We demonstrate that effective traveling-wave excitation of high-gain amplifiers requires velocities that are remarkably slower than the velocity of light. Experiments with a femtosecond-laser-pumped molecular hydrogen laser exhibit pronounced enhancement of the intensity if an excitation velocity that is slower than the velocity of light is employed. These results are directly scalable to shorter wavelengths, paving the way for a more effective pump setup for x-ray lasers. (C) 2001 Optical Society of America. [References: 14]
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