Transverse Mode Formation in Positive Branch Unstable Resonators.

The dynamics of transverse mode formation in positive branch unstable resonators with moderate magnification (1.32 < M < 2.7) and typical Fresnel numbers (279 < N(,T) < 10,800) used in high energy lasers has been investigated. The objective was to observe the far-field intensity distribution from the onset of pumping to the steady-state diffraction limited pattern. The research consisted of experimental and numerical analyses. The experimental measurements of mode formation time were performed using a pulsed CO(,2) laser and a pulsed XeF laser. Mode formation was deduced from observation of the far-field. The analytical effort consisted of a fast Fourier transform (FFT) wave optics computation which modeled the far-field pattern evolution. Results agree with the temporal mode formation geometrical theory of Anan'ev when laser saturation is achieved at the predicted mode formation time in a CO(,2) laser. Rapid saturation at one-third this time results in significantly earlier mode formation compared to the geometrical theory for a XeF laser.