Longitudinal modes in a fibre laser

[Niles]

Hi

In the Wiki article on longitudinal modes (http://en.wikipedia.org/wiki/Longitudinal_mode) they state that:

"The analysis of longitudinal modes is especially important in lasers with single transversal mode, for example, in single-mode fiber lasers. The number of longitudinal modes of such a laser can be estimated as ratio of the spectral width of gain to the spectral separation of longitudinal modes."

I can't quite see the why the bolded part is true. What I mean is that even in a fiber the gain curve is saturated when we have a laser with feedback (gain = loss), and if the gain curve is homogeneously broadened, in principle only a single longitudinal mode should oscillate (since the gain curve saturates homogeneously, so all other modes are below the gain curve). But they don't talk about that here. Is there something I am missing?Niles.

 

[eljose79]

Hello Niles,

Thank you for bringing up this question. The statement in the Wiki article is referring to lasers with single transversal mode, such as single-mode fiber lasers. In these types of lasers, the transverse mode is confined to a single spatial mode, but there can still be multiple longitudinal modes. The number of longitudinal modes is dependent on the spectral width of the gain, which is determined by the properties of the gain medium, and the spectral separation of the longitudinal modes, which is determined by the cavity length.

In a single-mode fiber laser, the gain curve is indeed saturated when there is feedback, but this does not mean that only a single longitudinal mode will oscillate. The gain curve may be homogeneously broadened, but there can still be multiple peaks within the broadened curve that correspond to different longitudinal modes. The separation between these peaks is determined by the cavity length, which is why the ratio of the spectral width of gain to the spectral separation of longitudinal modes can be used to estimate the number of longitudinal modes in a single-mode fiber laser.

I hope this helps to clarify the statement in the Wiki article. If you have any further questions, please let me know.