Abstract:
An active off-axis optic slip ring system is disclosed. The invention eliminates the huge number of fiber bundles and photodiodes in most published patents. A couple of conventional optical components such as mirrors and prisms are used to transmit optical signals with high, quality and low optic losses. The optical signal pick-up is realized through a pair of prisms mounted on the rotor of motors. It is an active, hi-directional rotational optical transmission device which could be used for multi-mode, or single mode fibers without the limitation to the through bore diameters.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention is related to off-axis multi-channel fiber optic slip ring to provide transmission of data in optic form between a mechanically rotational interface with a through bore. 
         [0003]    2. Description of Related Art 
         [0004]    It is well known that the devices to transmit optical data between two independently rotational members are called fiber optical rotary joints, or optical slip ring. There are single channel, two channel and multi-channel fiber optical rotary joints. However, most of them are categorized as on-axis fiber optical rotary joint because the optical paths are located along the axis of rotation, or occupy the central space along the axis of rotation. If the central space along the rotational axis is not accessible, the optical light paths would not be allowed to path through the central area along the rotational axis. Such devices are usually called off-axis optical slip ring. 
         [0005]    The simplest, off-axis slip ring has been described in U.S. Pat. No. 4,492,427, which comprises two opposed annular fiber bundles and increasing the number of such concentric annular bundles radially would make the device multi-channeled. The concentric, annular fiber bundle fiber optic slip rings are bi-directional but do have a modulated light loss dependent on the rotational angle. For minimizing the importance of the modulation, a digitized signal rather than an analog signal has to be used. This off-axis slip ring only could be used for multi-mode fibers, not single mode fibers. 
         [0006]    U.S. Pat. No. 4,460,242 discloses an optical slip ring employing optical fibers to allow light signals applied to any one or all of a number of inputs to be reproduced at a corresponding number of outputs of the slip ring in a continuous manner. It includes a rotatable output member, a stationary input member and a second rotatable member which is rotated at half the speed of the output member like a de-rotator. The input member having a plurality of equispaced light inputs and the output member having a corresponding number of light outputs and the second rotatable member having a coherent strip formed of a plurality of bundles of optical fibers for transmitting light from the light inputs on the input member to the light outputs. 
         [0007]    Another U.S. Pat. No. 4,943,137 assume the similar idea, where, a de-rotating, transmissive intermediate optical component with an array of lensed optical transmitters and receivers respectively mounted on the rotor and stator. The derotating, intermediate optical component comprises an image conduit, image transporter, or coherent optical fiber bundle of close-packed monotibers or multifibers. 
         [0008]    But actually, it is almost no way to handle and arrange so many fibers on said rotatable members, especially for large diameter slip ring. The optical loss is very obvious for multi-mode fibers. It is almost impossible to use single mode fibers. The effect of damaged fibers, the presence of debris, separation distances, component tolerances, or backlash in the gearing also cause problems. 
         [0009]    A more sophisticated approach can be found in U.S. Pat. No. 6,907,161. The patent uses multiple inputs and pick-ups to send and receive data across members that have large diameters. The use of multiple inputs and pick-ups is required to keep the optical signals at a level that is sufficiently high to permit the photodiode receivers to operate. Wave guides are employed. The multiple inputs and pick-ups also cause a rapid rise and fall of the signal because the signal reflects from one area of the waveguide to another. The drawback is to use photodiode receivers which is an electro-optical device, so that the output signal is electrical and the power must be high. Besides, there is a time jitter thus limiting the data rate. 
       SUMMARY OF THE INVENTION 
