Patent Publication Number: US-2009219545-A1

Title: Stitched waveguide for use in a fiber-optic gyroscope

Description:
BACKGROUND OF THE INVENTION 
     A fiber optic gyroscope (FOG) uses the interference of light to measure angular velocity. Rotation is sensed in a FOG with a large coil of optical fiber forming a Sagnac interferometer. To measure rotation, two light beams are introduced into the coil in opposite directions by an electro-optic modulating device such as an integrated optical circuit (IOC). If the coil is undergoing a rotation, then the beam traveling in the direction of rotation will experience a longer path to the other end of the fiber than the beam traveling against the rotation. This is known as the Sagnac effect. As the beams exit the fiber they are combined in the IOC, and the phase shift between the counter-rotating beams due to the Sagnac effect and modulation in the IOC causes the beams to interfere, resulting in a combined beam, the intensity and phase of which depends on the angular velocity of the coil. 
     When testing FOGs using a proton exchanged IOC in a vacuum environment, it has been found that a corruption of the electro-optic modulation occurred and grew with time, eventually rendering the FOG inoperable. The exact phenomenon that corrupts the modulation in FOG output is only partially understood and appears to involve ionic migration along the electric fields near the electrodes of the IOC. 
     Referring to  FIG. 1 , in a LiNbO 3  IOC, when a voltage is applied across a waveguide between electrodes parallel to the waveguide, the piezo-electric effect changes the spacing between the atoms in the poled molecules, which in turn changes the refractive index. This effect enables phase modulation, φ(t), of an electromagnetic wave transiting the waveguide. 
     Normally, in a LiNbO 3  IOC the response of the refractive index to the electric field applied to the electrodes follows the voltage very accurately. However, after soaking in a vacuum, the phenomenon called RDS manifests itself and corrupts the response. 
     More specifically, the voltage V φ (t), where t is time, across the electrodes changes the phase of light in a waveguide by Δφ. During normal IOC operation in air, Δφ(t) follows the shape of the trace of V φ (t) exactly. After the IOC has been in vacuum for a nominal time, instead of following V φ (t), Δφ(t) is corrupted, as shown in the lower trace as Δφ(t), and overshoots the desired Δφ at both the up and down voltage steps. 
     SUMMARY OF THE INVENTION 
     In an embodiment, an integrated optical circuit includes a first waveguide portion of a first material. The first waveguide portion includes an input-port section terminating in a junction section of the first waveguide portion from which first and second branch sections of the first waveguide portion are formed. Second and third waveguide portions are respectively coupled to the first and second branch sections. The second and third waveguide portions are diffused with a second material different from the first material. First and second modulators are respectively coupled to the second and third waveguide portions. Each of the modulators provides respective modulating voltages generating respective electric fields across the second and third waveguide portions. The second and third waveguide portions are coupled to the first and second branch sections at respective locations where the modulating electric fields are substantially zero. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  illustrates in graphical form the RDS effect of corrupted modulation in an IOC; 
         FIG. 2  illustrates a FOG according to an embodiment of the present invention; 
         FIG. 3  illustrates an IOC according to an embodiment of the present invention; and 
         FIG. 4  illustrates the extinction ratio of a misaligned stitch. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As will be described more fully hereinafter, an optical circuit of a Sagnac gyroscope according to an embodiment may be comprised of an IOC, a light source, polarizing circulator, detector, and fiber coil. An embodiment may employ crystalline LiNbO 3  integrated IOCs useful in navigation grade gyros because an applied voltage changes the refractive index of the LiNbO 3 . This property provides superior performance in closed loop gyros by allowing fast, accurate, and sophisticated modulation of light transiting the rate-sensing coil. 
     Early work with LiNbO 3  created IOCs by diffusing titanium (Ti) waveguides into LiNbO 3  designed to create a y-junction. A fiber containing light from a fiber laser is attached to the single tail of the y, and the two ends of a fiber coil were attached to the two outputs of the y-junction. Low voltage waveforms on the electrodes, parallel and close to the waveguides, change the refractive index of the waveguide and allow precise phase modulation between the clockwise and counterclockwise light beams propagating in the fiber coil. 
     Using a proton exchange process on LiNbO 3  to create waveguides has a distinct benefit: light of only one polarization is transmitted through the IOC. This greatly increases the precision of the fiber optic gyro rate measurement to a level necessary for the most demanding navigation requirements and, in an embodiment, eliminates the need for an external polarizer in the gyro circuit. 
