N×N single-mode optical waveguide coupler
TECHNICAL FIELD 
     The present invention relates to N×N evanescent wave optical star coupling devices, and more specifically, to single-mode optical waveguide star couplers. 
     DESCRIPTION OF THE PRIOR ART 
     In optical systems, information in the form of an optical signal may need to be divided into a plurality of equally powerful optical signals. Transmissive star couplers are used to divide signals in such a manner, and current designs of star couplers seek to provide an equal division of light signls without experiencing considerable power loss. 
     The basis of many larger N×N star coupler designs is 2×2 and 3×3 single-mode optical waveguide couplers which are of considerable importance in single-mode optical waveguide star coupler technology. Two methods of fabricating 2×2 or 3×3 single-mode couplers are the fusion process and the chemical etching process as described in &#34;Recent Advances in Single-Mode Fused Taper Coupler Technology&#34; by M. H. Slonecker et al. in Proceedings of SPIE, 1983, volume 412, pp. 50-53, and in &#34;Review of Optical Fiber Couplers&#34; written by A. K. Agarwal in volume 6, number 1 of Fiber and Integrated Optics 1987, pp. 29-33. However, disadvantages exist with both of these processes. The fusion fabrication process is stated to be highly labor intensive, a result of the relatively low-yield rate in manufacturing these couplers, according to the article &#34;Review of Single Mode Coupler Technology and Applications&#34; by A. Nelson in Proceedings: Paper Presented at the Ninth International Fiber Optic Communication and Local Area Networks Exposition in the USA Sept. 18-20, 1985, pp. 227-231. Within this same article, the chemical etching technique is stated to produce couplers that have environmentally unstable operation. These problems limit the widespread use of fused and chemically etched couplers. 
     Large N×N single-mode optical waveguide star couplers are needed and are currently being produced by cascading stages of smaller 2×2 or 3×3 couplers. An example of the cascading method is FIG. 1 of the article &#34;9×9 Single-Mode Fiber-Optic Star Couplers&#34; by C. C. Wang et al. in Optics Letters, volume 10, number 1, January 1985, pp. 49-51, in which 3×3 single-mode couplers are displayed in a cascaded design to form a 9×9 star coupler. The complexity of designing a large N×N star coupler by the method of cascading smaller star couplers increases as the number of ports of the star coupler increases. Even formal methods have been documented which describe how to interconnect smaller star couplers to produce arbitrarily large N×N single-mode star couplers with low power loss, e.g. &#34;Hierarchic and Combinatorial Star Couplers&#34; by M. E. Marchic in Optics Letters, August 1984, volume 9, number 8, pp. 368-370. 
     The problem remaining in the prior art is to provide a simpler approach in designing and fabricating large N×N single-mode star couplers. 
     SUMMARY OF THE INVENTION 
     The problem in the prior art has been solved in accordance with the present invention which relates to a single-mode optical waveguide coupler including an arrangement of optical waveguides, the arrangement facilitating coupling among all the waveguides. When a narrow frequency band optical signal is applied to an input port of this coupler, the power of the input signal is found to be nonuniformly distributed to the output ports of the coupler. This nonuniform output power level distribution is made uniform, in accordance with the present invention, by causing the components of the input signal of the coupler to experience both different propagation constants throughout, and different coupling coefficients between, the waveguides by either the application of a high-frequency, continuously changing magnitude, oscillatory electric or magnetic field or the introduction by each user of a predetermined broadband optical signal to the associated input port of the coupler. Also, the nonuniform output power distribution can be used to an advantage by introducing a predetermined narrow frequency band optical signal into a selected waveguide input port to achieve a distinct varied output power level distribution for purposes of selective switching of the input signal to certain output ports of the coupler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the preferred design of a coupler in accordance with the present invention showing the planar parallel arrangement and spacing of a plurality of N optical waveguides on a substrate; 
     FIG. 2 illustrates two exemplary alternative waveforms of a force field to be applied across the coupling region of the waveguides in FIG. 1 for one embodiment of the present invention; and 
     FIG. 3 illustrates an alternate design of a coupler in accordance with the present invention in which a plurality of optical waveguides are arranged in a bundle. 
