Abstract:
An apparatus having magnetic detection sensor deployed on a micro machined optical element is exposed to a magnetic field to sense change in property as the micro machined optical element is manipulated with respect to the magnetic field, and, conversely when the magnetic field is manipulated with respect to the micro machined optical element. The electrical, optical and/or mechanical change in sensor property varies according to said manipulation, and telemetry created by said property change tracks the manipulation of the moveable portion of the optical element. The system includes a configuration capable of compensating for temperature variation.

Description:
CROSS REFERENCE TO RELATED APPLICAITON  
       [0001]    This application is a division of and claims priority from U.S. patent application Ser. No. ______ , by Murali Chaparala entitled “MAGNETIC POSITION DETECTION FOR MICRO MACHINED OPTICAL ELEMENTS,” Agent&#39;s Docket No. ONX-117A, filed May 8, 2001, and which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to optical communications and more particularly to measuring the position of micro machined optical elements.  
         BACKGROUND OF THE INVENTION  
         [0003]    MEMS free-space optical switches can be categorized into two major branches: the planar matrix (2-dimensional) approach, and the beam-steering (3-dimensional) approach. The 2D approach typically involves mirrors that move between on and off position, while the 3-D approach typically involves mirrors that tilt over a continuous range of angles to deflect optical signals from one fiber array to another. The 3-D approach relies on accurate control of mirror position to minimize optical loss from the coupling of photons from one fiber to another.  
           [0004]    Fiber optic communications systems are subject to faults that interrupt signal traffic. The fault may occur in the optical switch or in some other part of the system. In both switching approaches it is useful for, fault detection purposes, to know whether a given mirror actuating mechanism has failed. One way to determine this is to sense the position of the mirror to determine whether it is in a desired state. If the mirror is not in the desired state, a fault in the mirror mechanism may be determined and signal traffic may be routed around the faulty mirror.  
           [0005]    Most of these MEMS optical elements have used some variation of sensing capacitance or piezoresistance as a means of detecting the angular position of the optical element. In the 2D approach, to perform accurate capacitance sensing the signal lines have to be shielded which adds significantly to the complexity of the MEMs die. Second, the capacitive sensing is highly non linear and the sensitivity degrades significantly at large angular deviations from the ideal final position. The piezoresistive sensors have smaller signal gain making them susceptible to noise and cross-talk.  
           [0006]    Thus, there is a need in the art, for a new method and apparatus for sensing the angular position of a MEMS optical element and an optical switch incorporating same.  
         SUMMARY OF THE INVENTION  
         [0007]    The disadvantages associated with the prior art are overcome by the present invention of a method and apparatus for measuring the position of a micro machined optical element.  
           [0008]    According to an embodiment of the invention, an apparatus includes a micro machined optical element and at least one magnetic sensor disposed on the movable portion of the micro machined optical element. The sense field may be generated by the actuation field used to manipulate the movable portion, or by a magnetic structure disposed on the fixed portion of the micro machined optical element. Alternatively, a sense field generating magnetic structures may be disposed on a moveable portion of the micro machined optical element and a magnetic sensor may be positioned on a nearby non-moveable portion of the micro machined optical element.  
           [0009]    Magnetic sensors, such as magnetoresistive elements, magnetostrictive elements, Hall-effect devices and sense coils provide for sensitive, reliable and robust measurement of the position or switching state of MEMS devices such as those used in optical switches. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a flow diagram of a method for measuring the position of a micro machined optical element according to a first embodiment of the present invention  
         [0011]    [0011]FIG. 2A an isometric schematic diagram of an apparatus according to a second embodiment of the present invention;  
         [0012]    [0012]FIG. 2B is a cross-sectional schematic diagram taken along line  2 B- 2 B of FIG. 2A:  
         [0013]    [0013]FIG. 3A is an isometric schematic diagram of an apparatus according to an alternative version of the second embodiment of the invention.  
         [0014]    [0014]FIG. 3B is a schematic diagram of a Wheatstone bridge circuit that may be used with the apparatus of FIG. 3A;  
         [0015]    [0015]FIG. 4 is an isometric schematic diagram of a MEMS optical switch according to a third embodiment of the invention;  
         [0016]    [0016]FIG. 5A is a plan view schematic diagram of an apparatus according to another alternative version of the second embodiment of the invention;  
         [0017]    [0017]FIG. 5B is a plan view schematic diagram of an apparatus according to another alternative version of the second embodiment of the invention;  
         [0018]    [0018]FIG. 5C is a cross-sectional schematic diagram of an apparatus according to another alternative version of the second embodiment of the invention;  
         [0019]    [0019]FIG. 5D is a plan view schematic diagram of an apparatus according to another alternative version of the second embodiment of the invention;  
         [0020]    [0020]FIG. 5E is a plan view schematic diagram of an apparatus according to another alternative version of the second embodiment of the invention; and  
         [0021]    [0021]FIG. 6 depicts an example schematic diagram of an optical switching system according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.  
       Theory  
       [0023]    Magnetic sensors may detect changes in a magnetic field by sensing a change in an electrical, mechanical and/or optical property of the sensor that result from changes in the magnetic field. The change in the electrical, mechanical and/or optical property may depend upon the strength of the magnetic field or the relative position of the field with respect to the sensor. Magnetic sensors include, but are not limited to magnetoresistive sensors, magnetostrictive sensors, Hall-effect sensors, flux sensing coils, magnetostriction sensors and magneto optic sensors.  
