Showing papers on "Laser published in 1989"

High-sensitivity, single-beam n(2) measurements.

Abstract: We present a simple yet highly sensitive single-beam experimental technique for the determination of both the sign and magnitude of n2. The sample is moved along the z direction of a focused Gaussian beam while the repetitively pulsed laser energy is held fixed. The resultant plot of transmittance through an aperture in the far field yields a dispersion-shaped curve from which n2 is easily calculated. A transmittance change of 1% corresponds to a phase distortion of ≃ λ/250. We demonstrate this method on several materials using both CO2 and Nd:YAG laser pulses.

Surface properties probed by second-harmonic and sum-frequency generation

Abstract: Optical second-harmonic generation and the related technique of infrared – visible light sum-frequency generation are extremely versatile tools for studies of many kinds of surfaces and interfaces. With the help of ultra-short laser pulses, they can be used to monitor surface dynamics and reactions with sub-picosecond time resolution.

Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers

Abstract: Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52- mu m mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems. >

Lasers without inversion: Interference of lifetime-broadened resonances

Abstract: While it is commonly believed that population inversion is a requirement for obtaining laser amplification, this is not so. We show that if two upper levels of a four-level laser system are purely lifetime broadened and decay to an identical continuum, there will be interference in the absorption profile of lower level atoms, and this interference will be absent from the stimulated emission profile of the upper level atoms. It is, therefore, possible to have a substantial gain cross section at frequencies where the absorption cross section is zero and to obtain laser amplification in conditions where the lower level population greatly exceeds the upper level population.

Degenerate quantum-beat laser: Lasing without inversion and inversion without lasing.

Abstract: A single lasing mode driven by a three-level ``quantum-beat'' atomic configuration can show gain without population inversion or optical absorption into an excited state without spontaneous or stimulated emission.

Improved laser‐beam uniformity using the angular dispersion of frequency‐modulated light

Abstract: A new technique is presented for obtaining highly smooth focused laser beams. This approach is consistent with the constraints on frequency tripling the light, and it will not produce any significant high‐intensity spikes within the laser chain, making the technique attractive for the high‐power glass lasers used in current fusion experiments. Smoothing is obtained by imposing a frequency‐modulated bandwidth on the laser beam using an electro‐optic crystal. A pair of gratings is used to disperse the frequencies across the beam, without distorting the temporal pulse shape. The beam is broken up into beamlets, using a phase plate, such that the beamlet diffraction‐limited focal spot is the size of the target. The time‐averaged interference between beamlets is greatly reduced because of the frequency differences between the beamlets, and the result is a relatively smooth diffraction‐limited intensity pattern on target.

The Physics of Laser Plasma Interactions

Abstract: (1989). The Physics of Laser Plasma Interactions. Journal of Modern Optics: Vol. 36, No. 3, pp. 417-418.

Experimental studies of the application of the Er:YAG laser on dental hard substances: II. Light microscopic and SEM investigations

Abstract: Many studies have been undertaken trying to use various laser systems as optical drills on dental enamel and dentin, but the high radiant exposure needed and subsequent high temperature rises lead to fractures of the hard substances and possible damages to the pulp. Compared to the other laser systems, the use of the Er:Yag laser has given encouraging results. Optical and scanning electron microscopy showed only minimal if any damage of the surrounding tissue.

Two-photon laser microscopy

Abstract: A laser scanning microscope produces molecular excitation in a target material by simultaneous absorption of two photons to thereby provide intrinsic three-dimensional resolution Fluorophores having single photon absorption in the short (ultraviolet or visible) wavelength range are excited by a stream of strongly focused subpicosecond pulses of laser light of relatively long (red or infrared) wavelength range The fluorophores absorb at about one half the laser wavelength to produce fluorescent images of living cells and other microscopic objects The fluorescent emission from the fluorophores increases quadratically with the excitation intensity so that by strongly focusing the laser light, fluorescence as well as photobleaching are confined to the vicinity of the focal plane This feature provides depth of field resolution comparable to that produced by confocal laser scanning microscopes, and in addition reduces photobleaching Scanning of the laser beam, by a laser scanning microscope, allows construction of images by collecting two- photon excited fluorescence from each point in the scanned object while still satisfying the requirement for very high excitation intensity obtained by focusing the laser beam and by pulse time compressing the beam The focused pulses also provide three-dimensional spatially resolved photochemistry which is particularly useful in photolytic release of caged effector molecules This invention was made with Government support under Grant Nos P41RR04224 awarded by the National Institute of Health; NSF-BBS-8714069 awarded by the National Science Foundation, and NSF-DMB-8609084 awarded by the National Science Foundation The Government has certain rights in the invention

