Abstract:
A wave-guiding system and method for mode-selective transmission of a spatially coherent light beam is disclosed. The system comprises a wave-guide with an inlet and an outlet for transmitting one or more modes; a first mode-shaping optical element for shaping the spatially coherent light beam before it enters through the inlet of the wave-guide to a desired mode of the wave-guide; and a second mode-shaping optical element for shaping light beam after it exits from the outlet of the wave-guide for obtaining a desired vectorial field.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase Application of PCT International Application No. PCT/IL2007/001063, International Filing Date Aug. 28, 2007, claiming priority of U.S. Patent Application 60/840,949, filed Aug. 30, 2006, both of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to optical waveguides. More particularly the present invention relates to apparatus and method for excitation of a single mode in a waveguide. 
     BACKGROUND OF THE INVENTION 
     Hollow waveguides present an alternative to solid core fibers at the infra-red (IR) regime where suitable optical materials are scarce. In addition, due to their air core, they can be used for broad-spectrum high power transmission as they suffer from small insertion losses. As a result, hollow waveguides are used in industrial and medical applications involving CO 2  and Er:YAG lasers as well as for spectroscopic and radiometric measurements. In 1897, Lord Rayleigh was the first to consider using hollow metallic waveguides for the propagation of electromagnetic radiation. However, he considered the metal to be a perfect conductor, thus his solution is inadequate in optical regimes where metal behaves more like a lossy dielectric. In 1961, Snitzer presented a general treatment for the propagation of electromagnetic fields inside cylindrical waveguides of arbitrary material. A convenient approximation for circular metallic waveguides was later presented by Marcatili and Schmeltzer in 1964. In 1984, Kawakami and Miyagi proposed an improved design in which an additional inner dielectric multilayer stack is used to reduce transmission losses. Recently, a new design for a circular hollow Bragg waveguide, which is a type of photonic band gap fiber, has been presented [S. Johnson, M. Ibanescu, M. Skorobogatiy, O. Weisberg, T. Engeness, M. Soljacic, S. Jacobs, J. Joannopoulos, and Y. Fink, “Low-loss asymptotically single-mode propagation in large-core OmniGuide fibers,” Opt. Express 9, 748-779 (2001), and B. Temelkuran, S. D. Hart, G. Benoit, J. D. Joannopoulos, Y. Fink, “Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO 2  laser transmission”, Nature 420, 650 (2002)]. In this case, guiding is obtained by reflection from a periodic dielectric layers rather than by metal cladding. Therefore, the waveguide performance is no longer limited by the metallic losses. 
     A hollow waveguide supports vectorial modes in a manner determined by its structure and material composition. It is customary to classify the modes: transverse electric-TE 0m , transverse magnetic-TM 0m , and hybrid-HE nm , and EH nm -modes. The integers n, m&gt;0 denote the azimuthal and radial mode orders. The commonly used modes of circular hollow waveguides are the TE 01  and HE 11 ; The TE 01  is an azimuthal linearly polarized vectorial vortex having a dark central core. This mode possesses the least amount of loss in a bare circular metallic hollow waveguide as well as in the circular hollow Bragg waveguide; The HE 11  mode is linearly polarized and has a bright central core. It is the lowest order mode in terms of waveguide cutoff. It is important to note that a general hollow waveguide mode has a spatially varying polarization state, with the exception of the linearly polarized HE 1m  set of modes. 
     While current applications of hollow waveguides, such as power delivery, rely on multimode operation, future applications might benefit from the ability to excite only a single waveguide mode. Such applications include hollow waveguide lasers and single TE 01  mode circular Bragg waveguides. We expect that additional applications of hollow waveguides might emerge once higher order modes are exploited. For example, the mode&#39;s dark core increment with azimuthal mode order n might prove useful for blue detuned atom guiding. Also higher order modes can be used for dispersion compensation. 
     Coupling a single hollow waveguide mode requires matching the phase, amplitude, and polarization state. Phase and amplitude matching methods are well developed and can be achieved by conventional optical devices, irregular waveguides, and diffractive or holographic optics. However, matching the polarization state is more challenging. Several techniques for this purpose exist such as liquid crystal spatial light modulators, interferometric techniques, and lasers with intra-cavity optical devices. However, all these methods are either cumbersome, have low power thresholds or inadequate in the IR regime. 
     Recently, we have demonstrated spatial polarization state manipulation by space-variant subwavelength gratings [E. Hasman, G. Biener, A. Niv, and V. Kleiner, “Space-variant polarization manipulation,” in Progress in Optics, vol. 47, E. Wolf ed. (Elsevier, Netherlands, Amsterdam, 2005)]. These devices act as waveplates with space-variant orientations and as inhomogeneous anisotropic subwavelength structures, they are particularly well suited for polarization manipulation. As the optical properties of these devices stem from the geometric Pancharatnam-Berry phase, they are called Pancharatnam-Berry phase optical elements (PBOEs). PBOEs are both compact and efficient optical devices. They were used for the formation of propagation invariant vectorial Bessel beams [A. Niv, G. Biener, V. Kleiner, and E. Hasman, “Propagation-invariant vectorial Bessel beams obtained by use of quantized Pancharatnam-Berry phase optical elements,” Opt. Lett. 29, 238-240], rotating vectorial vortices [A. Niv, G. Biener, V. Kleiner, and E. Hasman, “Rotating vectorial vortices produced by space-variant subwavelength gratings,” Opt. Lett. 30, 2933-2935 (2005)], and for the excitation of a vectorial hollow waveguide mode in the 1.55 μm wavelength regime [W. S. Mohammed, A. Mehta, M. Pitchumani, and E. G. Johnson, “Selective excitation of the TE 01  mode in hollow-glass waveguide using a subwavelength grating”, Photon. Technol. Lett. 17, 1441 (2005)]. Furthermore, we have presented the use of PBOEs for the coupling and inverse coupling of free-space linearly polarized beams to a hollow waveguide&#39;s azimuthally polarized vectorial TE 02  mode [Y. Yirmiyahu, A. Niv, G. Biener, V. Kleiner, and E. Hasman, “Vectorial vortex mode transformation for a hollow waveguide using Pancharatnam-Berry phase optical elements,” Opt. Lett. 31, 3252-3254 (2006)]. 