         [0024]    Magnetoresistive sensors utilize materials having an electrical resistance that changes in response to a change in a magnetic field. Magnetoresistivity in ferromagnetic materials was discovered in 1856 by Lord Kelvin, and has since been used in a variety of magnetic sensors to detect magnetic field strength and direction. The change in resistivity is dependent upon the strength of the magnetic field and the relative orientation of the field with respect to a conduction path through the magnetoresistive material. The change is usually a minimum when the field is perpendicular to the conduction path and is usually a maximum when the field is parallel to the conduction path. As the conduction path of a magnetoresistive sensor changes with respect to an external magnetic field (or vice-versa) the electrical resistance changes.  
         [0025]    The Hall effect is based on the deflection of moving electric charges by a magnetic field. In a Hall effect sensor, the electrical property may be a voltage, sometimes referred to as a Hall voltage. The Hall voltage is related to the strength of an electric field, referred to herein as the Hall electric field, that results from the interaction of an electric current with a magnetic field. The Hall electric field is generally directed perpendicular to both the magnetic field and the direction of flow of the electric current through the Hall effect sensor. As the direction of flow of the electric current through the Hall effect sensor changes with respect to an external magnetic field (or vice-versa) the Hall voltage changes.  
         [0026]    Flux sensing coils operate on the principle of electromagnetic induction. As the AC magnetic flux through the coil changes a voltage may be induced on the coil. The magnetic flux may change due to a change in intensity of an external magnetic field. Alternatively, the flux may change due to a change in the relative position of the coil with respect to the magnetic field. Flux sensing coils may be characterized by a property known as electrical inductance, which relates the voltage across the coil to the rate of change of electric current through the coil. The inductance of a coil may change, e.g., due to a change in proximity of magnetic material with respect to the coil.  
         [0027]    The term magnetostriction refers to the change in the physical dimensions caused by magnetization. Magnetostriction sensors utilize this effect to measure field strength. Magneto optic sensors utilize materials characterized by optical properties that depend on strength and/or orientation of an applied magnetic field. Such optical properties include, but are not limited to, polarizing direction, reflectivity etc. For example, in a Kerr or Faraday rotation, the polarization of optical signals is rotated by an amount that depends on the surface magnetization, which in turn depends on the strength and direction of the applied magnetic field. Thus, the amount of polarization rotation may be used as an indicator of magnetic field strength and/or orientation.  
       Magnetic Sensors to Sense MEMS Position  
       [0028]    According to an embodiment of the invention, a magnetic sense method includes disposing a magnetic sensor on the micro machined optical element, exposing the magnetic sensor to a magnetic field, and measuring a change in an electromagnetic, mechanical and/or optical property of the magnetic sensor as the position of the micro machined optical element changes. The micro machined optical element may comprise of a movable and fixed element made from single, composite, or multiple dies.  
         [0029]    [0029]FIG. 1 depicts a flow diagram illustrating an example of a method  100  for measuring the position of a micro machined (MEMS) optical element according to a first embodiment of the invention. At step  102  a magnetic sensor is disposed on a micro machined optical element. At step  104  the magnetic sensor is exposed to a magnetic field. At step  106  a change in an electrical, mechanical and/or optical property of the magnetic sensor is measured as an orientation of the MEMS optical element changes with respect to the magnetic field. As used herein, “position” may refer to relative spatial position, relative angular orientation, or some combination of both. Furthermore, the position of the MEMS optical element may change with respect to the magnetic field if the magnitude or direction of the magnetic field changes with respect to the MEMS optical element. The ON/OFF state of a 2D MEMS optical switch may be determined by comparing the value of the magnetic sensor property measured in step  106  with one or more predetermined values of the sensor property when the MEMS optical element is known to be in an ON and/or OFF position.  
         [0030]    FIGS.  2 A- 2 B depicts schematic diagrams of an apparatus  200  according to a second embodiment of the invention. The apparatus  200  includes a micro machined optical element  210  and a magnetic sensor  220  disposed on the micro machined optical element  210 . The magnetic sensor  220  may be coupled to a position detector  230 , e.g. by leads  231 ,  232 .  
         [0031]    By way of example, the micro machined optical element  210  may include a fixed portion, such as a base  212 , and a movable portion, such as a flap  214 . As used herein, the term “moveable” means capable of movement by translation or rotation or some combination of both. Translation includes translation with respect to one or more axes. Rotation includes rotation with respect to one or more axes. By way of example, the flap  214  may rotate about an axis  215 . The axis  215  may be oriented substantially parallel to a plane of the flap  214 . Alternatively, the axis  215  may be substantially perpendicular to the plane of the flap such that the flap is oriented substantially perpendicular to a plane of the base  212 . The flap  214  may be coupled to the base e.g. by one or more flexures, so that the flap  214  is movable out of the plane of the base  212 . The flexures may apply a torsional, or restoring force that returns the flap  214  to a rest position when an actuating force is removed. Other restoring forces may be applied to flap  214  to return the flap to the rest position. Such forces may be exerted on flap  214  by biasing mechanisms that operate via pneumatic, thermal, or magnetic principals, including coils that interact with an external magnetic field, electrostatic elements, such as gap closing electrodes, piezoelectric actuators and thermal actuators. Multiple restoring forces may also be used together, and the forces may operate along the same or opposing directions.  
         [0032]    A light-deflecting element  216  may be disposed on the flap  214  to deflect one or more optical signals. By way of example, the light-deflecting element  216  may be a simple plane reflecting (or partially reflecting) surface, curved reflecting (or partially reflecting) surface, prismatic reflector, refractive element, prism, lens, diffractive element, e.g. fresnel lens, a dichroic coated surface for wavelength specific and bandpass selectivity, or some combination of these.  
