Abstract:
This invention provides methods and apparatus for achieving wavelength sorting multiplexer/demultiplexer and its application to the implementation of planarized dynamic wavelength routing. Using integrated arrayed-waveguide gratings, sorting can be achieved by two configurations. In the first configuration channel wavelengths are properly selected and launched into prearranged input waveguides of an arrayed-waveguide grating such that channels at the same wavelength and from all inputs will be demultiplexed and routed to adjacent outputs. Operated in the reverse direction, the same device becomes a sorting multiplexer. The second configuration achieves wavelength sorting by using the cascade of multiple arrayed-waveguide gratings and can also be operated as a demultiplexer or a multiplexer. Combined with space switches, the wavelength sorting multi/demultiplexer are utilized to implement the planarized channel-selective dynamic wavelength router. The function of wavelength sorting eliminates on-chip waveguide crossings and therefore reduces losses and crosstalks. The sorting demultiplexer and multiplexer can further be implemented with a single arrayed-waveguide grating.

Description:
TECHNICAL FIELD 
     This invention relates to optical communication networks utilizing the technique of wavelength-division multiplexing. 
     BACKGROUND OF THE INVENTION 
     Channel-selective dynamic wavelength routing is a key function for all-optical networks which utilize the wavelength-division multiplexing (WDM) technique (See, for example, S. Alexander et al., Journal of Lightwave Technology, vol. 11, no. 5/6, pp. 714-735, 1993). A systematic diagram for realizing such a dynamic multi-wavelength routing network generally consists of the cascade of an input demultiplexing section, an intermediate space switch section, and an output multiplexing section. Each of the input demultiplexers separates various wavelength channels in one of the input fibers into various spatial positions. Channels at the same wavelength from all the input fibers are then fed into one of the intermediate space switches. The outputs of each space switch are individually connected to different multiplexers in the output multiplexing section. The internal switching states of the space switches are controlled according to the network routing instructions, which therefore achieve the overall required dynamic channel-selective wavelength routing between all wavelength channels in the input fibers. 
     One promising scheme to perform the multiplexing and demultiplexing functions as described above is to employ the integrated array-waveguide grating, for which a review of recent progress is given by B. H. Verbeek and M. K. Smit (Digest,1995 European Conference on Optical Communication, pp. 195-202). The arrayed-waveguide grating has been successfully implemented in various material systems including silica glasses, III-V semiconductors like InP, and polymers. The arrayed-waveguide grating typically consists of multiple input and output waveguides, and the input and output slab regions connected by multiple arrayed waveguides. Light at different wavelength channels from any of the input waveguides diffracts in the input slab and is captured by the arrayed waveguides. The lengths of the arrayed waveguides are designed to introduce wavelength-dependent incremental phase delays to provide a converging beam through the output slab at different directions for different wavelengths. The various wavelength channels are thus spatially separated and coupled to various output waveguides, which achieves the demultiplexing function. By reciprocity, when operated in the reverse direction, the arrayed-waveguide also performs the multiplexing function. 
     Full utilization of the multiple inputs and outputs of the arrayed-waveguide grating results in N×N interconnections between N input fibers and N output fibers (see, for example, Dragone, IEEE Photonics Technology Letters, pp. 896-899, October 1991), as well as the wavelength channel add/drop multi/demultiplexer. The latter arrangement, as first proposed by Y. Tachikawa et al. (Electronics Letters, vol. 29, no. 24, pp. 2133-2134, 1993), involves one input fiber, one output fiber, and a 16×16 arrayed-waveguide grating in which all but the center output waveguides are looped back to their corresponding inputs through external fibers. By opening the external fibers, up to 15 wavelength channels can be dropped and added. 
     The idea of using a single arrayed-waveguide grating and space switches for performing dynamic channel-selective wavelength routing between multiple input fibers and multiple output fibers was reported by S. Suzuki et al. (Electronics Letters, vol. 30, no. 13, pp. 1091-1092, 1994). For F input fibers and F output fibers with W wavelength channels in each fibers, the dynamic routing function can be achieved by an N×N arrayed-waveguide grating with N≧F(W+1) and a W array of F×F space switches. The loop-back paths were, however, still done by using external fibers. This configuration will be referred to in the following as an W-channel F×F dynamic wavelength router. When F=2 with one input and one output utilized as the add and the drop ports, respectively, it is also referred to as a W-channel optical add/drop multiplexer in the literature. 
