Abstract:
A MEMS-based optical steering apparatus for free space optical transmitters, receivers, and transceivers is disclosed. The MEMS device comprises actuators linked to an optic fiber, the actuators operable to maneuver the optical fiber in the X-Y plane at the focal point of the transmitter, receiver, or transceiver. The MEMS device may thus be used to replace gimbals for beam steering applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional patent application No. 60/609,420, entitled “MEMS-Based Optical Communications Beam Steering Apparatus” and filed on Sep. 13, 2004, and U.S. provisional patent application No. 60/609,413, entitled “Apparatus and Method for Free Space Optical Communications Beam Steering without Gimbals” and also filed on Sep. 13, 2004. The entire disclosures of these applications are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract no. F29601-02-C-0021 awarded by the United States Air Force Research Laboratory. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to the field of optical communications, and in particular to the field of beam steering for free space optical transceivers. More particularly, the present invention relates to a micro-electrical-mechanical systems (MEMS) apparatus for operation of the pointing and tracking system of an optical transceiver that does not utilize external gimbals.  
         [0004]     Laser communications systems are today employed in a vast array of applications, including without limitation communication with aircraft and satellites from ground positions. A laser communication system generally consists of a transmitting terminal and a receiving terminal. A transmitting terminal typically receives an electrical signal from a signal source, and converts the electrical signal into an optical signal. The transmitting terminal then transmits the resulting optical signal using a transmitting telescope. The receiving terminal receives the optical signal through a receiving telescope, which focuses the optical signal into an optical photodetector, and then converts the optical signal back into an electrical signal.  
         [0005]     In order for a receiving terminal to receive an optical signal from a transmitting terminal, the terminal telescopes must be properly aligned. This alignment process is known as beam steering. Generally, beam steering may be defined as changing the direction of the main lobe of a radiation pattern. In optical systems, beam steering is the manipulation of a transmitting telescope or receiving telescope, or both, to point in a desired direction. Other applications for beam steering, in addition to optical communications, include laser illumination, laser designation, laser radar, pointing and tracking, and remote optical sensing.  
         [0006]     Beam steering in optical systems may be accomplished by changing the refractive index of the medium through which the beam is transmitted, or by the use of mirrors or lenses. In particular, some existing non-gimballed beam-steering solutions include acousto-optics, liquid crystals, electro-optics, micro-optics, galvanometer or magnetic mirrors, and micro-mirror arrays. These types of systems, however, have generally proven to be unwieldy, or lack the speed, precision, and reliability necessary for high-speed, long-distance free-space optical communications. Thus the most common means for beam steering in optical communications systems is by the use of a motorized gimballing system. A gimbal is a mechanical apparatus to allow a suspended object to rotate freely along two simultaneous axes, within a defined angle of view. Gimbals are well known in the art, having been used, for example, since at least as early as the sixteenth century in the suspension of maritime compasses. A gimballing system used for the alignment of an optical transmitter or receiver typically moves the entire transmitting or receiving telescope through the required field of view. Often, the transmitter and receiver telescopes are mechanically coupled so that the transmitted beam is in the exact direction of an incoming optical beam for collection by the receiving telescope, the two telescopes thereby operating with a common gimballing system.  
         [0007]     Accurate alignment of the transceiver system is essential for free space laser communications systems. Therefore, gimballing systems must provide accurate alignment angular resolution in order for the receiver telescope to efficiently collect the incoming optical beam. Conversely, the transmitter telescope must be able to accurately point its beam so that a remote-receiving terminal can efficiently collect the optical signal for the photodetector. Mechanical gimballing systems have been favored in many free-space optical communications systems because they can provide very fast alignment times coupled with high angular resolution.  
         [0008]     Gimballed beam-steering systems do, however, suffer from several important disadvantages. Such systems are quite heavy due to the weight of the mechanical components, motors, and servos necessary for such a system. While weight may not be as important a factor in the design of a land-based system, weight is of paramount importance in aircraft and, especially, satellite design. Gimballing systems are also quite bulky due to the required mechanical components, which is also a significant disadvantage in the design of airborne and spaceborne systems. Finally, mechanical gimballing systems require the use of a great deal of electrical power, far more power than is typically consumed by the electronics associated with an optical receiver or transmitter system. Again, while power consumption may not be as important a factor in permanent ground-based systems, it is a critically important factor in airborne and spaceborne systems, as well as in mobile ground-based systems such as may be mounted on land vehicles.  
