Radial and azimuthal polarized all-fiber Raman oscillator

Summary form only given. The generation of radially and azimuthally polarized beams has attracted significant interests in recent years due to the outstanding benefits that they bring in material processing, microscope applications and optical tweezers. These two polarization types belong to the so-called cylindrical vector beams with spatially invariant polarization. The intensity profile of such beams is doughnut-shaped due to the presence of a polarization vortex at the centre of the beam. In general it is challenging to generate radially and azimuthally polarized beams, therefore, different sophisticated approaches have been developed such as the spatial light modulator [1], sub-wavelength gratings [2] and interferometric setups [3], just to name a few. All of these examples are free-space optics. However, in order to build compact and stable setups, all fiber-setups are preferred.We have developed a fiber based mode filter [4] consisting of a fiber with lifted polarization degeneracy and a Fiber-Bragg-Grating. This filter lends itself ideally for its integration in Raman fiber oscillators. There is a caveat, however, and this is that if the fiber core is not perfectly round, the beam profile will not be a doughnut anymore (see figure 1 (a)). In general, the core ellipticity can be quantified by the TE fraction value of the radially and azimuthally polarized beams. It is worth mentioning, however, that ellipticity only influences the intensity profile but not the polarization purity of the beams. In this work we discuss the influence of core ellipticity on the radially and azimuthally polarized fibers modes and on the lifting of the polarization degeneracy in strongly guiding fibers in view of the TE fraction value. The elliptical core can lead to a higher lifting of the polarization degeneracy, thus, allowing the separation of polarizations which would otherwise be nearly degenerated in perfectly circular fibers. We also demonstrate a way to compensate the impact of elliptical fiber cores by applying a force at the fiber end. With this force we introduce stress in such a way that the intensity profile of the radially or azimuthally polarized mode becomes doughnut-shaped. Finally, we demonstrate a Raman fiber oscillator that consists of a 14 m long fiber (Nufern UHNA7) with two highly reflective UV written Fiber-Bragg-Gratings (FBGs). The reflection wavelengths of the FBGs were chosen in such a way that the wavelengths of the radially and azimuthally polarized fiber modes coincide with the maximum Raman gain. This is the most important criterion to suppress the fundamental mode. The Raman oscillator was pumped with a linearly polarized fundamental mode and, by rotating its polarization, it is possible to switch between the radial and azimuthal polarization. It is worth mentioning that it would not have been possible to generate the radially polarized mode, if a perfectly circular fiber core had been used. Figure 1 (b) and (c) show the azimuthally and radially polarized beams from the Raman oscillator with compensation of the elliptical fiber core. The presented concept is well suited for a wide field of polarization sensitive applications requiring a compact and stable light source due, in part, to the wavelength independence of a Raman oscillator, i.e. the same medium (fiber) can be used for generating widely different wavelengths.