Unknown Pulse Research Articles

Crossed-beam spectral interferometry: a simple, high-spectral-resolution method for completely characterizing complex ultrashort pulses in real time

We present a high-spectral-resolution and experimentally simple version of spectral interferometry using optical fibers and crossed beams, which we call SEA TADPOLE. Rather than using collinear unknown and reference pulses separated in time to yield spectral fringes-and reduced spectral resolution-as in current versions, we use time-coincident pulses crossed at a small angle to generate spatial fringes. This allows the extraction of the spectral phase with the full spectrometer resolution, which allows the measurement of much longer and more complex pulses. In fact, SEA TADPOLE achieves spectral super-resolution, yielding the pulse spectrum with even better resolution. Avoiding collinear beams and using fiber coupling also vastly simplify alignment. We demonstrate SEA TADPOLE by measuring a chirped pulse, a double pulse separated by 14 ps, and a complex pulse comprising two trains of pulses with a time-bandwidth product of ~400.

Zero-additional-phase SPIDER: full characterization of visible and sub-20-fs ultraviolet pulses

We demonstrate a novel spectral-shearing interferometry setup for characterizing the temporal amplitude and phase of ultrashort optical pulses over an extremely wide wavelength region. By the mixing of two strongly chirped auxiliary pulses with the pulse to be characterized, two spectrally sheared replicas are generated, and their spectral interference is evaluated. We fully characterize 10-fs pulses in the visible region by sum-frequency mixing and 19-fs pulses in the ultraviolet region by difference-frequency mixing. The scheme is self-referencing and highly sensitive. The zero-additional-phase scheme does not alter the unknown pulses and yields the pulse shape at the interaction point of a spectroscopic experiment.

Middle Eocene to early Miocene environmental changes in the sub-Antarctic Southern Ocean: evidence from biogenic and terrigenous depositional patterns at ODP Site 1090

During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation. The base of the section is composed of middle Eocene nannofossil oozes mixed with red clays enriched in authigenic clinoptilolite and smectite, deposited at low sedimentation rates (≤2 cm ka −1). It indicates reduced terrigenous sediment input and moderate biological productivity during this preglacial warm climatic stage. The basal strata are overlain by an extended succession (100 m, 4 cm ka −1) of biosiliceous oozes and muds, comprising the upper middle Eocene, the entire late Eocene, and the lowermost early Oligocene. The opal pulse occurred between 37.5 and 33.5 Ma and documents the development of upwelling cells along topographic highs, and the utilization of a marine nutrient- and silica reservoir established during the pre-late Eocene through enhanced submarine hydrothermal activity and the introduction of terrigenous solutions from chemical weathering on adjacent continents. This palaeoceanographic overturn probably was initiated through the onset of increased meridional ocean circulation, caused by the diversion of the Indian equatorial current to the south. The opal pulse was accompanied by increased influxes of terrigenous detritus from southern African sources (illite), mediated by enhanced ocean particle advection in response to modified ocean circulation. The opal pulse ended because of frontal shifts to the south around the Eocene/Oligocene boundary, possibly in response to the opening of the Drake Passage and the incipient establishment of the Antarctic Circumpolar Current. Condensed sediments and a hiatus within the early Oligocene part of the section possibly point to an invigoration of the deep-reaching Antarctic Circumpolar Current. The mid-Oligocene to lower Miocene section on long time scale exhibits less pronounced lithological variations than the older section and points to relatively stable palaeoceanographic conditions after the dramatic changes in the late Eocene to early Oligocene.

Three dimensional imaging of short pulses

We exploit a slightly non-collinear second-harmonic cross-correlation scheme to map the 3D space–time intensity distribution of an unknown complex-shaped ultrashort optical pulse, down to a resolution of 10 μm in space and 200 fs in time. Moreover, we directly show that the spatial phase information is not lost in the process, thanks to the coherence of the non-linear interaction. This implies that the concept of second-harmonic holography can be employed down to the sub-picosecond time scale, and used to discuss the features of the technique in terms of the reconstructed fields.

Characterization of the spectral phase of ultrashort light pulses

The complete characterization of a short light pulse requires the measurement of the value of the electric field as a function of time, or conversely, as a function of the optical frequency. Many different techniques have been demonstrated for this purpose. They fall in two main categories, whether a known reference pulse whose spectrum encompasses that of the unknown pulse is available or not. In the first case, linear techniques such as time-domain or frequency-domain interferometry can be directly used, with the advantage of a high sensitivity. In the latter case, non-stationary filters can be implemented using optical non-linearities, in techniques such as Frequency Resolved Optical Gating (FROG) or Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER).

Online detection of pulse sequence in random noise using a wavelet

Two types of wavelet-based algorithms are proposed for an online detection of a train of unknown pulse signals corrupted by random noise. The mechanism of detecting singularities hidden in the noisy observation data is analyzed, and the performance of the proposed signal detectors is evaluated. Simulation studies are provided to confirm the effectiveness of the algorithms.

Cramer-Rao lower bounds on estimating the parameters of a filtered burst of sinusoid

Using Cramer-Rao lower bounds (CRLBs) for estimating time of arrival and frequency of a burst of sinusoid, we investigate the effect of the location and extent of the observation interval, relative to the support of the received signal pulse, and the effect of not knowing the time duration of the pulse. The effect of filtering the received waveform using a known bandpass filter is also studied and is found to be small. The tail portion of the filter response after the signal pulse ends provides almost no information about the signal parameters. The initial transient build-up portion of the signal pulse response provides specially important information about time of arrival. For observation intervals which include the whole pulse duration, there are significant potential performance differences between the cases of known and unknown pulse duration.