       [0010]    The prior patent, U.S. Pat. No. 7,792,400, by the same incentors, disclosed an invention for both single channel and multi-channel off-axis fiber optic slip ring, which is a passive fiber optic off-axis slip ring, including mirror, or mirror array, prisms, optic coupler and gears. The object of the present invention is to replace the gears by motors to provide an active, bidirectional, no time jitter, low-loss off-axis optic slip ring which could be used for multi-mode, or single mode fibers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1   a  &amp;  1   b  are the first embodiment of the invention. 
           [0012]      FIGS. 2   a  &amp;  2   b  illustrate the right hand rule for rotation vectors. 
           [0013]      FIG. 3  is an outline diagram of the off-axis slip ring in  FIGS. 1   a  &amp;  1   b.    
           [0014]      FIGS. 4   a  &amp;  4   b  demonstrate the second embodiment of current invention. 
           [0015]      FIG. 5  is the enlarged view for an on-axis multi-channel optic rotary joint used in  FIGS. 4   a  &amp;  4   b.    
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    As shown in  FIGS. 1   a  and  1   b , a typical embodiment of a single channel off-axis optic slip ring in the present invention comprises rotor  18 , stator  30 , mirror array  161  and  162 , rhomboid prisms  15  and  45 , right angle prisms  25  and  35 , motor  101  and  102 , collimators  10 , 11 , 12 , and coupler  13 . A pair of bearings  50  are mounted between rotor  18  and stator  30  to provide the main rotational interface. Motor bearings  52 , and  54  are used to rotationally support the motor shaft  24  and  34  in the stator  30 . Collimators  10  and more (depends on how many channel would be built), are mounted on rotor  18  in circumferential direction at a different distances to the common rotational axis  70 . The axis of the collimator  10 , are parallel to the main rotational axis  70 . The rotor  18  and the mirror frame  16  are hollow along said common rotational axis so that a through bore is provided, leaving the central part of the interface totally free. That means all the optical signals would not be allowed to pass through the through bore. 
         [0017]    A rhomboid prism  45  is attached on the motor rotor  34 . Said mirror frame  16  is co-axial with the common rotational axis  70  with two flat mirror surfaces  161  and  162 , which are perpendicular each other and symmetrical to the common rotational axis. The mirror frame  16  is stationary by fixed to stator  30  through holder  40 . 
         [0018]    Encoders  201  and  202  are used to detect the rotation speed and direction of rotor  18 . The signals from encoder  201  and  202  are transmitted to motor controllers  203  and  204  respectively to control the motion of motors  101  and  102 . 
         [0019]    The speed ratio between rotor  18  and motor shaft  24  and  34  is designed to 1:1. The rotation directions of motors are shown in  FIGS. 2   a  &amp;  2   b , where a right hand rule is applied on the rotation directions between rotor  18  and motor shaft  24  and  34 . Refers to  FIG. 2   a , if fingers of right hand point in the direction of rotation, the thumb points in the direction of rotation vector. Thus the direction of rotation vectors for rotor  18  and motor shaft  24  and  34  are represented by vector  1 ,  2 , and  3  respectively, as shown in  FIG. 2   b : all the rotation vectors either point inward, or point outward. 
         [0020]    When the collimator  10  rotates within 180° and 360°, the light beam emitted from collimator  10  will be reflected by the mirror surface  162  to rhomboid prism  45  and reflected two times by the paralleled surfaces of rhomboid prism  45  to the central hole of motor shaft  34 . Another similar right angle prism  35  fixed in the stator  30  would pickup the light beam to the collimator  11 , which is also fixed on stator  30 . Because the counterpart of the above described motor, rhomboid prisms, right angle prisms, and collimators are also symmetrically arranged to the common axis  70 , when the collimator  10  rotates between 0° and 180°, the light beam emitted from collimator  10  will be reflected by mirror surface  161 , prism  15  and  25 , then coupled to collimator  12 . 
         [0021]    As shown in s.  3 , the collimator  11  and  12  are connected to an optical coupler  13 , which is also fixed to stator  30  through cap  40 . 
         [0022]      FIG. 3  is an outline diagram of the off-axis slip ring in  FIGS. 1   a  &amp;  1   b , where, member  80  represents the opto-mechanical transformer, including all the motors, rhomboid prisms, right angle prisms, mirrors and bearings. Light beam would be transmitted from collimator  10  to coupler  13 , vise versa. If the power of optical signal from collimator  10  is P r , and the power of optical signal through collimator  11  and  12  are P 1  and P 2  respectively, then the power of optical signal to coupler  13 , P s , can be expressed as follows: 
         [0000]    
       
