Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/697,083, filed Jan. 29, 2010 and entitled “BROADBAND OPTICS FOR MANIPULATING LIGHT BEAMS AND IMAGES”. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with Government support under Contract No. W911QY-07-C-0032. 
       RIGHTS OF THE GOVERNMENT 
       [0003]    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. 
     
    
     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 over large angles 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 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 angular selectivity and efficiency. 
         [0008]    The cycloidal diffractive waveplates (DWs), 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. Y. 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 DW s, provides many advantages for manipulating with light beams and imaging. As shown in S. R. Nersisyan, N. Y. 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. That paper further showed that utilizing a pair of closely spaced DWs, one of them with switchable characteristics, it is possible to manipulate with transmission of unpolarized beams and images. This concept suggested and demonstrated in S. R. Nersisyan, N. Y. 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. P. Muth, M. Escuti, “A new beam steering concept: Riesley gratings,” Proc. SPIE, vol. 7466, pp. 74660JI-J8 (2009). 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Thus, the objective of the present invention is providing means for switching and manipulating with light beams and images in lateral and angular space using a set of DW s capable of deflecting nearly 100% of light using thin material layers for a broad band of wavelengths and divergence angles. 
         [0011]    The second objective of the present invention is incorporating in said set DWs with controlled characteristics of their optical properties for further enhancing optical manipulation capabilities of said systems. 
         [0012]    A further objective of the present invention is providing optical systems, incorporating said DW set, wherein manipulation of light and images with the DW set is transformed into transmission modulation of at the output of the optical system. 
     
    
     
