Showing papers on "Wavelength published in 2001"

Recent progress in quantum cascade lasers and applications

Abstract: Quantum cascade (`QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple - often several tens of - photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance. The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.

The Dependence of Sea Surface Roughness on the Height and Steepness of the Waves

Abstract: It is proposed that the sea surface roughness zo can be predicted from the height and steepness of the waves, zo/Hs = A(Hs/Lp)B, where Hs and Lp are the significant wave height and peak wavelength for the combined sea and swell spectrum; best estimates for the coefficients are A = 1200, B = 45 The proposed formula is shown to predict well the magnitude and behavior of the drag coefficient as observed in wave tanks, lakes, and the open ocean, thus reconciling observations that previously had appeared disparate Indeed, the formula suggests that changes in roughness due to limited duration or fetch are of order 10% or less Thus all deep water, pure windseas, regardless of fetch or duration, extract momentum from the air at a rate similar to that predicted for a fully developed sea This is confirmed using published field data for a wide range of conditions over lakes and coastal seas Only for field data corresponding to extremely young waves (U10/cp > 3) were there appreciable differences between the predicted and observed roughness values, the latter being larger on average Significant changes in roughness may be caused by shoaling or by swell A large increase in roughness is predicted for shoaling waves if the depth is less than about 02Lp The presence of swell in the open ocean acts, on average, to significantly decrease the effective wave steepness and hence the mean roughness compared to that for a pure windsea Thus the predicted open ocean roughness is, at most wind speeds, significantly less than is observed for pure wind waves on lakes Only at high wind speeds, such that the windsea dominates the swell, do the mean open ocean values reach those for a fully developed sea

Silicon‐Based Photonic Crystals

Abstract: In semiconductors electrons propagate in a periodic poten-tial, which originates from the atomic lattice. This modifiesthe dispersion relation of free electrons and a band structurewith a bandgap occurs in the case of semiconductors. Theincorporation of electrically active defects allows the manipu-lation of the electronic properties, which gave birth to a largevariety of electronic devices. There are distinct electrical andelectro-optical properties of the different semiconductormaterials, the dominant and most studied semiconductorbeing silicon.For more than ten years, the optical analogues to electronicsemiconductors, the so-called photonic crystals, have been thesubject of intense international research efforts. Photoniccrystals are materials with a periodically varying index ofrefraction. This allows the control of the propagation of elec-tromagnetic waves, similar to electrons in a semiconductorcrystal. By analogy with semiconductors, the periodicity of theunderlying lattice structure is of the same order of magnitudeas the wavelength of the electromagnetic radiation.Despite the far-reaching analogies between electronicwaves in semiconductors and electromagnetic waves in pho-tonic crystals, there are pronounced differences between thetwo as is noticeable from the corresponding equations of mo-tion. Electrons are described by a scalar wavefield. In con-trast, the electromagnetic field is vectorial by nature. Further-more, the time-independent Schrodinger equation allowssolutions with negative energy eigenvalues, whereas the corre-sponding wave equation in electrodynamics contains only thesquare of the eigenfrequencies, hence negative eigenvaluesare excluded from the outset. It may be inferred from the fewphotonic crystals that appear in nature, in contrast to ubiqui-tous semiconductor materials, that these differences have adisadvantageous effect on the likelihood of the formation ofphotonic bandgaps. From the multitude of the optical phe-nomena only, for example, the colorful speckles of opals, somecrystallites on the wings of butterflies and the spine of the sea-mouse

White light emitting phosphor blends for led devices

Abstract: There is provided a blue-green illumination system, comprising a light emitting diode (11), and at least one luminescent material (21) having at least two peak emission wavelengths, wherein the emission CIE color coordinates of the at least two peak emission wavelengths are located within an area of a pentagon on a CIE chromaticity diagram, whose corners have the following CIE color coordinates: e) x=0.0137 and y=0.4831; b) x=0.2240 and y=0.3890; c) x=0.2800 and y=0.4500; g) x=0.2879 and y=0.5196; and h) x=0.0108 and y=0.7220. The illumination system (51) may be used as the green light of a traffic signal (41). The luminescent material may be a blend of (Ba1-xEux)Mg2Al16O27 ('BAM') and (Ba1-xEux)Mg2-yMnyAl16O27 ('BAMMn') phosphors, where 0

