Temporal Intensity Profile Research Articles

Direct temporal reconstruction of picosecond pulse by cross-correlation in semiconductor device

Cross-correlation measurements using the two-photon absorption process in a semiconductor is experimentally demonstrated for two pulses of different wavelengths (shifted by ∼200 nm) and durations (20 times ratio). These measurements were found to be highly repeatable and fully suitable for the determination of the temporal intensity profile of picosecond (ps) pulses.

Self-referenced phase reconstruction proposal of GHz bandwidth non-periodical optical pulses by in-fiber semi-differintegration

We propose two new techniques able to retrieve the phase profile of a given temporal optical pulse based on the use of in-fiber semi-differintegral operators, where by semi-differintegration we mean either a 0.5th-order differentiation or integration. In both cases, the signal's temporal phase can be obtained by simple dividing two temporal intensity profiles, namely the intensities of the input and output pulses of a spectrally shifted semi-differintegral operator. In both cases, we obtained simple analytical expressions for the phase profile. The techniques are self-referenced and well-suited for real-time applications. We numerically prove the viability of these proposals.

All-optical nonlinear processing of both polarization state and intensity profile for 40 Gbit/s regeneration applications

In this paper, we report all-optical regeneration of the state of polarization of a 40-Gbit/s return-to-zero telecommunication signal as well as its temporal intensity profile and average power thanks to an easy-to-implement, all-fibered device. In particular, we experimentally demonstrate that it is possible to obtain simultaneously polarization stabilization and intensity profile regeneration of a degraded light beam thanks to the combined effects of counterpropagating four-wave mixing, self-phase modulation and normal chromatic dispersion taking place in a single segment of optical fiber. All-optical regeneration is confirmed by means of polarization and bit-error-rate measurements as well as real-time observation of the 40 Gbit/s telecommunication signal.

Coherent control of the vibrational state population in a nonpolar molecule

A coherent control scheme for the population distribution in the vibrational states of nonpolar molecules is proposed. Our theoretical analysis and results of numerical simulations for the interaction of the hydrogen molecular ion in its electronic ground state with an infrared laser pulse reveal a selective two-photon transition between the vibrational states via a coupling with the first excited dissociative state. We demonstrate that for a given temporal intensity profile the population transfer between vibrational states, or a superposition of vibrational states, can be made complete for a single chirped pulse or a train of chirped pulses, which accounts for the accumulated phase difference due to the ac Stark effect. Effects of a spatial intensity (or focal) averaging are discussed.

Generation of sub-3 fs pulses in the deep ultraviolet

We demonstrate generation and measurement of intense deep-ultraviolet light pulses with a duration of approximately 2.8 fs (FWHM of the intensity envelope) and a wavelength distribution between 230 and 290 nm. They emerge via direct frequency upconversion of sub-4 fs laser pulses of a carrier wavelength of approximately 750 nm focused into an Ne-filled, quasi-static gas cell. Dispersion-free, third-order autocorrelation measurements provide access to their temporal intensity profile.

Light storage in a doped solid enhanced by feedback-controlled pulse shaping

We report on experiments dealing with feedback-controlled pulse shaping to optimize the efficiency of light storage by electromagnetically induced transparency (EIT) in a Pr${}^{3+}$:Y${}_{2}$SiO${}_{5}$ crystal. A learning loop in combination with an evolutionary algorithm permits the automatic determination of optimal temporal profiles of intensities and frequencies in the driving laser pulses (i.e., the probe and coupling pulses). As a main advantage, the technique finds optimal solutions even in the complicated multilevel excitation scheme of a doped solid, involving large inhomogeneous broadening. The learning loop experimentally determines optimal temporal intensity profiles of the coupling pulses for a given probe pulse. The optimized intensity pulse shapes enhance the light-storage efficiency in the doped solid by a factor of $2$. The learning loop also determines a fast and efficient preparation pulse sequence, which serves to optically prepare the crystal prior to light-storage experiments. The optimized preparation sequence is 5 times faster than standard preparation sequences. Moreover, the optimized preparation sequence enhances the optical depth in the medium by a factor of $5$. As a consequence, the efficiency of light storage also increases by another factor of $3$. Our experimental data clearly demonstrate the considerable potential of feedback-controlled pulse shaping, applied to EIT-driven light storage in solid media.

