Patent Publication Number: US-2015071588-A1

Title: Low insertion loss, low back reflection fiber optic rotary joint with increased data throughput

Description:
BACKGROUND OF THE INVENTION 
     A typical rotary joint consists of a fixed collimator holder and a rotatable collimator holder which are relatively rotatable to each other to allow uninterrupted transmission of optical signals through the rotational interface from collimators in any one of the holders to the collimators in the other holder. 
     A multi-channel fiber optic rotary joint typically utilizes a dove prism and a gear box as the de-rotating mechanism between the fixed collimator holder and the rotatable collimator holder. It typically rotates at half the rotational speed of the rotatable collimator holder; effectively de-rotation the image. 
     The dove prism relies on refraction to bend the optical beam as part of the de-rotation, which theoretically should not cause a problem. However, in practice this refraction is a source of chromatic dispersion because optical signals have a finite spectral range. Since the angle of refraction is a partial function of the wavelength itself, each discrete wavelength in the optical signal travels a different distance in the dove prism which causes part of the signal to effectively fall behind other parts of the signal. This signal lag ultimately determines the maximum frequency at which a signal can be sent through the fiber optic rotary joint. One way to envision this problem is to picture a square wave, Each peak of the wave represents a discrete piece of information and the width of each peak represents the amount of dispersion in the signal. Now the only way to send the same information faster is to reduce the distance between signal peaks; however, if the distance between peaks is reduced to the point that the waves start to overlap then the information in the two waves starts to become scrambled. Therefore, the frequency at which a signal can be sent is limited by the width of each wave, or the amount of dispersion that occurs in the signal. 
     Another effect of dispersion is it effectively limits the wavelengths at which a particular device can be used. One of the results of different wavelengths traveling slightly different distances through the prism is each wavelength will emerge at different angles on the other side of the prism. As such, a device tuned at 1625 nm will not perform the same if used at 1310 mu. The degree to which the performance will degrade is directly related to the difference between the wavelengths in question. As such the unit will have better performance the closer the operating wavelength is to the wavelength at which the unit was tuned. Conversely, the performance of the unit will degrade the farther the operating wavelength is from the wavelength at which the unit was tuned. 
     This phenomena also sets a floor for the lowest achievable insertion loss. The cause of this lower limit is that in practice every light source has a finite spectral range around the nominal wavelength. The slight variation in the distance traveled by each wavelengths in the spectral range cause slight variations in the focal point for each wavelength in the spectral range. This theoretically reduces the efficiency with which the opposing collimator could recapture the signal, thereby putting a theoretical limit on the lowest achievable insertion loss. 
     All of the aforementioned problems can be resolved by using a dc-rotating prism that does not rely on refraction, such as the Pechan prism or the K-prism. However, as Ames correctly indicated in U.S. Pat. No. 5,157,745; the Pechan prism suffers from high back reflection. This problem leads Ames to conclude that the dove prism is preferable. The configuration embodied herein, solves the back reflection problem by applying an optical coating to the surfaces of the Pechan prism through which the optical signal must pass. This optical coating will eliminate/reduce the reflection at zero degree and function as a mirror at 45 degrees as well. Similarly, high reflection optical coatings can be applied to the surfaces of the K-prism through which the optical signal must pass for the same net result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Is the schematic drawing of de-rotating Dove prism; 
         FIG. 2  Is an outline diagram a coated Pechan prism in the present invention; 
         FIG. 3  Illustrates the principles of a coated Pechan prism for a multi-channel fiber optic rotary joint in the present invention; 
         FIG. 4  Depicts the position of coated Pechan prism relative to a stationary collimator array and a fiber collimator array in the present invention; 
         FIG. 5  Is a cross-sectional view of a multi-channel fiber optic rotary joint in the present invention, 
     
    
    
