Adjacent Pulses Research Articles

Chaotic synchronization and evolution of optical phase in a bidirectional solid-state ring laser.

We present results on experimental and theoretical studies of chaos in a solid-state ring laser with periodic pump modulation. We show that the synchronized chaos in the counter-propagating waves is observed for the values of pump modulation frequency fp satisfying the inequality f1 < fp < f2. The boundaries of this region, f1 and f2, depend on the pump-modulation depth. Inside the region of synchronized chaos we study not only dynamics of amplitudes of the counter-propagating waves but also the optical phases of them by mixing the fields of the counter-propagating waves and recording the intensity of the mixed signal. We demonstrate experimentally that in the regime of synchronized chaos the regular phase jumps appear during intervals between adjacent chaotic pulses. We improve the standard semi-classical model of a SSRL and consider an effect of spontaneous emission noise on the temporal evolution of intensities and phase dynamics in the regime of synchronized chaos. It is shown that at the parameters of the experimentally studied laser the noise strongly affects the temporal dependence of amplitudes of the counter-propagating waves.

Implementation of non-linearity management for Gaussian pulses in a fiber-optic link by means of second-harmonic-generating modules

We introduce a model for the non-linearity compensation (“management”), based on second-harmonic-generating [ χ (2)] elements, integrated with amplifiers, which are periodically inserted into a fiber-optic link. The link features the Kerr [ χ (3)] non-linearity, group-velocity dispersion (GVD), and loss. Simulations demonstrate that the system can directly provide for effective compensation of the fiber non-linearity for quasi-Gaussian pulses over the propagation distance z up to 10 fiber spans between the χ (2) modules, and the interaction between adjacent pulses does not manifest itself over the same propagation distance, provided that the separation between them is ≃5 widths of the pulse or more. The results are sensitive to the correct choice of the mismatch parameter in the χ (2) module, but not to variation of other parameters (for instance, peak power of the pulses). For larger values of z, distortion of the pulses commences under the action of GVD. This can be prevented by periodic dispersion compensation. Although the latter is not directly included into the model, it is demonstrated that the non-linearity and dispersion compensation can be implemented independently, as they take place at essentially different scales of z for typical NRZ pulses. Thus, the stable-transmission distance can be expanded much farther.

Inadvertent transmural India ink tattooing simulating intestinal infarction

An 80-year-old man with a history of colon carcinoma, which was resected for cure in 1985, presented with guaiac positive stools. A colonoscopy was performed and showed a new colonic primary in the distal sigmoid colon. At the time of endoscopy, biopsies were done and the tumor was marked with India ink to facilitate locating the lesion at laparotomy. Metastatic workup was unremarkable. The patient was otherwise asymptomatic. He was brought to the operating room subsequently. At laparotomy, a small segment of small bowel was found within a nest of abdominal adhesions and appeared necrotic (Fig. 1). After adhesiolysis, this segment was delivered through the incision and upon examination it was believed that there may have been an adhesive band that caused this (Fig. 2). On closer inspection the area in question was noted to have good peristalsis, and on palpating the adjacent mesentery palpable pulses were present. The serosa appeared healthy and not friable. Apart from dark discoloration, all other parameters of intestinal viability were normal. Further exploratory laparotomy did not reveal any further adhesive bands or any internal hernia defects. The colonic tumor was easily identified because there were a few scattered areas of India ink tattoos from the endoscopic markings on the serosa of the large bowel. The tumor location was verified with the colonoscope. It then became apparent that the infarcted-appearing bowel was a result of the injection of India ink traversing the large bowel and infiltrating layers of the small bowel. A resection of the large bowel tumor with primary anastomosis was carried out, and the discolored small bowel was left intact. Postoperatively the patient made an uneventful recovery.

