The behavior of transient oscillations has been studied experimentally for the first time in a broadly tunable ytterbium fiber laser. Spectroscopic study of the relaxation frequency allows one to distinguish three- and four-level transitions and provides a useful tool for controlling the dynamics of pulsed lasers. Particularly, the relaxation oscillation frequency depends on the occupation of the terminal level of the laser transition and clearly shows that the laser transition becomes four-level at the long-wavelength tail of the gain spectrum of ytterbium fiber (λ>1060 nm). The wavelength dependence of relaxation oscillations can be used to determine the parameters of the gain material such as transition cross-section.
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