Abstract:
Apparatus and methods are disclosed for selectively positioning a collimator body. The apparatus comprises support means adjustably supporting the collimator body; and adjustment means for selectively adjusting the collimator body, the adjustment means comprising an actuator component having a driver coil and a magnetic structure with a first gap formed therebetween; wherein an electrical current through the driver coil of the actuator component causes the collimator body to move perpendicular to a magnetic field created by the magnetic structure of the actuator component. The method comprises supporting the collimator; and adjusting the collimator body using an actuator component having a driver coil and a magnetic structure with a first gap formed therebetween; wherein an electrical current through the driver coil of the actuator component causes the collimator body to move perpendicular to a magnetic field created by the magnetic structure of the actuator component.

Description:
REFERENCE TO PENDING PRIOR APPLICATION  
       [0001]     This application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/368,300, filed Mar. 27, 2002 by Jack Foster et al. for LOW LOSS OPTICAL SWITCH USING MAGNETIC ACTUATION AND SENSING, which is hereby incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     This invention relates to optical switching apparatus and methods in general, and more particularly to actuation devices for optical switching.  
       BACKGROUND OF THE INVENTION  
       [0003]     Often it is desirable to have a relatively small switching fabric for a variety of purposes, such as optical add-drop or small switching fabrics for all-optical networks. A variety of techniques have been used for this purpose. For example, it is possible to use micromachined moving mirrors for free space optics switching devices. Typically, these mirrors are inserted between collimators so as to switch the beam between the collimators. Likewise, it is possible to move the fiber in front of the collimator lens and thereby steer the beam from one collimator to another. This actuation may be done by using piezoelectric, magnetic or other means. Or, conversely, the lens may be moved in front of a stationary fiber to achieve the same beam deflection, with similar actuation mechanisms, if desired.  
         [0004]     It is important that any actuation mechanisms be not susceptible to vibrations that may be occurring in the operating environment of the switch. In this respect, it is generally preferred to use balanced rotational mechanisms, such as properly designed mirrors, which are not susceptible to linear vibrations. This is because virtually all vibrations which occur in the operating environment are translational in nature. Mirrors also have the advantage that any angular rotation is multiplied by two.  
         [0005]     Most of the other systems described above, apart from mirrors, suffer adversely from these environmental vibrations and, hence, these systems require separate sensors and tight servo-controls to overcome environmental vibration problems. Systems that use relative movement of the fiber or the lens also suffer from the fact that the fibers are generally terminated with an 8 degree cut to avoid reflections. This configuration complicates effective coupling and, in turn, puts more stringent alignment requirements on the fiber and its motion.  
         [0006]     Recently, a system has been introduced by Polatis which rotates the collimators with respect to each other. See, for example, International Patent Application No. PCT WO 01/50176 A1. A connection is made when the collimators are properly pointing at each other. The system described uses arrays of piezo-electric torsional actuators, and possibly sensors, to rotate the collimators with respect to each other. This system has good optical characteristics. However, piezo actuators typically require a high voltage power source, and are prone to large drifts. In addition, this system is also quite expensive per port.  
         [0007]     It is, therefore, extremely desirable to construct a switching fabric that has very low loss, a low cost, and an ability to be expanded that can expand to a relatively large size (e.g. 256×256).  
       SUMMARY OF THE INVENTION  
       [0008]     A system of rotatable collimators is described, which are magnetically actuated and sensed. These collimators are oriented with respect to each other so that the undeflected beams converge in the center of the opposite fields, thereby reducing the required deflection angles by a factor of 2. A set of coils on the moving collimators interact with stationary permanent magnets such that rotation in two axes takes place. By measuring the inductance change of the coils, it is possible to measure the rotations of each coil, thereby providing a sensor output for the collimator, necessary to provide adequate positioning. The collimators are fixed, with the right orientation in an etched sheet which provides for the gimbal mounting of all these devices. The collimators are fixed at the center of mass so that no external reaction takes place when vibrations occur. The collimators used have very well controlled beam pointing abilities and are of the type described in U.S. patent application Ser. No. 09/715,917, which is hereby incorporated herein by reference. However, the tolerances on the rotatable pointing are substantially relaxed so as to provide inexpensive switching devices.  
