Efficient extracavity generation of radially and azimuthally polarized beams

Efficient extracavity generation of radially and azimuthally polarized beams

G.Machavariani,*Y.Lumer,I.Moshe,A.Meir,and S.Jackel

Soreq Nuclear Research Center,Electro-Optics Division,Yavne81800,Israel

*Corresponding author:galina@soreq.gov.il

Received January24,2007;revised March14,2007;accepted March14,2007;

posted March19,2007(Doc.ID79368);published May3,2007

We demonstrate an efficient transformation of a linearly polarized Gaussian beam to a radially or an azi-muthally polarized doughnut͑0,1͒*Laguerre–Gaussian beam of high purity.We use a spatially variable retardation plate,composed of eight sectors of a␭/2retardation plate,to transform a linear polarization distribution to radial/azimuthal distribution.We transformed an Nd:YAG Gaussian beam with M2=1.3to a radially and azimuthally polarized͑0,1͒*Laguerre–Gaussian beams with M2=2.5and degree of radial/ azimuthal polarization of96–98%.©2007Optical Society of America

OCIS codes:140.3300,230.5440.

Radially and azimuthally polarized beams have at-tracted high interest in recent years due to their unique properties,which are exploited in a variety of applications.A very important property of these beams is that their unique cylindrical-symmetry po-larization allows bypassing thermal birefringence-induced aberrations.This makes possible amplifica-tion of radially polarized beams in strongly pumped solid-state rods to obtain very high power and good beam quality[1].

Radially and azimuthally polarized beams can be generated inside a laser cavity[1–8].These methods, however,are based on complex resonator configura-tions,or require special fabricating techniques for nonstandard optical elements,or are applicable only in a limited range of pump powers.

Alternatively,radially or azimuthally polarized beams can be obtained directly from a Gaussian beam,outside the laser cavity,with specially de-signed mode converters.Such a conversion can be performed with different interferometric arrange-ments by interferometric superposition of linearly po-larized beams[9,10].However,these methods are very sensitive and require interferometrically precise alignment of discrete optical elements.A continuous single-element converter was realized with liquid crystals[11].Such a device,however,cannot be used for high powers.Another disadvantage of such device is the temporal instability of the orientation of the liquid-crystal molecules[12].

Instead of a continuous spatially varying retarder, an approximate device can be used.Such a device was used in[12,13].It consists of four quadrant sec-tors of half-wave plates,each one with different ori-entation of the crystal’s optical axis.When using four sectors,the transformation efficiency of the converter is75%[14],whereas the rest of the power is in higher-order Laguerre–Gaussian(LG)modes.To pu-rify the obtained beam,it was then sent through a confocal Fabry–Perot interferometer,which was op-erated as a mode cleaner when kept resonant only for the radially polarized͑0,1͒*LG mode.

In this Letter,we further develop the approach of a sectored spatially varying retarder(SVR),proposed in[12,13].We realize more efficient SVR,which con-sists of eight sectors of␭/2retardation plates,each one with different orientation of the“slow”axis of the birefringent crystal.Such a retarder is significantly closer to a“continuous”device,which makes the transformation significantly more efficient and does not require an additional mode cleaning Fabry–Perot interferometer.It is simple,robust,does not require interferometric accuracy in alignment,and can be used for high powers.We analyze the transformation theoretically and demonstrate experimentally trans-formation of a Gaussian beam to high-purity radially and azimuthally polarized LG͑0,1͒*modes.

The basic arrangement for transformation of a lin-early polarized Gaussian beam to a radially or azi-muthally polarized nearly LG͑0,1͒*beam is shown in Fig.1.The SVR device,which performs linear-to-radial polarization conversion,is shown in the inset. The device is composed of eight sectors of␭/2retar-dation plate,each one with different orientation of the crystal’s“slow”axis.The directions of the“slow”axes are shown by arrows,and the angles between the directions of the“slow”axes and the vertical di-rection are specified near each sector.When passing a linearly polarized Gaussian beam through the SVR, each sector will turn the polarization vector to a dif-ferent angle.The polarization distribution just after passing through the SVR,calculated using Jones ma-trices formalism is shown in Fig.2(a).It is close to ideal radial polarization distribution,except for the regions associated with the border areas between the sectors.The corresponding far-field(FF)polarization distribution is shown in Fig.2(b).FF intensity distri-bution,calculated by Fourier transforming of the near-field beam,is shown in Fig.2(c).The obtained FF intensity distribution is close to the ideal dough-nut distribution of a LG͑0,1͒*mode but has a slight eight-folded“star”halo surrounding the central doughnut,obviously due to the eight-sectored struc-ture of the SVR converter.Figure2(d)shows the FF

1468OPTICS LETTERS/Vol.32,No.11/June1,2007

0146-9592/07/111468-3/$15.00©2007Optical Society of America

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