Mode-locked erbium-doped fibre lasers are ultrashort pulsed sources widely studied due to their versatility and multiple applications in the near infrared range. Here we present the experimental study of the emission of a passive mode-locked erbium-doped fibre laser with an amplification stage outside the cavity by means of Frequency Resolved Optical Gating (FROG) and spectral interferometry. Due to shot-to-shot instabilities, the FROG traces can be understood as the combination of two different traces, corresponding to the coherent artifact and the average pulse characteristics. We have modified a Principal Components Generalized Projections Algorithm, in order to make it able to retrieve efficiently both the coherent artifact and the average pulse. In addition, we study the temporal dependence of the polarization, showing that the pulses present time-dependent polarization with a stable spectral relative phase between the horizontal and vertical projections. Up to our knowledge, this is the first experimental study that shows the FROG measurements of unstable pulse trains associated with the coherent artifact and analyses the time-dependent polarization in ultrafast fibre lasers.

锁模掺fiber光纤激光器由于其多功能性和在近红外范围内的多种应用而被广泛研究为超短脉冲光源。在这里，我们通过频率分辨光学选通（FROG）和光谱干涉法对腔内具有放大级的无源锁模掺光纤激光器的发射进行了实验研究。由于镜头之间的不稳定性，FROG轨迹可以理解为对应于相干伪像和平均脉冲特性的两个不同轨迹的组合。我们修改了主成分广义投影算法，以使其能够有效地检索相干伪像和平均脉冲。此外，我们研究极化的时间依赖性，表明脉冲呈现出随时间变化的极化，在水平和垂直投影之间具有稳定的光谱相对相位。据我们所知，这是第一项实验研究，显示了与相干伪像相关的不稳定脉冲序列的FROG测量，并分析了超快光纤激光器中随时间变化的偏振。