Showing papers on "Doppler broadening published in 2006"
TL;DR: This paper demonstrates experimentally that Brillouin slow light with an arbitrary large bandwidth can be readily obtained in conventional optical fibers using a simple and inexpensive pump spectral broadening technique and sees no limit to extend this technique to the delaying of GHz-bandwidth signals.
Abstract: Brillouin slow light in optical fibers is a promising technique for the development of all-optical buffers to be used in optical routers. The main drawback of this technique up to now has been its narrow bandwidth, normally restricted to 35 MHz in conventional single-mode optical fibers. In this paper we demonstrate experimentally that Brillouin slow light with an arbitrary large bandwidth can be readily obtained in conventional optical fibers using a simple and inexpensive pump spectral broadening technique. In our experiments, we show the delaying of 2.7 ns pulses over slightly more than one pulse length with only some residual broadening (<25%) of the pulse width. We see no limit to extend this technique to the delaying of GHz-bandwidth signals.
279 citations
IBM1
TL;DR: Comparison with theory indicates that an observed twofold increase of spectral broadening can be assigned to the dispersion of n(2) as well as first order- rather than second-order dispersion effects.
Abstract: We measure the transmission of ps-pulses through silicon-on-insulator submicron waveguides for excitation wavelengths between 1400 and 1650 nm and peak powers covering four orders of magnitude. Self-phase-modulation induced spectral broadening is found to be significant at coupled peak powers of even a few tens of mW. The nonlinear-index coefficient, extracted from the experimental data, is estimated as n(2) ~ 5*10(-18) m(2)/W at 1500 nm. The experimental results show good agreement with model calculations that take into account nonlinear phase shift, first- and second order dispersion, mode confinement, frequency dispersion of n(2), and dynamics of two-photon-absorption-generated free carriers. Comparison with theory indicates that an observed twofold increase of spectral broadening between 1400 and 1650 nm can be assigned to the dispersion of n(2) as well as first order- rather than second-order dispersion effects. The analysis of pulse broadening, spectral shift and transmission saturation allows estimating a power threshold for nonlinearity-induced signal impairment in nanophotonic devices.
225 citations
01 Jan 2006
TL;DR: In this article, the properties of highly non-linear optical fibers are discussed and four kinds of fibers that are developed to enhance the nonlinear effects are also reviewed, including self-phase modulation (SPM), XPM, and four-wave mixing (FWM).
Abstract: Publisher Summary
This chapter deals with the properties of highly non-linear fibers. It reviews the techniques used to measure the non-linear parameter (γ). The four kinds of fibers that are developed to enhance the non-linear effects are also reviewed. The three major non-linear effects occurring inside the optical fibers—the self-phase modulation (SPM), the cross-phase modulation (XPM), and the four-wave mixing (FWM), are governed by a single non-linear parameter γ. This parameter is fixed for each glass material. The chapter elaborates that the use of the non-silica glasses provides an alternative approach to designing non-linear fibers. The SPM technique uses the broadening of the pulse spectrum. It measures the maximum value of the non-linear phase shift. In place of deducing the SPM-induced phase shift from spectral broadening, a related technique deduces it from the spectral changes occurring when light from two lasers operating at slightly different wavelengths is transmitted through the fiber. Measurements in such an approach can be performed with CW lasers. The chapter summarizes that the combination of unusual dispersive properties and a high value of γ leads to a variety of novel non-linear effects.
150 citations
TL;DR: In this paper, the effect of space charge on photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source has been investigated, focusing on a quantitative evaluation of the induced spectral broadening.
Abstract: In this paper, we present experimental results on the effect of space charging in photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source. We particularly focus on a quantitative evaluation of the induced spectral broadening. Our results are compared with analytic calculations based on energy conservation considerations as well as with experimental results from measurements using picosecond pulses for the excitation process. As a measure of space charge effects, we monitored the angular and energy distributions of the photoemission from the occupied Shockley surface state of Cu(111) as a function of the total number N of the photoemitted electrons per laser pulse. Our results show that spectral distortions exist for the entire laser fluence regime probed. The energetic broadening of the surface state peak can be fitted with remarkable accuracy by a N dependence, in agreement with the theoretical predictions and different from the experimental picosecond results, where...