Bi‐soliton pulse in a dispersion‐managed optical transmission system and its application

AbstractIn this paper, the mutual interaction of adjacent pulses is studied as a cause for degradation of the transmission quality of a dispersion‐managed (DM) soliton. It is shown that the variation of the pulse spacing due to the mutual interaction between two DM solitons can be made extremely small by appropriately selecting the incident pulse spacing and the transmission line parameters. Next, the averaging method is applied to two DM solitons in the parameter region in which the effect of this mutual interaction becomes extremely small. It is shown that there exists a bi‐soliton as a steady solution with two main lobes. Its nature is discussed in detail. Further, a novel transmission line coding scheme using this bi‐soliton is worked out and a method to reduce the interpulse mutual interaction is proposed. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(3): 44–52, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10035

A software method for suppressing statistical fluctuations in preset count digital-rate meter algorithms

A method for suppression of statistical fluctuations of the mean count rate measurements based on limiting the time difference between two adjacent input pulses to a pre-assigned range, has been incorporated in the preset count digital-rate meter algorithm. The algorithm allows measurement of mean count rates with considerably lower levels of statistical fluctuations of the measurement results and significantly lower values of the preset count compared to the traditional algorithm. The exact relation between the lower and upper limits of the difference between two adjacent pulses has been derived. The lower limit of the preset count applicable to the algorithm has been identified. The response time of the algorithm to a sudden change of count rate has been determined.

Phase dynamics of radiation in chaotic lasing regimes of a solid-state ring laser

The radiation phase dynamics of a solid-state ring laser with a periodic pump modulation operating in a dynamic chaotic regime is studied theoretically and experimentally. It is found that, in the regime of synchronised dynamic chaos, the optical phases of counterpropagating waves change by π in intervals between two adjacent chaotic radiation pulses. It is shown that spontaneous radiation has a significant effect on the chaotic dynamics of the counterpropagating wave intensities, but its effect on the phase dynamics in a solid-state ring laser is negligible.

A Novel Method for Measuring the Absolute Phaseof a Few-Cycle High Intensity Laser

The effect of the absolute phase of the few-cycle driving laser fieldon the generation and measurement of high-order harmonic attosecondpulses is investigated theoretically. We find that the generatedattosecond soft-x-ray pulse is locked to the oscillations of thedriving laser field, but not to the envelope of the laser pulse, andthe intensity ratio of two adjacent attosecond pulses is exponential asa function of the absolute phase. Based on these results, we propose anovel method to detect the absolute phase of the driving laser field bymeasuring the spatial distribution of the photoelectrons induced by theattosecond soft-x-ray pulse and the driving laser field.

Direct measurement of the group refractive index of air with interferometry between adjacent femtosecond pulses.

The group refractive index of air in laboratory conditions is measured directly between adjacent femtosecond laser pulses by a new interferometry technique. Measurement of the repetition rate of the mode-locked pulse train that gives the maximum amplitude of the interference-signal envelope enables us to determine the group refractive index of air within a standard deviation of 2 x 10(-7). This simple method without vacuum reference is attractive for measuring the group refractive index needed for precise distance measurements in open fields.

Holographic recording of fast phenomena

We report on a holographic method for recording fast events whose speed is limited by the laser pulse duration if the recording material has sufficient sensitivity to reliably record a frame of the fast event with a single pulse. The method we describe uses the angular selectivity of thick holograms to resolve frames that are recorded with adjacent pulses. Two specially designed cavities are used to generate the signal and reference pulse trains. We experimentally demonstrate the system by recording laser induced shock waves with a temporal resolution of 5.9 ns, limited by the pulse width of the Q-switched Nd:yttrium–aluminum–garnet laser used in the experiments.

Intrachannel pulse interactions in dispersion-managed transmission systems: energy transfer

Energy transfer between adjacent pulses as a result of nonlinear intrachannel interactions in a transmission system with strong periodic dispersion management is analyzed. With small average dispersion, the energy change between nonzero bits grows linearly with respect to distance in the presence of loss and lumped amplification, whereas in a lossless system the energy change is found to be significantly suppressed. The analytical results compare favorably with the results of direct numerical simulation of the full system.

