Gaussian beams in hollow metal waveguides: experiment.

Gaussian beams have been widely used for propagating electromagnetic waves in free space and in certain other optical systems. It has been suggested that recurring forms of such beams might also be useful for propagation in planar or rectangular metal waveguides. Experimental verification of the recurrence of the Gaussian field distribution in metal waveguides is reported here.

Grazing reflection of Gaussian beams.

The reflectivities of most surfaces are higher for grazing or near-90-deg angles of incidence than for more perpendicular or near-zero-deg angles. Grazing-incidence configurations are especially important in the development of lasers and optical systems that operate in the far-ultraviolet and soft-x-ray regions of the spectrum, where transparent or highly reflecting media are almost unknown. Analytical solutions of the paraxial wave equation are obtained for the grazing reflection and complex interference effects that take place when a Gaussian beam interacts at shallow angles with a reflecting surface.

Enhanced laser thrombolysis with photomechanical drug delivery: an in vitro study.

BACKGROUND AND OBJECTIVE: Current techniques for laser thrombolysis are limited because they can not completely clear thrombotic occlusions in arteries, typically leaving residual thrombus on the walls of the artery. The objective of this study was to investigate the possibility of using photomechanical drug delivery to enhance laser thrombolysis by delivering drugs into mural thrombus during laser thrombolysis. STUDY DESIGN/MATERIALS AND METHODS: Three experimental protocols were performed in vitro to quantitatively compare the effectiveness of thrombolysis by 1) constant infusion of drug, 2) laser thrombolysis, and 3) photomechanical drug delivery. A fiber-optic flushing catheter delivered drug (a solution of 1 microm fluorescent microspheres) and light ( a 1 micros pulsed dye laser) into a gelatin-based thrombus model. The process of laser-thrombus interaction was visualized using flash photography and the laser-induced pressure waves were measured using an acoustic transducer. RESULTS: Lumen sizes generated by mechanically manipulating the catheter through the thrombus were smaller than those generated by laser ablation. The microspheres could be driven several hundred microns into the mural thrombus. CONCLUSION: Photomechanical drug delivery has potential for enhancement of laser thrombolysis. Two mechanisms seem to be involved in photomechanical drug delivery: 1) mural deposition of the drug at the ablation site and 2) increased exposure of the thrombus surface area to the drug.

Production and propagation of Hermite-sinusoidal-Gaussian laser beams.

Hermite-sinusoidal-Gaussian solutions to the wave equation have recently been obtained. In the limit of large Hermite-Gaussian beam size, the sinusoidal factors are dominant and reduce to the conventional modes of a rectangular waveguide. In the opposite limit the beams reduce to the familiar Hermite-Gaussian form. The propagation of these beams is examined in detail, and resonators are designed that will produce them. As an example, a special resonator is designed to produce hyperbolic-sine-Gaussian beams. This ring resonator contains a hyperbolic-cosine-Gaussian apodized aperture. The beam mode has finite energy and is perturbation stable.

Pressure impulses during microsecond laser ablation.

The collapse of laser-induced cavitation bubbles creates acoustic transients within the surrounding medium and also pressure impulses to the ablation target and light-delivery fiber during microsecond laser ablation. The impulses are investigated here with time-resolved flash photography, and they are found to occur whether or not the light-delivery fiber is in contact with the target. We demonstrate that the impulses depend primarily on the energy stored in the cavitation bubble. They are not directly dependent on the mode of light delivery (contact versus noncontact), and they are also not directly correlated to the other acoustic transients. The pressure impulses do seem to be associated with the bubble-driven jet formation caused by the bubble collapse.

Drug delivery with microsecond laser pulses into gelatin.

Photo acoustic drug delivery is a technique for localized drug delivery by laser-induced hydrodynamic pressure following cavitation bubble expansion and collapse. Photoacoustic drug delivery was investigated on gelatin-based thrombus models with planar and cylindrical geometries by use of one microsecond laser pulses. Solutions of a hydrophobic dye in mineral oil permitted monitoring of delivered colored oil into clear gelatin-based thrombus models. Cavitation bubble development and photoacoustic drug delivery were visualized with flash photography. This study demonstrated that cavitation is the governing mechanism for photoacoustic drug delivery, and the deepest penetration of colored oil in gels followed the bubble collapse. Spatial distribution measurements revealed that colored oil could be driven a few millimeters into the gels in both axial and radial directions, and the penetration was less than 500 µm when the gelatin structure was not fractured.

Beam propagation in parabolically tapered graded-index waveguides.