       FIG. 2  illustrates a FOG system  100  according to an embodiment of the present invention. A light source  105  provides an optical signal or beam to an optional coupler  110 , which may function to redirect a portion of the beam to a detector  120 . The remainder of the beam may be supplied to an IOC  115  of a sensing-loop assembly  125 , having a fiber coil  126 , via a circulator element  130  that is, in turn, coupled to the detector  120 . 
       FIG. 3  illustrates an IOC  115  according to an embodiment of the invention. The IOC  115  includes a first proton-exchanged waveguide portion  200 . The first waveguide portion  200  includes an input-port section  205  terminating in a junction section (y-junction)  210  from which first and second branch sections  215 ,  220  are formed. The first and second branch sections  215 ,  220  include respective bent regions  260 ,  265 . The bent regions  260 ,  265  may have an angular “elbow” configuration as illustrated in  FIG. 3 , or may be configured with a less-severe, more rounded radius of curvature than that illustrated. 
     Titanium-diffused waveguide portions  225 ,  230  are respectively coupled to the first and second branch sections  215 ,  220 . First and second modulators, such as electrodes  235 ,  240 , are respectively coupled to the waveguide portions  225 ,  230 . Each of the electrodes  235 ,  240  provide respective modulating voltages generating respective electric fields. The IOC  115  may further include second and third proton-exchanged waveguide portions  245 ,  250  coupled to the waveguide portions  225 ,  230 . 
     An approach to solving the RDS problem for gyroscopes includes, in an embodiment, a method called “stitching.” Stitching involves creating connected segments of Ti-diffused and proton-exchanged waveguides on the same substrate  255 . 
     Referring again to  FIG. 3 , in an embodiment, the waveguide portions  225 ,  230  are stitched, or otherwise coupled, to the first and second branch sections  215 ,  220  at respective locations  270 ,  275  where the electric fields produced by the electrodes  235 ,  240  are substantially zero. Additionally, the second and third proton-exchanged waveguide portions  245 ,  250  are stitched to the waveguide portions  225 ,  230  at respective locations  280 ,  285  where the electric fields produced by the electrodes  235 ,  240  are substantially zero. As such, the stitching occurs far enough from the electrodes  235 ,  240  such that the proton-exchanged waveguides  200 ,  245 ,  250  are unaffected by electric fields associated with modulation voltages. 
     Additionally, and preferably, the respective locations  270 ,  275  are approximately halfway between the electrodes  235 ,  240  and the bent regions  260 ,  265 . As such, the stitching occurs a distance away from the bent regions  260 ,  265  sufficient to avoid modal transition effects that may occur at the bent regions. 
     Further advantages to the approach illustrated in  FIG. 3  may be described in the following context: 
     Linearly polarized light propagating along the fast or slow axis of a birefringent material such as LiNbO 3  will remain in that axis, as coupling between the axes cannot occur for the reason that it is not possible to phase match the light in both beams simultaneously. 
     Since waveguides may be physically formed by well known processes for diffusing Ti or H+ along the crystal planes which develop the birefringence in the crystal, the angular alignment between the fast and slow axes of the stitched waveguides is virtually perfect, a property that maintains the very high extinction ratio provided by the proton exchange waveguides. 
     In anisotropic substances such as a birefringent crystal, electric vectors oscillate normal to the propagation vector in orthogonal planes (H and V). The azimuths and refractive indices of H and V are determined by the stoichiometric arrangement of the molecules comprising the crystal. The refractive index is proportional to the area density of atoms in the respective H and V planes (viz., atoms/mm 2 ); the birefringence is proportional to the difference of the refractive indices along the planes. 
       FIG. 4  illustrates the extinction ratio of a misaligned stitch. The extinction ratio obtained at a stitch is determined by angular misalignment ε of the field oscillations with respect to the axes in the crystal and is the ratio of the intensities in the H- and V-axes as follows: 
     
       
         
           
             
               
                 
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     for small ε. The extinction ratio is commonly expressed in decibels (dB) as 10·log [ε 2 ]. 
     In the embodiment illustrated in  FIG. 3 , the stitching occurs in portions of the waveguides that are parallel, or very nearly parallel, to the crystal planes. 
     Moreover, the LiNbO 3  crystal planes determine the alignment of both the birefringent axes in Ti-diffused waveguides, and the pass axis of the light in proton-exchanged waveguides. This makes the angular alignment at the stitch nearly perfect, thus avoiding gyro rate errors due to angular misalignments in the IOC. 
     Additionally, the extinction ratio of the stitched waveguide IOC  115 , which includes polarizing proton-exchanged waveguides and Ti-diffused waveguides, is substantially the same as that of a proton-exchanged IOC. 
     While the 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.