referring to fig1 there is shown a diagrammatic representation of an exemplary closed - loop alignment system according to an embodiment of the invention , designated generally at 20 , functioning with fiber optic cross - connect switch 23 . the basic structure of known cross - connect switch 23 comprises respective first and second two - dimensional mems mirror arrays 26 and 29 , each of which includes a plurality of respective individually tiltable mirrors 32 and 35 pivotally mounted on respective substrates 38 and 41 . mirrors 32 and 35 are used to direct a plurality of light beams , represented by signal beam 44 of light ( only one signal beam 44 being shown for simplicity ), emitted through ends 45 and 46 of a selected one of individual optical fibers 47 and 50 of respective first and second two - dimensional fiber optic arrays 53 and 56 , to a desired optical fiber 50 , 47 of the respective opposing fiber optic array 56 , 53 . the wavelength range typically used for optical telecommunications is about 1 , 250 to 1 , 650 nm , preferably at the infrared range of about 1500 nm . respective first and second lenslet arrays 59 and 62 , each comprising respective individual collimating lenslets 65 and 68 corresponding to a respective individual optical fiber 47 and 50 , can be used to focus signal beams 44 out of fiber ends 45 and 46 into the respective optical fiber 47 and 50 . note that the number of operational mirrors 32 and 35 , first and second optical fibers 47 and 50 , and lenslets 65 and 68 , are equal . lenslets 65 and 68 are optional in this system . fixed mirror 71 is disposed opposite mirrors 32 and 35 between fiber optic arrays 53 and 56 . while such cross - connect switches 23 , as described thus far , are typically calibrated for temperature , there is no way to adjust them in operation , such as by providing feedback as to the accuracy of the positioning of mirror arrays 26 and 29 relative to optical fibers 47 and 50 . closed - loop alignment system 20 of an embodiment of the invention , as shown in fig1 is employed to align the signal beams from the ends of fibers 47 and 50 to the ends of the fibers in the opposite array of optical fibers . the alignment system of the invention injects a visible wavelength alignment beam directly into the signal beams and detects visible wavelength combined fiber - ends images to determine the extent of alignment of the signals between the optical fiber arrays . this alignment system arrangement , by being coincident with the signal beams , compensates for time and temperature drift which can occur in such cross - connect switches . the alignment system of fig1 includes respective first and second selectively semi - transparent optical components or beam splitters 74 and 77 which are disposed , respectively , between first fiber optic array 53 and first mirror array 26 , and between second fiber optic array 56 and second mirror array 29 . the beam splitters are preferably dichroic mirrors but other functionally equivalent elements , such as diffraction gratings , could be used . light source 75 emits a probe or alignment beam 76 having a wavelength differing from the signal wavelength by at least 50 nm . the wavelength of beam 76 can range from about 400 nm to about 900 nm , and for practical purposes is preferably about 800 nm . because of the wavelength difference , this visible light beam does not interfere with signal beams 44 when they coincide . light beam 76 intersects with or impinges on first dichroic mirror 74 and at least a portion of this alignment beam is reflected onto fiber ends 45 . the images of the ends of fibers 47 form a plurality of light beams 78 which are reflected back to and partially through mirror 74 . these alignment light beams are coincident with signal beams 44 , which originate externally to fibers 47 and pass therethrough , and generally unimpeded through mirror 74 . dichroic mirror 77 at the output of the alignment system functions in a similar manner , as is explained below . dichroic mirrors 74 and 77 are designed with a known layering scheme ( not shown ) wherein light at certain predetermined wavelengths ( for example , signal beams 44 ) substantially pass therethrough without significant absorption or reflection . however , about fifty percent of light at other wavelengths , such as alignment beam 76 , at an incident angle of , for example , about forty - five degrees , is reflected at a complementary forty - five degree angle ( ninety - degree included angle ) and the remaining fifty percent passes therethrough . therefore , first dichroic mirror 74 acts to pass fifty percent of alignment beam 76 therethrough , which is lost , and fifty percent is reflected to optical fibers 47 . an alignment beam 78 reflects off end 45 of an optical fiber 47 and retraces its path as an image thereof to first dichroic mirror 74 , again with fifty percent lost ( being reflected back toward light source 75 ), and fifty percent passing therethrough to mirror array 26 . alignment beam 78 is reflected from its respective tiltable mirror 32 at the desired angle as controlled by processing and control system 92 , off fixed mirror 71 , and off a tiltable mirror 35 of second mirror array 29 , which is also controlled by processing and control system 92 , to second dichroic mirror 77 . second dichroic mirror 77 acts to reflect fifty percent of alignment beam 78 , which is lost , and pass fifty percent therethrough to optical fibers 50 . an alignment beam 78 reflects offend 46 of