Patent Publication Number: US-8111957-B2

Title: +cylindrical polarization beams

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/079,641, filed Jul. 10, 2008 and U.S. Provisional Patent Application No. 61/118,097, filed Nov. 26, 2008. 
    
    
     BACKGROUND 
     A cylindrically polarized light beam, and including a hybrid-azimuthal and a radial hybrid polarization beam as well as radial and azimuthal polarization beams generated from an input linearly polarized Gaussian beam using a spun optical waveguide device called HARP modes. 
     Light beams having optical vortices are desirable for a number of important applications. These applications include, for example, optical imaging, lithography, electron particle acceleration, material processing, efficient laser cutting, welding, and metrology. Further, radially and azimuthally polarized light enables the focusing of beams beyond the diffraction limit while generating stationary longitudinal electric and magnetic fields. 
     The radially and/or azimuthally polarized light beams, referred to as cylindrical vector beams, may be generated using several different methods including, for example, intra-cavity polarization manipulation, computer generated holograms, offset input to few-mode optical fibers, and excitation of optical fibers with higher order modes. For applications like optical trapping, the radial polarized beam is able to trap the ambient medium, while the azimuthally polarized beam works to trap particles with dielectric constants lower than the ambient medium. Switching between radial and azimuthal polarization can be done, for example by using two half-wave plates. 
     While these methods have been devised to generate vector beams, these methods are complicated for they involve using few mode fibers in conjunction with a number of micro optic components such as an asymmetric phase plate, half wave plates and polarization controllers. For example, one would first convert an input Gaussian beam to an asymmetric beam using a phase plate and then use a number of polarization components to enable conversion to a cylindrical polarization mode. 
     It is well known that spinning an optical fiber can cause coupling between the two linear polarization modes of the first spatial mode of an optical fiber, i.e. the LP01 mode. See, for example, A. J. Barlow et. al. Applied Optics 20, 2961, (1981); U.S. Pat. No. 5,298,047; and M. J. Li and D A. Nolan Optics Letters, Vol. 23, No. 21, pgs. 1659-1661, (1998). These spinning techniques and technologies have been well studied for the purpose of reducing polarization mode dispersion in long length optical fiber transmission lines. 
     SUMMARY 
     The present invention describes a method for generating cylindrically polarized light, including what we term to be hybrid-azimuthal polarization mode and also a hybrid radial polarization mode, from an input linearly polarized Gaussian beam. The hybrid-azimuthal polarization mode is a combination of the azimuthal polarization mode and an HE21 mode. The hybrid radial polarization mode is also a combination of the radial polarization mode and an HE21 mode. The conversion takes place in a spun few mode optical waveguide device. The waveguide is a spun fiber waveguide of an appropriate index distribution to enable propagation of the linearly polarized LP 11  superposition of vortex—vector polarized modes. In addition the core may be slightly elliptical and the core center slightly offset from the geometric center of the device. Spinning the fiber during the drawing process causes these asymmetries to rotate along the device length at the spin frequencies. Spinning the device at a superposition of high frequencies on the order of centimeters and less causes light to couple from the input fundamental mode to the LP 11  mode, which is a superposition of the vortex vector modes. These modes include the HE 21 , the radial and the azimuthal modes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a color line depiction of the polarization patterns of the four LP 11  modes, as follows: (a) is the polarization pattern for the TE 01  mode, (b) is the polarization pattern for the TM 01  mode, (c) is the polarization pattern for the HE 21   even  mode, and (d) is the polarization pattern for the HE 21   odd  mode; 
         FIG. 2  is a color line depictions for the radial and azimuthal polarization states, and showing the hybrid azimuthal polarization state according to the invention; 
         FIG. 3  illustrates an experimental arrangement to generate and analyze the cylindrical vector beams; 
         FIG. 4  illustrates the output of a spun fiber and the intensities that are seen after passing the output of the fiber through a linear polarizer set at (a) 0°, (b) 45°, and (c) 90°; 
         FIG. 5  illustrates a line depiction of the output polarization state from a spun fiber (a), and the intensity and polarization states when the output is passed through a linear polarizer set at (b) 0°, (c) 45°, and (d) 90°; and 
         FIG. 6  illustrates the conversion of the output from the spun fiber to a pure azimuthally polarized beam using a quarter wave plate oriented at 0° or 90°. 
     
