Abstract:
A novel free space optics transceiver is disclosed comprising at least one light source, at least one photodetector, corresponding positioning means that can independently adjust the position of the light source and the photodetector, and a controller that can send and receive alignment frames conforming to a communications protocol to and from a remote optical transceiver and can use information in the incoming alignment frames from the remote optical transceiver to appropriately adjust the position of the light source and the photodetector thereby maximizing the received power of light signals sent by the remote optical transceiver. Also, an alignment method is disclosed wherein the controller continuously adjusts the positions of the light source and the photodetector until a light signal is acquired from a remote optical transceiver. Then, the incoming alignment information is used to adjust the light source position to achieve bi-directional communication with the remote optical transceiver.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/714,085, filed on Sep. 2, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of free space optical communications systems and to techniques for aligning two free-space optical communication systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     In free space optics a first optical transceiver sends and receives optical signals from a second optical transceiver through free space. Alignment of the two optical transceivers is crucial for error free transmission of the signals exchanged between the two optical transceivers. In Prior Art, the optical transceivers are mounted on rotational stages than can rotate about horizontal and vertical axes. The initial alignment between the optical transceivers is done by operators adjusting the vertical and horizontal angles of the optical transceivers until a signal is detected by both optical transceivers, and then the signal is maximized by fine adjustments. After alignment, the optical transceivers are left at either a fixed position, or there is an active tracking mechanism to adjust their vertical and horizontal angles to maintain signal strength.  
         [0004]     However, this alignment method has several issues. First, it requires initially well trained operators to install the optical transceivers, and secondly, after initial alignment, if the optical transceivers become miss-aligned, manual intervention to re-align them may be needed. Further, even if the optical transceivers are aligned, optical aberrations due to weather conditions or aging of the optical or mechanical components of the optical transceivers can lead to improper focusing of the incoming beams, thus leading to errors in the transmission.  
       SUMMARY OF THE INVENTION  
       [0005]     In this invention a novel free space optics apparatus and alignment method are disclosed to eliminate the issues related to prior art alignment methods and apparatuses. Specifically, a light source and a photodetector are mounted on corresponding support bases that can independently position the light source and photodetector, thereby decoupling light emission from light detection and allowing maximum flexibility in aligning two optical transceivers to each other. Further an alignment method implemented by the free space apparatus of this invention is disclosed wherein a controller continuously transmits alignment information into free space and simultaneously adjusts the positions of the light source and the photodetector until a light signal is acquired from a remote optical transceiver. Then, the incoming alignment information from the remote optical transceiver is used to adjust the light source position to achieve bi-directional communication with the remote optical transceiver. 
     
    
     LIST OF FIGURES  
       [0006]      FIG. 1   a  shows one embodiment of the optical transceiver of this invention.  
         [0007]      FIG. 1   b  is a schematic diagram depicting proper alignment of two optical transceivers.  
         [0008]      FIG. 1   c  is a schematic diagram depicting improper alignment of two optical transceivers.  
         [0009]      FIG. 2   a  shows one embodiment of the receive optical elements of the optical transceiver of this invention.  
         [0010]      FIG. 2   b  shows one embodiment of the transmit optical elements of the optical transceiver of this invention.  
         [0011]      FIG. 3  shows one embodiment of the electronic elements of the optical transceiver of this invention.  
         [0012]      FIG. 4  shows a block diagram of one embodiment of the motor control algorithm implemented by the optical transceiver of this invention.  
         [0013]      FIG. 5  shows a block diagram of one embodiment of a Verilog program implemented by the optical transceiver of this invention to prioritize transmission of alignment and data frames to a remote optical transceiver.  
         [0014]      FIG. 6  shows a block diagram of one embodiment of a Verilog program implemented by the optical transceiver of this invention to process incoming alignment frames from a remote optical transceiver.  
         [0015]      FIG. 7  shows the fields of an alignment frame of this invention.  
