Inhomogeneous extraction of the energy of higher transverse modes from a gas laser

It is shown that the parameters of a light beam emerging from a helium–neon laser (λ = 633 nm) generating higher transverse modes can be improved considerably by the use of an inhomogeneous laser mirror whose transmission coefficient is significant only in a region opposite one of the segments of the transverse distribution of the mode field. This can be done using mirrors in which the phase of the reflection coefficient is constant over the whole surface facing the active medium. An example of such a system is described and photographs of the distribution of the intensity in a laser beam are given. It is shown that this method can yield an angular divergence and a far-field distribution of approximately the same type as for lower-order Gaussian beams. A report is given of the quantitative characteristics of the laser energy extracted through a tunable Fabry–Perot interferometer with an internal screen ensuring an inhomogeneous loading of segments of a transverse mode. The experimental results show that the inhomogeneity of the mirror does not affect the output power. The described experiments make it in principle possible to obtain high-quality laser beams from large-diameter discharge tubes without recourse to resonators with extremely long foci.