    
    
     DETAILED DESCRIPTION 
     As has been described in G. Volpe, D. Petrov,  Generation Of Cylindrical Vector Beams With Few - Mode Fibers Excited By Laguerre - Gaussian Beams , Opt. Commun 237 (2004) 89-95, and in other papers, cylindrical vector beams may be generated by exploiting the similarity between the polarization properties of the modes that propagate inside a step-index fiber and the cylindrical vector beam. A few-mode fiber supports the fundamental mode LP 01  (HE 11 ), in addition to the secondary modes LP 11  (TE 01 , TM 01  and HE 21 ). The transverse electric, TE 01 , TM 01  modes present, respectively, an azimuthally and radially polarized electric field, while the hybrid electric mode, HE 21 , has a hybrid structure. However, the modal power distribution over the cross-section of the fiber is different for the modes LP 01  and LP 11 . For LP 01  the power is concentrated in a small area around the axis of the fiber. By contrast, for LP 11  the power is distributed in a doughnut shape, for example, using a first order Laguerre-Gaussian LG 10  beam at the input of the fiber instead of a Gaussian beam. 
     The transverse electric fields of the four LP 11  modes, namely, TE 01 , TM 01 , HE 21   even , and HE 21   odd , is given as:
 
 e   TE   =F ( r ){sin(ø) {circumflex over (x)} −cos(ø) ŷ} 
 
 e   TM   =F ( r ){cos(ø) {circumflex over (x)} +sin(ø) ŷ} 
 
 e   HE   e   =F ( r ){cos(ø) {circumflex over (x)} −sin(ø) ŷ} 
 
 e   HE   o   =F ( r ){sin(ø) {circumflex over (x)} +cos(ø) ŷ} 
 
where F(r) is a radial function and {circumflex over (x)} and ŷ are unit vectors. The polarization patterns of these various modes are shown in  FIG. 1 . The state of total field polarization varies along the fiber because there are three propagation constants for these modes, namely, β HE  for even and odd HE 21  modes, β TE  for the TE 01  mode, and β TM  for the TM 01  mode.
 
     As shown above, the general expression for the polarization state of a radially or azimuthally polarized beam is given by: 
                   ε   =     {             a   ⁢           ⁢     sin   ⁡     (   ∅   )       ⁢     x   ^       +     b   ⁢           ⁢     cos   ⁡     (   ∅   )       ⁢     y   ^                     a   ⁢           ⁢     sin   ⁡     (   ∅   )       ⁢     x   ^       +     b   ⁢           ⁢     cos   ⁡     (   ∅   )       ⁢     y   ^                         [   1   ]               
respectively, where ø represents the angular position around the beam and a, b are coefficients allowing for circular or elliptical polarization states at some angles. With a=b=1, these two polarization states are shown in  FIGS. 2(   a ) and ( b ), respectively.  FIG. 2(   c ) shows the polarization state for:
 
ε=− a  sin(ø) {circumflex over (x)}+b  cos(ø) ŷ 
 
with a=1, and b=i.
 
     This is a new polarization state that we describe herein as the hybrid-azimuthal polarization state (HAPs), according to the invention that has an orthogonal state we describe as the hybrid-radial polarization state (HRPs). The HAPs mode has a polarization similar to azimuthal polarization at ø=0, π/2, π, 3π/2, and polarization similar to circular polarization at ø=π/4, 3π/4, 5π/4, and 7π/4. 
     The HRPs mode has a polarization similar to radial polarization at ø=0, π/2, π, 3π/2, and polarization similar to circular polarization at ø=π/4, 3π/4, 5π/4, and 7π/4. The HAPs and HRPs modes form a class we are calling HARP modes. 
     The hybrid-azimuthal polarization can be written as a superposition of the transverse electric, TE 01 , and the hybrid electric (even), HE 21  modes. The TE 01  is the same mode as the azimuthal polarization shown in both  FIGS. 1(   a ) and  2 ( b ). By contrast, the HE 21  mode is itself a hybrid mode and is shown in  FIG. 1(   d ). By combining the TE 01  mode with the HE 21  mode at a given phase difference (i) as follows:
 