     A fully integrated 16-channel optical add/drop multiplexer was reported by K. Okamoto et al. (1995 Optical Fiber Communication Conference, paper PD10-2) using 3 arrayed-waveguide gratings integrated with 16 Mach-Zehnder type thermo-optic 2×2 switches. The device fiber-to-fiber insertion losses were about 6-8 dB for signals routed from the main input port to the main output port or to the drop port, and 3-4 dB from the add port to the main output port. The worst-case crosstalk was -13 dB. Because the space switches were integrated with the multiplexer and demultiplexer, such a design represents an example of planarized implementation of the dynamic wavelength router which avoids external interconnecting fibers between the multi/demultiplexer and the array switches. The on-chip waveguide crossings (or overlappings) are, however, still an problem which, as F and W increase, greatly complicates the waveguide layout while at the same time introduces extra losses, loss nonuniformity, and crosstalks. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide means for achieving the function of wavelength sorting by using the arrayed-waveguide grating(s) with non-crossing and non-overlapping on-chip waveguides. A further object of this invention is to provide a planarized realization of a channel-selective dynamic wavelength router incorporating the function of wavelength sorting. 
     The preferred embodiment of the present invention provides means for realizing a wavelength sorting multiplexer/demultiplexer and its application to the implementation of a planarized dynamic wavelength router. Using the arrayed-waveguide grating (AWG), the sorting function can be achieved by two configurations. In the first configuration the WDM channel wavelengths are properly selected such that, when launched into prearranged input waveguides of the AWG, channels at the same wavelength and from all inputs will be routed to adjacent outputs, which therefore achieves the function of a sorting demultiplexer. By reversing the direction of flow of optical signals, the same arrangement becomes a sorting multiplexer. The second configuration achieves wavelength sorting by using the cascade of multiple AWGs with a different scheme of selecting channel wavelengths and can also be operated as a demultiplexer or as a multiplexer. Both configurations can accommodate the scalability of wavelength sorting and thus allow future increase in network capacity with no need to modify the configuration. 
     By incorporating the wavelength sorting multi/demultiplexer of the present invention and using an array of space switches, the dynamic wavelength router can be implemented in a planarized fashion free of external interconnecting fibers. The achieved wavelength sorting completely eliminates on-chip waveguide crossing (or overlapping) and therefore reduces signal losses and crosstalks. The planarized dynamic wavelength router is implemented when the switch array is either integrated with the sorting multi/demultiplexer or connected through direct waveguide-to-waveguide butt coupling. The sorting demultiplexer and multiplexer can further be implemented with a single AWG. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1(a) schematically illustrates the waveguide structure of a typical arrayed-waveguide grating. 
     FIG. 1(b) schematically illustrates a wavelength sorter using a single arrayed-waveguide grating. 
     FIG. 2(a) schematically illustrates a wavelength sorter using the cascade of multiple arrayed-waveguide gratings. 
     FIG. 2(b) schematically illustrates an example of the wavelength sorting using the cascade of three 12×12 arrayed-waveguide gratings. 
     FIG. 3 illustrates a configuration of a planarized dynamic wavelength router comprising a sorting demultiplexer, an array of space switches, and a sorting multiplexer. 
     FIG. 4 illustrates an alternate configuration of the planarized dynamic wavelength router comprising an array of space switches and an integrated sorting mult/demultiplexer. 
