Pulse Splitting Research Articles

Dynamics of fast and slow pulse propagation through a microsphere–optical-fiber system

We discuss the mechanism of fast and slow light generation when optical pulses propagate through a fiber taper coupled to an ultrahigh-Q microsphere system in the time domain. Fast and slow light are explained as interference effects between ballistic light through the fiber and circulated light within the sphere. One of the striking effects predicted by this model is dynamic pulse splitting in the transmitted pulse at the critical coupling condition, where the coupling strength between the sphere and fiber equals the round-trip loss in the sphere. By realizing this critical coupling condition experimentally, we observed this dynamic pulse splitting effect.

Spatio-temporal reshaping and X Wave dynamics in optical filaments.

<p> <a href="http://oe.osa.org/virtual_issue.cfm?vid=36">Focus Serial: Frontiers of Nonlinear Optics</a> </p>We investigate ultrashort laser pulse filamentation within the framework of spontaneous X Wave formation. After a brief overview of the filamentation process we study the case of an intense filament co-propagating with a weaker seed pulse. The filament is shown to induce strong Cross-Phase-Modulation (XPM) effects on the weak seed pulse: driven by the pump, the seed pulse undergoes pulse splitting with the daughter pulses slaved to their pump counterparts. They undergo strong spatio-temporal reshaping and are transformed into XWaves traveling at the same group velocities as the pump split-off pulses. In the presence of a gain mechanism such as Four-Wave-Mixing or Stimulated Raman Scattering, energy is then transferred from the pump filament leading to amplification of the seed X Wave and formation of a temporally compressed intensity peak.

Diffraction induced space–time splitting effects in ultra-short pulse propagation

Diffraction induced space–time splitting effects are studied in the propagation of ultra-short optical pulses by an optimized numerical simulation technique, avoiding approximations used in exact calculations. Point-wise detection as well as detection in a two-plus-one dimension of a pulse are simulated. Effects of diffraction on the spectral and spatial–temporal features of pulse evolution are analysed for a rotationally nonsymmetric pulse. Delays in the arrival of the portions of the pulse travelling off-axis are visualized with accurate estimation of pulse arrival time and amplitude variations. Subtle effects of pulse splitting and recombination in space and time both in near and far field are brought out clearly.

Reshaping periodic light pulses using cascaded uniform fiber Bragg gratings

The authors demonstrate the use of cascaded uniform fiber Bragg gratings (FBGs) for the generation of periodic optical pulses with arbitrary waveform. It is a significantly simplified structure compared to complex FBG shapes. The pulse shaping is based on splitting of the input pulses by low-reflecting FBGs into a number of replicas and their superposition with proper amplitude, time delay, and phase shift that depend on the FBG parameters. The reflection amplitude and phase of each grating are unambiguously determined by the needed pulse shape. This method was experimentally verified for converting sinusoidally phase-modulated radiation of continuous-wave laser diode into a Gaussian pulse train with a pulsewidth of 30 ps. A method for controlling the parameters of FBGs during their fabrication process is also presented. It is done by measuring the spectral interference between the reflections from the FBGs and the fiber end by an optical spectrum analyzer and performing a fast Fourier transform. The method allows correction of the FBGs until the needed parameters are obtained during the writing process, as well as at any time after that.

CONICAL EMISSION OF TI:SAPPHIRE FEMTOSECOND LASER PULSES PROPAGATING IN WATER

Conical emission is investigated for Ti :sapphire femtosecond laser pulses propagating in water. The colored rings can be observed in the forward direction due to the constructive and destructive interference of transverse wavevector, which are induced by the spatio-temporal gradient of the free-electron density. With increasing input laser energy, due to filamentation and pulse splitting induced by the plasma created by multiphoton excitation of electrons from the valence band to the conduction band, the on-axis spectrum of the conical emission is widely broadened and strongly modulated with respect to input laser spectrum, and finally remains fairly constant at higher laser energy due to intensity clamping in the filaments.

Splitting and broadening techniques for SFQ-pulse driver based on SQIF

We present the detailed analysis of the Single Flux Quantum (SFQ) pulse splitting structure for the output SFQ-pulse driver suggested earlier. The splitter tree is an important part of the driver based on series Superconducting Quantum Interference Filter (SQIF). Conflicting requirements to the circuit parameters and possible approaches to meet the challenges are presented and discussed. Importance of the pulse broadening technique for the driver operation is also discussed.

