Abstract:
Building-to-building over the air transmission of optical data is a growing area of data communications. The fast growing use of bandwidth mandates the use of over the air transmission equipment capable of similar performance as the performance of fiber optic transmission, for distances of 3-10 Km. Transparent transmission is important to enable seamless growth from low data-rare to Gbps rates, and then to Dense Wavelength Division Multiplexed (DWDM) transmission of several wavelengths. The only way to achieve the required performance is with narrow, directable beams. This patent application discloses a Micro-Electro-Mechanical-Systems (MEMS) mirror based, over the air, optical data transmission system. A narrow optical beam is used and a MEMS mirror fine-tunes the aiming of the beam to track building movement, vibrations etc.

Description:
This non-provisional application takes priority from U.S. Provisional Application Ser. No. 60/210,613 filed on Jun. 9, 2000. 

   BACKGROUND OF THE INVENTION 
   A description of some technologies related to embodiments of the invention follows: 
   U.S. Pat. No. 4,662,004 Fredriksen, et al. Fredriksen describes an optical communication link that includes a separate laser (in addition to the data transmission laser), which returns information about the level of the received signal to the transmitter. This separate laser is adjusted to emit power proportional to the received beam power. 
   U.S. Pat. No. 4,832,402 Brooks. Brooks describes a fast scanning mirror used to time-multiplex light beam into several steering mirrors, in which each of the steering mirrors aim the beam into one or a group of targets clustered together. The steering mirrors are slow due to the large angle required. Brooks also describes the use of “beacon transmitters” to said in target tracking (column 9 line 15). 
   U.S. Pat. No. 5,282,073 Defour, et al. Detour shows optical communications system with two galvanometer mirrors for beam steering, and a complex wide-angle lens to increase the angular scanning to a half-sphere. Defour also describes a target designation step, an iterative step of bilateral acquisition and a third step of exchanging data. 
   U.S. Pat. No. 5,390,040 Mayeux. Mayeux describes the use of one steerable mirror at the expanded beam location for aiming both the transmit beam and receive beam. Part of the surface of the mirror is used for transmission, and another part is used for reception. (Mayeux calls these parts of the mirror “field of views”, in contrast to common terminology.) 
   U.S. Pat. No. 5,448,391 Iriama, et at. Iriama describes the use of an optical Position Detector sensor (common art) to track the beam direction. A pair of mirrors is used for slow, large angle direction control and a fast lens is moved for fast corrections. 
   U.S. Pat. No. 5,646,761 Medved, et at. Medved describes an optical communications between a stationary location, like an airport gate, and a movable object, like an airplane parked at the gate. The optical units on the gate and the airplane are searching for each other, and stop this search when aligned. 
   U.S. Pat. No. 5,710,652 Bloom, et at. Bloom describes optical transmission equipment to interconnect low Earth orbit satellites. The whole transmitter and receiver unit is mounted on gimbals. Two lasers are used, one for tracking and one for data. A CCD optical detector detects a target location for tracking a servo control. 
   U.S. Pat. No. 5,768,923 Doucet, et al. Doucet discloses the distribution of television signals from one source to many receivers. The transmitter uses an X-Y beam deflector made of two galvanometer driven mirrors. This assembly is used to direct the beam into a specific receiver at a selected home. 
   U.S. Pat. No. 5,818,619 Medved, et al. Medved describes a communications network with air-links. A converter unit is converting the physical data transmission in the network to electricity, and drives an air-link transmitter. Similarly, the received beam is converted to electricity after reception. Medved also describes an optical switch to have one air-link serving plurality of networks between the same two locations. 
   EP 962796A2 Application Laor, et al. This application describes MEMS mirror construction. 
   SUMMARY OF THE INVENTION 
   An optical interconnect with light beams between buildings suffers from a difficulty associated with the movement of the buildings. The movements include waving in the wind, environmental vibrations, land shift, earthquakes, etc. Common over-the-air optical transmission equipment either uses narrow beam laser transmitters with tracking mechanisms or LED based wide beam transmitters with fixed aiming. 
