Abstract:
Pointing and positioning system of light beams and images comprising a plurality of cycloidal diffractive waveplates, each waveplate capable of deviating a generally broadband light beam over a predetermined angle. The lateral translation and deviation angles of the light beams are controlled by controlling the relative distance, rotational position, and the diffraction efficiency of at least one in the plurality of said waveplates.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under Contract No. W911QY-07-C-0032. 
       RIGHTS OF THE GOVERNMENT 
       [0002]    The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
     
    
     CLAIM OF PRIORITY 
       [0003]    This invention claims priority to the publication S. R. Nersisyan, N. V. Tabiryan, D. M. Steeves, B. R. Kimball, “Optical Axis Gratings in Liquid Crystals and their use for Polarization insensitive optical switching,” J. Nonlinear Opt. Phys. &amp; Mat., 18, 1-47 (2009), incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0004]    This invention relates to optical beam control and, in particular, to methods, systems, apparatus and devices for manipulating with light beams, including laser beams and beams with wide spectra and divergence angles, by translating them in the lateral direction and varying their propagation direction for optical switching, beam scanning, spectral modulation, optical tweezers, thermal seeker, imaging, information displays, and other photonics applications. 
       BACKGROUND OF THE INVENTION 
       [0005]    The present invention relates to optical systems for controlling with propagation of light beams. Pointing and positioning systems are enabling components for most laser applications. Conventionally, this is accomplished using mirrors, scan wheels, optical wedges, and other two-axis gimbal arrangements as exemplified, for example, in the U.S. Pat. No. 7,319,566 to Prince et al. These opto-mechanical systems are complex, bulky and heavy for large area beams. For example, the prism apex angle, hence its thickness is increased to achieve larger deflection angles. The electromechanical systems for rotation, translation or oscillation of such mirrors, prisms, and other optical components require high electrical power for their operation. They are relatively slow and have limited range of angles that could be covered within given time period. 
         [0006]    Thus, there is a need for thin, light-weight, fast, and inexpensive pointing, positioning, and switching systems for light beams, particularly, for laser beams. The state-of-the-art developments include all-electronics systems and rotating diffraction gratings. The all-electronics systems with no moving parts, as reviewed in P. F. McManamon, P. J. Bos, M. J. Escuti, J. Heikenfeld, S. Serati, H. Xie, E. A. Watson, A Review of Phased Array Steering for Narrow-Band Electrooptical Systems, Proceedings of the IEEE, Vol. 97, pages 1078-1096 (2009), require a large number of high efficiency diffraction gratings and spatial light modulators and/or electrically controlled waveplates. As a result, the overall transmission of these systems is reduced along with their radiation damage threshold, and their speed is limited by the speed of liquid crystal spatial light modulators and variable retarders. 
         [0007]    Rotating diffraction gratings as described in J. C. Wyant, “Rotating diffraction grating laser beam scanner,” Applied Optics, 14, pages 1057-1058 (1975), and in the U.S. Pat. No. 3,721,486 to Bramley, partially solves the problem of obtaining larger diffraction angle in thinner optical system, compared, for example to the system of Risley prisms. The light beam diffracted by the first grating in the path of the beam is further diffracted by the second grating. Depending on orientation of those gratings with respect to each other, the deflection angle of the beam can thus be varied between nearly 0 to double of the diffraction angle exhibited by a single grating. The problem with such systems is that phase gratings typically diffract light into multiple orders that need to be blocked along with the 0 th  order beam. High efficiency Bragg type gratings have narrow spectral and angular range as described in the U.S. Pat. No. 7,324,286 to Glebov et al., and can be used practically for laser beams only, expanded and collimated to minimize divergence. Blazed gratings such as proposed in the U.S. Pat. No. 6,792,028 to Cook et al., still exhibit a multitude of diffraction orders due to their discontinuous structure and do not improve considerably on the width of angular selectivity and diffraction efficiency. 
         [0008]    The cycloidal diffractive waveplates (CDWs), essentially, anisotropic plates meeting half-wave condition but with optical axis orientation rotating in the plane of the waveplate in a cycloidal manner, as described in the review S. R. Nersisyan, N. V. Tabiryan, D. M. Steeves, B. R. Kimball, “Optical Axis Gratings in Liquid Crystals and their use for Polarization insensitive optical switching,” J. Nonlinear Opt. Phys. &amp; Mat., 18, 1-47 (2009), do not have the disadvantages of conventional phase gratings. Moreover, DWs, referred to also as optical axis gratings and polarization gratings, can provide nearly 100% diffraction efficiency in micrometer thin layers. Furthermore, due to their waveplate nature, their diffraction spectrum is broadband, and can even be made practically achromatic. Due to their thinness and high transparency, they can be used in high power laser systems. 