Method for collagen treatment

Abstract: A method for controlled thermal shrinkage of collagen tissue by irradiation with coherent energy in the wavelength band of 1.80 to 2.55 microns as generated by a laser. A specific application to ophthalmological corneal reshaping is described.

Single-frequency microchip Nd lasers.

Abstract: Optically pumped, single-frequency, Nd-doped, solid-state lasers have been constructed using flat-flat cavities, which were diced from large dielectrically coated wafers of various crystals. For example, a Nd:YAG laser with a cavity length of 730 microm has operated at room temperature in a single longitudinal mode from a threshold of less than 1 mW to greater than 40 times the threshold. Theslope efficiency was greater than 30%. Heterodyne measurements showed an instrument-limited linewidth of 5 kHz. The microchip lasers demonstrate ways to reduce greatly the cost and complexity offabricating small lasers and electro-optic devices.

Light distributions in artery tissue: Monte Carlo simulations for finite-diameter laser beams.

Abstract: Finite-width light distributions in arterial tissue during Argon laser irradiation (476 nm) are simulated using the Monte Carlo method Edge effects caused by radial diffusion of the light extend +/- 15 mm inward from the perimeter of a uniform incident beam For beam diameters exceeding 3 mm the light distribution along the central axis can be described by the one-dimensional solution for an infinitely wide beam The overlapping edge effects for beam diameters smaller than 3 mm reduce the penetration of the irradiance in the tissue The beam profile influences the light distribution significantly The fluence rates near the surface for a Gaussian beam are two times higher on the central axis and decrease faster radially than for a flat profile The diverging light from a fiber penetrates tissue in a manner similar to collimated light

Theory of the optimally coupled Q-switched laser

Abstract: The general equations describing Q-switched laser operation are transcendental in nature and require numerical solutions, which greatly complicates the optimization of real devices. Here, it is shown that, using the mathematical technique of Lagrange multipliers, one can derive simple analytic expressions for all of the key parameters of the optimally coupled laser, i.e. one which uses an optimum reflector to obtain maximum laser efficiency for a given pump level. These parameters can all be expressed as functions of a single dimensionless variable z, defined as the ratio of the unsaturated small-signal gain to the dissipative (nonuseful) optical loss, multiplied by a few simple constants. Laser design tradeoff studies and performance projections can be accomplished quickly with the help of several graphs and a simple hand calculator. Sample calculations for a high-grain Nd:YAG and a low-gain alexandrite laser are presented as illustrations of the technique. >

A method for laser induced fluorescence of tissue.

Abstract: A method for laser induced fluorescence of tissue in which laser radiation is used to illuminate and induce fluorescence in the tissue under study to determine the chemical composition or pathologic condition of tissue. The laser radiation and the retrieved fluorescing radiation can be conveyed through a catheter using an array of optical fiber. The fluorescence spectrum of the tissue can be displayed and analyzed to obtain information regarding the chemical composition and medical condition of the tissue inside the human body.

Tunable optical parametric oscillator

Abstract: A broadly tunable femtosecond optical parametric generator includes a cavity having a pumping section in which is located a nonlinear crystal selected from the group CTA and RTA, cut for noncritical phasematching. The plane of the optical x axis and or the optical y axis of the crystal is aligned with the axis of the pumping section. A Ti:S laser pumping beam is directed into the pumping section either collinearly or noncollinearly with the pumping section axis and impinges on the crystal to produce signal and idler beams. The laser is tunable to produce OPO output beams between about 1 and 4 μm.

Selective laser sintering with assisted powder handling

Abstract: A method and apparatus for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder is deposited to the target area of the laser and attains high bulk density during sintering.