an optical fiber 50 and retraces its path as a combined image of ends 45 and 46 of an optical fibers 47 and 50 to second dichroic mirror 77 , with fifty percent passing therethrough , which is lost , and fifty percent being reflected therefrom as beam 79 onto detector array 83 as combined image 101 . note that a signal beam 44 may travel both from fiber optic array 53 to 56 , and from fiber optic array 56 to 53 , simultaneously in both directions in all of the optical fibers since alignment system 20 positions the tiltable mirrors 32 and 35 of mirror arrays 26 and 29 the same , regardless of the direction of travel of signal beam 44 . the purpose of the alignment is to have near complete overlap of the images of the ends 45 , 46 of fibers 47 , 50 , thereby ensuring optimal coupling of optical signals between the fibers . referring to fig2 and 3 , detector array 83 may comprise an array of relatively inexpensive silicon detectors . one example is a charge - coupled device ( ccd ) camera having a plurality of individual photosensitive detectors or sensors 119 such as in a 3000 × 3000 or a 5000 × 5000 array . an output signal ( os ) of each individual photosensitive sensor 119 is input to processing and control system 92 , which includes feedback controller 122 that compares the relative position , size , and shape of combined image 101 , each being comprised of an image 123 of end 45 of one optical fiber 47 , and an image 124 of end 46 of one optical fiber 50 , to an acceptable relative position , size , and shape of a combined image ( ci ) stored in image memory 128 . if an image 101 is out of tolerance in position , size , or shape , mirror position correction information is developed by feedback controller 122 and an appropriate feedback signal ( fs ) is sent to mirror driver 131 which integrates the feedback signal fs with newly arriving optical switching information ( osi ) being communicated to mirror driver 131 from master switching controller 134 . appropriately corrected drive signals ( cds ) are sent from mirror driver 131 to first or second mirror arrays 26 and 29 , or both , to properly position respective individual tiltable mirrors 32 and 35 based on the feedback signal fs . referring to fig4 therein is shown a diagrammatic representation of closed loop alignment system 220 in accordance with the invention , functioning with second fiber optic cross - connect switch 146 . cross - connect switch 146 comprises mirror arrays 26 and 29 , but note that there is no fixed mirror 71 disposed opposite the mirrors 32 and 35 between the fiber optic arrays 53 and 56 . in this switch embodiment , the respective mirror arrays 26 and 29 are tilted inwardly at about a forty - five degree angle to allow direct reflection of signal beams 44 emitted through respective optical fibers 47 and 50 directly from respective mirrors 32 and 35 to the other thereof , and to the desired optical fiber 47 and 50 of the respective opposing fiber optic array 53 and 56 . the respective , optional , first and second lenslet arrays 59 and 62 , having respective individual collimating lenslets 65 and 68 , function in the same manner as in the earlier embodiments . closed - loop alignment system 220 includes dichroic mirrors 74 and 77 which are disposed , respectively , between first fiber optic array 53 and first mirror array 26 , and between second fiber optic array 56 and second mirror array 29 . light source 75 emits the visible wavelength alignment beam 76 which impinges on the first dichroic mirror as previously described with respect to the fig1 embodiment . except for intermediate fixed mirror 71 , the fig4 embodiment operates in the same manner as the fig1 - 3 embodiment . alignment beams 78 are employed to align respective signal beams 44 without wavelength interference . referring to fig5 therein is shown a diagrammatic representation of closed loop alignment system 320 , functioning with third fiber optic cross - connect switch 149 . as with the previously described embodiments , cross - connect switch 149 comprises the mirror arrays 26 and 29 with the respective tiltable mirrors 32 and 35 . mirrors 32 and 35 are used to direct the signal beams 44 emitted through the optical fibers 47 and 50 of the respective fiber optic arrays 53 and 56 to the desired optical fiber 47 and 50 of the opposing fiber optic array 53 and 56 . lenslet arrays 59 and 62 function in the same manner as in the previously described embodiments . the fig5 embodiment is a different arrangement of the same elements as in fig4 which elements function in the same manner as before . referring to fig6 therein is shown a diagrammatic representation of closed loop alignment system 420 , functioning with fourth fiber optic cross - connect switch 152 . cross - connect switch 152 differs from the previously described embodiments in that it comprises mirror array 26 with tiltable mirrors 32 but does not include the second tiltable mirror array 29 . this is a more basic cross - connect switch embodiment . mirrors 32 are used to direct the signal beams 44 emitted through optical fibers 47 and 50 of fiber optic arrays 53 and 56 to the desired optical fiber 47 and 50 of the opposing fiber optic array 53 and 56 . the respective , optional , first and second lenslet arrays 59 and 62 , having respective individual collimating lenslets 65 and 68 , function in the same manner as in the previously described embodiments . note that this embodiment shows that the alignment system of this invention can function