Abstract:
By randomly varying the difference in the phase constants and/or the coupling coefficient along the lengths of two or more wavepaths, an equal division power divider is obtained whose operation is not critically dependent upon its dimensions.

Description:
TECHNICAL FIELD 
     This invention relates to power dividers. 
     BACKGROUND OF THE INVENTION 
     As described by S. E. Miller in his article entitled &#34;Coupled Wave Theory,&#34; published in the May 1954 issue of the Bell System Technical Journal, pp. 661-719, the power that is coupled between the waveguides of a directional coupler varies as a function of the length of the coupling interval, the coupling coefficient, and the propagation constants of the two waveguides. At microwave frequencies and below, it is relatively easy to design and construct a directional coupler having any prescribed power division ratio. At optical frequencies, however, the fabrication of a directional coupler having a particular set of operating characteristics is much more difficult. Accordingly, various means have been devised to compensate, for example, for inadvertent differences in propagation constants, and to adjust the effective length of the coupler so as to obtain the desired power division. (See, for example, U.S. Pat. No. 4,012,113.) While these various techniques are effective, they tend to complicate coupler structures. 
     The object of the present invention is to simplify the structure of directional couplers. 
     SUMMARY OF THE INVENTION 
     A directional coupler, in accordance with the present invention, comprises at least a pair of waveguides coupled together over a coupling interval L, characterized in that the incremental power transferred between wavepaths varies randomly as a function of the distance along said waveguides. This variation can be accomplished either by varying the difference in the phase constants (β 1  -β 2 ) of the two waveguides, and/or by varying the coefficient of coupling c. For the case of a pair of waveguides where the coupling is constant and the phase constant difference Δβ(z) varies randomly along the coupling interval about zero average value, it is shown that a 3 dB coupler of minimum length is obtained when B, the spectral density of the phase difference Δβ(z) at low spatial frequencies, is equal to eight times the coupling coefficient c. If, however, the coupler is longer than this minimum length, it is an advantage of the invention that none of the waveguide dimensions are critical to equal power division. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a prior art optical directional coupler; 
     FIG. 2 shows one embodiment of a directional coupler in accordance with the present invention; 
     FIG. 3 shows a family of curves illustrating the manner in which the power in one of the coupler strips varies along the coupler length for different values of the ratio B/c; and 
     FIG. 4 shows an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, FIG. 1 shows a typical prior art directional coupler 10 comprising a pair of transparent (low-loss) dielectric strips 11 and 12 embedded in a dielectric substrate 13 of lower refractive index. The strips, which are normally widely spaced apart, extend relatively close to each other over a coupling interval L. Typically, the spacing between the strips is uniform over the coupling interval, and coupling between the guides can be defined by a constant coefficient of coupling c. In addition, in the conventional coupler, the phase constant, β 1  and β 2 , of each of the strips is constant over the coupling interval. For each a coupler, power is coupl d back and forth between the two guides in an oscillatory manner as a function of the difference in phase constants (i.e., Δβ=β 1  -β 2 ), and the length L of the coupling interval. This back and forth coupling characteristic is illustrated by the curves on page 681 of the above-cited Miller article which shows the manner in which the signal amplitudes in the two waveguides vary as a function of distance along the coupling interval for different values of Δβ. In theory, these curves can be used to design a directional coupler having any prescribed power division ratio. At optical frequencies, however, it is relatively difficult to fabricate the guiding strips so as to achieve the design specifications. To compensate for any variations in coupler parameters produced during the fabrication process, one practice is to impress a voltage across the structure and utilize and electrooptic effect as a way of achieving the desired coupler characteristics. 
     One alternative to the above-described technique, in the case of a 3 dB coupler, is based upon the discovery that if the difference in the phase constants of the two guides is made to vary in a random manner, there is a critical coupler length beyond which the power division is substantially independent of the coupler dimensions. Thus, in a 3 dB optical coupler in accordance with the present invention, the phase constant of at least one of the optical strips is randomly varied along its length. This is illustrated in FIG. 2 which shows a 3 dB optical directional coupler 20 comprising, as in FIG. 1, a pair of dielectric strips 21 and 22 embedded in a substrate 23 of lower refractive index. Unlike the prior art coupler, in the embodiment of FIG. 2 at least one of the strips 21 is fabricated such that the phase constant along the strip varies randomly. This effect can be produced, for example, by inducing variations 24 in the cross-sectional dimensions of the strip as it is deposited onto substrate 23. The other strip 22 can either be made uniform over its length or, alternatively, can also be made to have random phase variations over the coupling interval. 
     If a signal is applied to input port 1 of strip 21, it can be shown that the average power, P 1 , at any point, z, along strip 21 is given by ##EQU1## where ##EQU2## 
     c is the coefficient of coupling between the strips; the average value of Δβ is zero; 
     and 
     B is the spectral density of Δβ(z) at low spatial frequencies. 
     Assuming negligible losses in the strips, the average power at any point z along waveguide 22 is 
     
