Arbitrary Pulse Shapes Research Articles

Continuous-Time Delta-Sigma Modulator Design Using the Method of Moments

We present a technique for determining the coefficients of the loop filter in a continuous-time delta sigma modulator (CTDSM) from the corresponding discrete time prototype. The method, which is based on computing the moments of the feedback DAC pulse shape, enables the determination of loop filter coefficients to an arbitrary DAC pulse shape without resorting to z-domain analysis. This not only simplifies the algebra, but also gives new insights into modulator operation. The technique helps derive topologies to compensate CTDSMs with FIR feedback DACs, as well as give closed form expressions for the coefficients of the compensated modulator. Finally, we give simple formulae for coefficients to compensate for excess loop delay in CTDSMs with arbitrary DAC pulses.

Imperfect relativistic mirrors in the quantum regime

The collective backscattering of intense laser radiation by energetic electron beams is considered in the relativistic quantum regime. Exact solutions for the radiation field are obtained, for arbitrary electron pulse shapes and laser intensities. The electron beams act as imperfect nonlinear mirrors on the incident laser radiation. This collective backscattering process can lead to the development of new sources of ultra-short pulse radiation in the gamma-ray domain. Numerical examples show that, for plausible experimental conditions, intense pulses of gamma-rays, due to the double Doppler shift of the harmonics of the incident laser radiation, can be produced using the available technology, with durations less than 1 as.

Analytical modeling of XPM in dispersion-managed coherent fiber-optic systems.

An analytical model for the cross-phase modulation (XPM) variance in dispersion-managed coherent fiber-optic systems is developed based on the first order perturbation theory. The XPM variance is analytically calculated for arbitrary pulse shapes. For a non-Gaussian pulse, the summation of time-shifted Gaussian pulses is used to fit the target pulse shape, which not only provides a good approximation of the non-Gaussian pulse but also allows explicit derivation of the XPM variance. The analytically estimated XPM variance is found to be in good agreement with numerical simulations.

Compression of ultrashort laser pulses via gated multiphoton intrapulse interference phase scans

Delivering femtosecond laser light in the focal plane of a high numerical aperture microscope objective is still a challenge, despite significant developments in the generation of ultrashort pulses. One of the most popular techniques, used to correct phase distortions resulting from propagation through transparent media, is the multiphoton intrapulse interference phase scan (MIIPS). The accuracy of MIIPS however is limited when higher order phase distortions are present. Here we introduce an improvement, called Gated-MIIPS, which avoids shortcomings of MIIPS, reduces the influence of higher order phase terms, and can be used to more efficiently compress broad band laser pulses even with a simple 4f pulse shaper setup. In this work we present analytical formulas for MIIPS and Gated-MIIPS valid for chirped Gaussian pulses; we show an approximated analytic expression for Gated-MIIPS valid for arbitrary pulse shapes; finally we demonstrate the increased accuracy of Gated-MIIPS via experiment and numerical simulation.

Arbitrary pulse shaping in Er-doped fiber amplifiers—Possibilities and limitations

Abstract A temporal deformation of nanosecond laser pulses occurring during an amplification process in a two-stage erbium-doped fiber amplifier operating in saturation is presented. We discuss distortion of rectangular laser pulses with duration ranging from 5 ns to 300 ns, generated at 20 kHz repetition rate. Calculations of a suitable peak power scaling factor for distorted pulses are presented. A simple compensation method of temporal pulse distortion, based on the direct current modulation with the use of arbitrary function generator (AFG), is also reported. An appropriate input pulse shape modification leads to obtaining an output amplified pulse with any desired shape. The main limitation of pulse shaping in a fiber amplifier seeded by a narrow linewidth laser diode turns out to be stimulated Brillouin scattering (SBS). However, the pulse shaping method can also improve the amplifier performance by increasing the power threshold of SBS. SBS phenomenon in the context of arbitrary pulse shaping have been also discussed.

