Change In Spectral Width Research Articles

Modeling of mode-locked fiber lasers and amplifiers with accurate saturable gain evaluation

We present a numerical model for mode-locked lasers and ultrafast nonlinear amplifiers in which the saturated gain profiles along active fibers are judiciously computed using experimental pump powers as input. This eliminates the need for approximating the gain profile by using a small-signal gain coefficient and saturation energy to simulate pulse propagation in active fibers. Our model shows good agreement with experiments involving mode-locked cavities at 1 µm with a silica glass host doped with ytterbium ions. Accurate results are also obtained for continuous-wave and mode-locked laser cavities around 2.8 µm, which uses ZBLAN fiber doped with erbium ions. In the case of Er:ZBLAN fiber, we use the model to show regions of stable mode-locking delivering a single pulse as output and how the spectral width changes with variation in doping concentration and fiber lengths. Our model enables accurate numerical modeling of mode-locked fiber lasers and ultrafast amplifiers, and can be useful in guiding the design of new architectures, understanding complex intracavity laser dynamics, and optimizing device output.

Photochemical Control of Exciton Superradiance in Light-Harvesting Nanotubes.

Photosynthetic antennae and organic electronic materials use topological, structural, and molecular control of delocalized excitons to enhance and direct energy transfer. Interactions between the transition dipoles of individual chromophore units allow for coherent delocalization across multiple molecular sites. This delocalization, for specific geometries, greatly enhances the transition dipole moment of the lowest energy excitonic state relative to the chromophore and increases its radiative rate, a phenomenon known as superradiance. In this study, we show that ordered, self-assembled light-harvesting nanotubes (LHNs) display excitation-induced photobrightening and photodarkening. These changes in quantum yield arise due to changes in energetic disorder, which in turn increases/decreases excitonic superradiance. Through a combination of experiment and modeling, we show that intense illumination induces different types of chemical change in LHNs that reproducibly alter absorption and fluorescence properties, indicating control over excitonic delocalization. We also show that changes in spectral width and shift can be sensitive measures of system dimensionality, illustrating the mixed 1-2D nature of LHN excitons. Our results demonstrate a path forward for mastery of energetic disorder in an excitonic antenna, with implications for fundamental studies of coherent energy transport.

Sensitivity to Auditory Spectral Width in the Fetus and Infant – An fMEG Study

Auditory change detection is crucial for the development of the auditory system and a prerequisite for language development. In neonates, stimuli with broad spectral width like white noise (WN) elicit the highest response compared to pure tone and combined tone stimuli. In the current study we addressed for the first time the question how fetuses react to “WN” stimulation. Twenty-five fetuses (Mage = 34.59 weeks GA, SD ± 2.35) and 28 healthy neonates and infants (Mage = 37.18 days, SD ± 15.52) were tested with the first paradigm, wherein 500 Hz tones, 750 Hz tones, and WN segments were randomly presented and auditory evoked responses (AERs) were measured using fetal magnetoencephalography (fMEG). In the second paradigm, 12 fetuses (Mage = 25.7 weeks GA, SD ± 2.4) and 6 healthy neonates (Mage = 23 days and SD ± 6.2) were presented with two auditory oddball conditions: condition 1 consisted of attenuated WN as standard and 500 Hz tones and WN as deviants. In condition 2, standard 500 Hz tones were intermixed with WN and attenuated WN. AERs to volume change and change in spectral width were evaluated. In both paradigms, significantly higher AER amplitudes to WN than to pure tones replicated prior findings in neonates and infants. In fetuses, no significant differences were found between the auditory evoked response amplitudes of WN segments and pure tones (both paradigms). A trend toward significance was reached when comparing the auditory evoked response amplitudes elicited by attenuated WN with those elicited by WN (loudness change, second paradigm). As expected, we observed high sensibility to spectral width in newborns and infants. However, in the group of fetuses, no sensibility to spectral width was observed. This negative finding may be caused by different attenuation levels of the maternal tissue for different frequency components.

