Pulse Propagation In Water Research Articles

Improved optical pulse propagation in water using an evolutionary algorithm

Optical pulse propagation in water is experimentally investigated using an evolutionary algorithm (EA) to control the shape of an optical pulse. The transmission efficiency (ratio of output to input optical power) is maximized by searching the combined amplitude and phase space governing an optical pulse shaper. The transmission efficiency of each tested pulse is physically determined by experiment during the course of the optimization. Combining the EA with an experiment in this manner is a powerful means of improving some figure of merit because no analytical or computational model is required-we optimize directly given the physics of the experiment. In addition, the EA is capable of efficiently searching a large parameter space. Here, we demonstrate improved linear optical pulse propagation near 800nm. Our results demonstrate a pulse with a dramatically narrower bandwidth that coincides with a local absorption minimum (near 800 nm) implying that the transmission efficiency is dominated by water's absorption spectrum.

Comparison of Optical Pulse Propagation in Water and Acetonitrile

Optical propagation in water has been studied primarily by light sources that have either a constant amplitude in time or have a modulated temporal width greater than a picosecond. Attenuation of light as it propagates through water has been experimentally measured throughout the optical spectrum with these light sources. Quantities such as absorption and scattering have been thoroughly characterized as a function of wavelength of light. Yet with this wealth of measurements, a model that describes the transparency of water in the visible region still does not exist. The reason has been attributed to the complexity in understanding the hydrogen bonding that occurs in water (1). Recently, an experimental study challenged the well established empirical knowledge of attenua- tion in water at near-infrared wavelengths. The study examined propagation of ultrafast optical pulses in water and demonstrated increased transmission of ultrafast light pulses as compared to longer pulses or cw light (2). In these experiments, pulses as short as 60fs with a center wavelength of 800nm were propagated through as much as 6 meters of water. To explain their observations, the authors suggested the possibility of light propagation by optical precursors. The question is: How and why were the optical precursors stimulated in the water? To better understand the propagation of ultrafast pulses of light in water, as demonstrated by Fox and ˜ Osterberg (2), we will compare measurements in water with those in acetonitrile. Both water and acetonitrile are polar solvents, but water molecules exhibit hydrogen bonding while acetonitrile molecules do not. In addition, this study will compare the difierences of the OH bond in water with the CH bond in acetonitrile. Harmonics of these molecular stretching frequencies in the near-infrared region are explored with a femtosecond laser system.

Dynamic nonlinear X waves for femtosecond pulse propagation in water.

Recent experiments involving femtosecond pulses in water displayed long-distance propagation analogous to that reported in air. We verify this phenomenon numerically and show that the propagation is dynamic as opposed to self-guided. Furthermore, we demonstrate that the propagation can be interpreted as being due to dynamic nonlinear X waves whose robustness and role in long-distance propagation follows from the interplay between nonlinearity and chromatic dispersion.

Wave effects on blockwork structures: Numerical models

Transient or fluctuating pressures, generated for example by a wave impact on a sea wall or a water jet plunging into a pool, have been shown to propagate into water filled cracks or fissures of structures and rock. Model studies revealed the characteristics of impact generated pressure pulses, which were observed to travel at very low speeds of 60–160 m/s and to attenuate, whereby higher frequencies were preferentially damped out. Other effects, such as reflection and dynamic amplification also indicated that the pulses constituted waves propagating through an elastic 2-phase medium consisting of water and a small amount of air. Based on these observations, concepts for a numerical model of pressure pulse propagation in water were developed and implemented. It was found that the numerical model approximates the physical model test results well, both in the linear and the non-linear range and including the transition from an initial steep pressure pulse to wave-like forms. The damping coefficient was found to be a constant, independent of the degree of aeration. The results from the numerical studies imply that, within very short time frames, water behaves like a visco-elastic solid rather than as a fluid.

Physical factors limiting the spectral extent and band gap dependence of supercontinuum generation.

Based on simulations of femtosecond pulse propagation in water we elucidate the physical factors that limit the attainable spectral extent of supercontinuum generation in bulk media. In contrast to the standard scenario, where arrest of self-focusing collapse by multiphoton absorption and plasma defocusing are viewed as imposing a limit to supercontinuum broadening, we show that linear chromatic dispersion also plays a major role. This insight provides an intuitive explanation of the observed band gap dependence of the attainable supercontinuum generation in condensed media.

Numerical simulation of ultrasonic shock wave propagation in lossy liquids obeying a frequency power law

Ultrasonic shock waves are applied for several noninvasive therapies like extracorporal lithotripsy. In order to design and to optimize therapeutical devices it is essential to predict accurately the shockwave propagation through the arrangement transducer/coupling medium/human tissue. An explicit high-order FDTD-discretization of a set of nonlinear acoustic equations has been shown to model the lithotripter pulse propagation in water in a quality that beats all other known methods. To consider attenuation caused by tissue structures, realistic broadband models of soft tissue attenuation are provided by frequency power laws that include strong dispersion. In this contribution a new method to include these frequency power laws into the explicit FDTD model is presented. It is based on a causal continuous time domain formulation of the attenuation law which contains a Riemann–Liouville derivative operator of fractional order. This operator is discretized by using a quadrature formula. The resulting FDTD algorithm is verified. It turns out to be an efficient tool to compute the dispersive and attenuating effects of soft tissue on the propagation of high-intensity ultrasonic pulses and shock waves. Exemplary nonlinear dispersive propagation of a lithotripter pulse through tissue is simulated.

Theory and simulation on the threshold of water breakdown induced by focused ultrashort laser pulses

A comprehensive model is developed for focused pulse propagation in water. The model incorporates self-focusing, group velocity dispersion, and laser-induced breakdown in which an electron plasma is generated via cascade and multiphoton ionization processes. The laser-induced breakdown is studied first without considering self-focusing to give a breakdown threshold of the light intensity, which compares favorably with existing experimental results. The simple study also yields the threshold dependence on pulse duration and input spot size, thus providing a framework to view the results of numerical simulations of the full model. The simulations establish the breakdown threshold in input power and reveal qualitatively different behavior for picoand femto-second pulses. For longer pulses, the cascade process provides the breakdown mechanism, while for shorter pulses the cooperation between the self-focusing and the multiphoton plasma generation dominates the breakdown threshold.