High-power diode-pumped passively mode-locked Yb:YAG lasers

We obtained 74-kW peak power and 3.5-W average output power in 1-ps pulses from a diode-pumped Yb:YAG laser at 1030 nm that was passively mode locked with a semiconductor saturable-absorber mirror. Another laser produced 57-kW peak power and as much as 8.1-W average output power in 2.2-ps pulses, split into two nearly diffraction-limited beams (M(2)<1.2) . To our knowledge, these are by far the highest reported peak and average output powers from a diode-pumped mode-locked laser in this pulse-duration regime.     

Echolocation in the Risso's dolphin,Grampus griseus

The Risso's dolphin (Grampus griseus) is an exclusively cephalopod-consuming delphinid with a distinctive vertical indentation along its forehead. To investigate whether or not the species echolocates, a female Risso's dolphin was trained to discriminate an aluminum cylinder from a nylon sphere (experiment 1) or an aluminum sphere (experiment 2) while wearing eyecups and free swimming in an open-water pen in Kaneohe Bay, Hawaii. The dolphin completed the task with little difficulty despite being blindfolded. Clicks emitted by the dolphin were acquired at average amplitudes of 192.6 dB re 1 microPa, with estimated sources levels up to 216 dB re 1 microPa-1 m. Clicks were acquired with peak frequencies as high as 104.7 kHz (Mf(p) = 47.9 kHz), center frequencies as high as 85.7 kHz (Mf(0) = 56.5 kHz), 3-dB bandwidths up to 94.1 kHz (M(BW) = 39.7 kHz), and root-mean-square bandwidths up to 32.8 kHz (M(RMS) = 23.3 kHz). Click durations were between 40 and 70 micros. The data establish that the Risso's dolphin echolocates, and that, aside from slightly lower amplitudes and frequencies, the clicks emitted by the dolphin were similar to those emitted by other echolocating odontocetes. The particular acoustic and behavioral findings in the study are discussed with respect to the possible direction of the sonar transmission beam of the species.     

Acoustic backscattering by Hawaiian lutjanid snappers. II. Broadband temporal and spectral structure

The characteristics of acoustic echoes from six species of deep-dwelling (up to 400 m) Hawaiian Lujanid snappers were determined by backscatter measurements at the surface. A broadband linear frequency-modulated signal and a short dolphinlike sonar signal were used as the incident signals. The fish were anesthetized and attached to a monofilament net that was attached to a rotor so echoes could be collected along the roll, tilt, and lateral axes of each fish. The temporal highlight structure of broadband echoes was determined by calculating the envelope of the cross-correlation function between the incident signal and the echoes. The echo waveforms were complex with many highlights and varied with the orientation of the fish. In the tilt plane, the strongest echoes occurred when the incident signal was perpendicular to the long axis of the swimbladder. The number of highlights was the fewest at this orientation. The number of echo highlights and the length of echoes increased as the fish was tilted from this orientation. The highlight structure of the echoes resulted in the transfer function being rippled, with local maxima and minima that changed with fish size and species. The echo structures in both the time and frequency domains were generally consistent within species and were easily distinguishable between species.     

Infrared multiphoton dissociation of unsubstituted metal carbonyls at 5 μm

A frequency-doubled carbon dioxide laser of modest output energy (1 mJ) has been used to study, for the first time, the infrared multiphoton absorption by, and dissociation of, the unsubstituted carbonyls of vanadium, chromium, iron, nickel, molybdenum, and tungsten. The multiphoton absorption cross-sections measured for Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, Mo(CO)₆, and V(CO)₆ are high (2×10^–17) and ensure facile multiphoton dissociation. In focussed beams a pressure independent reaction yield proportional to the 1.5 power of the beam energy is observed for Fe(CO)₅, Cr(CO)₆, and Mo(CO)₆ implying threshold fluences of only 32, 25, and 26 mJ cm^–2, respectively. The stoichiometry of the reaction, observed by a pressure measurement technique, is consistent with production of metal atoms and carbon monoxide as final products for Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, and Mo(CO)₆. This extensive decarbonylation along the ground state surface is consistent with recent studies of the photochemistry of these molecules from excited electronic states.NRC/Heriot-Watt Summer Student 1982On sabbatical leave from the Department of Chemistry, Heriot-Watt University, Edinburgh, United Kingdom     

Diode-pumped passively mode-locked lasers with high average power

We discuss challenges arising from the quest for high average powers from passively mode-locked diode-pumped lasers. The recently obtained detailed understanding of Q-switching instabilities in passively mode-locked lasers turns out to be a crucial element on the way towards higher powers. We give an overview on results achieved with Nd:YAG (10.7 W, 16 ps and 27 W, 19 ps), Yb:YAG (8.1 W, 2.2 ps and 16 W, 0.7 ps) and Nd:glass (1.4 W, 275 fs).     

Synthesis, characterization and activity of alumina-supported cobalt nitride for NO decomposition

A new type of transition metal nitride, viz. alumina-supported cobalt nitride, was synthesized for the first time by NH₃-temperature-programmed reaction, and its structure was characterized by BET, X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) techniques. The supported cobalt nitride performs much better than its bulk counterpart for NO decomposition, owing to its small crystal size, high thermal stability and big surface area.     

A primal-dual approach to inexact subgradient methods

For optimization problems with computationally demanding objective functions and subgradients, inexact subgradient methods (IXS) have been introduced by using successive approximation schemes within subgradient optimization methods (Au et al., 1994). In this paper, we develop alternative solution procedures when the primal-dual information of IXS is utilized. This approach is especially useful when the projection operation onto the feasible set is difficult. We also demonstrate its applicability to stochastic linear programs.     

Electromagnetic cross sections of double giant dipole resonances in 136Xe and 208Pb within the phonon damping model

The electromagnetic cross sections of the double giant dipole resonances (DGDR) in 136Xe and 208Pb are calculated using the strength functions obtained within the phonon damping model. The parameters of the model have been selected to describe reasonably well the single giant dipole resonance in these nuclei. The results are found in an overall agreement with the recent experimental data for the DGDR cross sections in exclusive measurements at near-relativistic energies.