         [0033]    Any conventional means may be used to provide an actuating force to move the flap  214 . For example, the flap  214  may contain a magnetically active element  225  to facilitate movement of the flap by interaction with an externally applied magnetic field. The magnetically active element may be a magnetically active material having, e.g. a fixed magnetic moment, i.e., it may be a permanent magnet. Magnetically active materials may include Nickel, Nickel—Iron, Iron—Cobalt, Aluminum—Nickel—Cobalt, Neodymium—Iron—Boron, etc., and, may be deposited in a uniform or stepped pattern. Alternatively, e.g. one or more vertical combdrive actuators may be used to tilt the flap  214  through a continuous range of angles in a controlled fashion. The magnetic sensor  220  may be used to sense the state or position of a flap such as the flap  214 . The magnetic sensor  220  may operate by sensing a change in an electrical property such as a resistance, reactance, or impedance of the sensor under the influence of a magnetic field B. The magnetic field B may be an external field that actuates movement of the flap by interaction with a magnetic material  225  on the flap  214 . Alternatively, the magnetic field may be a separate sense magnetic field, e.g. a magnetic field that is produced by the magnetic material  225 . The magnetic sensor  220  may include, but is not limited to, magnetoresistive sensors including giant magnetoresistance (GMR) sensors, such as spin valves, colossal magnetoresistance (CMR) sensors, anisotropic magnetoresistance (AMR) sensors, magnetic tunnel junction (MTJ) devices, and Hall effect sensors, flux sensing coils, magnetostriction sensors and magneto optic sensors.  
         [0034]    By way of example and without loss of generality, the magnetic sensor  220  may be a magnetoresistive sensor that includes a magnetoresistive material. Examples of magnetoresistive materials Include Cu, Ni, Fe, Co and their alloys, oxides and structures having multiple layers containing one or more of these. A magnetic sensor  220  in the form of a magneto resistive sensor may be formed by depositing magnetoresistive material and leads on the micro machined optical element  210 . Evaporation and annealing processes may be used for a multiple layer or GMR film. The magnetoresistive material may be deposited by suitable techniques including, but not limited to, sputter deposition, evaporation and electroplating  
         [0035]    [0035]FIG. 2B shows a cross-sectional schematic diagram of the apparatus  200  taken along line  2 B- 2 B. The flap  214  may make an angle θ with respect to the magnetic field B. A sense current I flows through the MR sensor  220 . The MR sensor  220  may have a thickness that is very small compared to its length and width to constrain the sense current I to flow in a path substantially within a plane. The sense current I is directed at an angle θ with respect to the magnetic field B. The sensor may be disposed on the flap  214  as shown in FIG. 2B, so that the angle θ changes as the flap  214  rotates with respect to the magnetic field B. Since the electrical property of the MR sensor  220  depends on both B and θ, changes in the angular orientation of the flap produce corresponding changes in the electrical property of the MR sensor  220 . Alternatively, the flap  214  may translate with respect to the magnetic field B. If the magnetic field B is non-uniform in either magnitude or direction, changes in the spatial position of the flap  214  may produce changes in the electrical property of the magnetic sensor  220 .  
         [0036]    The position detector  230  may measure changes in the electrical property of the magnetic sensor  220  that varies with changes in a magnetic flux through the magnetic sensor  220 . Where, for example, the relevant electrical property of the magnetic sensor is an electrical resistance, the position detector  230  may include a resistance measuring circuit. Such a circuit may supply a fixed sense current I to the magnetic sensor  220  and measure changes in the voltage across the magnetic sensor  220 . If the relevant electrical property of the MR sensor  220  is a Hall voltage, the position detector may supply a fixed current to the opposite ends of the magnetic sensor  220  and detect the Hall voltage that develops across the width of the detector. The position detector  230  may be implemented in hardware, software, firmware, or some combination of these. By way of example, the position detector  230  may be implemented as one or more application specific integrated circuits (ASIC&#39;s).  
         [0037]    More than one magnetic sensor may be disposed on the micro machined optical element. Furthermore, the magnetic sensor may be disposed on the fixed portion of the micro machined optical element. By way of example, FIG. 3 depicts an isometric schematic diagram of an apparatus  300  according to an alternative version of the second embodiment of the invention. Apparatus  300  may include a micro machined optical element  310  and first, second, third, and fourth magnetic sensors  320 A,  320 B,  320 C,  320 D disposed on the micro machined optical element  310 . The magnetic sensor  320  may be coupled to a bridge circuit  330 . The optical element  310  may include a fixed portion  312  and a moveable portion  314 . The magnetic sensors  320 A,  320 B,  320 C,  320 D may include, but are not limited to, giant magnetoresistance (GMR) sensors, spin valves, colossal magnetoresistance (CMR) sensors, anisotropic magnetoresistance (AMR) sensors, magnetic tunnel junction (MTJ) devices, and Hall effect sensors, flux sensor coils, magnetostriction sensors and magneto optic sensors.  
         [0038]    By way of example, the magnetic sensors  320 A,  320 B,  320 C,  320 D may be magnetoresistive (MR) sensors. The magnetoresistive sensors may be formed from a pattern of magnetoresistive material laid out on the micro machined optical element  310 , e.g., by photolithographic techniques. As the position of the movable portion  314  changes with respect to the magnetic field B during the actuation cycle, the orientation of the sensor  320 A with respect to the magnetic field B also changes, e.g., from a from parallel to a perpendicular orientation. In the version of the second embodiment depicted in FIG. 3A the first MR sensor  320 A may be disposed on the movable portion  314  of the micro machined element  310  and the other three sensors  320 B,  320 C,  320 D disposed on the fixed portion  312 . As the angular orientation of the movable portion  314  changes with respect to a magnetic field B an electrical property of the first sensor  320 A on the movable portion  314  changes correspondingly as described above. The electrical properties of the other three sensors  320 B,  320 C,  320 D, however, remain substantially fixed as the angular orientation of the movable portion changes with respect to the magnetic field B. The properties of all four sensors  320 A,  320 B,  320 C,  320 D change in proportion to changes in the magnetic field B. Thus, if all four sensors  320 A,  320 B,  320 C,  320 D are appropriately coupled to the bridge circuit  330  an output of the bridge circuit may be made sensitive to changes in the angular orientation of the movable portion  314  of the micro machined optical element  310 , but substantially insensitive to changes in the magnetic field B.  
