Abstract
We performed spectroscopic measurements of the relaxation oscillations in a tunable Nd3+‐doped fiber laser for the four‐level (4F3/24I11/2) and for the three‐level (4F3/24I9/2) 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.

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