Gaussian beam, focal spot size

The input beam was a spatially filtered beam from one of the laser sources described below. The spatial profile of the input beam usually showed a greater than 96% correlation with a gaussian profile. The pulse width was 2.5 nanoseconds. The experiments at low energies were conducted at 10 Hz. For incident fluences above about 10 mJ/cm, the repetition rate was reduced to 0.5 Hz. This removed any effect of persistent thermal effects. The laser intensity was controlled by wave plate/polarizer combination. The input beam was focused onto the sample using approximately f/45 optics. The focal spot size (beam radius) was measured to be 2.6 0.1 pm at 550 nm. 

The main advantage of LA is the direct ablation capability from preselected areas of a sample, thereby avoiding laborious sample preparation and contamination. The focal spot size achieved with UV lasers of adequate (Gaussian) beam profile can be reduced to a few micrometers or less, which enables the microanalysis of solids. Quasicontinuous sampling is possible if the laser is operated with an adequate repetition rate. 

The output beam of a laser oscillating in the fundamental TEMgQ-mode shows a Gaussian intensity profile (see Sect.5.11). If the Gaussian beam is properly matched to a focusing system with aperture d and focal length f, a diffraction-limited focal spot of diameter dQ 2(f/d)x can be reached. With a microscope objective for example, with f/d = 1, focal spot sizes of Wq = dQ/2 =... 

Note, however, that in the near-held region the center of curvature does not coincide with the center of the beam waist (Fig. 5.126). When a Gaussian beam is focused by a lens or a mirror with focal length /, the spot size in... 

A nearly parallel Gaussian beam is expanded by a telescope with two lenses of focal lengths / = 1 cm and fi = 10 cm. The spot size at the entrance lens is If = 1 mm. An aperture in the common focal plane of the two lenses acts as a spatial filter to improve the quality of the wavefront in the expanded beam (why ). What is the diameter of this aperture, if 95% of the intensity is transmitted ... 

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