Abstract:
A packaged micromirror assembly ( 21, 21′ ) is disclosed. The assembly ( 21, 21′ ) includes a mirror element ( 41 ) having a mirror surface ( 29 ) that can rotate in two axes. Magnets ( 53 ) are attached to the mirror element ( 41 ), to permit rotation of the mirror surface ( 29 ) responsive to the energizing of coil drivers ( 36 ). A sensor ( 63, 80 ) is disposed under the mirror surface ( 29 ) to detect mirror orientation. In one aspect of the invention, the sensor ( 63 ) includes a light source such as an LED ( 68 ) that imparts light through an aperture ( 66 ) at the underside of the mirror surface ( 29 ). Light detectors ( 65 ) are arranged at varying angles, and detect relative intensity of light reflected from the underside of the mirror surface ( 29 ), from which the rotational position of the mirror ( 29 ) can be derived.

Description:
This application claims priority under 35 USC §119 (e)(1) of Provisional Application No. 60/233,873, filed Sep. 20, 2000. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     This invention is in the field of optical communications, and is more specifically directed to micromirror assemblies as used in such communications. 
     Modern data communications technologies have greatly expanded the ability to communicate large amounts of data over many types of communications facilities. This explosion in communications capability not only permits the communications of large databases, but has also enabled the digital communications of audio and video content. This high bandwidth communication is now carried out over a variety of facilities, including telephone lines (fiber optic as well as twisted-pair), coaxial cable such as supported by cable television service providers, dedicated network cabling within an office or home location, satellite links, and wireless telephony. 
     Each of these conventional communications facilities involves certain limitations in their deployment. In the case of communications over the telephone network, high-speed data transmission, such as that provided by digital subscriber line (DSL) services, must be carried out at a specific frequency range to not interfere with voice traffic, and is currently limited in the distance that such high-frequency communications can travel. Of course, communications over “wired” networks, including the telephone network, cable network, or dedicated network, requires the running of the physical wires among the locations to be served. This physical installation and maintenance is costly, as well as limiting to the user of the communications network. 
     Wireless communication facilities of course overcome the limitation of physical wires and cabling, and provide great flexibility to the user. Conventional wireless technologies involve their own limitations, however. For example, in the case of wireless telephony, the frequencies at which communications may be carried out are regulated and controlled; furthermore, current wireless telephone communication of large data blocks, such as video, is prohibitively expensive, considering the per-unit-time charges for wireless services. Additionally, wireless telephone communications are subject to interference among the various users within the nearby area. Radio frequency data communication must also be carried out within specified frequencies, and is also vulnerable to interference from other transmissions. Satellite transmission is also currently expensive, particularly for bi-directional communications (i.e., beyond the passive reception of television programming). 
     A relatively new technology that has been proposed for data communications is the optical wireless network. According to this approach, data is transmitted by way of modulation of a light beam, in much the same manner as in the case of fiber optic telephone communications. A photoreceiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the waveguide of the fiber optic. 
     It is contemplated that the optical wireless network according to this approach will provide numerous important advantages. First, high frequency light can provide high bandwidth, for example ranging from on the order of 100 Mbps to several Gbps, using conventional technology. This high bandwidth need not be shared among users, when carried out over line-of-sight optical communications between transmitters and receivers. Without the other users on the link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing of multiple communications. Bi-directional communication can also be readily carried out according to this technology. Finally, optical frequencies are not currently regulated, and as such no licensing is required for the deployment of extra-premises networks. 
     These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector. 
     It will be apparent to those skilled in the art having reference to this specification that the ability to correctly aim the transmitted light beam to the receiver is of importance in this technology. Particularly for laser-generated collimated beams, which can have quite small spot sizes, the reliability and signal-to-noise ratio of the transmitted signal are degraded if the aim of the transmitting beam strays from the optimum point at the receiver. Especially considering that many contemplated applications of this technology are in connection with equipment that will not be precisely located, or that may move over time, the need exists to precisely aim and controllably adjust the aim of the light beam. 