T-scanner for measuring pulse durations

A method for ultrashort pulse-duration measurement is analysed both theoretically and experimentally. The approach is based on a time-resolved induced phase modulation of a probe beam-pulse in a two-colour Z-scan configuration. The experimental results presented are in good agreement with the simple analytical result derived for retrieving unknown pulse width.

Novel arbitrary-amplitude soliton solutions of the cubic-quintic complex Ginzburg-Landau equation.

We propose a method for finding stationary pulse solutions of the complex Ginzburg-Landau equation, which can be applied to both cubic and quintic models. In particular, we discover previously unknown arbitrary-amplitude pulse solutions for both the cubic and quintic models. Numerical simulations show that these solutions are stable relative to small perturbations.

Pulse arrival time estimation based on pulse sample ratios

Standard methods of determining the time of arrival (ToA) of an unknown pulse are adaptive thresholding (ATH) and double differentiation (DD). The former first finds the amplitude of the pulse, and uses a fraction, say 1/2, of the amplitude as a threshold (TH). The ToA estimate is the time when the leading edge of the pulse exceeds TH. In DD, the ToA estimate is the time at which the maximum slope of the leading edge occurs. In ATH, there is the requirement of storing measurements, until slightly past the pulse center, and of a peak detection. Due to differentiation, DD is unreliable at even moderate noise situations. By using the ratio of pulse samples as a detection statistic, the ratio thresholding (RTH) scheme requires only a constant TH. Simplicity in implementation is its main advantage over ATH. Its theoretical variance is derived and corroborated by simulation. The performance of RTH is superior to DD and close to ATH.

A robust correlation receiver for distance estimation

Many methods for distance estimation, such as the ultrasonic pulse-echo method, involve the estimation of a time-of-flight (TOF). In this paper, a signal model is developed that, apart from the TOF, accounts for an unknown, linear frequency dependent distortion as well as for additive noise. We derive a TOF estimator for this model based on the criteria of maximum likelihood. The resulting receiver can be seen as an extension or generalization of the well known cross-correlation, or matched filter, estimator described, e.g., by Nilsson. The novel receiver is found to be more robust against unknown pulse shape distortion than the cross-correlation estimator, giving less biased TOF estimates. Also, bias versus noise sensitivity can be controlled by proper model order selection. >

Detection of oscillatory and impulsive transients using higher-order correlations and spectra

Higher-order cross and ordinary correlation detectors are applied to four deterministic transients contaminated by uncorrelated Gaussian noise only. Histograms and moments are used to examine the properties of the signals and their effect on detector performance. Receiver operating characteristic (ROC) curve analysis and limiting signal-to-noise ratios for ‘‘good’’ detection provide comparative measures for different detectors. Probability density functions of detection ordinate values of signal-present and noise-only correlations are used to explain ROC curve behavior. Using a known source, the cross-correlation detector performs better than the higher-order correlation detectors for each transient studied. However, for an unknown narrow pulse source signal, the bicorrelation and tricorrelation detectors outperform the cross-correlation detector. In contrast, the bicorrelation detector performs very poorly for low-frequency narrow-band signals with a small third moment embedded in uncorrelated Gaussian noise. Rectification as part of the detection process improves the performance of the bicorrelation detector and also places the peak of maximum magnitude at the origin. This eliminates the problem in detection or time delay estimation that the existence of multiple peaks due to symmetries in the auto-bicorrelation or the bicorrelation of repeated signals may cause. The tricorrelation detector also performs better with rectification than without. For an unknown source, the bicorrelation and tricorrelation detectors with rectification perform on a level comparable to the cross-correlation detector for certain signals. Comparisons are made between repeating a known source and repeating noisy received signals in the bicorrelation.

On-line generation of sensitivity functions using correlation analysis

Abstract This paper presents a new algorithm for on-line generation of sensitivity functions which is based on correlation analysis. The algorithm is derived through system identification methods. Special attention is devoted to measuring the sensitivity function of a system with small parameter variations, which is significant for practical problem solving. The algorithm permits sensitivity functions to be calculated for systems with an unknown pulse response function during normal system operation

Detection Probabilities for Fluctuating Infrared Targets

Probability density functions for the projected area of a tumbling or precessing target are employed as prior distributions in the derivations of signal plus noise distributions for an ir scanner. The analysis treats several cases for known and unknown pulse arrival times using a matched filter and also an unspecified filter, where the sensor noise is stationary and Gaussian. The signal pulse shape is variously Gaussian or unspecified. The paper also provides the characteristic functions and detection probabilities corresponding to the signal plus noise densities.

Microwave Pulse-Power Measurements

This paper describes two techniques (crystal switch and pulsed receiver) for measuring peak power. Both systems sample the unknown RF pulse. The sample is then compared to an identical sample taken from an accurately known CW signal. A coherent detector is used to compare the levels of the two samples. Measurement can be made on pulse power levels down to -60 dbm. Neither measurement system is affected by the rise and fall time of the unknown RF pulse. Pulse widths of 0.3 μsec or greater can be measured with an accuracy approaching 3 per cent at 0 dbm and 5 per cent at -60 dbm.

The linear pulse stretching circuit

Accurate determination of the width of short-duration pulses, using common low-bandwidth equipment, is accomplished by the linear pulse stretching circuit. An unknown input pulse may be elongated by a known factor, and the wider pulse is more easily measured. Also included is an analysis of the associated wave-shaper, two methods of readout using available equipment, and the results of measurements.