         [0009]     This invention provides for a novel optical switching apparatus, specifically apparatus for selectively positioning a collimator body, the apparatus comprising: support means adjustably supporting the collimator body relative to a first position; and adjustment means for selectively adjusting the collimator body from the first position to a second position, the adjustment means comprising an actuator component having a driver coil and a magnetic structure with a first gap formed therebetween, one of the driver coil and the magnetic structure being in attachment to the selectively positionable collimator body and the other one of the driver coil and the magnetic structure being in attachment to a fixed support in connection with the support means adjustably supporting the collimator body; wherein an electrical current through the driver coil of the at least one actuator component causes the collimator body to move in a direction perpendicular to a magnetic field created by the magnetic structure of the at least one actuator component.  
         [0010]     This invention also provides for a novel optical switch, specifically a system for facilitating an optical cross-connect from a first region to a second region, the system comprising: a first collimator body and a second collimator body adjustably positioned at the first region and the second region, respectively, the first collimator body and the second collimator body each having a proximal end and a distal end, respectively, the proximal end of the first collimator body and the proximal end of the second collimator body being oriented toward one another, and first support means and second support means for adjustably supporting the first collimator body at a first position and the second collimator body at a second position, respectively; first adjustment means and second adjustment means for selectively adjusting the first collimator body from the first position to a third position and the second collimator body from the second position to a fourth position, respectively, the first adjustment means and the second adjustment means each comprising an actuator component having a driver coil and a magnetic structure with a gap formed therebetween, one of the driver coil and the magnetic structure of the first adjustment means being fixedly attached to the first collimator body and the other one of the driver coil and the magnetic structure of the first adjustment means being fixedly attached to the first support means, one of the driver coil and the magnetic structure of the second adjustment means being fixedly attached to the second collimator body and the other one of the driver coil and the magnetic structure of the second adjustment means being fixedly attached to the second support means; first current controller means and second current controller means for controlling a first electrical current and a second electrical current, respectively, the first current controller means selectively applying the first electrical current to the driver coil of the first adjustment means, the second current controller means selectively applying the second electrical current to the driver coil of the second adjustment means; first determiner means and second determiner means for determining a relative position of the first collimator body and a relative position of the second collimator body, respectively; and a first feedback loop and a second feedback loop connecting the first determiner means to the first current controller means and the second determiner means to the second current controller means, respectively.  
         [0011]     In another embodiment of the invention, there is provided a system for facilitating an optical cross-connection from a first region to a second region, the system comprising: a first collimator body and a second collimator body adjustably positioned at the first region and the second region, respectively, the first collimator body and the second collimator body each having a proximal end and a distal end, respectively, the proximal end of the first collimator body and the proximal end of the second collimator body being oriented toward one another; first support means and second support means for adjustably supporting the first collimator body at a center of mass thereof and the second collimator body at a center of mass thereof, respectively; and first adjustment means and second adjustment means for selectively adjusting the position of the first collimator body from the first position to a third position and the second collimator body from the second position to a fourth position.  
         [0012]     In another embodiment of the invention, there is provided a method for selectively positioning a collimator body, the method comprising: supporting the collimator body relative to a first position; and adjusting the collimator body from the first position to a second position, using adjustment means comprising an actuator component having a driver coil and a magnetic structure with a first gap formed therebetween, one of the driver coil and the magnetic structure being in attachment to the selectively positionable collimator body and the other one of the driver coil and the magnetic structure being in attachment to a fixed support in connection with the support means adjustably supporting the collimator body, wherein an electrical current through the driver coil of the at least one actuator component causes the collimator body to move in a direction perpendicular to a magnetic field created by the magnetic structure of the at least one actuator component.  