 TE   01   +iHE   21(even)  
 
gives rise to the HAP state. The factor of “i” before the HE 21  mode represents a π-phase difference between the two polarization states, which is what is responsible for the circular polarization components in the final polarization.
 
     Orthogonal to the HAP polarization is the hybrid radial polarization (HRP) mode. It is a superposition of the TM01 and the odd HE 21  mode
 
 TM 01+ iHE   21(odd)  
 
     The linear and angular momentum of the HAPs and the HRP modes can be calculated following the methods of L. Allen, M. J. Padgett, M. Babiker,  Progress in Optics XXIX, pp.  291-372 (1999). We consider in detail the HAP mode keeping in mind similar statements can be made concerning the HRP mode. As is shown from equation [1] above, the polarization state, c is a function of ø, and yet is shown with unit vectors {circumflex over (x)} and ŷ. If rewritten as r and ø, we would not allow for the constants a and b, which are the terms responsible for the manifestation of the HAP state. Assuming the vector potential takes the form,
 
 {right arrow over (A)} ( r )=(cos φ {circumflex over (x)} −sin φ ŷ ) u ( r )exp[ ikz ],
 
where u(r)=u o r exp [−r 2 ], φ=arctan(y/x) is the azimuthal position around the beam and r=√{square root over (x 2 +y 2 )}, the electric and magnetic fields calculated from the Lorenz gauge [10] are:
 
 {right arrow over (B)} ( {right arrow over (r)} )=− ik [(sin φ {circumflex over (x)}+i  cos φ ŷ ) u ( r )+( i/k )(sin φ d/dy+i  cos φ d/dx ) u ( r ){circumflex over (()} z )]  [3]
 
and
 
 {right arrow over (E)} ( {right arrow over (r)} )=− iw [(−sin φ {circumflex over (x)}+i  cos φ ŷ ) u ( r )+( i/k )(−sin φ d/dx+i  cos φ d/dy ) u ( r ){circumflex over (()} z )]  [4]
 
     Calculating the time averaged linear momentum, {right arrow over (p)}, using these fields 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     using the double angle identity, sin φ cos φ=(½) sin 2φ. Comparing this to the angular momentum calculated for a circularly polarized beam of light, we see that the “σ”, known as the spin angular momentum of a photon, that had a value of ±1 depending on the handedness of the circularly polarized light and 0 for linearly polarized light, now takes the spatially varying value of sin 2φ. This φ-dependence implies that locally, the angular momentum varies from 0 at φ=0 to +ℏ at φ=π/4 to 0 at φ=π/2 to −ℏ at φ=3π/2 and so on. Wherever there is a circular polarization in  FIG. 1(   c ), the angular momentum is ±ℏ. Calculating the total angular momentum by integrating across the beam gives:
 
 J   z   =∫∫rdrdφj   z =0,   [7]
 
as the integral over φ of j z  in Eq. 7 from 0 to 2π is zero. So, while the spatial variation in the angular momentum density for the hybrid polarization is non-zero, the total angular momentum is in fact, zero. This differs from the angular momentum from the cases of pure radial or azimuthally polarized light as the angular momentum density for both of these modes is zero, i.e. j z   radial =0 and j z   azimuthal =0.
 
