Fiber amplification of radially and azimuthally polarized laser light Moti Fridman $^{1}$, Micha Nixon$^{1}$, Mark Dubinskiy$^{2}$, Asher A. Friesem$^{1}$, and Nir Davidson1 1 Weizmann Institute of Science, Dept. of Physics of Complex Systems, Rehovot 76100, Israel $^{∗}$Corresponding author: moti.fridman@weizmann.ac.il 2 US Army Research Lab Adelphi, MD 20783, USA Compiled October 29, 2018 The results on amplifying either radially or azimuthally polarized light with a fiber amplifier are presented. Experimental results reveal that more than 85% polarization purity can be retained at the output even with 40dB amplification, and that efficient conversion of the amplified light to linear polarization can be obtained. c ○ 2018 Optical Society of America OCIS codes: 140.3280 (laser amplifiers), 260.5430 (polarization) Radially and azimuthally polarized laser light have unique properties and symmetries that are advantageous for high power lasers [1,2]. For example, their cross sec- tion intensity distributions have doughnut shapes whose peak intensities are significantly lower than that of light with a Gaussian distribution, so non-linear and damage effects through fibers core are significantly reduced [3]. Accordingly, several laser resonator configurations have been developed to produce radially and azimuthally po- larized light [3–10]. However, in all these the power of the output beam was relatively low, so that for some applications additional amplification is required [11,12]. Thus far, amplification of either radially or azimuthally polarized laser light was demonstrated with bulk solid state laser amplifiers [2], but not with fiber amplifiers, probably because of deleterious effects of their birefrin- gence and non-linearities. Here we present for the first time experimental results on amplifying radially and az- imuthally polarized light with a large mode area fiber amplifier, where we obtained an amplification of 40dB with over 85% polarization purity. We also show that the amplified beam can be efficiently converted back into a linearly polarized near-Gaussian beam, confirming the modal structure and polarization purity of the radially and azimuthally polarized amplified beam. The experimental arrangement for amplifying either radially or azimuthally polarized beam with a fiber am- plifier is presented schematically in Fig. 1, along with an arrangement for converting the amplified radial or azimuthal light distribution to a Gaussian distribution. In this configuration a weak linearly polarized Gaus- sian beam of 40 µW propagated through a space-variant retardation plate(SVR) to obtain either radially or az- imuthally polarized light that served as the input [3–5]. The SVR was comprised of eight sectors each having λ/ 2 retardation in different orientations, where the direction of the slow axis of each retardation sector is denoted by an arrow as shown in Fig. 1. When the SVR was oriented at 0 ◦ with respect to the polarization of the input beam, it converted the linearly polarized light into radially po- larized light, and when the SVR was oriented at 45 ◦ it We measured the amplified output power and found it to be 400mW, indicating that the amplification was 40dB. The polarization distributions of the input and amplified light beams were determined by measuring their intensity distributions after passing through three different sets of polarization elements, to obtain the four Stokes parameters [15]. Some representative results of three of the four Stokes parameters together with the resulting polarization state at each point in the beams, are shown in Fig. 2. Figures 2(a), (b) and (c) show the three Stokes parameters and the state of polarization for The radially or azimuthally polarized beam was then lunched into a 10 m long Ytterbium doped double clad fiber, with core diameter of 20 µm and numerical aper- ture of 0 . 07, that served as the amplifier. Such a fiber amplifier can support the propagation of four modes, and in particular the TE$_{01}$ and TM$_{01}$ modes of the ra- dial and azimuthal polarizations inside the fiber, which corresponds to the LG ∗ 10 mode in free space [13,14]. The fib er amplifier was co-pumped with a 915 nm diode laser. converted the linearly polarized light to azimuthally po- larized light. In both cases the spatial mode of the beam was converted into a LG ∗ 10 mode. arXiv:1001.2881v1 [physics.optics] 17 Jan 2010 Fig. 1. Experimental configuration for amplifying radi- ally and azimuthally polarized light with a fiber ampli- fier. Path A - space variant polarization measurement, path B - conversion arrangement to a linearly polarized light. Ytterbium doped  LMA fiber Pump Space-variant  polarization  measurement SVR CCD f f SVR 0 8 π 4 π − 4 π 8 π − 2 π 8 3 π 8 3 π − B A 1