Time Resolved Beam Divergence from a Copper Vapor Laser with Unstable Resonator

The temporal evolution of the far-field intensity distribution (and hence beam divergence) for the output of a CVL operating with both on-axis and off-axis unstable resonators is investigated in detail. The CVL output pulse consists of several temporally resolved components, where each successive component has lower divergence approaching the diffraction limit. A comprehensive model for the divergence of each temporal component from a CVL operating with a variety of unstable resonators is presented. In this model the resonator controls the output divergence by imposing geometric constraints on the propagation of spontaneous emission on repeated round-trips through the unstable resonator equivalent lensguide. Effects of resonator asymmetry and radial gain variations on the far-field intensity distribution (and hence divergence) are evaluated. Experimental measurements of the temporal evolution of output divergence from a CVL operating with both on-axis and off-axis unstable resonators and for a variety of excitation conditions are also presented. For CVL operation at high pulse repetition frequency the ASE at the start of the laser pulse has an annular profile. This annular gain distribution is found to modify the output far-field intensity distribution of the earlier temporal components of the laser pulse. For on-axis unstable resonators these components are found to have annular far-field intensity distributions, whereas for off-axis unstable resonators they have greatly reduced but anisotropic divergence.