Showing papers on "Laser published in 1985"

Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure

Abstract: The scattering force due to resonance radiation pressure was first detected by Frisch in 1933.[1] Later, Ashkin[2] pointed out that laser light can exert a substantial force suitable for the optical manipulation of atoms, and numerous proposals to cool and trap neutral atoms with laser light.[3] Atoms in an atomic beam have been stopped by light,[4] in which the final velocity spread corresponds to a temperature of 50−100 mK. We report here the confinement and cooling of atoms with laser light, in which the atoms are localized in a 0.2 cm volume for a time in excess of 0.1 second and cooled to a temperature of T = 2.4 × 10−4K.[5]

The biomedical effects of laser application.

Abstract: This paper briefly reviews the authors' experimental and clinical use of lasers over a 20-year period, during which laser effects on 15 biological systems were studied. Low-energy laser radiation was found to have a stimulating effect on cells, and high-energy radiation had an inhibiting effect. The application of lasers to stimulate wound healing in cases of nonhealing ulcers is recommended.

Solid sample introduction by laser ablation for inductively coupled plasma source mass spectrometry

Abstract: The results are described of a preliminary study of the mass spectrometry of solid samples using a ruby laser to ablate the sample into an inductively coupled plasma (ICP) source mass spectrometer. Standard rock samples were used, pelletted with a binder into the form of a disc. Some 200 ablation pits could be accommodated on each sample. Laser pulse energies of 0.3–1 J were used in the fixed Q mode and the ablated material transferred from the ablation cell into the plasma torch by means of the plasma injector gas flow. The mass spectrometer was used in the fixed ion mode using mean ion current detection to evaluate the reproducibility of successive pulses on major constituents and in the scanning mode at the rate of 10 scans s–1 to produce spectra using mean current detection for major elements and pulse counting detection for traces. Problems were experienced with saturation of the detection system in both the mean current and pulse counting modes owing to the transient nature of the sample pulse from the laser, when attempting to quantify major elements, but except where a major peak was saturated, reasonably uniform sensitivity for most elements across the mass range from 7 to 238 m/z was obtained. Isotope ratio measurements were made on lead at 29 µg g–1 and detection limits for the elements examined appear to be 10 ng g–1 or less.

Impulsive stimulated scattering: General importance in femtosecond laser pulse interactions with matter, and spectroscopic applications

Abstract: It is shown that ‘‘impulsive’’ stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter. ISRS excitation of coherent optic phonons, molecular vibrations, and other excitations (including rotational, electronic, and spin) may play important roles in femtosecond pulse interactions with molecules, crystals, glasses (including optical fibers), semiconductors, and metals. Spectroscopic applications of ISRS, including time‐resolved spectroscopy of vibrationally distorted molecules and crystals, are discussed.

Locking conditions and stability properties for a semiconductor laser with external light injection

Abstract: We present a theoretical and experimental investigation of injection locking of semiconductor lasers. The theoretical analysis takes into account the dependence of refractive index on the carrier density expressed by the linewidth enhancement factor α. Locking conditions and dynamic stability are analyzed. The nonzero value of α results in an increased locking bandwidth, where only part of the range corresponds to a dynamically stable state. Asymmetric characteristics are obtained for the locked power and phase as a function of frequency detuning between the master and slave laser. Outside the stable range, light injection gives rise to beat phenomena and intensity pulsations. The theoretical results were confirmed by experiments on 830 nm CSP lasers and 1.3 μm BH lasers. The experiments include the first measurements of locking bandwidth characteristics reported for 1.3 μm lasers. Power spectra are recorded under locked and near-locked conditions and compared with theory. The 1.3 μm lasers are found to have a better dynamic stability than 830 nm lasers. Even so, the stability problems may exclude the particular application of injection locking where phase modulation is generated for coherent transmission.

Generation of optical pulses as short as 27 femtoseconds directly from a laser balancing self-phase modulation, group-velocity dispersion, saturable absorption, and saturable gain

Abstract: We describe an ultrasbort-pulse laser that, under specific operating conditions, balances the mechanisms of conventional passive mode locking and solitonlike pulse shaping in a single resonator to generate optical pulses that are to our knowledge the shortest yet emitted directly from a laser, 27 fsec.

Second-order distributed feedback lasers with mode selection provided by first-order radiation losses

Abstract: We present a theoretical study of the effect of radiation losses on the mode selectivity of DFB lasers with second-order gratings. For a second-order grating, interference of the radiation due to first-order diffraction of oppositely propagating guided waves cancels the radiation loss at one of the edges of the spectrum gap. This provides threshold gain discrimination of order 10 cm-1against one of the two dominant modes occurring near the edges of the gap. This should allow fabrication of DFB lasers with properties that are nearly independent of the positions of the facets relative to the grating corrugations, which are uncontrolled. By applying antireflection coatings to the two ends, differential quantum efficiencies close to those of conventional Fabry-Perot lasers should be achievable.

