Abstract:
A non-imaging optical coupling device for use in i.e., optical communications, laser power delivery, laser radar (lidar), and other applications that is relatively immune from optical misalignment and therefore does not need sophisticated splicing or connectorization apparatus is described.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the fields of optics, telecommunications, and fiber optics and in particular to an apparatus that couples light from one or more fibers into another fiber with high efficiency and high tolerance to misalignment of the fibers on either side. 
       BACKGROUND OF THE INVENTION 
       [0002]    Traditional glass fiber optics require fusion splicing or connectorization involving polishing to achieve low loss optical interconnect between two distinct fibers. These options are time consuming, costly, and require considerable training for technicians to be able to perform the tasks well. The industry has long sought a fiberoptic equivalent of electrical connectors that twist, crimp, glue or otherwise mate two connectors through a simple “butt splice” method, but with fibers the issue of alignment and coupling must be addressed. This cannot be done using conventional optics such as lenses since these components do not provide the misalignment (both position and angle) tolerance needed for a robust field splice. 
         [0003]    In conventional fiber splices, the ends of the fiber are cleaved to create a clean, smooth surface and then joined by fusing the glass fibers together with an electric arc or similar means. This creates a continuous waveguide but the fibers must be precisely aligned to begin with, often using large and costly equipment. Fiber mechanical connectors often use “butt coupling,” where the ends of two fibers prepared with flat or angled facets (either cleaved or polished) are brought into very close proximity. This requires a system for connectorizing the fiber, which can also be costly and bulky. 
         [0004]    Grating couplers have been widely used in conventional beam combining technology, but always (to our knowledge) in free-space configurations or for coupling fiber to waveguides. Typically, multiple beams are directed at or focused onto a grating which is designed to diffract significant parts of each beam in the same output direction. The use of planar reflective gratings for coupling between fibers was disclosed by Bowen et al in U.S. Pat. No. 5,011,255 (Apr. 30, 1991), but their approach used fibers oriented in the same direction, with the reflection from the grating used to steer the beam from one fiber to the other. A similar approach was disclosed by Kompfner in U.S. Pat. No. 4,337,993 on Jul. 6, 1982. 
         [0005]    In-fiber gratings have also been used for coupling purposes, but these require the diffractive structure to be physically inscribed in photosensitive optical fiber using high laser fluence. An example of such a coupler is the prior art developed by Ohta et al (U.S. Pat. No. 7,113,674 issued Sep. 26, 2006), wherein two fibers are brought in proximity and inscribed with a slanted Bragg grating structure to achieve coupling. Similar approaches were disclosed by Kewitsch et al (U.S. Pat. No. 6,578,388, of Jun. 17, 2003), Stowe et al (U.S. Pat. No. 6,445,855, Sep. 3, 2002), by Lauzon (U.S. Pat. No. 5,764,831, of Jun. 9, 1998), Bricheno et al (U.S. Pat. No. 5,633,965, May 27, 1997), and by Snitzer (U.S. Pat. No. 5,459,801, Oct. 17, 1995). 
         [0006]    Fiber-to-waveguide coupling using diffractive structures was disclosed by Taillaert et al in U.S. Pat. No. 7,065,272 (issued Jun. 20, 2006), using a regular array of index variations in the slab waveguide material, such as a pattern of etched “dots” or “pillars” on the surface of the material. A similar approach was disclosed by Gunn et al in U.S. Pat. No. 7,006,732, issued Feb. 28, 2006. A transmissive diffractive lens structure was disclosed by Coleman in U.S. Pat. No. 6,956,992 (Oct. 18, 2005). 
         [0007]    The subject invention is a novel interconnect using micro-diffractive optics to achieve very high misalignment tolerance with low insertion loss. These coupling devices have the potential to not only allow efficient passive beam combining, but also to solve many illumination and laser delivery problems such as bending in hollow waveguides. Furthermore, the geometry of the device allows for multiple fiber sources to be combined in a very rugged way, free from the vibration and occlusion problems of free-space optics. 
         [0008]    All of the conventional systems described above have a large degree of misalignment sensitivity, which requires that the fibers be held in precise alignment with each other in order to preserve good coupling efficiency. The use of lenses or reflecting optics can help to some degree, but there is a basic limitation on how well this type of system can tolerate misalignment. 
         [0009]    The fundamental physical constraint on conventional optical coupling systems is the conservation of optical throughput, known from the so-called Lagrange invariant of geometric optics, which can be derived from first principles. In mathematical terms, the conservation of optical path between two media C 1  and C 2  with boundary K is governed by 
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         [0000]    where n is the refractive index, and s is the ray vector. The throughput, or the product of angular acceptance and optical aperture, in a non-diffractive optical system is limited by the component with the smallest throughput, so that 
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         [0010]    This formulation is equivalent to the so-called Liouville form of non-imaging optics, wherein conservation of refractive and reflective systems is often expressed as 
         [0000]      n 1 d 1  sin α=n 2 d 2  sin β,   (3) 
         [0000]    where n 1  and n 2  are the refractive indices of the media on either side of the system, d 1  and d 2  are the entrance and exit aperture widths of the system, respectively, and α and β are the angles over which the input and output beams are distributed. This derivation is based on the Liouville theorem, which applies to conformal transformations between three-dimensional spaces. Reflectors, lenses, Fresnel lenses, and similar optical instruments are all limited by this constraint. 
