Single-shot Signal Research Articles

Characterization of femtosecond laser-induced grating scattering of a continuous-wave laser light in air.

Nanosecond laser-induced grating scattering/spectroscopy (LIGS) technique has been widely applied for measuring thermodynamic parameters such as temperature and pressure in gaseous and liquid media. Recently, femtosecond (fs) laser was demonstrated to induce the grating and develop the fs-LIGS technique for gas thermometry. In this work, we systematically investigated the fs-LIGS signal generation using 35 fs, 800 nm laser pulses at 1 kHz repetition rate in ambient air by varying the pump laser energies, the probe laser powers and the temporal delays between two pump laser pulses. The stability of single-shot fs-LIGS signal was studied, from which we observed that the signal intensity exhibits a significant fluctuation while the oscillation frequency shows a much better stability. A 4.5% precision of the oscillation frequency was achieved over 100 single-shot signals. By using a previously-developed empirical model, the fs-LIGS signals were fitted using nonlinear least-squares fitting method, by which crucial time constants characterizing the signal decay process were extracted and their dependences on the pump laser energy were studied. From the measured results and theoretical analysis, we found that the appropriate range of the overall pump laser energy for reliable fs-LIGS measurements is approximately located within 80 ∼ 300 μJ. The limitations on the accuracy and precision of the fs-LIGS measurements, the origin of destructive influence of plasma generation on the signal generation as well as the electrostriction contribution were also discussed. Our investigations could contribute to a better understanding of the fs-LIGS process and further applications of the technique in single-shot gas thermometry and pressure measurements in various harsh conditions.

Attosecond Delays in Molecular Photoionization.

We report measurements of energy-dependent photoionization delays between the two outermost valence shells of N_{2}O and H_{2}O. The combination of single-shot signal referencing with the use of different metal foils to filter the attosecond pulse train enables us to extract delays from congested spectra. Remarkably large delays up to 160 as are observed in N_{2}O, whereas the delays in H_{2}O are all smaller than 50 as in the photon-energy range of 20-40eV. These results are interpreted by developing a theory of molecular photoionization delays. The long delays measured in N_{2}O are shown to reflect the population of molecular shape resonances that trap the photoelectron for a duration of up to ∼110 as. The unstructured continua of H_{2}O result in much smaller delays at the same photon energies. Our experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.

Cross-polarisation method for improvement of 14N NQR signal detectability

This is a study of the cross-polarization effects in the case of 14N quadrupolar spin-system with a long spin–lattice relaxation time. Two important benefits of the cross-polarization technique were demonstrated for PETN: (i) a polarization transfer resulting in increased NQR single shot signal response and (ii) a dynamic reduction in recovery time of the NQR system allowing scan repetition on a much shorter timescale. It was proved that this technique can reduce the optimal waiting time between pulse sequences up to 60 times through a significant reduction of the relaxation time of the quadrupolar spin-system. All experiments were carried out at room temperature using spin-locking multi-pulse sequences and small external magnetic field.

Multichannel single-shot transient signal measurements with a fiber delay line loop

An instrument has been developed to measure single-shot electrical transient signals, in two channels, with frequency responses above 10 GHz ( http://www.YYLabs.com). This instrument, which is named Fiber Loop Single-Shot Scope, utilizes an optical fiber recirculating delay line loop to regenerate captured single-shot signals, and then recovers the original single-shot signals with a sampling scope.

Parsing local signal evolution directly from a single-shot MRI signal: a new approach for fMRI.

In this work a new single-shot MRI method, single-shot parameter assessment by retrieval from signal encoding (SS-PARSE), is introduced. This method abandons a fundamental simplifying assumption that is used in conventional MRI methods. Established MRI methods implicitly assume that the local intrinsic signal does not change its amplitude or phase during signal acquisition, even though these changes may be substantial, especially during the relatively long signals used in single-shot image acquisitions. SS-PARSE, on the other hand, acknowledges local decay and phase evolution, and models each signal datum as a sample from (k,t)-space rather than k-space. Because of this more accurate signal model, SS-PARSE promises improved performance in terms of accuracy and robustness, but requires more intensive reconstruction computations. The theoretical properties of the method are discussed, and simulation results are presented that demonstrate more robust and accurate measurements of relaxation rate changes associated with brain activation in functional MRI (fMRI), freedom from geometric errors due to off-resonance frequencies, and better tolerance of the large susceptibility gradients that occur naturally in parts of the brain. In addition, this technique has the potential to assess nonexponential relaxation behavior during a single-shot signal.