140 citations
TL;DR: In this paper, the authors overview the recent development in applications of spectral broadening and supercontinuum generation in the field of optical communications, and special attention is dedicated to recent results obtained in their research groups.
138 citations
TL;DR: The results indicate that increasing the spectral linewidth of the pump can decrease the supercontinuum spectral width, and using a fiber with smaller dispersion at the pump wavelength reduces the required fiber length by decreasing the temporal width of the solitons formed from modulation instability.
Abstract: We numerically investigate supercontinuum generation using continuous-wave pumping. It is found that energy transfer during collision of solitons plays an important role. The relative influence of Raman gain on spectral broadening is shown to depend on the width of the calculation time window. Our results indicate that increasing the spectral linewidth of the pump can decrease the supercontinuum spectral width. Using a fiber with smaller dispersion at the pump wavelength reduces the required fiber length by decreasing the temporal width of the solitons formed from modulation instability. This also reduces the sensitivity to the pump spectral linewidth.
129 citations
TL;DR: In this paper, a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids is presented, based on the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states.
Abstract: Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material. Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement electron-positron correlation schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancytype defect on the ionic relaxations. DOI: 10.1103/PhysRevB.73.035103
96 citations
TL;DR: In this article, a single-mode cavity ring-down spectrometer was used to study rovibrational absorption line shapes within the and vibrational bands of water vapor, which has a noise-equivalent absorption coefficient of and frequency resolution of, enabling high-precision measurements of line shape effects and pressure shifting of relatively weak absorption transitions.
Abstract: A single-mode cavity ring-down spectrometer, which incorporates a stabilized and tuneable comb of resonant frequencies and a continuous-wave external-cavity diode probe laser, was used to study rovibrational absorption line shapes within the and vibrational bands of water vapor. This spectrometer, which has a noise-equivalent absorption coefficient of and frequency resolution of , enables high-precision measurements of line-shape effects and pressure shifting of relatively weak absorption transitions. We investigated the room-temperature pressure dependence over the range from of two transitions perturbed by He, , and . Foreign-gas broadening and pressure-shift coefficients were determined for a relatively strong transition at , and for a weaker transition at the self- and -broadening and pressure-shift parameters were measured. In the low-pressure limit the room-temperature Doppler width was measured within 0.2% of its expected value. Doppler-free saturation effects were also observed with linewidths below . The data were compared to semiclassical line-shape models that considered the influence of Dicke narrowing as well as the speed dependence of pressure broadening and pressure shifting. Taking both of these effects into account gave the best agreement with our observations and allowed us to model the observed asymmetries of experimental profiles. Hard- and soft-collision as well as billiard-ball collision models were considered. These results allowed us to quantify systematic errors in line intensity and in pressure broadening associated with oversimplified models of line shape.
93 citations
TL;DR: In this paper, the authors consider group delay and broadening using two strongly absorbing and widely spaced resonances and derive relations which show that very large pulse bandwidths coupled with large group delays and small broadening can be achieved.
Abstract: We consider group delay and broadening using two strongly absorbing and widely spaced resonances. We derive relations which show that very large pulse bandwidths coupled with large group delays and small broadening can be achieved. Unlike single resonance systems, the dispersive broadening dominates the absorptive broadening which leads to a dramatic increase in the possible group delay. We show that the double resonance systems are excellent candidates for realizing all-optical delay lines. We report on an experiment which achieved up to 50 pulse delays with 40% broadening.
90 citations
TL;DR: In this paper, the authors derived a general integral solution for the radiation field around resonance within the usual Fokker-Planck approximation, and showed explicitly that recoil and spin diffusivity source an absorption feature, whose magnitude increases with the relative importance of recoil compared to Doppler broadening.