     Copending application Ser. No. 09/310,284, filed May 12, 1999, entitled “Optical Switching Apparatus” now U.S. Pat. No. 6,295,154, commonly assigned herewith and incorporated herein by this reference, discloses a micromirror assembly for directing a light beam in an optical switching apparatus. As disclosed in this application, the micromirror reflects the light beam in a manner that may be precisely controlled by electrical signals. As disclosed in this patent application, the micromirror assembly includes a silicon mirror capable of rotating in two axes. One or more small magnets are attached to the micromirror itself; a set of four coil drivers are arranged in quadrants, and are current-controlled to attract or repel the micromirror magnets as desired, to tilt the micromirror in the desired direction. 
     Because the directed light beam, or laser beam, has an extremely small spot size, precise positioning of the mirror to aim the beam at the desired receiver is essential in establishing communication. This precision positioning is contemplated to be accomplished by way of calibration and feedback, so that the mirror is able to sense its position and make corrections. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a package for a micromirror assembly that includes sensing capability for the position of the micromirror. 
     It is a further object of the present invention to provide a method for making such a package. 
     It is a further object of the present invention to provide such a package and method that is relatively low-cost, and also well suited for high-volume production. 
     Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings. 
     The present invention may be implemented into a package for a micromirror assembly. The package is molded around a plurality of coil drivers, and their control wiring, for example by injection or transfer molding. A two-axis micromirror and magnet assembly is attached to a shelf overlying the coil drivers. Underlying the mirror is a sensor for sensing the angular position of the mirror. According to the preferred embodiment of the invention, a segmented conical capacitance sensor is provided, to measure variations in capacitance that are caused by the rotation of the mirror. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 a  is a schematic drawing of an optical wireless network using a packaged micromirror assembly. 
     FIG. 1 b  is a schematic drawing of an optical wireless network using a packaged micromirror assembly according to the preferred embodiments of the invention. 
     FIGS. 2 a  and  2   b  are plan and cross-sectional views, respectively, of a packaged micromirror assembly according to a first preferred embodiment of the invention. 
     FIG. 3 is a plan view of a mirror element in the packaged micromirror assembly according to the first preferred embodiment of the invention. 
     FIGS. 3 a  through  3   d  are cross-sectional views of the mirror element of FIG. 3, illustrating its operation. 
     FIGS. 4 a  and  4   b  are plan and cross-sectional views, respectively, of a packaged micromirror assembly according to a first preferred embodiment of the invention. 
     FIGS. 5 a  through  5   c  are various views of a packaged micromirror assembly according to a second preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in connection with its preferred embodiments, with an example of an application of these preferred embodiments in a communications network. It is contemplated, however, that the present invention may be realized not only in the manner described below, but also by way of various alternatives which will be apparent to those skilled in the art having reference to this specification. It is further contemplated that the present invention may be advantageously implemented and used in connection with a variety of applications besides those described below. It is therefore to be understood that the following description is presented by way of example only, and that this description is not to be construed to limit the true scope of the present invention as hereinafter claimed. 
     Referring first to FIG. 1 a , an example of an optical wireless network will be illustrated, to provide context for the present invention. In this simple example, unidirectional communications are to be carried out from computer  2  to server  20 , by way of modulated directed light. In this example, computer  2  is a conventional microprocessor based personal computer or workstation, including the appropriate network interface adapter for outputting the data to be communicated. Computer  2  is connected to transmitter optical module  5 , which aims a directed light beam at the desired receiver  17 , and which modulates the light beam to communicate the data. 
     Alternatively, the transmitting source may be a network switch or router, a source of video data such as a DVD player or a television set-top converter box, or the like, rather than computer  2  as shown. It is contemplated that the present invention may be used in connection with effectively any source of digital data. 
     In this example, transmitter optical module  5  includes modulating laser  6 , which generates a collimated coherent light beam of the desired wavelength (e.g., 850 nm) and power (e.g., on the order of 4 to 5 μW/cm 2  measured at 50 meters, with a spot size of on the order of 2.0 to 2.5 mm in diameter). Modulating laser  6  modulates this light beam according to the digital data being transmitted. The modulation scheme used preferably follows a conventional data communications standard, such as those used in connection with fiber optic communications for similar networks. The modulated laser beam exits modulating laser  6  and is reflected from micromirror assembly  10  toward receiver  17 . The construction of micromirror assembly  10  according to the preferred embodiments of the invention will be described in further detail below. 
