Patent Publication Number: US-9851510-B2

Title: Phase locking optical fiber coupler

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application hereby incorporates by reference in their entirety the following commonly assigned issued patents (as applicable): U.S. Pat. No. 7,308,173, U.S. Pat. No. 8,326,099, and U.S. Pat. No. 8,712,199. 
     FIELD OF THE INVENTION 
     The present invention relates generally to high power single mode laser sources, and to devices for coherent combining of multiple optical fiber lasers to produce multi-kilowatt single mode laser sources, and more particularly to phase locked optical fiber components of a monolithic design that may fabricated with a very high degree of control over precise positioning of even large quantities of plural waveguides, and that are configurable for optimization of the components&#39; fill factor (i.e., of the ratio of the mode field diameter of each waveguide at the “output” end thereof, to the distance between neighboring waveguides). 
     BACKGROUND OF THE INVENTION 
     Optical waveguide devices are indispensable in various high technology industrial applications, and especially in telecommunications. In recent years, these devices, including planar waveguides, and two or three dimensional photonic crystals are being used increasingly in conjunction with conventional optical fibers. In particular, optical waveguide devices based on high refractive index contrast or high numerical aperture (NA) waveguides are advantageous and desirable in applications in which conventional optical fibers are also utilized. However, there are significant challenges in interfacing optical high NA waveguide devices, including chiral optical fiber devices, with conventional low index contrast optical fibers. Typically, at least two major obstacles must be dealt with: (1) the difference between the sizes of the optical waveguide device and the conventional fiber (especially with respect to the differences in core sizes), and (2) the difference between the NAs of the optical waveguide device and the conventional fiber. Failure to properly address these obstacles results in increased insertion losses and a decreased coupling coefficient at each interface. 
     For example, conventional optical fiber based optical couplers, such as shown in  FIG. 6  (Prior Art) are typically configured by inserting standard optical fibers (used as input fibers) into a capillary tube comprised of a material with a refractive index lower than the cladding of the input fibers. There are a number of significant disadvantages to this approach. For example, a fiber cladding-capillary tube interface becomes a light guiding interface of a lower quality than interfaces inside standard optical fibers and, therefore, can be expected to introduce optical loss. Furthermore, the capillary tube must be fabricated using a costly fluorine-doped material, greatly increasing the expense of the coupler. 
     A commonly assigned U.S. Pat. No. 7,308,173, entitled “OPTICAL FIBER COUPLER WITH LOW LOSS AND HIGH COUPLING COEFFICIENT AND METHOD OF FABRICATION THEREOF”, which is hereby incorporated herein in its entirety, advantageously addressed all of the above issues by providing various embodiments of a novel optical fiber coupler capable of providing a low-loss, high-coupling coefficient interface between conventional optical fibers and optical waveguide devices. 
     Nevertheless, a number of challenges still remained. With the proliferation of optical devices with multiple waveguide interfaces (e.g., waveguide arrays), establishing low-loss high-accuracy connections to arrays of low or high NA waveguides often provide problematic, especially because the spacing between the waveguides is very small making coupling thereto all the more difficult. The commonly assigned U.S. Pat. No. 8,326,099, entitled “OPTICAL FIBER COUPLER ARRAY”, issued Dec. 4, 2012, which is hereby incorporated herein by reference in its entirety, addressed the above challenge by providing, in at least a portion of the embodiments thereof, an optical fiber coupler array that provides a high-coupling coefficient interface with high accuracy and easy alignment between an optical waveguide device having a plurality of closely spaced high NA waveguide interfaces, and a plurality of optical fibers each having low numerical apertures separated by at least a fiber diameter. While the &#39;099 Patent already teaches the coupler, which is capable to independently control waveguide NAs and channel-to-channel spacing, it did not specifically address the full extent of configurability with respect to interfacing with plurality of optical fibers, and with respect to adaptability of specially configured exemplary embodiments of the novel optical fiber coupler array for high power laser applications. 
     It is important to note that the practice of coherent combining of multiple fiber lasers has been advantageously utilized in development of multi-kilowatt single mode laser sources for a variety of applications, including, but not limited to, directed energy sources for military and defense applications, for free-space optical communications, for materials processing, and in many more industrial, scientific and even medical applications. 
     Combining multiple individual laser sources, versus creating a single high power laser source, allows for more efficient thermal management, which is one of the most significant limiting factor in high power laser systems. In order to achieve a coherent combination of multiple laser sources, the optical phases of the laser sources being combined must be synchronized (or “phase locked”). 
     One of the known and commonly used approaches to accomplish passive phase locking, is to utilize the “Talbot effect”, which defines a distance at which a periodic pattern recreates itself while propagating in free space. Therefore, Talbot laser cavities are used to lock the phases of Individual laser sources or amplifiers. To achieve the Talbot effect, it is critical that the array of coupled waveguides has good spatial periodicity, and that the distance from the face of a waveguide array to a reflector is precisely selected and maintained. At present, different elements of a commonly used Talbot cavity are free-space optical elements, mechanically held at predefined locations, which makes it very difficult to maintain environmental stability. 
     Accordingly, it would be advantageous to provide various embodiments of an inventive PROFA-based optical fiber link component that may be configured and optimized to achieve highly desirable phase locking characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, wherein like reference characters denote corresponding or similar elements throughout the various figures: 
         FIG. 1A  is a schematic diagram of a side view of a first exemplary embodiment of the optical fiber coupler array of the present invention, which comprises at least one vanishing core waveguide (VC waveguide), illustrated therein by way of example as a single VC waveguide, and at least one Non-VC waveguide, illustrated therein by way of example as a plurality of Non-VC waveguides, disposed symmetrically proximally to the exemplary single VC waveguide; 
         FIG. 1B  is a schematic diagram of a side view of a second exemplary embodiment of the optical fiber coupler array of the present invention, which comprises at least one vanishing core waveguide (VC waveguide), illustrated therein by way of example as a single VC waveguide, and at least one Non-VC waveguide, illustrated therein by way of example as a single Non-VC waveguide, disposed in parallel proximity to the exemplary single VC waveguide, where a portion of the inventive optical fiber coupler array has been configured to comprise a higher channel-to-channel spacing magnitude at its second (smaller) end than the corresponding channel-to-channel spacing magnitude at the second end of the optical fiber coupler array of  FIG. 1A ; 
         FIG. 1C  is a schematic diagram of a side view of a third exemplary embodiment of the optical fiber coupler array of the present invention, which comprises a plurality of VC waveguides, and a plurality of Non-VC waveguides, disposed longitudinally and asymmetrically to one another, and where at least a portion of the plural Non-VC waveguides are of different types and/or different characteristics; 
         FIG. 1D  is a schematic diagram of a side view of a fourth exemplary embodiment of the optical fiber coupler array of the present invention, configured for fan-in and fan-out connectivity and comprising a pair of novel optical fiber coupler components with a multi-core optical fiber element connected between the second (smaller sized) ends of the two optical fiber coupler components; 
         FIG. 2A  is a schematic diagram of a side view of a fifth exemplary embodiment of the optical fiber coupler array of the present invention, which comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure, wherein each plural VC waveguide is spliced, at a particular first splice location, to a corresponding elongated optical device (such as an optical fiber), at least a portion of which extends outside the single common housing structure by a predetermined length, and wherein each particular first splice location is disposed within the single common housing structure; 
         FIG. 2B  is a schematic diagram of a side view of a sixth exemplary embodiment of the optical fiber coupler array of the present invention, which comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure, wherein each plural VC waveguide is spliced, at a particular second splice location, to a corresponding elongated optical device (such as an optical fiber), at least a portion of which extends outside the single common housing structure by a predetermined length, and wherein each particular second splice location is disposed at an outer cross-sectional boundary region of the single common housing structure; 
         FIG. 2C  is a schematic diagram of a side view of a seventh exemplary embodiment of the optical fiber coupler array of the present invention, which comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure, wherein each plural VC waveguide is spliced, at a particular third splice location, to a corresponding elongated optical device (such as an optical fiber), at least a portion of which extends outside the single common housing structure by a predetermined length, and wherein each particular third splice location is disposed outside the single common housing structure; 
         FIG. 2D  is a schematic diagram of a side view of an alternative embodiment of the optical fiber coupler array of the present invention, comprising a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure, that is configured at its second end, to optimize optical coupling to a free-space-based optical device; 
