We introduce the most recent progress in the optimization of ultrafast fiber lasers for building ultralow timing jitter signal sources. Using a sub-20-attosecond-resolution timing jitter measurement technique, we optimize the timing jitter of optical pulse trains from mode-locked Er-fiber and Yb-fiber lasers to 70 attoseconds and 175 attoseconds, respectively, when integrated from 10 kHz to 40 MHz offset frequency. To our knowledge, these results correspond to the lowest rms timing jitter demonstrated from fiber lasers so far, the equivalent phase noise of which is comparable to that of the best microwave sources available, with much reduced cost and engineering complexity.