     It is a purpose of the present invention to provide a general approach for coupling free space beams to any of the hollow waveguide modes, thus enabling single mode operation. 
     SUMMARY OF THE INVENTION 
     There is thus provided, in accordance with some preferred embodiments of the present invention, a wave-guiding system for mode-selective transmission of a spatially coherent light beam, the system comprising: 
     a wave-guide with an inlet and an outlet for transmitting one or more modes; 
     a first mode-shaping optical element for shaping the spatially coherent light beam before it enters through the inlet of the wave-guide to a desired mode of the wave-guide; and 
     a second mode-shaping optical element for shaping light beam after it exits from the outlet of the wave-guide for obtaining a desired vectorial field. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the wave-guide comprises an optical fiber. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the optical fiber comprises a hollow fiber. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the optical fiber comprises a photonic band gap fiber. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the photonic band gap fiber comprises a circular hollow Bragg wave-guide. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the first mode-shaping optical element comprises a PBOE. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the second mode-shaping optical element comprises a PBOE. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the first and second mode-shaping optical elements comprise each a PBOE. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the system further comprises additional optical elements placed on an optical path of the system. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the additional optical elements comprise lenses. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the system further comprises a spatially coherent light source for generating the spatially coherent light beam. 
     Furthermore, in accordance with some preferred embodiments of the present invention, first mode-shaping optical element comprises an optical element that causes the light beam to be a light beam with azimuthal polarization state. 
     Furthermore, in accordance with some preferred embodiments of the present invention, there is provided a method for mode-selective transmission of a spatially coherent light beam, the method comprising: 
     mode-shaping the spatially coherent light beam to a desired mode of a wave-guide; 
     passing the light-beam with the desired mode through the wave-guide; and 
     mode-shaping the light beam after it exits from the wave-guide for obtaining a desired vectorial field. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the step of mode-shaping the spatially coherent light beam to a desired mode of a wave-guide comprises passing the light beam through a PBOE. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the step of mode-shaping the light beam after it exits from the wave-guide comprises passing the light beam through a PBOE. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the step of mode-shaping the spatially coherent light beam to a desired mode of a wave-guide comprises passing the light beam through a PBOE, and the step of mode-shaping the light beam after it exits from the wave-guide comprises passing the light beam through a PBOE. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the step of mode-shaping the spatially coherent light beam to a desired mode of a wave-guide comprises polarizing the beam to an azimuthal polarization state. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the step of mode-shaping the light beam after it exits from the wave-guide comprises polarizing the beam to a linear polarization state. 
     Furthermore, in accordance with some preferred embodiments of the present invention, the step of mode-shaping the light beam after it exits from the wave-guide comprises polarizing the beam to a radial polarization state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand the present invention, and appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals. 
         FIG. 1  shows a schematic presentation of our concept. A laser beam is coupled to a single waveguide mode by the subwavelength grating PBOE1 and the lens L1. Afterwards, the emerging beam is collimated and transformed into a free space mode using the lens L2 and PBOE2. The lens L3 is used for focusing the beam onto a two dimensional detector array. 
         FIG. 2  shows scanning electron microscope images of several PBOEs used for coupling a linearly polarized 10.6 μm wavelength beam to hollow waveguide modes of different azimuthal order n. 
         FIG. 3  shows measured results for exciting the TE 01  mode. 
         FIG. 4  shows measured results for exciting the  TM01  mode. 
         FIG. 5  shows the measured intensity and polarization at the fiber inlet and outlet for the EH 11 , EH 21 , and EH 31  modes. 
         FIG. 6  shows measured intensity and polarization for the inverse coupling and transformation of the TE 01  mode to: ( a ) a linearly polarized beam by use of PBOE with n=0, ( b ) a radially polarized beam by use of PBOE with n=1, ( c ) Azimuthally polarized beam obtained without a second PBOE. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hollow waveguides possess a discrete set of propagating modes labeled by their radial and angular indexes m, n. The general structure of these modes is well known. The mode&#39;s field distribution as well as propagation constant, loss, and dispersion can be found by solving a transcendental characteristic equation. We consider a waveguide with a core diameter much larger than the wavelength used, so that only the transverse electric part of the mode is treated. For the fundamental azimuthal dependence of n=0, this waveguide&#39;s modes split into separate sets: the transverse electric (TE) and transverse magnetic (TM). The TE modes are azimuthally polarized according to,
 