Plasma induced pulse breaking in filamentary self-compression

A plasma induced temporal break-up in filamentary propagation has recently been identified as one of the key events in the temporal self-compression of femtosecond laser pulses. An analysis of the Nonlinear Schrödinger Equation coupled to a noninstantaneous plasma response yields a set of stationary states. This analysis clearly indicates that the emergence of double-hump, characteristically asymmetric temporal on-axis intensity profiles in regimes where plasma defocusing saturates the optical collapse caused by Kerr self-focusing is an inherent property of the underlying dynamical model.

RETINAL VASCULAR TREE CONSTRUCTION WITH MULTIMODAL FLUORESCEIN ANGIOGRAM SEQUENCE

In this article, we present a method to construct retinal vascular tree with the spatial information and temporal intensity profile of vessels. The significance of the work is to assist in clinical studies of diagnosis of cardiovascular diseases, such as hypertension, which manifest abnormalities in either venous and/or arterial vascular systems. For temporal intensity profile, fluorescein angiogram (FA) is adopted to ensure continuity of vessel segments along time. We separate retinal vascular tree construction into two main steps — vessel extraction and vessel grouping. In vessel extraction, we use an existing algorithm that applies multiscale matched filtering and vessel likelihood measure to extract vessel segments and its spatial information in color retinal image. Vessel segments are grouped using extended Kalman filter to take into consideration continuities in curvature, width, and fluorescein angiogram sequence changes at the bifurcation or crossover point. The algorithm is evaluated on a clinical data set from Taipei Veterans' General Hospital, and the results are compared with the ground truth images provided by a physician. The results show that our method reaches an average correctness rate of 91.48%.

Measurements of femtosecond temporal speckle field of a random medium

The femtosecond temporal speckle field of a random medium is studied theoretically and experimentally. Femtosecond temporal speckle arises from the interference of multiple randomly scattered electric fields. The femtosecond temporal speckle field is measured with a cross-correlation frequency-resolved optical gating method. The spatial average of the speckle field yields a smooth transmitted profile. The speckle field is a circular complex Gaussian variable because the scattered light beams from different trajectories have no correlation with each other. The field and the intensity profiles of individual speckle spots fluctuate randomly in time. The ensemble average of the temporal intensity profiles converges, thereby yielding the photon travel time probability distribution function.

Experimental and theoretical study of free–free electron–helium scattering in a CO2 laser field

Free–free transitions during the scattering of electrons by helium in the presence of a linearly polarized CO2 laser field are investigated both experimentally and theoretically. Signals for laser-assisted scattering at 22 eV with absorption or emission of up to two photons are measured at scattering angles between 20° and 70°, and are compared to the values obtained from an 11-state R-matrix Floquet calculation and from the low-frequency approximation of Kroll and Watson. The two sets of theoretical results are found to be in very good agreement for the scattering geometries considered in the experiment. The order of magnitude of the experimental results is reproduced by calculations with intensities in the region of 107 W cm−2. Agreement is improved by averaging the theoretical results over the spatial distributions of the three beams as well as the temporal intensity profile of the laser pulse, and by allowing for some misalignment of the three beams in the experiment.

Single-shot real-time frequency chirp characterization of telecommunication optical signals based on balanced temporal optical differentiation

A simple single-shot frequency chirp characterization technique for telecommunication optical waveforms is introduced. The proposed technique is based on what we believe to be a novel balanced all-optical ultrafast differentiation scheme, and it enables a real-time conversion of the instantaneous frequency variation of an arbitrary complex (amplitude and phase) optical signal into a temporal intensity profile while simultaneously providing an efficient cancellation of the intensity noise. Single-shot real-time frequency chirp measurements of gigahertz-bandwidth phase-only and amplitude and phase temporal modulation waveforms are performed.