         [0039]    [0039]FIG. 3B illustrates a schematic diagram of an example of a bridge circuit  330 ′ that may be in conjunction with the apparatus  300 . Although the following relates to the use of a bridge circuit with magnetoresistive sensors, bridge circuits may also be used with other magnetic sensors such as Hall effect sensors, flux sensing coils, magnetostriction sensors and magneto optic sensors. The four magnetoresistive sensors  320 A,  320 B,  320 C,  320 D may be connected in a Wheatstone bridge fashion with one sensor  320 A being disposed on the movable portion  314  of the micro machined optical element  310 .  
         [0040]    By way of example, each of the four magnetoresistive sensors  320 A,  320 B,  320 C,  320 D may be respectively characterized by an electrical resistance R A , R B , R C , R D  that changes in response to changes in the magnetic field B. The first and third magnetoresistive sensors  320 A,  320 C may be electrically coupled at a first junction  331 . The second and fourth magnetoresistive sensors  320 B,  320 D may be electrically coupled at a second junction  332 . The first and second magnetoresistive sensors  320 A,  320 B may be electrically coupled at a third junction  333 . The third and fourth magnetoresistive sensors  320 C,  320 D may be electrically coupled at a fourth junction  334 . A current source  340  may be coupled between the first and second junctions  331 ,  332 , and null detector (N)  350  may be electrically coupled between the third and fourth junctions  333 ,  335 . The null detector  350  may be regarded as a sensitive electric current detector. By way of example, the resistance of the circuit between the second and third junctions, e.g., R B , may be varied to change the current through the null detector  350 . When the current through the null detector  350  is zero, it can be shown that the resistance of the magnetoresistive sensor  320 A may be given by:  
         R   A     =         R   C          R   B         R   D                             
 
         [0041]    Since R A , R C , R C , R D , are dependent magnetic field B changes in the magnetic field B tend to cancel out. However, in this example, only R A  depends on the angle θ. Thus, the bridge circuit  330 ′ may capture information regarding the angular position of the movable portion  314  of the micro machined optical element  310 . Although the foregoing discussion describes measurement of electrical resistance, Wheatstone bridge circuits may be utilized to measure other electrical properties such as Hall voltages. Other bridge circuits, such as Mueller bridge circuits may be used with the apparatus  300  to measure the resistance or other electrical property of one or more magnetic sensors. Furthermore, a single magnetic sensor may be coupled to a bridge circuit to sense a change in resistance or other relevant electrical property. One or more magnetic sensors can be employed as sense elements in a feedback loop to control the mirror angle, and to incorporate a diagnostic routine to inform a user of switch level malfunctions in the event that the control loop fails to move the mirror to the desired position.  
         [0042]    Embodiments of the present invention can be used to measure the angular position of the scanning MEMS micro mirrors used in fiber-optic switches for optical communication systems. According to one embodiment of the invention, an optical switch includes a plurality of micro machined optical elements and at least one magnetic sensor or magnetic sense field generator disposed on one or more of the movable micro machined optical elements and non-movable elements in the plurality. FIG. 4 depicts an isometric schematic diagram of an example of a MEMS optical switch  400 . According to a third embodiment of the invention, switch  400  may generally includes a plurality of micro machined optical elements  402  and magnetic sensors  404 . The magnetic sensors  404  may include, but are not limited to the various types of sensors described above, such as giant magnetoresistance sensors, colossal magnetoresistance sensors, anisotropic magnetoresistance sensors, magnetic tunnel junction devices, Hall effect sensors, flux sensing coils, , magnetostriction sensors, magneto optic sensors and the like. Each micro machined optical element  402  may include a movable portion  406 . The sensors  404  may be disposed on the movable portions  406  as described above. By way of example, the movable portion may rotate about an axis  407  relative to a fixed portion  408 . The fixed portion  408  may be a base common to all of the micro machined optical elements  402 .  
         [0043]    The movable portions  406  may include a light deflecting elements  416 . By way of example, the light-deflecting element  416  may be a simple plane reflecting (or partially reflecting) surface, curved reflecting (or partially reflecting) surface, prismatic reflector, refractive element, prism, lens, diffractive element, e.g. fresnel lens, a dichroic coated surface for wavelength specific and bandpass selectivity, or some combination of these. The light deflecting elements  416  may deflect optical signals to selectively couple the signals from one optical fiber to another. It must be stated that movable portion  406  is shown for example purposes only, that a plurality of movable element designs exist, and the present invention may be used on various MEMS optical mirror designs that utilize a movable optical element. The sensors  404  may be coupled to a switch controller  412 . The switch controller  412  may be implemented in hardware, software, firmware, or some combination of these. By way of example, the switch controller  412 , may be implemented as one or more application specific integrated circuits (ASIC&#39;s). The switch controller  412  may receive information on the angular position of the movable portions of the micro machined optical elements  402  from the sensors  404 . The switch controller may include a feedback loop to control the angle of the movable portions. Alternatively, the switch controller  412  may incorporate a diagnostic routine to inform a user of switch level malfunctions in the event that the control loop fails to move the micro machined optical element  402  to a desired position.  
