Direct generation of femtosecond cylindrical vector beams from a 2.8 <b> <i>μ</i> </b>m Er:ZBLAN fiber laser

Cylindrical vector beams (CVBs), which feature a unique spatially non-uniform polarization distribution with an axis symmetry, have been attracting increasing attention due to their various potential applications. However, mainstream CVB lasers demonstrated so far are emitting in the near-infrared wavelength region. The development of mid-infrared (MIR) CVB lasers can greatly broaden their application areas, especially when operating in the mode-locking regime where high peak power ultrafast pulses can be produced. Here, we present the direct generation of MIR femtosecond CVBs from an Er-doped fluoride fiber laser. The mode-locking operation is realized by the nonlinear polarization rotation technique, accompanying with an intra-cavity CVB converter based on two S-waveplates. Both radially and azimuthally polarized beams in the form of mode-locking pulse trains are achieved in the experiment with a typical pulse width of 336 fs and a peak power of 7.3 kW at a repetition of 42.24 MHz. The center wavelength locates at around 2795 nm. This work can provide a solution for the direct generation of femtosecond CVBs in the MIR wavelength region and facilitate its application in fields such as micro-manufacturing and nanoparticle manipulation.