Femtosecond pulse shaping technology and its applications

By controlling the spectral amplitude, phase and polarization of the femtosecond laser pulse in the frequency domain, a shaped femtosecond laser pulse with almost arbitrary pulse shape in time domain can be obtained, and this femtosecond laser pulse shaping technique provides a new experimental tool to study the nonlinear interaction between light and atoms or molecules. In this paper, we introduce the development history, technical method, control technique and relevant applications of the femtosecond laser pulse shaping technique, and also carry out a prospect on the research trends of this technique.

Integer and fractional charge Lorentzian voltage pulses analyzed in the framework of photon-assisted shot noise

We study the injection $n$ of electrons in a quantum conductor using voltage pulses applied on a contact. We particularly consider the case of Lorentzian voltage pulses. When carrying integer charge, they are known to provide electronic states with a minimal number of excitations, while any other type of pulses are accompanied with a neutral cloud of electron and hole excitations. We focus on the low-frequency shot noise arising when the excitations are partitioned by a single scatterer. Using periodic pulses, the physics can be discussed in the framework of the photon-assisted shot noise. Pulses of arbitrary shape and arbitrary charge are shown to give a marked minimum in the noise when the charge is an integer. The energy-domain characterization of the charge pulse excitations is also given using the shot-noise spectroscopy which reveals the asymmetrical energy spectrum of Lorentzian pulses. Finally, time-domain information is obtained from Hong-Ou-Mandel--type noise correlations when two trains of pulses generated on opposite contacts collide on the scatterer. For integer Lorentzian, the noise versus the time delay between pulse trains is shown to give a measure of the electron wave-packet autocorrelation function. In order to make contact with recent experiments, all the calculations are made at zero and finite temperatures.

Optimization of ultra-fast interactions using laser pulse temporal shaping controlled by a deterministic algorithm

Femtosecond laser pulse temporal shaping techniques have led to important advances in different research fields like photochemistry, laser physics, non-linear optics, biology, or materials processing. This success is partly related to the use of optimal control algorithms. Due to the high dimensionality of the solution and control spaces, evolutionary algorithms are extensively applied and, among them, genetic ones have reached the status of a standard adaptive strategy. Still, their use is normally accompanied by a reduction of the problem complexity by different modalities of parameterization of the spectral phase. Exploiting Rabitz and co-authors’ ideas about the topology of quantum landscapes, in this work we analyze the optimization of two different problems under a deterministic approach, using a multiple one-dimensional search (MODS) algorithm. In the first case we explore the determination of the optimal phase mask required for generating arbitrary temporal pulse shapes and compare the performance of the MODS algorithm to the standard iterative Gerchberg–Saxton algorithm. Based on the good performance achieved, the same method has been applied for optimizing two-photon absorption starting from temporally broadened laser pulses, or from laser pulses temporally and spectrally distorted by non-linear absorption in air, obtaining similarly good results which confirm the validity of the deterministic search approach.

Silicon-Organic Hybrid MZI Modulator Generating OOK, BPSK and 8-ASK Signals for Up to 84 Gbit/s

We report on high-speed multilevel signal generation and arbitrary pulse shaping with silicon-organic hybrid (SOH) Mach-Zehnder interferometer (MZI) modulators. Pure phase modulation exploiting the linear electrooptic effect allows the generation of multiple modulations formats at highest speed such as 40-Gbit/s on-off-keying (OOK) and binary-phase-shift keying (BPSK) and 28-Gbd 4-ASK and 8-ASK with data rates up to 84 Gbit/s. Additionally, beside NRZ pulse shaping, for the first time, Nyquist pulse shaping with silicon modulators is demonstrated to enable multiplexing at highest spectral efficiency.