         [0013]     In another embodiment of the invention, there is provided a method for facilitating an optical cross-connect from a first region to a second region, the method comprising: providing a first collimator body and a second collimator body adjustably positioned at the first region and the second region, respectively, the first collimator body and the second collimator body each having a proximal end and a distal end, respectively, the proximal end of the first collimator body and the proximal end of the second collimator body being oriented toward one another; supporting the first collimator body at a first position and the second collimator body at a second position, respectively; adjusting the first collimator body from the first position to a third position and the second collimator body from the second position to a fourth position, using first adjustment means and second adjustment means, respectively, the first adjustment means and the second adjustment means each comprising an actuator component having a driver coil and a magnetic structure with a gap formed therebetween, one of the driver coil and the magnetic structure of the first adjustment means being fixedly attached to the first collimator body and the other one of the driver coil and the magnetic structure of the first adjustment means being fixedly attached to the first support means, one of the driver coil and the magnetic structure of the second adjustment means being fixedly attached to the second collimator body and the other one of the driver coil and the magnetic structure of the second adjustment means being fixedly attached to the second support means; determining a relative position of the first collimator body and a relative position of the second collimator body, respectively; and applying a first electrical current to the driver coil of the first adjustment means based on the relative position of the first collimator body, and applying a second electrical current to the driver coil of second adjustment means based on the relative position of the second collimator body.  
         [0014]     In another embodiment of the invention, there is provided a method for facilitating an optical cross-connection from a first region to a second region, the method comprising: providing a first collimator body and a second collimator body adjustably positioned at the first region and the second region, respectively, the first collimator body and the second collimator body each having a proximal end and a distal end, respectively, the proximal end of the first collimator body and the proximal end of the second collimator body being oriented toward one another; supporting the first collimator body at a center of mass thereof and supporting the second collimator body at a center of mass thereof; and adjusting the position of the first collimator body from the first position to a third position and the second collimator body from the second position to a fourth position.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other objects and features of the present invention will be more fully disclosed by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:  
         [0016]      FIGS. 1A, 1B  and  1 C illustrate a preferred embodiment of the present invention comprising an array of collimators, which are shown oriented in a rest position;  
         [0017]      FIGS. 2A, 2B  and  2 C illustrate the array as shown in  FIGS. 1A, 1B  and  1 C, with a pair of the collimators rotated to make a connection;  
         [0018]      FIGS. 3A and 3B  illustrate a preferred embodiment of the present invention comprising one set of magnetic actuators used to rotate a collimator, wherein the coils are elongated along the axis of the collimator;  
         [0019]      FIGS. 4A and 4B  illustrate an alternative preferred embodiment of the present invention comprising a magnetic actuator suitable for large angles, wherein the planes of the coils are perpendicular to the collimator axis;  
         [0020]      FIG. 5  shows another alternative preferred embodiment of the present invention similar to that shown in  FIG. 4 ;  
         [0021]      FIG. 6  illustrates a detail of a preferred embodiment of a set of hinges used to adjustably anchor a collimator;  
         [0022]      FIG. 7  shows a mode spectrum of a preferred embodiment of the collimator actuator; and  
         [0023]      FIG. 8  shows a schematic of a preferred embodiment of a circuit used for position sensing. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     Both small-scale, and scale-free, space switching fabrics are important with respect to the development of all optical networks. By avoiding costly electrooptical converters, enhanced performance is provided at a decreased cost.  
         [0025]     Items that are of importance for an optical network switching fabric are the size of the fabric, the average insertion loss per connection, the variation in insertion loss, the polarization dependent loss (PDL loss) for each connection, the bandwidth of the system, the static and dynamic cross-coupling between ports, and the flue cost of the system per port. It is highly desirable to have a system that is large, has a low insertion loss, has a very low PDL loss, and has a very low cost per port.  
         [0026]     While micro mirror systems have several advantages for very large systems, such as those above 256×256, these advantages are diminished when smaller systems are considered, such as those that might be prevalent in some all-optical networks of the near future.  
         [0027]     More particularly, insertion loss becomes a very important factor if the full fiber (100-200 wavelengths), or substantial wavelength bands of the fiber, are switched, as this involves the loss of optical power over many wavelengths at the same time.  