         [0009]    Thus, replacing Risley prisms, wedges, mirrors and/or phase gratings with DWs, provides many advantages for manipulating with light beams and imaging. As shown in S. R. Nersisyan, N. V. Tabiryan, L. Hoke, D. M. Steeves, B. Kimball, Polarization insensitive imaging through polarization gratings, Optics Express, 17, 1817-1830 (2009), not only laser beams, but complex images can be steered over large angles without light attenuation or image deformation. The paper further showed that utilizing a pair of closely spaced CDWs, one of them with switchable diffraction, it is possible to manipulate with transmission of unpolarized beams and images through apertures. This concept suggested and demonstrated in S. R. Nersisyan, N. V. Tabiryan, L. Hoke, D. M. Steeves, B. Kimball, “Polarization insensitive imaging through polarization gratings,” Optics Express, 17, 1817-1830 (2009) was subsequently cited and tested in C. Oh, J. Kim, J. F. Muth, M. Escuti, “A new beam steering concept: Riesley gratings,” Proc. SPIE, vol. 7466, pp. 74660J1-18 (2009). 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Thus, the objective of the present invention is providing an optical system for translating light beams over predetermined distances and deviating over predetermined angles using a set of CDWs, the system generally being capable of controlling light beams of arbitrary polarization, wide wavelength spectrum and divergence angles, including images. 
         [0011]    The second objective of the present invention is providing means for controlling the optical properties of said set of CDWs mechanically, by varying the relative distance and angular positions between the CDWs, as well as by using other stimuli such as electromagnetic fields and temperature that vary the diffraction efficiency of at least one CDW in the set. 
         [0012]    A further objective of the present invention is providing an optical system wherein the light translated or deflected by the set of CDWs is further controlled with the aid of apertures, filters, and other optical elements. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]      FIG. 1A  schematically shows deflection of a circularly polarized light beam with a pair of diffractive waveplates. 
           [0014]      FIG. 1B  describes the pattern of spatial modulation of optical axis orientation of a cycloidal diffractive waveplate along a single coordinate axis. 
           [0015]      FIG. 1C  schematically shows the structure of cycloidal diffractive waveplates at different rotational positions. 
           [0016]      FIG. 1D  schematically shows the structure of cycloidal diffractive waveplates at anti-parallel rotational positions. 
           [0017]      FIG. 2A  shows sample dependence of the deflection angle of a light beam at the output of a pair of cycloidal diffractive waveplates as a function of the rotational angle between the waveplates. 
           [0018]      FIG. 2B  demonstrates the capability of a pair of cycloidal diffractive waveplates to steer with no distortions complex images carried by an unpolarized light. 
           [0019]      FIG. 3  schematically shows the displacement of a light beam by a pair of diffractive waveplates with parallel orientation of their optical axis modulation directions. 
           [0020]      FIG. 4A  schematically shows increasing of the deflection angle of a light beam by a set of four diffractive waveplates each arranged anti-parallel with respect to the previous one. 
           [0021]      FIG. 4B  demonstrates increasing deflection angle of a light beam by increasing the number of diffractive waveplates from one to four. 
           [0022]      FIG. 4C  shows increasing deflection angle of a light beam by a system of diffractive waveplates tilted with respect to each other. 
           [0023]      FIG. 5A  schematically shows a fragment of a cycloidal orientation pattern for molecules of an azobenzene liquid crystal in trans-isomer state. 
           [0024]      FIG. 5B  schematically shows transformation of the trans isomer of azobenzene liquid crystal molecules into cis isomer state due to absorption of radiation, and respective loss of cycloidal pattern. 
           [0025]      FIG. 5C  schematically shows randomization of liquid crystal orientation due to temperature induced phase transition to isotropic liquid state, nd related loss of cycloidal orientation pattern. 
           [0026]      FIG. 6A  schematically shows orientation of liquid crystal molecules between electrodes. 
           [0027]      FIG. 6B  schematically shows transformation of cycloidal orientation pattern into a homogeneous alignment pattern at the influence of an electric field. 
           [0028]      FIGS. 7A  and B schematically show switching between transmittive and deflective states of a pair of cycloidal diffractive waveplates when switching one of the diffractive waveplates into an optically homogeneous non-diffractive state. 
           [0029]      FIG. 8A  schematically shows two layers of cycloidal diffractive waveplates bonded with each other on a single substrate. 
           [0030]      FIG. 8B  schematically shows two layers of cycloidal diffractive waveplates bonded with each other with a liquid crystal cell acting as a spacer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not limitation. 