Rate equation analysis of microcavity lasers

Abstract: We describe the light output properties of single mode lasers having cavity dimensions on the order of the emitted wavelength. A simple rate equation formula is derived for a four‐level laser assuming enhanced spontaneous emission into the cavity. These rate equation analyses show that increasing the coupling of spontaneous emission into the cavity mode causes the lasing properties to become quite different from those of usual lasers having cavity dimensions much larger than a wavelength. We find that the lasing threshold disappears, the light emission efficiency increases, relaxation oscillations do not occur, and the dynamic response speed is improved. It is shown that the spontaneous emission rate alteration caused by the cavity plays an essentially important role for these characteristics.

Laser cooling below the one-photon recoil energy by velocity-selective coherent population trapping: theoretical analysis

Abstract: We present a theoretical analysis of a new one-dimensional laser-cooling scheme that was recently demonstrated on a beam of metastable 4He atoms. Both internal and translational degrees of freedom are treated quantum mechanically. Unlike semiclassical approaches, such a treatment can be applied to situations in which the atomic coherence length is of the same order of or larger than the laser wavelength, which is the case for atoms cooled below the one-photon recoil energy. We introduce families of states that are closed with respect to absorption and stimulated emission, and we establish the generalized optical Bloch equations that are satisfied by the corresponding matrix elements. The existence of velocity-selective trapping states that are linear combinations of states with different internal and translational quantum numbers is demonstrated, and the mechanism of accumulation of atoms in these trapping states by fluorescence cycles is analyzed. From a numerical solution of the generalized optical Bloch equations, we study in detail how the final atomic-momentum distribution depends on the various physical parameters: interaction time, width of the initial distribution, laser detuning, laser power, and imbalance between the two counterpropagating waves. We show that the final temperature decreases when the interaction time increases, so that there is no fundamental limit to the lowest temperature that can be achieved by such a method. Finally, possible extensions of this method to two-dimensional cooling are presented.

Amplification of spontaneous emission in erbium-doped single-mode fibers

Abstract: Amplification of spontaneous emission (ASE) in erbium-doped single-mode fiber amplifiers operating at lambda =1.53 mu m is studied theoretically and experimentally. The ASE noise spectra obtained from the theory are found to be in excellent quantitative agreement with the experimental data. The observed changes in ASE spectral shapes under different population inversion conditions are also explained. The model may be used to evaluate the performance of erbium-doped fiber lasers as well as to assess the noise characteristics of erbium-doped fiber amplifiers as applied to wavelength-division multiplexing optical communications. >

High‐brightness terahertz beams characterized with an ultrafast detector

Abstract: We have significantly improved the emission and detection of electromagnetic beams of single‐cycle 0.5 THz pulses, through the use of new dipolar antenna structures. The frequency response was extended to well beyond 1 THz, and the beam power was increased by more than 15 times. The antennas were located at the foci of sapphire lenses and were photoconductively driven by ultrafast laser pulses. An additional collimation by a paraboloidal mirror produced a beam with a 25 mrad divergence, and subsequent focusing by a second identical mirror improved the coupling between the transmitting and receiving antenna by orders of magnitude.

Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

Abstract: Vertical-cavity electrically driven lasers with three GaInAs quantum wells and diameters of several μm exhibit room-temperature pulsed current thresholds as low as 1.3mA with 958 nm output wavelength.

Ultraviolet Laser Ablation of Organic Polymer Films

Abstract: When a pulse (~ 14 nsec half-width) of laser radiation of 193 nm wavelength with a fluence above a threshold value falls on a polymer film, the material at the irradiation site is spontaneously etched away to a depth of 1000 A or more.[1,2] This process has been called Ablative Photo Decomposition’ [3]. The excimer laser which is the source of the 193 nm radiation is capable of providing radiation at other wavelengths such as 249 nm, 308 nm, and 351 nm. Spontaneous etching of the polymer films by the laser beam has been observed at all of these wavelengths [4–7].But there are quantitative differences in the etching process at different wavelengths and with different polymers.

Room-temperature continuous wave lasing characteristics of a GaAs vertical cavity surface-emitting laser

Abstract: Room‐temperature continuous wave (cw) operation of a GaAs vertical microcavity surface‐emitting laser has been achieved. An ultrashort cavity device with a cavity length of ∼5.5 μm was grown by metalorganic chemical vapor deposition. cw lasing characteristics such as mode properties and temperature characteristics were examined. Single longitudinal mode operation with a side mode suppression ratio of 35 dB was obtained. The temperature range for single mode operation was more than 50 K.