Abstract: We re-examine scattering of photons near the Lyα resonance in the intergalactic medium (IGM). We first derive a general integral solution for the radiation field around resonance within the usual Fokker–Planck approximation. Our solution shows explicitly that recoil and spin diffusivity source an absorption feature, whose magnitude increases with the relative importance of recoil compared to Doppler broadening. This spectrum depends on the Lyα line profile, but approximating it with the absorption profile appropriate to the Lorentzian wings of natural broadening accurately reproduces the results for a full Voigt profile so long as the IGM temperature is less than ~1000 K. This approximation allows us to obtain simple analytic formulae for the total scattering rate of Lyα photons and the accompanying energy exchange rate. Our power series solutions converge rapidly for photons that redshift into the Lyα resonance as well as for photons injected at line centre. We confirm previous calculations showing that heating through this mechanism is quite slow and probably negligible compared to other sources. We then show that energy exchange during the scattering of higher-order Lyman-series photons can be much more important than naively predicted by recoil arguments. However, the resulting heating is still completely negligible.
89 citations
TL;DR: In this article, it was shown that the parametric instability driven by ion sound quasimodes, which occur in the scrape-off plasma layer located near the antenna mouth, produces a significant broadening of the launched LH spectrum.
Abstract: In order to explain the results of the non-inductive current produced in the lower hybrid current drive (LHCD) experiments, a broadening of the radiofrequency (RF) power spectrum coupled to tokamak plasma needs to occur. The presented modelling, supported by diagnostic measurements, shows that the parametric instability (PI) driven by ion sound quasimodes, which occur in the scrape-off plasma layer located near the antenna mouth, produces a significant broadening of the launched LH spectrum. Considering the parameters of LHCD experiments of JET (Joint European Torus), and other machines as well, the PI growth rate is high enough for producing the compensation of the convective losses and, consequently, the broadening of a small fraction (of the order of 10%) of the launched power spectrum. Such a phenomenon is identified to be intrinsic to the RF power coupling in the LHCD experiments. As the principal implication of considering such spectral broadening in modelling the LH deposition profile, experiments of LHCD-sustained internal transport barriers in JET were successfully interpreted, which evidenced the effects of a well-defined LH deposition profile. The present work is important for addressing the long-lasting debate on the problem of the so-called spectral gap in LHCD. The design of LHCD scenarios relevant to the modern fusion research programme, an important requirement of which is the control of the plasma current profile in the outer half of plasma, can be properly achieved by considering PI-induced spectral broadening.
TL;DR: In this paper, a single atom was trapped in an optical dipole trap and the maximum number of captured atoms was locked to one by light-induced two-body collisions during the loading stage of the trap, which is confirmed by the observed photon antibunching in the detected fluorescence light.
Abstract: We describe a simple experimental technique which allows us to store a single $^{87}\mathrm{Rb}$ atom in an optical dipole trap. Due to light-induced two-body collisions during the loading stage of the trap the maximum number of captured atoms is locked to one. This collisional blockade effect is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom were studied with a narrow-band scanning cavity. We find that the atomic fluorescence spectrum is dominated by the spectral width of the exciting laser light field. In addition we observe a spectral broadening of the atomic fluorescence light due to the Doppler effect. This allows us to determine the mean kinetic energy of the trapped atom corresponding to a temperature of $105\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. This simple single-atom trap is the key element for the generation of atom-photon entanglement required for future applications in quantum communication and a first loophole-free test of Bell's inequality.
TL;DR: It is shown both theoretically and experimentally that the quasi-degenerate four-wave mixing between different longitudinal modes is the main broadening mechanism in the one-stage RFL at high powers.
Abstract: We present an analytical theory based on wave kinetic equations that describes a Raman fiber laser (RFL) generation spectrum. It is shown both theoretically and experimentally that the quasi-degenerate four-wave mixing between different longitudinal modes is the main broadening mechanism in the one-stage RFL at high powers. The shape and power dependence of the intracavity Stokes wave spectrum are in excellent quantitative agreement with predictions of the theory.
TL;DR: In this paper, a method for the calculation of spectral line broadening in plasma was developed and implemented based on a numerical simulation of the motion of the interacting plasma particles (both ions and electrons) and the use of the resulting time-dependent field to obtain the evolution of the radiator system.
