A simple laser beam characterization apparatus based on imaging

The development of an original low-cost hardware/software apparatus to optically control the laser beam and measure quality parameters by image analysis using a monochrome matrix sensor is described, along with experimental validation. Laser parameters, namely beam waist, divergence, Rayleigh range and M2, which are the most relevant in manufacturing operations (like optical measurement and laser processing), have been physically and mathematically characterized. Strategies for the apparatus size reduction are also presented, including the use of a prism to reduce by half the optical path, and a camera model to simulate the sensor resolution.

Turbulent Rayleigh–Bénard convection in an annular cell

We report an experimental study of turbulent Rayleigh–Bénard (RB) convection in an annular cell of water (Prandtl number $Pr=4.3$) with a radius ratio $\unicode[STIX]{x1D702}\simeq 0.5$. Global quantities, such as the Nusselt number $Nu$ and the Reynolds number $Re$, and local temperatures were measured over the Rayleigh range $4.2\times 10^{9}\leqslant Ra\leqslant 4.5\times 10^{10}$. It is found that the scaling behaviours of $Nu(Ra)$, $Re(Ra)$ and the temperature fluctuations remain the same as those in the traditional cylindrical cells; both the global and local properties of turbulent RB convection are insensitive to the change of cell geometry. A visualization study, as well as local temperature measurements, shows that in spite of the lack of the cylindrical core, there also exists a large-scale circulation (LSC) in the annular system: thermal plumes organize themselves with the ascending hot plumes on one side and the descending cold plumes on the opposite side. Near the upper and lower plates, the mean flow moves along the two circular branches. Our results further reveal that the dynamics of the LSC in this annular geometry is different from that in the traditional cylindrical cell, i.e. the orientation of the LSC oscillates in a narrow azimuthal angle range, and no cessations, reversals or net rotation were detected.

Spatial extension of the electromagnetic field from tightly focused ultra-short laser pulses

It is shown that in the focus of ultra-short pulses of duration t, the equivalent relation s = ct, where c is the speed of light and s the spatial extent of the pulse of the collimated pulse, does not hold. While the duration of one pulse is constant and independent of the measurement point, the spatial extension of the ultra-short pulse can be spatially shorter a factor more than 10 compared to the one obtained from the usual relation. The result is explained in correspondence with the extension of the Rayleigh range. Few femtosecond long gamma bursts can thus be generated in Thomson backscattering experiments performed in the lambda cube regime.