| E   φ,0m               =J   1 ( u   0m   r/a )|φ         ,  (1)

     while the TM 0m  modes are radial,
 
| E   r,0m               =−J   1 ( u   0m   r/a )| r             .   (2)

     In the case n≠0, this type of waveguide supports EH nm  and HE nm  set of modes with a transverse electric field given by,
 
| E   nm               =J   n∓1 ( u   nm   r/a )[±cos( nφ+φ   0 )| r           −sin( nφ+φ   0 )|φ         ],  (3)

     where upper or lower signs indicate either the HE nm  or EH nm  set of modes, respectively. Here, |r           =[exp(iφ)|R         +exp(−iφ)|L         ]/√{square root over (2)} and |φ         =i[exp(iφ)|R         −exp(−iφ)|L         ]/√{square root over (2)} stands for radial and azimuthal polarizations with |R         =(1,−i)/√{square root over (2)} and |L         =(1,i) T /√{square root over (2)} as right- and left-handed circular polarizations. Additionally, r and φ are the radius and azimuth polar coordinates in the transverse plane of the waveguide, a is the waveguide&#39;s air core radius, u nm /a is a complex transverse phase constant, and J l (x) is the first-kind Bessel function of order l.
     When the period of a dielectric grating is sufficiently smaller than the wavelength of the impinging beam, the grating behaves as a uniaxial crystal with optical axes parallel and perpendicular to the grating strips. Therefore, by controlling □ the local orientation of the dielectric subwavelength grating waveplates with a space-varying fast axis are achieved. This enables formation of beams with almost arbitrary space-variant polarization states. We have previously shown that for linearly polarized illumination, the beam that emerges from a π retardation PBOE is given by,
 
| E   out,n             =cos(2θ−φ)| r           +sin(2θ−φ)|φ         .  (4)

