Abstract:
A closed loop alignment system for cross-connect switches in optical telecommunications systems. Arrays of small tiltable mirrors within the switch direct the signal beams between optical fiber arrays connected to the switch. An external alignment system is incorporated into the cross-connect switch and is employed to provide an alignment beam of a wavelength different from the signal beams. Beam splitters such as dichroic mirrors, which pass the signal beams but pass only a portion of the alignment beams, are placed in the beam paths to image the ends of the optical fibers of the arrays. A combined image of the ends of the optical fibers from each array is reflected onto a photodetector array, output signals of which are sent to a feedback control system which compares the position, size, and shape of the combined image to a stored tolerance range of acceptable positions, sizes, and shapes, and consequently moves the tiltable mirrors so that the signal beams correctly enter the ends of the optical fibers in the opposite optical fiber array.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to fiber optic, cross-connect switches employing individual tiltable mirrors which direct signal beams within the switches between the desired optical fibers in telecommunications systems, and more specifically to an alignment system for such switches.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Optical telecommunications systems are increasingly replacing cable and other wire-based electronic telecommunications systems. This is directly related to the speed of light at which data is transmitted through optical fibers, the ability to transmit data in parallel using different wavelengths of light, the ability to simultaneously transmit data in both directions along each optical fiber, and the increased miniaturization and lower cost of the optical components necessary to build optical telecommunications systems.  
           [0005]    Optical telecommunications systems typically require switching of the data transmitted by the light from the end of one optical fiber to another which requires mechanical switching of the light path. Advances in miniaturization of optical switching based on micro-electromechanical system (MEMS) mirrors are now making optical communications systems both more economical to build and more reliable in use. Such MEMS mirrors are typically constructed as a two dimensional array of tiltable mirrors as part of an optical cross-connect switch, which mirrors direct light from an emitter end of one optical fiber to a target end of another optical fiber. Each end of an optical fiber can simultaneously be both an emitter end and a target end. The tiltable mirrors can simultaneously rotate about individual “X” and “Y” axes, each tiltable mirror being individually suspended above a base or substrate by a plurality of flexible suspension arms attached to the substrate. The signal beam travels using free-space light transmission between the optical fibers, making numerous configurations and sizes of optical cross-connect switches and systems possible. The tiltable mirrors are tilted by electrostatic, electro-magnetic, piezoelectric, or thermal actuation forces which are induced between the tiltable mirrors and the substrate through a controller. The MEMS mirror arrays may have on the order of 1000×1000 individual mirrors sized with a typical diameter or diagonal in the range of about 300 um to about 1000 um. The tiltable mirrors can be shaped as a circle, ellipse, polygon, or rectangle, and can be planar or curved, with planar being typical due to ease of construction. The signal beam is typically a circular beam. Respective circular collimating lenslets are typically, but optionally, disposed closely adjacent the ends of the optical fibers to focus the signal beams as they exit the emitter ends and enter the target ends thereof.  
           [0006]    Other components used in optical telecommunications systems may include beam combiners and beam splitters for multiplexing and demultiplexing optical signal beams having different wavelengths. Such beam combiners and beam splitters typically utilize a dichroic mirror disposed at an angle to the path of the signal beams. The dichroic mirrors function in a known manner, reflecting part or all of a preselected wavelength of light while transmitting the remaining wavelengths, or by transmitting the preselected wavelength of light and reflecting the remaining wavelengths. Therefore, either the preselected wavelength or the other wavelengths of light are passed through the dichroic mirror in a substantially straight path while part or all of the other wavelengths are reflected at a predetermined angle relative to the angle of incidence of the light beam.  
           [0007]    Another component used in various arts are charge-coupled device (CCD) cameras which comprise a large number of photosensitive detectors which are grouped, such as in a flat rectilinear array. Each detector includes a photosensitive front and a pair of electrical outputs through which electrons are emitted when light of a predetermined range of wavelengths shines on the photosensitive front. The outputs of the detectors of the CCD camera are input to a processing unit for analysis as needed. CCD cameras are used for various purposes in which the position, intensity, and wavelength of light needs to be recorded as an electrical signal to be analyzed electronically.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the present invention, a closed loop alignment system for fiber optic, cross-connect switches is used in order to ensure that signals are accurately coupled from optical fibers in one array or group to respective optical fibers in a second array or group. The optical fiber arrays may be spaced by any practical distance as the requirements of the cross-connect system may dictate or allow. A visual optical probe or alignment light beam passes through a first beam splitter and is superimposed on the ends of the signal-emitting or signal receiving optical fibers. Using that optical beam, the images of the ends of the signal-emitting optical fibers are then reflected as multiple light beams, through the use of the tiltable mirror arrays (MEMS) of the cross-connect switch, onto the ends of respective signal-receiving fibers, thereby ensuring full signal transfer or coupling between optical fibers at a distance from each other. The fibers in each array may be either signal-emitting or signal receiving, and may be both simultaneously. Dichroic mirrors are preferably employed as the beam splitters to partially reflect and partially pass the visual alignment beams, and to pass substantially all of the signal beams, which have a wavelength different from the visual alignment beams. A detector at the side of the cross-connect switch opposite to the probe beam source is employed to determine beam alignment.  
