Patent Application: US-25339899-A

Abstract:
the present invention provides n × n non - blocking switch modules using mmi - based switch elements . the arrangement requires a minimum of control elements to effect switching and uses no crossings of the signal waveguides . the switch control settings may be determined by following a simple and transparent algorithm for the setup procedure . very high - order switch fabrics comprising a variety of the taught nonblocking n × n switch modules are envisaged . determination of the appropriate values for ‘ n ’ is a practical consideration which trades - off the performance of the individual switch modules with the complexity required of the associated module interconnection fabric . the spatial switch fabrics that may by built from the non - blocking mmi - based switch arrangements may be combined with both wavelength - division switches and time - division switches to form any combination of higher order space - wavelength - time switch .

Description:
this description is divided into three parts . part i describes the nature and operation of n × n multimode interference couplers ( mmi couplers ) which are used as components in the inventive optical switches . part ii describes conventional optical switches which can be used as components in the inventive optical switches , and part iii describes optical switches in accordance with the invention . fig1 illustrates an n output mmi - coupler 10 comprising a free slab region 11 of waveguide and ports 12 of single moded waveguide . there are n input ports and n output ports . the relative phases of the output signals as well as the dimensional relationships between the output , input , and coupler waveguide sections are described by bachmann [ ref . 12 ]. the case where n = 3 is illustrated in fig2 for which the relative phases of the output optical signals are { 0 , − π , − 2π / 3 }. all outputs are of equal power , being ⅓of the input power ( ignoring switch losses ). fig3 shows the mmi - coupler of fig2 operated in reverse ; when equal - power signals are applied to the three inputs in the same phase relationship ( but opposite sign ) as those shown in fig2 a single output is obtained from the uppermost output , 1 . in the same manner , setting the input phases to correspond to the signals that emerge from the r . h . s . of the mmi - coupler in fig2 when the signal is supplied on input 2 results in a signal emerging from output 2 in fig3 . and similarly for input 3 in fig2 and output 3 of fig3 . a table of the required phase relations are collected in fig4 . fig5 ( a ) shows a conventional switch 50 formed by connecting two mmi - based couplers 10 a and 108 connected in series with connecting links 51 including phase control elements 52 ( designated f 1 , f 2 , . . . , fn ). as only the relative phases between the inputs of the second mmi - coupler are relevant , one phase control element ( e . g . f 1 ) may be omitted and only n - 1 phase control elements are required . by then appropriately setting the n - 1 phase control elements , an input presented to any of the n input ports of the l . h . s . mmi coupler can be switched to any of the n outputs on the r . h . s . of the r . h . s . mmi coupler . this is illustrated in the lower diagram of the fig5 ( b ). different physical realization of the phase control elements are possible , and may depend on the material system employed for the waveguide structures . for semiconductor - based waveguides , electro - optic or carrier effects may be employed ; for lithium niobate guides , electro - optic induced phase changes may be used ; and for doped silica guides , the thermo - optic effect may be used to effect the required phase changes . to maintain polarization insensitivity , the phase control elements should be polarization independent . the switch illustrated in fig5 may route a single input presented to the l . h . s . mmi coupler to any of the outputs of the r . h . s . mmi - coupler . with each such setting of the intermediate control elements 52 , signals presented to the other inputs of the l . h . s . mmi coupler are routed to the other r . h . s . mmi - coupler outputs . a table describing this routing is given in fig6 for the case when n = 3 . only 3 switch states are possible . for complete connectivity n ! switch states are required . this complete set of switch states may be obtained by concatenating identical mmi - couplers and phase control elements . for an n × n switch , n − 1 stages of n − 1 phase controllers per stage are required . fig7 illustrates this for a 3 × 3 switch requiring 3 ! = 6 states . here , two phase control stages , each with control elements 52 on two of the interconnecting waveguide links 51 , are employed between three 3 × 3 mmi couplers 10 a , 10 b , 10 c . the routing table for this switch arrangement is shown in fig8 . it has been seen that each of the switch states is provided by three independent combinations of the phase control elements . for output switching to occur , the different components of each signal passing through the cascaded lattice by different routes must arrive at the designated output in phase with each other . since each mmi coupler provides its outputs with signals of relative phases in multiples of π / n , and the output ports at the end of the mmi coupler lattice must be provided with signal components that are in phase , modulo 2 π , it follows that the phase control elements must provide phase corrections that are multiples of π / n . for a n × n switch which has n − 1 stages of n − 1 phase control elements per stage , a total of 2n ( n − 1 ) 2 potential phase control settings is possible . the interaction between the different concatenated mmi coupler elements restricts this total number to the solution set providing switching of each of the input signals to a single distinct output line . in the case of the 3 × 3 switch illustrated in fig7 and 8 , a total of 18 valid switch settings are possible , which provide a redundancy factor of 3 in the setting of the 3 !, i . e . 