     Experiments and Data 
     In order to cause coupling from one polarization mode of the LP 01  spatial mode to another it is required that the spin rate be sufficiently faster than the beat length of the fiber. The beat length is inversely related to the ellipiticity of the fiber core. The beat length is also related to the difference in propagation constants or effective indices of the two linearly polarized modes in a slightly elliptical core fiber. 
     Now the LP 11  mode is a superposition of the HE 21  mode as well as a radial and azimuthally polarization modes. These modes all have different propagation constants; see for example Vassallo,  Optical Waveguide Concepts , Elsevier, 1991. The propagation constants or indices of these modes are all slightly different. Spinning a fiber can cause coupling among all these modes. 
     The experimental setup to generate a vortex beam with the HAP state is shown in  FIG. 3 . A vertically (linearly) polarized HE-NE (helium-neon) laser  30  with a wavelength of 632.8 nm passes through a half-wave plate  32  to create a linearly polarized Gaussian beam that may be rotated to a desirable orientation. The linearly polarized Gaussian beam is then coupled into a spun fiber  34  via a 20× microscope objective  35 ( a ), which serves to focus the beam onto the core of the fiber. The output from the fiber is decoupled through a second 20× microscope objective  35 ( b ), which serves to expand the output mode to a beam. The beam is then passed through a linear polarizer  36  that acts as an analyzer to characterize the output polarization state. The intensity distribution is recorded on a black and white CCD camera  38 . 
     Although a vertically polarized He—Ne laser  30  is described as the source for generating a linearly polarized Gaussian beam, it is clear to one skilled in the art that any source for generating the linearly polarized Gaussian beam may be used. However, the source is preferably such that, once input into the fiber, the light propagates as an LP 11  mode. In the experiment described in  FIG. 3 , the elliptical core fiber  34  is spun at a rate of 20 turns per meter as it is made. However, the rate of twisting the fiber  34  may vary depending upon the ellipticity of the fiber and the length of the fiber in which one desires the conversion to occur. For example, a cylindrical fiber could theoretically be rotated at a lower rate, but preferably greater than 10 turns per meter. However, by using an elliptical core fiber, one could create the same mode-coupling effect at higher rotation rates. 
     Further, the length of the spun fiber  34  used in this experiment is approximately ten meters long. However, the length of the fiber may vary from a few centimeters to any desirable length, provided that the fiber is sufficiently long to allow the mode conversion to occur. As we reduce the length of the fiber, then we would need to increase the spin rate of the spun fiber, for a given ellipticity. Further, the experiment of  FIG. 3  describes a fiber having a cutoff wavelength of 740 nm. However, in order to generate the LP 11  modes, all that is required is that the cutoff wavelength be greater than the wavelength of the light propagating in the fiber. Since a laser  30  with a wavelength of 632.8 nm was used, a spun fiber having a wavelength cutoff of 740 nm was acceptable. However, it is preferred that the wavelength cutoff not be so great as to allow the propagation of HE modes of higher order than the HE 31  mode. According to the experiment described in  FIG. 3 , the spun fiber  34  has a one percent (1%) core delta and a core diameter of a few microns, resulting in the appropriate cutoff wavelength. As has just been described, a fiber having various combinations of core delta and core diameter may be used provided that the appropriate cutoff wavelength is achieved. 
     In the experiment described in  FIG. 3 , the spun core of the spun fiber is slightly elliptical. Although a cylindrical core may be used, the ellipticity, as the orientation of the ellipse rotates, enables superior coupling to and from the differing polarized modes. It is preferred that the ellipticity of the core of the spun fiber has an eccentricity of greater than five percent. The spun fiber used in the experiments further had a core center that is slightly offset from the geometric center of the fiber cladding. This offset enables the spatial mode distribution to change from the symmetric Gaussian mode to an asymmetric Laguerre-Gaussian mode. Preferably, the core center is offset from the geometric center of the fiber by at least ten percent of the core diameter. 
     A typical output from the spun fiber  34  with a linear input polarization passing through the linear polarizer  36  is shown in  FIG. 4 . The figure shows the vector state of the collimated output from the spun fiber  34  when the analyzer polarization is set at (a) 0°, (b) 45°, and (c) 90°, respectively. When the analyzer is set to 0° or 90° the intensity pattern resembles a first-order Hermite-Gaussian mode and when the analyzer is set to 45° the intensity distribution resembles a first-order Laguerre-Gaussian mode. This spun fiber  34  is effectively acting as a mode converter, transforming a zero-order mode into a first-order mode. The orientation of the incident polarization does not destroy the HAPs output from the spun fiber  34 . 
     In order to calculate the spin rates required for conversion of light among the LP 11  modes, one first needs to calculate the polarization corrections to the LP 11  mode [4], for these vector modes, i.e. the HE 21 , TM and TE modes. When the spin rate proceeds at a rate equivalent to the difference between two of these vector modes, light will rapidly and completely transfer from one of these vector modes to the other. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Dependence of spin rate of the fiber on output intensity and polarization. 
               