   MEMS is a technology that is used to manufacture small mechanical systems using common Silicon foundry processes. We describe here the use of narrow field of view transmission with a MEMS mirror being used to fine tune the beam direction. Since the MEMS mirror is rather small, 1-3 millimeters in diameter, it is difficult, if not impossible to use it to aim the expanded beam. In an embodiment of the invention, the MEMS mirror is installed near the light source, where the beam is small in diameter. This positioning enables only small angular deflection of the beam. The transmission equipment will be coarsely aimed either manually or with motors, and the MEMS mirror will do fine aiming with fast response. With coarse motorized aiming, the motors may be operated to search and find the other side of the communication link. After the MEMS mirror has begun aiming the beam, the motors could be adjusted slowly to hold the aim such that the MEMS mirror average angular deviation is around zero. This will maximize the correction capability of the MEMS mirror. 
   We will use the term “light” to mean all electromagnetic waves from the ultraviolate to infrared, and not only for the visible spectrum. This is a common use of the term. The common transmission wavelength is with light in the near infrared, and not only for the visible spectrum. This is a common use of the term. The common transmission wavelength is with light in the near infrared between 600 and 1600 nano-meters. 
   Another feature of the present invention is the use of optical fiber to carry light from a light source in data equipment to the optical beam transmitter positioned on the roof or in a window. Another optical fiber carries the light from an optical beam receiver on the roof or in a window to a detector in the data equipment. This facilitates the changing of data equipment, changing data rates, changing protocols, etc., without the need to replace the optical beam transmitter or beam receiver. The system may be upgraded to carry light in more then one wavelength using the same optical beam transmitter and receiver. For long transmission lengths, an optical fiber amplifier could be installed between the light source and the optical beam transmitter, or between the optical beam receiver and the detector, or both locations. For systems located in areas with common fog problems, such amplifiers could be set to activate when the transmission is fading. 
   Yet another feature is the use of two fast optical fiber 1×N switches to time-share the use of a network between several users. One network port will connect to the switches, with two fibers—transmit and receive. On the other side of the switches each pair of fibers will be connected to a pair of an optical transmitter and an optical receiver, aimed at one network user. This allows serving high data rate network interconnect to customers in a time-shared fashion, and adjusting the percentage of time used according to the needs of each customer. When the need arises, a dedicated network port could be used to direct-connect a customer for a full connection. This structure of the system having fully transparent optical transmitters and receivers allows for seamless transfer using dedicated fibers between the two locations when such fibers are installed. 
   A construction is described where the beam transmitter and the beam receiver share the use of one MEMS mirror. Servo control of the MEMS mirror angular position may be achieved with a separate servo LED source and a servo optical position detector. Close loop servo control is critical to the correct operation of the transmission system. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a beam transceiver in accordance with the present invention. 
       FIG. 2  is a schematic view showing the movement of an image at an optical fiber end shown in FIG.  1 . 
       FIG. 3  is a perspective view of a MEMS mirror positioned in the mirror package shown in FIG.  1 . 
       FIG. 4  is a schematic view of a beam transmitter in accordance with the present invention. 
       FIG. 5  is a perspective view of a beam transceiver with a coarse aiming mechanism in accordance with the present invention. 
       FIG. 6  is a perspective view of a beam transceiver with a coarse aiming mechanism in accordance with the present invention. 
       FIG. 7  is a schematic view of an optical link using beam transmitters in accordance with the present invention. 
       FIG. 8  is a schematic view of an optical link showing fiber amplifiers inserted into beam transmitters in accordance with the present invention. 
       FIG. 9  is a schematic view of a main network serving multiple sub-networks in accordance with the present invention. 
       FIG. 10  is a schematic view of a beam transceiver when a MEMS mirror controls both a transmitted beam and a received beam in accordance with the present invention. 
       FIG. 11  is a schematic view of a beam transceiver when a MEMS mirror controls both a transmitted beam and a received beam in accordance with the present invention. 
