Abstract:
An apparatus for optical coupling having a first array of individual waveguides optically communicating with a free space region at a first junction and each waveguide having a tapered region proximate the junction where a gap spacing between tapered regions of adjacent individual waveguides is substantially constant is proposed. The gap spacing is a minimum amount of space between adjacent individual waveguides. The first array of individual waveguides also has a horn region immediately proximate the tapered region and opposite the first junction with an increasing width whose maximum establishes the gap spacing.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to optical transmission systems and, more specifically, to low insertion loss optical couplers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical couplers are optical transmission system components used to connect planar arrangements of waveguides. As shown in FIG. 1, a star coupler  100  (discussed in detail in U.S. Pat. No. 4,904,042, issued Feb. 27, 1990 to Dragone, and herein incorporated by reference) has a free space region  110  bounded by an input junction  102  for receiving incoming signal(s) from a plurality of individual input waveguides  106  and an output junction  104  for power splitting the input signal(s) and/or coupling portions of the signal(s) to a plurality of individual output waveguides  108 . Insertion loss (a reduction in the power of a signal propagating through the coupler  100 ) typically occurs at the input junction  102  because of the abrupt change in the physical dimensions of the individual input waveguides as compared to the free space region  110 . The abrupt change causes a scattering of light associated with the signal, subsequently reducing power.  
           [0003]    In a theoretically ideal coupler, waveguides approaching a free space region are nearly parallel to one another. The waveguides are shaped so that they are narrow at first and then increase in width until the gap between them is zero (a point which defines for example, the input junction). Zero gap width along with non-converging waveguides provides for a theoretical insertion loss of zero.  
           [0004]    As shown in FIG. 2, the input junction  102  to the free space region  110  of conventional star coupler  100  is essentially arc-shaped. A first individual waveguide  106   1  and a second individual waveguide  106   2  are shown in detail as having respective uniform waveguide sections  202  and horn sections  204 . The uniform waveguide sections  202  have a substantially uniform waveguide width Wu. The horn sections  204  change to a different width Wt as they extend from the uniform waveguide sections  202  to the input junction  102 . The radial nature of the geometry of the individual waveguides along the input junction  102  forces them to converge too quickly for the transition to be gradual and thus truly have zero loss. Additionally, the limitations of lithography (a process used to fabricate coupler  100 ) create a non-zero gap  206  between the first and second individual waveguides  106   1 ,  106   2 . Even if lithography permitted a zero gap, an average value of this gap  206  as the waveguides converge would be finite which is counter to the desired theoretical zero value gap (i.e., the waveguides are overtly non-parallel); thus, creating the insertion loss condition.  
         SUMMARY OF THE INVENTION  
         [0005]    These and other deficiencies of the prior art are addressed by the present invention of an apparatus for optical coupling having a first array of individual waveguides optically communicating with a free space region at a first junction, each waveguide having a tapered region proximate the junction where a gap spacing between tapered regions of adjacent individual waveguides is substantially constant. In one embodiment, the tapered region of each individual waveguide has a length D of approximately 250 μm. The gap spacing is a minimum amount of space between adjacent individual waveguides and in one embodiment of the invention in the range of approximately 1.5-3.5 μm. The first array of individual waveguides also has a horn region immediately proximate the tapered region and opposite the first junction with a changing waveguide width. The first array of individual waveguides also has a waveguide region immediately proximate the horn region and opposite the tapered region with a width that is nearly uniform along its entire length. In one embodiment, the width of the tapered region is decreased (tapers) as a function of length as it extends away from the horn region. The apparatus may also have a second array of individual waveguides communicating with the free space region at a second junction. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0007]    [0007]FIG. 1 depicts a schematic diagram of an optical coupler according to the prior art;  
         [0008]    [0008]FIG. 2 depicts a detailed view of the optical coupler of FIG. 1;  
         [0009]    [0009]FIG. 3 depicts a diagram of an optical coupler according to one embodiment of the present invention; and  
         [0010]    [0010]FIG. 4 depicts a detailed view of the optical coupler of FIG. 3. 
     
    
       [0011]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The subject invention will be primarily described within the context of an optical star coupler which may be used in wavelength-division multiplexed (WDM) and dense WDM (DWDM) optical communications systems. However, it will be appreciated by those skilled in the art that the invention may be advantageously employed in any optical communications system in which it is desirable to minimize insertion losses at the coupling of two or more optical components.  
         [0013]    [0013]FIG. 3 depicts a diagram of an optical star coupler  300  according to the one embodiment of the present invention. Specifically, the coupler  300  of FIG. 3 comprises a plurality of individual input waveguides  302  defined by a plurality of waveguide walls  312 , a free space region  310  and a plurality of individual output waveguides  304  also defined by a plurality of waveguide walls  312 . The plurality of input waveguides  302  communicate with the free space region  310  at a first surface  306  that defines an input junction IJ. The plurality of output waveguides  304  communicate with the free space region  310  at a second surface  308  that defines an output junction OJ.  
         [0014]    Planar waveguides such as input waveguides  302  and output waveguides  304  comprise, illustratively, doped-silica waveguides fabricated with silicon optical bench (SiOB) technology and the like. Doped-silica construction is preferred due to a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber. However, this coupler can be made using any planar lightwave circuit technology, such as silicon-on-insulator, indium phosphide, polymer, and lithium niobate.  
