Generating beams of polarization order number +1 using a cellophane sheet

Abstract

Beams with polarization order number +1 represent radially or azimuthally polarized light beams. These polarization tailored beams have attracted immense attention in a wide array of scientific and technological disciplines due to their unique properties and potential applications in material processing, optical trapping, and optical data storage among many others. Numerous techniques investigated in the past for obtaining these beams have had certain difficulties. For example, the use of conical elements involved difficult and/or impractical refractive index matching techniques. Other methods reported are very expensive, cumbersome and even difficult to realize in practice. Contrary to these techniques, this research presents a robust, simple, cost effective and practical method of obtaining these beams using a cellophane sheet. The research began with a search for a cellophane sheet with birefringence property. Using a polariscope test, various cellophane sheets were tested. The sheet that displayed the largest variation in the transmitted light with rotation was selected and used for the fabrication of a polarization mask which consisted of four segments. The fast axis of each segment was oriented differently in order to rotate locally the polarization of the incident linearly polarized beam as desired. To ensure the correct operation of the polarization mask, the polarization state of the generated beam was tested by measuring the spatial distribution of the Stokes parameters. A frequency stabilized He-Ne laser operating at a wavelength λ = 632.8 nm was used as the source of light. The light beam was spatially filtered, expanded and then collimated. It was then passed through the fabricated polarization mask, which converted the linearly polarized beam into either a pseudo-radially or a pseudo-azimuthally polarized beam. To achieve a high mode purity of the generated beams, higher order modes were filtered out utilizing a spatial filter. To ensure that the beam emerging from the polarization mask was indeed radially or azimuthally polarized, it was passed through a rotating linear polarizer. The resulting image patterns or lobes were captured using a CCD camera interfaced with a computer. The number and the positions of the lobes agreed with the theoretical predictions. The polarization purity of generated and filtered radially polarized beam was quantified by performing a set of measurements and thereby calculated the Stokes parameters S0, S1, S2, S3. In addition, the spatial distributions of the experimentally normalized Stokes parameters S0, S1, S2, S3 of the filtered radially polarized beam were obtained and compared with the theoretically expected distributions. These (experimentally obtained) distributions were comparable with their respective theoretically calculated distributions; an indication that the beams generated using the cellophane-made converter is of good mode quality.