Abstract:
Effective relative intensity noise (RIN) subtraction systems and methods for improving ARW performance of a depolarized gyros. This invention taps the RIN detector light in the sensing loop, after the light transmits through the depolarizer and the coil but before it combines with the counter propagating lightwave. The tapped RIN lightwaves are polarized with pass-axis orientated in the same direction as that of the IOC, so that the RIN detector receives lightwaves with spectrum substantially identical to that of the rate detector, leading to more effective RIN subtraction.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Relative intensity noise (RIN) is one of the major contributors to interferometric fiber optic gyro (IFOG) angle random walk (ARW). Electric intensity-noise-subtraction has been used to reduce the RIN in order to improve the gyro performance. However, for depolarized single mode (SM) IFOGs, the RIN subtraction has not been as effective as that for polarization maintaining (PM) IFOGs due to mismatch between the lightwave spectra at the RIN detector and that at the rate detector. This spectrum mismatch is originated from the gyro depolarizer and the birefringence of the single mode coil fiber. 
         [0002]      FIG. 1  shows a typical prior art RIN subtraction scheme in a depolarized fiber optic gyroscope  100 . The lightwave emit from a light source  110  is directed by a 2×2 fiber coupler  120  into the input waveguide  131  of an integrated optical circuit (IOC)  130 . A Y junction splitter/combiner  132  of the IOC splits the input lightwaves into substantially equal parts, one of them (the CW light) is directed to waveguide  134  and the other (the CCW light) to waveguide  135 . The lightwaves in the waveguides  134 ,  135  are phase modulated by a modulator  133  and then coupled into a depolarizer section  140  having polarization maintaining fibers  141 ,  143 ,  144  and  146  that are spliced at splices  142  and  145  having 45° angles between the polarization axes of the adjacent PM fibers. Ends  151  and  152  of a non-PM single mode (SM) coil fiber  150  are connected to the PM fiber  143  and  146 , respectively. The returned CW and CCW lightwaves passing through the coil fiber  150  are recombined at Y junction (combiner  132 ) and propagate to a rate detector  160  after being directed by the 2×2 fiber coupler  120  from a port  122  to a port  123 . 
         [0003]    In this prior art, the light source intensity noise is typically measured at a port  124  of the 2×2 fiber coupler  120  by a RIN detector  170  and then electronically subtracted from the gyro rate detector signals after proper delays. This RIN tapping scheme works well for a PM gyro (not shown in  FIG. 1 ) which uses a PM fiber coil and no depolarizers, because the light spectra at the rate and RIN detector are substantially identical. In such a PM gyro, the polarization states of all the spectral components of the light source are aligned with the pass axis of the sensing loop and can reach the rate detector. However, this is not the case for the depolarized gyroscope. In the depolarized gyro shown in  FIG. 1 , the polarization states of different spectral components of the light source are not preserved along the optical path because the coil fiber is not polarization maintaining (PM) and there are 45° splices in the depolarizers. The polarization states of the spectral components may not be orientated parallel to the pass-axis of the IOC after transit through the optical circuit, thus being attenuated or blocked from entering the rate detector. As shown in  FIG. 2 , the lightwave spectrum  180  at the rate detector is spectrally modulated/channelized and significantly different from the lightwave spectrum  182  at the RIN detector. This spectral mismatch between rate and RIN causes significant degradation of RIN subtraction efficiency. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a more effective RIN subtraction method which will significantly improve the ARW performance of the depolarized gyros. This invention uses a scheme that taps the RIN detector light in the sensing loop, after the light transits through the depolarizer and the coil but before it combines with the counter propagating lightwave. The tapped RIN lightwaves are polarized by a following polarizer with pass axis orientated in the same direction as that of the IOC. In this way, the RIN detector receives lightwaves with spectrum substantially identical to that of the rate detector, leading to more effective RIN subtraction. Since the rotation and modulation induced lightwave phase variations are not converted to intensity variations at the RIN detector (interference with the counter propagating light does not happen), only the unwanted intensity noise is subtracted out from the rate signal. This scheme has an additional advantage of easy satisfaction of the delay requirement of the RIN signal relative to the rate signal without using a long delay fiber or additional electronics. Furthermore, since the RIN signals are tapped after the light transit through the coil fiber (up to a few kilometers), the intensity fluctuations originated from changes of coil loss are identical to both rate and RIN detector, leading to more efficient noise subtraction. 
