Patent Publication Number: US-2009226131-A1

Title: Fiber Optic Rotary Coupler

Description:
REFERENCES CITED  
     U.S. PATENT DOCUMENTS 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 5,039,193 
                 August 1991 
                 Snow et al. 
               
               
                   
                 4,124,272 
                 November 1978 
                 Henderson et al. 
               
               
                   
                 5,633,963 
                 May 1997 
                 Rickenbach et al. 
               
               
                   
                 5,949,929 
                 September 1999 
                 Hamm 
               
               
                   
                   
               
            
           
         
       
     
     OTHER PUBLICATIONS 
     “Fiber Optic Rotary Couplers-A Review”, by GLENN F. I. DORSEY.  IEEE Trans. Components, Hybrids, and Manufac. Technol.,  vol. CHMT-5, NO. 1, 1982, PP 39. 
     “Design and Implementation of a Broad Band OpticRotary Coupler Using C-lenses”, by Wencai Jing et al.,  Optics Express,  vol. 12, NO.17, 23 August 2004. PP 4088-4093. 
     BACKGROUND OF THE INVENTION  
     1. Field of the Invention 
     The invention is related to single channel fiber optic rotary coupler in the field of optic communication to ensure that the device has low insertion loss, small insertion loss variation, and high return loss. 
     2. Description of Related Art 
     The Fiber optic Rotary Coupler is the optic equivalent of the electrical slip ring. It allows uninterrupted transmission of an optic signal in a fiber guide through a rotational interface to a stationary apparatus. The Fiber optic Rotary Coupler is widely used in missile guidance systems, robotic systems, remotely operated vehicles, oil drilling systems, sensing systems, and many other field applications where a twist-free fiber cable is essential. Combined with electrical slip rings or fluid rotary couplers, Fiber optic Rotary Coupler adds a new dimension to traditional slip rings. As fiber optic technology advances, more and more traditional slip ring users will benefit from Fiber optic Rotary Coupler in their new fiber systems. 
     Comparing with its electrical counterpart, the electrical slip ring, the Fiber optic Rotary Coupler is not easy to fabricate because the transmission of the light beam through a fiber is strongly depend on its geometrical structure and related position. So it requires special design to ensure the transmission of light beam through a relative rotating coupler without suffering a large loss. A couple of prior inventions of single channel fiber optic rotary coupler are described in the following patents: U.S. Pat. No. 5,039,193, U.S. Pat. NO. 4,124,272, U.S. Pat. No. 5,633,963, and U.S. Pat. No. 5,949,929. Most of them employ the expanded beam technology, i.e., using lenses to expand the light beam and collimate it before transmitting to a rotary coupler. The beam is then refocused and aligned with the receiving fiber. The lenses include graded index rod lens, aspheric lens, and GRIN lens. This method has several significant drawbacks. First, this kind of rotary coupler require special fixture to have lenses aligned. Secondly, using high quality lenses would increase the sizes and cost of fiber optic rotary couplers. Further, to maintain the axial alignment is difficult so that this kind of rotary coupler is vulnerable in such environments as temperature change, vibration and shock. 
     SUMMARY OF THE INVENTION 
     The first object of the present invention is to minimize the need for maintaining precise axial alignment between the rotating and non-rotating elements of a single channel fiber optic rotary coupler so that it could be used in any harsh environments such as temperature change, vibration and shock. 
     Another object of the present invention is to provide a single channel fiber optic rotary coupler with a very low-profile and compact design. 
     A further objective of the preset invention is to reduce the insertion loss and increase return loss and to allow the rotary coupler to work at any ambient pressure by filling index-matching fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section view of one embodiment of the invention. There are a rotatable optic fiber and a stationary optic fiber to convey a light beam in a rotary interface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , a typical design of the present invention comprises a rotatable fiber holder  01  and a stationary fiber holder  08 . A pair of bearing  06   a  and  06   b  are mounted in the bore of stationary fiber holder  08  and on the shaft of rotatable fiber holder  01  so that the rotatable fiber holder  01  is able to rotate around the axis of the bore of stationary fiber holder  08 . 
     Both rotatable fiber holder  01  and a stationary fiber holder  08  are designed with a through central holes  01   h  and  08   h  respectively. A rotatable optic fiber  13 , having a tip  13   t , is fixed in the central hole  01   h  of the rotatable fiber holder  01  with the tip  13   t  protruded out of the rotatable fiber holder  01 . A stationary optic fiber  14 , having a tip  14   t,  is fixed in the central hole  08   h  of stationary fiber holder  08  with the tip  14   t  recessed in the central hole  08   h  of the stationary fiber holder  08 . The tip  13   t  and  14   t  are adjacent very closely. Because the diameter of hole  08   h  is slightly larger than the diameter of fiber  13 , the tip  13   t  of fiber  13  and the central hole  08   h  of the stationary fiber holder  08  mechanically forms a so-called “micro bearing, or “micro rotational interface. When the rotatable fiber holder  01  rotates relative to the stationary fiber holder  08 , the rotatable optic fiber  13  is able to rotate relatively to the stationary optic fiber  14  co-axially so as to transmit the optic signal from one fiber to another fiber bi-directionally. 
     The length of protrusion portion of the optic fiber  13  is deliberately designed to have enough flexibility to compensate the mechanical alignment error of the two fibers provided by bearings  06   a  and  06   b.  The mechanical alignment error of a fiber optical rotary coupler could be 10 to 20 um by a conventional fabrication and assembly procedure. For the present invention, the maximum alignment error of the fiber  13   a  and fiber  13   b  is only about 0.5 um so that the insertion loss is greatly improved. And by using of the “micro bearing, the whole size of the fiber optical rotary coupler could be greatly reduced. 
     The optic fibers,  13  and  14 , could be single mode, or multi-mode with a flat end surface, or an 8-degree facet to improve the return loss, or with a thermally expanded end surface. 
     The optic fibers,  13  and  14 , could also be Thermally Expanded Core (TEC) fiber, or micro-collimators with the similar diameter as the conventional optic fibers. 
     An index matching fluid is filled in the inner open space  08   s  of the stationary fiber holder  08 . The shaft seal  04  and o-ring  05  are utilized to seal the space  08   s.  One function of the index matching fluid is for the lubrication between bearings and the “micro bearing Another function of index matching fluid is for pressure compensating purposes. The whole space  08   s  inside the stationary fiber holder  08  could be used as the pressure compensation chamber. The shaft seal  04  is located between the shaft of rotatable fiber holder  01  and the bore of seal cover  02 . The space from seal  04  to bearing  06   a  is designed large enough to allow the shaft seal  04  to slid axially like a piston to balance ambient pressure with the pressure inside the stationary fiber holder  08 .