Impact of the Kerr effect on FM-to-AM conversion in high-power lasers.

In order to smooth the focal spot of high-power energetic lasers, pulses are phase-modulated. However, due to propagation impairments, phase modulation is partly converted into power modulation. This is called frequency modulation to amplitude modulation (FM-to-AM conversion). This effect may increase laser damage and thus increase operating costs. For the first time, to the best of our knowledge, we have studied the impact of the Kerr effect in this process. We have shown that when the Kerr effect is followed by a dispersive transfer function, a dramatic increase of FM-to-AM conversion may occur for a particular kind of FM-to-AM conversion that we have named "anomalous." Hence, we should remove or compensate for one of the items of the sequence: phase modulation, anomalous FM-to-AM conversion, Kerr effect, or the dispersive function. We have assessed all these solutions, and we have found an efficient inspection method to avoid anomalous FM-to-AM conversion.