while this invention is susceptible of embodiment in many different forms , there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated . in telecommunication applications , a condition called traffic reciprocity often exists . traffic reciprocity is defined as the condition where input b is connected to output a whenever input a is connected to output b . the exemplary connections illustrated in fig1 do not correspond to reciprocal traffic . specifically , input 1 is connected to output 2 whereas input 2 is not connected to output 1 as required by the definition of reciprocal traffic . because the module 10 supports all traffic connections ( both reciprocal and non - reciprocal ), it provides greater flexibility than is required in applications where traffic reciprocity exists . the price of this flexibility is the requirement to have n 2 − 1 switchable deflectors or mirrors for an n × n switch . by exploiting the presence of traffic reciprocity , an exemplary 4 × 4 ( n = 4 ) switch module 1 o a as in fig2 described below exhibits reduced switch complexity as compared to the crossbar switch of fig1 . switch module 1 o a includes input / output ports 28 - 1 , - 2 , - 3 ... - n . each port is coupled to at least one source medium , such as an input optical fiber and at least one destination medium , an output fiber . it will be understood that module 1 o a could be operated under the control of control circuits 1 o a - 1 . these control circuits could be part of a larger communications system without departing from the spirit and scope of the present invention . it will also be understood that multiple reciprocal - traffic switches , such as module 1 o a , can be included in larger single or multiple stage switches . when reciprocity exists , the inputs and outputs can be co - located and paired as illustrated in fig2 . it will also be understood that each input / output port , such as 28 - i , can be coupled to an input optical fiber and an output optical fibre . alternately , a single coupled fiber could be used to carry bidirectional traffic . for an n × n switch , the gray rectangles 30 a , b , c ... n denote fixed deflectors or mirrors . they always deflect an incident beam . black rectangles 32 a , b denote actuated deflectors or mirrors and dashed rectangles 34 a , b , c denote non - actuated deflectors or mirrors . deflectors 32 a , b and 34 a , b , c are all switchable between states . [ 0029 ] fig2 illustrates an example where port 28 - 1 is coupled optically to port 28 - 4 and port 28 - 2 is coupled optically to port 28 - 3 . the forward path and the reverse path of the reciprocal traffic are deflected off the same deflectors or mirrors . assume a pair of reciprocal connections is to be made between input number a and number b where a & lt ; b . then the two deflectors used for this pair of paths are the fixed deflector or mirror in row a and the actuated deflector or mirror in row b in column a . although fig2 illustrates a 4 × 4 switch 1 o a , this architecture can be extended to an nxn switch module . the required number of switchable deflectors or mirrors is [ n *( n − 1 )/ 2 ]− 1 . this is about half as many as those used by the crossbar module 10 . an improvement can be made to the switch 1 o a using staggered input and output ports 38 - 1 , - 2 , - 3 ... - n as illustrated by switch lo b , fig3 . in fig2 the path lengths of the paths are of unequal length . path lengths are directly related to the amount of loss an optical signal incurs . the loss is due to the divergence of the light . the light diverges even in the presence of good collimating lenses . therefore , it is desirable to make all path lengths equal , regardless of connection , in order to reduce the variability in insertion loss . switch lo b , fig3 provides equal path lengths . in fig3 fixed and switchable deflectors are represented using the same conventions as used in fig2 . deflectors 40 a , b , c , d are fixed . remaining deflectors 42 a , b , c , d , e are switchable . as illustrated in fig3 ports 38 - 1 and 38 - 3 are coupled together , and ports 38 - 2 and 38 - 4 are coupled together . these respective path lengths are of substantially the same length . pairs of staggered input and output ports create equal length light paths , for example the connection between ports a and b where a & lt ; b . the deflectors used are the fixed deflectors on row a and the actuated deflector or mirror on row b . the fixed deflector or mirror is n units away from the input to port a . the actuated deflector or mirror is n − b + a units away from an input to port b . the distance between the two deflectors or mirrors is b − a . therefore , the total length , in free space , of the light path is n +( n − b + a ) +( b − a ) = 2n which , is independent of the particular choice of a and b . in another embodiment , switch 1 o c , fig4 can be modified to include a loop - back function . loop - back is present when an input at a port is to be coupled with the corresponding output at the same port . . adding a fixed deflector , such as a v - shaped mirror at the end of each row , such as deflectors 50 a , b , c , d , provides a loop - back function . deflectors 52 a , b , c . . . i are switchable . in this embodiment , the constant path length property of module 1 o b is almost preserved with the exception that loop - back paths are slightly longer . as illustrated in fig4 ports 48 - 1 and 48 - 4 are coupled together . port 48 - 2 is looped - back on itself . port 48 - 3 is unused . the number of switchable deflectors or mirrors for an n × n switch as in fig4 is [ n ( n + 1 )/ 2 ]− 1 . this is slightly larger than the number used by the switch 1 o a of fig2 without loop - back . however , it is still approximately one / half the number required by the switch 10 . there are a variety of possible physical implementations . the deflectors or mirrors can move in and out of position by using either a sliding or tilting mechanism . they could be non - moving multi - state solid state deflectors . the input and output fibers should be rested on v - grooves for better alignment with the collimating lenses . the lenses , deflectors or mirrors , and v - grooves may all be part of a mems ( micro - electromechanical systems ) platform . it will be understood that the details of implementation of the various deflectors or mirrors are not limitations of the present invention . switch configurations , such as 1 o a , 1 o b and 1 o c can be used as building blocks to create larger multi - stage switches for reciprocal traffic . fig5 illustrates an exemplary multi - stage switch 60 of a known type as disclosed in published pct application wo 00 / 14583 , assigned to the assignee hereof . as illustrated in fig5 the switch 60 employs two groups of switching modules 62 and 64 . the first group of modules 62 includes a plurality of ( l , 2l − 1 )- way modules 1 − m . the ( l , 2l − 1 )- way modules 62 can be implemented in a variety of ways , as would be understood by those of skill in the art and are not a limitation of the present invention . the second group of modules 64 includes a plurality of m - way reciprocal switching modules 1 − 2l 31 1 . the m - way modules 64 can be implemented in accordance with the principles of any of the modules described above in connection with fig2 - 4 . the modules 62 are connected to the modules 64 so the externally disposed i / o ports 66 handle reciprocal traffic in a non - blocking fashion . to this end , the modules are interconnected by optical fibers as illustrated in the exemplary switch 60 of fig5 . it will be understood that a variety of switch architectures using modules 1 o a , 1 o b , 1 o c , could be implemented in multi - stage switches to support reciprocal switch traffic without departing from the spirit and scope of the present invention . from the foregoing , it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention . it is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred . it is , of course , intended to cover by the appended claims all such modifications as fall within the scope of the claims .