         FIG. 3A  is a schematic diagram of a cross-sectional view of a first alternative embodiment of the optical fiber coupler arrays of  FIGS. 1D to 2D , above, and optionally comprising a fiducial element operable to provide a visual identification of waveguide arrangement/characteristics (such as alignment), which may be disposed in one of several categories of cross-sectional regions; 
         FIG. 3B  is a schematic diagram of a cross-sectional view of a first alternative embodiment of the optical fiber coupler array of  FIG. 1A , above, in which at least one VC waveguide, illustrated therein by way of example as a single VC waveguide, is positioned along a central longitudinal axis of the single common housing structure, and surrounded by a plurality of parallel proximal symmetrically positioned Non-VC waveguides; 
         FIG. 3C  is a schematic diagram of a cross-sectional view of a first alternative embodiment of the optical fiber coupler array of  FIG. 3B  above, in which a volume of the single common housing structure medium surrounding the sections of all of the waveguides embedded therein, exceeds a total volume of the inner and outer cores of the section of the VC waveguide that is embedded within the single common housing structure; 
         FIG. 3D  is a schematic diagram of a cross-sectional view of a second alternative embodiment of the optical fiber coupler array of  FIG. 3B  above, in which the at least one VC waveguide positioned along the central longitudinal axis of the single common housing structure comprises a plurality of VC waveguides, and in a volume of the single common housing structure medium surrounding the sections of all of the waveguides embedded therein, exceeds a total volume of the inner and outer cores of the sections of the plural VC waveguides that are embedded within the single common housing structure; 
         FIG. 3E  is a schematic diagram of a cross-sectional view of a first alternative embodiment of the optical fiber coupler array of  FIG. 3D , further comprising a central waveguide channel operable to provide optical pumping functionality therethrough; 
         FIG. 3F  is a schematic diagram of a cross-sectional view of a second alternative embodiment of the optical fiber coupler array of  FIG. 3D , in which the plural VC waveguide that is positioned along the central longitudinal axis of the single common housing structure, is of a different type, and/or comprises different characteristics from the remaining plural VC waveguides, which, if selected to comprise enlarged inner cores, may be advantageously utilized for optimizing optical coupling to different types of optical pump channels of various optical devices; 
         FIG. 3G  is a schematic diagram of a cross-sectional view of a third alternative embodiment of the optical fiber coupler array of  FIG. 3B  above, in which at least one VC waveguide, illustrated therein by way of example as a single VC waveguide, is positioned as a side-channel, off-set from the central longitudinal axis of the single common housing structure, such that this embodiment of the inventive optical fiber coupler array may be readily used as a fiber optical amplifier and or a laser, when spliced to a double-clad optical fiber having a non-concentric core for improved optical pumping efficiency; 
         FIG. 3H  is a schematic diagram of a cross-sectional view of a first alternative embodiment of the optical fiber coupler array of  FIG. 3G , above, in which the at least one VC waveguide, illustrated therein by way of example as a side-channel off-center positioned single VC waveguide, comprises polarization maintaining properties and comprises a polarization axis that is aligned with respect to its transverse off-center location; 
         FIG. 3I  is a schematic diagram of a cross-sectional view of a fourth alternative embodiment of the optical fiber coupler array of  FIG. 3B , above, wherein each of the centrally positioned single VC waveguide, and the plural Non-VC waveguides, comprises polarization maintaining properties (shown by way of example only as being induced by rod stress members (and which may readily and alternately be induced by various other stress or equivalent means)), and a corresponding polarization axis, where all of the polarization axes are aligned to one another; 
         FIG. 3J  is a schematic diagram of a cross-sectional view of a first alternative embodiment of the optical fiber coupler array of  FIG. 3I , above, in which the polarization maintaining properties of all of the waveguides result only from a non-circular cross-sectional shape of each waveguide&#39;s core (or outer core in the case of the VC waveguide), shown by way of example only as being at least in part elliptical, and optionally comprising at least one waveguide arrangement indication element, positioned on an outer region of the single common housing structure, representative of the particular cross-sectional geometric arrangement of the optical coupler array&#39;s waveguides, such that a particular cross-sectional geometric waveguide arrangement may be readily identified from at least one of a visual and physical inspection of the single common coupler housing structure, the waveguide arrangement indication element being further operable to facilitate passive alignment of a second end of the optical coupler array to at least one optical device; 
         FIG. 3K  is a schematic diagram of a cross-sectional view of a fifth alternative embodiment of the optical fiber coupler array of  FIG. 3B , above, wherein the centrally positioned single VC waveguide, comprises polarization maintaining properties (shown by way of example only as being induced by rod stress members (and which may readily and alternately be induced by various other stress or equivalent means), and a corresponding polarization axis, and optionally comprising a plurality of optional waveguide arrangement indication elements of the same or of a different type, as described in greater detail in connection with  FIG. 3J ; 
         FIG. 3L  is a schematic diagram of a cross-sectional view of a second alternative embodiment of the optical fiber coupler array of  FIG. 3I , above, in which the single common housing structure comprises a cross section having a non-circular geometric shape (shown by way of example as a hexagon), and in which the polarization axes of the waveguides are aligned to one another and to the single common housing structure cross-section&#39;s geometric shape, and optionally further comprises a waveguide arrangement indication element, as described in greater detail in connection with  FIG. 3J ; 
         FIG. 4  is a schematic isometric view diagram illustrating an exemplary connection of a second end (i.e. “tip”) of the inventive optical fiber coupler array, in the process of connecting to plural vertical coupling elements of an optical device in a proximal open air optical coupling alignment configuration, that may be readily shifted into a butt-coupled configuration through full physical contact of the inventive optical fiber coupler array second end and the vertical coupling elements; 
         FIG. 5  is a schematic isometric view diagram illustrating an exemplary connection of a second end (i.e. “tip”) of the inventive optical fiber coupler array connected to plural edge coupling elements of an optical device in a butt-coupled configuration, that may be readily shifted into one of several alternative coupling configuration, including a proximal open air optical coupling alignment configuration, and or an angled alignment coupling configuration; 
         FIG. 6  is a schematic diagram of a cross-sectional view of a previously known optical fiber coupler having various drawbacks and disadvantages readily overcome by the various embodiments of the inventive optical fiber coupler array of  FIGS. 1A to 5 ; and 
         FIG. 7  is a schematic diagram of a phase locking optical fiber coupler comprising a monolithic pitch reducing optical fiber array (PROFA) component. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to an optical fiber coupler array capable of providing a low-loss, high-coupling coefficient interface with high accuracy and easy alignment between a plurality of optical fibers (or other optical devices) with a first channel-to-channel spacing, and an optical device having a plurality of waveguide interfaces with a second, smaller channel-to-channel spacing. Advantageously, in various embodiments of the present invention, each of a larger size end and a smaller size end of the optical fiber coupler array is configurable to have a correspondingly different (i.e., larger vs. smaller) channel-to-channel spacing, where the respective channel-to-channel spacing at each of the novel optical coupler array&#39;s larger and smaller ends may be readily matched to a corresponding respective first channel-to-channel spacing of the plural optical fibers at the larger optical coupler array end, and to a second channel-to-channel spacing of the optical device plural waveguide interfaces at the smaller optical coupler array end. 
     In various inventive embodiments thereof, the novel optical coupler array includes a plurality of waveguides (at least one of which may optionally be polarization maintaining), that comprises at least one gradually reduced “vanishing core fiber”, at least in part embedded within a common housing structure. Alternatively, in various additional inventive embodiments thereof, the novel coupler array may be configured for utilization with at least one of an optical fiber amplifier and an optical fiber laser. 
     Each of the various embodiments of the optical coupler array of the present invention advantageously comprises at least one “vanishing core” (VC) fiber waveguide, described in greater detail below in connection with a VC waveguide  30 A of the optical coupler array  10 A of  FIG. 1A . 
     It should also be noted that the term “optical device” as generally used herein, applies to virtually any single channel or multi-channel optical device, or to any type of optical fiber, including, but not being limited to, standard/conventional optical fibers. For example, optical devices with which the inventive coupler array may advantageously couple may include, but are not limited to, one or more of the following:
         a free-space-based optical device,   an optical circuit having at least one input/output edge coupling port,   an optical circuit having at least one optical port comprising vertical coupling elements,   a multi-mode (MM) optical fiber,   a double-clad optical fiber,   a multi-core (MC) optical fiber,   a large mode area (LMA) fiber,   a double-clad multi-core optical fiber,   a standard/conventional optical fiber,   a custom optical fiber, and/or   an additional optical coupler array.       