Abstract: A method for the calculation of spectral line broadening in plasma has been developed and implemented. The method is based on a numerical simulation of the motion of the interacting plasma particles (both ions and electrons) and the use of the resulting time-dependent field to obtain the evolution of the radiator system. The Fourier transform of the resulting radiator time-dependent dipole function then gives the spectral line shape. This approach thus naturally accounts for all frequency regions of the plasma-particle fields and for the effects of the particle interactions on the fields. Due to a rather general approach used for solving the Schrodinger equation, the method is applicable to line-shape calculations of isolated and overlapping spectral lines involving both dipole-allowed and dipole-forbidden radiative transitions. In addition, line shapes under a simultaneous influence of externally-applied (constant or time-dependent) electric and magnetic fields can be calculated in a self-consistent manner, and polarization properties of the emitted light, caused by such external fields, can be investigated. Part of the method capabilities is demonstrated. Results presented are for spectral lines of H- and He-like ions of C, Si, and Ar in non-magnetized plasmas. It is found that ion dynamics contributes to the line broadening significantly, in several cases exceeding the electron impact widths by a few times. Also, the interactions between the radiator and the perturbers cause a significant reduction in the line widths, even for overall-weakly-coupled plasmas; a relation between this effect and the radiator–perturber coupling is made.
TL;DR: Suppression of electroosmotic flow is proposed as a means of reducing micro-FFE band broadening due to hydrodynamic effects and maximizing resolution and peak capacity.
Abstract: The broadening mechanisms for micro-free flow electrophoresis (μ-FFE) have been investigated using a van Deemter analysis. Separation power, the product of electric field and residence time, is presented as a parameter for predicting the position of sample streams and for comparing separations under different conditions. Band broadening in μ-FFE is governed by diffusion at lower linear velocities and a migration distance-dependent mechanism at higher linear velocities. At higher linear velocities, the parabolic flow profile is elongated, generating a distribution of analyte residence times in the separation channel. This distribution of residence times gives rise to a distribution of migration distances in the lateral direction since analytes spend different amounts of time in the electric field. Equations were derived to predict the effect of electric field and buffer flow rate on broadening. Experimental data were collected to determine whether the derived equations were useful in explaining broadening ...
TL;DR: This work investigates experimentally and theoretically the broadening of the optical spectrum of a multimode cw field propagating in the normal dispersion regime of a single-mode fiber.
Abstract: We investigate experimentally and theoretically the broadening of the optical spectrum of a multimode cw field propagating in the normal dispersion regime of a single-mode fiber. The width of the optical spectrum is not a monotonic function of propagation length. This behavior arising from the interplay between the Kerr effect and group-velocity dispersion contrasts with spectral broadening of mode-locked pulses.
TL;DR: This report reports on the first spectroscopic observation of the rotational Doppler shift associated with light beams carrying orbital angular momentum when the two incident Laguerre-Gaussian laser beams have opposite topological charges.
Abstract: We report on the first spectroscopic observation of the rotational Doppler shift associated with light beams carrying orbital angular momentum. The effect is evidenced as the broadening of a Hanle electromagnetically induced transparency coherence resonance on Rb vapor when the two incident Laguerre-Gaussian laser beams have opposite topological charges. The observations closely agree with theoretical predictions.
TL;DR: A simple formula is presented by which the shape of the absorption spectrum of an aggregate of quantum "monomers" interacting via dipole-dipole forces can be calculated from the averaged spectrum of the quantum monomer itself.
Abstract: We present a simple formula by which the shape of the absorption spectrum of an aggregate of quantum "monomers" (cold atoms, molecules, quantum dots, nanoparticles, etc.) interacting via dipole-dipole forces can be calculated from the averaged spectrum of the quantum monomer itself. Spectral broadening, due to a wide variety of causes, is included explicitly so that the formula is applicable not only to the idealization of a discrete spectrum but also to the practical situation of a continuously broadened spectrum. In simple cases, analytic results are obtained showing the strong dependence of the aggregate spectrum on the precise nature of the broadening of the quantum monomer spectrum. The formula is compared with results of exact diagonalization of model aggregate Hamiltonians and with experiment.
TL;DR: In this paper, the problem of accurately representing the temperature dependence of neutron cross-section data in neutron transport problems when there are many nuclides and when the temperature distributions vary significantly with both space and time is investigated.