         [0011]    Of critical importance in Eq. 1 is the surface K, which in refractive and reflective optics cannot alter the wavevector ns. In diffractive optics, the surface K can cause discontinuity in ns, thereby allowing a different conservation relationship. Diffractive optics provide the only means by which this constraint may be relaxed to allow larger angles and areas to be converted to smaller angles and areas, or a larger mode distribution to be condensed into a smaller distribution of degenerate modes. 
         [0012]    Furthermore, the problem of coupling into fiber or waveguide structures has been typically approached from an imaging perspective; in essence, there is usually an implicit or explicit requirement to produce an image on the entrance facet of the waveguide. Non-imaging optics have not been used widely for this purpose, though there is an inherent match, since the entirety of the optical coupling process does not occur exactly at the entrance facet but is in fact distributed some depth into the waveguide. Non-imaging methods, which typically produce longer and less divergent “waist” regions thus have a better overlap with this extended coupling in waveguides than imaging methods, which commonly exhibit highly divergent evanescent penetration. 
         [0013]    Though a great deal of prior art exists in the general area of efficient coupling to fiber, there exists a continuing need for optical coupling structures providing high efficiency, while eliminating the need for fusion splicing, polishing, and precise alignment. Such structures would represent a significant advance in the art. The present invention represents a fundamental departure from prior art at the level of basic physical principles as well as structural design of the system. 
       SUMMARY OF THE INVENTION 
       [0014]    I have developed, in accordance with the principles of the invention, an optical coupling device for use in, i.e., telecommunications, fiber optic systems, laser power delivery, laser radar (lidar), and optical communications applications. In sharp contrast to prior art devices, my inventive coupler is a non-imaging, non-planar, high acceptance angle device. Consequently it is relatively immune from optical source incidence angles and therefore does not require precise alignment of the input and output fibers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A more complete understanding of the present invention may be realized by reference to the accompanying drawings in which: 
           [0016]      FIG. 1  shows a conceptual diagram of how the device steers light from one fiber to another with high tolerance for misalignment of the fiber; 
           [0017]      FIG. 2  shows a conceptual ray tracing of how light is diffracted inside the device; 
           [0018]      FIG. 3  shows the convention for different types and orders of reflection and diffraction; 
           [0019]      FIG. 4  shows an alternate embodiment of the device with different geometry in the diffracting region; 
           [0020]      FIG. 5  shows a device where multiple beams are combined and output into a single fiber; and 
           [0021]      FIG. 6  illustrates the use of a further diffractive structure or lens element to match a lower numerical aperture fiber. 
       
    
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  shows a conceptual view of the coupler. A hollow region  100  or tube is enclosed in a solid body  102  with a diffractive surface. The ends of solid body  102  are open so that optical fibers  104  and  106  can be inserted into the device and securely mated using a crimp sleeve  108 , mating to either the fiber jacket  110  or the fiber buffer or cladding  112  or both, or a similar mating device. 
         [0023]    Light emanating from fiber  104  spreads out in a conical distribution as determined by the numerical aperture of the fiber. Rays from fiber  104  strike the interior diffractive surface of the solid coupler body  102  and are either reflected or diffracted from the surface or absorbed by it, according to the diffractive characteristics of the surface. Rays which are diffracted are steered closer to the corresponding input cone of fiber  106 , determined by its numerical aperture. Rays which are reflected are then directed to another area of the diffractive surface on the interior of coupler body  102  and may thence be reflected or diffracted again. The cumulative effect of multiple reflection and diffraction events averaged over many light rays emanating from fiber  104  is to direct the majority of such rays into the acceptance cone of fiber  106  so that efficient capture of the light from fiber  104  by fiber  106  may occur. In general, as long as the exit apertures of the tube  100  remain approximately aligned with the fibers, efficient coupling will occur, so that the midsection of tube  100  can be allowed to flex. 
         [0024]      FIG. 2  illustrates the process by which a single ray may be directed from one fiber to the other. As noted above, the interior surface of hollow region  100  is diffractive, such that ray  200  striking the interior of the tube at an angle α with respect to the surface normal  202  or  204  is at least partially diffracted at a higher angle β. In the proximal section of tube  100  rays strike the diffractive surface at a glancing angle (such that angle α 1  is nearly 90 degrees) with respect to normal  202 , which in most cases will mean that little diffraction occurs. However, once the ray is past the center of hollow region  100  where the angle of the surface normal  204  of tube or cavity  100  with respect to the center axis  206  changes, it will strike the surface at a shallower angle. Thus, ray  200  striking the surface of hollow region  100  at an angle α 2  with respect to surface normal  204  can be diffracted in a positive direction toward the exit aperture  208  of hollow region  100 . 
