Arbitrary Pulse Shapes Research Articles

Parameterization of an acousto-optic programmable dispersive filter for closed-loop learning experiments

A representation of the search space in optical pulse shaping problems employing an acousto-optic programmable dispersive filter (AOPDF) is presented for use in closed-loop learning experiments where the optimal spectral phase function to some control problem is determined by an iterative learning algorithm. The representation allows the algorithm to select a value for the optical chirp at each frequency control point such that only acoustic grating functions which preserve the spectrum of the shaped pulses are tested. The limits of this space with respect to the rate of applied optical chirp, optical bandwidth and acoustic power are examined and tested through diffraction efficiency studies performed using a commercial AOPDF. The main benefits of this representation are the elimination of undesirable frequency mixing effects, reduction of diffraction efficiency variation between arbitrary pulse shapes and faster convergence of the evolutionary algorithm.

Flexible active cycle stress testing of smart power switches

Active cycle stress testing of smart power switches is conventionally performed either with current pulses of constant amplitude or with waveforms derived by switching inductive loads. A flexible test system is introduced that is capable of generating arbitrary current pulse shapes, which is verified experimentally on a typical smart power switch. It is demonstrated by a test run that pulses with different shape and amplitude but equal thermal stress derived from thermal simulation lead to comparable cycle life time.

An Exact Error Analysis for Low-Duty Pulsed DS-CDMA Systems in Flat Nakagami Fading

Based on the characteristic function method, an exact error analysis is presented for low-duty pulsed direct sequence code division multiple access (DS-CDMA) systems in flat Nakagami fading channel. The presented method is simple and good for any arbitrary pulse shape. The computational involvement of the method is simpler than the method based on improved Gaussian approximation (IGA), which is the only reliable method currently available for calculating error probabilities of such systems

A quantitative S-Matrix approach to high-order harmonic generation from multiphoton to tunneling regimes.

We present a S-matrix description of the process of high order harmonic generation during the interaction of atoms with strong electromagnetic fields. In contrast with the state-of-the-art approaches, our model does not employ the stationary phase approximation and accounts as well for the continuum-continuum transitions. Therefore we are able to reproduce quantitatively the higher frequency part of the spectrum for arbitrary pulse shapes, and for intensities corresponding to multiphoton, tunnel and soft over-the barrier ionization regimes. In addition this model can be implemented very efficiently in a Personal Computer to calculate the harmonic generation for the atom interacting with an eight-cycle pulse at lambda =800 nm in, roughly, ten minutes (a reduction of two orders of magnitude from the typical time requirements of the exact integration).

Shaping ability of all fiber coherent pulse stacker

Pulse stacking is an effective method to generate a long-shaped pulse from short pulses. In this paper, we study all-fiber coherent pulse stacking systematically; we show that the time delays and phase differences between the short pulses are the key parameters of the stacked pulse. The permitted variation of the time delay and phase difference are obtained. The ability of the stacker to produce arbitrary pulse shapes is discussed.

Efficient Picosecond Pulse Shaping by Programmable Bragg Gratings

We propose an efficient approach for arbitrary optical pulse shaping in the subhundred to a few hundreds picosecond regime. The designed device uses one-dimensional wave-guided Bragg gratings filled with an electrically controlled dielectric. One hundred electrodes are attached along the waveguide so that the refractive index pattern is programmable by applying voltages. The frequency response of this device with certain refractive index patterns is analyzed. Numerical simulations show that output pulses with desired shapes and high efficiency (>40% of theoretical maximum) can be obtained by using a genetic algorithm searching method augmented by an adiabatic following optimization scheme

Coherent control of collective spontaneous emission in an extended atomic ensemble and quantum storage

Coherent control of collective spontaneous emission in an extended atomic ensemble resonantly interacting with single-photon wave packets is analyzed. A scheme for coherent manipulation of collective atomic states is developed such that superradiant states of the atomic system can be converted into subradiant ones and vice versa. Possible applications of such a scheme for optical quantum-state storage and single-photon wave packet shaping are discussed. It is shown that also in the absence of inhomogeneous broadening of the resonant line, single-photon wave packets with arbitrary pulse shape may be recorded as a subradiant state and reconstructed even although the duration of the wave packets is larger than the superradiant lifetime. Specifically the applicability for storing time-bin qubits, which are used in quantum cryptography, is analyzed.

Analytical solutions for the electromagnetic fields of tightly focused laser beams of arbitrary pulse length

The analytical solution for a monochromatic focused laser beam was recently published [Opt. Lett.31, 1447 (2006)]. The effect of introducing bandwidth by including a finite-length temporal pulse envelope is included exactly. This is done formally first in the frequency domain for an arbitrary pulse shape, and the specific case of a cosine-squared envelope is then solved analytically for all pulse lengths, thereby decreasing the computation time by 2 orders of magnitude. The inclusion of longer wavelengths reduces the fraction of laser energy in the focus from 86.5% to 83.5% for a 5 fs Ti:sapphire laser and 72.7% in a single-cycle pulse.

