Pulse Shape Distortion Research Articles

Numerical studies on the electro-optic detection of femtosecond electron bunches

The electro-optic (EO) effect is a powerful diagnostic tool for determining the time profile of ultrashort relativistic electron bunches. When a relativistic bunch passes within a few mm of an electro-optic crystal, its transient electric field is equivalent to a half-cycle THz pulse passing through the crystal. The induced birefringence can be detected with polarized femtosecond laser pulses. A simulation code has been written in order to understand the faithfulness and the limitations of electron bunch shape reconstruction by EO sampling. The THz pulse and the laser pulse are propagated as wave packets through the EO crystal. Alternatively, the response function method is applied. Using experimental data on the material properties of zinc telluride (ZnTe) and gallium phosphide (GaP), the effects of velocity mismatch, pulse shape distortion, and signal broadening are explicitly taken into account. The simulations show that the most severe limitation on the time resolution is given by the transverse-optical (TO) lattice oscillation in the EO crystal. The lowest TO frequency is 5.3 THz in ZnTe and 11 THz in GaP. Only the Fourier components below the TO resonance are usable for the bunch shape reconstruction. This implies that the shortest rms bunch length which can be resolved with moderate distortion amounts to $\ensuremath{\sigma}\ensuremath{\approx}90\text{ }\text{ }\mathrm{fs}$ in ZnTe and $\ensuremath{\sigma}\ensuremath{\approx}50\text{ }\text{ }\mathrm{fs}$ in GaP. The influence of the crystal thickness on the amplitude and width of the EO signal is studied. The optimum thickness is in the range from 100 to $300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ for ZnTe and from 50 to $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ for GaP.

Nonlinear effects in pulse propagation through Doppler-broadened closed-loop atomic media

Nonlinear effects in pulse propagation through a medium consisting of four-level double-$\ensuremath{\Lambda}$-type systems are studied theoretically. We apply three continuous-wave driving fields and a pulsed probe field such that they form a closed interaction loop. Due to the closed loop and the finite frequency width of the probe pulses, the multiphoton resonance condition cannot be fulfilled, such that a time-dependent analysis is required. By identifying the different underlying physical processes we determine the parts of the solution relevant to calculate the linear and nonlinear response of the system. We find that the system can exhibit a strong intensity-dependent refractive index with small absorption over a range of several natural linewidths. For a realistic example we include Doppler and pressure broadening and calculate the nonlinear self-phase modulation in a gas cell with sodium vapor and argon buffer gas. We find that a self-phase modulation of $\ensuremath{\pi}$ is achieved after the propagation of a few centimeters through the medium while the absorption and pulse shape distortion in the corresponding spectral range is small.

Slow light with persistent spectral hole burning in waveguides

We model numerically the reshaping of a weak probe pulse propagating in an absorptive, optically dense, persistent spectral hole burning medium under conditions of slow group velocity. Saturated holes are burned in waveguide geometry by illumination in the transverse direction with low absorption, whereas the probing is carried out in longitudinal wave guiding directions with high absorption. We show that by choosing optimum hole spectrum, the Gaussian probe pulse may be delayed by several times its original duration, while overall pulse shape distortion is less than 1% and average energy loss is less than 10%.

Generalized time domain multipath channel and its application in ultra-wideband (UWB) wireless optimal receiver - part III: system performance analysis

This paper deals with the time domain characteristics of UWB multipath channel while considering UWB pulse shape distortion and its impact on system performance as well as optimal receiver design. It appears that no deterministic and analytical channel model in the time domain has ever appeared in the literature, to our best knowledge. A generalized channel model and some related exact solutions for some specific geometric configurations are discussed, based on the pulse propagation mechanisms of reflection and diffraction. In addition, a system performance framework integrated with the close-form channel model expressions is also provided, corresponding to the optimal receiver structure and the concrete geometric configurations of a UWB channel. This work is very important in the design of the optimal UWB systems

Optical pulse shaping by cross-phase modulation in a harmonic mode-locked semiconductor fiber ring laser under large cavity detuning