         [0044]    In some versions of the second embodiment of the invention, the magnetic sensor may be placed on a fixed portion of a micro machined optical element. FIGS.  5 A- 5 E depict several alternative versions of this embodiment. In these versions, a magnetic material is characterized by a permanent magnetic moment is disposed on a moveable portion and the magnetic sensor and its associated leads are disposed on a nearby fixed portion. The magnetic material may produce a magnetic flux that passes through a magnetoresistive sensor, Hall effect sensor or coil wherein the flux changes as the position of the magnetic material changes with respect to the sensor. Changes in flux through the sensor may cause changes an electrical property of the sensor, e.g. electrical resistance, Hall voltage or inductance. An advantage of this configuration is that an electrical connection to the moveable portion is not required. This greatly simplifies the manufacture of the apparatus and improves the robustness of its operation.  
         [0045]    [0045]FIG. 5A depicts a plan view of an apparatus  500  according to another alternative versions of the second embodiment of the invention. The apparatus  500  generally comprises a micro machined optical element having a fixed portion in the form of a substrate  502  and a moveable portion in the form of a flap  506 . The flap is movable, e.g. rotatable with respect to an axis  503 . The flap may include a light-deflecting element  507  One or more magnetic sensors  504 A,  504 B are disposed on the substrate  502  proximate the flap  506 . One or more magnetic elements  508 A,  508 B are disposed on the flap  506  near the sides thereof proximate the sensors  504 A,  504 B. The sensors  504 A,  504 B may be connected to detectors  501 A,  501 B through leads  505 A,  505 B,  505 C,  505 D. In the embodiment shown in FIG. 5A the sensors  504 A,  504 B and the magnetic materials  508 A,  508 B are oriented substantially parallel to each other and substantially perpendicular to the rotation axis  503 .  
         [0046]    The magnetic elements  508 A,  508 B may be magnetically active materials having, e.g. a fixed magnetic moment, i.e., they may be permanent magnets. Magnetically active materials may include Nickel, Nickel—Iron, Iron—Cobalt, Aluminum—Nickel—Cobalt, Neodymium—Iron—Boron, etc., and, may be deposited in a uniform or stepped pattern. The magnetic elements  508 A,  508 B may alternatively include one or more coils that carry electric current to provide a magnetic moment. Each magnetic element  508 A,  508 B may be characterized by a magnetic moment having a direction indicated by the arrows  509 A,  509 B. In the embodiment depicted in FIG. 5B the magnetic moments of the magnetic elements  508 A,  508 B are oriented substantially perpendicular to the axis  503 . As the flap  506  rotates about the axis  503  the change in the relative position and/or orientation of the magnetic field produced by the magnetic elements  508 A,  508 B with respect to the sensors  504 A,  504 B causes a change in the magnetic flux passing through the sensors  504 A,  504 B. The change in flux causes a change in an electrical property of one or more of the sensors  504 A,  504 B.  
         [0047]    In a preferred embodiment, the sensors  504 A,  504 B may have a C-shape that includes a gap. The sensors  504 A,  505 B “wrap around” the magnetic elements  508 A,  508 B. As the position of the flap  506  changes with respect to the substrate  502  the amount of magnetic flux produced by the magnetic elements  508 A,  508 B that is intercepted by the sensors  504 A,  504 B changes. Where the sensors  504 A,  504 B are magnetoresistive sensors, the change in intercepted flux produces a change in one or more sense signals detected at the detectors  501 A,  501 B. In the particular version of the second embodiment shown in FIG. 5A, the magnetic flux is a maximum when the flap  506  is substantially parallel to the substrate  502 . In this configuration, the magnetic elements  508 A,  508 B are disposed within the gaps in the sensors  504 A,  504 B.  
         [0048]    [0048]FIG. 5B depicts a plan view of an apparatus  510  according to another alternative version of the second embodiment of the invention. The apparatus  510  is a variation on the apparatus  500  of FIG. 5A. The apparatus  500  generally comprises a micro machined optical element having a fixed portion in the form of a substrate  512  and a moveable portion in the form of a flap  516 . A light-deflecting element  517  may be disposed on the flap  516 . The flap  516  is movable, e.g. rotatable with respect to an axis  513 . A magnetic sensor  514  may be disposed on the substrate  512  proximate an end of the flap  516 . A magnetic element  518  may be disposed on the flap  516  proximate the sensor  514 . The magnetic moment of the magnetic element  518  may be oriented substantially parallel to the axis  513 , as indicated by the arrow  519 . As in FIG. 5A the magnetic sensor  514  may be in the form of a magnetoresistive element having a C-shape with a gap. In the particular version of the second embodiment shown in FIG. 5A the magnetic element lies within the gap when the gap when the flap  516  is substantially parallel to the substrate  512 . The magnetic sensor  514  may be coupled to a detector  511 , e.g., by leads  515 A,  515 B.  
         [0049]    Some micro machined optical elements may use a top chip design to provide a sidewall for orienting the flap in an up or “on” position. FIG. 5C depicts a cross-sectional view of an apparatus  520  according to another alternative versions of the second embodiment of the invention. The apparatus  520  may be assimilated as a variation on those described with respect to FIGS.  5 A- 5 B. The apparatus  520  may generally comprises a micro machined optical element having fixed portions in the form of a base  522  and a top chip  525 . The micro machined optical element has a moveable portion in the form of a flap  526 .  
         [0050]    In some applications such a two-chip approach is used to align the optical element in an “up” or “on” position with the flap  526  oriented substantially perpendicular to a plane of the base  522 . The flap  526  may be formed from one or more layers of the substrate  522 . In an “off” or down-position (shown in phantom), the flap  526  is substantially parallel to the base  522 . The flap  526  may be attached for movement with respect to the substrate  522  by one or more flexures  533 . By way of example, the base  522  may be a silicon-on-insulator (SOI) substrate. The top-chip  525  has an opening  523  with perpendicular sidewalls  527 . The term “sidewall” as used herein refers generally to any surface that provides a reference stopping plane for the flap  526 . Although a sidewall  527  that is part of the substrate is shown in FIG. 5C the sidewall may alternatively be part of the substrate  522  or part of a separate structure formed on of the substrate  522  or on the top chip  525 .  