Splitting circuits for a single-flux-quantum-pulse driver based on a superconductingquantum interference filter

We present detailed analysis of the single-flux-quantum (SFQ) pulse splitting structure forthe output SFQ-pulse driver suggested before. The splitter tree is an important part of thedriver, based on a series superconducting quantum interference filter (SQIF). Conflictingrequirements to the circuit parameters and possible approaches to meet the challenges arepresented and discussed. The critical role of the pulse broadening technique for the driverdesign is discussed as well.

Nonlinear X-wave formation by femtosecond filamentation in Kerr media

We investigate the formation of X waves during filamentation in Kerr media. From the standard model developed for femtosecond filamentation in liquids, solids, and gases, the influence of several physical effects and parameters is numerically studied in the strongly nonlinear regime where group velocity dispersion alone is insufficient to arrest collapse. The collapse is shown to be arrested by multiphoton absorption and plasma defocusing, but not by dispersion. The postcollapse dynamics takes the form of a pulse splitting, which induces large gradients in the near field and seeds the formation of X waves, appearing both in the near and far fields. We discuss the universal features of the X-wave patterns, among which the long arms in the far field that follow the linear dispersive properties of the medium [Conti, Phys. Rev. Lett. 90, 170406 (2003); Kolesik, Phys. Rev. Lett. 92, 253901 (2004)] and are accompanied by a strong modulated axial emission.

Far-field spectral characterization of conical emission and filamentation in Kerr media

By use of an imaging spectrometer we map the far-field ($\theta-\lambda$) spectra of 200 fs optical pulses that have undergone beam collapse and filamentation in a Kerr medium. By studying the evolution of the spectra with increasing input power and using a model based on stationary linear asymptotic wave modes, we are able to trace a consistent model of optical beam collapse high-lighting the interplay between conical emission, multiple pulse splitting and other effects such as spatial chirp.

On one dimensional chemical diode and frequency generator constructed with an excitable surface reaction

The oxidation of carbon monoxide on a Pt(110) surface is considered as a medium for chemical information processing in which bits of information are represented by traveling pulses of high oxygen coverage. Using numerical simulations for a model of CO oxidation we demonstrate that in such system one dimensional chemical signal diode can be realized by setting a proper profile of temperature. We also show that a pulse splitting can occur on a temperature inhomogeneity. The phenomenon of pulse splitting can be used to construct one dimensional generator of a train of pulses with adjustable frequency.

Phase measurement of periodic optical pulses using temporal fractional Talbot effect

Proposed and demonstrated is a novel method for time-domain interferometry of optical pulse trains that does not need light pulse splitting. The interference is between time-shifted pulse-train replicas produced in a dispersive delay line via the temporal fractional Talbot effect. A built-in advantage of the method is its low sensitivity to environmental disturbance. In the experiment the method was used to demonstrate phase characterisation of periodic optical pulses.

Generation and nonlinear dynamics of X waves of the Schrödinger equation

The generation of finite-energy packets of X waves is analyzed in normally dispersive cubic media by using an X-wave expansion. The three-dimensional nonlinear Schrödinger model is reduced to a one-dimensional equation with anomalous dispersion. Pulse splitting and beam replenishment as observed in experiments with water and Kerr media are explained in terms of a higher-order breathing soliton. The results presented also hold in periodic media and Bose-condensed gases.

Measurements and numerical analysis for femtosecond pulse deformations after propagation of hundreds of meters in air with water-vapor absorption lines

We have clarified the influences of water-vapor absorption lines in air on femtosecond pulse propagations from experimental and theoretical points of view. Precise measurements for the femtosecond pulse shapes after propagation of as much as 300 m through air have been made in a semiunderground optical testing tunnel. We observed the pulse splitting and the enhancement of the pulse broadening due to the 100-m propagation in air. The experimental results are in good agreement with the theoretical analysis by use of the HITRAN database at the edge and the center of the water-vapor absorption regions in air. Measured autocorrelation traces are mostly reproduced by those calculated with only the real part (dispersion) of the refractive index modulated by water-vapor absorption lines in air. This fact enables us to conclude that the pulse deformations due to water absorption lines in air are caused mainly by the real part (dispersion), not by the imaginary part (absorption), of the refractive index of air.

Nonlinear space-time dynamics of ultrashort wave packets in water.

We have monitored the space-time transformation of a 150-fs pulse undergoing self-focusing and filamentation in water, by means of the nonlinear gating technique. We have observed that pulse splitting and subsequent recombination apply to axial temporal intensity only, whereas the space-integrated pulse profile preserves its original shape.

