Nonlinear effects in coherently coupled laser resonators

A procedure is described to accurately measure the self phase-tuning in a coupled fiber laser. A fiber is designed and fabricated to eliminate the effects of self-tuning from wavelength shifting, thermal expansion, and thermally induced index change, allowing us to study the phase effects produced by Kramers-Kronig phase shifting as a function of various cavity parameters. For sufficient pump power, we observe that the Kramers-Kronig effect is capable of compensating for all path length errors introduced into the cavity, resulting in efficient lasing under all path length conditions. We have directly measured the Kramers-Kronig-induced phase shift and present experimental evidence that this additional phase compensates for the applied phase error and promotes efficient lasing.