Abstract: A system for cooling a fiber amplifier includes a fiber amplifier assembly, which, in turn, includes a longitudinally-extending fiber amplifier, a jacket and a retaining structure. The jacket surrounds the fiber amplifier and extends at least partially longitudinally therealong. In this regard, the jacket surrounds the fiber amplifier such that the fiber amplifier assembly defines a passage between the jacket and the fiber amplifier for the circulation of coolant therethrough. To facilitate the circulation of coolant, the retaining structure is disposed within the passage defined by the fiber amplifier assembly for at least partially maintaining a spacing between the fiber amplifier and jacket. The retaining structure and coolant can both comprise an emulsion of phase change material.