     FIG. 5 illustrates another configuration of the planarized dynamic wavelength router comprising an array of space switches and an integrated sorting multi/demultiplexer implemented by a single array-waveguide grating. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The integrated waveguide structure of a typical N×N AWG is schematically illustrated in FIG. 1(a) fabricated on a substrate 100. There are N input waveguides 101 (denoted by I 1 , I 2  . . ., I N ) and N output waveguides 102 (denoted by O 1 , 2 , . . . , O N ) Wavelength channels launched into I i  through the input fiber are guided and directed to the input slab 111. The diffracted light in 111 is captured by a plurality of arrayed waveguides 120 of an incremental length difference ΔL=l k+1  -l k , which are then connected to the output slab 112. This arrangement results in a phased array antennas radiating in the slab 112. ΔL can be selected such that a particular wavelength channel 1 0  will be routed from I i  to O j  for specific i and j. The AWG structural parameters can be designed such that if λ 0   is routed from I i  to O j , then λ 0  will also be routed from I i+1  to O j-1 , while λ 0  +Δλ (with Δλ&gt;0) will be routed from I i  to O j+l  if ΔL&gt;0 or to O j-1  if ΔL&lt;0. Such a rule of input-output wavelength routing can be summarized by 
     
         If λ.sub.0 :I.sub.i to O.sub.j for any i and j, 
    
     
         then λ.sub.0 :I.sub.i+l to O.sub.j-1                (1) 
    
     
         and λ.sub.0 +Δλ: I.sub.i to O.sub.j+1 for ΔL&gt;0or I.sub.i to O.sub.j-1 for ΔL&lt;0 
    
     In addition, the wavelength routing is periodical with a period called free spectral range (fsr). Δλ is the AWG device channel spacing related to other parameters by ##EQU1## where n c , n s , and n g  are the waveguide effective index, the slab effective index, and the group index, respectively, d is the waveguide spacing at the slab boundary, L f  is the slab focal length, and M is the order of the grating. Details of the device operation principle and design method of AWG were discussed, for example, by H. Takahashi et al. (Journal of Lightwave Technology, vol. 13, no. 3, pp. 447-455, March 1995). 
     Based on the properties of wavelength routing in an N×N AWG, the first method for achieving wavelength sorting of the preferred embodiment of this invention is illustrated in FIG. (b) where the N×N AWG is represented by a box 100 which has the internal waveguide structure illustrated by FIG. 1(a). Consider that F incoming fibers are connected to the top F input waveguides 103 and each fiber carries W wavelength-multiplexed channels. Let λ ki  denote the channel at the wavelength λ k , launched into input I l . In other words (λ 11 ,λ 21 ..., λ W1 ) 131 are launched into I l , (λ 12 ,λ 22 , . . . ,λ W2 ) 132 into I 2 , . . . , and (λ 1f ,λ 2F , . . . ,λ WF ) 133 into I F . The wavelength spacing δλ(=λ k+1  -λ k ) of the W wavelengths is selected as δλ=FΔλ 141 with Δλ given by equation (2). Suppose the AWG 100 is designed to route λ 11  to O F , then according to equation (1), λ 12  will be routed to O F-l ,λ 13  (not shown) routed to O F-2 , . . . , and λ lF  routed to O 1 . All λ 1  channels are thus routed to adjacent outputs (O F , O F-1 , . . . ,O 1 ) 105. Also according to equation (1) with ΔL &gt;0, λ 21  will be routed to O 2F  due to the selection of δλ. The same argument concludes that all λ 2  channels are routed to adjacent outputs (O 2F , O 2F-1 , . . . , O F+1 ) 106. The highest wavelengths λ W  will then be routed to (O WF , O WF-1 , . . . , O.sub.(W-1)F+1) 107. Such an arrangement thus functions as a sorting demultiplexer. The required number of input/output waveguides of the AWG should be N≧WF 141 (actually only F waveguides are used at the input side). In order for the sorting to function properly, the total wavelength span (λ W  -λ 1 ) should be less than the AWG fsr so that the routing periodicity has no effect on wavelength sorting, i.e. we require fsr&gt;Wδλ 142. When FIG. 1(b) is operated in the reverse direction, i.e. when sorted wavelength channels, are launched into 100 from the output ports 105, 106, and 107, they will be multiplexed onto 103 due to the reciprocity of AWGs built with passive material. FIG. 1(b) can therefore also be used as a sorting multiplexer. Moreover, if the AWG waveguide structure in FIG. 1(a) is designed symmetric with respect to the plane A--A&#39;, the properties of wavelength sorting will also be symmetric. Therefore, FIG. 1(b) can be operated as a sorting demultiplexer in both the forward and the reverse directions, which is also true when used as a sorting multiplexer. 