Recent Progress On Laser-Induced Modifications And Intrinsic Bulk Damage Of Wide-Gap Optical Materials

Abstract: In this paper we provide a comprehensive review of our recent work on the nonlinear interaction between high intensity pulsed laser beams and transparent solids. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelengths. These measurements, performed in specially purified materials, have allowed the discovery of the roles of laser-induced excitations in energy absorption, leading to the conclusion that virtually all lattice heating occurs via a nonlinear absorption of laser photons by multi-photon-excited free electrons. These results yield an experimentally confirmed theoretical definition of intrinsic, single pulse laser damage thresholds at 532 nm wavelength in three- and four-photon bandgap alkali halides. Extending this work to multipulse effects in the subthreshold intensity regime, we have formulated a new model of bulk damage based on thermomechanical stress induced by accumulation of multiphoton-generated lattice defects.

Lasers without inversion: interference of dressed lifetime-broadened states.

Abstract: We describe the use of a coupling electromagnetic field to provide a general method of producing inversion-free laser systems. The interference between dressed states produces a zero in absorption while allowing gains of the order of that of the uncoupled system.

High density plasmas produced by ultrafast laser pulses.

Abstract: We describe temporal and spectroscopic measurements of high-density plasmas produced by focusing intense, 160-fsec laser pulses on solids. Soft x-ray emission with a duration of 2 +- 2 psec is observed up to photon energies of a kilovolt. We observe reduced emission from long-lived spectral lines, indicating the presence of a short-lived, high-density plasma. Reflectivity measurements indicate that absorption of the laser pulse occurs at the surface of the solid before it expands.

Femtosecond pulse generation in a laser with a nonlinear external resonator.

Abstract: A simple model is presented to describe mode locking in a laser coupled to a nonlinear resonator. It reveals a new mechanism for pulse shortening and shows that shortening does not rely on dispersion in the auxiliary cavity. Experimental results are given to support the basic predictions of the model.

Actively mode-locked semiconductor lasers

Abstract: Measurements of actively mode-locked semiconductor lasers are described and compared to calculations of the mode-locking process using three coupled traveling-wave rate equations for the electron and photon densities. The dependence of pulse width on the modulation current and frequency are described. A limitation to minimum achievable pulse widths in mode-locked semiconductor lasers is shown to be dynamic tuning due to gain saturation. Techniques to achieve subpicosecond pulses are described, together with ways to reduce multiple pulse outputs. The amplitude and phase noise of linear- and ring-cavity semiconductor lasers were measured and fond to be tens of dB smaller than YAG and argon lasers and limited by the noise from the microwave oscillator. High-frequency phase noise is only measurable in detuned cavities, and is below -110 dBc (1 Hz) in optimally tuned cavities. The prospects for novel ways to achieve even shorter pulses are discussed. >

Corneal Ablation by Nanosecond, Picosecond, and Femtosecond Lasers at 532 and 625 nm

Abstract: • We produced corneal excisions with nanosecond (ns)-, picosecond-, and femtosecond (fs)-pulsed lasers at visible wavelengths. The threshold energy for ablation was proportional to the square root of the pulse duration and varied from 2.5 microjoules (μJ) at 100 fs to 500 μJ at 8 ns. Excisions made with picosecond and femtosecond lasers were ultrastructurally superior to those made with nanosecond lasers and, at pulse energies near threshold, showed almost as little tissue damage as excisions made with excimer lasers at 193 nm. We conclude that ultra-short-pulsed lasers at visible and near-infrared wavelengths are a possible alternative to excimer lasers for corneal surgery and might have advantages over conventional ophthalmic neodymium-YAG lasers for some intraocular applications.

Cold-plasma production for recombination extreme-ultraviolet lasers by optical-field-induced ionization

Abstract: For high-intensity, linearly polarized light, above-threshold ionization results from the time difference between the instant of ionization and the electric field maximum. Cold plasmas suitable for recombination extreme-ultraviolet (XUV) lasers can be produced if appropriate wavelength-ionizing radiation is used. Multiphoton ionization intensities are calculated for representative XUV laser schemes.