In the book Quantum Optics by Scully and Zubairy it is mentioned that the finite linewidth of the laser is essentially due to spontaneous emission events that randomize the phase. My question is; why does spontaneous emission randomize the phase, but stimulated emission doesn't? Their model uses a single cavity mode coupled to a thermal bath at zero temperature.
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The spontaneous emission events occur randomly in time, so phase is equally distributed over the unit circle.
Stimulated emission is induced as a jump between two electron states by the dipole field of the external em field. The electron jump simply acts as a creation operator in the incoming mode, adding one energy quantum of the same $(\omega, k)$ in phase.
