Abstract:
A method of constructing a fiber-optic gyroscope includes optically coupling first and second optical fibers to an optical path of an interferometer having an outer surface, coupling at least a portion of the first and second fibers to the outer surface, and optically coupling the first and second fibers to an optical path of an integrated optics chip (IOC).

Description:
BACKGROUND OF THE INVENTION 
   Fiber optic gyroscopes (FOGs) have become widely used technologies in many systems to sense the rotation and angular orientation of various objects, such as aerospace vehicles. FOGs work by directing light in opposite directions around a closed optical path enclosing an area whose normal is along an axis of rotation. If the device is rotated about the axis of rotation, the optical path length for the light traveling in one direction will be reduced, while the optical path length for the light traveling in the opposite direction will be increased. The change in path length causes a phase shift between the two light waves that is proportional to the rate of rotation. 
   Referring to  FIG. 1 , a typical FOG  10  includes a light source  15 , a rate detector  20 , a coupler  25 , an integrated optics chip (IOC)  30 , and an interferometer, such as a sensing coil  35 . As shown in  FIG. 1 , a red fiber service lead  40  and blue fiber service lead  45  of the IOC  30  is spliced  50 ,  55  to the red fiber service lead  60  and blue fiber service lead  65  of the fiber coil interferometer  35 . Each of these service leads are often approximately two meters in length to allow optical splicing needed in the build process. Since these fiber service leads for splicing are functionally part of the interferometer  35 , the manner in which the service leads are stowed is a key gyroscope performance parameter. 
   In conventional FOG builds, these lead fibers are stowed in a thread-like winding pattern in a holding compartment having independent thermal characteristics from the interferometer  35 . Specifically, in such an approach, waves counter-propagating through the coil  35  may “see” different environment effects at different points in time. High-performance polarization maintaining gyroscopes must have Lorentz reciprocity between the counter-propagating waves. Lorentz reciprocity requires light propagating in a medium to have identical effects independent of the direction of light propagation. Environmental effects can easily degrade Lorentz reciprocity and gyroscope performance. As such, these conventional approaches typically have degraded Lorentz reciprocity caused by environmental effects. 
   SUMMARY OF THE INVENTION 
   In an embodiment, a method of constructing a fiber-optic gyroscope includes optically coupling first and second optical fibers to an optical path of an interferometer having an outer surface, coupling at least a portion of the first and second fibers to the outer surface, and optically coupling the first and second fibers to an optical path of an integrated optics chip (IOC). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is a schematic view of a conventional FOG in which embodiments of the present invention may be implement; 
       FIG. 2  is a front plan view of service lead routing according to an embodiment of the invention; 
       FIG. 3  is a rear plan view of service lead routing according to an embodiment of the invention; and 
       FIG. 4  is a top perspective cross-sectional view of an interferometer with service lead routing according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment provides for a service-lead stowage location on the outer layer of an interferometric fiber coil, thereby providing improved performance as compared to a separate compartment storage. 
   An embodiment of a bifilar service-lead routing method provides improved performance under time-varying thermal gradients and reduces the Shupe effect of these service leads. 
   An embodiment provides bifilar fiber routing method that ensures service leads are confined to a single outer layer of a coil and not additional layers, which would be subject to more stress over temperature changes. 
   An embodiment provides a reduction of the number of points where optical fiber crosses over itself, as well as twists in the fiber, thereby improving gyroscope bias stability. 
   By routing the fibers in such a bifilar-pair fashion, points equal and opposite in the sensing loop are physically adjacent to each other and see the same effects over varying environments. 
   Referring now to  FIG. 2 , illustrated is a front view of the outer surface of an interferometer  200  along which are routed, according to an embodiment, a red service lead fiber  210  (illustrated in  FIG. 2  as a solid fiber element) and a blue service lead fiber  220  (illustrated in  FIG. 2  as a cross-hatched fiber element) connecting an optical path of the interferometer  200  to the optical path of an IOC (not shown). 
   In an embodiment of the invention, a first portion of the red fiber  210  directly coupled to the optical path of the interferometer  200  is seen emerging from the center of the interferometer and is oriented along a left-to-right path along the outer surface. Similarly, a first portion of the blue fiber  220  directly coupled to the optical path of the interferometer  200  is seen emerging from the center of the interferometer and is oriented along a right-to-left path along the outer surface. As such, a second portion of the red fiber  210  and a second portion of the blue fiber  220  converge toward one another so as to form a “Y”-junction  230 . 
   Subsequently, beginning at a region  240  of the outer surface, multiple turns of respective third portions of the red fiber  210  and blue fiber  220  are wrapped around the outer surface a predetermined integer number of times to form a winding  250 . As best seen in  FIG. 4 , the winding is formed so as to form only a single layer along the outer surface of the interferometer  200  in a direction normal to a center axis  400  of the interferometer. Additionally, it should be noted that, in forming the winding  250 , the red and blue fibers  210 ,  220  do not cross or otherwise overlap each other. 
   After forming the winding  250 , a fourth portion of the red fiber  210  and a fourth portion of the blue fiber  220  are routed up and away from the winding and in the opposite direction so as to form a “U-turn” configuration  260 , or perhaps an even more rounded, “lasso” type configuration (not shown). In an embodiment, if there is a disparity in length between the red and blue fibers  210 ,  220 , the distance between the red and blue fibers may be increased at the “U” portion of the configuration  260  to accommodate the longer fiber. 
   After forming the U-turn configuration  260 , as best illustrated in  FIG. 3 , the red and blue fibers  210 ,  220  are oriented in a substantially sinusoidal, or serpentine, configuration  300  along the outer surface, also in the direction opposite of the direction in which the winding  250  was formed. The red and blue fibers  210 ,  220  may then be subsequently coupled to the optical path of the IOC. 
   In an embodiment, the red and blue fibers  210 ,  220  are coupled to the outer surface of the interferometer  200  in a manner that reduces or eliminates light cross-coupling. 
   It should be further noted that the only time the red and blue fibers  210 ,  220  cross each other or otherwise overlap is at the points of transition from the winding  250  to the “U-turn” configuration  260  as illustrated in the example of  FIG. 2 . 
   While a preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.