         [0016]      FIG. 8  shows an alternative motorized mounting support for the transmit optics of this invention.  
         [0017]      FIG. 9  shows an alternative configuration for the transmit optics of this invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1   a  shows one embodiment of the optical transceiver of this invention. It includes a servo motor  1 , such as a Futaba FUTM0255, with its shaft  2  attached to a plate  3 . A second servomotor  4 , such as a Futaba FUTM0255, is attached to the plate  3  and its shaft  5  is attached to a tube  6 . Tube  6  is attached to a second tube  7 . Servomotor  1  rotates tubes  6  and  7  about the vertical axis defined by shaft  2 , and servomotor  4  rotates tubes  6  and  7  about the horizontal axis defined by shaft  5 . In general the servomotors and tubes form a support base with at least two degrees of freedom to support the optical and electronic components of this invention.  
         [0019]      FIG. 1   b  shows proper alignment between two optical transceivers  61  and  62 , to achieve error free transmission. Both optical transceivers have the same construction, as described in this invention.  
         [0020]      FIG. 2   a  shows one embodiment of the receive optics of the optical transceiver of this invention. It includes a lens  22 , such as a plano-convex lens from Thorlabs, attached to the front end of tube  6 . A system of lenses  23  (plano-convex) and  24  (ball lens), along with lens  22  focus an incoming laser beam  21  onto a photodetector  25 , such as a Fermionics FD100S2. Several lens vendors offer lenses that can be used in this invention, such as those from Thorlabs or Edmund Optics. The photodetector  25  is attached to a motorized X-Y-Z stage  26 , such as a set of three MX80L Parker Daedal stages in an X-Y-Z configuration with an associated driver such as a ViX driver. In general, numerous stages, motors and controllers are available from multiple vendors to accomplish this task, such as stages from Newport, motors from Sanyo-Denki, motor drivers from Allegromicro. A removable band-pass optical filter  29 , such as an Edmund Optics DWDM dielectric band-pass filter, is also used in the optical path to allow only predetermined optical wavelengths to pass through.  
         [0021]      FIG. 2   b  shows one embodiment of the transmit optics of the optical transceiver of this invention. It includes an optical fiber  35  entering one end of tube  7 . A system of lenses  34 ,  33  and  32  expands the laser beam carried by the fiber and emits it into free space  31 .  
         [0022]      FIG. 3  shows one embodiment of the electronic components of this invention The output  28  of the photodetector  25  of  FIG. 2   a  is input into a transimpedance amplifier (TIA)  202 , such as a Maxim MAX3793. The TIA  202  has a differential output  206  that is input into a limiting amplifier  207 , such as a Maxim MAX3265, and a photodiode average current output  203  that is converted to voltage by means of a resistor  205  and is input into an analog to digital converter (ADC)  204 , such a National Semiconductor ADC0801. The output of the ADC  204  is input into a field programmable gate array (FPGA)  209 , such as a Xilinx Virtex FPGA.  
         [0023]     The FPGA  209  receives the output of the ADC  204 , and generates control signals  27  that drive the servo motors  1  and  4 , and the motors of the X-Y-Z stage  26 , to maximize the photodiode current  203 . The FPGA can either implement a Verilog code, or have an embedded CPU that executes instructions to generate the appropriate motor control signals. There are several Prior Art techniques for controlling a set of servomotors to maximize a feedback variable. For example, the FPGA controls each servomotor sequentially. Referring to  FIG. 4 , for each servomotor, it sets the motor to step a prescribed amount (angle for conventional servomotors and distance for linear servomotors). If the photodiode current increases it continues stepping the motor in the same direction, otherwise it reverses direction. The feedback loop can be repeated a number of times (e.g. 10) before the currently controlled servomotor is fixed at its optimized position and the next servomotor is then actively being controlled. If at any time the photodiode current drops below a certain threshold level (i.e. signal is lost), a complete scan is performed to find the signal.  
