Cylindrical vector (CV) beams are optical beams whose polarization states exhibit cylindrical symmetry across their cross sections (see Figure 1). This is different from traditional polarization types, such as linearly, elliptically, and circularly polarized beams, where the polarization state is spatially homogeneous across the beam's cross section.1 CV beams exhibit unique focusing properties through high-numerical-aperture lenses. Beams with radial polarization can be focused onto a smaller spot than traditionally polarized beams characterized by strong and localized longitudinal components. A focal spot as small as 0.16λ2 (where λ is the operational wavelength) has been obtained for radial polarization using an annular aperture. For azimuthally polarized beams, the focused field yields a doughnut-shaped spot with a symmetrical polarization distribution.1 The unique focusing properties of CV beams facilitate novel applications of cylindrically polarized beams, including particle trapping,2 material processing,3 and surface-plasmon excitation.4
Figure 1. Polarization distribution of cylindrical vector (CV) beams. (a) Radially polarized. (b) Azimuthally polarized. (c) CV beam as a linear superposition of (a) and (b). Arrows denote polarization directions.
Fiber lasers have attracted significant interest during the most recent two decades because of their high efficiency, compact design, and flexibility. CV beams can also be generated in optical fibers. According to waveguide theory, the lowest-order transverse-electric (TE01) and transverse-magnetic (TM01) modes in step-index fibers are characterized by cylindrical polarization5 (see Figure 2). To generate a CV beam, we only need to excite TE01 and TM01 modes in a few-mode fiber.
Figure 2. Polarization distribution of LP11 linearly polarized modes in step-index fibers. TM, TE: Transverse magnetic, electric. HE: Hybrid electric.
We present a very simple method to fabricate an all-fiber laser that generates CV beams6,7 (see Figure 3). It consist of a section of ytterbium (Yb) fiber, a Sagnac loop mirror, a 980/1060nm wavelength division multiplexing coupler, fiber collimators with operational wavelengths of 1060 and 1550nm, a section of SMF-28 single-mode fiber, and a 980nm laser diode. The Yb fiber is used as gain medium, while the SMF-28 fiber (highlighted in red in Figure 3) acts as few-mode fiber for the 1060nm band. We obtained CV beams by exciting TE01 or TM01 modes in the few-mode fiber. This was implemented through adjusting the angles and transverse dimensions of the fiber collimators. The radially and azimuthally polarized beams can be switched conveniently by simply applying twists or pressure to the few-mode fiber. Figure 4 shows the doughnut-shaped intensity distribution of the CV beam (recorded on a CCD). To measure the polarization distribution of the laser beam, we inserted a tunable linear polarizer in the light path prior to the CCD recorder.
Figure 3. Experimental setup for CV-beam generation. WDM: Wavelength division multiplexer. Yb: Ytterbium.
Figure 4. Intensity distribution of the CV beam.
In general, when a polarizer is placed in the light path after a CV beam has been generated, two lobes appear in the resulting intensity profile. Figures 5 and 6 illustrate the radial and azimuthal polarization states, respectively. White arrows indicate the polarization directions. Radial polarization is indicated by the dark line between the two lobes orthogonal to the polarization direction. The reverse occurs in the presence of azimuthal polarization (see Figure 6).
Figure 5. Radially polarized beam with different polarization directions.
Figure 6. Azimuthally polarized beam with different polarization directions.
In summary, we obtained CV beams from a new all-fiber laser with two fiber collimators as mode filter. Different CV-beam states, characterized by radial and azimuthal polarization, are switchable, while the CV-beam fiber laser is also very compact and easy to fabricate. We will further develop our all-fiber laser for practical use.
Lixin Xu, Rui Zheng, Chun Gu, Anting Wang, Hai Ming
University of Science and Technology of China
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