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Light carries both spin and orbital angular momentum and the superpositions of these two dynamical properties have found many applications. Many techniques exist to create such light sources but none allow their creation at the femtosecond laser. Here we report on a novel mode-locked ytterbium-doped fiber laser that generates femtosecond pulses with higher-order Poincaré sphere beams. The controlled generation of such pulses such as azimuthally and radially polarized light with definite orbital angular momentum modes are demonstrated. A unidirectional ring cavity constructed with the Yb-doped fiber placed at the end of the fiber section to reduces unnecessary nonlinear effects is employed for self-starting operation. Two pairs of diffraction gratings are used for compensating the normal group velocity dispersion of the fiber and other elements. Mode-locked operation is achieved based on nonlinear polarization evolution, which is mainly implemented with the single mode fiber, the bulk wave plates and the variable spiral plates (q-plate with topological charge q=0.5). The conversion from spin angular momentum to the OAM and reverse inside the laser cavity are realized by means of a quarter-wave plate and a q-plate so that the polarization control was mapped to OAM mode control.
To demonstrate the total spatiotemporal and vectoral characterization of the new type femtosecond laser beams, here, a polarization-sensitive Mach–Zehnder interferometer temporal scan technique was used, at the first time, to capture the complete information of the pulse. The corresponding measurement device, placed on the collimated and attenuated beam at the laser output and simply consists of a special Mach–Zehnder interferometer, a polarizing beam-splitter and two charge-coupled device (CCD) cameras. The reference beam with vertical polarization is exported from the cavity and attenuated to a suitable intensity by using a neutral-density filter. After Fourier filtering, removal of the reference curvature, correction of achromatic wave-front distortions, spectral phase and amplitude reconstructions, as well as the measurement for the intrinsic phase of the reference pulse, the complete information of the pulse will be obtained. This new measurement capability opens the way to in-depth characterization and optimization of the vector vortex femtosecond laser pulse and ultimately to the detection of new phenomena of the interactions between materials and structured femtosecond laser beams in space–time and polarization.
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