     It is evident that the arrangement of FIG. 1(b) with the conditions 140, 141, and 142 will still achieve wavelength sorting when less than F input fibers are connected. Equivalently if we use an AWG with N=W(F+F scal ), we preserve the possibility of wavelength sorting for additional F scal  fibers. In other words the sorting scalability in the number of fibers is F scal  143. Similarly FIG. 1(b) can handle more wavelength channels in the input fibers by using N greater than WF. Specifically, if N-WF=W scal  F, FIG. 1(b) will still achieve wavelength sorting when each input fiber carries additional W scal  wavelength channels. W scal  144 will be referred to as the sorting scalability in the number of wavelength channels. 
     The second method for achieving wavelength sorting of the preferred embodiment is schematically illustrated in FIG. 2(a) by using a cascade of F N×N AWGs 200 for F input fibers and W wavelength channels in each fiber. All AWGs satisfy the conditions N≧WF 251 and fsr &gt;Wδλ 253. The selected W wavelength channels have spacing matched to the AWG device wavelength spacing, i.e. δλ=λ k+1  -λ k  =Δλ 252 with Δλ given by equation (2). Consider the function of sorting demultiplexer first, (λ 11 ,λ 21 , . . . ,λ W1 ) 211 are launched into input I 1 , (λ 12 ,λ 22 , . . . ,λ W2 ) 212 into I 2 ,..., and (λ 1F , . . . ,λ WF ) 213 into I F . Each of the cascaded AWGs 231, 232, . . . , 233, and 234 contributes to a partial sorting according to equation (1). At the outputs of AWG-1 231, there will be F+(W-1) wavelength signal lines 241. When outputs of the first AWG-1 231 are fed into the inputs of AWG-2 232, there will be F+2(W-1) wavelength signal lines 242 at the outputs of 232. This process proceeds such that at the inputs of the last AWG (AWG-F) 234, we have F+(F-1)(W-1) wavelength signal lines 243, and AWG-F 234 completes the required sorting: channels of the same wavelength and from all F input fibers are routed to adjacent output waveguide. The distributions are (λ 11 ,λ 12 , . . . ,λ 1F ) 221, (λ 21 , λ 22 ,...,λ 2F ) at (O F+1 , O F+2 , . . . , O +2F ) 222, . . . , and (λ W1 ,λ W2 , . . . , λ WF ) at (O.sub.(W-1)F+PO.sub.(W-1)F+2, . . . ,O 2F ) 223. By reversing the direction of flow of optical signals, the configuration of FIG. 2(a) becomes a sorting multiplexer. It should be pointed out that the configuration of FIG. 2(a) requires an alternate arrangement of positive and negative ΔL structures of the identical AWGs. 
     An example of the sorting demultiplexer of the configuration FIG. 2(a) is illustrated by FIG. 2(b) for W=4 and F=3. Three 12×12 AWGs are cascaded with ΔL&gt;0 for AWG-1 and AWG-3, and ΔL&lt;0 for AWG-2 (AWG-2 is simply the structure of FIG. 1(a) reversed upside down). When (λ 11 , λ 21 , λ 31 , λ 41 ) are launched into I 1 ,the AWG is designed to route λ 41  to the 12-th output of AWG-1, equation (1) with ΔL&gt;0 indicates that (λ 11 , λ 21 , λ 31 , λ 41 ) will exit AWG-1 as shown by the column a 1 . After passing through AWG-2 with ΔL&lt;0 (which is identical to AWG-1 but reversed upside down), λ 41  will exit at the first output. Applying equation (1) results in column b 1 . AWG-3 with ΔL&gt;0 then routes wavelength channels in b 1  to c 1 . Similarly, (λ 12 , λ 22 , λ 32 , λ 43 ) from I 2  will be routed to columns a 2 , b 2 , and c 2 , while (λ 13 , λ 23 , λ 33 , λ 41 ) from I 3  will be routed to columns a 3 , b 3 , and c 3 . The function of sorting demultiplexer is clearly achieved. 