         [0051]    The top chip  525  is aligned with the substrate  522  such that flap aligns with the opening  523  and the substrate  522  and top-chip  525  are bonded together. The opening  523  receives the flap  526  when the flap is in an “on” state, i.e., substantially perpendicular to a plane of the substrate  522 . The flap  526  may be clamped against a sidewall  527  of the top chip  525  when the flap is in the “on” state as shown in FIG. 5C. When the top-chip  525  is properly aligned and bonded to the susbtrate  522  the sidewalls  527  of the openings  523  can serve as reference stopping planes to fix the up-position of the flap. In addition, the sidewalls  527  may also serve as electrodes to hold the mirrors in the up-position by electrostatic attraction. A “top chip” having openings with almost perfectly perpendicular sidewalls may be formed, e.g., by etching a &lt;110&gt; silicon wafer with an anisotropic etchant.  
         [0052]    One or more magnetic sensors  524  may be disposed on the top chip  525  proximate the flap  526 . Although FIG. 5C shows the sensor  524  disposed on a surface of the top chip  525 , a sensor  524 ′ may alternatively be disposed on the sidewall  527 . The sensors  524 ,  524 ′ may be coupled to a detector  521 , e.g., via leads  529 A,  529 B. A magnetic element  528 . such as a magnetic material, may be disposed on the flap  526  to provide a sense magnetic field that is detected by the sensors  524 ,  524 ′. Alternatively one or more of the sensors  524 ,  524 ′ may be disposed on the flap  526  and the magnetic material may be disposed on the substrate  522 , the top chip  525  or the sidewalls  527 . It need be stated that the top chip associated with each micro machined optical element may also be comprised of two high-aspect-ratio deep vertical walls separated by an air gap.  
         [0053]    Several orientations of the sensors and magnetic elements are possible. Two particular configurations are depicted in FIG. 5D and FIG. 5E. FIG. 5D depicts a plan view of an apparatus  530  according to another alternative versions of the second embodiment of the invention. The apparatus  530  generally comprises a micro machined optical element having fixed portions in the form of a substrate  532  and a top chip  535 . The micro machined optical element includes a moveable portion in the form of a flap  536 . One or more magnetic sensors  534 A,  534 B are disposed on the top chip  535  proximate the flap  536 . The sensors  534 A,  534 B may be coupled to a detector  531 , e.g., via leads  539 A,  539 B. The sensors  534 A,  534 B may be in the form of serpentine coils of magnetic material. The serpentine shape allows a greater length for the sensors, which increases their sensitivity to changes in magnetic flux. One or more magnetic elements  538 A,  538 B are disposed on the flap  536  near the sides thereof. The magnetic elements  538 A,  538 B may be positioned such that they are proximate the sensors  534 A,  534 B when the flap  536  is clamped against the top chip  535 . In this position, the magnetic flux though the sensors  534 A,  534 B from the magnetic elements  538 A,  538 B may be maximized.  
         [0054]    [0054]FIG. 5E depicts a plan view of an apparatus  540  according to another alternative version of the second embodiment of the invention. The apparatus  540  generally comprises a micro machined optical element having fixed portions in the form of a substrate  542  and top chip  545 . The micro machined optical element may include a moveable portion in the form of a flap  546 . A magnetic sensor  544  may be disposed on the top chip  545  proximate the flap  546 . The magnetic sensor  544  may be coupled to a detector  541 , e.g. through leads  547 A,  547 B. The magnetic sensor  544  may be in the form of a serpentine pattern of magnetoresistive material having features in common with the serpentine patter described with respect to FIG. 5D. One or more magnetic elements  548  may be disposed on the flap  516  proximate an end thereof. The magnetic element  548  may be positioned on the flap  546  such that it is proximate the magnetic sensor  544  when the flap is in an “on” position.  
         [0055]    Other variations are possible on the above embodiments. For example, the magnetic sensor element may include an inductive coil disposed on either a fixed or moveable portion of a micro machined optical element. Changes in the position of the moveable portion may lead to changes in an inductance of the coil. The change in inductance may be correlated to the change in position. Changes in inductance may be less susceptible to noise than changes in capacitance.  
         [0056]    [0056]FIG. 6 depicts a block diagram depicting an optical communications system  600  according to a fourth embodiment of the invention. In the system  600 , a method having features in common with the method  100  of FIG. 1 may be implemented as a computer program code  605  running on a processor of a computer controlled apparatus having features in common with the MEMS optical switch  400  described above with respect to FIG. 4. In the embodiment shown, the program code  605  controls the operation of one or more MEMS optical elements  632 A,  632 B,  632 C,  632 D in an optical switch  630 . Although the program  605  is described herein with respect to a MEMS optical switch, those skilled in the art will recognize that programs embodying the method of the present invention may be applied to any MEMS device. The optical elements  632 A,  632 B,  632 C,  632 D may have features in common with the optical elements described above. The optical switch  630  may have features in common with the type of switch  400  shown in FIG. 4. By way of example, the switch  630  may be a 2D MEMS optical switch. Each optical element  632 A,  632 B,  632 C,  632 D may include a moveable portion that is moveably coupled to a substrate and actuated by, for example, electrostatic, pneumatic thermal, acoustic or magnetic actuators  634 A,  634 B,  634 C,  634 D. The optical elements  632 A,  632 B,  632 C,  632 D may be clamped in vertical or horizontal position by voltages applied to clamping electrodes (not shown).  