            
           
           
               
               
               
            
               
                 Spin Rate 
                 Output Intensity 
                 Output Polarization 
               
               
                   
               
               
                  0 turns/meter 
                 HG 10  or HG 01   
                 Linear 
               
               
                 10 turns/meter 
                 Almost vortex 
                 Slight Superposition 
               
               
                 20 turns/meter 
                 Vortex 
                 Total Superposition (linear + 
               
               
                   
                   
                 azimuthal) = HAPs 
               
               
                   
               
            
           
         
       
     
     The dependence of the spin rate on the output state of the spun fiber  34  is shown in Table 1. In general, although this invention would cover a spun fiber waveguide device that is cylindrical, it is preferred to use a slightly elliptical waveguide having a core center slightly offset from the geometric center of the device. Given this configuration, spinning the fiber during the fiber drawing process causes the asymmetries to rotate along the length of the fiber at the spin frequency. Accordingly, when light is coupled into a fiber with the same cutoff wavelength and indices of refraction in not spin, i.e. a spin rate of 0 turns per meter, no vortex is seen. Rather, a Hermite-Gaussian mode is seen that is linearly polarized. Referring to  FIG. 5 , the output of an un-spun fiber has a HG mode structure like  FIG. 5(   b ) or ( d ). Depending upon the alignment of the coupling into the fiber, the output is either a HG 10  or a HG 01i  mode.  FIGS. 5(   b - d ) show schematically what the polarization states are after passing through a linear polarizer set at set at (b) 0°, (c) 45°, and (d) 90°. 
     However, when light is coupled into a fiber with a spin rate of 10 turns per meter, the output starts to show signs of conversion to a cylindrical vector mode that has an optical vortex, and looks like a blend between a vortex and a HG mode, where the polarization is not quite linear and not quite the hybrid-azimuthal polarization. 
     When light is coupled into a fiber with a spin rate of 20 turns per meter shows complete conversion to the HE mode, as depicted by the line schematic in  FIG. 5(   a ) and by the polarizer output depiction of  FIG. 4(   a ). As shown, the output of the spun fiber is an optical vortex that is in a vector superposition state composed of linear and circular polarization states. Hence the HAP state is shown to have been created. Accordingly, 
     The cylindrical vector beam shown and described in  FIGS. 4 and 5  is the experimental observation of the hybrid-azimuthal polarization with circular polarization states every 90° around the beam profile. Referring now to  FIG. 6 , the hybrid-azimuthal beam can be transformed into a pure azimuthal polarization state by inserting a quarter-wave plate oriented at 0° or 90°. This will transform the regions of circular polarization into linear polarization states oriented at 45° or 135° depending on the handedness of the circular polarization and not affect the linear polarization states. This beam can be converted further to a pure radially polarized mode by inserting a half-wave plate oriented at 45° in the path of the beam, as shown in  FIG. 6 . 
     Further, it was found experimentally that fiber spinning during the drawing process can cause light to couple to the hybrid azimuthal polarization mode rather than the azimuthal polarization mode directly. This results because the hybrid is a combination of the HE 21  mode and the azimuthal mode, making it more likely that both modes be occupied rather than one mode alone. 
     What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every combination of components or methodologies for purposed of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.