         [0009]     MEMS technology is today used to develop mechanical and electromechanical systems on a microscopic scale. MEMS devices are constructed using fabrication processes that are similar to those used for the construction of integrated electrical/electronic circuits (ICs). Such processes include ultraviolet lithography, thin film material deposition, and selective etching. Each of these processes are known in the art for the construction of both ICs and MEMS devices. MEMS devices have been used in a variety of applications, such as miniaturized macroscopic elements including mirrors, pressure sensors, accelerometers, and strain gauges. MEMS devices have been incorporated into a number of existing technologies in widely various fields, including microfluidics, ink jet printing heads, and drug delivery patches. MEMS offer many of the same advantages that ICs offer over macroscopic electronic components, including greatly reduced weight and bulk, lower power consumption, and economies of scale that allow them to be mass produced economically.  
         [0010]     What is desired then is a device for beam pointing and tracking in an optical communications system that provides high speed and high angular resolution, with reduced size, weight, and power consumption as compared to traditional gimballing systems now employed in laser communications terminals. In particular, the inventors recognized that it would be desirable to develop a MEMS-based device for this purpose in order to take advantage of the very small size, weight, and power requirements of MEMS-based devices.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed to a MEMS-based device to provide angular adjustment to an optical fiber situated in the focal plane of an optical beam pointing and tracking system. The MEMS device employs surface micro-machined actuators to move an optical fiber through the optical plane of an optical signal receiver or transmitter. The fiber is fed through a bore in the MEMS device, which allows the actuators to manipulate the optical fiber in the X-Y plane of the receiver or transmitter. In the case of a receiver, this movement allows for signal tracking in order that the maximum signal strength may be received. In the case of a transmitter, this allows the signal to be pointed in precisely the desired direction. The MEMS device may be manipulated by various electrical signals that drive the actuators. A yoke may be employed to link the actuators with the optical fiber. By moving the optical fiber within the focal plane of a receiver or transmitter, signals may be received or sent, respectively, along a precisely defined direction.  
         [0012]     By use of this MEMS-based device for beam steering, no gimbals are required in the pointing and tracking system, thereby greatly reducing the size, weight, and power consumption of the pointing and tracking system. In addition, the MEMS-device may improve performance by providing faster response times to a signal to change the direction of a receiver or transmitter. The result is a laser communications system that is more practical for low-cost, high-bandwidth application markets. Such a system is also capable of providing the high performance necessary for mission-critical applications such as military avionics and space-based inter-satellite communications, without decreasing reliability or functionality. In fact, reliability may improve over the complex mechanical systems required when gimbals are employed, since MEMS devices are typically more reliable than such systems.  
         [0013]     It is therefore an object of the present invention to provide for a MEMS-based device for optical beam steering that achieves high speed and angular precision without the use of gimbals.  
         [0014]     It is a further object of the present invention to provide for a MEMS-based device for optical beam steering that consumes relatively little electrical power during operation.  
         [0015]     It is also an object of the present invention to provide for a MEMS-based device for optical beam steering that is of a relatively small size and weight.  
         [0016]     It is also an object of the present invention to provide for a MEMS-based device for optical beam steering that has a relatively low production cost.  
         [0017]     It is also an object of the present invention to provide for a MEMS-based device for optical beam steering that has a decreased response time to a re-directed signal, thereby allowing faster acquisition and tracking of a signal source.  
         [0018]     It is also an object of the present invention to provide for a MEMS-based device for optical beam steering that utilizes a MEMS device fabricated according to traditional IC/MEMS processing steps and equipment.  
         [0019]     These and other features, objects and advantages of the present invention will become better understood from a consideration of the following detailed description of the preferred embodiments and appended claims, in conjunction with the drawings as described following: 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic diagram showing the layout of a MEMS-based device according to a preferred embodiment of the present invention  
         [0021]      FIG. 2  is a detail schematic diagram showing the fiber yoke portion of the layout of a MEMS-based device according to a preferred embodiment of the present invention.  
         [0022]      FIG. 3  is a cross-sectional view showing the fiber yoke portion of the layout of a MEMS-based device according to a preferred embodiment of the present invention.  
         [0023]      FIG. 4  is a cross-sectional view of a MEMS-based device package including the epoxy staking and optical fiber according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     With reference to  FIG. 1 , the preferred embodiment of the present invention may be described. MEMS device  5  is constructed using IC/MEMS fabrication techniques, preferably successive selective deposition and etching using ultraviolet (UV) photolithography on a single crystal silicon wafer. Electrical signals propagated into device  5  enter through one of bond pads  10 . Each bond pad  10  is connected by wirebonding to electrical conductive paths  12 . Bond pads  10  and conductive paths  12  may be constructed of metal, highly doped polysilicon, or other conductive materials. Conductive paths  12  are, in turn, electrically connected to electrostatic comb-drive actuators  15 . As a result of this arrangement, a signal voltage applied at a bond pad  10  is propagated to one or more electrostatic comb-drive actuators  15 .  