Light qubit storage and retrieval using macroscopic atomic ensembles

We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the well-established Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer long delay lines between the passages. Furthermore, we introduce a time dependent interaction strength which enables storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by explicit calculations for a photonic qubit.

Identification of continuous‐time delayed systems using an input pulse of arbitrary shape

This article resolves difficulties of continuous identification often encountered in field testing, such as unsteady initial states, unknown disturbances, and noise‐corrupted measurements. The unique feature of the proposed method is that it divides the identification problem into two simpler estimation problems based on pulse testing of arbitrary shape. Sequential algorithms are developed to deal with the first estimation problem subjected to the zero portion of the input and then to solve the remaining problem subjected to the arbitrary portion of the input. The method provides a convenient way to fit model predictions to output measurements in the face of unknown initial states and static disturbances. Simulation results demonstrate that the use of the integral filter renders the method rather robust with respect to noise and model structure mismatch.

A receiver model for optical fiber communication systems with arbitrarily polarized noise

The authors have derived a receiver model that provides an explicit relationship between the Q factor and the optical signal-to-noise ratio (OSNR) in optical fiber communication systems for arbitrary pulse shapes, realistic receiver filters, and arbitrarily polarized noise. It is shown how the system performance depends on both the degree of polarization of the noise and the angle between the Stokes' vectors of the signal and the noise. The results demonstrate that the relationship between the OSNR and the Q factor is not unique when the noise is partially polarized. This paper defines the enhancement factor and three other parameters that explicitly quantify the relative performance of different modulation formats in a receiver. The theoretical and experimental results show that the performance of the return-to-zero format is less sensitive to variations in the receiver characteristics than is the performance of the nonreturn-to-zero format. Finally, a validation of the formula is presented for computing the Q factor from the OSNR and the Stokes vectors of the signal and the noise by comparison with both experiments and Monte Carlo simulations.

Accumulative effects in the coherence of three-level atoms excited by femtosecond-laser frequency combs

We investigate coherent accumulation processes in three-level atoms excited by a train of ultrashort pulses in the case where the atomic relaxation times are greater than the laser repetition period. In this situation the resonances of the laser field with the atomic system are determined by the laser frequency comb rather than by the spectrum of a single pulse. Using the density matrix formalism, we develop a perturbative theory that is valid for arbitrary pulse shapes. The excitation of a Doppler-broadened atomic vapor by hyperbolic-secant pulses and $0\ensuremath{\pi}$ pulses is analyzed. It is shown that pulse shape has a great influence on the accumulation process and can change the spectral periodicity of the pattern impressed on the Doppler profile of the medium due to the two-photon absorption process. The effect of interpulse phases is also investigated, and we show that the atomic populations can vary by more than one order of magnitude with small variations of the laser repetition rate, while being insensitive to variations of the laser offset. Finally, the theory is adapted for the temporal-coherent-control technique, and its results are compared with previously reported experimental data.

Analytical Evaluation of the Temporal Focal Shift for Arbitrary Pulse Shapes

In this letter, we analyze the propagation of linearly chirped arbitrary-shaped light pulses through a parabolic dispersive medium to derive an analytical formula of assessing the location of the transverse plane where the pulse root-mean-square width is minimum. Closed form expressions for compressed pulses, which are independent of the input pulse shape, are demonstrated. In this way, we demonstrate that both the relative temporal focal shift and the minimum pulsewidth are solely determined by two factors, the temporal equivalent of the Fresnel number of the geometry and the pulse quality factor, i.e., the temporal analogue of the spatial M/sup 2/ beam quality factor. Some examples are discussed.

Theory of reflection and absorption of light by low-dimensional semiconductor objects in a strong magnetic field under monochromatic and pulsed excitations

The theoretical principles of reflection and absorption of light by low-dimensional semiconductor objects (quantum wells, quantum wires, quantum dots) under monochromatic and pulsed excitations with an arbitrary pulse shape are developed. A semiconductor object can be placed in a strong constant magnetic field. The normal incidence of light on a quantum well whose width can be comparable to the light wavelength and for which the number of levels of electronic excitations can be arbitrary is considered as an example. An integral equation similar to the Dyson equation is derived for the Fourier components of the electric fields. The solutions to this equation are given for a number of special cases.

Predicting the Failure Threshold of Dose Rate for ICs Exposed to Pulsed Ionizing Radiation of Arbitrary Pulse Shape

A computer simulation is conducted of IC response to one or more ionizing radiation pulses of arbitrary shape and width. The failure threshold of dose rate is examined as a function of pulse width. A method for estimating the failure threshold is devised and validated. It depends on experimental evaluation of the failure threshold for a very short and a very long radiation pulse. The theoretical results are confirmed by experimental data on the failure threshold of dose rate subject to pulse shape and width.