     DETAIL DESCRIPTION OF INVENTION 
     Dove prisms are used to invert an image and when they are rotated about their longitudinal axis, the transmitted image rotates at twice the rate of the prism (see  FIG. 1 ). Therefore, if the prism rotates at half the rate of a rotating object, the image, after passing through the prism, will appear to be stationary.  FIG. 1  is the schematic drawing of de-rotating Dove prism in the prior art. The image ( 2 ) of an object ( 1 ) is inverted by the Dove prism ( 10 ). Furthermore, if the prism ( 10 ) is rotated about the optic axis ( 3 ), the image ( 2 ) rotates at twice the rate of rotation of Dove prism ( 10 ). 
       FIG. 2  illustrates the imaging principle of the coated Pechan prism in the present invention. The image ( 152 ) of an object ( 151 ) on the entrance side of a coated Pechan prism ( 157 ) is inverted in a similar way as the Dove prism ( 10 ) in  FIG. 1 . However, there is one critical difference; there are no refractive elements in the optical path of the coated Pechan Prism. As a result, there will not be any dispersion of the optical beam. It is also important to note that the prism surfaces through which the beam passes ( 153 ,  154 ,  155  and  156 ) must be coated with an optical coating to reduce the back reflection to an acceptable number. 
       FIG. 3  depicts how the coated Pechan prism ( 101 ) can be used as a de-rotating mechanism for the multi-channel fiber optic rotary joint in the present invention. Suppose the coated Pechan prism ( 101 ) rotates an angle “b” around its axis “Z” from position “1” to position “2”, e.g., from  101 “1” to  101 “2”. The co-ordinates of the object ( 4 ) in position “1”, e.g., 4 “1”, is (X 1 , Y 1 ). According to  FIG. 2 , because the image ( 5 ) is inverted symmetrically relative to the axis “Z”, the co-ordinates of the image ( 5 ) in position “1” are (−X 1 , Y 1 ). If the object rotates an angle “2b” around axis “Z” in the same direction as the coated Pechan prism ( 101 ), the co-ordinates of the object ( 4 ) in position “2”, 4 “2”, are (X 2 , Y 2 ). It&#39;s easy to get that the co-ordinates of the image ( 5 ) in position “2” are (−X 2 , Y 2 ). So the absolute position of the image ( 5 ) remains s the same before and after the rotation. That means that if the coated Pechan prism rotates at half the speed of a rotating object ( 4 ), its image ( 5 ) passing through the coated Pechan prism ( 101 ), will remain to be stationary. 
     In  FIG. 4 , a coated Pechan prism de-rotating mechanism ( 12 ) its the present invention is positioned between a stationary collimator array ( 13 ) and a rotary collimator array ( 11 ). The rotary collimator array ( 11 ), the stationary collimator array ( 13 ) and the coated Pechan prism de-rotating mechanism ( 12 ) are rotatable around a common axis ( 15 ). All the collimators ( 111 ,  112 ,  113 ,  114 ,  115 ,  116  . . . ) in said stationary collimator array ( 13 ) and said rotary collimator array ( 11 ) are arranged parallel to the common axis ( 15 ). If the coated Pechan prism de-rotating mechanism ( 12 ) rotates at half the speed of rotation of said rotary collimator array ( 11 ) around the common axis ( 15 ), the optical signals from the rotary collimator array ( 11 ) would be passed through the coated Pechan prism de-rotating mechanism ( 12 ) and be transmitted to the related channel of the stationary collimator array ( 13 ) respectively, e.g., the first channel optical signal can be transmitted between collimator ( 111 ) and ( 112 ); the second channel optical signal can be transmitted between collimator ( 115 ) and ( 116 ); the third channel optical signal can be transmitted between collimator ( 113 ) and ( 114 ), so as to provide a continuous, bi-directional, multi-channel electro-magnetic signal transmission between two collimator arrays. 
       FIG. 5  depicts one of embodiments of a multi-channel fiber optic rotary joint of the present invention. A speed reduction mechanism includes gears ( 24 ,  25 ,  26 , and  27 ) in which two gears ( 26  and  27 ) are rotatable around the common axis ( 15 ), while the other two gears ( 24  and  25 ) are rotatable around a parallel axis ( 16 ). The gear ratio i from gears  26  to gear  27  can be determined as follows: 
     
       
         
           
             i 
             = 
             
               
                 
                   Z 
                   24 
                 
                  
                 
                   Z 
                   27 
                 
               
               
                 
                   Z 
                   26 
                 
                  
                 
                   Z 
                   28 
                 
               
             
           
         
       
     
     where, Z 24 , Z 25 , Z 26  and Z 27  are the number gear teeth number for gears  24 ,  25 ,  26  and  27  respectively. If the gear ratio i=2:1, that means gear  27  will rotate at half the speed of the rotation of gear  26 . 
     As illustrated in  FIG. 5 , the coated Pechan prism de-rotating mechanism ( 12 ) is in the center of the cylinder ( 28 ). The relative position between the coated Pechan prism de-rotating mechanism ( 12 ), the stationary collimator array ( 13 ) and the rotary collimator array ( 11 ) are the same as depicted in  FIG. 4 . The rotor ( 21 ) is part of a gear ( 26 ), which is rotatable relative to the stator ( 22 ) through the bearings ( 31  and  32 ), The cylinder ( 28 ) is part of a gear ( 27 ), which is rotatable relative to the stator ( 22 ) through the bearings ( 33  and  34 ). Two gears ( 24  and  25 ) are physically connected to the common shaft ( 23 ), which is rotatable around the parallel axis ( 16 ) relative to the stator ( 22 ) through two bearings ( 35  and  36 ). As stated above, a gear ratio i=2:1 would assure that the coated Pechan prism de-rotating mechanism ( 12 ) will rotate at half the speed of the rotation of the rotary collimator array ( 11 ).