         [0025]     In the preferred embodiment, device  5  comprises four electrostatic comb-drive actuators  15 . Comb-drive actuators operate on the principle of electrostatic repulsion between two “combs” having interleaved fingers, with one comb being free to move. Imparting charge to such a device causes the free-moving comb to move away from the fixed comb, the effect achieved being that of a microscale linear actuator. MEMS manufacturing facilities can construct such devices, such as the facilities maintained at Sandia National Laboratories in Sandia, N. Mex. Such devices are used for a wide variety of applications in the optical communications field, such as in connection with switching elements in optical networks. As will be seen in  FIG. 1 , actuators  15  of the preferred embodiment each embody multiple sets of combs in order to achieve the depth of linear movement desired for this application.  
         [0026]     Actuators  15  drive arms  20 , which are pivotally linked to both a corresponding actuator  15  and fiber yoke  25 . Arms  20  move about on top of ground plane substrate  30 , which is preferably constructed of polysilicon. Arms  20  have pin or flex joints  35  at each end to allow yoke  25  to move freely in the X-Y plane above ground plane substrate  30 , including movement at non-orthogonal angles.  
         [0027]     Electrical drive signals reach each of actuators  15  through the corresponding bond pads  10  and the corresponding conducting paths  12 . Four different types of signals are employed in the preferred embodiment: up, down, left, and right. These signals are labeled “U,” “D,” “L,” “R,” respectively, in  FIG. 1 . A ground signal is also required, which is labeled as a down arrow in  FIG. 1 . (Note that while only a single ground signal is illustrated in  FIG. 1  for clarity, the preferred embodiment would include a ground line connected to each of actuators  15 .) Each of the “U,” “D,” “L,” and “R” signals may preferably be coded as a voltage applied at the corresponding bond pad  10 .  
         [0028]     A “U” signal causes the activation of the appropriate actuator  15  such that the arm  20  oriented in the Y-direction moves in the positive Y-direction, that is, in an upward direction, thereby causing yoke  25  to deflect upward. A “D” signal causes the activation of that same actuator  15  as activated by the “U” signal, but in this case the corresponding arm  20  moves in the negative Y-direction, that is, in a downward direction, thereby causing yoke  25  to deflect downward. An “L” signal causes the activation of each of the appropriate actuators  15  such that the arms  20  that are oriented in the X-direction move in the negative X-direction, that is, to the left, thereby causing yoke  25  to deflect to the left. It may be noted that this movement requires the leftward arm  20  to retract while the rightward arm  20  extends. Conversely, a “R” signal causes the activation of each of these actuators  15  such that the arms  20  that are oriented in the X-direction move in the positive X-direction, that is, to the right, thereby causing yoke  25  to deflect to the right. It may be noted that this movement requires the leftward arm  20  to extend while the rightward arm  20  retracts.  
         [0029]     It may be seen from  FIG. 1  and the above description that yoke  25  may be moved about on substrate  30  to any X-Y position within its range of motion by a combination of U, D, L, and R signals. For example, a simultaneous “U” and “R” signal will cause yoke  25  to deflect to the upper-right portion of substrate  30 . In this way, yoke  25  may be moved to any desired position by the proper combination of signals, just as may be performed with gimballed steering and pointing systems.  
         [0030]     In the preferred embodiment, both the Y-axis and X-axis actuation is provided by a pair of actuators  15  oriented to move linearly in the Y and X directions, respectively. In alternative embodiments, a different number of actuators  15  may be employed in either direction. For example, in one alternative embodiment the Y-axis actuation is provided by a single actuator  15  oriented to move linearly in the Y direction. A single actuator  15  was chosen for the Y direction in this alternative embodiment due to space requirements in the initial fabrication process. The X-direction movement in this alternative embodiment is provided by two actuators  15 , despite the fact that only one actuator  15  is employed for movement in the Y-direction. In still another alternative embodiment, only one actuator  15  may be employed in each of the Y and X directions.  
         [0031]     In the preferred embodiment, each actuator  15  providing drive in the same linear direction is controlled together such that only a single set of “U,” “D,” “L,” and “R” drive signal inputs pads  10  is required. For example, only a single “L” signal is required in this arrangement to operate both actuators  15  that provide movement in the negative X-direction. Alternatively, separate pads  10  and conducting paths  12  could be provided for the drive signals directed to each actuator. In still another embodiment, both combined drive signals and a separate drive signal line to each actuator  15  could be implemented in the same device, providing application flexibility to the designer seeking to integrate device  5  into a desired mechanism.  