Near-Field Distribution of Optical Transmission of Periodic Subwavelength Holes in a Metal Film

Abstract: Recent experimental discovery of the enhanced optical transmission through metal films with periodic subwavelength holes has given rise to a considerable interest in the optical properties of such structures due to their possible numerous applications in optics and optoelectronics as well as rich physics behind the phenomenon of the transmission enhancement [1–4]. The transmission of a subwavelength aperture is very low and proportional to the fourth power of the ratio of its diameter and light wavelength. However, if a metal film is perforated with a periodic array of such holes, the optical transmission can be significantly enhanced [1]. Being normalized to the total area of the illuminated holes, the transmission coefficient corresponds to an enhancement up to 3 orders of magnitude compared to the transmission of the same number of individual holes. This enhancement depends on the array geometry (hole diameter and periodicity), light wavelength, angle of incidence, as well as material of a film.

Electrohydrodynamic instabilities in polymer films

Abstract: We have studied the influence of electric fields on highly viscous polymer films An electrohydrodynamic (EHD) instability causes a wave pattern with a characteristic wavelength λ, leading to an array of polymer columns which span the gap of a capacitor device When represented as a master curve, the data is quantitatively described by an EHD model, without any adjustable parameters Our results suggest that EHD experiments using polymer films are well suited to study non-equilibrium pattern formation in quasi-two-dimensional systems

Shape of the particulate beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution.

Abstract: The link between the spectral shape of the beam attenuation spectrum and the shape of the particle size distribution (PSD) of oceanic particles is revisited to evaluate the extent to which one can be predicted from the other. Assuming a hyperbolic (power-law) PSD, N(D) ∝ D-ξ, past studies have found for an infinite distribution of nonabsorbing spheres with a constant index of refraction that the attenuation spectrum is hyperbolic and that the attenuation spectral slope γ is related to the PSD slope ξ by ξ = γ + 3. Here we add a correction to this model because of the finite size of the biggest particle in the population. This inversion model is given by ξ = γ + 3 - 0.5 exp(-6γ). In most oceanic observations ξ > 3, and the deviation between these two models is negligible. To test the robustness of this inversion, we perturbed its assumptions by allowing for populations of particles that are nonspherical, or absorbing, or with an index of refraction that changes with wavelength. We found the model to provide a good fit for the range of parameters most often encountered in the ocean. In addition, we found that the particulate attenuation spectrum, cp(λ), is well described by a hyperbolic relation to the wavelength cp ∝ λ-γ throughout the range of the investigated parameters, even when the inversion model does not apply. This implies that knowledge of the particulate attenuation at two visible wavelengths could provide, to a high degree of accuracy, the particulate attenuation at other wavelengths in the visible spectrum.

Increase of the SBS threshold in a short highly nonlinear fiber by applying a temperature distribution

Abstract: We evaluate numerically and experimentally the stimulated Brillouin scattering (SBS) threshold increase for a short, highly nonlinear GeO/sub 2/-doped fiber by applying different temperature distributions along the fiber. The temperature coefficient for the Brillouin frequency downshift is measured to 1.2 MHz//spl deg/C. A threefold SBS threshold increase is obtained for a 100-m long highly nonlinear fiber with a 140/spl deg/C temperature gradient. The proposed scheme is implemented in a wavelength converter based on fiber optical four-wave mixing (FWM). The SBS suppression scheme shows negligible influence on the FWM efficiency as well as the wavelength conversion bandwidth. The temperature coefficient for the zero dispersion wavelength is measured to 0.062 nm//spl deg/C.

