Effect of the Number of Longitudinal Modes on Spectral Broadening in a High Power Fiber Amplifier

The dependence of the spectral broadening characteristics on the number of longitudinal modes is studied experimentally in a bi-directional pumping fiber amplifier. It is found that the linewidth of the amplifier decreases with reducing the longitudinal mode number of the seed source. In this letter, we present a 68 pm-linewidth kilowatt-level fiber laser using a simple master oscillator power amplifier structure. Meanwhile, we theoretically analyze the spectral broadening characteristics of the seeds with different longitudinal mode numbers. The conclusions are in agreement with the experimental results. This provides a simple and efficient method for obtaining the high power narrow linewidth fiber laser used in spectral beam combining.