Abstract:
In a star coupler, a Free Propagation Region (FPR) is bounded by a first interface and a second opposing interface, and guides an input signal launched from the first interface in a predetermined first plane while allowing the input signal to travel unguided in a predetermined second plane in the FPR which is orthogonal to the first plane. A plurality of output waveguides are formed in an array and terminate at the second interface of the FPR. The axis of each output waveguide at the second interface is separated from an axis of an adjacent output waveguide by a predetermined distance “t”. An input waveguide is split into a plurality of subsections which each terminate at the first interface of the FPR. The subsections of the input waveguide are arranged for simultaneously launching parts of the input signal into the FPR which diffracts and produces mode patterns at the second interface having a maximum intensity at inputs of each of the output waveguides, and a low intensity elsewhere.

Description:
FIELD OF THE INVENTION 
     The present invention relates to method and apparatus for providing reduced insertion loss in planar star couplers. 
     BACKGROUND OF THE INVENTION 
     Referring now to  FIG. 1 , there is shown a exemplary prior art planar star coupler  10  (or one input-to-n output signal coupler) comprising a Free Propagation Region (FPR)  12 , an input waveguide  13 , and a plurality of “i” output waveguides  14   a - 14   i  for use with lightwave transmission. The FPR  12  comprises a first interface  15  having a partial cylindrical shape on which an output of the input waveguide  13  is terminated, and a second opposing interface  16  having a partial cylindrical shape. The second opposing interface  16  has a radius related to that of the first interface and is spaced apart at a distance R from the first interface  15 . The second opposing interface  16  terminates the inputs to the plurality of “i” output waveguides  14   a - 14   i . Inputs  18   a - 18   i  to the plurality of “i” waveguides  14   a - 14   i , respectively, have their axis spaced apart from the axis of any one of the adjacent output waveguides  14   a - 14   i  by a distance “t”. The input waveguide  13  has a width W1, and an input of each one of the output waveguides  14   a - 14   i  has a width W2 at the second interface  16 , and then tapers to a width W3. 
     The FPR  12  of the planar star coupler  10  is essentially a slab waveguide, and tends to guide light in a vertical plane (in a direction out of the paper in  FIG. 1 ) but allows light to travel unguided in the horizontal plane (in the plane of the paper of FIG.  1 ). In operation, an optical signal is launched into the input of the waveguide  13  and propagates in the input waveguide  13  to the output at the first interface  15  of the FPR  12 . Upon entering the FPR  12 , the optical signal diffracts freely in the horizontal plane. As the light reaches the second interface  16 , each of the output waveguides  14   a - 14   i  is placed to accept a portion of light incident upon its input  18   a - 18   i , respectively. The fraction of light that overlaps the inputs  18   a - 18   i  of the waveguides  14   a - 14   i , respectively, then gets coupled into the respective waveguides  14   a - 14   i . The light pattern at the second interface  16  can be defined as a Fourier transform of the light pattern at the input waveguide  13 . 
     Referring now to  FIG. 2 , there is shown a mode profile  20  of an input signal at the first interface  15  of the FPR  12 , and a mode profile  22  at the second interface  16  of the star coupler  10  of FIG.  1 . In a conventional waveguide, the mode profile is nominally Gaussian shaped and, accordingly, the diffracted pattern has a nominal Gaussian profile as is shown for the mode profiles  20  and  22 . 
     Referring now to  FIG. 3 , there is shown a diagram of a first circle  25  and a second inner circle  26 , called the Rowland circles, which illustrate the technique of forming the FPR  12  of FIG.  1 . The first circle  25  has a radius r, where a portion of the circumference thereof forms the arc of the second interface  16  of the FPR  12 . The second inner circle  26  has a diameter R which may or may not (depending on the design) intersect the center and the circumference of the first circle where the second interface  16  is located as is shown in  FIG. 3. A  portion of the circumference of the second inner circle  26  opposite the second interface  16  forms the arc of the first interface  15  of the FPR  12 . 
     Referring now to  FIG. 4 , there is shown a function of a mode profile  22  (shown in  FIG. 2 ) of light found at the second interface  16  of the star coupler  10  of  FIG. 1 , and periodic mode profiles  24   a - 24   i  of light at the inputs  18   a - 18   i  of the output waveguides  14   a - 14   i , respectively, at the second interface  16  of the FPR  12  of FIG.  1 . The X axis is shown in units of microns, whereas the Y axis is shown in normalized values of intensity. With the mode profiles  24   a - 24   i , the light incident on the portion of the second interface  16  where there is no input to any one of the waveguides  14   a - 14   i  is not coupled and is lost by reflection and leakage and, therefore, produces an insertion loss. 
     It is desirable to provide a planar star coupler with a reduced insertion loss from that found in prior art star couplers. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to method and apparatus for providing reduced insertion loss in planar star couplers from that found in conventional star couplers. 