Solar wind magnetic fluctuation spectra: Dispersion versus damping

Abstract: Solar wind magnetic fluctuation power spectra at frequencies ƒ < 1 Hz are commonly observed to have the approximate power law dependence ƒ−5/3. These observations may be described by Kolmogorov diffusion in wavenumber space which defines what is called the inertial range. At wavelengths shorter than the inertial range, spectra often are observed to have steeper power laws. This intermediate wavelength regime is sometimes called the dissipation range because it has been suggested that this steeper slope is caused by collisionless damping of Alfven and magnetosonic waves. Here it is argued that, at intermediate wavenumbers, Alfven fluctuations are suppressed by proton cyclotron damping, so the observed power spectra are likely to consist of weakly damped magnetosonic/whistler waves which have an increased wavenumber diffusion rate due to their dispersion. Numerical calculations at βp ≲ 2.5 with a model representing this picture yield fluctuation spectra with steep power laws at intermediate wavenumbers and with sharp cutoffs due to electron cyclotron damping at still shorter wavelengths. The term “dispersion range” is more appropriate to describe steep power law spectra in the intermediate wavenumber regime.

Force field particle filter, combining ultrasound standing waves and laminar flow

Abstract: A continuous flow microparticle filter that combines megahertz frequency ultrasonic standing waves and laminar flow is described. The filter has a 0.25 mm, single half wavelength, acoustic pathlength at right angles to the flow. Standing wave radiation pressure on suspended particles drives them towards the centre of the acoustic pathlength. Clarified suspending phase from the region closest to the filter wall is drawn away through a downstream outlet. Experimental tests achieved >1000-fold clearance of 5 μm yeast cells, at a sample flow rate of 6 ml min −1 , from which the clarified aliquot is 1 ml min −1 . At this flow rate the average residence time in the sound field was 1 h was less than 1 K. The design criteria considered in the fabrication of this high performance device are discussed. A theoretical model of the filter’s efficiency, which considers the action of primary radiation force and the particle distribution across a laminar flow profile is presented here. The model predicts that totally clarified filtrate (i.e. zero suspended particles) may be drawn from the downstream outlet. The system described offers a generic approach to automated filtration in some applications. It is continuous flow thereby solving many of the problems of automation presented by batch filter methods and centrifuges. It could be developed for both larger scale and microfluidic applications.

Ice absorption features in the 5-8 μm region toward embedded protostars

Abstract: We have obtained 5{8 m spectra towards 10 embedded protostars using the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO-SWS) with the aim of studying the composition of interstellar ices. The spectra are dominated by absorption bands at 6.0 m and 6.85 m. The observed peak positions, widths and relative intensities of these bands vary dramatically along the dierent lines of sight. On the basis of comparison with laboratory spectra, the bulk of the 6.0 m absorption band is assigned to amorphous H2O ice. Additional absorption, in this band, is seen toward 5 sources on the short wavelength wing, near 5.8 m, and the long wavelength side near 6.2 m. We attribute the short wavelength absorption to a combination of formic acid (HCOOH) and formaldehyde (H2CO), while the long wavelength absorption has been assigned to the C{C stretching mode of aromatic structures. From an analysis of the 6.85 m band, we conclude that this band is composed of two components: a volatile component centered near 6.75 m and a more refractory component at 6.95 m. From a comparison with various temperature tracers of the thermal history of interstellar ices, we conclude that the two 6.85 m components are related through thermal processing. We explore several possible carriers of the 6.85 m absorption band, but no satisfactory identication can be made at present. Finally, we discuss the possible implications for the origin and evolution of interstellar ices that arise from these new results.

