# Longitudinal modes in a fibre laser

1. Nov 19, 2011

### 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?

Best,
Niles.

Last edited: Nov 19, 2011