Bismuth triborate (BiB3O6) optical parametric oscillators

We report the phase matching of parametric frequency conversion in the nonlinear material BiB3O6 (BiBO) and on an investigation of optical parametric oscillators (OPOs) of this new crystal. Based on the calculation of collinear type I and type II phase matching within the refractive-index planes, the most favorable directions for phase matching are identified for OPOs pumped by the fundamental or the harmonics of 1064-nm Nd-doped lasers. Based on these results, pulsed 532-nm-pumped ns OPOs are realized. The pump source is either a Q-switched high repetition rate (10 kHz) Nd:YVO4 laser (with a pulse energy of 24 μJ) or a low repetition rate (10 Hz), high pulse energy (120 mJ) Nd:YAG laser system. The BiBO OPO pumped by the Nd:YVO4 laser showed a very low threshold of 0.047 J/cm2. At an average pump power of 2.4 W the total OPO output power was 630 mW. By changing the phase-matching angle Θ within the yz plane from 0 to 11.6° the signal wavelength was tuned from 735 nm to 970 nm, while the spectral width changed from 0.2 nm to 1.4 nm. By pumping the OPO with the Nd:YAG laser, the OPO had a threshold of 0.12 J/cm2, a steep slope (59%) and a high total efficiency (of up to 48%). Due to divergence broadening the spectral width changes from 8.5 nm at 800 nm to 70 nm near degeneracy. The properties of BiBO determined from the experimental results are compared with those of well-known nonlinear materials such as BBO, LBO and KTP.

A statistical comparison of SuperDARN spectral width boundaries and DMSP particle precipitation boundaries in the nightside ionosphere

The boundary between quasi‐dipolar (closed) geomagnetic field lines and those connected to the interplanetary magnetic field (open) is a key diagnostic for the magnetospheric system. This study presents an HF radar technique for determining the location of this boundary in the nightside ionosphere and calibrates it with the boundary determined from simultaneous particle precipitation data. The latitudes of spectral width boundaries (SWBs) identified from 5 years (1997–2001) of data from the Halley HF radar of the Super Dual Auroral Radar Network (SuperDARN), in the range 1800–0200 magnetic local time (MLT), have been compared with nightside particle precipitation boundaries (PPBs) identified from measurements from the Defense Meteorological Satellite Program (DMSP) spacecraft. Latitudinal differences were measured between the PPBs and the nearest SWB within a ±10 min UT window and within a ±1 hr MLT window. These differences were then organised into 2‐hour MLT sectors. The statistical distributions of the latitudinal differences show that the nightside SWB correlates best with the b6 PPB, which is a good marker for the open‐closed field line boundary (OCB) in the nightside ionosphere. When considering SWBs determined using different spectral width threshold values, those determined using the lower threshold values (<150 m/s) are offset ∼1–2° equatorward of the b6 boundary suggesting that there is a gradual latitudinal change in spectral width between the b5 and b6 boundaries rather than a step change at the b6 boundary. The results enable the OCB to be determined with confidence in the 1800–0200 MLT sector using SuperDARN radar data.

In situ optical sensing of diesel exhaust particulates using a polychromaticLED source

A novel optical probe for in situ monitoring of diesel exhaust particulates hasbeen developed and is demonstrated. The probe uses a transmissive configurationto interrogate a particulate stream using polychromatic (white) light froman electroluminescent LED, the probe being mounted transversely tothe exhaust flow in the tailpipe. Consequently the effects of scatteringand absorption are measured on the transmission spectrum. The opticalsignal is relayed via an optical fibre bundle to a remote detection and datalogging unit. Photodetector signals representing red, green and blue spectralinformation are monitored using detectors having overlapping spectralresponsivities. Data collection is by red–green–blue (RGB) tristimulusdetection and spectral analysis uses the hue–lightness–saturation (HLS)algorithms. It is shown that changes in spectral width may be used torepresent particulate bursts (scattering) via the saturation parameterwith a compensation algorithm being used to compensate for shifts in theoperating point due to contamination of the optics. As polychromatic light isused it is demonstrated that variation in the mean particle size may berepresented in terms of the ratios of polydisperse extinction for adjacentspectral bands. The potential advantages of chromatic sensing for signalprocessing in terms of reduced susceptibility to vapour content and opticalcontamination is demonstrated by comparison with an intensity based sensor.

The I*(2P1/2)−I*(2P1/2) Contact Pair Emission in Condensed Media: A Molecular Spring-Gauge for Cavity Sizing

The I*I*(0g + ) f I*I(B(0u + )) emission spectra obtained in neat Ar, Kr, Xe, and in mixed Kr/Xe matrixes is reported. The emission from the strictly cage-bound initial state shifts linearly with Kr/Xe mixing fraction, without significant change in spectral width. The line shapes in mixed-hosts are understood in terms of the statistics of sites in the solid solutions. The pure-host line shapes are reproduced through Monte Carlo simulations using accurately known I-Rg potentials. The analysis yields a refined I*I* potential and provides a calibration curve to readout the cavity size of isolation from the observed emission peak shift.

Portal vein pulsatility and spectral width changes in patients with portal hypertension: relation to the severity of liver disease.