Abstract: This work considers the problem of accurately representing the temperature dependence of neutron cross-section data in neutron transport problems when there are many nuclides and when the temperature distributions vary significantly with both space and time. An approach involving interpolation between nuclear data libraries at various reference temperatures is investigated. Reference nuclear data libraries are obtained by Doppler broadening cross sections to the desired temperatures using the NJOY code system. Several interpolation schemes over various temperature intervals are studied. Interpolated values at intermediate temperatures are compared to NJOY Doppler broadened results for the same temperature. Differences relative to the Doppler broadened results are calculated in order to judge the suitability of the interpolation scheme and temperature interval. The total, elastic scattering, capture, and fission (if applicable) reactions for {sup 238}U, {sup 235}U, natural Zr, {sup 16}O, {sup 10}B and {sup 1}H are considered in this study, over a temperature range of 294 K to 811 K ({approx}70 F to {approx}1000 F). The nuclides and temperature range are selected to best represent typical light water reactor calculations. This work covers only the free-atom cross section and does not explore the many nuances of temperature treatment of nuclear data in the thermalmore » energy range for nuclides where molecular binding effects are significant, e.g., water, beryllium, graphite. Additionally, dilute-average cross sections are used in the unresolved resonance range (URR) for this study. Temperature treatment of probabilistic methods used to construct cross sections in the URR are not considered for this work. The study shows that cross sections can be interpolated within an accuracy of 0.1% over a temperature interval of 111 K (200 F) for {sup 1}H, {sup 10}B, and {sup 16}O. Smaller intervals are required for nuclides with more complex resonance behavior. Some values of the interpolated cross sections for natural Zr, {sup 238}U and {sup 235}U, remain greater than the target 0.1% relative difference even with a 28 K (50 F) interval, suggesting a smaller interval is necessary for these nuclides.« less
TL;DR: In this article, the effect of Cu on the matrix damage accumulation under neutron irradiation was investigated by Coincidence Doppler Broadening of positron annihilation radiation (CDBAR), a well established tool for studying defects in materials.
TL;DR: In this paper, the authors investigated the information about neutron star structure and the geometry of the emission region that can be obtained by analyzing the profiles of atomic lines formed at the surface of the star.
Abstract: Motivated by the report by Cottam et al. of iron resonance scattering lines in the spectra of thermonuclear bursts from EXO 0748-676, we have investigated the information about neutron star structure and the geometry of the emission region that can be obtained by analyzing the profiles of atomic lines formed at the surface of the star. We have calculated the detailed profiles of such lines, taking into account the star's spin and the full effects of special and general relativity, including light bending and frame dragging. We discuss the line shapes produced by rotational Doppler broadening and magnetic splitting of atomic lines for the spin rates and magnetic fields expected in neutron stars in low-mass X-ray binary systems. We show that narrow lines are possible even for rapidly spinning stars if the emission region or the line of sight are close to the spin axis. For most neutron stars in low-mass systems, magnetic splitting is too small to obscure the effects of special and general relativity. We show that the ratio of the star's mass to its equatorial radius can be determined to within 5% using atomic line profiles, even if the lines are broad and skewed. This is the precision required to constrain strongly the equation of state of neutron star matter. We show further that if the radius and latitude of emission are known to ~5%-10% accuracy, then frame dragging has a potentially detectable effect on the profiles of atomic lines formed at the stellar surface.
TL;DR: In this paper, the spectral and spatial characteristics of gyrosynchrotron emission and the polarization of solar bursts in a highly inhomogeneous medium were analyzed using a modified version of Ramaty's code.