         [0032]    The preferred embodiment of the present invention includes two CDWs, marked with numerals  103  and  105  in  FIG. 1A , arranged parallel to each other in close proximity such as the light diffracted by the first CDW  103  is fully captured by the aperture of the second CDW  105 . At the output of the set of CDWs  103  and  105 , the pointing direction of the light beam  108  circularly polarized as indicated by spiral  107  is, in general, different from the propagation direction of the incident light beam  101  circularly polarized as indicated by spiral  102 . The deflection angle of the beam is controlled by mechanical rotation of the CDWs schematically shown by arrows  104  and  106 . 
         [0033]    It is convenient to depict the spatially modulated orientation direction of the optical axis in a CDW by elongated ellipses  111  as shown in  FIG. 1B . The optical axis orientation angle α in a CDW varies along a single coordinate axis x′: α=qx′. The modulation period Λ defined by the wavevector q, Λ=π/q, determines the magnitude of the diffraction angle of the CDWs. The orientation angles β and γ of the x′-axes of the CDWs with respect to a fixed x-axis in the laboratory coordinate system,  FIG. 1C , determine the diffraction direction. The minimum deflection angle is 0 and it is achieved for parallel arrangement of the CDWs wherein α=qx′ for both CDWs. This corresponds to the case where β=γ in  FIG. 1C . The largest deflection angle is double of the diffraction angle produced by individual CDWs, and it is achieved for their anti-parallel arrangement, schematically shown in  FIG. 1D , wherein γ=β+π. 
         [0034]    The plot of output angles measured for a sample system as a function of angular position between the CDWs in S. R. Nersisyan, N. V. Tabiryan, L. Hoke, D. M. Steeves, B. Kimball, “Polarization insensitive imaging through polarization gratings,” Optics Express, 17 (3), 1817-1830 (2009) is shown in  FIG. 2A  for normal incidence of the beam on the first CDW. In the setup shown in  FIG. 1A , the polarization of the incident beam is assumed circular, as schematically shown by the spiral  102 . The output beam  108  in this case maintains the circular polarization state  107 . In case of incident unpolarized or linearly polarized beam, two beams of orthogonal circular polarization are generated at the output of the system of two CDWs,  FIG. 2B . The angle between those beams changes from 0 to four times the diffraction angle when the relative rotational position between the CDWs is varied from parallel to anti-parallel. This situation, along with the photos of the two diffracted beams corresponding to some intermediate relative rotational positions of the CDWs is demonstrated in  FIG. 2B  for beams carrying a complex image. No image distortions occurs in this process. 
         [0035]    In the preferred embodiment, CDWs are made of liquid crystal polymers though other optically anisotropic materials and material structures such as subwavelength gratings can be used as well. In general, the layer of CDW, typically only a few micrometer thick, is obtained as a coating on a substrate for stability and robustness. The substrate can be made of a material adequate for the particular application. As an example, a fused silica can be used when controlling UV light beams, and ZnSe, BaF 2  and silicon can be used for controlling laser beams of infrared wavelengths. 
         [0036]    Varying the distance Δz between two parallel arranged identical CDWs  302  and  304 ,  FIG. 3 , introduces transverse shift Δx of the beam  305  emerging from the system with respect to the position of the input beam  301  as a result of deflection of the beam  301  by the first CDW  302  into the beam  303  before it is further diffracted by the CDW  304 . Said emerging beam  305  propagates parallel to the input beam  301 . The beam can be translated over larger distances or steered over larger angles by adding CDWs into the set. Four CDWs,  406 - 409 , are shown in  FIG. 4A  as an example. The input light  401  undergoes four deflections,  402 - 405 . In order for each subsequent deflection to further increase the resultant deflection angle, the CDWs  407  and  409  have to be arranged anti-parallel to CDWs  406  and  408 . A demonstration of light deflection by such a system of four CDWs is shown in  FIG. 4B . CDWs can be tilted with respect to each other such as each of the subsequent CDWs is nearly perpendicular to the beam deflected by the previous CDW. The CDWs  407  and  409  in  FIG. 4C  are anti-parallel to the CDWs  406  and  408 , and all four deflected beams  402 - 405  of the input beam  401  subsequently increase total deflection angle. 