         [0024]     A complete scan is done by positioning all 5 servo motors  1 ,  4  and the motors of the X-Y-Z stage  26  through all their predetermined positions and finding the position that maximizes the photodiode current  203 . For example, if each servomotor is allowed to have 10 discrete positions then there are 100,000 distinct positions for all five servomotors. In general, the position of each servomotor can be adjusted in a few milliseconds, and therefore the entire scan will take several minutes. In general, the rate at which each servomotor is stepped from one position to the next can be adjusted. Also, two optical transceivers that attempt to communicate with each other can have different scan rates.  
         [0025]     Further in  FIG. 3 , the limiting amplifier  207  is connected to a Serializer/Deserializer transceiver IC (SERDES)  208 , such as a Texas Instruments TLK. The SERDES  208  is connected to the FPGA  209 . The FPGA  209  is connected to a Gigabit Ethernet transceiver IC  210 , such as a National Semiconductor DP83865DVH. The Gigabit Ethernet transceiver IC  210  is connected to a set of four isolation transformers  211 , one for each channel of the 1000 Base-T signal, such as four Pulse H-5007, which in turn are connected to a RJ-45 connector  212 . The SERDES  208  is also connected to a laser driver  213 , such as a Maxim MAX3273 that drives a single mode laser module  214 , such as a Sumitomo SLT4460. The fiber  35  is connected at one end to the output of the laser module  214  and coupled at the other end to the transmit optics  7 .  
         [0026]     The FPGA  209  implements a Verilog program shown in a high-level block diagram form in  FIG. 5  to transmit frames to the other optical transceiver. By implementing this program the FPGA accomplishes a number of functions in parallel. First, it generates an alignment frame every 0.5 seconds (the time could be adjusted to any other appropriate value). Referring to  FIG. 7 , the timing frame has a preamble field  401  of 56 bits of alternating 1&#39;s and 0&#39;s, identical to those of an Ethernet frame. It has a Start Frame delimiter field  402  of 8 bits, 10101011, again identical to those of an Ethernet frame. It has a destination address field  403  of 48 bits. This address is a local address recognized only by the two optical transceivers. For example, the first optical transceiver  61  of  FIG. 1   b,  can have address  1  (that is the first 47 bits of the destination Address field are 0 and the last bit is 1), and the second optical transceiver  62  of  FIG. 1   b  can have address  2  (that is the first 46 bits of the destination address field are 0 and the last two bits are 1 and 0). The Source address field  404  contains the address of the source optical transceiver. All bits of the type field  405  are zero, and the first 8 bits of the data field  406  contain the digital value of the photodiode average current value  203  as determined by the ADC  204 . The next 8 bits of the alignment frame contain the photodiode average current value of the other optical transceiver, if an alignment frame sent by the other optical transceiver  62  was received. The remainder of the fields  404  to  307  of the alignment frame have the value zero (that is all bits are 0). In parallel, the FPGA  209 , receives and stores in its RAM frames received from the Ethernet transceiver  210 . It sends these frames to SERDES  208 , when an alignment frame is not being transmitted.  
         [0027]     By implementing this Verilog program, each optical transceiver sends periodically status information to the other optical transceiver, such as the value of the photodiode current they measure, which is the strength of the received signal. This status information can be used by each optical transceiver in a feedback loop, for example the feedback algorithm shown in  FIG. 4  to control their position and thus achieve or optimize their alignment.  
         [0028]     The FPGA  209  also implements a Verilog program shown in a high-level block diagram form in  FIG. 6  to process incoming frames from the other optical transceiver. Each optical transceiver receives frames from the other optical transceiver and determines if the incoming frame is an Ethernet frame or alignment frame, by examining the fields of the frame. If the incoming frame is an Ethernet frame it is forwarded to the Local Area Network  218  of  FIG. 3 . If it is an alignment frame it is used by the FPGA  209  to adjust the position of the optical transceiver and thus optimize alignment.  