PHARO: A Near‐Infrared Camera for the Palomar Adaptive Optics System

Abstract: We describe Cornell's near-infrared camera system PHARO (Palomar High Angular Resolution Observer) built for use with the JPL Palomar Adaptive Optics System on the 5 m Hale telescope. PHARO uses a HgCdTe HAWAII detector for observations between 1 and 2.5 mm wavelength. An all-reflecting 1024 # 1024

Dramatic extension of the high-order harmonic cutoff by using a long-wavelength driving field

Abstract: We present an experimental demonstration of extending the high-order harmonic cutoff photon energy more than a factor of 2 when the driving-field wavelength is changed from 0.8 to 1.51 μm with an optical parametric amplifier. With argon gas, the cutoff has been extended from 64 to 160 eV. We predict that coherent keV x rays can be generated by exciting helium gas with the long-wavelength driving pulses. Experiments on xenon gas with several pump wavelengths also showed the dramatic cutoff extension, as well as full tunability of the generated XUV wavelengths.

Self-Focusing and Defocusing in Waveguide Arrays

Abstract: We show that two regimes of diffraction exist in arrays of waveguides, depending upon the input conditions. At higher powers, normal diffraction leads to self-focusing and to the formation of bright solitons through the nonlinear Kerr effect. By slightly changing the input conditions, light experiences anomalous diffraction and is nonlinearly defocused. For the first time, self-focusing and self-defocusing have been achieved for the same medium, structure, and wavelength.

Production of ordered silicon nanocrystals by low-energy ion sputtering

Abstract: We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si(100) substrate with 1.2 keV Ar ions at normal incidence. Atomic Force and High-Resolution Transmission Electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 hours) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to the sputtering yield dependence on the local surface curvature

Thermal radiation from two-dimensionally confined modes in microcavities

Abstract: A two-dimensional array of a microcavity with a high aspect ratio is made on a Cr-coated Si surface using the micromachining technology. The thermal emission spectra whose wavelength is close to the dimension of cavity aperture (5 μm) are measured on samples with a different aspect ratio. The clear selective emission bands corresponding to the two-dimensionally confined electromagnetic modes are demonstrated experimentally. It is found that the low emissivity of the base material is essential to obtain the high spectral selectivity of thermal radiation. The direction and polarization properties are also examined. The dominant peaks of the emission spectra can be explained by a simple cavity resonator model.

First confirmation that water ice is the primary component of polar mesospheric clouds

Abstract: Polar mesospheric clouds (PMCs) have been measured in the infrared for the first time by the Halogen Occultation Experiment (HALOE). PMC extinctions retrieved from measurements at eight wavelengths show remarkable agreement with model spectra based on ice particle extinction. The infrared spectrum of ice has a unique signature, and the HALOE-model agreement thus provides the first physical confirmation that water ice is the primary component of PMCs. PMC particle effective radii were estimated from the HALOE extinctions based on a first order fit of model extinctions.

Stochastic ion heating at the magnetopause due to kinetic Alfvén waves

Abstract: The magnetopause and boundary layer are typically characterized by large amplitude transverse wave activity with frequency below the ion cyclotron frequency. The signatures of the transverse waves suggest that they are kinetic Alfven waves with wavelength on the order of the ion gyroradius. We investigate ion motion in the presence of large amplitude kinetic Alfven waves with wavelength the order of rho(subscript ''i'') and demonstrate that for sufficiently large wave amplitude (delta B(subscript ''perpendicular'')/B(subscript ''0'') > 0.05) the particle orbits become stochastic. As a result, low energy particles in the core of the ion distribution can migrate to higher energy through the stochastic sea leading to an increase in T(subscript ''perpendicular'') and a broadening of the distribution. This process can explain transverse ion energization and formation of conics which have been observed in the low-latitude boundary layer.

