  We derive semiclassical laser equations valid in all orders of nonlinearity.
With the help of a diagrammatic representation, the perturbation series in
powers of electric field can be resummed in terms of a certain class of
diagrams. The resummation makes it possible to take into account a weak effect
of population pulsations in a controlled way, while treating the nonlinearity
exactly. The proposed laser theory reproduces the all-order nonlinear equations
in the approximation of constant population inversion and the third-order
equations with population-pulsation terms, as special cases. The theory can be
applied to arbitrarily open and irregular lasers, such as random lasers.