         [0057]    One or more magnetic sensors  636 A,  636 B,  636 C,  636 D may be respectively coupled to moveable and/or fixed portions of the optical elements  632 A,  632 B,  632 C,  632 D. The magnetic sensors  636 A,  636 B,  636 C,  636 D may be of any of the types described above. The magnetic sensors  636 A,  636 B,  636 C,  636 D sense changes in the position or state of the optical elements  632 A,  632 B,  632 C,  632 D with respect to a magnetic field B′ provided, e.g., by a magnet  638 . If the actuators  634 A,  634 B,  634 C,  634 D are magnetic actuators, the magnetic field B′ may be the same magnetic field that drives the actuators. Alternatively, the magnetic field B′ may be a separate sense magnetic field. In some embodiments, a single magnet  638  may be used to actuate all the optical elements  632 A,  632 B,  632 C,  632 D. In such a situation, the actuators  634 A,  634 B,  634 C,  634 D may include electrodes for clamping moveable portions of the optical elements  632 A,  632 B,  632 C,  632 D in their respective “ON” or “OFF” states. The switch  630  may optionally include a temperature sensor  620  disposed in proximity to switch  630  or positioned in thermal contact with a portion of the switch, e.g. one or more of the optical elements  632 A,  632 B,  632 C,  632 D. The temperature sensor may produce a signal that is proportional to a temperature of the switch  630 . By way of example, the temperature sensor  620  may be a thermocouple, thermistor, infrared (IR) temperature sensor, etc.  
         [0058]    One or more input fibers  607 A,  607 B and output fibers  607 C,  607 D may be optically coupled to the optical switch  630 . Optical sources (OS)  603 A,  603 B may provide optical signals to the input fibers  607 A,  607 B while optical detectors (OD)  609 A,  609 D may be optically coupled to the output fibers  607 C,  607 D to establish, for example, that the micro machined optical elements in the switch are in a known state. Alternatively, the optical sources and detectors may be replaced with optical transceivers to allow two-way signal traffic through the switch  630 .  
         [0059]    A switching sub-system  600  may typically include a switch  630  combined with a controller  601 . The controller  601  may be a self contained microcontroller such as the PICK Microchip, or controller  601  may be configured to include a CPU  602 , memory  604  (e.g., RAM, DRAM, ROM, and the like), clock  614  and well-known support circuits  610  such as power supplies  612 , input/output (I/O) functions  618  coupled to a control system bus  608 . The memory  604  may contain instructions that the processor unit  602  executes to facilitate the performance of the apparatus  600 . The instructions in the memory  604  may be in the form of the program code  605 . The code  605  may conform to any one of a number of different programming languages such as Assembly, C++, JAVA or a number of other languages. The controller  601  typically operates the apparatus  600  through I/O functions  618  in response to data and program code instructions stored and retrieved by the memory  604 .  
         [0060]    The CPU  602  may be coupled to the elements of the system  600  via the system bus  608  and the I/O functions  618 . The elements of system  600  may include the following: one or more detector circuits (DC)  635  coupled to one or more of the magnetic sensors  636 A,  636 B,  636 C,  636 D, and one or more actuator drivers (AD)  633  coupled to one or more of the actuators  634 A,  634 B,  634 C,  634 D. If the magnet  638  is an electromagnet, a magnet driver (MD)  637  may be coupled to the magnet. For the sake of clarity, connection is shown to only one of the magnetic sensors  636 D and one of the actuators  634 D. In practice, all the magnetic sensors  636 A,  636 B,  636 C,  636 D and actuators  634 A,  634 B,  634 C,  634 D may be coupled to the I/O functions  618 . One or more clamping voltage sources may be optionally coupled between clamping electrodes in the switch  601  and the I/O functions  618 . The optical sources  603 A,  603 B and the optical detectors  609 A,  609 B may also be coupled to the I/O functions  618  and system controller  601  may provide control to switch optical signals between the input fibers  607 A,  607 B and the output fibers  607 C,  607 D. The support circuits  610  may also include a temperature detector (TD)  639  coupled to the temperature sensor  620  and the I/O functions  618 .  
         [0061]    It should be stated that depending on the configuration or selection of controller  601  and system  600 , the conditioning circuits, including actuator driver  633 , temperature detector  639 , magnetic driver  637  and/or detector circuit  635  may be implemented in software form,, e.g., within code  605 , such that I/O functions  618  may directly connect to each respective switch component.  
         [0062]    The system  600  may be a subsystem or component of a network element (not shown). The network element may be part of a network (not shown). The microcontroller  601  may include network element interface  606  which may be implemented in software e.g. in a subroutine in memory  604  or hardware to allow the system  600  to communicate with the network element. Such communication may include, but is not limited to, switching commands issued from the network element to the system  600  and switch state data from the system  600  to the network element.  
         [0063]    Certain steps of the method described above with respect to FIG. 1 may be implemented by a suitable computer program code  605  running on the CPU  602  of the controller  601 . The CPU  602  may form a general-purpose computer that becomes a specific purpose computer when executing programs such as the program  605 . Although the invention is described herein as being implemented in software and executed upon a general purpose computer, those skilled in the art will realize that the invention could be implemented using hardware such as an application specific integrated circuit (ASIC), microcontroller or other hardware circuitry. As such, it should be understood that the invention can be implemented, in whole or in part, in software, hardware or both.  
