Regular pulsations of the intensity of stimulated radiation emitted by a neodymium glass fiber laser

An investigation was made of regular pulsations of the emission intensity (spikes) resulting from the appearance of absorption centers generated by the ultraviolet part of the pumping radiation. The concentration of these absorption centers was varied by altering the concentration of an aqueous solution of K₂Cr₂O₇ which absorbed the ultraviolet part of the pumping spectrum. A study was made of the dependence of the spiking period and the delay time of laser emission on the pumping power. An anomalous increase in the spiking period was observed when the pumping power was increased: this was due to the formation of absorption centers in the glass fiber. The rate equations and the experimental results were used to find the dependences of the threshold value of the population inversion of the active levels on the pumping power and on the relative concentration of the absorption centers. The ratio of the threshold value of the difference between the level populations to the total number of neodymium atoms was 0.7–0.9 for a fiber laser. A steady-state value of the absorption coefficient was established at low concentrations of the absorption centers.