To a good approximation, the electromagnetic-propagation characteristics of graded-index waveguides can be written in terms of polynomial-Gaussian modes. For uniform quadratic-index waveguides the behavior of these modes is well known. However, there are sometimes practical reasons for using tapered waveguides, but detailed propagation solutions are known for only a few specific taper functions. The parabolic taper is perhaps the most important special case, and the solution-generating techniques that we generalize are used to obtain analytic solutions for this case.

Pump-pulse effects in hybridly mode-locked dye lasers.

The pump pulse of a hybridly mode-locked laser provides additional parameters for system optimization. We use a semiclassical model to explore numerically the role of the pump pulse in the ultrashort pulse production of a hybridly mode-locked dye laser. Pump pulses of lengths less than ~1 psec are found to correspond to Dirac delta functions and deliver the shortest output pulses. We conclude that existing dye-laser systems would yield a twofold to threefold shortening in output pulse widths and a threefold to fivefold increase in peak power if pump pulses of 1 psec were used. We note that a 1-psec pump pulse is attainable using fiber-compression techniques.

Direct current-excited cw CO(2) metal waveguide laser.

A novel design for a dc-excited cw CO(2) metal waveguide laser has been developed in which a slotted hollow cathode also doubles as a metal waveguide for the cavity modes. This design has been implemented in a compact structure that produces over 1 W of cw 10.6-microm radiation. The discharge characteristics, laser gain, and laser output have been studied as functions of various discharge parameters. The advantages of the transverse discharge of the slotted hollow cathode geometry include low voltage, positive impedance, rugged structure, and high optical gain. Overall efficiency is comparable with that of conventional longitudinal CO(2) lasers. The output laser modes are clean low-order Hermite-Gaussian or Airy function modes.

Modes of a laser resonator with a retroreflecting corner cube mirror.

The self-consistent integral equation for the field distribution of the resonant modes in a resonator with a tilted retroreflecting corner cube mirror is solved. The corner cube acts like a convex lens with radius of curvature -L cot(2)theta in the rotation direction (L is the cavity length and theta the rotation angle) and like a flat plane in the direction of the rotation axis. The field distribution can be described in terms of Hermite-Gaussian functions, and these results have been confirmed experimentally using an Ar-ion laser. The equivalent beam matrix for a reflecting corner cube is also found.

Modes of a laser resonator with a retroreflecting roof mirror.

The self-consistent integral equation for the field distribution of the resonant modes in a resonator with a tilted retroreflecting roof mirror is solved. The field distribution in the direction of the roof can be described in terms of Hermite-Gaussian functions. The beam matrix for a retroreflecting roof is found, and a new type of resonator which does not need precise alignment is proposed.

Modes of a laser resonator with a retroreflective mirror.

The self-consistent integral equation for the field distribution of the resonant modes in an inclined retroreflective grating resonator is solved in the limit of large Fresnel numbers. The transverse field distribution in the direction perpendicular to the grating grooves can be described in terms of Hermite-Gaussian functions provided that lambda << d << w, where lambda is the wavelength, d is the grating spacing, and w is the beam spot size.

Synthesis of Gaussian beam optical systems.

Systematic procedures are presented for determining the the optical components needed to obtain an arbitrary transformation of a propagating light ray or Gaussian beam.

Electromagnetic modes of an inhomogeneous sphere.

In many cases the electromagnetic modes of a sphere can be obtained exactly. Special simplifications occur when the conductivity, complex permittivity, and complex permeability vary only in the radial direction, and static and time dependent examples are discussed. With quadratic or Coulomblike permittivity variations, the wave functions resemble the quantum mechanical harmonic oscillator or hydrogen atom wave functions, and similar angular momentum operators are obtained.

Laser power calculations: sources of error.

The physical phenomena that dominate the power characteristics of a laser depend on the detailed nature of the amplifying medium and the resonator structure. In predicting the power characteristics, numerous approximations are always required. The most important approximations are considered here in detail, and error estimates are presented so that a designer can select the appropriate model for a particular application. Emphasis is placed on analytic solutions and specific phenomena considered include longitudinal and transverse spatial hole burning, large single-pass gain, and mixed line broadening.

Scattering of focused beams by tenuous particles.

This paper deals with the problem of the scattering of focused laser beams by tenuous particles using an iterative technique. The results are shown to be accurate provided that (a) the polarizability of the particle medium is small and (b) the phase shift of the central ray is less than 2. It was found that when the size of the incident beam waist is close to that of the scatterer, the scattered field deviates significantly from that for the incident plane wave case. Specific examples are given.