with an optical cross - connect switch having a single mirror array . it also reveals that the relative angles of the components are quite flexible , it only being necessary to direct signal beams from one fiber array to another . the alignment apparatus can be arranged to conform to any cross - connect switch configuration . this embodiment of closed loop alignment system 420 includes dichroic mirrors 74 and 77 which are disposed , respectively , between first fiber optic array 53 and mirror array 26 , and between second fiber optic array 56 and mirror array 26 . light source 75 emits the alignment beam 76 , as before , which impinges on first dichroic mirror 74 . light detector array 83 is affixed adjacent second dichroic mirror 77 to receive reflected light therefrom . as before , signal beams between optical fibers 47 and 50 are aligned when the optical images of ends 45 and 46 of the respective optical fibers are aligned , as detected by detectors 119 on array 83 . referring to fig7 therein is shown a diagrammatic representation of closed loop alignment system 520 , functioning with fifth fiber optic cross - connect switch 161 . cross - connect switch 161 comprises mirror array 26 and mirror array 164 , comprising single mirror element 35 pivotally mounted on substrate 167 . mirrors 32 and 35 are used to direct signal beams 44 , emitted through optical fibers 47 of fiber optic array 53 , and optical fiber array 56 , containing single second optical fiber 50 , to a desired optical fiber 47 and 50 of the respective opposing fiber optic array 56 and single optical fiber 50 . respective , optional , lenslet arrays 59 and 62 function in the same manner as in the previously described embodiments . closed loop alignment system 520 includes dichroic mirrors 74 and 77 which are disposed , respectively , between first fiber optic array 53 and first mirror array 26 , and between second fiber optic array 56 ( optical fiber 50 ) and second mirror array 164 ( mirror 35 ). light source 75 emits visual alignment beam 76 which impinges on first dichroic mirror 74 . light detector array 83 is affixed adjacent second dichroic mirror 77 to receive reflected light therefrom . the optical alignment system operates in the same manner as has been previously described with respect to other embodiments of the cross - connect switch . the primary difference is that one optical fiber array consists of a plurality of fibers and the other array is a single fiber . in this embodiment , all signal beams from optical fiber array 53 are aligned with single fiber 50 . one purpose for this optical switch is to mulitplex a selectable subset of different wavelength signals from different optical fibers 47 into single fiber 50 . it should be understood that the cross - connect switch can function as a demultiplexer where the optical signals enter through the single optical fiber and are dispersed as multiple signals through the array of multiple fibers . appropriate changes as to the relative locations of the alignment beam source and the detector array can be made as needed . it can be seen that the alignment system of the invention is external to and can be adapted to any optical fiber cross - connect switch , and any practical combination of angles can be accommodated . it is even possible to employ the present alignment system with a cross - connect switch having the input and the output optical fibers bundled together in adjacent arrays . the ends of the fibers could all be oriented in the same direction or they could be oriented so that the coupling ends of the input and output fibers are 1800 apart , with the fibers all in parallel . for purposes of providing practical examples , probe or alignment beam source 75 can be any of several different types . it could be an arc lamp , a laser diode , an led , or an incandescent lamp , among others . where there are unwanted wavelengths a filter can be employed at the output of the light source . beam 76 is preferably collimated . it could be a coherent light beam , but that is not necessary . the wavelength of the probe beam should be matched with the most efficient detection wavelength of detector 83 . silicon sensors are preferred because they are effective and inexpensive , and a light beam at about 800 nm is easily detected by them . in reality it is envisioned that the probe beam can range in wavelength from the visible to the near - infrared . with the losses created in the alignment beam by the beam splitters , only about 6 % of the intensity of the original beam arrives at detector 83 . this does not matter , as long as there is sufficient light to accomplish the alignment purpose . because of their construction , the beam splitters pass the signal beams with only nominal or insignificant losses in intensity . in any or all of the embodiments shown and described , processing and control system 92 can be any suitable device such as a microcomputer or a pc , among others . it need only be able to accomplish the functions described and no specific device or element is necessary for this invention . whereas this invention is here illustrated and described with reference to embodiments thereof presently contemplated as the best mode of carrying out the invention , in actual practice it is to be understood that various changes may be made in adapting the invention to different embodiments without departing from the broader inventive concepts disclosed herein and comprehended by the claims that follow , and their equivalents .