     On the receiver end, receiver  17  captures the incoming directed light beam, and converts the modulated light energy to an electrical signal; for example, receiver  17  may include a photodiode, which modulates an electrical signal in response to the intensity of detected light. Such other conventional receiver circuitry, such as demodulators, filters, and the line, are also provided. The demodulated communicated electrical signal is then forwarded from receiver  17  to router  18 , and thus into the receiving network, for eventual distribution to server  20 , in this example. 
     As evident from FIG. 1 a  and the foregoing description, this example illustrates a unidirectional, or simplex, communications approach, for ease of this description. It will be appreciated by those skilled in the art that bi-directional, or duplex, communications may be carried out by providing another transmitter-receiver pair for communicating signals in the opposite direction (router  18  to computer  2 ). 
     The communications arrangement of FIG. 1 a  may be utilized in connection with a wide range of applications, beyond the simple computer-to-network example suggested by FIG. 1 a . For example, it is contemplated that each of multiple computers in an office or other workspace may communicate with one another and with a larger network by way of modulated light to a central receiver within the room, and also between rooms by way of relayed communications along hallways or in a space frame. Other indoor applications for this optical wireless communications may include the communication of video signals from a computer or DVD player to a large-screen projector. It is further contemplated that optical wireless communications in this fashion may be carried out in this manner but on a larger scale, for example between or among buildings. 
     The positioning of micromirror assembly  10  must be precisely controlled to aim the modulated laser beam at receiver  17 , and thus optimize the signal-to-noise ratio of the transmitted signals. It is contemplated that this precision positioning is preferably accomplished by way of calibration and feedback, so that the mirror is able to sense its position and make corrections. Previous efforts toward providing such positioning, in connection with the present invention, have included external sensors such as illustrated in FIG. 1 a.    
     As shown in this example, the reflected laser beam impinges beam splitter  12 . Beam splitter  12  transmits the majority of the energy to receiver  17 , but reflects a portion of the energy to position sensitive detector (PSD)  15 . PSD  15  provides signals to control circuitry  14 , indicating the position of the reflected light that it receives. Control circuitry  14  then issues control signals to micromirror assembly  10  to direct its angle of reflection in response to the signals from PSD  15 , optimizing the aim of the directed laser beam at receiver  17 . In one example, during setup of the transmission, micromirror assembly  10  and PSD  15  “sweeps” the aim of the directed laser beam across the general area of receiver  17 . In response, receiver  17  issues signals to control circuitry  14  over a secondary communications channel (not shown), indicating the received energy over time. These “pings” may be compared with the instantaneous position of micromirror assembly  10  as measured by PSD  15 , to calibrate and optimize the aim of micromirror assembly  10  to achieve maximum energy transmission. Once this aim is set, communications may then be carried out. It is contemplated, however, that adjustments may be necessary due to external factors such as building or equipment movement and the like. These adjustments may be carried out by way of feedback from receiver  17  (either over the secondary channel or as transmit mode feedback in a duplex arrangement), or by periodically repeating the measurement and sweeping. 
     The provision of beam splitter  12  and PSD  15  in transmitter optical module  5  as shown in FIG. 1 a  provides the desired feedback control of the reflection of the laser beam. However, from a cost and reliability standpoint, it is desired to reduce the number of components in the transmitter optical module. Referring now to FIG. 1 b , a communications system according to the preferred embodiments of the invention, in which the packaged micromirror assembly includes integral position sensing capability, will now be described. 
     As shown in FIG. 1 b , transmitter optical module  25  includes packaged micromirror assembly  21  that directly reflects the laser beam from laser  6  to receiver  17 , without a beam splitter or other external components interposed in the path of the beam. Packaged micromirror assembly  21 , as will be described in detail below, includes sensing capability by way of which the orientation of the mirror is detected and a signal generated that is applied to control circuit  24 . In response, control circuit  24  provides electrical signals to packaged micromirror assembly  21  to position the mirror, so that the beam may impinge receiver  17  in an optimal manner. 
