Abstract:
A method for terminating a plurality of optical fibers arranged in a two-dimensional arrangement comprises inserting the plurality of optical fibers into and through a fiber ferrule, where the fiber ferrule has a plurality of parallel channels extending from an entry surface through to a polish surface; polishing the polish surface including an end of each of the plurality of optical fibers to form a coplanar surface at a polish angle relative to a reference plane perpendicular to the parallel channels; and affixing a glass plate to the polish surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/325,538, entitled “Apparatus and Method for Terminating an Array of Optical Fibers,” which was filed on Jul. 8, 2014, and is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention is generally related to terminating an array of optical fibers, and more particularly, to terminating a two-dimensional fiber array using single-plane fiber termination. 
       BACKGROUND OF THE INVENTION 
       [0003]    Arrays of optical fibers, or “fiber arrays,” are widely used in fields such as imaging, optical communications, remote sensing, and astronomy. One-dimensional (“1D”) fiber arrays (i.e., N-by-1 fiber arrays) integrate multiple fibers in a line in a compact optical device and offer multiplexing capability. Two-dimensional (“2D”) fiber arrays (i.e., N-by-M fiber arrays) enhance compactness and multiplexing capabilities by increasing packing density and further, provide an ability to address two dimensional spatial information in a straight-forward manner. Conventional array structures such as silicon v-grooves, glass v-grooves, glass ferrules provide precise fiber positioning yet are efficient and cost effective to manufacture. 
         [0004]    A uncoated optical fiber end suffers from an approximate four percent (4%) Fresnel reflection which then couples back into the fiber if the fiber is perpendicularly terminated. In many applications, ends of optical fibers require different termination techniques to reduce insertion loss and/or increase return rejection. Return rejection is a concern when a laser cavity or optical amplifier is sensitive to feedback of a coupling fiber. High return loss is required in lidar (i.e., laser radar) remote sensing applications, because a small amount of surface reflection coupling back into the fiber may overwhelm as sensed return signal. 
         [0005]    Conventional techniques for addressing return rejection and/or insertion loss include anti-reflective coatings or angle polishing/cleaving techniques. Requirements for high return loss are difficult to meet solely by applying AR coating on a terminated fiber array. Although angle termination is suitable for a single fiber or a 1D fiber array, angle terminating a 2D fiber array can be very challenging if the 2D fiber array is needed for lens imaging applications. As illustrated in  FIG. 5 , the fiber ends of a two-by-two fiber array have to be arranged step wise one row next to another, such that the optimal imaging can be achieved by positioning all fiber ends on a desired plane defined by the lens imaging system. As illustrated, the ends of the fibers in a given row share a common plane; but the ends of the fibers in different rows are on different planes. However, fabricating the stepwise angle terminated fiber array involves complex processes or special tools. 
         [0006]    What is needed is an improved mechanism for terminating a two-dimensional fiber array that does not suffer the performance or manufacturing drawbacks of conventional systems. 
       SUMMARY OF THE INVENTION 
       [0007]    According to various implementations of the invention, a method for terminating a plurality of optical fibers arranged in a two-dimensional arrangement comprises inserting the plurality of optical fibers into and through a fiber ferrule, where the fiber ferrule has a plurality of parallel channels extending from an entry surface through to a polish surface; polishing the polish surface including an end of each of the plurality of optical fibers to form a coplanar surface at a polish angle relative to a reference plane perpendicular to the parallel channels; and affixing a glass plate to the polish surface. 
         [0008]    These implementations, their features and other aspects of the invention are described in further detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a termination assembly with a 2D fiber array terminated according to various implementations of the invention. 
           [0010]      FIG. 2  illustrates a 2D fiber array terminated in a single plane according to various implementations of the invention. 
           [0011]      FIG. 3  illustrates a termination assembly with a 2D fiber array terminated according to various implementations of the invention. 
           [0012]      FIG. 4  illustrates a process for forming a termination assembly to various implementations of the invention. 
           [0013]      FIG. 5  illustrates a conventional solution for terminating a 2D fiber array. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  illustrates a termination assembly  100  useful in connection with a two-dimensional arrangement  140  of optical fibers  150 . In some implementations of the invention, termination assembly  100  includes a fiber ferrule  110  and a compensating wedge plate  130 . In some implementations of the invention, a fiber array  105  is formed by inserting optical fibers  150  through multi-channel fiber ferrule  110  and by affixing them therein using conventional techniques. In some implementations of the invention, fiber ferrule  110  has circular holes, or other regulating channels, such as triangular or hexagonal channels, formed therein in a desired two-dimensional arrangement, where each of such holes/channels accommodates a single optical fiber  150 . In some implementations of the invention, fibers  150  are bundled next to each other with minimal gaps to form a high density fiber array. A surface  120  (also referred to herein as a “single-plane”) of fiber ferrule  110  is formed by grinding and/or polishing surface  120  of fiber ferrule  110  (now also referred to as fiber array  105 ), including ends of fibers  150  at a polish angle θ 1 . According the various implementations of the invention, fiber array  105 , due to angled, polished surface  120 , provides high return loss capabilities. In other words, that portion of optical signals travelling through fiber  150  (also referred to herein as “optical beam(s)”) that are reflected off terminated end  220  (illustrated in  FIG. 2 ) do so at an angle that reduces or eliminates such reflected signals from being reflected back into fiber  150 . 
         [0015]      FIG. 2  illustrates surface  120  from perspective perpendicular to surface  120  after fiber ferrule  110  and fibers  150  are polished (or more particularly, ends  220  of fibers  150  are polished). In some implementations of the invention, surface  120  of fiber array  105  includes two-dimensional arrangement  140  of terminated ends  220  of fibers  150 . As illustrated in  FIG. 2 , surface  120  includes a 2-row-by-3-column arrangement  140  of fibers  150 ; other two-dimensional arrangements  140  may be used including two-dimensional arrangements other than row-by-column arrangement as would be appreciated. 
         [0016]    Compensating glass plate or wedge plate  130  has a mating surface that mates wedge plate  130  to surface  120  of fiber ferrule  110 , including ends  220  of fibers  150 . According to various implementations of the invention, wedge plate  130  is formed having a wedge angle θ 2  between mating surface and an emergent surface  170  as will be described in further detail below. In some implementations of the invention, wedge plate  130  is formed from a material that matches various optical and mechanical properties of fibers  150 . In some implementations of the invention, wedge plate  130  is formed from silica glass to match various optical and mechanical properties of fibers  150  also formed from silica glass. Other materials may be used as would be appreciated. 
         [0017]    In some implementations of the invention, wedge plate  130  is attached to fiber array  105 . In some implementations of the invention, wedge plate  130  is affixed to fiber array  105  using epoxy or other affixing agents. In some implementations of the invention, the epoxy or other affixing agents matches an index of wedge plate  130  and fibers  150  to minimize insertion loss as would be appreciated. In some implementations of the invention, the epoxy or other affixing agent encloses ends  220  of fibers  150  and/or conceal any imperfections in the surfaces of ends  220  of fibers  150  to further improve return loss performance. 
         [0018]    In some implementations of the invention, ends  220  of fibers  150  directly affix to wedge plate  130  (via epoxy or other affixing agent). In some implementations of the invention, ends  220  of fibers  150  may be detached from wedge plate  130 ; doing so should not significantly affect return loss performance or imaging condition. 
         [0019]    Optical beams carried by fibers  150  embedded in fiber ferrule  110  emerge from ends  220  of fibers  150  and enter wedge plate  130  as optical beams  155 . In some implementations, these optical beams  155  pass through epoxy or other affixing agents after emerging from ends  220  of fibers  150  and prior to entering wedge plate  130 . Optical beams  155  pass through wedge plate  130  and emerge from emergent surface  170  as optical beams  160  at an angle θ 3  from an original path of fibers  150 . 
         [0020]    From a perspective in an exterior medium (i.e., from a medium outside of wedge plate  130 , such as air), each of ends  220  projects back into wedge plate  130  onto a single apparent plane  180 . Apparent plane  180  may be adjusted (i.e., tilted) by changing wedge angle, θ 2 . In some implementations of the invention, an optimal wedge angle, θ 2 , occurs when apparent plane  180  is normal to (i.e., perpendicular to) a chief ray direction of optical beams  160  emergent from wedge plate  130  as illustrated in  FIG. 1 . When an imaging lens (not otherwise illustrated) is aligned to the chief ray directions of optical beams  160 , optical beams  160  may be focused onto a target plane normal to the optical axis with minimal image degradation. However, in some implementations of the invention, optical beams  160  emerging from wedge plate  130  may be bent relative to the parallel fibers  150 . 
         [0021]    In some implementations of the invention, other wedge angles may be used to tilt the target plane for a various reasons, including, but not limited to compensating for aberration or to accommodate various optical components such as lens arrays, Fresnel lens structures or grating structures (none of which are otherwise illustrated). 