         [0028]     Referring to  FIGS. 1A, 1B , and  1 C, in a preferred embodiment of the present invention, there is provided a cross-connect system  5  having a first array  5  and a second array  5 ′ of precision collimators  15 ,  20 ,  25 ,  30  and precision collimators  15 ′,  20 ′,  25 ′,  30 ′, respectively. Array  5  and array  5 ′ are each arranged in such a way that precision collimators  15 ,  20 ,  25 ,  30  and precision collimators  15 ′,  20 ′,  25 ′,  30 ′, respectively, can be oriented with great precision towards each other by servo controlled precision mechanisms. In this configuration, the loss associated with a connection is simply the insertion loss associated with two collimators, which is a very low loss. Typically, such losses are lower than 1 dB and, with care, such losses can be less than 0.5 dB. By using a dual gimbal system, it is possible to position the collimator (and the associated driving coils) with its center of mass at the coincidence of the two rotation axes, and provide great suppression, if not full isolation, for lateral vibrations.  
         [0029]      FIG. 1A  illustrates a schematic side view of array  10  and array  10 ′.  FIGS. 1B and 1C  schematically illustrates a two dimensional front view of array  10  and array  10 ′, respectively. Array  10  of a transmission portion of cross-connect system  5  shows four rows of collimators  15 ,  20 ,  25 ,  30  arranged in an array aa through dd ( FIG. 1B ). Likewise, array  10 ′ of a receiving side of cross-connect system  5  shows four rows of collimators  15 ′,  20 ′,  25 ′,  30 ′, which are arranged in an array aa′ to dd′ ( FIG. 1C ). An actuator coil and magnet assembly  35  are operatively connected with each collimator  15 ,  20 ,  25 ,  30  of array  10  and each collimator  15 ′,  20 ′  25 ′,  30 ′ of array  10 ′, respectively. The spacing between actuator coil and magnet assemblies  35  is adjusted such that the collimators can move freely over the desired deflecting angles.  
         [0030]     Undeflected beams  40 ,  45 ,  50 ,  55 , exiting from collimators  15 ,  20 ,  25 ,  30  are arranged to converge toward point  60 , which is in the center of the exit plane of the opposite collimators  15 ′,  20 ′,  25 ′,  30 ′. A symmetrical arrangement holds true for the orientation of collimators  15 ′,  20 ′,  25 ′,  30 ′ in that undeflected beams  65 ,  70 ,  75 ,  80  converge toward point  85 , which is in the center of the exit plane of the opposite collimators  15 ,  20 ,  25 ,  30 .  
         [0031]     A plate  90  comprises several sets of two dimensional gimbals ( FIG. 6 ) for the deflection of collimators  15 ,  20 ,  25 ,  30  of array  10 . Plate  90 ′ on the opposite side of system  5  comprises several sets of two dimensional gimbals ( FIG. 6 ) for the deflection of collimators  15 ′,  20 ′,  25 ′,  30 ′ of array  10 ′. The sets of two dimensional gimbals of plate  90  and plate  90 ′ allow gross adjustment of collimators  15 ,  20 ,  25 ,  30  and collimators  15 ′,  20 ′,  25 ′,  30 ′ with respect to one another. Their operation and construction are described in detail hereinbelow.  
         [0032]     The optical axis of each collimator is made to coincide with its center of rotation at plate  90  or plate  90 ′. This configuration permits beam rotation without causing any translation during the rotation of a set of collimators, e.g., collimator  15  and collimator  30 ′. The convergent arrangement of collimators  15 ,  20 ,  25 ,  30  and collimators  15 ′,  20 ′,  25 ′,  30 ′, respectively, reduces by half the required angle of deflection that is needed in both directions. For example, collimator  15  and collimator  30 ′ are each rotated until beam  40  and beam  80  are in alignment with one another, thereby allowing beam  40  to enter collimator  30 ′, or beam  80  to enter collimator  15 , if the direction of the light beam is reversed.  
         [0033]     Referring now to  FIG. 2A , collimator  15  and collimator  30 ′ are shown in alignment with one another after appropriate rotation from the configuration shown in  FIG. 1A . Once a connection is made, an optical feedback loop (not illustrated) is used to adjust its set point. In a preferred embodiment of the present invention, the magnets of assembly  35  are stationary and the coils of assembly  35  rotate together with collimators  15  and  30 ′.  
