Abstract:
A device and method for detecting rotational drift of mirror elements in a MEMS tilt mirror array used in an optical crossconnect. The optical crossconnect directs optical signals from an input fiber to an output fiber along an optical path by rotatably positioning mirror elements in desired positions. A monitoring device disposed outside of the optical path is used to obtain images of the MEMS array or to transmit and receive a test signal through the crossconnect for detecting the presence of mirror element drift.

Description:
This application is based on U.S. Provisional Application Ser. No. 60/164,459 filed on Nov. 10, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to fiber optic communications systems and, more particularly, to monitoring devices and methods for monitoring shifts in optical crossconnect configurations utilizing micro electromechanical systems (MEMS) tilting mirror arrays. 
     2. Description of the Related Art 
     In fiber optic communication systems, signal routing is essential for directing an optical signal carrying data to an intended location. Existing routing techniques typically experience optical power loss due to inefficient coupling of optic signals between input and output fibers. This increases the dependence on optical power sources (e.g., pump lasers) which are used to compensate for power losses by injecting optical power back into the optical system. The need for optical power sources increases the overall cost of the optical system. 
     Another criteria for signal routing is the ability to direct a signal received from one of a plurality of input fibers or ports to any of a plurality of output fibers or ports without regard to the frequency of the optical signal. 
     Free-space optical crossconnects allow interconnecting among input and output ports in a reconfigurable switch fabric. An example of such an optical crossconnect utilizing mirco-electromechanical systems (MEMS) tilting mirror devices is disclosed in commonly owned and copending U.S. patent application Ser. No. 09/410,586, filed Oct. 1, 1999. By adjusting the tilt angles of the MEMS mirror devices, optical signals can be directed to various destinations, i.e. to numerous output fibers. 
     MEMS devices and, in particular, tilting mirror devices are susceptible to unwanted movement or drift due to external factors such as temperature changes and mechanical fatigue experienced by actuator elements used to deploy and control the individual mirror elements. As a result, optical signal power may be lost due to misalignment of the reflected optical signal with its intended target (e.g. an output fiber). Accordingly, a system is desired to monitor MEMS optical crossconnect configuration to provide for displacement adjustment. 
     SUMMARY OF THE INVENTION 
     An optical crossconnect device having a monitoring feature for detecting optical signal drift is provided. The device provides optical connection of optic signals between input fibers and output fibers by using a MEMS tilt mirror array. The MEMS array includes a plurality of tiltable mirror elements which are positionable in an intended orientation for directing optical signals, but which are susceptible to drift that causes degradation in the optical coupling of the signals to the output fibers. A monitoring device positioned outside of the optical path dynamically monitors the position of one or more of the mirror elements to detect drift. 
     In a preferred embodiment, the monitoring device is a camera for obtaining an image of one or more mirror elements. 
     In another embodiment, the monitoring device comprises an optical transmitter and an optical receiver for transmitting a test signal through the optical crossconnect to monitor mirror position drift. 
     In yet another embodiment, a pattern is formed on one or more of the mirror elements and an image or reflection of the pattern is obtained for determining the presence of mirror drift. 
     A method is also described for monitoring mirror element positions of mirror elements in a MEMS tilt mirror array used in an optical crossconnect. The method is used with a MEMS mirror array having mirror elements disposed at desired tilt positions for crossconnecting an optic signal between an input fiber and an output fiber along an optical path. A monitoring device disposed outside of the optical path monitors the positions of the mirror elements to detect when position drift occurs. The mirror positions are then adjusted by forming control signals based on the detected drift and applying the control signals to the drifted mirror elements. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate and explain the structures and procedures described herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, wherein like reference numerals denote similar elements throughout the several views. 
