Actual modal power distributions in multimode optical fibers and their effect on modal noise

The modal power distribution, which specifies the amount of power flowing in each mode of a multimode fiber, influences the level of modal noise in optical-fiber systems. Generally, previous work has assumed that power in multimode fibers was distributed uniformly among the fiber modes. By considering the coupling of a semiconductor laser to a lensed fiber, we show that this assumption is grossly incorrect for practical excitations. Typically, half of the power is in just 3 modes, even though the fiber supports 49 modes. This leads to much lower modal noise than would be expected if the power were uniformly distributed. The dependence of the modal noise level on mode excitation is found to be much more complex than previously suggested.