         [0029]     Numerous other embodiments of the present invention are also possible. For example, although this embodiment uses a 1000Base-T connection to the switching hub of the base station, many other connections of different speeds or connection media are possible, such as 10-BaseT, or 100-BaseTX, or 1000Base-T connections, that use electrical interfaces, or 1000Base-X, or 10Gibabit/s connections using optical interfaces. Further, in other embodiments, the data frames could be in the form of SONET or ATM frames or any other type of data units or frames transmitted through a network.  
         [0030]     Also, the data field  406  of the alignment frame can include information about the position of the motors of the optical transceiver that sends the alignment frame and the position of the motors of the second optical transceiver, as present in the last received incoming alignment frame. In servomotors, the angular position of their shaft is determined by the width of a pulse sent to their signal wire by the motor controller, the FPGA  209  in this embodiment. Therefore, the alignment frame may include the width of the control pulses sent to each servomotor by the controller in some pre-determined order. For example, the data field of the alignment frame can include the width of the pulse sent to servo motor  1 , followed by the width of the pulse sent to servo motor  4 , followed by the value of the control signals sent to the motors of the X-Y-Z stage  26 , followed by the data present in the last alignment frame received. In general, any type of information available to the optical transceiver can be included in the alignment frame. Also such frames can be requested by and sent to the other optical transceiver or requested by and sent to any other device in the network to which the optical transceiver is connected.  
         [0031]     Also, the photodetector  25  could be replaced or supplemented by a multi-element detector, such as a PSD1 quadrant detector from Thorlabs, that also provides positioning information. In such embodiment, the photocurrent values for each photodetector quadrant will be included in the alignment frame in a specific order, such as top quadrant first followed by the other quadrants in a clockwise direction. The photodetector quadrant values are use by the second optical transceiver to adjust the transmit optics position, thus optimizing alignment between the two optical transceivers  61  and  62 . This is done by rotating the transmit optics tube  7  by a small amount, e.g. 0.1 degrees, in the direction of the quadrants with the lowest photocurrent values.  
         [0032]     Also, the receive and transmit optics can be replaced by a different lens or system of lenses or mirrors (singlets, doublets, parabolic reflectors) that accomplish the same functions. In the simplest case, a single lens can focus incoming light onto the photodetector, or expand the beam carried by the fiber. Also, the servomotors can be replaced by numerous other types of motors, such as stepper motors, and controllers. Further, in other embodiments, motor control can be supplemented with or substituted by manual control of the rotation of the shafts  2  and  5  and/or the position of the X-Y-Z stage  26 . This can be done, for example, by adding hand cranks and micrometers. In general, numerous prior art methods exist to control the angular and linear motion of shafts and stages, as for example the manual or motorized swivel bases found in camera tripods.  
         [0033]     Also, the FPGA  209  could be replaced by an application specific IC (ASIC) or by a DSP or CPU programmed to perform the functions of the FPGA. Also, the SERDES  208  could be replaced by a transceiver IC, such as, for example, an Intel LXT971 for 100 Mbits/s transmissions. Many combinations of the above are also possible. Also, the Ethernet transceiver IC  210  could be replaced by an Ethernet controller IC, such as an Intel 82540EM Gigabit Ethernet controller, and the connection to FPGA  209  could be done by a PCI bus. Other controllers and bus types could also be used. Also, the Ethernet transceiver functions could be integrated in the FPGA.  
         [0034]     In another embodiment of this invention, a multi-channel radio receiver, such as a Futaba R156F, can be attached to the optical transceiver. The radio receiver is connected to the servomotors  1  and  4 . An operator can remotely then control the position of the optical transceivers, by using a radio controller, such as a Futaba 6FGK FM. In this manner, an operator can align the optical transceivers remotely, during initial installation or if alignment is lost. Alternatively, the radio receiver signals can be input into the FPGA, and control of the motors can be accomplished through the FPGA. A visible light source, such as a laser diode, can also be mounted onto the optical transceivers to assist the operator in aligning the devices.  
