Abstract:
A multi-channel 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 gear transmission systems. It is a true passive, bi-directional rotational optical transmission device which could be used for both multi-mode and single mode fibers without the limitation to the through bore diameters.

Description:
REFERENCES CITED 
   U.S. Patent Documents 
   U.S. Pat. No. 4,460,242 July 1984 Birch, et al. 
   U.S. Pat. No. 4,492,427 January 1985 Lewis, et al. 
   U.S. Pat. No. 4,943,137 July 1990 Speer 
   U.S. Pat. No. 4,934,783 June 1990 Jacobson 
   U.S. Pat. No. 6,907,161 July 2005 Bowman 
   OTHER PUBLICATIONS 
   
       
       “Fiber Optic Rotary Joints-A Review”, by GLENN F. I. DORSEY.  IEEE Trans. Components, Hybrids, and Manufac. Technol ., vol. CHMT-5, NO. 1, 1982, PP39. 
       “Mechanism design, analysis and synthesis, volume 1” by Arthur G. Erdman and George N. Sandor. Third Edition. 1997. 
     
  
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   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. 
   2. Description of Related Art 
   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. 
   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. 
   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. 
   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 monofibers or multifibers. 
   But actually, it is almost no way to handle and arrange so many fibers on the 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. 
   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 
   The object of the present invention is to eliminate the huge number of fiber bundles and photodiodes in most prior arts, to provide a true passive, bidirectional, no time jitter, low-loss off-axis optic slip ring which could be used for both multi-mode and single mode fibers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is preferred embodiment of the invention. 
       FIG. 2  is an outline diagram of the off-axis slip ring in  FIG. 1 . 
       FIG. 3  shows the mirror array in the invention. 
       FIG. 4  illustrates another arrangement of the mirror array in the invention. 
       FIG. 5  represents the position changes for the collimators on stator. 
       FIG. 6  shows another embodiment of the gear transmission in the invention. 
       FIG. 7  demonstrates a different way to build a multi-channel off-axis optic slip ring. 
       FIG. 8  is the enlarged view for an on-axis multi-channel optic rotary joint used in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIG. 1 , a typical embodiment of a multi-channel off-axis optic slip ring in the present invention comprises rotor  18 , stator  30 , mirror array  16 ,  26 ,  36 ,  46 , rhomboid prisms  15 ,  45 , right angle prisms  25 , 35 , gears  19 , 22 ,  23 , 24 , collimators  10 ,  20 ,  11 ,  12 , and coupler  13 . A pair of bearings  50  are mounted between rotor  18  and stator  30  to provide the main rotational interface. Other bearings  51 ,  52 ,  53 , and  54  are used to rotationally support the gears  22 ,  23 ,  24 ;  32 ,  33 , and  34  in the stator  30 . Collimators  10 ,  20 , 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 collimators  10 , and  20  are parallel to the main rotational axis  70 . The rotor  18  and the mirror holder  60  are hollow along the 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. On the inward end part of rotor  18  is a bevel gear  19 , which is engaged with another bevel gear  32 . A spur gear  33  is fixed with the bevel gear  32  and rotatable through the bearings  53 , thus driving the next spur gear  34  to rotate through the bearings  54 . A rhomboid prism  45  is attached on the gear  34  thus rotating with gear  34 . A folded mirror  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 (as shown in  FIG. 3 ). The mirror array  16 ,  26 ,  36  and  46  are stationary by fixed to stator  30  through holder  60  and cover  40 . The gear ratio between gear  19  and  34  is designed to 1:1. The rotation direction of the gear  34  is the same as that of rotor  18 . 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 gear  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 gears, 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 . Finally, the collimator  11  and  12  are connected to an optical coupler  13 , which is also fixed to stator  30  through cap  40 . 
     FIG. 2  is an outline diagram of the off-axis slip ring in  FIG. 1 , where, member  80  represents the opto-mechanical transformer, including all the gears, rhomboid prisms, right angle prisms, mirrors and bearings. In the first channel, light beam would be transmitted from collimator  10  to coupler  13 , vise versa. In the second channel, light beam would be transmitted from collimator  20  to coupler  63 , vise versa, in the same way. Mirror  26  is for second channel (as shown  FIG. 1 ,  FIG. 3  and  FIG. 4 ). The gears and prisms for the second channel are not shown in the  FIG. 1 , but they have the same opto-mechanical structure as the first channel. As illustrated in  FIG. 2 , 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: 
               P   s     =               P   2     /   2     ,     --     --     --     (     0   ∼     180   ⁢   °       )                           P   1     /   2     ,     --     --     -     (       180   ⁢   °     ∼     360   ⁢   °       )                     ,         
where, P 2 ≈P r , - - - (0˜180°), P 1 ≈P r , - - - (180°˜360°),
 