6 , unique switch settings . although the concatenated n × n mmi coupler switches can provide full n × n switching functionality , the algorithmic complexity of determining the control element settings required to effect any particular switch configuration , along with the complex interactions that occur between the phase control elements of one switching stage and the phase control elements of the other stages which inhibits an efficient and effective means of determining the phase control element settings in practical devices , means that a simpler n × n switching configuration is desirable . fig9 schematically illustrates a simplified n × n non - blocking optical routing switch in accordance with the invention comprising a sequence of connected optical switches 91 ( 2 × 2 ), 91 ( 3 × 3 ), . . . , 91 ( n × n ) forming a sequential series of switches of unitary increasing switch dimension . by non - blocking it is meant that all possible combinations of output routing of the input signals can be effected by an appropriate setting of the switch . one or more , and preferably all of the switches 91 are switches as shown in figs ,. 5 ( a ) or 5 ( b ), comprising a pair of multiport self - imaging multimode interference couplers interconnected by a plurality of optical pathways including a respective plurality of phase controlling elements ( as shown in fig5 ( a ) and 5 ( b ). the first switch 91 ( 2 × 2 ) is a 2 × 2 switch , the second is a ( 3 × 3 ) switch . the dimensions of the switches increment unitarily until the last switch is n × n . in practical embodiments n is typically ≧ 4 and advantageously ≧ 8 . in examining the behavior of the switch illustrated in fig9 we notice that if the output routing combinations provided by the 3 × 3 mmi coupler of the type illustrated in fig5 and written out in fig6 are examined and compared with the full set of possible routing combinations shown in fig1 , it is seen that the additional routing combinations are provided by simply swapping the input signals provided to two of the input lines . in fig1 this is illustrated by keeping input port 1 fixed and then for each combination adding the case where input ports 2 and 3 are swapped . the swapped combinations are shown in italics . thus , it is seen that by including the case in which inputs 2 and 3 are reversed , the complete set of 3 ! switch states are achieved . that this is inevitable is understood by considering the fact that a single input of the 3 × 3 is routed to the different output ports by the different settings of the controllers and that each such setting routes the other two inputs to the two remaining output ports in a certain fixed manner . if , however , these two inputs can be presented to the two input ports of the 3 × 3 then both of the two possible output routing combinations for these two inputs can be accessed . if we now increase the switch module size to a 4 × 4 by adding a subsequent 4 × 4switching stage according to fig9 the same argument may again be made . for each of the four routing settings of the 4 × 4 switching stage the single uppermost input signal is routed to a certain output and the remaining lower three inputs are routed to the other three outputs in a certain prescribed manner . if these three inputs can now be rearranged to feed the inputs of the 4 × 4 switching stage in all of the possible combinations , then all routing possibilities for these three inputs can be achieved and all of routing combinations of the four inputs of the overall 4 × 4 switch can be accessed . such rearrangement of the three lower inputs is provided by the cascade of a 2 × 2 and a 3 × 3 switching stages , as just described , above . fig1 ( a ) illustrates in greater detail a fully configurable 3 × 3 switch comprising a cascade of a 2 × 2 and a 3 × 3 switching element , and fig1 ( b ) shows a fully configurable 4 × 4 switch comprising a cascade of a 2 × 2 , 3 × 3 , and 4 × 4 element , according to the above description . in this manner , a n × n non - blocking switch may be realized by cascading n − 1 switching elements of the type illustrated in fig5 where the switching elements increases from a 2 × 2 to a ( n − 1 )×( n − 1 ), as illustrated in fig9 . the number of phase control elements required by this incrementing cascade switch architecture arrangement is seen to be n ( n − 1 )/ 2 . thus , where n − 1 elements are used in each n × n stage ( recognizing the fact that only relative phases are important and that one connecting link therefore does not have to bear a control element ), a 16 × 16 switch , just 120 control elements are required . this number is significantly less than the ( n − 1 ) 2 , or 225 control required for the lattice switch arrangement illustrated in fig7 and the n 2 , or 256 , required for a matrix switch such as that of goh [ ref 8 ]. the switching table for the 3 × 3 non - blocking switch shown in fig1 is given in fig1 . it is an expansion of the table provided in fig1 and shows explicitly the phase settings of the control element in the 2 × 2 switching stage that effects the reversal of inputs 2 and 3 to the 3 × 3 switching stage element illustrated in the arrangement of fig1 . it is seen from fig1 that symmetry considerations limit the number of combined states employed by the phase control elements . these are the same symmetry considerations that determine the acceptable states of the individual phase elements in the cascade . in the case illustrated , there are 3 distinct states employed by the phase control elements of the 3 × 3 element and 2 states employed by the 2 × 2 element , for a total of 3 ! switch states . in practice , the electrical control circuitry may set the combination of phase controllers according to the switch state that has been selected , rather than set the phase settings of each individual phase controller element separately . in the above example of a 3 × 3 switch , this corresponds to 3 control settings for the 3 × 3 stage and 2 settings for the 2 × 2 stage . a total of just 2 × 3 , or 6 , six control settings are thus required , corresponding to the six routing settings of the overall 3 × 3 switch . in practice , the phase control elements of the switch fabric are not fabricated with sufficient accuracy to provide a - priori knowledge of their operational characteristics ; their performance must be determined after formation , inferred from the behavior of the overall switch module . in the case of the lattice switch of fig7 ( or in general for complex matrix switches where access to the individual switching