     In addition, while the term “fusion splice” is utilized in the various descriptions of the exemplary embodiments of the novel coupler array provided below, in reference to interconnections between various novel optical coupler array components, and connections between various novel optical coupler array components and optical device(s), it should be noted, that any other form of waveguide or other coupler array component connectivity technique or methodology may be readily selected and utilized as a matter of design choice or necessity, without departing from the spirit of the invention, including but not limited to mechanical connections. 
     Referring now to  FIG. 1A , a first exemplary embodiment of an optical fiber coupler array of the present invention is shown as an optical coupler array  10 A, which comprises a common housing structure  14 A (described in greater detail below), at least one VC waveguide, shown in  FIG. 1A  by way of example, as a single VC waveguide  30 A, and at least one Non-VC waveguide, shown in  FIG. 1A  by way of example, as a pair of Non-VC waveguides  32 A- 1 ,  32 A- 2 , each positioned symmetrically proximally to one of the sides of of the exemplary single VC waveguide  30 A, wherein the section of the VC waveguide  30 A, located between positions B and D of  FIG. 1A  is embedded in the common housing structure  14 A. 
     Before describing the coupler array  10 A and its components in greater detail, it would be useful to provide a detailed overview of the inventive VC waveguide  30 A, the exemplary embodiments and alternative embodiments of which, are advantageously utilized in each of the various embodiments of the inventive coupler arrays of  FIGS. 1A to 5 . 
     The VC waveguide  30 A has a larger end (proximal to position B shown in  FIG. 1A ), and a tapered, smaller end (proximal to position C shown in  FIG. 1A ), and comprises an inner core  20 A (composed of a material with an effective refractive index of N- 1 ), an outer core  22 A (composed of a material with an effective refractive index of N- 2 , smaller than N- 1 ), and a cladding  24 A (composed of a material with an effective refractive index of N- 3 , smaller than N- 2 ). 
     Advantageously, the outer core  22 A serves as the effective cladding at the VC waveguide  30 A large end at which the VC waveguide  30 A supports “M1” spatial propagating modes within the inner core  20 A, where M1 is larger than 0. The indices of refraction N- 1  and N- 2 , are preferably chosen so that the numerical aperture (NA) at the VC waveguide  30 A large end matches the NA of an optical device (e.g. an optical fiber) to which it is connected (such as an optical device  34 A- 1 , for example, comprising a standard/conventional optical fiber connected to the VC waveguide  30 A at a connection position  36 A- 1  (e.g., by a fusion splice, a mechanical connection, or by other fiber connection means), while the dimensions of the inner and outer cores ( 20 A,  22 A), are preferably chosen so that the connected optical device (e.g., the optical device  34 A- 1 ), has substantially the same mode field dimensions (MFD). Here and below we use mode field dimensions instead of commonly used mode field diameter (also MFD) due to the case that the cross section of the VC or Non-VC waveguides may not be circular, resulting in a non-circular mode profile. Thus, the mode field dimensions include both the mode size and the mode shape and equal to the mode field diameter in the case of a circularly symmetrical mode. 
     During fabrication of the coupler array  10 A from an appropriately configured preform (comprising the VC waveguide  30 A preform having the corresponding inner and outer cores  20 A,  22 A, and cladding  24 A), as the coupler array  10 A preform is tapered in accordance with at least one predetermined reduction profile, the inner core  20 A becomes too small to support all M1 modes. The number of spatial modes, supported by the inner core at the second (tapered) end is M2, where M2&lt;M1. In the case of a single mode waveguide, where M1=1 (corresponding to 2 polarization modes), M2=0, meaning that inner core is too small to support light propagation. The VC waveguide  30 A then acts as if comprised a fiber with a single core of an effective refractive index close to N- 2 , surrounded by a cladding of lower index N- 3 . 
     During fabrication of the coupler array  10 A, a channel-to-channel spacing S- 1  at the coupler array  10 A larger end (at position B,  FIG. 1A ), decreases in value to a channel-to-channel spacing S- 2  at the coupler array  10 A smaller end (at position C,  FIG. 1A ), in proportion to a draw ratio selected for fabrication, while the MFD value (or the inversed NA value of the VC waveguide  30 A) can be either reduced, increased or preserved depending on a selected differences in refractive indices, (N- 1 -N- 2 ) and (N- 2 -N- 3 ), which depends upon the desired application for the optical coupler array  10 A, as described in greater detail below. 
     The capability of independently controlling the channel-to-channel spacing and the MFD values at each end of the inventive optical coupler array is a unique and highly advantageous feature of the present invention. Additionally, the capability to match MFD and NA values through a corresponding selection of the sizes and shapes of inner  20 A and outer  22 A cores and values of N- 1 , N- 2 , and N- 3 , makes it possible to utilize the novel optical coupler array to couple to various waveguides without the need to use a lens. 
     In various embodiments thereof, the property of the inventive VC waveguide permitting light to continue to propagate through the waveguide core along the length thereof when its diameter is significantly reduced, advantageously, reduces optical loss from interfacial imperfection or contamination, and allows the use of a wide range of materials for a medium  28 A of the common housing structure  14 A (described in greater detail below), including, but not limited to:
         (a) non-optical materials (since the light is concentrated inside the waveguide core),   (b) absorbing or scattering materials or materials with refractive index larger than the refractive index of standard/conventional fibers for reducing or increasing the crosstalk between the channels, and   (c) pure-silica (e.g., the same material as is used in most standard/conventional fiber claddings, to facilitate splicing to multi-core, double-clad, or multi-mode fiber.       