     Comparing this result to the waveguide&#39;s modes in Eqs. (1), (2), and (3) we find that by setting the PBOE local groove orientation to be, 
     
       
         
           
             
               
                 
                   
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                     = 
                     
                       
                         
                           
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                           φ 
                           0 
                         
                         2 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   5 
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     The polarization state of the waveguides modes is exactly reproduced. The upper ‘−’ and lower ‘+’ signs correspond to the HE nm  and EH nm  set of modes, respectively. It is important to note that in order to avoid limitations imposed by the continuous groove orientation, it is approximated by a piecewise continuous orientation composed of constant orientated zones. We have shown that when 16 discrete zones are used, diffraction is negligible (more than 98% to the first diffraction order). 
     After having achieved the desired polarization state, in order to obtain an efficient coupling as well, the magnitude and phase distribution of the incident beam should also match those of the waveguide mode. We show that this can be achieved, to a satisfactory degree, by using a simple focusing lens. Immediately after the PBOE, the beam |E out,n              has uniform intensity and the desired polarization state. By placing the PBOE at the front focal plane of a lens, we obtain at the back focal plane a field of the form,
 
| E   f,n             =A   n ( r )| E   out,n           ,  (6)

     where, 
     
       
         
           
             
               
                 
                   
                     
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     Here, λ is the wavelength, R 0  is the aperture radius, f is the lens focal length, and the ‘−’ and ‘+’ signs correspond to TE/HE and TM/EH sets of modes, respectively. The field in Eqs. (6)-(7) represents concentric intensity rings that bear a close resemblance to the hollow waveguide modes. In order to evaluate the coupling efficiencies between this field and the waveguide mode we define a coupling coefficient according to, 
     
       
         
           
             
               
                 
