All-fiber Raman oscillator for the generation of radially and azimuthally polarized beams

In this paper we demonstrate a Raman fiber oscillator for the generation of radially and azimuthally polarized beams. The Raman fiber oscillator comprises a high NA fiber and two Fiber-Bragg Gratings (FBGs). Due to the high NA of the fiber, radially and azimuthally polarized modes are guided with their own effective refractive indexes, i.e. they are not degenerated. Therefore, the FBGs reflect these modes at different wavelengths. The mode that oscillates in the resonator can be selected by controlling the coupling lens and the polarization of the pump beam. Unfortunately, at the output of the fiber oscillator the output beams exhibit a non-circularly symmetric intensity profile as a result of a slightly elliptical fiber core. Consequently, the impact of elliptical cores on the polarization degeneracy has been analyzed in detail. In order to compensate for the elliptical core we applied a transverse force on the last few cm of the fiber. With this force the waveguide characteristic of the fiber is changed in such a way that a radially or azimuthally polarized doughnut-shaped beam profile is observed. Thereby an output power of 480mW (400mW) was reached for the azimuthal (radial) polarization. The presented concept is wavelength agile and suitable for all-fiber microscopic setups, especially for STED-microscopy.