Quantum mechanical explanation of spontaneous emission K-factor

The rate of spontaneous photon emission in an injection laser depends on whether the laser modes are gain-guided or index-guided. For gain-guided modes the ratio of spontaneous to stimulated emission is greater by a factor K to the corresponding ratio for index-guided modes. Recently, the validity of the K-factor was questioned by means of a quantum mechanical argument. It is the purpose of this letter to show that quantum theory does not contradict the classical derivation of the K-factor but instead confirms its validity and sheds new light on the physics of spontaneous emission into gain-guided laser modes.