The authors experimentally investigate the phenomena of the pulse-shape distortion due to the drift of cavity length or modulation frequency in a harmonically mode-locked semiconductor fiber ring laser (SFRL). Also, it is experimentally demonstrated that the optical pulse shaping by cross-phase modulation (XPM) can be applied to enhance the robustness of a harmonic mode-locked semiconductor ring laser to the environmental factors, e.g., the modulation-frequency drift and/or the cavity-length drift. The mechanism of pulse-shape distortion due to the modulation-frequency drift or cavity-length drift is explained and experimentally verified. As the locking condition of the SFRL goes to the rational harmonic mode-locking (RHML) regime, the pulse-shape distortion becomes dominant. To validate the explanation, a simple equation for estimating and comparing the amount of cavity-length detuning in terms of frequency is proposed for comparison, which is called the equivalent frequency detuning (EFD). The distortion mostly appeared in the trailing edge of the pulse. By selectively filtering the XPM-induced red-chirped wavelength components in the rising edge of the pulse, the mode-locked pulse train with large pulse-shape distortion can be reshaped to regenerate a mode-locked pulse train with less pulse-shape distortion than the pulse train without the proposed scheme. This method can be used to reduce the pulse-shape distortion effectively, maintaining the pulse repetition rate under the large modulation-frequency detuning, or equivalently, large cavity-length detuning. The proposed scheme is able to generate the pulse-trains robust to the modulation-frequency drift and/or cavity-length drift of the mode-locked SFRL.

Engineering superpositions of coherent states in coherent optical pulses through cavity-assisted interaction

We propose a scheme to engineer quantum superpositions of coherent states (``Schr\"odinger-cat states'') of propagating optical pulses. Multidimensional and multipartite cat states can be generated simply by reflecting coherent optical pulses successively from a single-atom cavity. The influences of various sources of noise, including atomic spontaneous emission and pulse-shape distortion, are characterized through detailed numerical simulation, which demonstrates the practicality of this scheme within the reach of current experimental technology.

The Effects of Salinity on the Accuracy and Uncertainty of Water Content Measurement

We used an automatic network analyzer (ANA) operated in both time and frequency domain modes to investigate the measurement accuracy of metallic time domain reflectometry (TDR) probes operated in sands saturated with NaCl solutions of varying electrical conductivity (EC). We chose to use time domain transmission (TDT) measurements for this investigation to separate the effect of the bulk soil‐probe interaction from the effect of the large reflection typically found at the air–soil boundary for a TDR configuration. Pulse travel times and their variability increase with increasing pore‐water EC. The source of travel time variability arises from the extreme variability in pulse shape, thereby introducing a high degree of uncertainty to curve fitting routines used to determine travel times. Pulse shape distortion is due primarily to attenuation of high frequency components through conductive loss rather than by dispersion. There is generally good correspondence between pulse rise time and the average water content measurement error over a 0‐ to 40‐dS m−1 EC range. For rise times <6 ns, measurement errors are <0.1 m3 m−3 Given that rise times can be determined easily, we recommend that they be reported routinely as indicators of data quality. Because the variability of travel time measurements at a given EC is on the same order as the average difference between the travel time measured at that EC and that measured at zero salinity, pulse rise times cannot be used to correct for individual travel time measurements at high salinity. Similarly, knowledge of the EC of the medium may allow for identification of erroneous water content measurements, but does not allow for error correction.

Influence of Temporal Pulse Shape on Second Harmonic Generation

Frequency conversion technique using nonlinear optical crystals in indispensable to solid-state laser applications. It is well known that the conversion efficiency is very sensitive to temperature change of crystal, which is caused by laser absorption. We have been analyzing problems of second harmonic generation (SHG) by coupling the electric field to the temperature field. In this study, temporal pulse-shape dependence of SHG was investigated theoretically, and Gaussian pulse and rectangular pulse were compared. Main conclusions obtained are as follows : (1) Both conversion efficiencies with Gaussian and rectangular pulses fluctuate with irradiation time, however the former fluctuation is slower and more gentle. (2) Pulse of output second harmonic fluctuates most remarkably at the center, where power density of incident fundamental is highest, in the Gaussian pulse. (3) As a result, pre-pulse and post-pulse are geserated before and after the main pulse. (4) Such distortion of output pulse shape becomes more remarkable as power density increases.