         [0025]    It will be apparent to skilled practitioners that this process will occur in fundamentally the same manner if the center axis  206  of hollow region  100  is bent or curved in a given direction. In fact, some advantage may be gained by increasing the average angle α 2  at which the rays are incident on the distal portion of the tube. This characteristic is wholly different from a reflective structure of the same geometry. In the case of a pure reflective structure, a multitude of cases exist where all or a majority of rays will not reach exit aperture  208  if center axis  206  is bent or curved. Even when center axis  206  is predominantly unbent, the diffractive effect of the surface of region  100  will ensure that the total angular spread Ω of rays is smaller than the corresponding spread for a reflective surface. 
         [0026]    It will be apparent to skilled technicians that the diffractive surface  300  on the interior of hollow region  100  must be optimized to reduce negative diffraction, or diffraction of rays in a direction proximal of the specular ray  304  with respect to the incident ray  302 , as shown in  FIG. 3 . Equivalently, the desired diffractive effect is a positive one, where diffracted rays  306  are directed along an angle greater than the specularly reflected ray  302 , rather than in a negative direction as indicated by ray  308 . Proper grating design to maximize positive diffraction will in many cases also have the effect of directing scattered light substantially more toward the output of the tube rather than the input. 
         [0027]    For a diffraction grating, there will be an angular cutoff such that more oblique incidence angles cannot satisfy the grating equation 
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         [0000]    where m is the diffraction order, λ is the incident wavelength, d is the grating spacing, α is the incident angle and β is the diffracted angle. The angular cutoff occurs at 
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         [0000]    so that for a particular design wavelength λ and grating spacing d, the most oblique angle at which an incident ray can be diffracted will occur when m=1. One simple method to increase diffraction at high angles is to use a second order grating, where the peak diffraction efficiency at shallow incidence is in the 2 nd  order, while the peak at higher incidence angles is in the first order. This approach can also obviously be extended to third order and higher, though at some cost of overall efficiency and unwanted orders. 
         [0028]    Several grating design variations may also be used to optimize the multiple diffraction effect. In particular, the angular distribution of rays striking the inner surface of hollow region  100  near the midsection will be greater than the angular distribution of rays striking the inner surface of hollow region  100  near exit aperture  208 , due to the diffractive effect. This means that while the grating must be designed to diffract efficiently over a wide range of angles, at the midsection of hollow region  100 , it can be designed for much higher efficiency at glancing incidence farther down the tube, closer to exit aperture  208 . 
         [0029]    Likewise, the length and width of the tube can be optimized for given materials and geometries. As is known from the technology of hollow waveguides, longer tubes will result in greater interaction of the light with the sides of the tube, or a greater number of reflections or diffractions and thus greater loss. At the same time, the multiple diffraction effect will require a certain number of diffraction events in order to confine a given percentage of incident beams into a cone of a given output angle or numerical aperture (NA). Several variations of this basic concept are also encompassed within the present invention, including taper profiles for the basic conic shape, which may be parabolic, hyperbolic, exponential, or a general power series function. As illustrated in  FIG. 4 , geometries such as an exponential taper  400  may improve efficiency by decreasing the output NA. This effect can be understood by observing that diffracted rays  402  will more closely approach parallelism with waveguide axis  404  as the surface normal  406  more closely approaches perpendicularity with waveguide axis  404 . Near-collimated rays produced by an exponential taper  400  may also be more easily refocused by a fixed optic such as a lens or diffractive optical element, as described further below. 
         [0030]    A second embodiment of the device is shown in  FIG. 5 . In this case, a half-conic shape  500  can be used to guide light from a plurality of fibers or sources  502  into a single exit aperture  504  and thence into a single fiber or waveguide  506 . The basic physics of this process can be well understood by skilled practitioners given the preceding discussion. It will also be apparent that this device does not possess the same symmetry as the first embodiment; light emanating from fiber or waveguide  506  will not be coupled as efficiently into a plurality of guides  502 , due to limitations of subtended area (packing fraction) and NA matching. Other methods such as coupling lenses, lenslets, diffractive optical elements or similar means may be used to improve the symmetry of this device and are also an object of this invention. 
         [0031]    A further modification of my invention, as shown in  FIG. 6 , uses a focusing optic  600  to couple into a waveguide  602  of smaller area cross sectional area, such as a single mode or small-core fiber. It will be appreciated by skilled practitioners that a diffractive hollow tube  604  tightly coupled to a focusing optic will produce a more stable output than a corresponding imaging or relay system consisting of multiple focusing optics. Thus, the spot or circle of least confusion  606  produced by this method will move less than a corresponding spot produced by a relay configuration, relative to the input aperture  608  of the waveguide  602 . Mechanical alignment of the waveguide  602  within a given tolerance, for example using a metal ferrule to house the fiber and the coupler, to spot  606  can then be used to achieve efficient coupling into a small guide. 
         [0032]    At this point, while I have discussed and described my invention using some specific examples, those skilled in the art will recognize that my teachings are not so limited. Accordingly, my invention should be only limited by the scope of the claims attached hereto.