    
    
     DETAILED DESCRIPTION 
     Numerous single-mode optical waveguide coupler designs in accordance with the present invention are possible, two of which are coupler 10 of FIG. 1 and coupler 26 of FIG. 3. Referring to FIG. 1, coupler 10, the preferred coupler design of an N×N single-mode optical waveguide coupler, is shown as including a planar arrangement of N mutually parallel optical waveguides 11 1  -11 N  embedded in a substrate 12 having a predetermined refractive index less than that of the waveguide material. A planar arrangement of waveguides is preferable because of the relative ease in the manufacturing process as compared to the nonplanar arrangement of coupler 26 of FIG. 3. Waveguides 11 1  -11 N  include input ports 15 1  -15 N  adjacent to a first end of waveguides 11 1  -11 N , and output ports 16 1  -16 N  adjacent to a second end of waveguides 11 1  -11 N . To achieve sufficient coupling between waveguides 11 1  -11 N , spacing 13 between any pair of immediately adjacent waveguides 11 i  and 11 i ±1 ; should preferably be a fraction of the width of individual waveguides 11 1  -11 N , and coupling region or length 14 should preferably be at least twice the 3-dB coupling length of waveguides 11 1  -11 N . The 3-dB coupling length is defined as the length of a pair of adjacent waveguides 11 i  and 11 i ±1 necessary to allow half of the input signal introduced into one of waveguides 11 i  or 11 i ±1 to be transmitted to the other waveguide 11 i ±1 or 11 i , respectively. 
     When an optical signal of a predetermined frequency band, for example, substantially less than 100 gigahertz, is introduced to one of the input ports 15 1  -15 N , coupling occurs between each of the waveguides 11 1  -11 N  to yield a nonuniform power distribution at output ports 16 1  -16 N . This nonuniform output power distribution can be used to an advantage or made substantially uniform by several means. 
     In accordance with a first embodiment of the present invention, the application of an optical signal of a predetermined narrow frequency band, e.g. appreciably less than 100 gigahertz, to any one of waveguide input ports 15 1  -15 N  of FIG. 1 is found to produce a nonuniform power distribution among output ports 16 1  -16 N . The distribution of an input signal&#39;s power to output ports 16 1  -16 N  of coupler 10 is dependent on both the frequency of the input signal and the position of waveguide 11 i , the waveguide to which the input signal is introduced, relative to the other waveguides. For example, if an optical signal of a specific frequency is applied to input port 15 3 , a unique nonuniform power distribution appears at output ports 16 1  -16 N . If an optical signal of the same frequency is applied to an input port other than input port 15 3 , or if an optical signal of a substantially different frequency is applied to the same port 15 3 , a different nonuniform power distribution is produced at output ports 16.sub. 1 -16 N . Experimentally determining how optical signals of different frequencies, introduced at each of input ports 15 1  -15 N , will appear with sufficient power at only certain output ports 16 1  -16 N  enables the use of the present invention as a selective switching device. 
     In accordance with a second embodiment of the present invention, the application of a predetermined broadband optical signal to any one of the waveguide input ports 15 1  -15 N  of FIG. 1 produces a uniform output power distribution at output ports 16 1  -16 N . The propagation constant of waveguides 11 1  -11 N  varies in the frequency domain of an applied signal, and changes in propagation constants of waveguide material results in changes in the distribution of power among output ports 16 1  -16 N . Therefore, each frequency component of the predetermined broadband optical signal generates a distinct power distribution among output ports 16 1  -16 N  as described above for the predetermined narrow frequency band optical signal. For example, if a single frequency component of a predetermined broadband optical signal is applied to input port 15 4 , a unique nonuniform output power distribution results wherein, for example, output port 16 6  receives two-fifths of the input signal&#39;s power and output port 16 1  receives one-third of the power while the remaining output ports receive the remainder of the power. Each of the other frequency components of this predetermined broadband optical signal, when applied to the same input port 15 4 , produces a different output power distribution. Averaging these different power distributions over the entire broadband of the optical input signal on exemplary input port 15 4  yields an appreciably uniform output power distribution. Therefore, if an input signal that possesses a broad range of frequency components, for example, appreciably broader than 100 gigahertz, is introduced to an input port 15 i , the average of the different output power distributions yields a substantially uniform power distribution among output ports 16 1  -16 N  of coupler 10. 
