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

: Jocher, Christoph; Jauregui, Cesar; Becker, Martin; Rothhardt, Manfred; Limpert, Jens; Tünnermann, Andreas


Ramachandran, S. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Fiber Lasers XI. Technology, Systems, and Applications : 3 - 6 February 2014, San Francisco, California, United States
Bellingham, WA: SPIE, 2014 (Proceedings of SPIE 8961)
ISBN: 978-0-8194-9874-8
Paper 896122
Conference "Fiber Lasers - Technology, Systems, and Applications" <11, 2014, San Francisco/Calif.>
Fraunhofer IOF ()
fiber optic; radially and azimuthally polarization; stimulated Raman scattering; fiber-bragg grating

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 doughnutshaped 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.