     FIG. 1 is a planar view of an example of a MEMS mirror array used in connection with the present invention; 
     FIG. 2 is a schematic representation of an optical crossconnect monitoring device in accordance with one embodiment of the present invention; and 
     FIG. 3 is a schematic representation of a monitoring device for a “folded” optical crossconnect in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Arrays of two-axis tilt mirrors implemented using micro-electromechanical systems (MEMS) technology in accordance with the invention allow for the construction of large scale optical crossconnects for use in optical systems. Optical crossconnects are commonly employed to connect a number of input optical paths to a number of output optical paths. A typical requirements of optical crossconnects is that any input be capable of being connected to any output. One example of a MEMS mirror array  10  is depicted in FIG.  1 . The mirror array  10  includes a plurality of tilt mirrors  12  formed on a substrate  11 , mounted to actuation members or springs  14  and controlled by electrodes (not shown). Each mirror  12  is approximately 100-500 Microns across, may be shaped as square, circular or elliptical, and is capable of operatively rotating or tilting about orthogonal X-Y axes, with the tilt angle being selectively determined by the amount of voltage applied to the control electrodes. Further details of the operation of the MEMS mirror array  10  are found in copending U.S. patent application Ser. No. 09/415,178, filed Oct. 8, 1999, the entire contents of which are incorporated herein by reference. The general concept of utilizing two or more such tilt mirror arrays  10  to form an optical crossconnect is disclosed in copending U.S. patent application Ser. No. 09/410,586, filed Oct. 1, 1999, the entire contents of which are also incorporated herein by reference. 
     The use of one or more MEMS tilt mirror arrays in conjunction with a lens array is disclosed in co-pending U.S. patent application Ser. No. 09/512,174, filed Feb. 24, 2000, the entire content of which is also incorporated herein by reference. As disclosed in that application, various optical crossconnect configurations of compact size (i.e. minimal spacing between crossconnect components) and exhibiting minimal optical power loss can be realized. One such optical crossconnect  100  discussed in the aforementioned application is depicted in FIG.  2 . Crossconnect  100  receives input optic signals  108  through a plurality of optic fibers  112 , preferably formed in an array as is well known in the art. For ease of illustration fiber array  110  is shown as a one-dimensional array having four fibers  112   a ,  112   b ,  112   c ,  112   d . It is in any event to be understood that fiber array  112  as well as other fiber arrays discussed herein are preferably two-dimensional arrays such as, for example, N×N arrays. 
     Fiber array  112  transmits the optical signals  108  to an array of lenses  114  that function as collimating lenses. The lens array  114  is positioned relative to fiber array  112  so that each lens communicates with a corresponding fiber for producing pencil beams  116  from the optic signals  118 . Thus, beam  116   a  is produced from a signal carried by fiber  112   a , beam  116   d  is produced from a signal carried by fiber  112   d , etc. 
     A first MEMS tilt mirror array  118 , also referred to as the input array, is positioned in alignment with lens array  114  so that each mirror element  12  will receive a corresponding beam  116 . The mirror elements are operatively tilted, in a manner discussed in application Ser. No. 09/415,178, to reflect the respective beams  116  to a second or output MEMS mirror array  122  positioned in optical communication with MEMS array  118 . Depending on the tilt angle of each mirror element in input MEMS array  118 , the reflected signals can be selectively directed to specific mirror elements in output MEMS array  122 . To illustrate this principle, beam  116   a  is shown in FIG. 2 generating reflection beams  120   a  and  120   a ′ and beam  116   d  is shown in the figure generating reflection beams  120   d  and  120   d ′. These beams are received by mirror elements in the output MEMS array  122  and are directed as beams  124  to an output lens array  126 . An output fiber array  128  is aligned with lens array  126  to receive and output optical signals  129 . Thus, lens array  126  couples beams  124  into the output fiber array  128 . 
     The rotatable positions or orientations of the individual mirror elements  12  of arrays  118  and  122  are, however, affected by environmental conditions such as temperature changes. As a result, once the positions of the mirror elements  12  are set, those intended positions may drift or change due (for example) to temperature variations, thereby adversely causing inefficient or unintended signal routing and associated power losses. A similar problem may be caused by mechanical fatigue and stress on the actuators used to control mirror position, and by electric charging effects on the actuators. These variations can result in conditions referred to as macro-drift, wherein all of the mirror elements in an array drift by an equal amount, and micro-drift, in which only some of the mirror element positions unintendedly change. 