       FIG. 12  is a schematic view of a servo LED being used as a light source in accordance with the present invention. 
       FIG. 13  is a schematic view of a position sensor in accordance with the present invention. 
       FIG. 14  is a perspective view of a beam transceiver in accordance with the present invention. 
       FIG. 15  is an elevational view of the beam transceiver shown in FIG.  14 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention comprises a method and apparatus for a MEMS based over-the-air optical data transmission system. In the following description, numerous specific details are set forth to provide a more thorough description of embodiments of the invention, It will be apparent, however, to one skilled in the art, that the invention may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to obscure the invention. 
     FIG. 1  shows the construction of a beam transceiver  20  in accordance with one embodiment of the invention. The beam transceiver  20  may operate as a beam transmitter or as a beam receiver, or as both. In the beam transceiver  20  shown in  FIG. 1 , a light beam  22  that propagates in the optical fiber  24  exists in the fiber end  28  in a cone  28 . The optical fiber  24  is a common single-mode telecommunications fiber, with a core diameter of approximately 10 microns and a cladding diameter of 125 microns. The cone  28  of light hits a MEMS mirror  30  and is deflected toward a lens  32 , which collimates the beam  22  for transmission. The collimation may not be exact, as larger or smaller beam angles may be required. As shown in  FIG. 1 , the mirror  30  is enclosed in a mirror package  34 . The mirror  30  may be rotated in two degrees of freedom over two perpendicular axes (not shown) which are parallel to a mirror surface  36 . An image  38   a  or  38   b  ( FIG. 2 ) of the optical fiber end  26  is thus moved in space. By moving the image  38 a or  38 b of the optical fiber  24 , the beam  22  that emerges from the lens  32  changes direction. 
     FIG. 2  is a schematic drawing showing the movement of the image  38   a  or  38   b  to the optical fiber end  26  in accordance with the present invention. A light cone  28  emerges from the fiber core at the fiber end  26 . The cone  28  is reflected by the MEMS mirror  30 . The mirror  30  is rotatable around the axis  37  shown, and the second axis is not shown for clarity. When the mirror  30  is in position A, the mirror  30  creates an image  38   a  and the light beam  22  exits in a cone  40   a . When the mirror  30  is in position B, the mirror  30  creates an image  38   b  and the light beam  22  exits in cone  40   b . Since image  38   a  and image  38   b  are in different positions, the lens  32  will collimate light beam  22  exiting from these images  38   a  or  38   b  in different directions. Two exiting cones  40   a  and  40   b  have some beam wander on the lens  32 , requiring somewhat larger lens diameter. 
   In  FIG. 3 , the MEMS mirror  30  is drawn showing only the mirror  30  and the mirror package  34 . The mirror package  34  is a mechanical structure that holds and protects the MEMS mirror  30 . The mirror package  34  may have a window that enables hermetic sealing, not shown here for clarity. The MEMS mirror  30  can be controlled to rotate in the horizontal and vertical axis. A detailed description of a type of MEMS mirror useful for this application may be found in “Optical Switch Demos in Cross-Connect” by David Krozier and Alan Richards, Electronic Engineering Times, May 13, 1999, p. 80 and in EP 962796A2. The MEMS mirror dimensions are reported to be approximately 3 mm×4 mm. The size is larger than a typical MEMS mirror and is quite useful for the construction of the beam transceiver  20 . A smaller MEMS mirror  30  will require the optical fiber  24  ( FIG. 1 ) to be very near to the mirror  30 , which may be obstructing part of the beam  22  (FIG.  1 ). Also, a small mirror  30  will create only a small deviation of the position of the image  38   a  or  38   b  on the optical fiber  24 , and will achieve a small active angle of aiming. However, the size of the MEMS mirror  30  may vary in accordance with different embodiments of the present invention. 
