Lasers beam profile from

Fig. 8.5. Evolution of the laser beam profile in an ionizing medium calculated by 3D simulation. L is the propagation distance measured from the entrance plane of a 9-mm gas jet. The gas jet positions are az = 0 and b z = —18 mm...

A summary of the decomposition results is shown in Fig. 16. The rate coefficients are bounded by the upper [61] and lower [13] broken lines for Oi Ar mixtures. Least squares analysis yielded values for fe, and of eqn. (B) with M and O2 respectively as collision partners. The fact that fea did not depend upon the identity of M lends additional support to the linearity assumption. The value of 2 was deduced from an analysis of the laser beam profile following the dip and was found to agree within a factor of two with other reports [61—63]. The ratio of 2/ 3, a measure of the atom efficiency, is of the order of 5—7. 

The light from the source, in this case a laser diode, is transferred to the fibre input cross section by a transfer lens system. The first lens is the laser collimator, with a focal length, fl, which is normally a few mm. If the collimated beam is focused into a fibre by a lens of a longer focal length, 12, all aberrations in the laser beam profile are magnified by a factor M = 12 / fl. This requires a fibre of a eorrespondingly large diameter. However, the NA of the beam coupled into the fibre, and eonsequently the pulse dispersion in the fibre, is reduced by the same ratio. 

In order to maintain the good beam quality of the cw dye laser during its amplification by transversely pumped amplifier cells, the spatial distribution of the inversion density in these cells should be as uniform as possible. Special designs (Fig. 5.91) of prismatic cells, where the pump beam traverses the dye several times after being reflected from the prism end faces, considerably improves the quality of the amplified laser beam profile. 

The laser interferometry technique is widely used for the study of the detonation wave time profile and structure due to its exceptionally good time resolution. The laser interferometry operating principle is based on the Doppler effect. The technique records the position and time dependence of the interferometric fields obtained due to the Doppler shift in wavelength of the reflected laser beam, resulting from the thin metal shim motion. The metal shim, 15-25 pm thick, is placed between the explosive charge and windows that are made of an inert optically transparent material, such as water, lithium fluoride, or polymethylmethacrylate. On the basis of the velocity of the explosive/metal shim interface as a function of time, it is possible to calculate the values of detonation parameters of the explosive (Gimenez et al., 1985, 1989 Hemsing, 1985 Leeetal., 1985). 

Fig. 7.4 Spatial profile of sodium atomic beam focused by the gradient force of a copropagating Gaussian laser beam. (Reprinted from Bjorkhohn et al. 1980 with courtesy and permission of IEEE (USA).)...

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

A commercial fs-laser (CPA-10 Clark-MXR, MI, USA) was used for ablation. The parameters used for the laser output pulses were central wavelength 775 nm pulse energy -0.5 mj pulse duration 170-200 fs and repetition rate from single pulse operation up to 10 Hz. In these experiments the laser with Gaussian beam profile was used because of the lack of commercial beam homogenizers for femtosecond lasers. 

Fig. 1—Profile measurement technique of Champper 2000+. A surface measurement is made with a linearly polarized laser beam that passes to translation stage which contains a penta-prism. The beam then passes through a Nomarski prism which shears the beam into two orthogonally polarized beam components. They recombine at the Nomarski prism. The polarization state of the recombined beam includes the phase information from the two reflected beams. The beam then passes to the nonpolarizing beam splitter which directs the beam to a polarizing beam splitter. This polarizing beam splitter splits the two reflected components to detectors A and B, respectively. The surface height difference at the two focal spots is directly related to the phase difference between the two reflected beams, and is proportional to the voltage difference between the two detectors. Each measurement point yields the local surface slope [7].

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