A simple technique for directly generating a radially-polarized output beam from an ytterbium-doped fiber laser using an intracavity spatially-variant waveplate is reported. The laser yielded 32W of output with a corresponding slope efficiency of 65.8% in a radially-polarised beam with beam propagation factor ~2.1 and polarization purity &gt;95%

Beresna, M.

Clarkson, W.A.

Daniel, J.M.O.

Gecevičius, M.

Kazansky, P.G.

Lin, Di

English

Southampton (e-Prints Soton)

     High power radially-polarized Yb-doped fiber laser  D. Lin, J. M. O. Daniel, M. Gecevičius, M. Beresna, P. G. Kazansky and W. A. Clarkson  Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK  Radially-polarized light is characterized by a donut-shaped intensity profile and exhibits axially symmetry both in field-amplitude and polarization direction. Radially-poalrized beams can produce a strong longitudinal electric field component along the beam axis under the condition of tight focusing. This unique property leads to a range of applications in material processing, high-resolution microscopy, particle acceleration and trapping.  Here, we present an alternative approach for efficiently generating radially-polarized output directly in an Yb-doped fiber laser by employing an S-waveplate in an external feedback cavity arrangement. The S-waveplate consists  of  spatially-variant  sub-wavelength  gratings  which  are  produced  by  femtosecond  laser  pulse  direct writing in a fused-silica window. These grating structures induce form birefringence with slow and fast axes aligned parallel and perpendicular to the grating direction respectively, which is aligned at an angle 𝜑 2 ⁄  from the azimuthal angle 𝜑 [1]. The experiment set-up, illustrated in Fig. 1, consisted of ~ 1m length of non-polarization maintaining Yb-doped with an external feedback cavity. The fiber had a core diameter and numerical aperture of 20µm and 0.08 respectively. The calculated V-number of this fiber was 4.79 and hence the fiber could support a few lower order modes. The end of fiber adjacent to the external cavity was cleaved at 8° to suppress the broadband feedback and hence parasitic lasing between the fiber end facets. The opposite end was perpendicularly-cleaved to serve as the output coupler. The Yb fiber was cladding-pumped by a 60W laser diode. An S-waveplate and a polarizing beam splitter  (PBS)  were  placed  in  the  external  cavity  between  rear  mirror  and  fiber  collimating  lens.  The  PBS allowed only p-polarized light to pass through it, which was sunsequently incident on the S-waveplate. When the orientation  of  the  S-waveplate,  shown  by  the  arrow  in  Fig.1(a),  is  aligned  parallel  to  the  p-polarization,  it converts the p-polarized beam into a radially-polarized beam, which is then coupled into the fiber to excite the radially-polarized TM01 mode. Fig.2(a) shows the output power as a function of the launched pump power. The output power increased linearly with pump power with a slope efficiency of 65.8% with respect to launched pumpe power.  The laser yielded a maximum output power of 32W at 1050nm for the maximum available pump power of 50W. The typical output beam intensity distribution is shown in Fig.2(b) confirms that the output beam has a donut-shaped intensity profile with a null in intensity at the centre of the beam. Figs.2(c)-(e) show the two-lobe structures of output  beam  beam  after  passing  through  a  polarizer  oriented  at  different  angles  thus  verifying  that  the polarization is indeed radial. The polarization purity was measured to be > 95% over the full range of output power. The beam propagation factor (M2) was measured to be 2.1 and hence in close agreement with the theory confirming the high quality and purity of radially-polarized TM01 mode that was generated. To the best of our knowledge, this is the highest power and highest slope efficiency for radially-polarized fiber laser to date. The prospects for further improvement in efficiency and higher output power will be discussed.  References [1] M. Beresna, M. Gecevicius, and P. G. Kazansky, "Polarization sensitive elements fabricated by femtosecond laser nanostructuring of glass [Invited]," Opt Mater Express 1, 783-795 (2011). Fig. 1 (a) Schematic of nanogratings ; (b) experimental laser set-up. Fig. 2 (a) Laser output power measurement ; (b) beam intensity distribution in far-field ; (c)-(d) beam profile after passage through a rotated linear polarizer. 

Polarization sensitive elements fabricated by femtosecond laser nanostructuring of glass [Invited],&quot;

High power radially-polarized Yb-doped fiber laser

High power radially-polarized Yb-doped fiber laser

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