     From a first apparatus aspect, the present invention is a one-to-n signal coupler comprising a Free Propagation Region (FPR). The FPR comprises edge interfaces which define at least two input ports which terminate at least two separate subsections from a single input waveguide, and a plurality of n output ports which provide inputs to a plurality of n output waveguides, respectively, where n is an integer greater than 1. Each of the n output ports are separated from each other and intersect different areas of a second portion of the edge interfaces defining the FPR. The at least two input ports intersect a first portion of the edge interfaces opposite the second portion thereof and are separated by a predetermined distance from each other. When portions of an input signal are launched into the FPR from each of the at least first and second input ports, the signal diffracts in the FPR to generate a high intensity signal at each of the n output ports and a low intensity signal elsewhere. 
     From a second apparatus aspect, the present invention is a star coupler comprising a Free Propagation Region (FPR), a plurality of n output waveguides, and an input waveguide. The FPR comprises a first interface and a second opposing interface for guiding an input signal launched from the first interface in a predetermined first plane while allowing an input signal to travel unguided in a predetermined second plane in the FPR which is orthogonal to the first plane. The plurality of n output waveguides are formed in an array which terminate at the second interface of the FPR where an axis of each output waveguide is separated from an axis of an adjacent output waveguide by a predetermined distance “t”. The input waveguide is split into a plurality of subsections which each terminate at the first interface of the FPR. The input waveguide is arranged for simultaneously launching parts of the input signal into the FPR from the plurality of subsections that produce mode patterns at the second interface having a maximum intensity at inputs of each of the n output waveguides and having a low intensity elsewhere. 
     From a third apparatus aspect, the present invention is a planar star coupler comprising a Free Propagation Region (FPR), a plurality of n output waveguides, and an input waveguide. The FPR comprises a first interface and a second opposing interface for guiding an input signal launched from the first interface in a predetermined first plane while allowing the input signal to travel unguided in a predetermined second plane in the FPR which is orthogonal to the first plane. The plurality of n output waveguides are formed in an array which terminate at the second interface of the FPR. The axis of each output waveguide at the second interface is separated from an axis of an adjacent output waveguide by a predetermined distance “t”. The input waveguide is split into at least first and second subsections which terminate at the first interface of the FPR. The input waveguide is arranged for simultaneously launching parts of the input signal into the FPR from the at least first and second subsections that produce mode patterns at the second interface having a maximum intensity at inputs of each of the output waveguides and having a low intensity elsewhere. 
     From a fourth apparatus aspect, the present invention is a one-to-n coupler comprising a housing member, an input port, and a plurality of n separated output ports. The housing member has walls which enclose a Free Propagation Region (FPR) and define the input port and the plurality of n separated output ports. The input port comprises at least two separated input waveguide channels which couple the input port into predetermined different areas of a first portion of the walls of the housing member. The plurality of n output ports are separated from each other and intersect different areas of a second portion of the walls. The first and second portions of the walls are located opposite each other such that, with a signal introduced at the input port, essentially signals enter the FPR from the at least two input waveguide channels and generate at each of the n output ports a high intensity signal and a low intensity signal elsewhere. 
     From a method aspect, the present invention is a method of transmitting a single input signal to n output ports in a star coupler. In the method, portions of the single input signal are concurrently transmitted from at least two input ports into a Free Propagating Region (FPR) of the star coupler. In the FPR, each of the portions of the input signal are caused to diffract such that each of the n output ports receives a high intensity signal and a low intensity signal is received elsewhere. 
     The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an arrangement of an exemplary prior art planar star coupler with one input waveguide and a plurality of output waveguides; 
         FIG. 2  illustrates mode profiles of an input signal at first and second interfaces, respectively, of the planar star coupler of  FIG. 1 ; 
         FIG. 3  shows a diagram of a first circle  25  and a second inner circle  26 , called the Rowland circles, which illustrate the technique of forming the FPR  12  of  FIG. 1 ; 
         FIG. 4  shows a function of the far field diffraction pattern found at the second interface, and periodic mode profiles of light at inputs of arrayed output waveguides at the second interface of the prior art star coupler of  FIG. 1 ; 
         FIG. 5  shows a cross-sectional view of an input waveguide for use in an exemplary planar optical star coupler in accordance with the present invention; 
         FIG. 6  shows an exemplary planar optical star coupler including the input waveguide of  FIG. 4  in accordance with the present invention; 
         FIG. 7  shows enlarged mode profiles of the far field diffraction pattern two coherent in-phase input signals in a Free Propagating Region (FPR) and periodic mode profiles of arrayed output waveguides in the star coupler of  FIG. 6  in accordance with the present invention; and 
         FIG. 8  shows enlarged mode profiles of the far field diffraction pattern of two coherent input signals with 10 degrees phase difference in a Free Propagating Region (FPR) and periodic mode profiles of arrayed output waveguides in the star coupler of  FIG. 6  in accordance with the present invention. 
     