     Because of the in-package positional feedback provided by packaged micromirror assembly  21 , transmitter optical module  25  may be manufactured at significantly lower cost. In addition, by eliminating the beam splitter, transmitter optical module  25  avoids the inherent loss of beam intensity that is otherwise directed to the position sensitive detector. 
     Referring now to FIGS. 2 a  and  2   b , packaged micromirror assembly  21  according to a first preferred embodiment of the invention will now be described. As shown in FIGS. 2 a  and  2   b  and as will be described in further detail below, mirror element  41  is formed of a single piece of material, preferably single-crystal silicon, photolithographically etched in the desired pattern, to form mirror surface  29  and its supporting hinges and frame. To improve the reflectivity of mirror surface  29 , mirror element  41  is preferably plated with a metal, such as gold or aluminum. In its assembled form, as shown in FIGS. 2 a  and  2   b , four permanent magnets  53  are attached to mirror element  41 , at a 90° relative orientation from one another, to provide the appropriate rotation. Magnets  53  may be formed of any permanently magnetizable material, a preferred example of which is neodymium-iron-boron. 
     FIGS. 3 and 3 a  through  3   d  illustrate mirror element  41  in further detail. Mirror element  41  includes a frame portion, an intermediate gimbals portion, and an inner mirror portion, all preferably formed from one piece of crystal material such as silicon. In its fabrication, silicon is etched to provide outer frame portion  43  forming an opening in which intermediate annular gimbals portion  45  is attached at opposing hinge locations  55  along first axis  31 . Inner, centrally disposed mirror portion  47 , having a mirror  29  centrally located thereon, is attached to gimbals portion  45  at hinge portions  55  on a second axis  35 , 90 degrees from the first axis. Mirror  29 , which is on the order of 100 microns in thickness, is suitably polished on its upper surface to provide a specular surface. Preferably, this polished surface is plated with a metal, such as aluminum or gold, to provide further reflectivity. In order to provide necessary flatness, the mirror is formed with a radius of curvature greater than approximately 2 meters, with increasing optical path lengths requiring increasing radius of curvature. The radius of curvature can be controlled by known stress control techniques such as, by polishing on both opposite faces and deposition techniques for stress controlled thin films. If desired, a coating of suitable material can be placed on the mirror portion to enhance its reflectivity for specific radiation wavelengths. 
     Mirror element  41  includes a first pair of permanent magnets  53  mounted on gimbals portion  45  along the second axis, and a second pair of permanent magnets  53  mounted on extensions  51 , which extend outwardly from mirror portion  47  along the first axis. In order to symmetrically distribute mass about the two axes of rotation to thereby minimize oscillation under shock and vibration, each permanent magnet  53  preferably comprises a set of an upper magnet  53   a  mounted on the top surface of the mirror element  41  using conventional attachment techniques such as indium bonding, and an aligned lower magnet  53   b  similarly attached to the lower surface of the mirror assembly as shown in FIGS. 3 a  through  3   d . The magnets of each set are arranged serially such as the north/south pole arrangement indicated in FIG. 3 c . There are several possible arrangements of the four sets of magnets which may be used, such as all like poles up, or two sets of like poles up, two sets of like poles down; or three sets of like poles up, one set of like pole down, depending upon magnetic characteristics desired. 
     By mounting gimbals portion  45  to frame portion  43  by means of hinges  55 , motion of the gimbals portion  45  about the first axis  31  is provided and by mounting mirror portion  47  to gimbals portion  45  via hinges  55 , motion of the mirror portion relative to the gimbals portion is obtained about the second axis  35 , thereby allowing independent, selected movement of the mirror portion  47  along two different axes. 
     The middle or neutral position of mirror element  41  is shown in FIG. 3 a , which is a section taken through the assembly along line A—A of FIG.  3 . Rotation of mirror portion  47  about axis  35  independent of gimbals portion  45  and/or frame portion  43  is shown in FIG. 3 b  as indicated by the arrow. FIG. 3 c  shows the middle position of the mirror element  41 , similar to that shown in FIG. 3 a , but taken along line B—B of FIG.  3 . Rotation off the gimbals portion  45  and mirror portion  47  about axis  31  independent of frame portion  43  is shown in FIG. 3 d  as indicated by the arrow. The above independent rotation of mirror  29  of mirror portion  47  about the two axes allows direction of optical beam  13  as needed by the optical switch units. 