Using Doppler ultrasound in patients with chronic liver disease (CLD) and portal hypertension, portal vein pulsatility pattern and spectral width changes were assessed and evaluated in relation to the severity of liver disease as determined according to the Child-Pugh score. The pulsatility index (PI) was significantly lower in CLD patients (mean+/-SD: 0.23+/-0.08) compared with healthy subjects (0.39+/-0.1) (p<0.001) and lower in Child-Pugh class C compared with Child-Pugh class A patients (0.21+/-0.07 vs 0.25+/-0.08, respectively) (p<0.05). The spectral width index was significantly higher in CLD patients vs healthy subjects (0.91+/-0.16 vs 0.60+/-0.12, respectively) (p<0.001). The difference was also noted in the early stage (Child-Pugh A patients) when compared with healthy subjects (0.88+/-0.17). In conclusion, portal vein pulsatility and spectral width indices can reflect the early haemodynamic changes in CLD patients. These changes become more pronounced with the progression of liver disease.

Pulsed Doppler velocity patterns produced by arterial anastomoses

Centerstream velocity waveforms produced by end-to-end and end-to-side anastomoses constructed in the dog illeofemoral arterial system were studied with a 20 MHz pulsed Doppler velocimeter combined with spectral analysis. Flow disturbance was identified by changes in spectral width during the systolic phase of the cardiac cycle. Measurement of the maximum frequency and the spectral width at peak systole was used to quantify the magnitude of flow disturbance at varying locations proximal and distal to the anastomoses. Disruption of the normal laminar flow pattern observed in the unoperated dog artery was evident distal to both anastomotic configurations. An increase in spectral width reflecting disturbed flow was maximal during the deceleration phase of systole. Flow disturbance was localized to a zone within one diameter distal to the anastomosis and dissipated rapidly downstream. The velocity spectrum changes observed downstream of an anastomosis resemble the flow disturbances produced by low grade, nonpressure reducing arterial stenoses. This study suggests that spectral analysis of pulsed Doppler waveforms is a potentially useful method of anastomosis assessment both to rule out major flow disruption produced by technical error, and to provide insight into the role of turbulence in the development of anastomotic intimal hyperplasia.

Incoherent scatter observations of mesospheric dynamics at Arecibo

Incoherent scatter spectral observations at Arecibo have been extended down to 59 km. For the first time vertical velocities in the mesosphere have been observed using this technique. The velocities unambiguously reveal a wavelike structure growing with height and varying in time. Above around 75 km, the vertical wavelength is around 16 km and the time period about 4 hours ± 20 minutes. Vertical velocities near 90 km were as large as 1 ms−1 and the apparent vertical energy flux was reasonably constant above 75 km. Below 75 km the energy flux was substantially lower, suggesting a mechanism where the gravity waves acquire significant energy from the diurnal tidal component trapped below the mesopause.The estimated adiabatic density and temperature changes near 90 km, estimated using simple idealized theories, were around 11%. The observed changes in spectral width were also found to be in harmony with estimated density and temperature changes to within 25%.

Early Results of the 1961 JPL Venus Radar Experiment.

view Abstract Citations (10) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Early Results of the 1961 JPL Venus Radar Experiment. Muhleman, Duane O. Abstract A strong radar contact with Venus was made during the 1961 Venus conjunction utilizing the Goldstone, California, radar facilities of the Jet Propulsion Laboratory. Observations were made nearly daily from March to through May to. A t2.5-cm CW signal was transmitted with an 85-ft parabolic antenna, and the returned signal was received with a second 85-ft antenna. The transmitted frequency stability was held to within 1 cps for the signal-delay time. This reference signal was used to synchronously detect the returned signal and to determine the Doppler shift with an accuracy of 1 cps which corresponds to 0.07 m/sec. The transmitter was also keyed off and on for purposes of making unambiguous range measurements (time-of-flight measurements). Preliminary results of an astronomical unit determination yield a value approximately 60 000 km larger than the value determined by Kabe (1960). The accuracy of the data suggests the possible improvement of the Venus ephemeris. Analysis of the returned signal indicates a reflectivity relative to a perfectly conducting sphere of about 10% at this wavelength as compared with the lunar reflectivity of about 2 %. Doppler spreading of the returned signal due to the rotation of Venus strongly indicates synchronous rotation of the planet. The spectrum of the Doppler-spread signal is very similar in shape to the spectrum computed from a lunar-reflected signal at the same wavelength indicating similar backscattering characteristics. The analysis of the change in spectral width as a function of time for purposes of determining the Venus rotation axis orientation has not been completed as yet. Publication: The Astronomical Journal Pub Date: September 1961 DOI: 10.1086/108422 Bibcode: 1961AJ.....66..292M full text sources ADS |