         [0034]     Referring now to  FIGS. 3A, 3B ,  4 A and  4 B, in a preferred embodiment of the present invention, there is provided a sensor system  92  for providing a position feedback system of one of the collimators, e.g., collimator  15 . Sensor system  92  operates in conjunction with an optical feedback loop (not shown) that analyzes light flowing through cross-connect system  5  between two of the collimators, e.g., collimator  15  and collimator  30 ′ (see  FIG. 2A ). Alternatively, sensor system  92  may operate independently of, or in the absence of, an optical feedback loop (not shown). Such a system requires no high voltages, thus making its driving circuitry easily integratable and low cost.  
         [0035]     Referring to  FIGS. 3A and 3B , in a preferred embodiment of the present invention, there is provided a coil arrangement  95  having a first coil  100 , a second coil  105 , a third coil  110 , and a fourth coil  115  disposed lengthwise along a longitudinal axis  120  of collimator  15 . A frame  125  attaches coils  100 ,  105 ,  110 ,  115  to collimator  15 . In a preferred embodiment of the present invention, the coils  100 ,  105 ,  110 ,  115  are elongated in the direction of axis  120  so as to maximize the torque and minimize the lateral extend of collimator  15 . Magnetic structures  130 ,  135 ,  140 ,  145  are mounted adjacent to coils  100 ,  105 ,  110 ,  115 , respectively, with a gap disposed therebetween. The actuators operate on the voice coil principle. Coils  100 ,  105 ,  110 ,  115  are surrounded by magnetic fields perpendicular to the path of current flow.  
         [0036]     In an alternative embodiment of the present invention, the top and bottom ends  150 ,  155  may be removed for simplicity (as used herein, the terms “top” and “bottom” are intended to be understood in the context of the orientation shown in  FIG. 3B ). The direction of the local magnetic fields are indicated by arrows  160 . For example, if coil  100  is actuated it will move perpendicular to the orientation of the local field of magnetic structure  130 . This produces collimator rotation in the x-direction. Coil  105 , when actuated properly at the same time as coil  100 , produces augmented motion in the x-direction. Likewise, coils  110  and  115  when actuated alone, or in tandem, produce motion in the y-direction.  
         [0037]     In a preferred embodiment of the present invention, magnetic structures  130 ,  135 ,  140 ,  145  are made of permanent magnets and magnetic keeper material so as to create a gap field as high as possible, as is well known to those skilled in the art. The gap between magnetic structures  130 ,  135 ,  140 ,  145  and coils  100 ,  105 ,  110 ,  115 , respectively, is configured wide enough to accommodate the rotation of the collimator  15  as it rotates around its axis in the x-direction and the y-direction. Because the motion of collimator  15  is conical with respect to the rotation point, the required distance between coils  100 ,  105 ,  110 ,  115  and magnetic structures  130 ,  135 ,  140 ,  145 , respectively, increases along the length of each coil from top end  150  to bottom end  155 , which in turn decreases the magnetic field.  
         [0038]     In a preferred embodiment of the present invention (not shown), magnet structure  145  and coil  115  may be tapered with respect to longitudinal axis  120 . The gap between coil  115  and magnetic structure  145  is decreased at top end  150  of coil  115 , which is near the rotation point, and increased at the bottom end  155 , so as to accommodate the larger travel of the distal end of collimator  15 .  
         [0039]     Referring now to  FIGS. 4A and 4B , in another preferred embodiment of the present invention, there is shown an actuator device  160  with coils  165  and  170  attached to collimator body  15 , and magnetic structures  175  and  180  in surrounding configuration to coils  165  and  170 , respectively. Magnetic structures  175  and  180  produce magnetic fields that are generally perpendicular to the current flowing in coils  165  and  170 . Actuation of coil  165  produces motion of collimator body  15  in the x-direction, while actuation of the coil  170  produces motion of collimator body  5  in the y-direction. Because these motions are each in a plane that coincides with the plane of coils  165  and  170 , the vertical air gap between the coils  165  and  170  and the magnetic structures  175  and  180 , respectively, can be quite small. This configuration allows high magnetic fields and magnetic torques.  
