Picosecond pulses from a cryogenically cooled, composite amplifier using Yb:YAG and Yb:GSAG

A cryogenic Yb amplifier using two laser materials, Gd3Sc2Al3O12 and Y3Al5O12 (YAG), has been used to obtain 70 W average power at 5 kHz pulse repetition frequency; the output was compressed to 1.6 ps, compared with an input compressible to 1.4 ps. The gain broadening obtained by combining two media enables shorter pulses than using Yb:YAG alone but retains the power-scaling advantages of cryogenic Yb:YAG.     

Effects of tip gap and amplitude of piezoelectric fans on the performance of heat sinks in microelectronic cooling

Piezoelectric fan is a promising option for cooling microelectronic devices owing to its unique features such as no electromagnetic noise, low power consumption and minimum space requirement. The recent interest is to integrate the piezoelectric fans (piezofans) with heat sink; this idea is widely accepted and researches are still underway. This article presents experimental analysis on the effects of tip gap (δ) and amplitude of piezofan vibration (α) on the heat transfer characteristics of finned heat sinks. Two heat sink configurations, namely A and B (with two and four fins respectively) each of which is arranged with three piezofans, are considered for the study. The transient temperature distributions for cases with and without piezofans are obtained for both the configurations, and compared. The heat transfer coefficient, thermal impedance, Nusselt number and Reynolds number are investigated as functions of δ and α. The effect of α on the fan effectiveness is also analyzed. It is observed that the configuration B has better cooling performance compared to A. Among the tested ranges of δ and α, the case with least tip gap (δ?=?0.03) and highest amplitude (α?=?5.29) is found to be the best; at this setting, the fan effectiveness is increased to almost 4?times compared to the case without piezofans.     

Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam

Five 500 W fiber amplifiers were coherently combined using a diffractive optical element combiner, generating a 1.93 kW beam whose M(2)=1.1 beam quality exceeded that of the inputs. Combining efficiency near 90% at low powers degraded to 79% at full power owing to thermal expansion of the fiber tip array.     

New hydroxy-pyrazoline intermediates, subtle regio-selectivity and relative reaction rate variations observed during acid catalyzed and neutral pyrazole cyclization

Arylhydrazines 4 and 1,3-dicarbonyl compounds 5 react to form pyrazoles by loss of water via hydrazone isomer pairs 6 and 7 which give rise to two possible regio-isomers. Occasionally, 3-hydroxy-3,4-dihydropyrazoles or hydroxy-pyrazolines 9 and 9 are observed as stable isolatable intermediates that can be fully characterized prior to loss of the second molecule of water that gives rise to pyrazoles 10 and 11. Fully characterized examples of intermediates of type 8 and 9 are relatively rare. We studied the reaction series where R = CH3, CHF2 and CF3 and Ar = Ph and 5-methanesulfonylpyridin-2-yl, (Scheme 2), and observed differences in properties between kinetic behavior and regio-isomerism depending on the degree of electron-withdrawing capability of the R and Ar substituents. The reaction conditions that caused cyclization to pyrazoles varied from direct condensation of the hydrazine and 1,3-dicarbonyl compounds, to reactions requiring catalytic quantities of sulfuric acid to sulfuric acid in excess. Unexpected regio-selectivity was observed in the case of R = CF3 that depended upon the reaction conditions.     

Sub-picosecond pulses at 100 W average power from a Yb:YLF chirped-pulse amplification system

We present a high-repetition-frequency, diode-pumped, and chirped-pulse amplification system operating at 106 W average output power with excellent beam quality (M(2)=1.3), based on cryogenically cooled Yb:YLF. 1 nJ seed pulses, derived from a mode-locked Ti:sapphire laser, are first amplified to 1 mJ pulse energy at 10 kHz repetition frequency in a regenerative amplifier. The second-stage, multipass amplifier increases the pulse energy to 10.6 mJ, resulting in a spectral width of 2.2 nm. The pulses are compressed to 865 fs in duration, which is 1.26 times the transform limit.     

Images of unstable Taylor-Sedov blast waves propagating through auniform gas

We present images from the first observation and measurements of an instability in Taylor-Sedov blast waves propagating through a uniform background gas. The instability occurs when the adiabatic index of the background gas is low. Images of the expanding blast waves are dark field shadowgraphs obtained using a frequency-doubled (527 nm) pulsed YAG laser and black and white photographic film. The film images are digitized, and image processing routines are used for analysis and presentation