         
           
             
               P 
               s 
             
             = 
             
               
                 
                   
                     
                       P 
                       2 
                     
                     , 
                     
                       -- 
                       
                         -- 
                         
                           - 
                           
                             ( 
                             
                               
                                 0 
                                 ~ 
                                 180 
                               
                                
                               ° 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   
                     
                       P 
                       1 
                     
                     , 
                     
                       -- 
                       
                         -- 
                         
                           - 
                           
                             ( 
                             
                               180 
                                
                               
                                 ° 
                                 ~ 
                                 360 
                               
                                
                               ° 
                             
                             ) 
                           
                         
                       
                     
                     , 
                   
                 
               
             
           
         
       
       
         
           
             where, P 2 ≈P r , - - - (0˜180°), P 1 ≈P r , - - - (180°˜360°), 
           
         
       
     
         [0024]    (Note: the angle refers to the rotation position of rotor  18  in  FIG. 1   b ) 
         [0000]    Due to the opto-mechanical transmission error, usually, P 1 ≠P 2 , and P 1 −P 2 ≦1 dB. 
         [0025]    In  FIGS. 4   a  and  4   b , a preferred embodiment of the current invention for multi-channel off-axis fiber optic slip ring is illustrated, where, two on-axis multi-channel fiber optic rotary joints are integrated in motor  99  and  100 . They are co-axially arranged with motor shaft  34  and  24  respectively. To compare with  FIG. 1   a  and  FIG. 1   b , almost all the opto-mechanical members are the same in  FIG. 4   a  and  FIG. 4   b  as in  FIG. 1   a  and  FIG. 1   b . The collimator  10  in  FIG. 1   a  and  FIG. 1   b  is replaced by a collimator bundle  20  in  FIG. 4   a  and  FIG. 4   b  in the same position on rotor  18 . The collimator  11 , or  12  in  FIG. 1   b  becomes a multi-collimator bundle  111 , or  112  in  FIG. 4   b  in the similar position on stator  30 . The collimator bundle  20  could transmit multi-channel optical signals. The light beams emitted from collimator bundle  20  should be parallel one another. For example, the paralleled light beams from the collimator bundle  20  would be reflected by the flat mirror surface  162 , or  161 , and then reflected two times by the rhomboid prism  15 , or  45 , to get into the central bore of the motor shaft  24 , or  34  along the rotational axis of gear  34 , or gear  24 . When the collimator bundle  20  rotates with the rotor  18  around the common rotational axis  70 , the paralleled light beams from the collimator bundle  20  will rotate around the axis of motor shaft  24 , or  34 , in a stable pattern after transmitted by the mirror  16  and rhomboid prism  15 , or  45 . The on-axis fiber optic rotary joint integrated in motor  99 , or  100 , will allow the rotating paralleled light beams from the collimator bundle  20  to be coupled with the multi-collimator bundle  111 ,  112 , which is fixed to the stator  30 . Like in  FIG. 1   b , a coupler bundle  133  will couple the corresponding fibers from collimator bundle  111  and  112 . 
         [0026]    To explain how the on-axis fiber optic rotary joint (FORJ) integrated in motor  99 , or  100  works, the cross section view of a preferred on-axis fiber optic rotary joint is enlarged in  FIG. 5 . The motor shaft  34 , or  24 , is also the rotor of FORJ. A sun gear  118  is fixed with rotor  34 , which is engaged with planet gear  119 , while another planet gear  120  is engaged with an internal gear  122 , which is fixed with motor housing  99 . A Dove prism  115  is co-axially fixed inside the through bore of carrier  116 . The planet gear system is such designed so that the carrier  116  will rotate at the half speed as that of the rotor  34  and in the same rotational direction. In this way, the rotating paralleled light beams on the rotor  34  will be coupled into corresponding collimators in the collimator bundle  111 , or  112  after pass through the Dove prism  115 . 
         [0027]    The on-axis fiber optic rotary joint in  FIG. 5  is only one typical on-axis fiber optic rotary join. Any other types of on-axis fiber optic rotary joint could be used in present invention in the same manner as the on-axis fiber optic rotary joints in  FIG. 4   a  and  FIG. 4   b.