OTDM transmitter using WDM-TDM conversion with an electroabsorption wavelength converter

An all-optical multiplexing technique using wavelength division multiplexing (WDM)-time division multiplexing (TDM) conversion with an electroabsorption wavelength converter has been proposed and demonstrated. The effectiveness of this WDM-TDM conversion technique for various pulsewidth settings was experimentally investigated. The fluctuation of the signal performance, which was inevitably caused by the coherent crosstalk between adjacent pulses in the conventional optical time division multiplexing (OTDM) technique, were successfully suppressed, even in the case of wide pulse duration. High Q-factor performance has been maintained for a wide range of duty ration from 36% to 74%. By introducing this technique to the optical time division multiplexer, a highly stable and high-quality 40-Gb/s optical signal can be effectively produced without generating the short pulse or setting two tributaries at orthogonal polarization states, and without introducing high-speed electronics for signal multiplexing. The WDM-TDM conversion with an electroabsorption wavelength converter was extended to 60-Gb/s operation by using three 20-Gb/s tributaries. A clear eye opening was confirmed for a waveform after the WDM-TDM conversion of the 60-Gb/s signal.

Polarization-mode dispersion-induced outages in soliton transmission systems

The bit-error-rate (BER) degradation of conventional soliton systems due to polarization-mode dispersion (PMD) is investigated. It is found that the interplay between the dispersive waves generated by PMD and adjacent soliton pulses will seriously degrade the BER of soliton systems, and make them even worse than linear systems if all other transmission impairments are neglected. In order to achieve soliton robustness to PMD, some techniques to eliminate or reduce the dispersive waves must be employed, such as soliton control methods or dispersion-managed solitons. Different systems are estimated and compared in terms of PMD-induced outage probability.

Intrachannel four-wave mixing in dispersion managed RZ systems

We model the interaction of three consecutive pulses in a dispersion managed transmission system, and obtain an analytical expression for the energy exchange. The results show that interaction can be suppressed using unequally spaced pulses, which has been verified by the computer simulations. The central pulse pumps energy to the adjacent pulses if the phase shift is zero, and the energy exchange process is reversed if the central pulse has a phase shift of /spl pi//2.

Stability of multiple pulses in discrete systems.

The stability of multiple-pulse solutions to the discrete nonlinear Schrödinger equation is considered. A bound state of widely separated single pulses is rigorously shown to be unstable, unless the phase shift Delta phi between adjacent pulses satisfies Delta phi=pi. This instability is accounted for by positive real eigenvalues in the linearized system. The analysis leading to the instability result does not, however, determine the linear stability of those multiple pulses for which Delta phi=pi between adjacent pulses. A direct variational approach for a two-pulse predicts that it is linearly stable if Delta phi=pi, and if the separation between the individual pulses satisfies a certain condition. The variational approach can easily be generalized to study the stability of N pulses for any N>or=3. The analysis is supplemented with a detailed numerical stability analysis.

Prescaled clock recovery based on small timing misalignment of data pulses

A simple and robust prescaled clock recovery technique is analyzed and demonstrated. An electrical clock is extracted from an ultra-high-speed time-division multiplexed (TDM) RZ signal using a classic approach to clock recovery with a detector and a bandpass filter (BPF). A subharmonic tone at the base rate frequency is generated by inducing a small misalignment between adjacent pulses in the transmitted data. The subharmonic tone is recovered as a clock signal at the receiver. Numerical calculations clarify the effect of filter bandwidth, word length, and strength of timing shift on the received timing jitter. Furthermore, it is found numerically that correlated TDM channels will decrease the jitter of the recovered clock considerably. A clock recovery circuit is implemented into an experimental 40 Gb/s and 80 Gb/s optical TDM (O-TDM) system without any observed penalty. Measurements of the timing jitter of the recovered prescaled clock have been performed to verify the numerical results. A 10 GHz clock signal with subpicosecond root-mean-square timing jitter is recovered from a 40-Gb/s O-TDM sequence without a phase-locked loop (PLL) configuration. By using a PLL-configuration, the timing jitter is reduced further by 50%. A discussion on the influence on transmission capacity is performed in general and for nonlinear optical communication systems in particular.