Holey optical fibres

Abstract: The percentage fraction of fundamental mode power located in the cladding holes of different holey fibres (PFholes) is shown as a function of wavelength in microns of the fundamental mode (μ). Properties of two groups of holey fibres are shown. The upper group of three curves shows embodiments of the invention with Μ=0.75 νm and d/Μ=0.6, 0.7 & 0.8 respectively, where d is the hole diameter and Μ the hole spacing or pitch. The lower group of curves, which are almost superimposed on each other, show properties of holey fibres representative of the prior art with Μ=3.2 νm and d/Μ=0.6, 0.7 & 0.8 respectively. A huge improvement in the mode power present in the holes is evident. In the prior art curves, the mode power fraction is generally less than 1%, whereas with the illustrated embodiments of the invention, holey fibres with 10-40% of the fundamental mode power in the holes are achieved. Generally the holey fibre should be structured such that the wavelength of the guided light μ > 2.2Μ. For telecommunications wavelengths, this means that the hole spacing should be smaller than typical in the prior art, i.e. around 1νm or less, and the hole diameter should be as large as possible in relation to the hole spacing, preferably d/Μ > =0.6.

Quantitative analysis of bending efficiency in photonic-crystal waveguide bends at lambda = 1.55 mum wavelengths.

Abstract: Based on a photonic-crystal slab structure, a 60° photonic-crystal waveguide bend is successfully fabricated. Its bending efficiency within the photonic bandgap is measured, and near 100% efficiency is observed at certain frequencies near the valence band edge. The bending radius is ∼1 μm at a wavelength of λ∼1.55 μm. The measured η spectrum also agrees well with a finite-difference time-domain simulation.

Periodic light coupler gratings in amorphous thin film solar cells

Abstract: Efficient light trapping structures for amorphous hydrogenated silicon (a-Si:H) solar cells have been realized using periodically structured aluminum doped zinc oxide (ZnO:Al) with periods between 390 and 980 nm as a transparent front contact. Atomic force microscopy, optical reflection, and diffraction efficiency measurements were applied to characterize solar cells deposited on such gratings. A simple formula for the threshold wavelength of total internal reflection is derived. Periodic light coupler gratings reduce the reflectance to a value below 10% in the wavelength range of 400–800 nm which is comparable to cells with an optimized statistical texture. Diffraction efficiency measurements and theoretical considerations indicate that a combination of transmission and reflection gratings contribute to the observed reduction of the reflectance.

Light emitting diode (LED) device that produces white light by performing phosphor conversion on all of the primary radiation emitted by the light emitting structure of the LED device

Abstract: Presented is an LED device that produces white light by performing phosphor conversion on substantially all of the primary light emitted by the light emitting structure of the LED device. The LED device comprises a light emitting structure and at least one phosphor-converting element located to receive and absorb substantially all of the primary light. The phosphor-converting element emits secondary light at second and third wavelengths that combine to produce white light. Some embodiments include an additional phosphor-converting element, which receives light from a phosphor-converting element and emits light at a fourth wavelength. In the embodiments including an additional phosphor-converting element, the second, third, and fourth wavelengths combine to produce white light. Each phosphor-converting element includes at least one host material doped with at least one dopant. The phosphor-converting element may be a phosphor thin film, a substrate for the light emitting structure, or a phosphor powder layer.

Optical binding of particles with or without the presence of a flat dielectric surface

Abstract: Optical fields can induce forces between microscopic objects, thus giving rise to different structures of matter. We study theoretically these optical forces between two spheres, either isolated in water, or in the presence of a flat dielectric surface. We observe different behavior in the binding force between particles at large and at small distances (in comparison with the wavelength) from each other. This is due to the great contribution of evanescent waves at short distances. We analyze how the optical binding depends on the size of the particles, the material composing them, the wavelength, and, above all, the polarization of the incident beam. We also show that depending on the polarization the force between small particles at small distances changes its sign. Finally, the presence of a substrate surface is analyzed, showing that it only slightly changes the magnitudes of the forces, but not their qualitative nature, except when one employs total internal reflection, in which case the particles are induced to move together along the surface.

Sound Emission due to Superfluid Vortex Reconnections

Abstract: By performing numerical simulations based on the Gross-Pitaevskii equation, we make direct quantitative measurements of the sound energy released due to superfluid vortex reconnections. We show that the energy radiated expressed in terms of the loss of vortex line length is a simple function of the reconnection angle. In addition, we study the temporal and spatial distribution of the radiation and show that energy is emitted in the form of a sound pulse with a wavelength of a few healing lengths.

