thermal load due at least in part to absorption of the pump light over the first length wherein the second gain fiber segment has a second thermal load due at least in part to absorption of the pump light over the second length means for propagating signal light of a signal light wavelength in the first core means for coupling a first amplified version of the signal light into the second core from the first core in a first direction means for coupling a second amplified version of the signal light out of the second core as an output signal beam and means for configuring the first and second lengths such that the second thermal load stays below a value at which a modal instability occurs it is specifically contemplated that the present invention includes embodiments having combinations and subcombinations of the various embodiments and features that are individually described herein including the various embodiments described by patent applications and patents incorporated by reference herein i e rather than listing every combinatorial of the elements this specification includes descriptions of representative embodiments and contemplates embodiments that include some of the features from one embodiment combined with some of the features of another embodiment further some embodiments include fewer than all the components described as part of any one of the embodiments described herein it is to be understood that the above description is intended to be illustrative and not restrictive although numerous characteristics and advantages of various embodiments as described herein have been.