(Note: the Angle Refers to the Rotation Position of Rotor  18  in  FIG. 1 )
 
Due to the opto-mechanical transmission error, usually, P 1 ≠P 2 , and P 1 -P 2 ≦1 dB.
 
   Another embodiment of mirror array is illustrated in  FIG. 4  if the gear systems for the even number of channel are arranged to perpendicular to the odd number of channel. For example, mirror  16  is for channel one, mirror  36  for channel  3 , mirror  26  and  46  for channel  2  and channel  4  respectively. In this way, the axis of gears for channel  1  and  3  would be perpendicular to the axis of gears for channel  2  and  4  in order to save space. 
   In  FIG. 5 , the optical signals would be directly coupled to collimator  11  and  12  respectively instead of using right angle prisms  25  and  35  like in  FIG. 1 . 
   An alternative embodiment of the invention is illustrated in  FIG. 6 , where the gear transmission is arranged in a different way as in  FIG. 1 . The gear engagement between  19  and  24 , (or between  19  and  34 ), is in such an order as from spur gear to bevel gear, while in  FIG. 1  it is from bevel gear to spur gear. The gear engagement order would not change the light path and the performance of the invention, but affect the mechanical dimensions of the invention. 
   In  FIG. 7 , a preferred embodiment of the invention for multi-channel off-axis fiber optic slip ring is illustrated, where, two on-axis multi-channel fiber optic rotary joints  99  and  100  are utilized. They are co-axially arranged with gear  34  and gear  24  respectively. To compare with  FIG. 1  and  FIG. 5 , almost all the opto-mechanical members are the same in  FIG. 7  as in  FIG. 1  and  FIG. 5 , but only one mirror  16  is needed for this embodiment. The collimator  10  in  FIG. 1  and  FIG. 5  becomes a multi-collimator bundle  1000  in  FIG. 7  in the same position on rotor  18 . The collimator  11 , or  12  in  FIG. 1  and  FIG. 5  becomes a multi-collimator bundle  111 , or  112  in  FIG. 7  in the similar position on stator  30 . The multi-collimator bundle  1000  could transmit multi-channel optical signals. The light beams emitted from multi-collimator bundle  1000  should be parallel one another. For example, the paralleled light beams from the multi-collimator bundle  1000  would be reflected by the flat mirror surface  162 , or  161 , and then reflected two times by the rhomboid prism  45 , or  15 , to get into the central bore of the gear  34 , or gear  24  along the rotational axis of gear  34 , or gear  24 . When the multi-collimator bundle  1000  rotates with the rotor  18  around the common rotational axis  70 , the paralleled light beams from the multi-collimator bundle  1000  will rotate around the axis of gear  34 , or gear  24 , in a stable pattern after transmitted by the mirror  16  and rhomboid prism  45 , or  15 . The on-axis fiber optic rotary joint  99 , or  100 , will allow the rotating paralleled light beams from the multi-collimator bundle  1000  to be coupled with the multi-collimator bundle  111 ,  112 , which is fixed to the stator  30 . Like in  FIG. 1  and  FIG. 5 , a coupler bundle  133  will couple the corresponding fibers from collimator bundle  111  and  112 . 
   To explain how the on-axis fiber optic rotary joint (FORJ)  99 , or  100  works, the cross section view of a preferred on-axis fiber optic rotary joint  99 , or  100  is enlarged in  FIG. 8 . The gear  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 part of stator  99  of the FORJ. 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. 
   The on-axis fiber optic rotary joint in  FIG. 8  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. 7 .