     A third embodiment of the present invention uses the application of a high-frequency, continuously changing magnitude, oscillatory electric or magnetic field 17 throughout coupling region 14 of optical waveguides 11 1  -11 N  to produce an appreciably uniform power distribution among output ports 16 1  -16 N , applied field 17 originating from source 18. This embodiment requires that waveguides 11 1  -11 N  be made of electrooptic material, e.g. lithium niobate, in the case of applied electric field 17 or magnetooptic material, e.g. yttrium iron garnet, in the case of applied magnetic field 17. The electric or magnetic field oscillation can be in the form of a triangular wave, a sinusoidal wave, or other waves which have continuously changing magnitudes; a square wave is an example of a wave that is not desirable because its magnitude does not change continuously. The continuously changing magnitude of applied field 17 causes instantaneous variations in the propagation constant and coupling coefficients of the field sensitive waveguide material and, in turn, instantaneous variations in the output power distribution of coupler 10. The instantaneous variations in the output power distributions averaged over the period of the input signal, the period being the length of a bit of optical information, can yield a substantially uniform power distribution among output ports 16 1  -16 N  of waveguides 11 1  -11 N  depending on the frequency of applied field 17 relative to the bit rate of the input signal. In general, a sinusoidal waveform 20 shown in FIG. 2 when applied throughout coupling region 14 should have a frequency of one or at least two times the bit rate--the number of bits per unit of time--of the incoming optical signal. In the case of triangular waveform 22 shown in FIG. 22, applied field 17 should have a frequency greater than or equal to the bit rate of the incoming optical signal. There is a difference in the lower bound frequency requirements of waveforms 20 and 22, because waveforms 20 and 22 have different slope characteristics which affect the way in which waveguides 11 1  -11 N  experience coupling. Moreover, the higher the frequency and the greater the magnitude of applied field 17, the greater the uniformity in the power distribution of the output ports 16 1  -16 N  will be. 
     The second and third embodiments of the present invention produce near uniform power distributions among output ports 16 1  -16 N . The power distribution is substantially uniform for all output ports except output port 16 h  of waveguide 11 h  which receives the optical input signal at its input port 15 h . For example, if input port 15 2  of FIG. 1 is used to introduce an optical signal to coupler 10, the output power observed at output port 16 2  would not be equal to the power found at any of the other output ports. In general, the equation that describes the output power distribution for each of the waveguides other than that of the input waveguide, e.g. waveguide 11 2 , is 
     
         P.sub.o =[1/(N+1)]P.sub.i 
    
     where P i  is the input power and P o  is the output power. The power at the output port of the input waveguide, e.g. exemplary waveguide 11 2 , is the remainder of the input power that has not been distributed among the other output ports or simply two times the output power at any one of the other output ports, [2/(N+1)]P i . 
     Coupler 26 of FIG. 3 mentioned above as an exemplary alternate design of coupler 10 includes an arrangement of an arbitrary number of waveguides greater than three, in this case twelve waveguides 30 1  -30 12  arranged in a bundle in such a way to allow sufficient evanescent wave coupling among all waveguides 30 1  -30 12 . The embodiments of coupler 10 can be applied directly to coupler 26 or any other design that consists of a plurality of waveguides wherein the waveguides are sufficiently close to one another to facilitate evanescent wave coupling. Coupler 26 produces a nonuniform output power distribution when a predetermined narrow frequency band optical signal is applied to the input ports of the waveguides 30 1  -30 12 . A substantially uniform power distribution can be achieved by either applying a high frequency, continuously changing magnitude, oscillatory electric or magnetic field 17 across coupling region 14 of waveguides 30 1  -30 12  where waveguides 30 1  -30 12  are made of electrooptic material in the case of an applied electric field 17 or magnetooptic material in the case of an applied magnetic field 17, or introducing a predetermined broad frequency band input to any one of the input ports of coupler 26.