Quenching of He-induced intensity enhancement effect in H and D emission produced by Nd-doped yttrium aluminum garnet laser irradiation on solid targets in low pressure helium gas

An experimental study was performed on the N2-induced quenching of He-induced intensity enhancement effect in reduced-pressure plasma emission produced by Nd-YAG irradiation on solid zircaloy and porous fossil samples. The spatial distributions and temporal variations in the emission intensities show pronounced intensity quenching effects on the He I 667.9 nm, H I 656.2 nm, and D I 656.1 nm emission lines in both samples when a tiny amount (5% by volume) of nitrogen was added to helium gas, while leaving the spatial and temporal intensity profiles of the heavier Zr and Ca atoms virtually unaffected. In both cases of different ambient gases, the spatial and temporal variations in the He, H, and D emission intensities exhibit distinct features and changes, which are clearly distinguishable from those observed on the Zr and Ca emission lines, which were mainly produced by the shock-wave induced thermal excitation process. The analysis of these data unambiguously revealed the presence of an additional and distinct “He-assisted” excitation mechanism in the He plasma, which was further suggested to be related to the He metastable excited state. The quenching effect was therefore explained as a consequence of energy depletion of the He metastable excited state triggered by the Penning ionization process induced by the presence of nitrogen. This also explains the relatively insensitive response of the Zr and Ca emission intensity profiles to nitrogen addition despite the increased plasma electron density resulting from the ionization process.

Group Delay Characterization of Dispersive Devices Using a Pulse Temporal Intensity Measurement Setup

We propose and demonstrate a novel technique for group delay (GD) characterization of dispersive devices. The proposed technique involves the measurement of the temporal intensity profiles of a dispersed ultrashort optical pulse (i.e., after the device under test) and its first time derivative, which can be obtained by simple linear propagation through an all-fiber optical filter. The GD can be reconstructed using a direct and noniterative algorithm. We present experimental results for the measurement of the GD (including group-delay ripple) of high-dispersion and low-dispersion chirped fiber Bragg gratings.

Complete temporal pulse characterization based on phase reconstruction using optical ultrafast differentiation (PROUD)

A simple and general technique for recovering the phase profile of a given optical waveform from temporal intensity measurements is introduced and experimentally demonstrated. The proposed method involves the measurement of the temporal intensity profiles at the input and output of a linear optical time differentiator. The signal phase profile can be unambiguously recovered from these intensity measurements using a direct and noniterative algorithm. Given that ultrafast optical differentiators can be readily implemented in all-fiber or free-space platforms, the proposed technique could be applied over time waveforms with durations ranging from the subpicosecond to the nanosecond regime.

Intense 1.5-cycle near infrared laser waveforms and their use for the generation of ultra-broadband soft-x-ray harmonic continua

We demonstrate sub-millijoule-energy, sub-4 fs-duration near-infrared laser pulses with a controlled waveform comprised of approximately 1.5 optical cycles within the full-width at half-maximum (FWHM) of their temporal intensity profile. We further demonstrate the utility of these pulses for producing high-order harmonic continua of unprecedented bandwidth at photon energies around 100 eV. Ultra-broadband coherent continua extending from 90 eV to more than 130 eV with smooth spectral intensity distributions that exhibit dramatic, never-before-observed sensitivity to the carrier-envelope offset (CEO) phase of the driver laser pulse were generated. These results suggest the feasibility of sub-100-attosecond XUV pulse generation for attosecond spectroscopy in the 100 eV range, and of a simple yet highly sensitive on-line CEO phase detector with sub-50-ms response time.

Discrete Changes in Cortical Activation during Experimentally Induced Referred Muscle Pain: A Single-Trial fMRI Study

Noxious stimulation of skeletal muscle evokes pain that is often referred into distal areas. Despite referred pain being of significant clinical importance, the brain regions responsible for the perception of referred pain remain unexplored. The aim of this investigation is to define these regions using functional magnetic resonance imaging. We induced muscle pain by hypertonic saline injections (0.5 ml) into the tibialis anterior (TA) or flexor carpi radialis (FCR) muscle. TA injections evoked pain that was referred to the ankle/foot in 10/17 subjects, whereas FCR injections evoked pain that was projected into the wrist/hand in 6/12 subjects. Regional brain responses were statistically tested by convolving the temporal profile of the subjective pain intensity rating with the hemodynamic response function. For all subjects, signal increased in the region of primary somatosensory cortex (SI), which represents the leg or arm, that is, the area corresponding to the injection site. However, for those subjects who reported referred pain, signal intensity increases also occurred in the SI region representing the foot or hand. Interestingly, differential signal changes also occurred in anterior cingulate, cerebellar, and insular cortices. This is the first study to provide evidence of cortical differentiation in the processing of primary and referred pain.