Abstract: Aims. In this study we have analyzed the spectral and spatial characteristics of gyrosynchrotron emission and the polarization of solar bursts in a highly inhomogeneous medium. Our main goal was to investigate the effects of the geometry of the source on the resulting spectrum. Methods. The SOHO/EIT and TRACE high resolution images revealed structured magnetic configurations in detail over solar active regions. In our method, we represent the magnetic field geometry by three-dimensional structures obtained from magnetic field extrapolation, tomography, or any geometry that resembles the observed structures. The gyrosynchrotron radiation was numerically calculated through a modified version of Ramaty’s code. Results. We calculated the radiation produced by non-thermal electron distributions in this complex environment and solved the radiative transfer equation. The results, presented in brightness distribution maps, polarization maps, and spectra, are discussed. Our numerical results agree with statistical analyses of observed spectra present in the literature. We note a spectral broadening due to the spatial and intensity inhomogeneity of the magnetic field and no center-to-limb variations, which cannot be explained by homogeneous source models. The computed maps revealed a non uniform brightness distribution, with small-scale structures, and different spatial characteristics in each frequency. Also, we found different spectral characteristics at different regions of the emitting source. Polarization maps reveal its high dependence on geometry and on the position of the source on the solar disk.
TL;DR: In this paper, several vacancy-solute complexes in iron are investigated theoretically from the viewpoint of positron annihilation and the results obtained are discussed in the context of experimental data available in the literature.
TL;DR: In this paper, the authors presented a new kind of broadband continuous-wave source which outperforms any other broadband superluminescent or amplified spontaneous emission source both in terms of output spectral density and bandwidth.
Abstract: We present a new kind of broadband continuous-wave source which outperforms any other broadband superluminescent or amplified spontaneous emission source both in terms of output spectral density and bandwidth. Our source covers the wavelength band of interest for fibre applications (from 1450 to 1625 nm) and has an output power of approximately 1.3 W. The source is obtained by pumping a conventional non-zero dispersion-shifted fibre with a continuous-wave Raman fibre laser tuned to the region of small anomalous dispersion of the fibre. The laser beam undergoes an extreme spectral broadening in the fibre. Our experimental results show clearly that the modulation instability (MI)-induced soliton fission is the key element leading to this spectral broadening. Modulation instability is seeded by fast intensity instabilities present in the laser output. We show that this source features good power stability and we believe that it might have very interesting applications in fibre sensing, for instance to avoid the need of amplification in the interrogation of remote Bragg gratings or to improve the resolution and dynamic range of optical coherence tomography setups.
TL;DR: In this paper, the nuclear quadrupole hyperfine structure of H14N12C was resolved in the laboratory for the first time using millimeter-wave absorption spectroscopy.
Abstract: The nuclear quadrupole hyperfine structure of H14N12C and D14N12C has been resolved in the laboratory for the first time using millimeter-wave absorption spectroscopy. The transient species were produced in a pulsed DC discharge nozzle, and Doppler broadening effects were minimized by propagating the millimeter waves coaxially with the supersonic molecular beam. New rest frequencies for the J = 1-0, J = 2-1, and J = 3-2 rotational transitions of the ground vibrational state were determined. The nuclear quadrupole coupling constants derived from the spectra are (eQq)N = 264.5 ± 4.6 kHz for H14N12C and (eQq)N = 294.7 ± 13.1 kHz and (eQq)D = 261.9 ± 14.5 kHz for D14N12C.
TL;DR: A generalized amplitude modulation signal model was developed to collectively account for the transit-time effects of the ultrasound beam geometry and the range gate characteristics and it was found that, for scatterers passing though the center of the sample volume, the broadening extent mainly depends on beamforming characteristics at higher beam-flow angles, but it is more dependent on range gate parameters at smaller angles.
Abstract: In Doppler ultrasound, transit-time broadening arises from the finite scatterer transit time through the sample volume. As a unifying description of this broadening mechanism, a generalized amplitude modulation signal model was developed to collectively account for the transit-time effects of the ultrasound beam geometry and the range gate characteristics. Simulations based on a pulsed linear-array system also were performed to study the broadening extent for different scatterer flow lines. With our signal model and simulation results, some generalized insights were obtained on the characteristics of transit-time broadening. First, as consistent with previous findings, we found that, for scatterers passing though the center of the sample volume, the broadening extent mainly depends on beamforming characteristics at higher beam-flow angles, but it is more dependent on range gate parameters at smaller angles. Second, for the central flow line, a transition angle exists in which a significant change occurs in the governing parameters of transit-time broadening. Third, for the general case in which scatterers undertake an off-central path through the sample volume, the broadening extent depends on both the beam geometry and the range gate. Bandwidth skewing and further spectral broadening also can be seen for these off-central flow lines
TL;DR: Stehle et al. as discussed by the authors analyzed Stark broadening of the lines corresponding to 2−n and 3−m transitions with principal quantum numbers n=7−12 and m=10−12 using recent model microfield method calculations.
