Applications of ultrashort pulses at 1.9 \({\upmu }\) m, such as micromachining, nonlinear imaging, and harmonic generation, require knowledge of the amplitude, phase, and polarization during the pulse. Therefore, the aim of this work is to characterize these parameters for pulses generated by a hybrid mode-locked thulium-doped fiber laser using the tomographic ultrafast reconstruction of transverse light fields (TURTLE) principle. The measurements are performed in two cases: directly at the laser output and after a polarization-independent isolator. Even at the laser output, the pulses exhibit a time-dependent polarization, confirming the intrinsic vector nature of nonlinear polarization rotation inherent in hybrid mode-locked lasers. Numerical simulations based on the coupled nonlinear Schrödinger equations reproduce this behavior qualitatively. Introducing a polarization-insensitive isolator significantly modifies the evolution of the pulse polarization due to uncompensated polarization mode dispersion, which splits the transmitted pulse into two pulses with opposite directions of electric field rotation. By using a polarization controller in front of the isolator, it is possible to adjust the power ratio between the two separated pulses, effectively suppressing the splitting process. These results demonstrate the importance of measuring and controlling the vectorial structure of ultrashort pulses in lasers based on non-polarization-maintaining fibers.
