Angular momentum of a light field as superposition of Hermite-Gaussian modes

The orbital angular momentum (OAM) for an arbitrary light field as superposition of Hermite-Gaussian modes is considered. An analytical formula for the OAM of such fields is obtained. It is shown that under equal initial conditions, OAM of a linear combination of two Hermite-Gaussian beams derived in this work is equal to that obtained earlier [JOSA A, 31, 274 (2014)]. Relations for more complex combinations of Hermite-Gaussian modes are also derived. It is found that unlike two-mode superposition, in three-mode superposition the OAM can be larger than the maximum index of the superposition for structurally stable light fields.