Solution of the Master Equation in Laser Process Under the Action of Linear Resonance Force

The master equation of density operator in laser process under action of linear resonance force can be concisely solved by virtue of thermo-entangled state representation and the technique of integration within an ordered product of operators. We obtain the infinitive operator-sum representation of density operator. As the application of this method, the evolution of thermal field and that of vacuum state in this model are discussed. The results show that thermal field maintains its original character, and vacuum state evolves thermal field in laser process.