Hi Often when I read papers about state-of-the-art lasers (in terms of linewidth and stability), many mention (e.g. here, figure 1: http://arxiv.org/pdf/physics/0512096.pdf) that the Hydrogen-maser works in the bad-cavity limit. I don't know why that is and I can't seem to find any good litterature explaining this. Can anyone recommend a book that explains this matter in some detail? Best, Niles.