Fast monitoring of spectral broadening in seed sources for narrow-linewidth fiber lasers

Line-broadening of narrow-linewidth seed-sources is an effective method for Brillouin threshold enhancement. Fast monitoring of spectral broadening and carrier suppression is critical. In addition to characterizing and stabilizing the linewidth, fast monitoring also enables quick control signals to turn the system off when degradation or failure of linebroadener is observed. This is critical since failure of line-broadening can lead to catastrophic damage to the amplifier chain. Here we present a fast and easily integrable method of monitoring the line-broadening by modulating a tag frequency to a small tapped-component of the initial seed-source and interfering with a small tapped-component of the line-broadened seed. By observing the power in the tagged frequency component as line-broadening is varied (through drive voltage to phase modulator), linewidth and carrier suppression can be inferred. We demonstrate this experimentally with a white-noise and pseudo-random binary sequence (PRBS)- based noise source as line-broadener. Experimental data matches well with our theoretical model. Further, we demonstrate that the speed of detection is ⁓200 ns, which gives us a sub-microsecond window for an error signal generation for shutting-off the modules.