Impact of Chromatic and Polarization-Mode Dispersions on DPSK Systems Using Interferometric Demodulation and Direct Detection

We study the impact of chromatic dispersion (CD) and first-order polarization-mode dispersion (PMD) on systems using binary differential phase-shift keying (2-DPSK) or quaternary DPSK (4-DPSK) with nonreturn-to-zero (NRZ) or return-to-zero (RZ) formats. These signals are received using optical preamplification, interferometric demodulation, and direct detection. We consider the linear propagation regime and compute optical power penalties at fixed bit-error ratio (BER). In order to evaluate the BER precisely taking account amplifier noise, arbitrary pulse shapes, arbitrary optical and electrical filtering, CD, and PMD, we introduce a novel model for DPSK systems and compute the BER using a method recently proposed by Forestieri for on-off keying (OOK) systems. We show that when properly applied, the method yields highly accurate results for DPSK systems. We have found that when either the NRZ or RZ format is used, 2-DPSK exhibits lower power penalties than OOK in the presence of CD and first-order PMD. RZ-2-DPSK, as compared with NRZ-2-DPSK, incurs smaller penalties due to PMD, but offers no advantage in terms of CD. 4-DPSK, as it has twice the symbol duration of OOK or 2-DPSK for a given bit rate, incurs much lower CD and PMD power penalties than either of these techniques. RZ-4-DPSK is especially promising, as it offers CD and PMD penalties significantly smaller than all other techniques, including NRZ-4-DPSK.

Response theory for time-resolved second-harmonic generation and two-photonphotoemission

A unified response theory for the time-resolved nonlinear light generation and two-photonphotoemission (2PPE) from metal surfaces is presented. The theory allows one to describethe dependence of the nonlinear optical response and the photoelectron yield, respectively,on the time dependence of the exciting light field. Quantum-mechanical interference effectsaffect the results significantly. Contributions to 2PPE due to the optical nonlinearity of thesurface region are derived and shown to be relevant close to a plasmon resonance. Theinterplay between pulse shape, relaxation times of excited electrons, and band structure isanalysed directly in the time domain. While our theory works for arbitrary pulse shapes,we mainly focus on the case of two pulses of the same mean frequency. Difficultiesin extracting relaxation rates from pump–probe experiments are discussed—forexample due to the effect of detuning of intermediate states on the interference. Thetheory also allows one to determine the range of validity of the optical Blochequations and of semiclassical rate equations, respectively. Finally, we discuss howcollective plasma excitations affect the nonlinear optical response and 2PPE.

Phonon-assisted damping of Rabi oscillations in semiconductor quantum dots.

Electron-phonon interaction is a major source of optical dephasing in semiconductor quantum dots. Within a density matrix theory the electron-phonon interaction is considered up to the second order of a correlation expansion, allowing the calculation of the quantum kinetic dephasing dynamics of optically induced nonlinearities in GaAs quantum dots for arbitrary pulse strengths and shapes. We find Rabi oscillations renormalized and a damping that depends on the input pulse strength, a behavior not known from exponential dephasing mechanisms.

Cherenkov radiation emitted by ultrafast laser pulses and the generation of coherent polaritons

We report on the generation of coherent phonon polaritons in ZnTe, GaP and LiTaO$_{3}$ using ultrafast optical pulses. These polaritons are coupled modes consisting of mostly far-infrared radiation and a small phonon component, which are excited through nonlinear optical processes involving the Raman and the second-order susceptibilities (difference frequency generation). We probe their associated hybrid vibrational-electric field, in the THz range, by electro-optic sampling methods. The measured field patterns agree very well with calculations for the field due to a distribution of dipoles that follows the shape and moves with the group velocity of the optical pulses. For a tightly focused pulse, the pattern is identical to that of classical Cherenkov radiation by a moving dipole. Results for other shapes and, in particular, for the planar and transient-grating geometries, are accounted for by a convolution of the Cherenkov field due to a point dipole with the function describing the slowly-varying intensity of the pulse. Hence, polariton fields resulting from pulses of arbitrary shape can be described quantitatively in terms of expressions for the Cherenkov radiation emitted by an extended source. Using the Cherenkov approach, we recover the phase-matching conditions that lead to the selection of specific polariton wavevectors in the planar and transient grating geometry as well as the Cherenkov angle itself. The formalism can be easily extended to media exhibiting dispersion in the THz range. Calculations and experimental data for point-like and planar sources reveal significant differences between the so-called superluminal and subluminal cases where the group velocity of the optical pulses is, respectively, above and below the highest phase velocity in the infrared.

Evaluating the error probability in lightwave systems with chromatic dispersion, arbitrary pulse shape and pre- and postdetection filtering

A novel approach to analytically evaluate the bit error probability in optically preamplified direct-detection systems is presented, which can take into account the effects of pulse shaping, chirping, filtering at the transmitter and the receiver, both pre- and postdetection, chromatic dispersion, and ASE noise. The method is computationally very fast in that the saddle point integration method for solving the resulting line integral of a particular moment generating function is adopted. A closed-form approximation for the bit error probability is also provided, which is within 0.01 dB from the exact numerical results.