     In order to protect hinges  55  from in-plane shock during handling and shipping, stops  57  may be provided, as described in the above-incorporated application Ser. No. 09/310,284. According to another optional feature of the invention, lock down tabs associated with each hinge are provided, also as described in the above-incorporated application Ser. No. 09/310,284, now U.S. Pat. No. 6,295,154. 
     Referring back to FIG. 3, extensions  51  are preferably provided with laterally extending tabs  51   a , which can be used to clamp down the mirror portion during assembly to thereby provide additional stress protection. 
     Mirror element  41 , in this embodiment of the invention, rests upon and is attached to shelf  34  of body  30 . Shelf  34  lies inwardly of window shelf  32 , upon which transparent window  31  rests and is attached. Window  31  may be formed of conventional microscope slide glass, or of a transparent plastic such as LEXAN plastic. It is highly preferred that the dimensions and locations of shelves  32 ,  34 , as well as the bottom well of body  30 , be selected so that the maximum deflection of mirror  29  is stopped by one of magnets  53  impacting body  30  without mirror  29  itself impacting the inner surface of window  31 . Additionally, it is preferred that the maximum deflection of mirror  29  is limited, by body  30 , to an angle that is well below that which overstresses hinges  55 . 
     Further detail regarding the construction and method of manufacturing packaged micromirror assembly  21  according to the preferred embodiments of the invention, including alternative methods for such manufacture, is provided in copending provisional application No. 60/234,074, filed Sep. 20, 2000, entitled “Molded Packages for Optical Wireless Network Micromirror Assemblies”, now U.S. patent application Ser. No. 09/955,506 filed Sep. 18, 2001, both commonly assigned and incorporated herein by this reference. 
     As shown in the cross-section of FIG. 2 b , packaged micromirror assembly  21  includes position sensor  63  physically disposed between mirror assembly  41  and driver coils  36 , and thus in close proximity to mirror element  41 . Sensor  63  is preferably mounted to body  30  prior to the attachment of mirror element  41 , as sensor  63  is positioned between body  30  and mirror element  41 . Position sensor  63  is electrically connected to leads  61 , to provide electrical signals (or response, in the passive sense) to external circuitry such as control circuitry  24  in transmitter optical module  25 . In this example, therefore, packaged micromirror assembly  21  provides position sensing signals to control circuitry  24  on leads  61 , and receives position input signals on leads  39 . The complete feedback sensing and control response is thus provided within packaged micromirror assembly  21  itself, according to the present invention. 
     According to a first preferred embodiment of the invention, as shown in FIGS. 4 a  and  4   b , the mechanism by way of which the position of mirror  29  is sensed uses incident light produced below the surface of mirror  29  and reflected from its underside. As shown in FIG. 4 a , position sensor  63  according to this first embodiment of the invention is a printed circuit board having light-emitting-diode (LED)  68  that emits light through point aperture  66  toward the underside of mirror  29 . As shown in FIG. 4 b , this arrangement of LED  68  and aperture  66  provides distribution  71  of light intensity that is substantially Gaussian in shape, with the light intensity imparted to mirror  29  being at a maximum at its center point, and falling off sharply away from the center. For example, a standard deviation of on the order of 40° for distribution  71  is contemplated to be suitable for use in connection with this embodiment of the invention. This distribution  71  of light intensity is used to determine the position of mirror  29 , as will be described below. 
     Sensor  63  also includes four light detectors  65 , spaced 90° from one another as shown in FIG. 4 a . In this example, detectors  65 x+,  65 x− are located along the positive and negative horizontal axes, using aperture  66  as the origin; detectors  65 y+,  65 y− are located along the positive and negative vertical axes in similar fashion. It is preferred that detectors  65  are placed at the same radial distance from aperture  66 . Detectors  65  are preferably electrically coupled or connected to external circuitry, for example via leads  61  (FIG. 2 b ). 