Microcavities in polymeric photonic crystals

Abstract: We report the fabrication and characteristics of planar microcavities in a log-pile photonic crystal structure formed using light-induced photopolymerization of resin. A planar defect was introduced into the middle of the log-pile structure as a single layer with every second rod missing. The existence of confined cavity states was confirmed experimentally and by numeric simulations. The cavity resonance found at the midgap wavelength λM∼4.0 μm had a quality factor of about 130.

Scattering optics of foam

Abstract: The multiple scattering of light by aqueous foams is systematically studied as a function of wavelength, bubble size, and liquid fraction. Results are analyzed in terms of the transport mean free path of the photons and an extrapolation length ratio for the diffuse photon concentration field. The wavelength dependence is minimal and may be attributed entirely to the wavelength dependence of the refractive index of water rather than thin-film interference effects. The transport mean free path is found to be proportional to the bubble diameter and the reciprocal of the square root of liquid fraction. The extrapolation length ratio varies almost linearly with liquid fraction between the values for water-glass-air and air-glass-air interfaces.

Formation of conical microstructures upon laser evaporation of solids

Abstract: The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼104) at laser fluence of 1–2 J/cm2 below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 105 Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti.

Dispersion and damping of a two-dimensional plasmon in a metallic surface-state band.

Abstract: We have studied, for the first time, the energy and the linewidth dispersion of a plasmon in a dense two-dimensional electron system in a metallic surface-state band on a silicon surface. As expected from the considerably high effective density and long Fermi wavelength of the system, the plasmon energy dispersion exhibited an excellent agreement with the nearly free-electron theory. However, in a small wave number region below the Landau edge, we have observed an anomalous linewidth dispersion which nearly free-electron theories do not predict.

An experimental study of two-dimensional surface water waves propagating on depth-varying currents. Part 1. Regular waves

Abstract: This paper describes an experimental study of two-dimensional surface water waves propagating on a depth-varying current with a non-uniform vorticity distribution. The investigation is divided into two parts. The first concerns the ‘equilibrium’ conditions in which the oscillatory wave motion and the current co-exist. Measurements of the water-surface elevation, the water-particle kinematics, and the near-bed pressure fluctuations are compared to a number of wave and wave–current solutions including a nonlinear model capable of incorporating the vertical structure of the current profile. These comparisons confirm that the near-surface vorticity leads to an important modification of the dispersion equation, and thus affects the nature of the wave-induced orbital motion over the entire water depth. However, the inclusion of vorticity-dependent terms within the dispersion equation is not sufficient to define the combined wave–current flow. The present results suggest that vorticity may lead to a significant change in the water-surface profile. If a current is positively sheared, dU/dz > 0, with negative vorticity at the water surface, as would be the case in a wind-driven current, a wave propagating in the same direction as the current will experience increased crest–trough asymmetry due to the vorticity distribution. With higher and sharper wave crests there is a corresponding increase in both the maximum water-particle accelerations and the maximum horizontal water-particle velocities. These results are consistent with previous theoretical calculations involving uniform vorticity distributions (Simmen & Saffman 1985 and Teles da Silva & Peregrine 1988).The second part of the study addresses the ‘gradually varying’ problem in which there are changes in the current, the wavelength and the wave height due to the initial interaction between the wave and the current. These data show that there is a large and non-uniform change in the current profile that is dependent upon both the steepness of the waves and the vorticity distribution. Furthermore, comparisons between the measured wave height change and a number of solutions based on the conservation of wave action, confirm that the vorticity distribution plays a dominant role. In the absence of a conservation equation for wave action appropriate for nonlinear waves on a depth-varying current, an alternative approach based on the conservation of total energy flux, first proposed by Longuet-Higgins & Stewart (1960), is shown to be in good agreement with the measured data.