Continuous-phase elements can improve laser beam quality. 
Siegman [Opt.  Lett.  18, 675 (1993)] showed that binary-phase plates cannot improve laser beam quality.  We demonstrate that continuous spiral phase elements can improve the quality of beams that originate from a laser operating with a pure high-order transverse mode.  A theoretical analysis is presented, along with experimental results obtained with a CO(2) laser.  The results reveal that a nearly optimal Gaussian output beam can be obtained with only a small decrease in the output power.