         [0015]    The performance of the coupler  300  is contingent upon the waveguide dimensions, such as the height and width of the waveguide core and the refractional difference of the refractive index between the core and the cladding of the waveguide. However, it is the gap spacing GS between the individual waveguides (input  302  and/or output  304 ) and more specifically the waveguide walls  312  that is the primary focus of the subject invention. Specifically, the individual waveguides ( 302  and/or  304 ) are fashioned such that their widths are increased until the gap spacing GS between each individual waveguide is at an optimum distance for maintaining substantially parallel waveguide walls proximate the input junction IJ. In one embodiment of the invention, the gap spacing GS is in the range of approximately 0.5-3.5 μm and in one embodiment, preferably 2.5 μm. The gap spacing in another embodiment is the minimum spacing attainable by lithography techniques and may be any width considered applicable and optimal to achieve the desired low insertion loss characteristics.  
         [0016]    Once the optimal gap spacing GS has been established, it is maintained along adjacent individual waveguides for a predetermined distance D. In one embodiment of the invention the desired distance is in the range of approximately 100-2000 μm and in one embodiment preferably 250 μm. The optimal gap spacing GS is maintained until the individual waveguides reach the free space region  310 . This improvement is realized by reshaping each of the individual waveguides in such a manner so as to slow down the rapid convergence of the waveguide walls near the free space regions thus reducing the average gap spacing therebetween to a minimum. Although FIG. 3 draws the coupler as being symmetric, it is possible that only one side of the coupler uses the proposed tapering design.  
         [0017]    [0017]FIG. 4 depicts a detailed view of a portion of the optical coupler  300  of FIG. 3 to more clearly depict the physical characteristics described above. Specifically, a portion of the plurality of individual input waveguides  302  are shown proximate the first surface  306  that defines the input junction IJ. Each of the individual input waveguides  302  are segmented into three regions. A first region  402  comprises a primary waveguide region that extends for a majority of the waveguide&#39;s length at a substantially uniform width Wu to propagate signals in the expected manner. A second region  404  comprises a horn region that extends from an end of the primary waveguide region  402  to a beginning of a third region  406  and has an increasing width, typically with a quadratic change in width with length. The third region  406  comprises a tapered region that extends from the end of the horn region  404  to the interface junction IJ. The width of the horn region  404  increases from the waveguide&#39;s uniform width Wu to a maximum width W MX  at the beginning of the third region  406 . This maximum width W MX  is realized in each of the plurality of individual input waveguides  302  such that W MX  corresponds to the maximum width of the waveguide resulting in the optimal amount of gap spacing GS. Preferably, the optimal gap spacing GS is that which is possible under known lithographic techniques and is finite as discussed above.  
         [0018]    The third (tapered) region  406  exhibits an overall width W MT  that is varied amongst the input waveguides  302  so as to maintain the gap spacing GS between each of the individual input waveguides  302  along their respective predetermined distances D. That is, the tapered region  406  width W MT  is varied slightly in each individual input waveguide  302  to compensate for the small differences in the linear path of each individual input waveguide  302  as it follows along the arced surface  306  that forms the input junction IJ. In one embodiment of the invention, W MT  is decreased along length D as the individual waveguides extend away from the second region  404  in order to substantially maintain a constant gap. In one embodiment of the invention, W MT  is decreased in the range of approximately 0.5-1.0 μm with respect to W MX . In summation, a relationship between W MT  and D is realized to optimize the invention.  
         [0019]    Specifically,  
       D   =       (     3        W   MT   2       )     λ                           
 
         [0020]    gives a reasonable approximation of D where λ is the wavelength of the light propagating through the specific waveguide. It will be understood by those skilled in the art that similar physical arrangement and descriptions are applicable (but not necessary) to a portion of the plurality of individual output waveguides  304  as they meet the second surface  308  that defines the output junction OJ.  
         [0021]    The subject invention and arrangement of individual waveguides discussed above is effective for minimizing insertion loss. Two-dimensional simulations have shown that signal strength may be improved by ˜0.3 dB per coupler when the device is fabricated with the physical construction as described. By keeping the gap spacing GS nearly constant along the predetermined distance D, the individual waveguides have a more parallel physical orientation to one another than realized in the prior art. This physical orientation lowers the average (or effective) gap between individual waveguides in the coupler as compared to the gap  206  of the prior art coupler of FIG. 2. Thus the transition between the waveguides and the free space region is less abrupt, resulting in less optical light scattered into unwanted spatial diffraction orders at the waveguide-free space boundary. In other words, the subject invention creates individual waveguides that do not converge as quickly as the prior art; thus resulting in a junction to the free space region that is not as abrupt as previously realizable. This physical characteristic results in reduced dispersion of light emanating from the individual waveguides at the input junction IJ and lower insertion loss.  
         [0022]    This loss reduction technique can be combined with other techniques, such as waveguide segmentation described in U.S. Pat. No. 5,745,618, issued Apr. 28, 1988 to Y. P. Li in order to further reduce the loss.  
         [0023]    Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.