Abstract: Spatially resolved measurements of deuterium Balmer and Paschen line emission have been performed in the divertor region of the National Spherical Torus Experiment using a commercial 0.5m Czerny-Turner spectrometer. While the Balmer emission lines, as well as the Balmer and Paschen continua in the ultraviolet and visible regions have been extensively used for tokamak divertor plasma temperature and density measurements, the diagnostic potential of infrared Paschen lines has been largely overlooked. We analyze Stark broadening of the lines corresponding to 2−n and 3−m transitions with principal quantum numbers n=7–12 and m=10–12 using recent model microfield method calculations [C. Stehle and R. Hutcheon, Astron. Astrophys., Suppl. Ser. 140, 93 (1999)]. Densities in the range (5–50)×1019m−3 are obtained in the recombining inner divertor plasma in 2–6MW neutral beam heated H-mode discharges. The measured Paschen line profiles show good sensitivity to Stark effects and low sensitivity to instrumental and Dop...
01 Jan 2006
TL;DR: Self-phase modulation (SPM) as mentioned in this paper is the phenomenon in which a laser beam propagating in a medium interacts with the medium and imposes a phase modulation on itself, resulting in self-diffraction.
Abstract: Self-phase modulation refers to the phenomenon in which a laser beam propagating in a medium interacts with the medium and imposes a phase modulation on itself. It is one of those very fascinating effects discovered in the early days of nonlinear optics (Bloembergen and Lallemand, 1966; Brewer, 1967; Cheung et al., 1968; Lallemand, 1966; Jones and Stoicheff, 1964; Shimizu, 1967; Stoicheff, 1963). The physical origin of the phenomenon lies in the fact that the strong field of a laser beam is capable of inducing an appreciable intensity-dependent refractive index change in the medium. The medium then reacts back and inflicts a phase change on the incoming wave, resulting in self-phase modulation (SPM). Since a laser beam has a finite cross -section, and hence a transverse intensity profile, SPM on the beam should have a transverse spatial dependence, equivalent to a distortion of the wave front. Consequently, the beam will appear to have self-diffracted. Such a self-diffraction action, resulting from SPM in space, is responsible for the well-known nonlinear optical phenomena of self-focusing and self-defocusing (Marburger, 1975; Shen, 1975). It can give rise to a multiple ring structure in the diffracted beam if the SPM is sufficiently strong (Durbin et al., 1981; Santamato and Shen, 1984). In the case of a pulsed laser input, the temporal variation of the laser intensity leads to an SPM in time. Since the time derivative of the phase of a wave is simply the angular frequency of the wave, SPM also appears as a frequency modulation. Thus, the output beam appears with a self-induced spectral broadening (Cheung et al., 1968; Gustafson et al., 1969; Shimizu, 1967).
21 May 2006
TL;DR: In this paper, a novel method for generating few-cycle pulses with extremely high peak powers >100 GW was experimentally demonstrated, which relies on plasma-induced spectral broadening and does not require any additional means for dispersion compensation.
Abstract: A novel method for generating few-cycle pulses with extremely high peak powers >100 GW is experimentally demonstrated. This method relies on plasma-induced spectral broadening and does not require any additional means for dispersion compensation.
TL;DR: In this paper, the influence of laser intensity on absorption line broadening was investigated, and the predicted variation in broadening width with laser intensity showed a good agreement with the measurement.
Abstract: The influence of laser intensity on absorption line broadening was investigated. Laser absorption spectroscopy was applied to low-pressure plasma, and the translational temperature deduced from the Doppler width was found to increase with laser intensity; this was in contrast to the conventional laser theory. Consequently, the dependency of absorption saturation on the Doppler frequency was considered. The predicted variation in broadening width with laser intensity showed a good agreement with the measurement. In addition, we obtained a correction factor for the temperature measurement at high laser intensity.