                   
                     η 
                     
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     For example, for the TE 0m  modes we obtain coupling efficiencies close to 1, 0.55, and 0.48 for πa/(λf # )=5.4, 9,12, which corresponds to a radial mode order of m=1, 2, and 3, respectively. We also calculated coupling efficiencies close to unity (about 0.99) with πa/(λf # )=3.7, 7, 8.4, 9.7 for the HE 11 , EH 11 , EH 21 , and EH 31  modes, respectively. Therefore, it is possible to achieve high coupling coefficients for modes of radial order m=1 by choosing an appropriate focal length f. 
     In order to test our approach, we used the apparatus schematically depicted in  FIG. 1 . A 10.6 μm wavelength beam from a CO 2  laser source  10  illuminated PBOEs  12 ,  14  whose azimuthal order ranged from n=0 to 3. The PBOEs, 10 mm in diameter, were realized upon GaAs wafers and had a subwavelength period of Λ=2 μm, a fill factor of 0.5, and a nominal etching depth of 5 μm so as to achieve the desired π retardation. The desired groove orientation, given by Eq. (5), was approximated by 16 discrete zones for which we calculated diffraction efficiency greater than 98%.  FIG. 2  shows scanning electron microscope images of several PBOEs used for coupling a linearly polarized 10.6 μm wavelength beam to hollow waveguide modes of different azimuthal order n. The discrete changes in the groove orientation as well as the high aspect ratio and rectangular shape of the grooves are clearly observed. The beam emerging from PBOE 1   12 , now having the correct polarization distribution, was focused onto the waveguide  18  inlet using lens L1  16 . The lens aperture and the focal length were adjusted to obtain optimal coupling according to Eq. (8) and  FIG. 3 . We used a 61 cm commercially available Ag/AgI hollow silica waveguide with a 300 μm inner bore diameter (Polymicro HWCA300750). Finally, lenses L2  20  and L3  22  together with PBOE2  14  were used to modify the waveguide mode to either a diffraction limited focus spot, or a radially polarized vectorial vortex. We demonstrate our approach by measuring the full polarization state at the waveguide entrance and outlet ports. The results for excitation of different waveguide modes are shown in  FIGS. 3-5  using the following template: The upper and lower rows relate to the waveguide&#39;s input and outlet ports, respectively. The first and second columns show false color images of the initial intensity and the intensity after a polarizer, respectively. The dashed line indicates the waveguides circumference while yellow arrows show the polarizer&#39;s orientation. The third column shows the measured local polarization ellipse&#39;s orientation. Finally, the fourth column shows a typical measured (dots) and predicted (solid line, from Eq. (7)) intensity cross-section. 
     Measured results for exciting the TE 01  mode are shown in  FIG. 3 . The power at the waveguide inlet was measured to be 170 mW. The doughnut-shaped intensity results from the vectorial vortex induced by the PBOE. The propeller-like intensity fringes after the polarizer indicate the existence of an azimuthally polarized vectorial vortex at the waveguide entrance. Measured intensity and polarization for coupling to the TE 01  mode at the waveguide&#39;s inlet (top) and outlet (bottom) ports. The first column depicts false color intensity representation with dashed line indicating the waveguide&#39;s inner circumference. The second column depicts false color representation of the intensity after a linear polarizer whose orientation is given by the yellow arrows. The third column shows the measured polarization ellipse&#39;s orientation, with bar length indicating intensity. The fourth column shows the measured (dots) and predicted (solid lines) intensity cross sections. 
     This result is supported by the polarization ellipse orientation whose typical deviation from the desired orientation, as calculated by Eq. (1), was 0.12 radians with a similar value for the mean ellipticity. Therefore, we obtained the required linearly polarized azimuthally oriented vectorial vortex at the waveguide entrance. In addition, the calculated and measured cross-sections indicate that the correct intensity profile was also achieved. At the waveguide&#39;s outlet, the single intensity ring and the evident propeller-like intensity behind the polarizer demonstrate a close resemblance to the TE 01  mode. The deviation of the polarization orientation from its desired value was 0.2 radians, with the ellipticity deviation less than 0.3 radians. Therefore, the field at the outlet port has the correct linear azimuthal n=0 polarization structure. Turning to the cross-section plot, a good agreement is found between the predicted and measured values, thereby verifying the required intensity profile of radial index m=1, and thus the existence of a single TE 01  mode. Comparing the measured equivalent losses (insertion+propagation) of 4.7 db/m at the waveguide outlet to the calculated 4.13 db/m TE 01  mode losses indicates the low insertion losses of our apparatus. Therefore, an efficient single mode excitation is demonstrated. 
       FIG. 4  demonstrates coupling of the TM 01  mode in much the same way as in  FIG. 3 . As can be seen from the measurements at the waveguide inlet, this mode differs from the TE 01  only by the local polarization orientation, namely radial instead of azimuthal. Consequently, it is coupled using an identical PBOE of order n=0, while either the PBOE or the incoming beam is rotated by a right angle. The intensity and polarization measurements performed at the waveguide&#39;s inlet and outlet indicate successful coupling of this mode. Measured intensity and polarization for coupling to the TM 01  mode at the waveguide&#39;s inlet (top) and outlet (bottom) ports. The first to fourth columns depict the intensity (dashed line indicates the waveguide inner circumference), intensity after a polarizer (arrows indicate polarizer orientation), measured polarization orientation, and intensity cross-section (dots-measured, solid line-predicted), respectively. 
