The method of producing at least quasi-continuous lasing operation between initial and terminal lasing states comprising the steps of selecting a laser medium and cavity configuration (lasing frequency) with the laser medium containing upconverting material which provides for the exchange of energy between sufficient number of electrons at the terminal lasing state (manifold) so as to maintain population inversion between the initial and terminal lasing states during the lasing operation; and applying excitation energy of a suitable amount to the laser medium during at least a quasi-continuous time period.