     FIG. 4  shows a different optical design of a beam transceiver  20  in accordance with the present invention. The light beam  22  emerging from the optical fiber  24  in a cone  28  is collimated by an “on-axis” lens  42 . The collimated beam  44  is reflected by the MEMS mirror  30  into an “eyepiece” lens  46 . The eyepiece lens  46  focuses the collimated beam  44  into a real image focal spot  48  at or near the focal plane of the lens  32 . The lens  32  creates a collimated or nearly collimated light beam  22  for transmission. By rotating the MEMS mirror  30 , the location of the real image focal spot  48  can be adjusted, thereby adjusting the direction of the transmitted light beam  22 . 
   It is common knowledge that for any path taken by a beam of light, the reverse path is also a possible path for another beam. Therefore,  FIGS. 1-4  which were used above to describe the beam transceiver  20  operating as a beam transmitter will also be used to explain operation of the beam transceiver  20  as a beam receiver. Referring to  FIG. 1 , a light beam  22  arrives at a lens  32  and being focused and directed to a fiber end  26  of an optical fiber  24  by a MEMS mirror  30 . The direction from where the optical fiber  24  will accept a light beam  22  is controlled by the MEMS mirror  30 . The optical fiber  24  in the beam transceiver  20  operating as a beam receiver may be identical to the optical fiber  24  in the beam transceiver  20  operating as a beam transmitter, but it may also be a common multi-mode fiber with a core diameter of 50 or 62.5 microns and a clad diameter of 125 microns. A larger core diameter will allow relaxed aiming accuracy, but will limit the data rate if the fiber is long, due to modal dispersion. 
   A pair of beam transceivers  20 , one operating as a beam transmitter and one operating as a beam receiver, together create a one-way optical link. The distance between the beam transceivers  20  could be several kilometers. For two-way communications, light beams  22  can be made to propagate in the optical fibers  24  in both directions simultaneously. Alternatively, two beam transceivers  20 , each operating as both a beam transmitter and a beam receiver, can be used to create a full duplex optical link. 
   The beam steering by the MEMS mirror  30  is limited in angular deviation. Only a few degrees of angular deviation are typically possible. In some designs, only a fraction of a degree of adjustment is possible. Therefore, a mechanism for coarse aiming is required, which is capable of aiming in 360 degrees in azimuth and approximately +/−45 degrees in elevation.  FIG. 5  shows the beam transceiver  20  mounted in a coarse aiming mechanism  50 . The beam transceiver  20  is mounted onto a mount  52 , with a motor that controls the horizontal axis of rotation of the beam transceiver  20 . The motor enables the movement of the beam  22  ( FIG. 4 ) in elevation. The exact design of the motor and a drive mechanism  50  are not shown. The mount  52  is attached to a base  54  with a similar drive mechanism, which enables rotation around the vertical axis, for adjusting the beam  22  in an azimuth direction. The motors are capable of aiming the beam  22  generally to a target, but are neither fast nor accurate enough to track building movements. 
     FIG. 6  shows a different structure for adjusting the light beam  22  in an azimuth direction. The beam transceiver  20  is mounted on the base  54  facing up. A large folding mirror  56  directs the light beam  22  in a general horizontal direction. The beam transceiver  20  and the folding mirror  56  rotate around the vertical axis for azimuth control. It is possible that only the folding mirror  56  will rotate to achieve azimuth control. The folding mirror  56  aims the light beam  22  in elevation by rotating around a horizontal axis. Again, the motor drive is not shown. 
     FIG. 7  shows a network system  110  using the beam transceivers  120   a  and  120   b , which are described above as transceiver  20 . A main network  112  needs to interconnect with a sub-network  114 . The main network  112  and the sub-network  114  are located in different buildings with free line-of-sight between them. It is also possible to interconnect the main network  112  to the sub-network  114  between different floors of the same building by sending light beams  122   a  and  122   b  vertically. A network element  116   a , such as a switch, router and the like, is attached to the main network  112 . A port  118   a  in the network element  116   a  is connected to the beam transceiver  120   a  with a pair of optical fibers  124   a  and  124   b . A laser or LED transmitter and a PIN or avalanche photodiode detector at the network element  116   a  or  116   b  performs the light generation and detection respectively, commonly marked TX and RX. The beam transceiver  120   a  is mounted on the roof or in a window, and aimed at the beam transceiver  120   b , which is connected to the sub-network  114  with optical fibers  124   c  and  124   d . When the beam transceivers  120   a  and  120   b  are correctly aimed at each other, light from the respective TX units  126   a  and  126   b  at each respective network element  116   a  and  116   b  is passed via the respective optical fibers  124   a  and  124   c  to the respective beam transmitters  120   a  and  120   b , over the air to the respective beam transceivers  120   b  and  120   a  and to the respective RX units  128   b  and  128   a  at the other respective network elements  116   b  and  116   a . Accordingly, a full duplex communication is established. 