     Preferably, in accordance with the present invention, the desired relative values of NA- 1  and NA- 2  (each at a corresponding end of the coupler array  10 A, for example, NA- 1  corresponding to the coupler array  10 A large end, and NA- 2  corresponding to the coupler array  10 A small end), and, optionally, the desired value of each of NA- 1  and NA- 2 ), may be determined by selecting the values of the refractive indices N1, N2, and N3 of the coupler array  10 A, and configuring them in accordance with at least one of the following relationships, selected based on the desired relative numerical aperture magnitudes at each end of the coupler array  10 A: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Desired NA-1/NA-2 
                 Corresponding 
               
               
                   
                 Relative Magnitude 
                 Relationship bet N1, N2, N3 
               
               
                   
                   
               
             
            
               
                   
                 NA-1 (lrg. end) &gt; NA-2 
                 (N1 − N2 &gt; N2 − N3) 
               
               
                   
                 (sm. end) 
                   
               
               
                   
                 NA-1 (lrg. end) = NA-2  
                 (N1 − N2 = N2 − N3) 
               
               
                   
                 (sm. end) 
                   
               
               
                   
                 NA-1 (lrg. end) &lt; NA-2  
                 (N1− N2 &lt; N2 − N3) 
               
               
                   
                 (sm. end) 
               
               
                   
                   
               
            
           
         
       
     
     Commonly the NA of any type of fiber is determined by the following expression:
 
 NA =√{square root over ( n   core   2   −n   clad   2 )},
         where n core  and n clad  are the refractive indices of fiber core and cladding respectively.       

     It should be noted that when the above expression is used, the connection between the NA and the acceptance angle of the fiber is only an approximation. In particular, fiber manufacturers often quote “NA” for single-mode (SM) fibers based on the above expression, even though the acceptance angle for a single-mode fiber is quite different and cannot be determined from the indices of refraction alone. 
     In accordance with the present invention, as used herein, the various NA values are preferably determined utilizing effective indices of refraction for both n core  and n cladding , because the effective indices determine the light propagation and are more meaningful in the case of structured waveguides utilized in various embodiments of the present invention. Also, a transverse refractive index profile inside a waveguide may not be flat, but rather varying around the value N1, N2, N3, or N4. In addition, the transition between regions having refractive indices N1, N2, N3, and N4 may not be as sharp as a step function due to dopant diffusion or some other intentional or non-intentional factors, and may be a smooth function, connecting the values of N1, N2, N3, and N4. Coupling optimization requires to change both the values of N1, N2, N3, and N4 and the sizes and shapes of the regions having respective indices. 
     Returning now to  FIG. 1A , the common coupling structure  14 A, comprises the medium  28 A, in which the section of the VC waveguide  30 A located between positions B and D of  FIG. 1A  is embedded, and which may include, but is not limited to, at least one of the following materials:
         a material, having properties prohibiting propagation of light therethrough,   a material having light-absorbing optical properties,   a material having light scattering optical properties,   a material having optical properties selected such that said fourth refractive index (N- 4 ) is greater than said third refractive index (N- 3 ), and/or   a material having optical properties selected such that said fourth refractive index (N- 4 ) is substantially equal to said third refractive index (N- 3 ).       

