Fundamental limits on1/ffrequency noise in rare-earth-metal-doped fiber lasers due to spontaneous emission

It is proposed that observed $1/f$ laser frequency noise in rare-earth-metal-doped optical fiber lasers is caused by diffusion of local entropy fluctuations associated with random spontaneous emission events. This heat generation is directly associated with the broad emission spectrum of rare-earth-metal-doped laser glasses. Using data from a well characterized erbium-doped fiber laser, it is shown that the power spectral density of frequency fluctuations resulting from this mechanism is able to achieve excellent agreement with experiment. The proposed theory is expected to be generally applicable to broadly emitting solid-state gain media.