Visible fluorescence spectral diagnostic for laser angiosurgery

Abstract: A method of diagnosis of the type of tissue in an artery, including distinguishing artery wall from atheromateous plaque is described, in which a catheter with one or more optical fibers is enclosed at a distal end by an optical shield transparent to light radiation and wherein the proximal end of the catheter and optical fibers are coupled to a source of optical radiation. The catheter is inserted into an artery until the optical shield is brought into contact with a suspected arterial lesion or other tissue to be diagnosed. An optical fiber is selected and light coupled from the source at a wavelength of about 480 nm enters the proximal end of the optical fiber and is transmitted by the selected optical fiber to the distal end out the optical shield and impineges on the suspected lesion. The scattered and fluorescence light is returned to the proximal end of the selected optical fiber, and is coupled to a spectral analyzer wherein the fluorescence or scattered light excited by said light is analyzed to determine if the material contacted by the optical shield and, in particular, the portion irradiated by the incident light from the selected optical fiber is healthy arterial tissue or plaque or other material. This determination is made by observing at the proximal end of the selected fiber the fluorescence excited by the light to analyze the spectral profile in the range of 520-610 nm, where peaks occur. A determination is made from the ratios of fluorescence peak heights at 550 nm and 600 nm and the valley at 580 nm, or by analyzing other fluorescence wavelengths, whether the tissue being analyzed is artery wall, plaque, blood, or other tissue.

Semiconductor laser device

Abstract: PURPOSE:To improve an S/N ratio by driving a semiconductor laser by the signal on which high-frequency current is superposed and oscillating the laser with non-single mode. CONSTITUTION:The high-frequency current having the frequency, for example, 2.5-4GHz for returning the oscillation mode of the semiconductor laser LD to multimodes prior to lapse of the time when said mode changes to single mode is emitted from a high-frequency current source G and is superposed on the output current of a DC current source I via a capacitor C. The laser LD is thus driven by the signal on which the high frequency is superposed. The amplitude of the current source G is so set as to cut off thoroughly the threshold current of the laser LD. The characteristic having the stable S/N ratio is thus obtd. even if the optical length and the frequency of the high-frequency current vary to some extent.

Laser manipulation of atomic beam velocities: Demonstration of stopped atoms and velocity reversal.

Abstract: Experience de ralentissement d'un faisceau d'atomes Na a des vitesses nulle ou negatives en utilisant le rayonnement d'un laser qui se propage en sens inverse et dont la frequence varie de facon precise par modulation electrooptique. Obtention d'une temperature de ∼50 mK avec une densite superieure a 10 6 atomes/cm 3 . Possibilites de la methode pour ralentir, deflechir ou stocker des atomes

Theory of gain, modulation response, and spectral linewidth in AlGaAs quantum well lasers

Abstract: We investigate theoretically a number of important issues related to the performance of AlGaAs quantum well (QW) semiconductor lasers. These include a basic derivation of the laser gain, the linewidth enhancement factor α, and the differential gain constant in single and multiple QW structures. The results reveal the existence of gain saturation with current in structures with a small number of wells. They also point to a possible two-fold increase in modulation bandwidth and a ten-fold decrease in the spectral laser linewidth in a thin QW laser compared to a conventional double heterostructure laser.

Density-matrix theory of semiconductor lasers with relaxation broadening model - Gain and gain-suppression in semiconductor lasers

Abstract: The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Density-matrix theory of semiconductor lasers with relaxation broadening model-gain and gain-suppression in semiconductor lasers

Abstract: The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Method and apparatus for transmyocardial revascularization using a laser

Abstract: A surgical carbon dioxide laser is disclosed which includes a handpiece for directing a laser beam to a desired location. Mounted on the forward end of the handpiece is a hollow needle precisely aligned along the laser path. This permits the laser to be used in surgical applications where the needle perforates a porton of tissue to provide the laser beam direct access to distal tissue. The needle is mounted to the handpiece in a manner which provides for forward and lateral adjustment of the needle relative to the laser beam path which insures co-axiality between the laser beam and the needle lumen. This is crucial for the safe use of the laser. The needle is adjusted such that the focal point of the laser beam is approximately at the tip of the needle. This provides maximum energy at the tip of the needle where it is required. This apparatus is particularly useful in transmyocardial devices utilized to provide ischemic endocardial tissue direct access to blood within the ventricular cavity. The device is inserted within the epicardium of the heart; the laser is then activated to vaporize a channel of tissue through the endocardium. This provides for perforation of the endocardium with minimal damage to the epicardium.

Analysis and application of theoretical gain curves to the design of multi-quantum-well lasers

Abstract: Gain/current curves for a single quantum well are calculated. The optimum well number, cavity length, threshold current, and current density of multi-quantum-well (MQW) lasers are derived in terms of this gain curve. The limiting performance of MQW lasers is found to be better than that of graded refractive index (GRIN) lasers, assuming comparable efficiencies and spontaneous emission linewidths. The optimum threshold current for an MQW laser with a 7 μm cavity and 90 percent facet reflectivity is \sim50 \mu A/μm.

Neodymium-doped silica single-mode fibre lasers

Abstract: We report the first CW operation of a neodymium-doped silica single-mode fibre laser pumped by a GaAIAs laser diode. A laser threshold of less than 1 mW has been obtained.