         [0035]     In another embodiment of this invention, and referring to  FIG. 8 , the transmit optics tube  7  is mounted onto the shaft  805  of a servomotor  804 . Servomotor  804  is mounted onto the shaft  802  of servomotor  801 . The servomotor  801  is attached to the receive optics tube  6  of  FIG. 1 . There are numerous prior art methods of attaching mechanical components and motors together, such as using mounting brackets and screws, welding and so on. In this embodiment, the transmit optics tube  7  can have additional degrees of freedom than those allowed by the arrangement of  FIG. 1 . In other embodiments tube  6  could also be mounted on one or more servomotors similarly to tube  7 . Also in this invention, tubes  6  and  7  could be substituted by any platform capable of supporting the receive and transmit optics respectively, such as plates of various shapes and sizes or any other appropriate combination of mechanical parts.  
         [0036]     In another embodiment, the laser module  214  and transmit optics  215  of  FIG. 3  can be replaced by a free space laser diode  901  of  FIG. 9 , such as a Mitsubishi ML925B6F diode, mounted inside tube  7 , that emits a free space laser beam  903  that is collimated by a plano-convex lens  902 . Also, the laser diode could be mounted on a linear travel stage, such as a Thorlabs NF2AP50 stage, to adjust its distance from lens  902 , thus adjusting the divergence of beam  903 . Also, the laser diode  903  can be replaced with other types of single mode or multi-mode laser diodes or LEDS. Multiple tubes  7  that contain transmit optics can also be used to increase output light power.  
         [0037]     In another embodiment, the scan in the flowchart of  FIG. 4  performed by the two optical transceivers  61  and  62  to find the signal when the photodiode current is below threshold can include the following steps: 
    (i) The shafts of servomotors  1  and  2  are set initially to a pre-determined angular position, so that each optical transceiver approximately faces the other. The initial pre-determined angular position can either be stored in the memory of the controller, or it can be received from the network, or it can be set manually by an operator, or it can be calculated from Global Positioning System (GPS) coordinates input to the controller from a GPS receiver, such as an Atmel ATR0630 connected to the controller  209 , of the two optical transceivers. An electronic compass, such as a Dinsmore 1525 connected to a National Semiconductor ADC0801 that is connected to the controller  209 , can also be used to assist with initial positioning. In general, the optical transceiver can include any number of position awareness means for locating its position within a pre-determined reference frame.     (ii) The shafts  802  and  805  of servomotors  801  and  804  are then stepped by the controller through a pre-determined number of angular positions. For example, initially the shafts are set to a 45-degree angle with respect to their zero position. Then shaft  802  is rotated one degree every 2 seconds from its initial position up to the 125-degree position, while shaft  805  is kept stationary. Then shaft  805  is advanced 1 degree, and shaft  802  performs another sweep from 45 to 125 degrees, and so on until shaft  805  has also completed its sweep range.     (iii) Simultaneously but independently of the movement of shafts  802  and  805 , linear stage  26  is continuously stepped through its range of available positions. The time that it takes for stage  26  to step through all its positions should be equal to or less than a predetermined value, in this example 2 seconds.     (iv) If at any time an incoming alignment frame is detected, then the next outgoing alignment frame transmitted by the optical transceiver will include the position of the motors of the optical transceiver followed by the position of the motors of the remote optical transceiver as reported in the last incoming alignment frame. Also, the position of linear stage  26  is fixed to the position it has at the time the incoming alignment frame is received.     (v) If the detected incoming frame has information on the position of the motors of this optical transceiver then the scan stops, and the motors are set to the values contained in the incoming frame     (vi) If an incoming alignment frame has not been detected since the beginning of the scan, the outgoing alignment frames do not contain any information on the position of the motors of the other optical transceiver, i.e. the corresponding data fields of the outgoing frame are left blank.     (vii) If at the end of the scan no incoming frames have been detected, then the scan is repeated.