Numerical investigation of relative intensity noise in frequency-doubled multimode fiber lasers

In this paper, we study the intensity noise performance of frequency-doubled lasers. In particular, we investigate how the relative intensity noise (RIN) of multimode fiber lasers is affected by the second harmonic generation process. We first develop an analytical approach and show that, in contrast with a single-mode laser, the low frequency RIN (or excess noise) of a two-mode laser can increase of more than 6 dB after the frequency-doubling operation. This occurs when the intensities of both modes are different and this is explained by a nonlinear coupling between noise and intensity of modes. To deal with more commonly-used multimode fiber lasers, we have extended our study to any number of modes. For this purpose, we have developed a model to numerically simulate the dynamics of a multimode fiber laser. This model includes noise sources and mode competition dynamics due to spatial hole burning. It gives access to the complex amplitude of the electric field of the laser. Using this model, we have confirmed that the excess noise of frequency-doubled multimode fiber lasers can be more than 6 dB higher than the excess noise of the laser before frequency-doubling.