Suppression of pulse shape distortion caused by frequency drift in a harmonic mode-locked semiconductor ring laser

We propose and demonstrate a novel method to generate an optical pulse train in a harmonic mode-locked semiconductor ring laser that is not prone to the frequency drifts of locking elements. This technique is based on wavelength conversion by cross-phase modulation in a semiconductor optical amplifier inside the ring laser and external filtering of the induced red-chirped frequency components. Though the pulsewidth increased from 45 to 65 ps at a repetition rate of 2.5 GHz due to the characteristics of the external optical filter, the frequency locking range was increased by a factor of 2.9 compared with that of a conventional harmonic mode-locked semiconductor ring laser. In addition, the pulse train over the frequency detuning range of -250 to +250 kHz did not show any distortion in pulse shape.

Effects of transmitter imperfection on polarization mode dispersion impairments

We study the influences of transmitter characteristics on polarization mode dispersion (PMD) impairments in optical transmission systems with both experiments and simulations. We show that transmitter imperfection, such as timing and amplitude jitters and pulse shape distortion, can significantly increase PMD impairments, especially when the differential group delay in the systems is large. Our simulation further shows that the increased PMD impairments due to transmitter imperfection will considerably reduce the effectiveness of using large system optical signal-to-noise ratio margin to combat PMD.

Ion Motion Stability Diagram for Distorted Square Waveform Trapping Voltage

Ion motion in a periodic radio frequency (RF) quadrupole electric field is analysed theoretically by the matrix method and direct trajectory calculation. General properties of the ion motion: stability condition, oscillation spectrum and secular frequency are derived analytically from the elements of the transformation matrix. Stability diagrams for ion motion in the Paul ion trap are presented for rectangular waveforms with different duty cycles (duration of pulse over period). Calculation of the secular frequencies of the ion motion in the ion trap is performed for the first time. The relation of radial and axial secular frequencies along the RF scan line was found to be practically identical in both the square waveform and harmonic voltage cases. Pulse shape distortions, due to resistive-inductive-capacitive filtering in real devices, are considered. Stability diagrams of ion motion in the ion trap with distorted voltage waveforms are calculated. Distortion of the waveform is shown to introduce minor changes in the diagram shape with respect to the diagram for an ideal square wave. Within the first stable region, distortion of the waveform does not lead to any auxiliary parametric resonances of the ion motion. Ion trapping with a pure random pulsed voltage is investigated by means of direct trajectory simulations. It is shown that, in this case, the ion motion can be conditionally stable for a considerable length of time.

Ultrafast switching dynamics of Mach-Zehnder interferometer switches

We present results of cross-gain and cross-phase modulation experiments on InGaAsP-based semiconductor optical amplifiers and Mach-Zehnder interferometer (MZI) switches. For 150-fs input pulses, we observe strong spectral and pulse shape distortions of the signal pulse, especially in the pulse overlap regime. These pulse deteriorations are induced by subpicosecond gain and refractive index nonlinearities. In consequence, the ON-OFF ratio of the MZI switch is drastically reduced for subpicosecond switching windows. In the copropagation arrangement, the switching window with maximum transmission is shown to be limited to about 1.5 ps.

Ultracompact autocorrelator for femtosecond laser pulses

We present a novel optical and mechanical design of an autocorrelator which allows unmatched compactness. The device, capable of measuring laser pulses in the femtosecond regime, consists of the minimum number of elements, resulting in little distortion in pulse shape. A Mach–Zehnder interferometer provides operation for a wide range of input beam directions, making this autocorrelator a reliable tool for femtosecond lasers diagnostics.

