Laser beam profile

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...

The laser beam profile at the end of the medium can be direct evidence of the profile flattening. Figure 8.6b and c shows the laser profiles at the gas jet positions z = 0 and z = —18 mm. At z = 0, the laser profile was severely distorted and weakened after the propagation. On the other hand, at z = —18 mm, the laser beam formed a flattened profile with a radius of 60 pm, which closely matched the 3-D calculation. It is thus clear that a proper selection of the gas jet position is critical for the profile flattening and self-guiding of laser pulses. 

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 volumes of product S atoms sampled by the ionization detector in the pulsed beam mode and in the gas mode are different. The determination of the intersection volume of the ionization laser beam and the CH3SCH3 beam, as well as that of the ionization laser beam and the CS molecules in the gas cell, is necessary for the calibration. The ionization dye laser beam profile is again assumed to conform with the Gaussian profile. The size of the CH3SCH3 beam is determined by the skimmer opening and the nozzle-skimmer distance. 

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. 

An improvement introduced in the LPAS operation is a simultaneous calibration of the detection signal to increase the precision of tte measurement [40]. The photoacoustic signal of the sample is normalu by the signal of a reference standard solution of known absorbance. Both sample and reference solutions are measured simultaneously by the dual channel LPAS set-up (see Fig. 5). By this method, the influence of long term instability of laser energy and laser beam profile on the photoacoustic signal are eliminated and at the same time the spectrum is converted to absolute absorbance units. 

Since the temperature dependences of M and have been experimentally determined one has to make assumptions regarding the radial temperature distribution in the heated spot to find the corresponding radial distribution of M and R. If the intensity distribution of the laser beam is Gaussian the temperature distribution in the film can be represented by means of isotherms of elhpsoidal character (Umer-Wille et al., 1980) such as schematically indicated in fig. 53. A Gaussian laser beam profile was also used in the numerical calculations of Huth (1974), presented below. 

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. 

Figure 10 Spectral and spatial modes of a laser. (A) Atomic line enclosing several longitudinal resonator modes. (B) Transverse spatial modes of a laser beam. The fundamental TEMoo mode is the Gaussian laser beam profile. The arrows indicate the direction of the beam s electric field at a given instant of time.

The properties of the laser and the laser instrumentation which make the system suitable for the ablation process required for sample introduction for elemental analysis are the laser wavelength, power density, pulse-to-pulse stability, pulse frequency, pulse duration, laser beam profile and ablation spot size. 

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