   Since the network elements  116   a  and  116   b  see standard fiber attachments, it is very simple to correct direct point-to-point optical fibers  124  between the network elements  116   a  and  116   b  when available, replacing the over-the-air link. This feature allows for seamless growth of the network system  110 . 
   Optical transmissions from the respective TX units  126   a  and  126   b  to the respective RX units  128   b  and  128   a  will suffer losses, due to loss in the optical fibers  124   a-d , optical abberrations and diffraction in the beam transceivers  120   a  and  120   b , a receiver aperture being smaller in diameter than the beam  122   a  or  122   b  generated by the respective beam transceivers  120   a  and  120   b , inaccuracies in the aiming mechanism for both transmitter and receiver, and optical absorption and scattering in the atmosphere, etc. In common 2.5 Gbps transmission equipment, such loss is allowed to reach 20-30 dB, i.e. only 1/100 to 1/1000 of the light transmitted by the laser should arrive at the detector to achieve low error rate transmission. If the link loss is excessive, optical fiber amplifiers  130  may be inserted in the link  132  as shown in FIG.  8 . The optical fiber amplifiers  130  that are commonly used are Erbium Doped Fiber Amplifiers (EDFA). Optical fiber amplifiers  130  may be inserted into the link  132  after the lasers in the TX units  126   a  and  126   b  boost the transmitter power, dr before the receivers in the RX units  128   a  and  128   b  increase the received optical power, or in both locations. If a high loss is a phenomenon related only to fog conditions, the amplifiers  130  may be inserted actively when the bit error rate deteriorates. 
     FIG. 9  shows a network system  110  where several sub-networks  114  are served by one main network  112 . A 1×N fiber optic switch  134   a  is attached to the TX unit  126   a  of the port  118   a  in the main network  112 . The switch  134   a  is serving light to one of the beam transceivers  120   a  at a time. A second switch  134   b  is connected to the RX unit  128   a  of the port  118   a . Each sub-network  114  operates for a short time, and then is disconnected for a longer time. For example, the switching time may be 5 mS and each sub-network  114  could be served for 100 mS at a time. If there are 5 sub-networks  114 , there will be a gap of 425 mS between connections for any specific sub-network  114 . Some messages may be delayed, but this may be tolerated. If the link loss is different to different sub-networks  114 , the gain of the corresponding optical amplifier  130  may be adjusted to each sub-network  114  differently. Fast AGC is required on all the RX units  128   a  and  128   b . This construction enables the installation of standard transmission equipment, for example Gigabit Ethernet, in all the network elements  116   a  and  116   b , even when the communications need is lower, and adjusting the main network  112  connect time to each sub-network  114  according to the needs. An advantage is the use of only two optical fiber amplifiers  130 , which are expensive. Another advantage is that the connectivity to each sub-network  114  may be adjusted without the need for a physical equipment change, and remotely. The user of the sub-network  114  may be charged for network services according to the average data rate he uses. Only when a particular sub-network  114  needs full connectivity at the main network data rate, the particular sub-network may be assigned a particular port  118   a  in the main network and directly connected to the particular port  118   a  instead of via the fiber switches  134   a  and  134   b.    