           [0009]    The detector employed to receive the probe beams is an array of a plurality of sensors for detecting light beams which impinge on it, the detector array being disposed in the cross-connect switch adjacent a second beam splitter. A portion of the probe beams of the combined alignment and signal beams passes through the second beam splitter and reflects off the ends of the optical fibers of the second fiber optic array to form an image of the ends of the optical fibers of the first fiber optic array superimposed upon the image of the ends of the optical fibers of the second fiber optic array. The alignment beam carrying the combined probe beam image passes back to the second beam splitter, and a portion of that beam reflects from the second beam splitter onto the detector array as the combined image.  
           [0010]    A feedback control system receives output signals from the photosensitive sensors of the detector array. The feedback control system compares the position, size, and shape of the combined image to a stored tolerance range of acceptable relative positions, sizes, and shapes for the combined image. The feedback control system generates and sends corrective feedback signals to mirror drivers as mirror positioning signals to correct for any deviation between the actual position, size, and shape of the combined image and the desired tolerance range of acceptable positions, sizes, and shapes of the images to correctly aim the tiltable mirrors of each mirror arrays. When properly aligned so that the images are coincident, signals are fully coupled between the respective fibers in the optical arrays.  
           [0011]    The invention also includes the method of providing closed-loop feedback in fiber optic, cross-connect switches employing the structures described.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0012]    The objects, features and advantages of the invention will be more clearly perceived from the following detailed description, when read in conjunction with the accompanying drawing, in which:  
         [0013]    [0013]FIG. 1 is a diagrammatic view of a closed-loop alignment system according to an embodiment of the invention as part of a first fiber optic cross-connect switch;  
         [0014]    [0014]FIG. 2 is a diagrammatic view on an enlarged scale of a combined image on a detector array of the system of FIG. 1;  
         [0015]    [0015]FIG. 3 is a block diagram of exemplary electronics for the system of FIG. 1;  
         [0016]    [0016]FIG. 4 is a diagrammatic view of a second embodiment of a closed-loop alignment system in accordance with the invention, the alignment system functioning with a second fiber optic cross-connect switch embodiment;  
         [0017]    [0017]FIG. 5 is a diagrammatic view of a third embodiment of a closed-loop alignment system in accordance with the invention as part of a third fiber optic cross-connect switch embodiment;  
         [0018]    [0018]FIG. 6 is a diagrammatic view of a fourth embodiment of a closed-loop alignment system in accordance with the invention, the alignment system functioning with a fourth fiber optic cross-connect switch embodiment; and  
         [0019]    [0019]FIG. 7 is a diagrammatic view of a fifth embodiment of a closed-loop alignment system in accordance with the invention as part of a fifth fiber optic cross-connect switch embodiment.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Referring to FIG. 1, 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 .  
         [0021]    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 .  
         [0022]    Closed-loop alignment system  20  of an embodiment of the invention, as shown in FIG. 1, 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.  
         [0023]    The alignment system of FIG. 1 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.  
         [0024]    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.  
         [0025]    Referring to FIGS. 2 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.  
         [0026]    Referring to FIG. 4, 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 .  
         [0027]    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.  
         [0028]    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 FIG. 1 embodiment. Except for intermediate fixed mirror  71 , the FIG. 4 embodiment operates in the same manner as the FIGS. 1-3 embodiment. Alignment beams  78  are employed to align respective signal beams  44  without wavelength interference.  
         [0029]    Referring to FIG. 5, therein is shown a diagrammatic representation of closed loop alignment system  320 , functioning with third fiber optic cross-connect switch  149 .  
         [0030]    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 FIG. 5 embodiment is a different arrangement of the same elements as in FIG. 4, which elements function in the same manner as before.  
         [0031]    Referring to FIG. 6, therein is shown a diagrammatic representation of closed loop alignment system  420 , functioning with fourth fiber optic cross-connect switch  152 .  
         [0032]    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.  
         [0033]    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 .  
         [0034]    Referring to FIG. 7, therein is shown a diagrammatic representation of closed loop alignment system  520 , functioning with fifth fiber optic cross-connect switch  161 .  
         [0035]    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.  
         [0036]    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.  
         [0037]    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 .  
         [0038]    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.  
         [0039]    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.  
         [0040]    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.  
         [0041]    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.  
         [0042]    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.  
         [0043]    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.  
         [0044]    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.