     At the optical coupler array  10 A large end (proximally to position B in  FIG. 1A ), the VC waveguide  30 A is spliced, at a particular splice location  36 A- 1  (shown by way of example as positioned inside the common housing structure  14 A), to a corresponding respective elongated optical device  34 A- 1  (for example, such as an optical fiber), at least a portion of which extends outside the common housing structure  14 A by a predetermined length  12 A, while the Non-VC waveguides  32 A- 1 ,  32 A- 2  are spliced, at particular splice locations  36 A- 2 ,  36 A- 3 , respectively (disposed outside of the common housing structure  104 C), to corresponding respective elongated optical devices  34 A- 2 ,  34 A- 3  (such as optical fibers), and extending outside the common housing structure  14 A by a predetermined length  12 A. 
     Optionally, the novel coupler array  10 A may also include a substantially uniform diameter tip  16 A (shown between positions C and D in  FIG. 1 ) for coupling, at an array interface  18 A with the interface  42 A of an optical waveguide device  40 A. The uniform diameter tip  16 A may be useful in certain interface applications, such as for example shown in  FIGS. 1D, 4 and 5 . Alternatively, the novel coupler array  10 A may be fabricated without the tip  16 A (or have the tip  16 A removed after fabrication), such that coupling with the optical device interface  42 A, occurs at a coupler array  10 A interface at position C of  FIG. 1A . 
     In an alternative embodiment of the present invention, if the optical device  40 A comprises a double-clad fiber, when the small end of the coupler array  10 A is coupled (for example, fusion spliced) to the optical device interface  42 A, at least a portion of the common housing structure  14 A proximal to the splice position (such as at least a portion of the tip  16 A), may be coated with a low index medium (not shown), extending over the splice position and up to the double-clad fiber optical device  40 A outer cladding (and optionally extending over a portion of the double-dad fiber optical device  40 A outer cladding that is proximal to the splice position). 
     Referring now to  FIG. 1B , a second exemplary embodiment of the optical fiber coupler array of the present invention, is shown as a coupler array  10 B. The coupler array  10 B comprises a common housing structure  14 B, at least one VC waveguide, shown in  FIG. 1B  by way of example, as a single VC waveguide  30 B, and at least one Non-VC waveguide, shown in  FIG. 1B  by way of example, as a single Non-VC waveguide  32 B, disposed in parallel proximity to the VC waveguide  30 B, where a portion of the optical coupler array  10 B, has been configured to comprise a larger channel-to-channel spacing value S 2 ′ at its small end, than the corresponding channel-to-channel spacing value S 2  at the small end of the optical coupler array  10 A, of  FIG. 1A . This configuration may be readily implemented by transversely cutting the optical fiber array  10 A at a position C′, thus producing the common housing structure  14 B that is shorter than the common housing structure  14 A and resulting in a new, larger diameter array interface  18 B, having the larger channel-to-channel spacing value S 2 ′. 
     Referring now to  FIG. 1C , a third exemplary embodiment of the optical fiber coupler array of the present invention, is shown as a coupler array  10 C. The coupler array  10 C comprises a plurality of VC waveguides, shown in  FIG. 1C  as VC waveguides  30 C- 1 , and  30 C- 2 , and a plurality of Non-VC waveguides, shown in  FIG. 1C  as Non-VC waveguides  32 C- 1 ,  32 C- 2 , and  32 C-a, all disposed longitudinally and asymmetrically to one another, wherein at least a portion of the plural Non-VC waveguides are of different types and/or different characteristics (such as singlemode or multimode or polarization maintaining etc)—for example, Non-VC waveguides  32 C- 1 ,  32 C- 2  are of a different type, or comprise different characteristics from the Non-VC waveguide  32 C-a. Additionally, any of the VC or Non-VC waveguides (such as, for example, the Non-VC waveguide  32 C-a) can readily extend beyond the coupler array  10 C common housing structure by any desired length, and need to be spliced to an optical device proximally thereto. 
     Referring now to  FIG. 1D , a fourth exemplary embodiment of the optical fiber coupler array of the present invention that is configured for multi-core fan-in and fan-out connectivity, and shown as a coupler array  50 . The coupler array  50  comprises a pair of novel optical fiber coupler array components ( 10 D- 1  and  10 D- 2 ), with a multi-core optical fiber element  52  connected (e.g., by fusion splicing at positions  54 - 1  and  54 - 2 ) between the second (smaller sized) ends of the two optical fiber coupler array components ( 10 D- 1 ,  10 D- 2 ). Preferably, at least one of the VC waveguides in each of the coupler array components ( 10 D- 1 ,  10 D- 2 ) is configured to maximize optical coupling to a corresponding selected core of the multi-core optical fiber element  52 , while minimizing optical coupling to all other cores thereof. 
     Referring now to  FIG. 2A , a fifth exemplary embodiment of the optical fiber coupler array of the present invention, is shown as a coupler array  100 A. The coupler array  100 A comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure  104 A, shown by way of example only, as plural VC waveguides  130 A- 1 ,  130 A- 2 . Each plural VC waveguide  130 A- 1 ,  130 A- 2  is spliced, at a particular splice location  132 A- 1 ,  132 A- 2 , respectively, to a corresponding respective elongated optical device  134 A- 1 ,  134 A- 2  (such as an optical fiber), at least a portion of which extends outside the common housing structure  104 A by a predetermined length  102 A, and wherein each particular splice location  132 A- 1 ,  132 A- 2  is disposed within the common housing structure  104 A. 
     Referring now to  FIG. 2B , a sixths exemplary embodiment of the optical fiber coupler array of the present invention, is shown as a coupler array  1008 . The coupler array  1008  comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure  104 B, shown by way of example only, as plural VC waveguides  1308 - 1 ,  130 B- 2 . Each plural VC waveguide  1308 - 1 ,  130 B- 2  is spliced, at a particular splice location  132 B- 1 ,  132 B- 2 , respectively, to a corresponding respective elongated optical device  1348 - 1 ,  1348 - 2  (such as an optical fiber), at least a portion of which extends outside the common housing structure  1048  by a predetermined length  102 B, and wherein each particular splice location  132 B- 1 ,  132 B- 2  is disposed at an outer cross-sectional boundary region of the common housing structure  1048 . 
     Referring now to  FIG. 2C , a seventh exemplary embodiment of the optical fiber coupler array of the present invention, is shown as a coupler array  100 C. 
     The coupler array  100 C comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure  104 C, shown by way of example only, as plural VC waveguides  130 C- 1 ,  130 C- 2 . Each plural VC waveguide  130 C- 1 ,  130 C- 2  is spliced, at a particular splice location  132 C- 1 ,  132 C- 2 , respectively, to a corresponding respective elongated optical device  134 C- 1 ,  134 C- 2  (such as an optical fiber), at least a portion of which extends outside the common housing structure  104 C by a predetermined length  102 C, and wherein each particular splice location  132 C- 1 ,  132 C- 2  is disposed outside of the common housing structure  104 C. 
     Referring now to  FIG. 2D , an alternative embodiment of the optical fiber coupler array of the present invention, is shown as a coupler array  150 . The coupler array  150  comprises a plurality of longitudinally proximal VC waveguides at least partially embedded in a single common housing structure, that is configured at its second end, to optimize optical coupling to a free-space-based optical device  152 . The free-space-based optical device  152  may comprise a lens  154  followed by an additional optical device component  156 , which may comprise, by way of example, a MEMS mirror or volume Bragg grating. The combination of the coupler and the free-space-based optical device  152  may be used as an optical switch or WDM device for spectral combining or splitting of light signals  160   b  (representative of the light coupler array  150  output light signals  160   a  after they have passed through the lens  154 .) In this case, one of the fibers may be used as an input and all others for an output or vise versa. 
     Prior to describing the various embodiments of the present invention shown in  FIGS. 3A to 3L  in greater detail, it should be understood that whenever a “plurality” or “at least one” coupler component/element is indicated below, the specific quantity of such coupler components/elements that may be provided in the corresponding embodiment of the novel coupler array, may be selected as a matter of necessity, or design choice (for example, based on the intended industrial application of the coupler array), without departing from the spirit of the present invention. Accordingly, in the various  FIGS. 3A to 3L , single or individual coupler array components/elements are identified by a single reference number, while each plurality of the coupler component/elements is identified by a reference number followed by a “( 1  . . . n)” designation, with “n” being a desired number of plural coupler elements/components (and which may have a different value in any particular inventive coupler array embodiment described below). 
     Also, all the waveguides VC and Non-VC are shown with a circular cross-section of the inner and outer core and cladding only by example. Other shapes of the cross-sections of the inner and outer core and cladding (for example, hexagonal, rectangular or squared) may be utilized without departure from the current invention. The specific choice of shape is based on various requirements, such as channel shape of the optical device, channel positional geometry (for example, hexagonal, rectangular or square lattice), or axial polarization alignment mode. 
     Similarly, unless otherwise indicated below, as long as various relationships/requirements set forth below (for example, the relative volume relationship requirement set forth below with respect to optical coupler arrays  200 C and  200 D of  FIGS. 3C and 3D , respectively, and the requirement, set forth below in connection with the coupler array  200 H of  FIG. 3H , that the PM VC waveguide  204 H be positioned longitudinally off-centered transversely from the coupler array  200 H central longitudinal axis), are adhered to, the sizes, relative sizes, relative positions and choices of composition materials, are not limited to the exemplary sizes, relative sizes, relative positions and choices of composition materials, indicated bellow in connection with the detailed descriptions of the novel coupler array embodiments of  FIGS. 3A to 3L , but rather they may be selected by one skilled in the art as a matter of convenience or design choice, without departing from the spirit of the present invention. 
     Finally, it should be noted that each of the various single common housing structure components  202 A to  202 L, of the various coupler arrays  200 A to  200 L of  FIGS. 3A to 3L , respectively, may be composed of a medium having the refractive index N- 4  value in accordance with an applicable one of the above-described relationships with the values of other coupler array component refractive indices N- 1 , N- 2 , and N- 3 , and having properties and characteristics selected from the various contemplated exemplary medium composition parameters described above in connection with medium  28 A of  FIG. 1A . 
     Referring now to  FIG. 3A , a first alternative embodiment of the novel optical fiber coupler array embodiments of  FIGS. 1D to 2D , is shown as a coupler array  200 A in which all waveguides are VC waveguides. The coupler array  200 A comprises a single common housing  202 A, and plurality of VC waveguides  204 A-( 1  . . . n), with n being equal to 19 by way of example only, disposed centrally along the central longitudinal axis of the housing  202 A. The coupler array  200 A may also comprise an optional at least one fiducial element  210 A, operable to provide one or more useful properties to the novel coupler array, including, but not limited to:
         enabling visual identification (at at least one of the coupler array&#39;s ends) of the coupler array waveguide arrangement; and   facilitating passive alignment of at least one of the inventive coupler array ends to at least one optical device.       

     Furthermore, when deployed in inventive optical coupler array embodiments that comprise at least one polarization maintaining VC waveguide (such as the optical coupler array embodiments described below in connection with  FIGS. 3H-3L ), a fiducial element is further operable to:
         enable visual identification of the optical coupler array&#39;s particular polarization axes alignment mode (described in greater detail below in connection with  FIGS. 3H-3L ); and   serve as a geometrically positioned reference point for alignment thereto, of one or more polarization axis of PM waveguides in a particular optical coupler array.       

