Analysis and experimental demonstration of spatial mode selection in a coherently combined fiber laser

Self-phasing due to spatial mode selection in a two-element passively coupled fiber laser is studied. We find that the addition of a second supermode in a coupled resonator results in a 90% increase in the average output power and nearly π/2 radians of passive phase adjustment versus applied phase errors between the gain elements. These results require a phase of zero (modulo 2π) between the beams in the external cavity. These findings are supported by an eigenmode analysis of the resonator and show that beam recycling is a useful resonator design feature but must be appropriately implemented to obtain beneficial results.