It has been known for a long time that unstable waveguiding can occur
in planar, gain guided stripe lasers. The possible existence of instab
ilities can be demonstrated by a very simple waveguide calculation. In
this letter the demonstration of possible instabilities is extended t
o structures with cylindrical geometry. On the basis of these results
is concluded that unstable waveguiding may exist in cylindrical vertic
al cavity surface emitting lasers.