     In operation, sensor  63  is able to detect changes in the orientation of mirror  29  from variations in the light intensity sensed by detectors  65 , as will now be described relative to FIG. 4 b . The example of FIG. 4 b  illustrates the operation of sensor  63  for rotation in one axis only, for clarity of description; it will of course be understood by those skilled in the art, from this example, that the operation of sensor  63  in two axes will be similar. In any rotation, the intensity of the light emitted by LED  68  through aperture  66  will be applied to the underside of mirror  29  in a manner with the point of highest intensity at the center of mirror  29 . However, the location of mirror  29  from which light reflects to each of detectors  65  and, given distribution  71  of this light, the intensity of the light reflected to detectors  65 , varies with the rotational orientation of mirror  29 . In the null, or flat, position N of mirror  29 , as shown in FIG. 4 b , the light that will be reflected by mirror  29  to each of sensors  65  originates from aperture  66  at substantially the same angle relative to the normal (i.e., perpendicular to the surface of LED  68 ), as shown by paths  73   0 + and  73   0 − to sensors  65 x+,  65 x−, respectively. From this position of mirror  29 , equal light intensity will be sensed by detectors  65 x+,  65 x−. 
     Upon the rotation of mirror  29  into a rotated position  29 ′, as shown in FIG. 4 b , detectors  65 x+,  65 x− will sense different magnitudes of light intensity. This differential magnitude is due to the change in the paths traveled by the emitted light. As shown in this example, with mirror  29  rotated toward detector  65 x−, path  73 ′− traveled by light reflected to detector  65 x− originates from aperture  66  at an angle farther from normal than when in the null position. As a result, detector  65 x− will receive lower intensity reflected light from mirror  29  in rotated position  29 ′. On the other hand, path  73 ′+ traveled by light reflected to detector  65 x+ with mirror  29  in position  29 ′ originates from aperture  66  at an angle closer to normal than in the null position. Because of the significantly higher intensity of light imparted by LED  68  through aperture  66  at points nearer the center of mirror  29 , detector  65 x+ will receive a higher magnitude of light with mirror  29  in rotated position  29 ′ than from mirror  29  in its null position. 
     As a result of the variations of detected reflected light with the rotational position of mirror  29 , as sensed by detectors  65 , sensor  63  can generate or modulate electrical signals indicative of the intensity of detected light. These signals can then be used to determine the rotational position of mirror  29 , and thus to control the positioning of mirror  29  to aim the laser beam toward the desired receiver. 
     In the foregoing implementation of the first preferred embodiment of the invention, a single light source is defined by aperture  66 , with multiple detectors  65  angularly arranged at a common radius from the center axis. Alternatively, multiple light sources may be angularly arranged away from the center axis, for example at the locations of detectors  65  illustrated in FIG. 4 a , in combination with a single detector located at the center axis (i.e., at the location of aperture  66  in FIG. 4 a ). According to this alternative, the multiple light sources (e.g., LEDs and apertures) would illuminate the underside of mirror  29  in a pattern from which the path intensities could be derived. For example, the multiple LEDs could be activated in a temporal sequence, with the detected reflected light analyzed according to a synchronized sequence in order to determine the relative light intensities along each path. The multiple light sources could emit their light simultaneously, but at different modulation frequencies, in which case signals corresponding to the detected light would be demodulated to determine a frequency spectrum indicative of the relative path intensities. 
     In any case, additional intelligence may be provided within sensor  63 , if desired, to facilitate the feedback and control of mirror  29 . Referring back go FIG. 2 b , memory  77  is provided on the printed circuit board of sensor  63 , for storing calibration information. Because of manufacturing tolerances, it is contemplated that a flat, null, position of mirror  29  may not correspond to a balanced light intensity reading among detectors  65 ; as a result, the intensity readings may require calibration, from assembly to assembly, to ensure that a “zero” electrical signal corresponds to a flat mirror orientation, even if the detected light intensities are not balanced. Memory  77  is therefore optionally provided to store calibration data for each of detectors  65 , or for the detectors  65  in the aggregate, so that the resulting electrical signal presented by assembly  21  corresponds to the deviation of mirror  29  from null. In this regard, memory  77  is preferably a non-volatile read/write memory, such as an EEPROM. For example, the manufacturer may pre-calibrate each assembly  21  in factory testing, and store the calibration values in memory  77 . The users of assemblies  21  can then rely on the electrical signals to indicate mirror orientation, without performing additional calibration at the system application. 