         [0022]    A total apparent length of a refracted optical array is given by Σ i l i /n i , where l i  is the segmental ray distance and n i  is the local refractive index. The apparent lengths may be equalized even through optical signals travel along different paths. Employing Snell&#39;s law and some elementary geometry, an optimal relation between surface  120  and emergent surface  170  may be expressed as 2 sin θ 2 =n 2  sin 2(θ 2 −θ 1 ), where n is the common refractive index of fibers  150  and wedge plate  130 . For a standard polishing angle of eight degrees (i.e., θ 1 =8°), wedge angle θ 2  is approximately fifteen degrees (i.e., θ 2 ≈15° and optical signals  160  emerge from emergent surface  170  bent at an angle of approximately three and one half degrees (i.e., θ 3 ≈3.5°. 
         [0023]    In some implementations of the invention, because the compensation provided by wedge plate  130  is not affected by translation of wedge plate  130  and not sensitive to a roll of wedge plate  130 , aligning and affixing fiber ferrule  110  with wedge plate  130  may be quite straight-forward and robust. 
         [0024]    In some implementations of the invention, emergent surface  130  may be coated with an anti-reflective coating to reduce insertion loss. In some implementations of the invention, emergent surface  130  may be left uncoated for attaching additional optical components as would be appreciated. 
         [0025]      FIG. 4  illustrates a process  400  for forming a termination assembly according to various implementations of the invention. In an operation  410 , a plurality of optical fibers  150  are inserted into a fiber ferrule  110 . In an operation  420 , a surface  120  of fiber array  105  (including a surface of ferrule  110  and ends  220  of fibers  150 ) are ground and/or polished at polish angle θ 1 . In an operation  430 , a wedge plate  130  is affixed to fiber array  105 , where wedge plate  130  has a wedge angle of θ 2 . 
         [0026]      FIG. 3  illustrates a termination assembly  300  useful in connection with a two-dimensional arrangement  140  of optical fibers  150 . In some implementations of the invention, termination assembly  300  includes a polished, single-plane fiber ferrule  310  and a glass plate or plane window  330 . In some implementations of the invention, a fiber array  305  is formed by inserting optical fibers  150  through a multi-channel ferrule  310  and by affixing them therein using conventional techniques. In some implementations of the invention, ferrule  310  has circular holes or other regulating channels, such as triangular or hexagonal channels, formed therein in a desired two-dimensional arrangement, where each of such holes/channels accommodates a single optical fiber  150 . In some implementations of the invention, fibers  150  are bundled together with minimal gaps. A surface  320  (also referred to herein as a “single-plane”) of single-plane fiber ferrule  310  is formed by grinding and/or polishing fiber ferrule  110 , including ends  220  of fibers  150  at a polish angle θ 1 , which in these implementations of the invention, is zero degrees (i.e., θ 1 =0°). 
         [0027]    In some implementations of the invention, plane window  330  may be directly affixed to surface  320  of fiber ferrule  310  (now also referred to as fiber array  305 ). In some implementations of the invention, plane window  330  may be directly affixed to surface  320  of fiber array  305   310  using index matching agents to minimize surface reflection (i.e., reflection of optical signals off of plane window  330  and back into fibers  150 ). In some implementations of the invention, plane window  330  is formed from silica glass. In some implementations of the invention, for a single mode fiber or a small core fiber, whose Raleigh range is roughly 100 μm, plane window  330  may be a few millimeters thick. Such a thin plane window  330  should result in little, if any, Fresnel reflection that would couple back to fibers  150 . 
         [0028]    In some implementations of the invention, emergent surface  380  of plane window  330  may be coated with an anti-reflective coating to reduce any return loss from emergent surface  380 . In some implementations of the invention, further improvements may be achieved if an index matching film  340  is precisely controlled to create destructive interference between two Fresnel reflections occurring at index matching film  340 . When fibers  150  and plane window  330  are formed from the same material, complete cancellation may occur. In some implementations, a thickness of index matching film  340  may be actively controlled during manufacturing by using a sensor to monitor a return loss as would be appreciated. In some implementations of the invention, UV epoxy may be used because its curing process may be readily controlled as would be appreciated. 
         [0029]    Implementations of the invention illustrated generally in  FIG. 1  may have return losses better than −60 dB, whereas implementations of the invention illustrated generally in  FIG. 3  may have return losses approaching −50 dB. Hence, for less stringent applications, the implementations of  FIG. 3  may be attractive over the implementations of  FIG. 1  due to simpler manufacturing processes. Further, the implementations of  FIG. 3  provide a straight-line optical path through termination assembly  300  whereas, the optical path is bent by termination assembly  100 . 
         [0030]    While the invention has been described herein in terms of various implementations, it is not so limited and is limited only by the scope of the following claims, as would be apparent to one skilled in the art. These and other implementations of the invention will become apparent upon consideration of the disclosure provided above and the accompanying figures. In addition, various components and features described with respect to one implementation of the invention may be used in other implementations as well.