         [0040]     Looking now at  FIG. 5 , in another preferred embodiment of the present invention, there is shown an actuator device  185  having coils  190  and  195  configured on top of each other, and a magnetic structure  200  built around coils  190  and  195  so as to serve both coils  190  and  195  at the same time. This configuration allows for a compact arrangement of coils  190  and  195  and magnetic structure  200 , thereby providing for an almost equal torque on both axes with equal current and dissipation. Here, collimator  15  is surrounded by magnetic structure  200  that includes magnetic paths for magnets  205  and  210 . Magnets  205  and  210  provide fields that are perpendicular to coils  190  and  195 , respectively. This allows lateral motion in two independent directions, while maintaining small air gaps between magnets  205  and  210  and coils  190  and  195 , respectively, which gives rise to a strong field and hence requires only modest drive currents. Coils  215  and  220  provide an inductive sensor for the motion of coil  190 . Coils  225  and  230  provide for sensing of the motion of coil  195 . Differential readout of the output of coils  215  and  220  provides a voltage that is almost linear with the displacement of primary coil  190  when excited at high frequencies.  
         [0041]     In both of these cases, the area of coils  215  and  220  is restricted as much as possible in order to create a cell as small as possible. Each cell consists of a collimator, e.g. collimator  15 , a set of coils  190  and  195 , and the magnetic structure  205  and  210  attached to the surrounding cell wall (not shown). The cell walls (not shown) form a rectangular honeycomb array of intersecting lines. The honeycomb cells (not shown) are aligned with, and converge toward, the convergence point  60 ,  85  ( FIG. 1A ), respectively, of each array  10 ,  10 ′ of collimators  15 ,  20 ,  25 ,  30  and collimators  15 ′,  20 ′  25 ′,  30 ′, respectively. The typical convergence angle of a cell is 0.8 degrees, with each successive collimator outward from the center of the array having an increasing convergence angle, i.e., by 0.8 degrees.  
         [0042]     Now looking at  FIG. 6 , in a preferred embodiment of the present invention, there is provided a sheet  235  having a hole  240 , and hinges  245  and  250 , therein. Collimator  15  rotates along two orthogonal axes. These degrees of freedom are provided as collimator  15  is disposed through hole  240  and is adjustably supported by sheet  235 . Hinges  245  and  250  provide two degrees of freedom for rotation in two orthogonal directions. As illustrated, hinges  245  and  250  are of the folded type, and provide increased lateral stiffness for the same rotational stiffness. Hole  240 , and hinges  245  and  250 , are typically etched, simultaneously, in one large sheet for supporting multiple collimators (e.g. 64 or 256 collimators).  
         [0043]     In a preferred embodiment of the present invention (not shown), sheets  235  are fabricated by stacking together several ones of sheet  235  and then machining the stacked sheets  235  by electrical discharge machining (EDM). When etched, hole  240  may be etched in several sections that fold away upon insertion of the collimator such that the resulting flaps are used to attach collimator  15  to sheet  235 .  
         [0044]     Still referring to  FIG. 6 , in a preferred embodiment of the present invention, several sets of hinges  245  and  250  are etched into a flat sheet of metal so as to form plate  90  or  90 ′ ( FIG. 1A ) comprising several sets of dual gimbal and attachment means. Hinges  245  and  250  are etched inexpensively with great precision so as to thereby provide a very economical cross-connect system  5 . Cross-connect system  5  can operate in very adverse environmental conditions with very little interference. The beams of arrays  10  and  10 ′ are made to converge during fabrication so as to decrease the required angle of deflection.  
         [0045]     Sheet  235  may be made out of any suitable metal such as stainless steel, titanium, etc. In a preferred embodiment of the present invention, sheet  235  is a few mils thick. Typically, hinges  245  and  250  may be 1.7 mm long, with a 200 micron wide center hinge and 100 micron wide return hinges. With a typical aluminum collimator, which is about 2.8 mm in diameter and about 18 mm in length, the torsional resonance frequencies in both axes are on the order of 50 to 60 Hz. The next higher mode, which consists of vertical pumping mode, is in the neighborhood of 250-300 Hz.  