       FIG. 5  shows the measured intensity and polarization at the fiber inlet and outlet for the EH 11 , EH 21 , and EH 31  modes. Good agreement of the results with the desired intensity distribution, polarization orientation, and cross-section indicates the ability to obtain a single high order excitation. The distortion of the high order modes at the waveguide inlet results from greater variation in the required polarization distribution. Further distortion at the waveguide outlet is attributed to the higher propagation losses of these modes (after Ref. 5: 10.25 db/m, 15.82 db/m, and 22.38 db/m for the EH 11 , EH 21 , and EH 31  modes, respectively) thus reducing the signal-to-noise ratio in these cases. Measured intensity and polarization for coupling the high order modes at the waveguide&#39;s inlet and outlet ports. The first to forth columns depict the intensity (dashed line indicates the waveguide&#39;s inner circumference), intensity after a polarizer (arrows indicate polarizer orientation), measured polarization orientation, and intensity cross-section (dots-measured, solid line-predicted), respectively. 
     For many applications, manipulation of the fiber mode at the waveguide outlet is desired. An example is the transformation of a waveguide mode to a uniformly polarized free space beam so as to achieve high focusability, or to a radially polarized vectorial vortex for efficient metal cutting. The required polarization manipulation can be obtained by inverting our coupling mechanism, as depicted in  FIG. 1 . In this case, the 1″ focal length lenses L2 and L3, serve as a 4-f system with PBOE2 placed in an intermediate focal plane. The PBOE modifies the polarization of the hollow waveguide mode to any desired polarization. 
     Transforming the azimuthal polarization of the TE 01  mode to a linear polarization can be obtained by a PBOE with n=0. This arises from a general property of π-retardation PBOEs, namely that T 2 =−I where T is the PBOE transmission matrix and I is the unit matrix.  FIG. 6(   a ) shows, in the usual manner, the measured results at the focus of the L3 lens. The absence of a distinctive doughnut intensity pattern results from the polarization manipulations of PBOE 2 . The focusability factor of the intensity spot was measured to be M 2 =1.7. This is a considerable improvement over the M 2 =2.2 of the TE 01  mode. Furthermore, by removing the intensity side lobes, seen in the intensity cross-section, we can achieve a focusability of M 2 =1.1 with a mere 11% intensity loss. Thus, the ability to obtain a diffraction-limited spot by using PBOEs is demonstrated. In  FIG. 6  measured intensity and polarization for the inverse coupling and transformation of the TE 01  mode to: ( a ) a linearly polarized beam by use of PBOE with n=0 (red line shows the focus of a Gaussian beam having a similar width.), ( b ) a radially polarized beam by use of PBOE with n=1. ( c ) Azimuthally polarized beam obtained without a second PBOE. The first to fourth columns depicts the intensity, intensity after a polarizer (arrows indicate polarizer orientation), measured polarization orientation, and intensity cross-section (dots-measured, solid line-predicted), respectively. 
     For comparison,  FIG. 6(   b ) shows the intensity without a PBOE inserted in the intermediate focal plane of the L2 and L3 lenses. Note the larger focal spot of the azimuthal linearly polarized vectorial vortex with respect to the linearly polarized focus of  FIG. 6(   a ).  FIG. 6(   c ) demonstrates transformation of the TE 01  waveguide mode to a radially polarized vectorial vortex by using a PBOE with n=1 as PBOE2. The results indicate that the desired radially polarized vectorial vortex was achieved. We therefore demonstrate the ability of a PBOE (together with lenses) to efficiently couple a waveguide mode to a desired free-space mode. 
     We have proposed PBOEs as an efficient means of obtaining the space-variant polarization state of a waveguide mode, thereby enabling its efficient coupling to high order waveguide modes. We have experimentally demonstrated the coupling of linearly polarized light at a wavelength of 10.6 μm to single vectorial modes of various orders within a hollow metallic waveguide by use of PBOEs. In addition, we demonstrated inverse transformation of the fundamental low order TE 01  mode to linearly and radially polarized free space modes. In a similar manner, any waveguide vectorial mode can be excited by a properly designed PBOE. 
     Throughout the present specification the term “waveguide”, although the above account referred to hollow waveguides, includes (without limiting the scope of the invention), for example, optical fiber, single-mode or multi-mode, filled core (filled=material other than air, vacuum), hollow fiber, photonic band gap fiber. 
     Optical elements suitable for a system according to the poresent invention are typically PBOE but not limited to these elements. Generally, the first optical element comprises a mode-shaping optical element aimed at shaping the incoming spatially coherent light beam to a desired mode of the fiber, whereas the second optical element comprises a mode-shaping optical element for shaping the exiting light beam to desired vectorial field. In fact we refer to inhomogeneous anisotripic polarization optical elements of which PBOEs are inhomogeneous anisotripic subwavelength optical elements. 
     All measures, values and parameters hereinabove are given as examples for explanatory purposes and in no way limit the scope of the present invention. 
     It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope. 
     It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the present invention.