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We report on an experimental characterization of Bessel beams with finite apertures. We show that re Bessel beams can be generated with intensity profiles that closely resemble the ideal [Mathematical Expression Omitted] transverse-intensity distribution of Bessel beams. We also show interferometrically that these beams have planar phase fronts with [Pi]-phase shifts from one Bessel ring to the next. We report tolerance conditions Bessel beam generation and give an example of this generation that uses an unstable resonator as the source., An experiment that investigated the features of Bessel beams with finite apertures at different laser wavelengths and pulse intervals is reported. The study shows the results of interferometric measurements of a Bessel beam's phase front, longitudinal and transverse-intensity distributions. It also gives proof on the use of unstable resonators as Bessel beam sources under specific circumstances.

This year marks the 50th anniversary of the invention of the laser. The Optical Society of America is publishing this feature issue to celebrate this auspicious birthday.

Nonexponential decay of the 4F(3/2) upper laser state in Nd:YLF was observed with time-resolved fluorescence spectroscopy and small-signal gain probes in low-doped (1.16-at. %) samples when intensely pumped to high population inversions. A rapid initial decay is observed after Q-switched laser pumping, followed by a longer nonexponential decay that is clearly identified as a phase of migration-assisted energy-transfer upconversion (ETU) in the hopping regime. During this phase, a rate-equation treatment is valid and the macroscopic ETU coefficient, alpha2 = (0.98 +/- 0.03) x 10(-16) cm3/s, is directly evaluated from the decay kinetics. The ETU and the migration microparameters are also estimated to be C(da)* approximately 200 x 10(-40) cm6/s and C(dd) approximately 1000 x 10(-40) cm6/s, respectively, and are compared with theoretical values found in the literature. On the basis of these values, rate-equation treatments of intensely pumped Nd:YLF are not strictly valid.

The light-scattering properties of dental enamel and dentin were measured at 543, 632, and 1053 nm. Angularly resolved scattering distributions for these materials were measured from 0° to 180° using a rotating goniometer. Surface scattering was minimized by immersing the samples in an index-matching bath. The scattering and absorption coefficients and the scattering phase function were deduced by comparing the measured scattering data with angularly resolved Monte Carlo light-scattering simulations. Enamel and dentin were best represented by a linear combination of a highly forward-peaked Henyey-Greenstein (HG) phase function and an isotropic phase function. Enamel weakly scatters light between 543 nm and 1.06 µm, with the scattering coefficient (µ(s)) ranging from µ(s) = 15 to 105 cm(-1). The phase function is a combination of a HG function with g = 0.96 and a 30-60% isotropic phase function. For enamel, absorption is negligible. Dentin scatters strongly in the visible and near IR (µ(s)≅260 cm(-1)) and absorbs weakly (µ(a) ≅ 4 cm(-1)). The scattering phase function for dentin is described by a HG function with g = 0.93 and a very weak isotropic scattering component (˜ 2%).