A novel technique for testing pulsed RF MSK data communication devices

The paper discusses a novel technique for testing Minimum Shift Keying (MSK) impulsive radio-frequency (RF) signal transmitters. This technique utilizes the acquired samples of a single shot signal to determine the trajectories of the frequency and phase, by means of a time domain analysis. The theoretical foundation of this technique is given, discussing the application to a pulsed RF MSK signal modulated with a bit-rate of 5 Mb/s, having a total duration of 6 /spl mu/s and whose central frequency falls in the band going from 960 to 1206 MHz. The performance of the measurement algorithm (robustness, influence of the digitizer resolution, time base jitter, and noise on the precision of the measurement) has been evaluated by means of simulation tests. The method has been experimentally verified for the characterization of identification friend or foe (IFF) time division multiple access (TDMA) actual devices.

Compact tunable Cr:LiSAF laser for infrared matrix-assisted laser desorption/ionization.

A tunable Cr:LiSAF laser-pumped optical parametric oscillator was used for mid-infrared matrix-assisted laser desorption/ionization (MALDI) mass spectrometry experiments. The mass spectra of substance P, bovine insulin and pd(T)10 in the 2.88-2.96 microns range showed excellent single shot signal quality (signal-to-noise-ratio, resolution) but poor shot-to-shot reproducibility. The reproducibility is expected to improve with more stable laser design. No correlation was found between the absorption spectrum of the matrix and the MALDI response.

Near-source characterization of the seismic wavefield radiated from quarry blasts

SUMMARY A series of controlled seismic experiments performed in a limestone quarry demonstrate the utility of high-precision electronic detonators in studying source characteristics of multiple explosive arrays. At near-source ranges (80-130m), where source dimensions are on the same order as source-receiver distances, the influence of the difference in travel path-length among individual explosions on the seismograms is significant. Focusing of the seismic energy is observed as a function of station location with respect to the source array and is attributed to the extended source length (68-94 m) and firing time of the source (380-544 ms). We examine two methods for modelling ripple-fired explosions at near-source ranges using the principles of superpositioning. The first method is based primarily on acquisition of an adequate single shot signal and requires well-constrained shot times. Amplitude variations which result from travel path differences are not modelled, which restricts use of this technique for purposes of blast vibration reduction to larger distances (>2-3 source dimensions) where the spatial finiteness effects of the source begin to diminish. For near-source distances (≤2 source dimensions), we successfully model multiple-source seismograms by convolving a synthetic seismic source signal for a single explosion with individual half-space Green's functions calculated for each explosion in the array. Our single-source model for a cylindrically shaped single charge (borehole length of 17.5 m and diameter of 90 mm) of 68 kg consists of a modified Mueller-Murphy approximation which utilizes source parameter estimates taken from chemical explosion study results. Model parameters include a final cavity radius of 0.25 m and an elastic radius of 18 m. The final model is obtained by convolving the simulated single-source time series with half-space Green's functions calculated for several source depths and superposed to approximate the spatial extent of the borehole. The relative amplitude and phase characteristics of the observed single-source signal at the same distance (80.6 m) are reproduced by this model. Multiple-source synthetic seismograms contain individual Green's functions for each source-receiver distance but utilize identical sources for the explosive array. Focusing effects are shown to be due to the effect of propagation path differences between individual explosions in agreement with the results of Anderson & Stump (1989) in simulating multiple-source seismograms. Good fits to the measured production shot amplitude spectra are obtained with the synthetic spectra. Spectral peaks are well-matched due to precision of the firing times which were controlled by electronic detonators. Our example of delay time variances for 32 ms production shot (Appendix) argues for better constraint of firing times for contolled seismic experiments. Such constraint requires a 1 per cent error or less in cap firing times which can be realized by the use of firing systems with an order-of-magnitude increase in precision compared to pyrotechnic detonators.

Modelling of blast vibrations

A hybrid method for modelling the time history of blast vibrations from production blasts is developed. The seismic signal produced by the detonation of a single hole charge is recorded in situ. This record contains information about the complex mechanism of radiating seismic energy from an explosive source as well as the filtering effect of the travel path of the signal. Assuming a linear superposition of the seismic effects of the individual holes in a row shot, the vibrations produced by a production blast are simulated. The single shot signal is convolved with an impulse series that represents the firing sequence. By a systematic variation of firing times in the simulation procedure, their effects on the vibrations are studied. The peak amplitudes of vibrations can be reduced. The application of an electronic initiation system allows the realization of optimized firing times in production blasts and a successful validation of the synthetic seismograms.