Critical behavior of erbium fiber lasers in pulsed and relaxation oscillations regimes

Miniaturization of erbium fiber lasers is a crucial task, which implies a use of heavily doped fibers. In dense ensemble, interaction of erbium ions changes a configuration of quantum levels of gain medium and leads to pulsed generation. As a result, heavily doped erbium lasers demonstrate two thresholds, the first one associated with an onset of lasing in the pulsed regime, and the second with a transition to CW. Operation features near these two thresholds have been established experimentally. For the first time, a power-law behavior of the system parameters – pulses frequency, duration and peak intensity – was revealed in a wide range of pump rates around both thresholds. The power exponents were associated with critical indices of phase transition. Their values were convincingly determined different from integers and half-integers. Critical indexes were shown weakly dependent on the Fabry-Perot and distributed feedback (DFB) laser cavity parameters, which made it possible to experimentally establish the universal dependence of the pulse frequency and duration on the lasing power. The results of the work are extremely useful for determining and predicting the parameters of the designed erbium lasers, due to universality of the critical indices.