         [0037]    In another embodiment, spatial positioning of a light beam is controlled by incorporating in a set of CDWs one or more CDWs with variable diffraction efficiency and spectrum, particularly, switchable between diffractive and non-diffractive states at the influence of stimuli such as optical, thermal, electrical, or mechanical. For example, the variable CDW can be made of azobenzene liquid crystal that can be transformed into isotropic state due to trans-cis photoisomerization as shown in S. R. Nersisyan, N. V. Tabiryan, D. M. Steeves, B. R. Kimball, “Optical Axis Gratings in Liquid Crystals and their use for Polarization insensitive optical switching,” J. Nonlinear Opt. Phys. &amp; Mat., 18, 1-47 (2009).  FIG. 7  demonstrates the effect of photoisomerization and temperature on the cycloidal alignment pattern of a CDW structure shown in  FIG. 7A . In case of photoisomerization,  FIG. 7B , the molecules of azobenzene liquid crystal isomerize into molecular structure with no mesogenic ability. Thus, the optical anisotropy of the material is reduced with accumulation of those cis-isomers and is eventually lost at sufficiently high concentration levels. For commercially available materials such as room temperature azobenzene liquid crystal 1005 (BEAM Co.), the energy required for full transformation into the isotropic state is of the order of 0.4 J/cm 2  for a light beam of 409 nm wavelength according to the product specifications (www.beamco.com). Azobenzene liquid crystal may also be used as a dopant to randomize a host liquid crystal orientation as a result of photoisomerization. 
         [0038]    Similar process, reducing optical anisotropy till its complete disappearance may take place also when heating the liquid crystal to the isotropic state. In this case, the molecules of the liquid crystal do not isomerize, but lose the orientational order as shown in  FIG. 7C . This phase transition temperature varies for different materials. For example, it is nearly equal to 35° C. for the nematic liquid crystal 4-pentyl-4′-cyanobiphenyl widely known under the trade name 5CB. 
         [0039]    Alternatively, spatially modulated orientation pattern in a CDW in a set can be transformed into homogeneous orientation state by electrical fields. In the preferred embodiment shown in  FIG. 6 , the electrodes  611  are deposited on one of the substrates  610  of a cell with cycloidal orientation of a liquid crystal  620 . Application of an electric field  612  through the electrodes  611  aligns the liquid crystal molecules along the electric field thus transforming the diffractive structure of spatially modulated liquid crystal orientation into a homogeneous orientated non-diffractive state  630 . Sinusoidal electric field at around 1 kHz frequency can be used for realignment with the strength of the field varying from nearly 1 V to 100 V depending on material properties and electrode spacing. 
         [0040]    A preferred embodiment of a system for positioning a light beam with the aid of a variable CDW is shown in  FIG. 7  when a CDW  703  with a fixed diffractive property is paired with a variable CDW  702  in parallel arrangement. As described above, according to in S. R. Nersisyan, N. V. Tabiryan, L, Hoke, D. M. Steeves, B. Kimball, “Polarization insensitive imaging through polarization gratings,” Optics Express, 17 (3), 1817-1830 (2009), the incident light beam  700  in this case,  FIG. 7A , emerges from the set of CDWs as the beam  701  propagating along the propagation direction of the incident beam  700 . 
         [0041]    In case the CDW  602  is transformed into a non-diffractive state  606 ,  FIG. 7B , the incident light  700  is deflected by the CDW  703  into the beams  707  and  708  for circular polarized beams of different handedness. Both diffraction orders,  607  and  608 , shown in  FIG. 7B  are present for unpolarized or linearly polarized incident beam. 
         [0042]    One advantage for controlling with pointing and positioning of light beams with the aid of variable CDWs is the opportunity for having a compact system where CDWs are physically attached to each other as schematically shown in  FIG. 8A . Since each CDW layer is only a few micrometer thick, multiple layers of desired orientation and sequence can be deposited on a single support substrate. In a preferred embodiment, a variable liquid crystal polymer CDW  801  is deposited on a support substrate  810  and serves as basis for a second, not stimuli responsive CDW layer  802 . In another preferred embodiment, the two CDW layers are separated with a spacer layer that may, in general, be a functional layer by itself for performing functions such as spectral filtering and phase modulation. In the preferred embodiment shown in  FIG. 8B  the functional spacer is a liquid crystal  830  sandwiched between glass substrates  811  and  812  and acting as an electrically or optically controlled phase modulator.  FIG. 8B  shows the case of electrical control with said substrates having transparent electrodes  821  and  822 . 
         [0043]    The pointing and positioning functionality of the set of CDWs of the present invention can be extended to new applications by incorporating other optical components in the system, particularly, at its output. An optical system with variable transmission is an example of such functionality obtained, for example, by arranging an aperture  705  at the output of the set of CDWs shown in  FIG. 7 . In the preferred embodiment, the system is in high transmission state when both CDWs,  702  and  703 , are in diffractive state. The system undergoes switching onto a low transmission state,  FIG. 7B , as a result of switching the structure of one of the CDWs from diffractive state  703  onto a non-diffractive state  706 . Since the deflected beams are blocked by the aperture  705 . 
         [0044]    Although the present invention has been described above by way of a preferred embodiment, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.