Modeling and Analysis of Ladder-Network Transmission Lines with Capacitive and Inductive Lumped Elements

This paper examines the properties of wave propagation in transmission lines with periodic LC and CL cells, taking into account ohmic losses in resistors connected in series to lumped capacitors and inductors. First order time differential equations are derived for current and charge, thus allowing analysis of transient regimes of the lines being excited by a pulse of arbitrary shape. In particular we examine the propagation characteristics of periodic lines in which identical unit cells are repeated periodically and also discuss the interpretation of positive and negative phase velocities associated with the LC and CL topologies. Loss effects on the propagation bandwidths of both lines are also discussed, and it is shown that in the left-handed transmission line (CL configuration) the phase advance of the crest of the transmitted signal with respect to the source signal is due to the intrinsic dispersive nature of the CL line which, in contrast to the LC line, is highly dispersive at low propagation factors.

A low-power, generic biostimulator with arbitrary pulse shape, based on a central control core

A low-power, generic biostimulator with arbitrary pulse shape, based on a central control core

Parametrically shaped femtosecond pulses in the nonlinear regime obtained by reverse propagation in an optical fiber

We present the experimental realization of a method to generate predetermined, arbitrary pulse shapes after transmission through an optical fiber in the nonlinear regime. The method is based on simulating the reverse propagation of the desired pulse shape in the fiber. First, linear and nonlinear parameters of a single-mode step-index fiber required for the simulation are determined. The calculated pulse shapes are then generated in a pulse shaper.

Nonlinear Shannon Limit in Pseudolinear Coherent Systems

In this paper, we develop a general first-order perturbation theory of the propagation of a signal in an optical fiber in the presence of amplification and Kerr nonlinearity, valid for arbitrary pulse shapes. We obtain a general expression of the sampled signal after optical filtering, coherent detection, and optimal sampling. We include intrachannel and as well as interchannel nonlinear effects. We obtain simplified expressions in the case in which the accumulated dispersion is high (equivalent to the far-field limit in paraxial optics). This general theory is applied in detail to the special case of spectral-efficient sinc pulses. This exercise shows that the characteristics of the neighboring wavelength-division multiplexed channels are essential in determining the nonlinear impairments.

Quantum motion in superposition of Aharonov–Bohm with some additional electromagnetic fields

The structure of additional electromagnetic fields to the Aharonov-Bohm field, for which the Schr\"odinger, Klein-Gordon, and Dirac equations can be solved exactly are described and the corresponding exact solutions are found. It is demonstrated that aside from the known cases (a constant and uniform magnetic field that is parallel to the Aharonov-Bohm solenoid, a static spherically symmetrical electric field, and the field of a magnetic monopole), there are broad classes of additional fields. Among these new additional fields we have physically interesting electric fields acting during a finite time, or localized in a restricted region of space. There are additional time-dependent uniform and isotropic electric fields that allow exact solutions of the Schrodinger equation. In the relativistic case there are additional electric fields propagating along the Aharonov-Bohm solenoid with arbitrary electric pulse shape.

Gain without population inversion in a yoked superfluorescence scheme

We consider a medium composed of three-level (cascade scheme) atoms prepared with coherence between the upper and the ground states (a yoked superfluorescence system). We obtain an analytical solution for propagation of seed pulses which are resonant with the upper and lower transitions for arbitrary level populations and pulse shapes. We find that if the initial coherence is large enough and the intermediate level is populated the system can have gain without population inversion. Coherence can also yield gain suppression in the inverted medium. We obtain conditions for the gain in terms of level populations and coherence.

Pulse propagation in an elastic medium with quadratic nonlinearity (L)

This letter examines the propagation of an acoustic pulse in an elastic medium with weak quadratic nonlinearity. Both a displacement pulse and a stress pulse of arbitrary shapes are used to generate the wave motion in the solid. By obtaining the explicit solutions for arbitrary pulse shapes, it is shown that for a sinusoidal tone-burst, in addition to a second order harmonic field, a radiation induced static strain field is also generated. These results help clarify some confusion in the recent literature regarding the shape of the propagating static displacement pulse.

