A fiber laser system and a related method for its use, in which one or more fiber laser amplifiers are cryogenically cooled and optimized to operate at a desirably high efficiency. Versions of the laser system using either thulium or erbium doped fibers are disclosed. In a high power version of the system, the outputs of multiple fiber lasers are coherently combined. Cooling by a selected liquefied gas, such as nitrogen, is applied to the fiber laser amplifiers and, optionally, to pump diodes and to optical elements used to combine the outputs of the fiber laser amplifiers.