     The fiducial element  210 A may comprise any of the various types of fiducial elements known in the art, selected as a matter of design choice or convenience without departing from the spirit of the invention—for example, it may be a dedicated elongated element positioned longitudinally within the common housing structure  202 A in one of various cross-sectional positions (such as positions X or Y, shown in  FIG. 3A . Alternatively, the fiducial element  210 A may comprise a dedicated channel not used for non-fiducial purposes, for example, replacing one of the waveguides  204 A-( 1  . . . n), shown by way of example only at position Z in  FIG. 3A . 
     Referring now to  FIG. 3B , a first alternative embodiment of the novel optical fiber coupler array  10 A of  FIG. 1A , above, is shown as a coupler array  200 B, that comprises a single housing structure  202 B, and at least one VC waveguide, shown in  FIG. 3B  by way of example as a VC waveguide  204 B, and a plurality of Non-VC waveguides  206 B-( 1  . . . n), with n being equal to 18 by way of example only. The VC waveguide  204 B is positioned along a central longitudinal axis of the common housing structure  202 B, and circumferentially and symmetrically surrounded by proximal parallel plural Non-VC waveguides  206 B-( 1  . . . n). 
     Referring now to  FIG. 3C , a first alternative embodiment of the novel optical fiber coupler array  200 B of  FIG. 3B , above, is shown as a coupler array  200 C that comprises a single housing structure  202 C, a VC waveguide  204 C, and a plurality of Non-VC waveguides  206 C-( 1  . . . n), with n being equal to 18 by way of example only. The VC waveguide  204 C is positioned along a central longitudinal axis of the common housing structure  202 C, and circumferentially and symmetrically surrounded by proximal parallel plural Non-VC waveguides  206 C-( 1  . . . n). The coupler array  200 C is configured such that a volume of the common housing structure  202 C medium, surrounding the sections of all of the waveguides embedded therein (i.e., the VC waveguide  204 C and the plural Non-VC waveguides  206 C-( 1  . . . n)), exceeds a total volume of the inner and outer cores of the section of the VC waveguide  204 C that is embedded within the single common housing structure  202 C. 
     Referring now to  FIG. 3D , a first alternative embodiment of the novel optical fiber coupler array  200 C of  FIG. 3C , above, is shown as a coupler array  200 D that comprises a single housing structure  202 D, a plurality of VC waveguides  204 D-( 1 -N), with N being equal to 7 by way of example only, and a plurality of Non-VC waveguides  206 D-( 1  . . . n), with n being equal to 12 by way of example only. The plural VC waveguides  204 D-( 1 -N) are positioned along a central longitudinal axis of the common housing structure  202 D, and circumferentially and symmetrically surrounded by proximal parallel plural Non-VC waveguides  206 D-( 1  . . . n). The coupler array  200 D is configured such that a volume of the common housing structure  202 D medium, surrounding the sections of all of the waveguides embedded therein (i.e., the plural VC waveguides  204 D-( 1 -N), and the plural Non-VC waveguides  206 D-( 1  . . . n)), exceeds a total volume of the inner and outer cores of the section of the plural VC waveguides  204 D-( 1 -N) that are embedded within the single common housing structure  202 D. 
     Referring now to  FIG. 3E , a first alternative embodiment of the novel optical fiber coupler array  200 D of  FIG. 3D , above, is shown as a coupler array  200 E, that comprises a single housing structure  202 E, a plurality of VC waveguides  204 E-( 1 -N), with N being equal to 7 by way of example only, a plurality of Non-VC waveguides  206 E-( 1  . . . n), with n being equal to 11 by way of example only, and a separate single Non-VC waveguide  206 E′. The Non-VC waveguide  206 E′, is preferably operable to provide optical pumping functionality therethrough, and is positioned along a central longitudinal axis of the common housing structure  202 E and circumferentially and symmetrically surrounded by proximal parallel plural VC waveguides  204 E-( 1 -N), that are in turn circumferentially and symmetrically surrounded by proximal parallel plural Non-VC waveguides  206 E-( 1  . . . n). 
     Referring now to  FIG. 3F , a second alternative embodiment of the novel optical fiber coupler array  200 B of  FIG. 3B , above, is shown as a coupler array  200 F, that comprises a single housing structure  202 F, a plurality of VC waveguides  204 F-( 1 -N), with N being equal to 6 by way of example only, a separate single VC waveguide  204 F′, and a plurality of Non-VC waveguides  206 F-( 1  . . . n), with n being equal to 12 by way of example only, that preferably each comprise enlarged inner cores of sufficient diameter to optimize optical coupling to different types of optical pump channels of various optical devices, to which the coupler array  200 F may be advantageously coupled. The VC waveguide  204 F′, is positioned along a central longitudinal axis of the common housing structure  202 F, and circumferentially and symmetrically surrounded by proximal parallel plural VC waveguides  204 F-( 1 -N), that are in turn circumferentially and symmetrically surrounded by proximal parallel plural Non-VC waveguides  206 F-( 1  . . . n). 
     Referring now to  FIG. 3G , a third alternative embodiment of the novel optical fiber coupler array  200 B of  FIG. 3B , above, is shown as a coupler array  200 G, that comprises a single housing structure  202 G, and at least one VC waveguide, shown in  FIG. 3G  by way of example as a VC waveguide  204 G, and a plurality of Non-VC waveguides  206 G-( 1  . . . n), with n being equal to 18 by way of example only. The VC waveguide  204 G is positioned as a side-channel, off-set from the central longitudinal axis of the single common housing structure  202 G, such that optical fiber coupler array  200 G may be readily used as a fiber optical amplifier and or a laser, when spliced to a double-clad optical fiber (not shown) having a non-concentric core for improved optical pumping efficiency. It should be noted that because a double-dad fiber is a fiber in which both the core and the inner cladding have light guiding properties, most optical fiber types, such as SM, MM, LMA, or MC (multi-core), whether polarization maintaining or not, and even standard (e.g., conventional) single mode optical fibers, can be converted into a double-clad fiber by coating (or recoating) the fiber with a low index medium (forming the outer cladding). 
     Optionally, when the second end of the coupler array  200 G is spliced to a double-clad fiber (non shown), at least a portion of the common housing structure  202 G proximal to the splice point with the double-clad fiber (not-shown), may be coated with a low index medium extending over the splice point and up to the double-clad fiber&#39;s outer cladding (and optionally extending over a portion of the outer cladding that is proximal to the splice point) 
     Referring now to  FIGS. 3H to 3L , in various alternative exemplary embodiments of the optical coupler of the present invention, at least one of the VC waveguides utilized therein, and, in certain embodiments, optionally at least one of the Non-VC waveguides, may comprise a polarization maintaining (PM) property. By way of example, the PM property of a VC waveguide may result from a pair of longitudinal stress rods disposed within the VC waveguide outside of its inner core and either inside, or outside, of the outer core (or through other stress elements), or the PM property may result from a noncircular inner or outer core shape, or from other PM-inducing optical fiber configurations (such as in bow-tie or elliptically clad PM fibers). In various embodiments of the inventive optical fiber in which at least one PM waveguide (VC and/or Non-VC) is utilized, an axial alignment of the PM waveguides (or waveguide), in accordance with a particular polarization axes alignment mode may be required. 
     In accordance with the present invention, a polarization axes alignment mode may comprise, but is not limited to, at least one of the following:
         axial alignment of a PM waveguide&#39;s polarization axis to the polarization axes of other PM waveguides in the optical coupler;   when a PM waveguide is positioned off-center, axial alignment of a PM waveguide&#39;s polarization axis to its transverse cross-sectional (geometric) position within the optical coupler;   when the single common housing structure of the optical coupler comprises a non-circular geometric shape (such as shown by way of example in  FIG. 3L ): axial alignment of a PM waveguide&#39;s polarization axis to a geometric feature of the common housing structure outer shape;   in optical coupler embodiments comprising one or more waveguide arrangement indicators, described in greater detail below, in connection with  FIGS. 3J-3L : axial alignment of a PM waveguide&#39;s polarization axis to at least one geometric characteristic thereof;   in optical coupler embodiments comprising at least one fiducial element  210 A, as described in greater detail above in connection with  FIG. 3A : axial alignment of a PM waveguide&#39;s polarization axis to a geometric position of the at least one fiducial element  210 A;       