     To detect such unwanted mirror drift in optical crossconnects in accordance with the present invention to compensate for actual mirror positions, one or more monitoring devices  130 ,  132  are included in the crossconnect system  100  shown in FIG.  2 . The monitoring devices may be used to detect both macro-drift and micro-drift conditions of the MEMS mirror arrays  118 ,  122 . For example, each monitoring device may be a camera or other imaging devices which operates independently of other cameras. Each camera is shown in FIG. 2 positioned outside of the optical path of the crossconnect (i.e. the path in which optical signal  116  travels through the crosssconnect to fiber array  128 ) and obtains an image of its respective MEMS array. Thus, camera  130  is focussed on MEMS array  118  and camera  132  is focussed on MEMS array  122 . The resulting images are then compared to reference images of mirror array positions stored, for example, in a controller block  500  containing a processor and a database (not shown) in a manner well-known to those having ordinary skill in the art. In the event that an unacceptable amount of drift is detected for the entire mirror array, feedback control signals can be generated by the control block  500  for adjusting the tilt angles to compensate for drift by applying appropriate voltages to the mirror actuators. If on the other hand only certain mirror elements need to be adjusted, these mirrors can be identified, through the aforementioned image comparison with a reference image, and then re-positioned by applying appropriate voltages to the desired actuators. 
     The monitoring system of FIG. 2 can also be employed in connection with a folded crossconnect configuration, as for example shown in FIG. 3, wherein a single input/output fiber array  312 , single MEMS mirror array  318 , and reflective surface element  330  comprise the folded configuration. A camera  340  positioned outside of the optical path  316  obtains an image  342  of the mirror elements in the array  318  for use in calculating and compensating for detected drift. 
     As an alternative or in addition to the use of cameras, device  130  (FIG. 2) may comprise one or more illuminators (not shown) for producing, for example, one or more infra-red beams  131 ,  133  directed at mirror arrays  118 ,  122  and devices  130 ,  132  may comprise an infra-red detector for detecting the reflected infra-red beams. The illumination source may produce a test signal having a different wavelength from the signal wavelength or can be modulated to discriminate and distinguish it from the signal wavelength. The infrared beams  131 ,  133  may be pencil beams for illuminating a single mirror element which may be designated as a reference element, such as element  16  in FIG.  1 . The reflected infra-red signal will pass through the optical crossconnect for receipt by its respective infra-red detector. For example, for an infra-red test beam directed at a mirror element in array  118 , the test beam will be reflected and directed to detector  130 , and for an infrared beam directed at a mirror element in array  122 , the test beam will be received by detector  132 . Depending on the characteristics of the reflected and received infra-red beams—such as a reduction in beam power or intensity and/or a change of position on the detector at which the beam is received, etc.—macrodrift can be dynamically detected. For example, and as a result of a temperature change, drift may occur among all mirror elements in mirror arrays  118 ,  122 . By measuring and detecting drift from a reference mirror element (e.g. mirror  16 ), the mirror arrays can be adjusted to compensate for drift by generating appropriate feedback signals from control blocks  500  to be applied to mirror control actuators. 
     It will be appreciated that both devices  130 ,  132  can operate as combined or dual-function source/receiver devices wherein each device produces a signal for receipt by the other and receives a signal produced by the other. Likewise, and in connection with the folded configuration of FIG. 3, device  340  can be implemented by or supplemented with a detector/receiver for receiving reflected test signals  342 ,  343  generated by a source such as an infrared source  350  for illuminating one or more mirror elements  12 . 
     For micro-drift compensation, the devices  130 ,  132  in the system  100  of FIG.  2  and the device  340  in the system  300  of FIG. 3 can be connected to a scanning device which may be found in controller block  500  for changing the position of the test beam (beam  130  in FIG.  2  and beam  342  in FIG. 3) to illuminate multiple mirror elements. For example, the scanner can adjust the test beam position to illuminate one mirror element  12  at any given time for determining the tilt angle of each illuminated mirror. 
     As another alternative, the reference mirror element  16  may be formed with an imaging pattern  14 , as for example by surface etching. This modification allows for the use of pattern recognition techniques wherein a generated pattern is received or monitored by a detector or camera. Detected movement of the pattern indicates mirror drift. Pattern  14  may be specifically oriented to generate a unique pattern that is observable in scattered light so as to provide an enhanced signature when a light beam is centered on mirror  16 . A single unique pattern may be used for all mirrors, or each mirror can be coated with its own unique pattern. Entire pathways through the mirror array may be defined by unique patterning, thus helping to guide light beams through the array during switching. 
     Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods disclosed and devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.