     Referring now to FIGS. 5 a  through  5   c , packaged micromirror assembly  21 ′ according to a second preferred embodiment of the invention will now be described in detail. The components of packaged micromirror assembly  21 ′ according to this embodiment of the invention are, for the most part, similar to those described above in packaged micromirror assembly  21 . By way of example, the body of packaged micromirror assembly  21 ′ is formed by transfer molded approach, as described in the above-incorporated provisional application No. 60/234,074, and patent application Ser. No. 09/955,506; of course, other packaging techniques may also be used in connection with the present invention. According to this preferred embodiment of the invention, however, packaged micromirror assembly  21 ′ includes capacitive sensor  80 , for detecting the rotational position of mirror  29  by variations in capacitance. 
     As shown in FIG. 5 a , capacitive sensor  80  has a conical upper shape, and is disposed between driver coils  36  and mirror element  41 . The vertex of capacitive sensor  80  is disposed under the center point of mirror  29 , so that this center point, relative to which the potential rotations of mirror  29  are made, remains in a fixed position relative to this vertex, with a small space between sensor  80  and mirror  29  at this point. 
     FIG. 5 b  illustrates capacitive sensor  80  in plan view. As shown in FIG. 5 b , sensor  80  is segmented into multiple sections that are electrically isolated from one another. In this example, sensor  80  has four segments, each corresponding to a quadrant of the plane defined by the two axes of rotation of mirror  29 . In this example, each of magnets  53  are centered within one of the segments of sensor  80  (rather than between segments). 
     As a result of this construction of sensor  80 , rotations of mirror  29  from a null position will place mirror  29  closer to one or more of the sensor segments than others. In the example shown in FIG. 5 c , mirror  29  is rotated into rotated position  29 ′, in which case mirror  29  is nearer the right-hand segment of sensor  80  (as shown in FIG. 5 c ) than to the left-hand segment. 
     The variation in the distance between mirror  29  and sensor  80  may be converted into an electrical signal by considering mirror  29  as one plate of a capacitor, and each of the segments of sensor  80  as opposing plates of multiple capacitors. The distance between mirror  29  and each segment of sensor  80  will determine the value of capacitance between these two plates. As is fundamental in the art, capacitance is inversely proportional to the dielectric distance. This capacitance value is then measured by conventional techniques, for example by the application of a high frequency input signal between mirror  29 , on one hand, and each of the segments of sensor  80 , on the other hand; the response of the capacitors to the high frequency input signal will indicate the value of capacitance between each segment of sensor  80  and mirror  29 . Alternatively, the capacitors established by the various segments of sensor  80  may be arranged into a conventional capacitance bridge, by way of which the various capacitance legs may be determined in the known manner. The resulting measurements may be communicated from packaged micromirror assembly  21 ′ by way of external leads (e.g., leads  61  of FIG. 2 b ). 
     Additionally, a memory device may optionally be provided within packaged micromirror assembly  21 ′, for storing calibration values in the manner discussed above relative to assembly  21 . 
     Alternatively, sensor  80  may be flat, rather than conical. This construction would provide a lower cost sensor, but would likely result in reduced sensitivity because of the reduction in capacitance between the flat sensor and mirror  29 . 
     Further in the alternative, it is contemplated that relative inductance between sensor  80  and mirror  29  may be detected and used to measure the relative orientation of mirror  29 . 
     Similarly as described above relative to the first embodiment of the invention, the capacitance measurement approach of packaged micromirror assembly  21 ′ provides direct feedback of the position of mirror  29 , without requiring external components such as a beam splitter and a position sensitive detector as shown in FIG. 1 a . This provides an optical transmitter module that can be fabricated at lower cost, with higher reliability. Additionally, the full intensity of the directed laser beam may be used for data transmission, without the losses inherent in the use of a beam splitter. 
     While the present invention has been described according to its preferred embodiments, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives obtaining the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein.