Study of Various modes in a Solid State laser

The light emitted by most lasers contains several discrete optical frequencies, separated from each other by frequency differences, which can be associated with different modes of the optical resonator. It is common practice to distinguish two types of resonator modesLongitudinal modes differ from one another only in their oscillation frequency; and Transverse modes differ from one another not only in their oscillation frequency, but also in their field distribution in a plane perpendicular to the direction of propagation. Corresponding to a given transverse mode are a number of longitudinal modes which have the same field distribution as the given transverse mode but which differ in frequency.