         [0064]    A computer program  605  may be devised to implement steps  104  and  106  described above with respect to FIG. 1. The program  605  is suitable for monitoring and controlling the position or state of the optical elements  603 A,  603 B,  603 C,  603 D of the optical switch  601  in accordance with embodiments of the present invention. By way of example, the program  605  may implement fault detection in the system  600 . For example, suppose that only when the optical element  632 B is in an “ON” state, optical element  632 B deflects optical signals from input fiber  607 B to output fiber  607 C. The state of optical element  632 B may be determined by sending an optical signal towards optical element  632 B from the source  603 B to input fiber  607 B and monitoring the optical signal at output fiber  607 C with optical detector  609 A. If the optical signal from the optical source  603 B is detected by the optical detector  609 A optical element  632 B is presumably in the “ON” state. While the optical element  632 B is known to be in the “ON” state, the property of the magnetic sensor associated with thereto may be recorded through I/O function  618  and stored in a look-up table in memory  604 . This step may occur when the magnet  638  is turned on to provide a sense field for the magnetic sensors  636 A,  636 B,  636 C,  636 D or when the magnet  638  is turned on to perform a switching event. Alternatively, a signal from the magnetic sensor  636 B disposed proximate the optical element  632 B may be measured when the movable element associated with the magnetic sensor  636 B is in a known state. . Signals from sensors  636  may be measured in batch or selectively addressed in response to code  605  and through I/O functions  618  when they are in a known state.  
         [0065]    The position of optical element  632 B changes when it moves from the “ON” state to the “OFF” state. Consequently, the magnetic sensor  636 B may produce a different signal when the optical element  632 B is in the OFF state. The other magnetic sensors  636 A,  636 C,  636 D may also produce different signals. In a manner similar to that described above, a set of signals from the sensors  636 A,  636 B,  636 C,  636 D may be correlated to the “OFF” state of the optical element  632 B. In a similar fashion, the known “ON” and “OFF” states of the other optical elements  632 A,  632 C,  632 D may be correlated to measured signals from the magnetic sensors  636 A,  636 B,  636 C,  636 D.  
         [0066]    These signals from the magnetic sensors  636 A,  636 B,  636 C,  636 D may be organized by the program  605  as a set of predetermined signals, e.g. in a look-up table stored in memory  604 . The program  605  may index the aforementioned look-up table after reading the value or values associated with the magnetic sensor property to determine that the state of the switch is configured according to the requests of network element interface  606 .  
         [0067]    The properties of the magnetic sensors  636 A,  636 B,  636 C,  636 D may be temperature dependent. Consequently, signals from the magnetic sensors  636 A,  636 B,  636 C,  636 D may drift as the temperature of the switch  630  changes. To compensate for such drift, the program  605  may include instructions for temperature compensation. By way of example, such instructions may include measuring the signal from the magnetic sensors  636 A,  636 B,  636 C,  636 D for the “ON” and off states of the optical elements  632 A,  632 B,  632 C,  632 D at different temperatures measured by the temperature sensor  620 . The program may then determine ranges for the values of the magnetic sensor signals that correspond to the “ON” and “OFF” states. If, over a certain temperature range, the two ranges do not overlap the state of an optical element may be determined by measuring that magnetic sensor signal to see whether it falls in the “ON” range or the “OFF” range.  
         [0068]    It must be stated that the look-up table storing the predetermined magnetic sensor property values associated with each micro machined movable element, may be configured to allow a test value to fall within a range of predetermined values for added stability. For example, the magnetic sensor property values read into memory  604  through I/O functions  618 , when the optical element is in a known state to achieve the predetermined value for the lookup table, may be configured in code  605  such that test values read into memory  604  through I/O functions  618  when the optical element is not in a known state may be substantially equal to the lookup values in the table. This approach results in added stability and may be used to compensate for temperature variation effects.  
         [0069]    If there is substantial overlap between the “ON” and “OFF” ranges it may be desirable to correct for thermal drift in real time. The program  605  may correct for thermal drift by relating the measured magnetic sensor signals in the “ON” and “OFF” states to temperature measurements made during operation of the switch  630 . The relationship may be stored in the form of a look-up table. Alternatively, the relationship may be in the form of a temperature correction equation. For example, in the case of a linear relationship between temperature and magnetic sensor signal, the program may calculate a temperature drift coefficient. The temperature drift coefficient may be used to adjust the predetermined magnetic sensor signals for changes in temperature.  
         [0070]    Alternatively, the system controller  601  may be coupled to a temperature regulator (not shown) coupled, e.g. through the I/O circuits  618 . The program  605  may instruct the temperature regulator to maintain the temperature of the switch  630  within a desired temperature range in response to temperature measurements from the temperature sensor  620 . Preferably, the desired temperature range is sufficiently narrow that any thermal drift of magnetic sensor signal may be neglected. Furthermore, the system  600  may employ some combination of thermal drift correction and temperature regulation to compensate for changes in temperature.  
         [0071]    It should also be stated that magnetic sensors may be connected together e.g. through a bridge circuit and the output of the connected sensors may be batch read by the controller  601  to determine the individual state of each movable portion in the batch of elements. This can be accomplished by designing or tuning the sensors to produce a unique value in each known ON and OFF state. For example, a magnetoresistive element associated with each micro machined optical element may be designed to produce a unique prime resistance value when turned ON or OFF. Magnetic sensors may be connected in series or parallel and grouped according to, but not limited, rows or columns. As so, the program code  605  may engage in a row or column select to pull the combined sensor property value into memory for post processing by the CPU. Program code  605  may then perform data processing on the recorded property value to discern the individual state of all member optical flaps contained in group of sensors. Memory  604  may store the predetermined prime values associated with the plurality of sensors and the program  605  may engage in an process whereby the recorded value of the combined sensor group is compared against various prime number combinations associated with the group, until a match is found. When a match is determined, the micromachine optical elements associated with the match prime numbers set will share the same ON or OFF state, and the individual states of the batch group can be determined. While the above is a complete description of several embodiments of the present invention, it is possible to use various alternatives, modifications and equivalents. For example, the magnetic switch state tracking method described above with respect to FIG. 6 may be readily applied to tracking the state of a 3D steered beam optical switch having optical elements that may move through a continuous range of positions. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents.