High-resolution wideband measurement of amplitude and phase noises of a GHz-repetition rate actively mode-locked Er-doped fiber laser by time-domain demodulation

Using a time-domain demodulation technique, we measured with good accuracy amplitude and phase noises of a 2.56-GHz optical pulse train generated from an actively mode-locked Er-doped fiber laser. This technique requires only the measurement of the 1st harmonic of the signal, reducing the bandwidth requirements of the measurement setup. Without any restrictive hypothesis on the pulse train, we obtained, with very good resolution and unprecedented dynamic range, a perfect separation of amplitude and phase noises, allowing us to understand clearly the physical origin of each noise contribution. We concluded that the generator, which is of relatively poor quality, determines both amplitude and phase noises of the pulse train. In particular, at very low frequencies (below 10 Hz), we observed the highest noise values, adding a large contribution to the global noise and justifying the interest for their measurement