         P.sub.2 =1-P.sub.1                                         (3) 
    
     As seen in equation (1), there are three important ranges of interest defined by the ratio B/c. For B/c less than 8, the expression under the radical is positive; for B/c=0, this expression is zero; and for B/c greater than 8, it is negative. 
     For the special case of uniform coupling and no variations in Δβ (i.e., β=0), the expressions for the average powers are 
     
         P.sub.1 =cos .sup.2 2cz                                    (4) 
    
     and 
     
         P.sub.2 =sin .sup.2 2cz                                    (5) 
    
     These are the expressions for an ideal coupler as given by Miller. 
     For the case of Δβ≠0 and values of B/c greater than zero but less than 8, P 1  is a damped, oscillatory function which approaches one-half for large values of cz. 
     For B/c equal to 8, P 1  reduces to ##EQU3## 
     For B/c greater than 8, P 1  is given by ##EQU4## where ##EQU5## 
     FIG. 3 is a plot of P 1  for these various values of B/c. It will be noted that all of the curves for which B/c is greater than zero asymptomatically approach one-half. That is, half of the power remains on the input strip and half is coupled to the other strip. Thus, any directional coupler, if long enough, will serve as an equal power splitter by the simple expedient of randomly varying the phase constant of one or both of the strips making up the coupler. However, the shortest coupler is obtained when B/c is approximately equal to eight. Accordingly, where length is an important consideration, the density and magnitude of the phase variations are advantageously such that the ratio of B to c is eight or slightly greater. 
     Techniques for fabricating integrated optical circuits are well known in the art. See, for example, the article by D. R. Herriott et al. entitled &#34;EEBS: A Practical Electron Lithographic System&#34; published in the July 1975 issue of the IEEE Transactions on Electron Devices, Vol. ED-22, No. 7, wherein a method for fabricating masks is described. Of particular interest is the variable spot scanning technique, described in the copending application by R. J. Collier et al., Ser. No. 855,608, filed Nov. 29, 1977, and assigned to applicants&#39; assignee, which can be employed to vary the dimensions of a mask. This technique permits the generation of electron beams of variable cross-sectional configurations. For the purposes of the present invention, the beam cross-section is caused to vary in a random manner. A mask, thus produced, can then be used to fabricate a coupler in accordance with the present invention. 
     In the embodiment of FIG. 2, the phase constant of only one of the waveguides 21 is shown to be varied. Alternatively, the phase constants of both guides can be varied simultaneously in &#34;push-pull&#34; fashion. That is, the cross-section of one guide is increased as the cross-section of the other is decreased. To minimize losses due to scattering, the spatial wavelength of these variations is made long compared to the optical wavelength. On the other hand, they occur rapidly relative to the overall length of the coupler. 
     As indicated hereinabove, a minimum length coupler is produced when the ratio of B to c is approximately 8. As a first approximation in the design of an optimum length coupler, the overall length L is specified. From FIG. 3, it is noted that for B/c=8, the power P 1  is reduced to half at about cz=π. Thus, for the given z=L, we obtain 
     
         c=π/L                                                   (9) 
    
     which specifies the coefficient of coupling. Substituting this value of c, the spectral density is then computed to be 
     
         B=8π/L                                                  (10) 
    
     FIG. 4 illustrates an extension of the principles of the invention to a three-way power split. In this embodiment, a first waveguide 40 is simultaneously coupled over a common coupling interval to two wavelengths 41 and 42 located on opposite sides of waveguide 40. As before, random perturbations 43 in the phase constants (and/or coupling coefficients) of the three guides result in the incident power propagating along waveguide 40 being divided equally among the three waveguides. In theory, the power can be divided equally among n waveguides. 
     As indicated hereinabove, the incremental power transfer can be varied either by varying the phase constant of one or more of the coupled guides and/or the coupling coefficient. When parameter is chosen to be varied will depend upon the particular embodiment of the coupler and the method of fabrication employed. 
     In the particular embodiments described, reference was made to optical couplers wherein the cross-sectional dimensions of one or more strip waveguides was changed as a means of varying the phase constant. Alternatively, the refractive index of the strip can be varied or, if one sought to vary c, the distance between coupled strips can be changed. Clearly, that technique which produces the most effective change at the lowest cost is most advantageously employed.