We performed spectroscopic measurements of the relaxation oscillations in a tunable Nd3+-doped fiber laser for the four-level (F-4(3/2)-I-4(11/2)) and for the three-level (F-4(3/2)-I-4(9/2)) transitions. We observed that the relaxation oscillation frequency for the three-level transition exhibited a strong wavelength dependence; in contrast, the relaxation oscillation frequency for the four-level transition did not exhibit any wavelength dependence. A simple theory to explain this remarkable feature was developed, confirming that the wavelength dependence is related to the thermal laser level population, which is significant in three-level systems but negligible in four-level systems. This should be taken into account for a careful description of laser systems, and is also instrumental for predicting oscillator or amplifier performance.