elements is not possible ) the interaction between the control elements of each stage is complex and the individual characteristic of any given control element has to be extracted in an exacting and non - transparent manner from the overall performance of the complete switch fabric . in stark contrast , the performance of the individual switch elements of the arrangement of fig9 may be examined independent of each other by examining the routing behavior of the signal applied to the single uppermost input port ; each individual switch element is available to simple external examination and the control elements of the entire n × n switch may be set up according to a simple algorithm . a simple set - up procedure begins with a signal applied to input 1 with no inputs applied to the inputs 2 through n . the control elements of the last , n × n , switch element are then configured so that this input signal can be routed to each of its n output ports . by routing the input signal in sequence to the n outputs , the n − 1individual control elements of this n × n mmi - based switch element in the n − 1 &# 39 ; th stage of the switch may be tuned in to their optimum values . with the control elements on this last switch element optimized , the input signal on input line 1 may be removed and replaced by a signal on input line 2 . this signal passes directly to the ( n − 1 )×( n − 1 ) switch element and then through the final n × n switch element . with the control elements of the last n × n switch element now optimized and set to a known state , the control elements on the ( n − 1 ) x ( n − 1 ) switch may now be optimized by running through the routing states of that switch . once the control elements of this ( n − 1 ) x ( n − 1 ) switching stage are optimized , the control elements of the ( n − 2 ) x ( n − 2 ) switch may be optimized . and so on , until the solatory control element of the 2 × 2 switch is optimized . in this way , all the n ( n − 1 )/ 2 control elements of the fully non - blocking n × n switch may be readily optimized . waveguide crossings always cause a certain amount of the transmitted signal power to be lost and also introduces some cross - talk as some light from one path leaks into the other . they also require significant device area as large - radius waveguide bends are required in order to route without incurring bend - related power losses . waveguide crossings in switch elements should thus be keep to a minimum whenever practicable . the taught implementation of a non - blocking n × n routing switch , in contrast to traditional n × n switching elements , has no waveguide crossings at all , and is thus highly advantageous . it is recognized that the multimode interference regions of adjacent switches may be cojoined without the intermediate use of a connecting optical waveguide path . this makes for a more compact device structure and may offer the advantage of reduced optical insertion loss . this arrangement , illustrated for a 3 × 3 switch , is shown in fig1 . the cojoined slab region 130 substitutes for the 10 b coupler of the 2 × 2switch and the 10 a coupler of the 3 × 3 switch . it is also recognized that in order to reduce cross talk it might be advantageous to introduce intentionally optically absorbing or dispersing structures between the sequential switching elements to capture and eliminate stray light scattered from previous optical switching units . such absorber or dispersive structures may be provided by suitable deposited or grown materials or by the introduction of reflection facets formed by a suitable fabrication process such as etching . such structures would be placed so as to be optically distant from the interconnecting optical pathways but disposed so as to substantially block entry of stray light into area of the subsequent multimode coupler regions . a possible arrangement is illustrated in fig1 , with the absorbing structures 140 advantageously disposed between successive cascaded switches 91 . any convenient planar waveguide material system may be employed : silica planar waveguides [ ref . 1 ], ion - exchanged glass and dielectric waveguides [ ref . 2 ], or semiconductor - based waveguides [ ref . 3 ] the means of providing the phase control on the elements connecting the multiport couplers may be various and will depend on the waveguide material system employed . in the case of semiconductor - based waveguides , optical phase control may be effected by means of a voltage - induced movement of the semiconductor band - edge or by carrier injection ( or depletion ) within a section of the connecting waveguide . in the case of a dielectric waveguides such as lithium niobate , an applied voltage may be used to induce a refractive index change in the waveguide phase - control section . for silica - based wavguides , thermo - optic heating may be employed , in which the waveguide phase control section is heated and an optical phase change results from the consequent change in the waveguide refractive index . it is understood that this invention is not restricted to any specific planar waveguide material system , nor to any particular means of providing the phase control in the waveguide sections connecting the multi - post couplers . it is understood that the invention , although described with respect to its planar implementation , includes those realizations in three - dimensional systems such as may be provided by multi - layer planar waveguide devices or by fused fiber devices or bulk optical devices . it is understood that the invention includes all ‘ higher order ’ spatial switch architectures formed through the inter - connection , according to normal practice , of the basic switch elements described here . in addition to the space switching function considered in detail above , by arranging the optical path lengths of interconnections between linked mmi - couplers comprising a switch to differ by pre - determined multiples of the optical wavelength , the transmission function of the switch may be made wavelength sensitive . this provides a wavelength selective element and forms a wavelength division multiplexer , as have been reported [ refs . 18 , 19 , 20 , 21 ]. by introducing the switching function , a composite wavelength - 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