     FIG. 10  shows another embodiment of the beam transceiver  20  using the MEMS mirror  30  to control both a transmitted beam  22   a  and a received beam  22   b . The transmit optical fiber  24   a  shown has a Numerical Aperture (NA) of 0.1, which is common for Single Mode fibers, and creates an opening of the beam at about 5.7 degrees from the axis. The transmitted beam  22   a  reflects from the MEMS mirror  30  and is aimed at a transmit lens  32   a  via a fixed mirror  58 . The receive optical fiber  24   b  shown has an NA of 0.26, which is common for multi-mode fibers with a core diameter of 62.5 microns. The receive lens  32   b  focuses the received beam  22   b  on the MEMS mirror  30 . The received beam  22   b  will have a radius of about 15 degrees. Since it is intended to use the same area of the MEMS mirror  30  for both transmission and reception, the transmit and receive cones  28   a  and  28   b  can not have parallel axes at the MEMS mirror  30 . The fixed mirror  58  is used, therefore, to make the transmit and receive beams  22   a  and  22   b  parallel outside of beam transceiver  20 . 
     FIG. 11  shows the design of a MEMS mirror  30  serving both transmission and reception, where the collimated beams  44   a  and  44   b  at the MEMS mirror  30  are substantially collimated. The description of each optical path, for transmission and reception, is essentially the same as described above for FIG.  4  and FIG.  10 . However, the position of the fixed mirror  58  and the transmit lens  32   a  are swapped. Eyepiece lens  46   a  and on-axis lens  42   a  control the transmit beam  22   a , and eyepiece lens  46   b  and on-axis lens  42   b  control the receive beam  22   b.    
   The operation of the atmospheric optical link depends critically on the correct aim of the transmit and receive beams  22   a  and  22   b . A servo control system  59  (see  FIG. 15 ) must be employed to aim the beams  22   a  and  22   b . The servo control system  59  should have a different mechanism to align the beams  22   a  and  22   b  and many different ways are known an described in the prior art. We need, however, a mechanism that makes use of the positioning of the same MEMS mirror  30  as the transmit and receive beams  22   a  and  22   b . The essential parts of such a servo system  60  are shown in  FIGS. 12 and 13 . In  FIG. 12 , a servo LED  62  is used as the light source. A laser could also be used as the light source. The servo LED  62  emits light in a servo light beam  64   a  modulated at relatively low speed, enabling detection with low received power. A servo LED lens  66  creates a wide cone of light  68  from the servo light beam  64   a  emitted by the servo LED  62 . This cone  68  may be several degrees wide, so the aiming is very simple and the amount of detected radiation is not sensitive to small movements of this beam.  FIG. 13  shops a servo sensor of the servo system  60 , which uses the same MEMS mirror  30  as described before. The servo light beam  64   b  is focused on the MEMS mirror  30  with a servo sensor lens  70 . The servo sensor of the servo system  60  uses an optical position detector  72 , which is a common art and includes a Silicone diode with several outputs. The electrical signals outputted from the detector  72  are sensitive to the intensity of an optical signal in a received servo light beam  64   b  and to the exact location of the optical signal on the detector  72 . The electrical signals indicate if the MEMS mirror is aiming the servo light team  64   a  directly at an opposing servo LED  62 . If there is an error in aiming, the electrical signal outputted from the detector  72  indicates the direction and magnitude of the error. The servo system  60  will then adjust the MEMS mirror  30  correctly. 
     FIG. 14  shows the outside view of an optical system  74  incorporating the beam transceiver  20 . In  FIG. 15 , a flattened drawing of the optical system  74  of  FIG. 14  is shown. The optical beams are shown by the central beam only, for clarity. As shown in  FIG. 15 , one MEMS mirror  30  is used to control three beams  22   a ,  22   b  and  64   b  concurrently. Accordingly, fixed mirror  58   a  reflects the transmit light beam  22   a  onto the MEMS mirror  30 , and fixed mirror  58   b  reflects the servo light beam  64   b  onto the MEMS mirror  30 . 
   Thus, a method and apparatus for MEMS based over-the-air optical data transmission system has been described. However, the claims and the full scope of their equivalents describe the invention.