         [0005]    In the present invention, there are two preferred embodiments of the invention. One is to insert a PM-coupler at one of the IOC output fibers connected to the depolarizer. This coupler couples a small amount of light that transits through the coil and directs it to the RIN detector. Properly polarizing the RIN light and isolating it from returning to the sensing loop are necessary. Another embodiment requires a new design of the integrated optical circuit (IOC) which incorporates the RIN tap coupler and the polarizer inside the IOC. This scheme increases the level of component integration and helps to realize a more compact gyro design. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0007]      FIG. 1  is a schematic view of a depolarized gyroscope with a typical prior art RIN subtraction scheme; 
           [0008]      FIG. 2  is a plot of lightwave spectra at RIN and rate detector for a typical depolarized gyroscope; 
           [0009]      FIG. 3  is a schematic view of an optical circuit according to an embodiment of the invention; and 
           [0010]      FIG. 4  is a schematic view of an optical circuit according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Relative intensity noise (RIN) of a broadband light source originates from beating of the different optical frequency components contained in the light source. For an interferometric fiber optic gyroscope (IFOG), the RIN at the rate detector is determined by the light source spectrum reached the detector. In order to effectively subtract the intensity noise from the rate detector, another detector (RIN detector) dedicated to record the RIN of the light source at a different place of the optical circuit is desirable to receive light with substantially the same spectrum as that of the rate detector. The present invention describes systems and methods for effective RIN subtraction in depolarized gyros using matched light spectra at RIN and rate detectors. 
         [0012]    Referring to  FIG. 3 , according to one embodiment of the present invention, a depolarized gyro  200  includes a light source  210 , a directional coupler  220 , an integrated optical circuit (IOC)  230 , and a fiber loop  250 . These elements may be identical to the elements  110 ,  120 ,  130 , and  150  shown in  FIG. 1 , respectively. Lightwaves emitted from source  210  are coupled into an input waveguide  231  of the IOC  230  and split at a Y-shape splitter/combiner  232  into CW and CCW propagating waves. 
         [0013]    Before being coupled into the fiber coil  250 , the CW (CCW) light in a waveguide  234  ( 235 ) first propagates to an upper (lower) of a depolarizer  240  section that includes a PM fiber  241  ( 284 ,  285 ) and  243  ( 246 ). The polarization pass-axis of the IOC  230  is aligned with that of the PM fiber  241  ( 284 ,  285 ), and the polarization axes of  241  ( 284 ) and  243  ( 246 ) are orientated 45° with respect to each other at the fiber splice  242  ( 245 ). In such a configuration, each wavelength component of the broadband light source launched into the coil will have a different polarization state ranging from linear to elliptical to circular shapes that in total form a nearly depolarized light. CW (CCW) lightwaves exiting end  252  ( 251 ) of the fiber coil is coupled into the lower (upper) depolarizer section that comprises PM fiber  246  ( 243 ),  284  ( 241 ) and the 45° splice  245  ( 242 ) connecting them. 
         [0014]    The CW light recombines with the CCW light at splitter/combiner  232 . Only wavelength components with non-zero intensity along the pass-axis of the IOC  230  reach the rate detector  260  after being directed by the coupler  220 . A typical light spectrum at the rate detector is shown by the light spectrum  180  in  FIG. 2 . 
         [0015]    Different from the prior art shown in  FIG. 1 , a PM coupler  281  is inserted between the second IOC waveguide  235  and the 45° splice  245  in the lower depolarizer as shown in  FIG. 3 . The coupler  281  passes a substantial amount of light propagating from the second lower PM fiber  284  to the first lower PM fiber  285  and vice versa. The coupler  281  couples a small fraction of CW light (propagating from the second lower PM fiber  284  to the first lower PM fiber  285 ) into a port  283 . A polarizer  287  with polarization axis orientated identical to that of the IOC  230  passes the same wavelength components to an RIN detector  270  as those reaching a rate detector  260 . An isolator  286  prevents any back-reflected light from entering the sensing loop. 
         [0016]    It can be theoretically proved that the lightwave spectrum at the RIN detector  270  is identical to that at the rate detector  260 . The lightwaves from the broadband light source  210  are unpolarized. The lightwaves are polarized by the waveguides of the IOC  230 . The CW lightwave at the combiner  232  after transmitting through the whole fiber loop  250  can be expressed by the Jones matrix method. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       E 
                       CW 
                     
