ABSTRACT: The first topic of this pot pourri concerns the phenomenon known as filamentation in air. A macroscopic beam appears to collapse in a 200 micron non-diffracting beam. There have been numerous theories about the nature of filaments, from the moving focus (Shen 1971) to the Bessel beam model (Dubietis 2004) to the self-induced waveguide model(Akhmanov 1966, Mysyrowicz, Chin and countless others). Since, so far, no experiment could distinguish these contradictory descriptions, the debate has been reduced to theoretical arguments. By controlling the initial conditions, we are able to resolve experimentally the debate. The initial conditions were:
--- Filaments focused in air
--- Filaments launched from a 200 micron beam waist in vacuum
--- Filaments prepared in air and launched in vacuum.
Measurements are presented that unequivocally identify the correct model for filamentation in air. The result is a surprise for everybody...

The second topic the use of a laser as an intracavity phase interferometer. The laser is mode-locked with two intracavity pulses circulating in its cavity. The difference in carrier to envelope phase shift of the two pulse trains emitted by the laser enable the measurements of phase shifts smaller than 10 E-7. Applications are measurements of nonlinear indices, magnetic fields, displacements, etc...