Continuous wave-controlled shape and chirp oscillations of optical solitons

Interactions between solitons and continuous waves (CW) are studied in terms of a perturbation method and systematic numerical simulations as well. The critical dependence of the interactions on the initial phase difference between the pulse and the CW is shown to result in two distinct types of evolution under propagation. Pulse shape and chirp oscillations occur for specific configurations of pulse and CW initial amplitudes and phase difference, while pulse destruction and splitting occurs for other possible configurations. The effect of modulational instability, emerging from the interactions, is also considered.

Second-harmonic generation pulse splitting in quartz observed by frequency-domain interferometry

Second-harmonic generation from a thin quartz wedge is studied using short laser pulses. Under phase mismatched condition a temporal splitting of the second-harmonic field occurs due to the different group velocities of the free and forced modes. By measuring the temporal splitting as a function of the quartz crystal thickness using spectral interference a non-zero time-difference between the two second-harmonic pulses at the incident surface is predicted. This is interpreted as a phase-difference between the homogeneous and the inhomogeneous solution of the wave equation at the incident surface.

Control of the refractive index change in fused silica glasses induced by a loosely focused femtosecond laser

We demonstrate waveguide fabrication using of a loosely focused femtosecond laser that induces both nonlinear ionization and nonlinear propagation in silica glasses without any scanning process. The numerical aperture is chosen to be 0.007 to avoid spatial splitting of the laser pulses during the nonlinear propagation of the femtosecond laser pulses inside the fused silica glass. It also enables a uniform cylindrical refractive index change, which acts as an optical waveguide, to be induced. We found that the induction of irregular refractive index changes is related to the free electron density of the focused area and is controlled by decreasing the NA. The length, position, and core diameter of the fabricated waveguide can be controlled by the pulse-width, energy, and irradiation time of the incident laser. By using this technique, we varied the length of the fabricated waveguides between 20 μm and 6 mm, while keeping the core diameter at around 5 μm.

Experimental EOS determination of aluminum at Mbar pressure

A shock wave is driven by a laser pulse of 1.2 ps duration (FWHM), with the intensity of ~10 14 W/cm 2 at 785 nm, irradiating a 500 nm thick aluminum foil. A chirped laser pulse split from the main pulse is used to detect the shock breakout process at the rear surface of the target based on frequency domain interferometry. The mean shock velocity determination benefits from the precise synchronization (<100fs resolution) of the shock pump and probe laser pulses, which is calculated from the time the shock takes to travel the 500 nm thick aluminum. The released particle velocity determination benefits from the chirped pulse frequency domain interferometry. The average shock velocity is 15.15 km/s and the shock release particle velocity is 15.24 km/s, and the corresponding pressure after shock is 3.12 Mbar under our experimental condition.

One-dimensional numerical simulations of random sound impulses

We perform one-dimensional numerical simulations of small-amplitude acoustic pulses in space- and time-dependent random mass density and time-dependent velocity fields. Numerical results reveal that: (a) random fields affect the speeds, amplitudes and, consequently, shapes of sound pulses; (b) for weak random fields and short propagation times the numerical data converge with the analytical results of the mean field theory which says that a space-dependent (time-dependent) random field leads to wave attenuation (amplification) and all random fields speed up sound pulses; (c) for sufficiently strong random fields and long propagation times numerical simulations reveal pulse splitting into smaller components, parts of which propagate much slower than a wave pulse in a non-random medium. These slow waves build an initial stage of a wave localization phenomenon. However, this effect can be very weak in a real three-dimensional medium.

Simultaneous measurement of laser-induced shock wave and released particle velocities at Mbar pressure

We show the feasibility of simultaneous measurement of shock velocity and released particle velocity after shock at Mbar pressure. The shock wave is driven by a laser pulse of 1.2 ps duration (full width at half maximum), with the intensity of ∼1014 W/cm2 at 785 nm, irradiating a 500-nm-thick aluminum foil. A chirped laser pulse split from the main pulse is applied to detect the shock breakout process at the rear surface of the target based on frequency domain interferometry. The mean shock velocity determination benefits from the precise synchronization (&amp;lt;100 fs resolution) of the shock pump and probe laser pulse, which is calculated from the time the shock takes to travel the 500-nm-thick aluminum. The released particle velocity determination takes advantage of the chirped pulse frequency domain interferometry. The two measured parameters are self-consistent.