No abstract

We have demonstrated that a simple diaphragm can play the role of a very cheap reconfigurable diffractive optical element allowing the generation of interesting laser beam patterns. For instance, a truncated Laguerre-Gauss LG 10 beam (one peak surrounded by one ring) can be transformed in the focal plane of a lens into an optical bottle beam (OBB) which consists to a dark (or minimal intensity) region surrounded by higher intensity light in the three principal directions. Depending on the beam truncation, one can also observe a flat-top intensity profile.

Diffraction of a high-order radial Laguerre-Gauss LG p0 beam truncated by a circular aperture is considered. In contrast with the truncated usual Gaussian LG 00 beam, which is not Gaussian in the near-field and quasi-Gaussian in the far-field, the truncated LG p0 beam (for p = 1 to 4) behaves very differently. Depending on the diaphragm size, the radial intensity distribution of a truncated LG p0 beam in the far-field (focal plane of a lens) can take the shape of (i) a flat-top, (ii) a hollow beam and (iii) one central peak and a ring having the same intensity. In addition, clipping-even weakly-of an LG p0 beam splits the usual focal point into two.

A single rf-compensated cylindrical Langmuir probe has been developed in order to characterise a plasma RF discharge. A circuit using radiofrequency filtering and the passive compensation method was employed to minimize the probe curve distortions. The effect of the rfcompensation on the probe measurements was discussed. The latter were performed at power and pressure of 50 W and 510-2–1.2 mbar, respectively. Compensated measurements of the electron energy distribution function (EEDF) and plasma parameters were conducted at powers ranging from 5 to 120 W and pressure of 0.3 mbar. They exhibit a transition between the stochastic electron heating mode operating at low powers and the secondary-electron emission heating mode (γ) operating at high powers. The electronic density increases from 1.5×109 to 3.2×1010 cm-3 while the effective electron temperature decreases from 3.7 to 2.3 eV. The EEDFs were found to be Druyvesteyn-like in the range of 5–80W and then evolve to the Maxwellian beyond 90W.

A method to determine the variation of the average refractive index in a ruby laser caused by the direct coupling of Cr3+ ions to the average strains of the crystal lattice is presented. The method involves creation of a spatial variation of the population inversion density in the laser rod, and then measurement of the resulting dynamic divergence of the laser. The results obtained are in reasonable agreement with the values determined by other methods reported previously.