Spectral filtering of femtosecond laser pulses by interference filters

Phase properties of optical thin film interference filters are discussed from the aspect of their usage for phase-error free wavelength separation of broadband femtosecond laser pulses. It is shown that both transmissive or reflective interference filters with high contrast ratios exhibit high cubic phase shifts on transmission or reflection, respectively, causing intolerable distortion in the temporal pulse shape. We show, however, that high efficiency wavelength separation of broadband femtosecond laser pulses can be achieved by using low contrast, properly designed reflective optical interference filters directly built into the cavity of the broad spectrum, femtosecond pulse lasers or parametric oscillators. For demonstrative purposes, we implemented the idea for a Kerr-lens mode-locked Ti:sapphire laser, and obtained two-color, inherently synchronized, unchirped, femtosecond pulse outputs from a single laser oscillator.

Fast analog signal transmission for an air Cherenkov photomultiplier camera using optical fibers

A system to demonstrate fast analog signal transmission through optical fibers is presented. The dynamic range of output voltages is 2mV to 1.0V at a bandwidth of 150MHz. The intended future application is to transmit nanosecond photomultiplier pulses over distances of 150m with minimal pulse shape distortions. The advantages, problems and the further development potential are discussed.

Distortion of a chirped short pulse in type II second-harmonic generation

We theoretically studied temporal properties of the second-harmonic generation of a chirped short pulse. Transient coupling-wave equations are used in describing the time-dependent nonlinear process. The pulse distortion of a fundamental wave occurs at the leading or trailing edge, which is determined by the difference of the group velocity between fundamental and harmonic waves. Pulse chirp and phase mismatch disturb the pulse shape and cause phase distortion. Even in exact phase matching, phase distortion is expected because of pulse chirp. Both distortion of pulse shape and phase shift are dependent on the sign and the quantity of the chirp.

Optical phase conjugation of multiwavelength signals in a dispersion-shifted fiber

Optical phase conjugation (OPC) of multiwavelength signals in a dispersion-shifted fiber (DSF), which can be used for dispersion compensation in the wavelength division multiplexing communication system, is theoretically studied. The multiwavelength phase-conjugate signals are generated by four-wave mixing (FWM) in the dispersion-shifted fiber. There are the pulse-shape distortion and the induced frequency chirping in the phase-conjugate signals owing to pump depletion and the cross-phase modulation among the signals and phase-conjugate signals, respectively. The FWM among the pump wave, signals, and phase-conjugate signals causes unequal conversion efficiencies for the multiwavelength signals and enhances the induced frequency chirping in the phase-conjugate signals. The induced frequency chirping may deteriorate the restoration of the pulse shape. Both the pulse-shape distortion and induced frequency chirping increase with the signal and pump powers. The formula for the induced frequency chirping that is only caused by the signals through cross-phase modulation is derived. The requirement of the signal power in the dispersion-shifted fiber for the restoration of the pulse shape by the optical phase conjugation is estimated and numerically verified.

Pulse distortion in MR and GMR playback heads

Distortion of the playback pulse shape due to saturation is studied via a 3-D micromagnetic model for both MR/SAL and spin-valve heads. The deviation from linearity is examined as a function of both the Mr/spl delta/ of the recording medium and the off-track position of the device. Playback voltages for isolated transitions of both polarities are calculated and their shapes are compared to linear references. The differences in structure between MR and GMR sensors result in very dissimilar distortions of the playback pulse shape under large applied fields.

Effect of carrier depletion on optical phase conjugation in a semiconductor laser amplifier

The temporal effect of optical phase conjugation in a semiconductor laser amplifier is studied numerically. The conjugate pulse has distortion and frequency chirping owing to the carrier depletion induced by the signal pulse. Both the pulse-shape distortion and frequency chirping are enhanced as the signal power and the injection current increase. We can reduce these two effects by increasing the pump power to reduce the carrier-density depletion induced by the signal pulse.

Margin evaluation of PRML channels: non-linear distortions, misequalization and off-track noise performance

We present a margin-type procedure for predicting the error rate of PRML channels. Experimental error margin plots which demonstrate the influence of misequalization, non-linear distortions and off-track noise on system performance are presented. Pulse shape distortions caused by misequalization and non-linearities produce a distinct shift of the error margin plots, without changing the slope. However, off-track measurements introduce additional noise into the system changing the slope of the plots. Interpretation of experimental results is presented. It is demonstrated that error margin analysis is an effective tool for evaluating performance of PRML channels, heads and media.