    
    
     The drawings are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 5 , there is shown a view of an input waveguide  30  for use in an exemplary planar optical star coupler (e.g., the planar star coupler  10  of  FIG. 1 ) in accordance with the present invention. The input waveguide  30  comprises a width W1 for propagating an input signal along a section  31 , and splits into a first subsection  31   a  terminating at a first output port  32   a , and a second subsection  31   b  terminating at second output port  32   b  which are spaced apart by a predetermined distance “d” at ports  32   a  and  32   b . In operation, an input signal propagating in section  31  is divided into first and second parts which propagate along subsections  31   a  and  31   b  and are launched from the first and second output ports  32   a  and  32   b , respectively. Although only first and second ports  32   a  and  32   b  are shown for purposes of simplicity, it is to be understood that the input waveguide  30  can comprise more than two waveguide subsections and associated output ports (not shown). The splitting of input waveguide  31  to a plurality of waveguides can be made using different methods such as Y-junctions, directional couplers, multimode interference (MMI), etc. 
     Referring now to  FIG. 6 , there is shown a cross-sectional view of an exemplary planar optical star coupler  40  comprising the input waveguide  30  of  FIG. 5  in accordance with the present invention. The star coupler  40  comprises a Free Propagation Region (FPR)  42 , the input waveguide  30 , and a plurality of “i” output waveguides  44   a  to  44   i . The FPR  42  comprises a first interface  45  having a partial cylindrical shape on which the output of the input waveguide  30  is located, and a second opposing interface  46  having a partial cylindrical shape. The second opposing cylindrical interface  46  has a corresponding radius to that of the first cylindrical interface  45  and is spaced apart at a radial distance R from the first interface  45 . The second opposing interface  46  includes inputs  48   a - 48   i  with one input being in communication with one of the plurality of i output waveguides  44   a - 44   i , and with the centers of the inputs  48   a - 48   i  being spaced apart by a distance “t”. The inputs  48   a - 48   i  each have a width W2, and the output of each of the waveguides  44   a - 44   i  then tapers to a width W3. This taper is not necessary, but a general engineering practice is to reduce loss by including the taper in the conventional devices as are presently made. A main portion of the coupler  40  may be considered a housing member having walls  45  and  46  which define the input ports  32   a  and  32   b  from input channels  31   a  and  31   b , n separated output ports  48   a - 48   i , and the FPR  42 . 
     The FPR  42  of the star coupler  40  is essentially a slab waveguide, and tends to guide light in a vertical plane (in a direction out of the paper in  FIG. 6 ) but allows light to travel unguided in the horizontal plane (in the plane of the paper of FIG.  6 ). In operation, an optical signal propagating in the input waveguide  30  is divided into two parts and propagates to the first and second ports  32   a  and  32   b  at the first interface  45  of the FPR  42 . Upon entering the FPR  42 , the optical signal diffracts freely in the horizontal plane. As the light reaches the second interface  46 , each of the output waveguides  44   a - 44   i  is positioned to accept a portion of light incident upon its input  48   a - 48   i , respectively. The fraction of light that overlaps the inputs  48   a - 48   i  of each of the waveguides waveguide  44   a - 44   i  then gets coupled into the respective waveguide  44   a - 44   i . The light pattern at the second interface  46  can be defined as a Fourier transform of the light pattern launched by output ports  32   a  and  32   b  of the input waveguide  30 . 
     In accordance with the present invention, a mechanism is provided where the light pattern incident on the second interface  46  is modified to match the mode profile of the array of output waveguides  44   a - 44   i  such that the intensity of the launched light is maximum at the inputs  48   a - 48   i  of the waveguides  44   a - 44   i , respectively, and a low intensity at locations where a waveguide  44   a - 44   i  is absent. This objective is achieved by the splitting of the input waveguide  30  into the two separate waveguides (channels) in a way that the light is launched into the FPR  42  from more than one source (output ports  32   a  and  32   b ). As the light from the two adjacent ports  32   a  and  32   b  diffracts and arrives at the second interface  46 , the light from each of the ports  32   a  and  32   b  interfere with each other and form an interference pattern. By designing the separation “d” of ports  32   a  and  32   b  of the input waveguide  30 , it is possible to construct an interference pattern at the second interface  46  to match the mode profile of the output waveguides  44   a - 44   i . As will be clear from the below discussion, this results in a substantial reduction of the insertion loss from that obtained by the conventional planar start coupler  10  shown in FIG.  1 . 
     The intensity of the light along the second interface  46  of the star coupler  40  is proportional to cos 2 (δ/2), where δ=(2πnxd)/(λR), where n is the index of refraction for the FPR  42 , x is the distance along the arc of the second interface  46 , d is the distance between the input ports  32   a  and  32   b , and R is the diameter of the outer of two circles called the Rowland circles forming the FPR  42  as is shown in FIG.  3 . The light interference pattern maxima and minima are obtained when (δ/2)=mπ which yields maxima of x=(mRλ)/(nd) where |m|=0, 1, 2 . . . , and minima of x=(mRλ)/(nd) where m=½, {fraction (3/2)}, {fraction (5/2)}. . . . To ensure that light maxima are aligned with the output waveguides  44   a - 44   i , it is necessary to ensure that periodicity of interference pattern matches that of output waveguide profiles  24   a - 24   i , t=(mRλ)/(nd), and the phase difference between two coherent sources at  32   a  and  32   b  interfaces is adjusted accordingly. Exemplary values for the start coupler  40  of  FIG. 6  are t=16.5 μm, R=5 mm, d=300 μm, λ=1.55 μm, and n=1.55. 
     Referring now to  FIG. 7 , there are shown an enlarged periodic mode profile  52  of the arrayed output waveguides  44   a - 44   i  in the star coupler  40  of  FIG. 6 , and an enlarged mode profile  53  of the far field diffraction pattern of two coherent in-phase input signals from input port  32   a  and  32   b  in the Free Propagation Region  42  of  FIG. 6  in accordance with the present invention.  FIG. 7  displays a same information to that shown in  FIG. 4  for the conventional star coupler  10  of FIG.  1 . The overlap between mode profiles  52  and  53  for the present star coupler  40  with two input ports  32   a  and  32   b  is larger than the overlap between mode profiles  20  and  24   a - 24   i  with the one input port  13  of FIG.  4 . 
     Referring now to  FIG. 8 , there are shown an enlarged mode profiles  62  of the arrayed output waveguides  44   a - 44   i  in the star coupler  40  of  FIG. 6 , and a mode profile  63  of the far field diffraction pattern of two coherent input signals with a 10 degree phase error from input ports  32   a  and  32   b  in the Free Propagation Region (FPR)  42  in  FIG. 6  in accordance with the present invention.  FIG. 8  displays a same information to that shown in  FIG. 4  for the conventional star coupler  10  of FIG.  1 . The overlap between mode profiles  62  and  63  for the present star coupler  40  with two input ports  32   a  and  32   b  are larger than the overlap between mode profiles  20  and  24   a - 24   i  of FIG.  4 . This shows that the current design is relatively insensitive to the phase fluctuations between two coherent input sources. Therefore, with the present planar star coupler  40  having two input ports  32   a  and  32   b , there is an excellent match of the mode profiles shown in  FIGS. 6 and 7 , and lower loss is achieved from that found in the conventional start coupler  10  of FIG.  1 . 
     The loss reduction can be exhibited by calculating the overlap integral for the mode profile (M) for the case where there is only one source (a conventional star coupler as shown in FIG.  1 ), and the overlapping integral where there are two separate waveguides (present star coupler shown in  FIG. 6 ) using the following formula (1).
 