     Using the wavelength sorting multi/demultiplexer of the present invention, a planarized dynamic wavelength router can be implemented as schematically illustrated by FIG. 3. The configuration comprises the input sorting demultiplexer 301, the intermediate switch array 302, and the output sorting multiplexer 303. For W selected wavelengths (λ 1 , λ 2 , . . . , λ W ), this system dynamically switches any λ k  channel between inputs (I 1 , λ 2 , λ F ) 311 giving switched outputs at (O 1 , O 2 , . . . ,O F ) 312. The sorting multi/demultiplexer 303 and 301 can be realized by the arrangement of either FIG. 1(b) or FIG. 2(a). The switch array 302 comprises W F×F space switches which are preferably integrated on one chip with the input and output waveguides arranged along lines B--B&#39; and C--C&#39;, respectively. In general various types of switches (electro-optic, thermo-optic, mechanical etc.) can be employed for 302. At the output of sorting demultiplexer 301, the sorted λ 1  channels 321 are fed to the inputs 351 of λ 1  -switch 341,λ 2  channels 322 to the inputs 352 of λ 2  -switch 342, . . . , and λ W  channels 323 to the inputs 353 of λ W  -switch 343. Similarly, at the outputs of switch array 331, 332, . . . , and 333, they are fed to the sorting multiplexer 303. An ideal implementation of FIG. 3 is to integrate the switch array 302 with the sorters 301 and 303. If they have to be fabricated separately due to for example size consideration, the preferred interconnecting scheme is to directly butt couple the chip 302 to the chips 301 and 303 along the lines B--B&#39; and C--C&#39; using packaging techniques with matched waveguide layouts on both sides. It is evident that the achieved wavelength sorting of the present invention completely eliminates on-chip waveguide crossing (or overlapping) from the input fibers at 311 to the inputs of switch array 351, 352, 353 and from the outputs of switch array 331, 332, 333 to the output fibers at 312. 
     FIG. 4 schematically illustrates a second configuration of the planarized dynamic wavelength router incorporating wavelength sorting of the preferred embodiment. In this configuration, the sorting demultiplexer 301 and multiplexer 303 are integrated on one chip 401 while the switch array 302 is either also integrated on the same chip or on another 402. The sorted wavelength channels 321, 322, and 323 are fed to the inputs of switch array at 431, 432, and 433 with the connection done along the line D--D&#39;, while the outputs of switch array at 421, 422, and 433 are designed turning back and connected to the inputs of the sorting demultiplexer 303 along the same line D--D&#39;. If the switch array is on another chip 402, the preferred interconnecting scheme is to directly butt couple the chips 402 and 401 along the line D--D&#39; with matched waveguide layouts on both sides. 
     The preferred embodiment of another configuration the planarized dynamic wavelength router incorporating wavelength sorting is illustrated in FIG. 5 where the sorting demultiplexer and the sorting multiplexer are implemented with a single N×N AWG with N≧F(W+1) 521. The selected W channel wavelengths have a spacing the same as in FIG. 1(b). i.e. δλ=λ k+1  -λ k  =FΔλ, and the AWG has its free spectral range fsr &gt;Wδλ 522. When W wavelength channels are launched into each of the input waveguides (I 1 , I 2 , . . . , I F ) 501, the AWG can be designed, in the same principle as recited pertaining to FIG. 1(b), such that all wavelength channels will be routed to (O F+1 , O F+2 , O F (W+1)) 512 in a sorted fashion. These sorted wavelength signals are fed into the inputs of the switch array at 321, 322, . . . , and 323. The outputs of switch array at 421, 422,..., and 423 are fed back to waveguides (I F+1 , I F+2 , . . . , I F (W+1)) 511. The symmetrically designed AWG 500 will multiplex them onto the outputs (O 1 , O 2 , . . . , O  F ) 502, which completes the required function of dynamic wavelength routing. Again it is evident that the function of wavelength sorting of the present invention totally eliminates on-chip waveguide crossings (or overlapping) and removes the associated losses and crosstalks.