Modeling of time-resolved laser-induced incandescence transients for particle sizing in high-pressure spray combustion environments: a comparative study

In this study experimental single-pulse, time-resolved laser-induced incandescence (TIRE-LII) signal intensity profiles acquired during transient Diesel combustion events at high pressure were processed. Experiments were performed between 0.6 and 7 MPa using a high-temperature high-pressure constant volume cell and a heavy-duty Diesel engine, respectively. Three currently available LII sub-model functions were investigated in their performance for extracting ensemble mean soot particle diameters using a least-squares fitting routine, and a “quick-fit” interpolation approach, respectively. In the calculations a particle size distribution as well as the temporal and spatial intensity profile of the heating laser was taken into account. For the poorly characterized sample environments of this work, some deficiencies in these state-of-the-art data evaluation procedures were revealed. Depending on the implemented model function, significant differences in the extracted particle size parameters are apparent. We also observe that the obtained “best-fit” size parameters in the fitting procedure are biased by the choice of their respective “first-guess” initial values. This behavior may be caused by the smooth temporal profile of the LII cooling curve, giving rise to shallow local minima on the multi-parameter least squares residuals, surface sampled during the regression analysis procedure. Knowledge of the gas phase temperature of the probed medium is considered important for obtaining unbiased size parameter information from TIRE-LII measurements.

Ultrafast adaptive optical near-field control

Simultaneous control of the spatial and temporal properties of the optical near field in the vicinity of a nanostructure is achieved by illumination with broadband optimally polarization-shaped femtosecond light pulses. Here we demonstrate the spatial control of the local linear and nonlinear fluence, the local spectral distribution, and the local temporal intensity profile on a subdiffraction length scale. The boundary-element method is used for a self-consistent solution of Maxwell's equations in the frequency domain. Particular control objectives for spatial field distribution and temporal evolution are expressed as fitness functions in an evolutionary algorithm that optimizes adaptively the polarization-shaped input light pulses. Substantial control according to different goals is demonstrated and the limits of controllability are investigated. The dominating control mechanism is local interference of near-field modes that are excited with the two independent polarization components of the incident light pulses and hence polarization pulse shaping is essential to achieve substantial control in the optical near field. The influence of other control mechanisms is discussed and a number of applications are presented.

Coherent control of pump-probe signals of helical structures by adaptive pulse polarizations

The simplification of the pump-probe spectrum of excitons by pure-phase-polarization pulse shaping is investigated by a simulation study. The state of light is manipulated by varying the phases of two perpendicular polarization components of the pump, holding its total spectral and temporal intensity profiles fixed. Genetic and iterative Fourier transform algorithms are used to search for pulse phase functions that optimize the ratio of the signal at two frequencies. New features are extracted from the congested pump-probe spectrum of a helical pentamer by selecting a combination of Liouville space pathways. Tensor components which dominate the optimized spectra are identified.

Anomalous spectral behaviour of diffracted chirped Gaussian pulses in the near field

By using the Fourier transform method, analytical expressions for the axial power spectrum and near-field intensity in the spacetime domain of chirped Gaussian pulses diffracted at an aperture are derived, which permit us to study changes in spectral and temporal profiles of the chirped Gaussian pulses both analytically and numerically. Detailed numerical results and physical analysis show that spectral anomalies take place in the neighbourhood of certain critical distances and the shifting of maximum and splitting of temporal intensity profiles appear. In particular, for ultrashort chirped pulses, there exists also spectral switch. Besides the truncation parameter, the chirp parameter and pulse duration affect the behaviour of spectral switches.