         [0046]     Referring now to  FIG. 7 , in a preferred embodiment of the present invention, there is shown a mode spectrum  255 . This is a very desirable mode spectrum for actuation of an actuator assembly  35  ( FIG. 1A ), with the lowest torsional modes  260  and  265  ( FIG. 7 ) being very well separated from the next higher order mode  270 , which is perpendicular to the rotational control directions. While it is also possible to use hinges of different types which include, for example, bending hinges, generally the resonant spectra are not as desirable, and are not as well separated as in the preferred embodiment of the present invention. It is highly desirable to have the torsional spectra well separated from the next mode, and to have the next mode as one where the collimator does not rotate and, hence, does not greatly affect the established optical link. Since the next mode is a vertical pumping mode, it affects the coupling between collimators very little and, hence, it is of little consequence. Higher modes involving transverse motion of the hinge structures are typically in the neighborhood of 800 to 2000 Hz, which is well separated from the frequencies used in control system.  
         [0047]     During assembly, in order to orient the collimators in the appropriate convergent direction, collimators  15 ,  20 ,  25 ,  30  (or collimators  15 ′,  20 ′,  25 ′,  30 ′) are positioned in a second, thick aligned guiding plate (not shown), which has an array of conical holes oriented such that the desired convergence is forced on array  10  of collimators  15 ,  20 ,  25 ,  30  (or array  10 ′ of collimators  15 ′,  20 ′,  25 ′,  30 ′). Hinges  245  and  250  remain undeflected during insertion, and collimator  15  is then glued in place at hole  240 . The convergence plate (not shown) is removed after collimator  15  is positioned at the correct orientation.  
         [0048]     In another preferred embodiment of the present invention, and referring now to  FIG. 8 , there is shown a sensor arrangement  275  for independently measuring the angular deflection of collimator  15  ( FIG. 3A ) about its axes. This may be accomplished in a variety of ways. Referring to  FIG. 3A , the inductance of coil  100  increases as coil  100  moves in the x-direction toward magnetic structure  100 , and the inductance of coil  105  decreases at the same time as coil  105  moves in the x-direction away from magnetic structure  135 . Hence, the x-position of coils  100  and  105 , and the angular position of collimator  15 , are derived by measuring the differential inductance of coils  100  and  105 . Likewise, the differential inductance of coils  110  and  115  gives a measure of the y-position of collimator  15 . There are several systems, which are well known in the art, that may be used to deduce a sensing signal from this differential output. Coils  100 ,  105 ,  110 ,  115  are operated under DC power so as to produce deflection, while coils  100 ,  105 ,  110 ,  115  may also be operated under AC current at high frequencies such as, for example, several MHz, so as to produce sensing signals without affecting the drive of the actuator assembly.  
         [0049]     Referring again to  FIG. 8 , there is shown sensor arrangement  275  having driver amplifiers  280  and  285  in the same integrated circuit and operating in a push-pull arrangement, respectively. Coil  100  and coil  105  each have a lead connected to a bias voltage  290 , also referred to hereinbelow as Vbias  290 . For example, a typical bias voltage, Vbias  290 , is 2.5 vdc. The other end of coil  100  and coil  105  are driven by driving amplifiers  280  and  285  through RF chokes  295  and  300 , respectively. The driver outputs swing symmetrically around Vbias  290 , providing a bipolar current in each coil  100  and  105  for positioning collimator  15  (see  FIG. 3A ). An RF source  305 , V 1 , is applied to coil  100  and coil  105  through the RC networks  310  and  315 . The RF chokes act to keep the driver decoupled from the coils at RF frequencies. The circuit comprising coil  100 , RC network  310 , coil  105 , and RC network  315  forms a bridge excited by V 1   305 . The bridge output at X+320 and X−325 will have a differential AC output that depends on the bridge balance. As the inductance of coil  100  and the inductance of coil  105 , respectively, change with position, the bridge output at X+320 and X−325, respectively, will vary in amplitude and polarity. Well-known methods such as synchronous demodulation use the reference AC signal  305  to recover position information from the bridge output at X+320 and X−325. This method provides a very high S/N ratio that is advantageous with small signals in such an environment. The circuitry is duplicated for the y-axis.  
         [0050]     In another preferred embodiment of the present invention (not shown), and referring again to  FIG. 5 , at least one of drive coils  190  and  195  is also supplied with an RF signal, and the sensing coils  215  and  220  are wound on the magnetic structure  200 . By taking the difference between the induced RF signals in the coils, it is possible to measure the position of the collimator  15  in the x-direction. A similar arrangement may also be applied in the y-direction so as to provide full position encoding.