     The selection of a specific type of polarization axes alignment mode for the various embodiments of the inventive optical coupler is preferably governed by at least one axes alignment criterion, which may include, but which is not limited to: alignment of PM waveguides&#39; polarization axes in a geometric arrangement that maximizes PM properties thereof; and/or satisfying at least one requirement of one or more intended industrial application for the novel coupler array. 
     Referring now to  FIG. 3H , a first alternative embodiment of the novel optical fiber coupler array  200 G of  FIG. 3G , above, is shown as a coupler array  200 H, that comprises a single housing structure  202 H, and at least one VC waveguide, shown in  FIG. 3H  by way of example as a PM VC waveguide  204 H having polarization maintaining properties, and a plurality of Non-VC waveguides  206 H-( 1  . . . n), with n being equal to 18 by way of example only. The PM VC waveguide  204 H is positioned as a side-channel, off-set from the central longitudinal axis of the single common housing structure  202 H, and comprises a polarization axis that is aligned, by way of example, with respect to the transverse off-center location of the PM VC waveguide  204 H. 
     Referring now to  FIG. 3I , a fourth alternative embodiment of the novel optical fiber coupler array  200 B of  FIG. 3B , above, is shown as a coupler array  200 I, that comprises a single housing structure  202 I, and at least one VC waveguide, shown in  FIG. 3I  by way of example as a PM VC waveguide  204 I having polarization maintaining properties, and a plurality of PM Non-VC waveguides  206 I-( 1  . . . n), with n being equal to 18 by way of example only, each also having polarization maintaining properties. The PM VC waveguide  204 I is positioned along a central longitudinal axis of the common housing structure  202 I, and circumferentially and symmetrically surrounded by proximal parallel plural PM Non-VC waveguides  206 I-( 1  . . . n). By way of example, the coupler array  200 I comprises a polarization axes alignment mode in which the polarization axes of each of the PM VC waveguide  204 I and of the plural PM Non-VC waveguides  206 I-( 1  . . . n) are aligned to one another. The PM properties of the PM VC waveguide  204 I and of the plural PM Non-VC waveguides  206 I-( 1  . . . n) are shown, by way of example only, as being induced by rod stress members (and which may readily and alternately be induced by various other stress, or equivalent means)). 
     Referring now to  FIG. 3J , a first alternative embodiment of the novel optical fiber coupler array  200 I of  FIG. 3I , above, is shown as a coupler array  200 J, that comprises a single housing structure  202 J, and at least one VC waveguide, shown in  FIG. 3J  by way of example as a PM VC waveguide  204 J having polarization maintaining properties, and a plurality of PM Non-VC waveguides  206 J-( 1  . . . n), with n being equal to 18 by way of example only, each also having polarization maintaining properties. The PM VC waveguide  204 J is positioned along a central longitudinal axis of the common housing structure  202 J, and circumferentially and symmetrically surrounded by proximal parallel plural PM Non-VC waveguides  206 J-( 1  . . . n). The PM properties of the PM VC waveguide  204 J and of the plural PM Non-VC waveguides  206 J-( 1  . . . n) are shown, by way of example only, as resulting only from a non-circular cross-sectional shape (shown by way of example only as being at least in part elliptical), of each plural PM Non-VC waveguide  206 J-( 1  . . . n) core (and from a non-circular cross-sectional shape of the outer core of the PM VC waveguide  204 J). 
     The coupler array  200 J optionally comprises at least one waveguide arrangement indication element  208 J, positioned on an outer region of the common housing structure  202 J, that is representative of the particular cross-sectional geometric arrangement of the optical coupler array  200 J waveguides (i.e., of the PM VC waveguide  204 J and of the plural PM Non-VC waveguides  206 J-( 1  . . . n)), such that a particular cross-sectional geometric waveguide arrangement may be readily identified from at least one of a visual and physical inspection of the common coupler housing structure  202 J that is sufficient to examine the waveguide arrangement indication element  208 J. Preferably, the waveguide arrangement indication element  208 J may be configured to be further operable to facilitate passive alignment of a second end of the optical coupler array  200 J to at least one optical device (not shown). 
     The waveguide arrangement indication element  208 J, may comprise, but is not limited to, one or more of the following, applied to the common housing structure  202 J outer surface: a color marking, and/or a physical indicia (such as an groove or other modification of the common housing structure  202 J outer surface, or an element or other member positioned thereon). Alternatively, the waveguide arrangement indication element  208 J may actually comprise a specific modification to, or definition of, the cross-sectional geometric shape of the common housing structure  202 J (for example, such as a hexagonal shape of a common housing structure  202 L of  FIG. 3L , below, or another geometric shape). 
     By way of example, the coupler array  200 J may comprise a polarization axes alignment mode in which the polarization axes of each of the PM VC waveguide  204 J and of the plural PM Non-VC waveguides  206 J-( 1  . . . n) are aligned to one another, or to the waveguide arrangement indication element  208 J. 
     Referring now to  FIG. 3K , a fifth alternative embodiment of the novel optical fiber coupler array  200 B of  FIG. 3B , above, is shown as a coupler array  200 K, that comprises a single housing structure  202 K, and at least one VC waveguide, shown in  FIG. 3K  by way of example as a PM VC waveguide  204 K having polarization maintaining properties, and a plurality of Non-VC waveguides  206 K-( 1  . . . n), with n being equal to 18 by way of example only. The PM VC waveguide  204 K is positioned along a central longitudinal axis of the common housing structure  202 K, and circumferentially and symmetrically surrounded by proximal parallel plural PM Non-VC waveguides  206 K-( 1  . . . n). The PM properties of the PM VC waveguide  204 K are shown, by way of example only, as being induced by rod stress members (and which may readily and alternately be induced by various other stress, or equivalent means)). The coupler array  200 K, may optionally comprise a plurality of waveguide arrangement indication elements—shown by way of example only, as waveguide arrangement indication elements  208 K-a and  208 K-b, which may each be of the same, or of a different type, as described in greater detail above, in connection with the waveguide arrangement indication element  208 J of  FIG. 3J . 
     Referring now to  FIG. 3L , a second alternative embodiment of the optical fiber coupler array  200 I of  FIG. 3I , above, is shown as a coupler array  200 L, that comprises a single housing structure  202 L comprising a cross section having a non-circular geometric shape (shown by way of example as a hexagon), and at least one VC waveguide, shown in  FIG. 3L  by way of example as a PM VC waveguide  204 L having polarization maintaining properties, and a plurality of PM Non-VC waveguides  206 L-( 1  . . . n), with n being equal to 18 by way of example only, each also having polarization maintaining properties. The PM VC waveguide  204 L is positioned along a central longitudinal axis of the common housing structure  202 L, and circumferentially and symmetrically surrounded by proximal parallel plural PM Non-VC waveguides  206 L-( 1  . . . n). 
     By way of example, the coupler array  200 L comprises a polarization axes alignment mode in which the polarization axes of each of the PM VC waveguide  204 L and of the plural PM Non-VC waveguides  206 L-( 1  . . . n) are aligned to one another, and to the common housing structure  202 L cross-sectional geometric shape. The PM properties of the PM VC waveguide  204 L and of the plural PM Non-VC waveguides  206 L-( 1  . . . n) are shown, by way of example only, as being induced by rod stress members (and which may readily and alternately be induced by various other stress, or equivalent means)). The coupler array  200 K, may optionally comprise a waveguide arrangement indication element  208 L-a which may comprise any of the configurations described in greater detail above, in connection with the waveguide arrangement indication element  208 J of  FIG. 3J . 
     Referring now to  FIG. 4 , a second end  302  (i.e. “tip”) of the inventive optical fiber coupler array is shown, by way of example, as being in the process of connecting to plural vertical coupling elements  306  of an optical device  304  in a proximal open air optical coupling alignment configuration, that may be readily shifted into a butt-coupled configuration through full physical contact of the inventive optical fiber coupler array second end  302  and the vertical coupling elements  306 . 
     Referring now to  FIG. 5  a second end  322  (i.e. “tip”) of the inventive optical fiber coupler array is shown, by way of example, as being in the process of connecting to plural edge coupling elements  326  of an optical device  324  in a butt-coupled configuration, that may be readily shifted into one of several alternative coupling configuration, including a proximal open air optical coupling alignment configuration, and or an angled alignment coupling configuration. 
     In at least one alternative embodiment of the present invention, the inventive optical coupler array (i.e., such as optical coupler arrays  200 D to  200 L of  FIGS. 3C to 3L ) may be readily configured to pump optical fiber lasers, and/or optical fiber amplifiers (or equivalent devices). In a preferred embodiment thereof, a novel pumping-enabled coupler array comprises a central channel (i.e., waveguide), configured to transmit a signal (i.e., serving as a “signal channel”) which will thereafter be amplified or utilized to generate lasing, and further comprises at least one additional channel (i.e., waveguide), configured to provide optical pumping functionality (i.e., each serving as a “pump channel”). In various exemplary alternative embodiments thereof, the novel pumping-enabled coupler array may comprise the following in any desired combination thereof:
         at least one of the following signal channels: a single mode signal channel configured for optimum coupling to a single mode amplifying fiber at at least one predetermined signal or lasing wavelength, a multimode signal channel configured for optimum coupling to a multimode amplifying fiber at at least one predetermined signal or lasing wavelength, and   at least one of the following pumping channels: a single mode pumping channel configured for optimum coupling to a single mode pump source at at least one predetermined pumping wavelength, a multimode pumping channel configured for optimum coupling to a multimode pump source at at least one predetermined pumping wavelength.       

     Optionally, to maximize pumping efficiency, the novel pumping-enabled coupler array may be configured to selectively utilize less than all the available pumping channels. 
     It should also be noted that, as a matter of design choice, and without departing from the spirit of the invention, the novel pumping-enabled coupler array may be configured to comprise:
         a. At least one signal channel, each disposed in a predetermined desired position in the coupler array structure;   b. At least one pumping channel, each disposed in a predetermined desired position in the coupler array structure; and   c. Optionally—at least one additional waveguide for at least one additional purpose other than signal transmission or pumping (e.g., such as a fiducial marker for alignment, for fault detection, for data transmission, etc.)       

     Advantageously, the pump channels could be positioned in any transverse position within the coupler, including along the central longitudinal axis. The pump channels may also comprise, but are not limited to, at least one of any of the following optical fiber types: SM, MM, LMA, or VC waveguides. Optionally, any of the optical fiber(s) being utilized as an optical pump channel (regardless of the fiber type) in the novel coupler may comprise polarization maintaining properties. 
     In yet another exemplary embodiment of the present invention, the novel pumping-enabled coupler array may be configured to be optimized for coupling to a double-clad fiber—in this case, the signal channel of the coupler array would be optimized for coupling to the signal channel of the double-clad fiber, while each of the at least one pumping channels would be optimized to couple to the inner cladding of the double-clad fiber. 
     In essence, the novel optical coupler arrays, shown by way of example in various embodiments of the present invention, may also be readily implemented as high density, multi-channel, optical input/output (I/O) for fiber-to-chip and fiber-to-optical waveguides. The inventive optical fiber couplers may readily comprise at least the following features:
         Dramatically reduced channel spacing and device footprint (as compared to previously known solutions)   Scalable channel count   All-glass optical path   Readily butt-coupled or spliced at their high density face without the need of a lens, air gap, or a beam spreading medium   May be fabricated through a semi-automated production process   Broad range of customizable parameters: wavelength, mode field size, channel spacing, array configuration, fiber type.       

     The inventive optical fiber couplers may be advantageously utilized for at least the following applications, as a matter of design choice or convenience, without departing from the spirit of the invention:
         Coupling to waveguides:
           PIC or PCB-based (single-mode or multimode)   Multicore fibers   Chip edge (ID) or chip face (2D) coupling   NA optimized for the application, factoring in:
               Packaging alignment needs   Chip processing needs/waveguide up-tapering   
               Polarization maintaining properties may be readily configured   
           Coupling to chip-based devices: e.g. VCSELs, photodiodes, vertically coupled gratings   Laser diode coupling   High density equipment Input/Output (I/O)       

     In conclusion, when implemented, the various exemplary embodiments of the inventive optical fiber couplers comprise at least the following advantages, as compared to currently available competitive solutions:
         Unprecedented density   Low-loss coupling (≦0.5 dB)   Operational stability   Form factor support   Broad spectral range   Matching NA   Scalable channel count   Polarization maintenance       

     Referring now to  FIG. 7 , at least one exemplary embodiment of the inventive optical coupler is shown as a phase locking optical fiber coupler  450 , configured for use in applications requiring coherent combining of multiple fiber laser sources to produce superior high power single mode laser sources. 
     The vanishing core approach, described in &#39;099 Patent, allows the creation of a pitch reducing optical fiber array (PROFA) interconnect—an optical device, which is fusion spliceable to an output glass optical element, comprising a reflector, serving as a laser output surface. As is shown in  FIG. 7 , this creates a monolithic structure, which maintains stability with respect to environmental fluctuations, including temperature variations and mechanical shock and vibration. 
     While in operation, the phase of all optical waveguides of the phase locking optical fiber coupler  450  become locked at the output surface of the common reflector, creating a high intensity in the far field. Some other benefits of utilizing a PROFA as a component of a Talbot cavity, include flexibility in the design parameters, enabling optimization of the array geometry and fill factor to achieve high efficiency. Also, the PROFA fabrication process enables highly accurate placement of the optical waveguides, which is a critical parameter in the maximizing Talbot cavity efficiency. For example, a better than one micron positional accuracy has been demonstrated for PROFA components with over sixty (60) waveguide channels 
     In various embodiments thereof, the phase locking optical fiber coupler  450  can be readily utilized in multiple laser configurations, including a single master oscillator power amplifier (MOPA) configuration for one of the channels and amplifying cavities for the other channels, multiple MOPAs, or multiple laser cavities. 
     Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.