the laser energy flux, pulse duration, and the beam and target surface areas. Results are compared with recent experimental data obtained using a high‐power CO2 laser.

Laser induced surface damage.

Abstract: A summary of recent investigations of surface damage of transparent dielectrics is presented. Damage threshold measurements made on Owens-Illinois ED-2 laser glass at normal incidence and at Brewster’s angle are reported. For 30-nsec pulses at normal incidence, exit surface damage thresholds are typically 100 J/cm2 for ED-2 glass. The observed ratio between entrance and exit damage thresholds for the two geometries can be explained by considering the electric field strengths at the surfaces and including interference between incident and reflected light waves. A similar analysis is applied to surface damage that occurs during total internal reflection. Finally the morphology of surface damage of ED-2 laser glass is described, and a model based upon reflections from the laser induced plasma is proposed to explain the observations.

Harmonic generation and parametric excitation of waves in a laser-created plasma

Abstract: When focusing an Nd glass laser (frequency ωo) onto a cryogenic target (D2 or H2) quite intense lines with frequencies 2ωo, 3ω0/2, ω0/2 are found in the reflected light in both backward and 45° directions. The lines at 2ω0 and 3ω0/2 are broadened on the low frequency side only. The occurrence and broadening of such lines may be related to parametric excitation of waves.

Gain-current relation for GaAs lasers with n-type and undoped active layers

Abstract: Results for the optical gain coefficient as a function of the nominal current density are given for GaAs lasers with compensated n-type active layers and with undoped active layers. The results suggest that n-type layers may give lower threshold currents near room temperature than do comparable p-type layers.

Optically pumped GaAs surface laser with corrugation feedback

Abstract: A GaAs distributed‐feedback laser was fabricated and pumped optically. A narrow stimulated spectrum was obtained around 0.83 μ with threshold pumping power of ∼2 × 105 W/cm2.

Nd‐ultraphosphate laser

Abstract: Laser action at λ=1.051 μm with a bandwidth of 1.4 nm is reported in a 35‐μm‐thick platelet of a neodymium‐ultraphosphate crystal (NdUP) placed within an optical resonator. Longitudinally pumped with a pulsed rhodamine 6G laser (λ=0.58 μm) the room‐temperature threshold was 40 μJ and the quantum efficiency slope was ∼18%. Also in glass of the same chemical composition laser action was observed.

Gaussian light beams in inhomogeneous media.

Abstract: Vector wave solutions are obtained for the propagation of beams of light in media having slow spatial variations of the gain, loss, or index of refraction. The formalism developed here is applicable to a wide range of problems, and an example considered in detail is the propagation of off-axis beams in lenslike laser materials and optical waveguides. A procedure is also described for the diagnosis of localized dielectric inhomogeneities such as plasmas by means of Gaussian laser beams.

Nonlinear Behavior of Stimulated Brillouin and Raman Scattering in Laser-Irradiated Plasmas

Abstract: Both analytic theory and inhomogeneous plasma simulations are presented to show the nonlinear development of the Brillouin and Raman backscatter instabilities. Nonlinear fluid behavior predicts a backscatter energy on the order of the incident laser energy, but particle trapping and heating effects associated with the excited electrostatic wave can significantly reduce this.

Apparatus for heat treating a surface

Abstract: Apparatus using a high power laser beam for heat treating a surface, such as a metal surface to harden the surface scans the metal surface which has been coated with radiation absorbing material such as carbon, so that the scanned areas of the metal surface are rapidly heated to above the critical temperature of the metal. Since the heating is very shallow, the heated surface cools very rapidly and so the surface is hardened. The intensity profile of a typical high power laser beam is not flat and so as the beam scans the metal surface, heating of the surface across the width of the beam is not uniform. A flat beam intensity profile is obtained in the present invention by dithering (spacially oscillating) the beam continuously as it sweeps the metal surface. The cross-section area of the dithered beam is substantially greater than the cross-section area of the initial beam from the high power laser and the intensity profile across the dithered beam can be tailored and is dependent upon the initial beam intensity profile and the configuration of the dither. The intensity profile across the dithered beam can be further tailored by blocking portions of the dithered beam.

Laser emission at 3 μ from Dy3+ in BaY2F8

Abstract: A unique combination of properties in the host crystal BaY2F8 has made it possible to obtain laser emission from the Dy3+ ion at 3.022 μ. The laser line lies close to the electronic transition from the lowest level of the 6H13/2 excited state at 3520 cm−1 to a ground manifold level at 216 cm−1, but the oscillation frequency is shifted by 5 cm−1 due to overlapping vibronic lines. The intensity of Dy3+ emission is enhanced by energy transfer from Er3+, Yb3+, Tm3+, and Ho3+. The restrictions imposed on the multiphonon decay rates for the operation of a Dy3+ laser at 3 μ are discussed. A comparison of radiative and nonradiative relaxation rates for rare‐earth ions in BaY2F8 indicates that an extension of laser emission to beyond 4 μ is unlikely.