Novel Orthogonal Codes for Spectrally-Encoded CDMA Systems in Fading Channels

Orthogonal spreading codes play an essential role in code-division multiple-access (CDMA) systems by eliminating multiple-access interference (MAI). In this paper, a novel orthogonal spreading code has been proposed for spectrally-encoded (SE) CDMA, a.k.a., spread-time (ST) CDMA with arbitrary pulse shape. It has been shown that it is possible to retain the orthogonality of the code in the presence of tail truncation by time windowing and in a general multipath fading channel in which users experience different frequency selectivity just by modifying the user codewords. Simulation results show that the proposed codes can achieve single user performance when the code length is twice the number of users.

Time dependent model of gain saturation in microchannel plates and channel electron multipliers

Abstract We present and discuss a time dependent solution of the transmission line model of a channel electron multiplier, introduced in a previous paper and already solved in steady-state conditions. The model is applicable to all the situations in which the multiplier input current is sufficiently large so that the statistical variations of the gain for each electron can be ignored and it does not apply to photon counting detectors. By introducing the appropriate boundary conditions the time dependent non-linear equations of the model are reduced to an integral equation in implicit form, whose solution can be calculated numerically by a perturbative approach. In this way the multiplying current signal i(z,t) and the voltage V(z,t) are found as functions of the position z along the channel, and of the time t during the pulse itself, for any arbitrary shape of the input current waveform. The important case of the amplification of input current pulses with a short duration compared to the multiplier recovery time is investigated in detail, showing that the non-linear behavior can be entirely described by a general function of a conveniently defined saturation parameter and that this function is characteristic of any uniform channel multiplier. The model is then used to investigate the recovery of the multiplier after a saturating pulse, and it is found that the gain recovery from weak or moderate saturation levels is exponential to a very good approximation, but with a time constant different from the characteristic time constant RC of the multiplier. Finally the case of pulses of arbitrary shape and duration is considered and examples are given of the amplification of step pulses and of a regular sequence of identical pulses. A remarkable feature of the model is that the solution can be calculated from the time shape of the output pulse, rather than from the input. This makes possible to implement methods for pulse restoration, i.e. for recovering the original input pulse shape from a measured saturated output.

Precise BER Computation for Binary Data Detection in Bandlimited White Laplace Noise

Investigation into the characteristics and behaviours of Laplace noise is of crucial importance for evaluating the performance of communication systems operating in impulsive noise, as well as for ultra-wideband wireless systems operating in the presence of multi-user interference. The bit error rate performances of a binary data communication system operating in the presence of additive bandlimited white Laplace noise is analyzed theoretically. A theoretical expression for the average bit error rate is derived using the Beaulieu series for the optimal soft-limiting detector and the matched-filter detector when arbitrary pulse shapes are used. The bit error rate performance of a reduced complexity version of the optimal detector, the hard-limiting detector, and a reduced complexity version of the matched-filter detector, the sum-of-samples detector, are also analyzed. The analytical expressions for the bit error rate are validated by numerical examples.

Development and Performance of a Controllable Autoloading Needle-Free Jet Injector

A jet injector platform technology that provides improved performance over existing jet injectors through the use of a controllable linear Lorentz-force actuator and software-based control system has been developed. Injectors designed on this platform are capable of delivering injections using arbitrary pressure pulse shaping. Pulse shaping has been shown to allow a wide degree of control over the depth to which the injection is delivered. A software-based injector control system improves repeatability and allows for automatic reloading of the injector, a task that would be difficult to implement using existing jet injector platforms. A design for a prototype autoloading controllable jet injector (cJI) based on this platform is detailed. The injection capability of this cJI was evaluated both in-vitro and in-vivo using a tissue analog, excised porcine tissue, and ovine tissue. An analysis of the cJI’s performance indicates that this design is capable of delivering a controllable volume of fluid to a controllable depth based entirely on the parameter’s input into the control software.