         [0032]     The MEMS device  5  of  FIG. 1  is preferably fabricated as five layers of polycrystalline silicon (polysilicon) deposited to form the structural layers of the preferred embodiment, with silicon dioxide (oxide) used as the sacrificial material that is fully removed by etching as a final process step, thereby creating the gaps and spacing needed for moving elements to operate. One of these layers is preferably reserved for use as a ground plane to dissipate charge accumulation under moving structures under high potential. Each layer of polysilicon and oxide is preferably deposited as a continuous thin film of material on the wafer, and then a UV-sensitive polymer photoresist is used to create a stencil through which the selected material was removed by etching. Each layer is patterned by one or more optical masks that may be preferably created from CAD artwork and are superimposed upon one other to generate the final working device  5 .  
         [0033]     It may be noted that while MEMS features are generally only a few microns along a minimum dimension, they may have very large aspect ratios, with, for example, lengths that exceed their height or width by a factor of 500 or more. Although traditional IC fabrication processes such as the UV photolithography of the preferred embodiment are used in the fabrication of MEMS devices, the processes used in MEMS are generally larger in footprint, thickness, and pitch. This lower resolution requirement means that older equipment may be utilized in MEMS manufacturing. This equipment is generally operated much harder per cycle, however, than is required for IC fabrication in order to achieve the thicker, larger films and features. As a result, a preferred fabrication facility may be one that is outmoded for modern IC fabrication, and thus the equipment value may be less and the loss from equipment degradation correspondingly less related to the equipment&#39;s value. Thus the cost of producing device  5  may be further reduced relative to alternative technologies for beam steering.  
         [0034]     Referring now to  FIG. 2 , fiber yoke  25  and a portion of arms  20  according to a preferred embodiment may be described in greater detail. Yoke frame  60  is preferably of a roughly square shape and, like the other MEMS elements of device  5 , is fabricated from subsequent deposition of polysilicon films. Yoke hole  65  is sized to receive an optical fiber (not shown in  FIG. 2  for clarity). The standard optical fiber outside diameter of 125 microns is employed in the preferred embodiment, such that the size of yoke hole  65  in the preferred embodiment is preferably about 130 microns to snugly receive the 125 micron fiber. Arms  20  attach to yoke frame  60  at pivot joints  35 . These joints allow arms  20  to pivot in the X-Y plane with respect to yoke frame  60 , thereby allowing yoke frame  60  to move freely within the X-Y plane within a defined area passing over substrate  30 .  
         [0035]     Referring now to  FIG. 3 , fiber yoke  25  and its related components may be seen in profile, showing the manner in which the polysilicon film layers are built up during the fabrication of device  5 . Yoke hole  65  is shown in the center portion of  FIG. 3 , with the layered elements on either side being yoke frame  60 . The gaps in the polysilicon layers of yoke frame  60  are filled with silicon dioxide in the preferred embodiment. Each layer of yoke frame  60  is preferably about two microns thick. A two-micron clearance  80  is preferably formed between the lower surface of yoke frame  60  and the upper surface of substrate  30 . This clearance allows fiber yoke  25  to glide over substrate  30  as it translates the optical fiber in the focal plane of the transmitter or receiver. As explained above, yoke  25  is drive by actuators  15 , which respond to signals that are sequenced and applied to the various actuators  15  to create the desired motion through the associated arms  20 .  
         [0036]     Turning now to  FIG. 4 , the system package assembly for device  5  is shown in profile. Device  5  has a substrate passage  100  formed at its center in order to allow the passage and deflection of optical fiber  110 . In the preferred embodiment, substrate passage  100  has a diameter of approximately 250 microns. Substrate passage  100  is preferably formed by a standard chemical etching technique to a silicon oxide layer that serves as an etch stop. Device  5  is preferably held in place by epoxy or other permanent means on the package or printed circuit board (PCB)  95 . Substrate passage  100  in device  5  must be properly aligned with PCB passage  105  during attachment. Fiber  110  is fed through PCB passage  105 , substrate passage  100 , and into yoke  25  (not shown for clarity in  FIG. 4 ). Fiber  110  may preferably be staked into place by means of epoxy or other permanent adhesive  115  at the bottom surface of PCB  95 . The components are sealed and protected by the application of a lid  120 , which may be formed of glass or another sufficiently strong and transparent material. Lid  120  is sealed into place with sealing ring material  125 , which may in the preferred embodiment be an epoxy. The resulting assembly may then be mounted into the optical transmitter or receiver, with the fiber pigtail connected to a laser communications signal processor. It may be noted that while the preferred embodiment has been described for use with respect to a dedicated transmitter or receiver, the preferred embodiment may also be employed in a transceiver arrangement, where the same optical fiber is used to both send and receive optical signals.  
         [0037]     The present invention has been described with reference to certain preferred and alternative embodiments that are intended to be exemplary only and not limiting to the full scope of the present invention as set forth in the appended claims.