 M=|∫E   1   E   2   *ds|   2   +|∫E   1   E   1   *ds∫E   2   E   2   *ds|   (1) 
 
Table I shows the calculated mode profile overlap M for each case where W1=W2=5 μm, d=300 μm, t=16.5 μm, and R=5 mm.
 
                                                       TABLE I                       Mode Overlap   Loss   Loss           (M)   (dB)   Reduction                                    Conventional Approach   0.468   3.40           Double Waveguide   0.660   2.18   1.22       Double Waveguide (10° phase error)   0.657   2.20   1.30                    
From Table I it can be seen that the theoretical loss can be reduced by 1.22 dB (25%) using dual input waveguide ports  32   a  and  32   b . There is also shown that if there is a 10 degree phase error between the signals in the two waveguide ports  32   a  and  32   b , the loss reduction is changed by only 0.02 dB which indicates that the robustness of the present design.
 
     It is to be appreciated and understood that the specific embodiments of the present invention described hereinabove are merely illustrative of the general principles of the present invention. Various modifications may be made by those skilled in the art which are consistent with the principles set forth. For example, for a better control over the output spectrum, the present invention can be implemented using more than two input waveguides. The dual, or multiple, modes at the input waveguide can also be obtained using a Long Period Grating (LPG) mode transformer, or any other method, to convert a single mode shape into a higher order mode such as a super-mode coupler having two or more adjacent waveguides. Still further, the present invention is not limited to the transmission of lightwave signals, and can be applied to, for example, the transmission of microwaves as in antennas, or multiplexers and demultiplexers.