Efficient, frequency-stable laser-diode-pumped Nd:YAG laser

Abstract: We have designed and tested a laser-diode-pumped monolithic Nd:YAG oscillator. The electrical-to-optical slope efficiency was 6.5%. The frequency jitter was less than 10 kHz over a 0.3-sec period, the best frequency stability reported for a Nd:YAG laser to date.

Residual amplitude modulation in laser electro-optic phase modulation

Abstract: An important practical limitation to the sensitivity of frequency-modulation spectroscopy arises because of imperfect phase modulation of the laser beam. This imperfection manifests itself as residual amplitude modulation (RAM), and we present data from a careful series of experiments designed to elucidate its origin. Experimentally, we find two components to the RAM, one that depends for its intensity on the laser frequency and one that does not. We present a model that correctly accounts for the intensity and rf phase and frequency behavior of the laser-frequency-dependent component. We are unable to offer a plausible explanation for the laser-frequency-independent component.

Interaction of powerful laser radiation with the surfaces of semiconductors and metals: nonlinear optical effects and nonlinear optical diagnostics

Abstract: This article reviews the present status of research on strongly nonequilibrium physical effects produced near and at the surfaces of metals, semiconductors, and dielectrics by high-power pulsed laser radiation, and their diagnostics by nonlinear-optical methods. The basic stages of the absorption and relaxation of the energy of laser radiation in the electronic and phonon subsystems of semiconductors and heating of the lattice are studied. The questions associated with fast laser-induced phase transformations in surface layers are discussed: melting-solidification and amorphous-solid-to-crystal and vice versa phase transitions, occurring with nano- and picosecond laser pulse durations (laser annealing). Different methods of laser diagnostics of the surfaces and nonequilibrium ultrafast processes on the surfaces of metals and semiconductors are reviewed. Special attention is devoted to nonlinear-optical methods of surface diagnostics (generation of second and third harmonics and difference frequencies in reflection, and Raman scattering of light). The high information content and sensitivity of these methods compared with other methods for studying surfaces is pointed out. The problem of the appearance of spatially periodic surface structures induced by laser radiation is examined. The theories of different laser-induced instabilities of the surface relief, giving rise to the formation of these structures (generation of surface acoustic and capillary waves and evaporation waves), are reviewed. Theoretical and experimental results are compared.

Correlated spontaneous-emission lasers: Quenching of quantum fluctuations in the relative phase angle.

Abstract: In quantum-beat and Hanle-effect experiments, spontaneous-emission events from two coherently excited states are strongly correlated. A doubly resonant laser cavity driven by such atomic configurations can have vanishing diffusion coefficient for the relative phase angle.

Microwave radiation from a high-gain free-electron laser amplifier.

Abstract: A high-gain, high--extraction-efficiency, linearly polarized free-electron laser amplifier has been operated at 34.6 GHz. At low signal levels, exponential gain of 13.4 dB/m has been measured. With a 3-=kW input signal, saturation was observed with an 80-MW output and a 5% extraction efficiency. The results are in good agreement with linear models at small signal levels and nonlinear models at large signal levels.

Laser light dynamics

Abstract: Preface. List of symbols. 1. Introduction. 2. Basic properties and types of lasers. 3. Laser resonators. 4. The intensity of laser light. Rate equations. 5. The basic equations of the semiclassical laser theory. 6. Applications of semiclassical theory. 7. Ultrashort pulses. 8. Instability hierachies of laser light. Chaos, and routes to chaos. 9. Optical bistability. 10. Quantum theory of the laser I. A first approach via quantum mechanical Langevin equations. Coherence, noise and photon-statistics. 11. Quantum theory of the laser II. A second approach via the density matrix equation and quantum classical correspondence. 12 A theoretical treatment of the two-photon laser. 13. The laser - trailblazer of synergetics. References. Subject index.

Femtosecond imaging of melting and evaporation at a photoexcited silicon surface

Abstract: We describe a simple imaging technique that can be used to photograph ultrafast processes with time resolution determined by the duration of pump and probe laser pulses. We demonstrate this technique by photographs having 100-fsec time resolution of a silicon surface undergoing melting and evaporation following intense excitation by an ultrashort laser pulse. These photographs resolve the increase in surface reflectivity caused by surface melting both temporally and spatially. Material evaporation from the melted surface further alters the image of the surface by absorbing and scattering the illuminating laser light. Our analysis of this selectively imaged light suggests that the evaporated material emerges as liquid droplets several hundred angstroms in diameter, which atomize in less than a nanosecond.

Instantaneous temperature field measurements using planar laser-induced fluorescence

Abstract: A single-pulse, laser-induced-fluorescence diagnostic for the measurement of two-dimensional temperature fields in combustion flows is described. The method uses sheet illumination from a tunable laser to excite planar laser-induced fluorescence in a stable tracer molecule, seeded at constant mole fraction into the flow field. The temporal resolution of this technique is determined by the laser pulse length. Experimental results are presented for a rod-stabilized, premixed methane–air flame, using the Q1(22) line of the nitric oxide A2 ∑+(v = 0) ← X2Π1/2(v = 0) transition (λ ≃ 225.6 nm).