Autoregressive modeling of transfer functions in frequency domain to determine complex travel times

We present a method to determine the complex travel times of impulses in the time domain on the basis of an autoregressive (AR) modeling of superimposed sinusoids in a finite complex series in the frequency domain. We assume that the complex frequency series consists of signals represented by a complex AR equation with additional noise. The AR model in the frequency domain corresponds to a complex Lorentzian in the time domain. In a similar way to the Sompi or extended Prony method, the complex travel times are given by solutions of a characteristic equation of complex AR coefficients, which are obtained as the eigenvector corresponding to a minimum eigenvalue in an eigenvalue problem of non-Toeplitz autocovariance matrix of the complex series. Our method is tested for synthetic frequency series of transfer functions, which show that (1) the complex travel times of closely adjacent pulses in the time domain are clearly resolved, and that (2) the frequency dependence of the complex travel times for physical and structural dispersions is precisely determined by the analysis within a narrow frequency window. These results demonstrate the usefulness of our method with high resolvability and accuracy in the analysis of impulse sequences.

Optimization of the average-dispersion range for long-haul dispersion-managed soliton systems

We consider limitations on unfiltered transmission of dispersion-managed solitons, arising from the Gordon-Haus jitter, adjacent pulse interaction, and signal-to-noise degradation. We maximize the range of allowed values of average dispersion, thereby providing the first step in optimization of dispersion maps for wavelength-division-multiplexed lightwave systems. As specific examples, we consider dispersion maps made of several different types of optical fiber and study their performance for transmission of 10-40 Gb/s channels over distances in the range from 3000 km to 10000 km.

The Late Weichselian glaciation of the Franz Victoria Trough, northern Barents Sea: ice sheet extent and timing

High resolution seismic profiles (PARASOUND, 4 kHz) and three sediment cores from the Franz Victoria Trough and the adjacent continental slope were studied in order to constrain the timing and extent of the northern Svalbard/Barents Sea ice sheet during the Late Weichselian glaciation. Stacked debris flow lobes and layers of glacimarine diamicton on the lower continental slope indicate that large quantities of glacially derived sediments were deposited by the northern Svalbard/Barents Sea ice sheet directly onto the upper continental slope at approximately 23 14 C ka. A grounding-line advance to the shelf break is supported by the identification of diamicton, interpreted as till, in the seismic profile near the shelf break. After several ice sheet instabilities marked by significant input of ice rafted detritus to the continental margin, the disintegration of the northern Svalbard/Barents Sea ice sheet (Termination Ia) is indicated by a distinct pulse of ice rafted detritus at 15.4 14 C ka and the transition to an isotopically defined meltwater signal. The drastic change in sedimentary pattern on the upper continental slope, dated to about 13.4 14 C ka, is interpreted as grounding-line retreat from the shelf edge. A further stepwise retreat of the northern Svalbard/Barents Sea ice sheet is indicated by pulses of ice rafted detritus which appear to be contemporaneous with the onset of distinct ice rafting events in adjacent areas and pulses of glacimarine sedimentation in the southwestern Barents Sea. q 2000 Elsevier Science B.V. All rights reserved.

Chaotic pulse position modulation: a robust method of communicating with chaos

In this letter we investigate a communication strategy for digital ultra-wide bandwidth impulse radio, where the separation between the adjacent pulses is chaotic arising from a dynamical system with irregular behavior. A pulse position method is used to modulate binary information onto the carrier. The receiver is synchronized to the chaotic pulse train, thus providing the time reference for information extraction. We characterize the performance of this scheme in terms of error probability versus E/sub b//N/sub 0/ by numerically simulating its operation in the presence of noise and filtering.

Stable multisoliton pulses in dispersion management with fiber Bragg gratings

We have studied the propagation of prechirped Gaussian pulse pairs in a fiber Bragg grating dispersion-managed system. We discovered that, under quite general conditions, a number of individual pulses evolve to a stable bound multisoliton solution, with fixed values for the phase difference and the distance between adjacent pulses. These stable multisoliton solutions may propagate for long distances without deformation, with the ultimate distance limitation imposed by the noise amplification.