       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  schematically shows the structure of diffractive waveplates at different rotational positions. 
           [0015]      FIG. 2A  shows sample dependence of the propagation angle of a light beam at the output of a pair of diffractive waveplates as a function of the rotational position between the waveplates. 
           [0016]      FIG. 2B  demonstrates the capability of a pair of diffractive waveplates to steer with no distortions complex images carried by an unpolarized light. 
           [0017]      FIG. 3A  schematically shows the displacement of a light beam by a pair of diffractive waveplates with parallel orientation of their optical axis modulation directions. 
           [0018]      FIG. 3B  schematically shows the increase in the resultant deflection angle of a light beam by a pair of diffractive waveplates with anti-parallel orientation of their optical axis modulation directions. 
           [0019]      FIG. 3C  shows the optical axis orientation pattern in diffractive waveplates with antiparallel 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, and comparing them to the original propagation direction of the beam. 
           [0022]      FIG. 5  shows increasing deflection angle of a light beam by a system of diffractive waveplates tilted with respect to each other. 
           [0023]      FIGS. 6A ,  6 B and  6 C schematically show switching between transmittive and deflective states of a pair of diffractive waveplates when switching one of the diffractive waveplates into an optically homogeneous non-diffractive state as shown in  6 C. 
           [0024]      FIG. 7  shows a schematic of a beam combining function of a pair of diffractive waveplates. 
           [0025]      FIGS. 8A ,  8 B and  8 C show a schematic of a system for controlling the spectrum of a light beam with the aid of a set of diffractive waveplates. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    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. 
         [0027]    The preferred embodiment of the present invention includes two DWs, marked with numerals  103  and  105  in  FIG. 1A , arranged parallel to each other in close proximity. At the output of the system of DWs  103  and  105 , the pointing direction of the light beam  108 , circularly polarized as shown by spirals  102  and  107 , is, in general, different from that of the propagation direction of the light beam  101  incident on the system, controlled with relative rotational positions of the DWs as schematically shown by arrows  104  and  106 . The optical axis orientation pattern corresponding to different rotational positions of said DWs is shown in  FIG. 1B  wherein the axes of elongated ellipses  109  correspond to local optical axis orientation direction. In the preferred embodiment, DWs 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 DW, typically only a few micrometer thick, is coated on a substrate  110  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 highly transparent glass materials with low absorption can be used for controlling high power laser beams. 
         [0028]    The plot of output angles measured for a sample system as a function of angular position between the DWs in S. R. Nersisyan, N. Y. 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 DW. 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 DWs, and the angle between them changes from nearly 0 to nearly double of the diffraction angle depending on relative rotational positions between the DWs as shown in  FIG. 2B  for light beam carrying a complex image. No image distortions occurs in this process. 
         [0029]    Increasing the distance Δ          between two identical DWs  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 by the first DW  302 . Said emerging beam  305  propagates parallel to the input beam  301  in case the optical axis modulation directions of DW s  302  and  304  are parallel,  FIG. 3A , and it also changes in propagation direction when the DW  304  is rotated with respect to DW  302  into a new position  306 ,  FIG. 3B . The overall deflection angle of the beam can be maximized positioning the output DW  306  anti-parallel with respect to the input DW  302 . The optical axis alignment patterns for anti-parallel DWs  302  and  306  are schematically shown in  FIG. 3C . The beam can be steered over arbitrarily large angles by adding DWs into the system. Four DWs,  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 DWs  407  and  409  have to be arranged anti-parallel to DWs  406  and  408 . A demonstration of light deflection by such a system of four DWs is shown in  FIG. 4B . In general, DWs can be tilted with respect to each other such as each of the subsequent DWs is nearly perpendicular to the beam deflected by the previous DW. The DWs  507  and  509  are anti-parallel to the DWs  506  and  508 , and all four deflected beam  502 - 505  of the input beam  501  result in increasing total deflection angle. 
         [0030]    In another embodiment, one or more DWs in a system can be switched between diffractive and non-diffractive states, optically, thermally, electrically, mechanically, or by any other means, due the effect of external stimuli on optical anisotropy and optical axis orientation modulation pattern. For example, the DW can be made of azobenzene liquid crystal polymer that can be transformed into isotropic state or realigned by light beams as discussed in S. R. Nersisyan, N. Y. 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). Alternatively, DWs can be transformed into homogeneous orientation state by electrical fields if they are made of liquid crystals or liquid crystal polymer network materials. 
         [0031]    Particularly important is the case shown in  FIG. 6  when a DW  603  with a fixed diffractive property is paired with a controllable DW  602  in configuration when their optical axis modulation directions are parallel. As noted in S. R. Nersisyan, N. Y. Tabiryan, L. Hoke, D. M. Steeves, B. Kimball, “Polarization insensitive imaging through polarization gratings,” Optics Express, 17 (3), 1817-1830 (2009), this state corresponds to total cancellation of diffraction, and such a pair allows transmitting the light beam  601  through the system as shown schematically in  FIG. 6A . An image sensor  604  furnished with an aperture  605  large enough not to block the transmitted beam would not register any distortions to the beam. In case the DW  602  is transformed into a non-diffractive state  606 , the diffraction of, generally, an unpolarized light on the remaining DW  603  redirects the input beam  601  into diffracted beams  607  and  608  as shown in  FIG. 6B , diffracting it into orthogonal circular polarized components in case of unpolarized or linearly polarized incident beam. No beam is acting on an image sensor  604  in this case provided the deflected beams propagate beyond the receiving aperture of the image sensor. Thus, the system described in  FIG. 6  undergoes switching from high transmission to no or low-transmission state as a result of switching the structure of one of the DWs in the system from diffractive state  603  into a non-diffractive state  606 ,  FIG. 6C . Indeed, such change in transmission through particular aperture can be obtained also by mechanically changing the rotational position of the DWs or the distance between them. 
         [0032]    Paired DWs and their systems can have many applications in photonics. A setup for beam combining is shown in  FIG. 7 . Two parallel propagating light beams of orthogonal circular polarizations  701  and  704 , after being deflected by the first DW  707  are further deflected into beams  702  and  705 , emerging as overlapping beams of the same propagation direction  703  and  706  by the second DW  707  in  FIG. 7 . 
         [0033]    Given the thinness of individual DW layers, a multilayer system can be designed for spectrally selective switching without compromising the high throughput and the small size of the system. In the embodiment shown in  FIG. 8 , a set of DW pairs is used for controlling with the spectral content of the transmitted light by allowing light at different portions of the spectrum at least partially be deflected out of the system. The beams  801  and  804  in  FIG. 8  are assumed to possess with different, non-overlapping, spectral content. The individual DWs in the first pair  807  are optimized for diffracting the light beam  801  while having diffraction spectrum out of the spectral range of the beam  804 . The individual DWs in the second pair  808  are optimized for diffracting the light beam  804  while having diffraction spectrum out of the spectral range of the beam  801 . Thus, when DWs in both pairs are parallel aligned with respect to their optical axis modulation direction, all the light is transmitted, and the spectral content of the output light is the same as in the input light. In this case shown in FIG. SA, the input light  801  propagates through the first DW pair into the beam  802  without changing its propagation direction due to diffraction on both DWs constituting the pair  807 . The beam  802  further propagates through the second DW pair  808  into the beam  803  without deflection since its spectrum is out of the diffraction spectrum of the second DW pair  808 . Similarly, the input light  804  propagates through the first DW pair into the beam  805  without changing its propagation direction since its spectrum is out of the diffraction spectrum of the first DW pair  807 . The beam  805  further propagates through the second DW pair  808  into the beam  806  due to the diffraction on both DWs constituting the pair  808 . 
         [0034]    In case one of the DWs constituting the first pair  807  is switched into nondiffractive state  809 , or is rotated to double the diffraction angle of the beam  801  by the first DW in the pair  807 , the beam  801  is diffracted out of the optical system into a beam  810 . Propagation of the beam  804  is not affected by that. Thus the light spectrum obtained at the output of the optical system coincides with that of the beam  804 ,  FIG. 8B . 
         [0035]    In case one of the DWs constituting the second pair  808  is switched into nondiffractive state  811 , or is rotated to double the diffraction angle of the beam  805  by the first DW in the pair  808 , the beam  805  is diffracted out of the optical system into a beam  812 . Propagation of the beam  802  is not affected by that. Thus the light spectrum obtained at the output of the optical system coincides with that of the beam  801 ,  FIG. 8C . 
         [0036]    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.

Technology Category: 3