                     = 
                     
                       
                         ( 
                         
                           
                             
                               
                                  
                                 
                                    
                                    
                                   
                                       
                                   
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               ɛ 
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     4 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                           
                             
                               
                                 
                                   - 
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     3 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           ( 
                           
                             
                               
                                 A 
                               
                               
                                 B 
                               
                             
                             
                               
                                 C 
                               
                               
                                 D 
                               
                             
                           
                           ) 
                         
                         · 
                         
                           ( 
                           
                             
                               
                                 1 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                    
                                   
                                     
                                       - 
                                        
                                     
                                      
                                     
                                         
                                     
                                      
                                     
                                       t 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                           
                             
                               
                                 
                                   - 
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     1 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                  
                                 
                                    
                                    
                                   
                                       
                                   
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               ɛ 
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 E 
                                 
                                   0 
                                    
                                   x 
                                 
                               
                             
                           
                           
                             
                               
                                 E 
                                 
                                   0 
                                    
                                   y 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                       
                   
                    
                   
                     
                       E 
                       CWx 
                     
                     = 
                     
                       
                         1 
                         2 
                       
                        
                       
                         E 
                         
                           0 
                            
                           x 
                         
                       
                        
                       
                         
                            
                           
                             
                                
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 φ 
                                 B 
                               
                             
                             + 
                             
                               φ 
                               R 
                             
                           
                         
                          
                         
                           ( 
                           
                             A 
                             - 
                             
                               B 
                                
                               
                                   
                               
                                
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     2 
                                   
                                 
                               
                             
                             + 
                             
                               C 
                                
                               
                                   
                               
                                
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     3 
                                   
                                 
                               
                             
                             - 
                             
                               D 
                                
                               
                                   
                               
                                
                               
                                  
                                 
                                   
                                     
                                       - 
                                        
                                     
                                      
                                     
                                         
                                     
                                      
                                     
                                       t 
                                       2 
                                     
                                   
                                   - 
                                   
                                      
                                      
                                     
                                         
                                     
                                      
                                     
                                       t 
                                       3 
                                     
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0017]    In the above expression, E 0x  and E 0y  are the electric field of the input light polarized along pass- and block-axis of the IOC  230 . Without lost of generality, it is assumed here that the x-polarized light E 0x  is orientated along the IOC pass-axis, and the y-polarized light E 0y  is orientated along the IOC block axis. t 1 , t 2 , t 3 , and t 4  are the phase delays incurred by the birefringent slow axis of ( 234 + 241 ),  243 ,  246  and ( 235 + 285 + 284 ) relative to their corresponding fast axis. φ B  is the bias modulation phase applied at a modulator  233 , and φ R  is the rotation induced Sagnac phase. A, B, C, and D are the wavelength dependent Jones matrix elements of the SM coil in the fiber loop  250  which can be measured or simulated. ε is the polarization amplitude extinction ratio of the IOC  230 . When the IOC  230  has high polarization extinction ratio, the y-component of the electric field is negligibly small. Only the x-polarized light will reach the detector. 
         [0018]    The CCW lightwave at the combiner  232  after it is transmitted through the fiber loop  250  can be similarly expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       E 
                       CCW 
                     
                     = 
                     
                       
                         ( 
                         
                           
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               ɛ 
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     1 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 
                                   - 
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                           
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     2 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           ( 
                           
                             
                               
                                 A 
                               
                               
                                 B 
                               
                             
                             
                               
                                 C 
                               
                               
                                 D 
                               
                             
                           
                           ) 
                         
                         · 
                         
                           ( 
                           
                             
                               
                                 1 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                    
                                   
                                     
                                       - 
                                        
                                     
                                      
                                     
                                         
                                     
                                      
                                     
                                       t 
                                       3 
                                     
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 
                                   - 
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                           
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                             
                               
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 45 
                                  
                                 ° 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               1 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     4 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                  
                                 
                                    
                                    
                                   
                                       
                                   
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               ɛ 
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 E 
                                 
                                   0 
                                    
                                   x 
                                 
                               
                             
                           
                           
                             
                               
                                 E 
                                 
                                   0 
                                    
                                   y 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                       
                   
                    
                   
                     
                       E 
                       CCWx 
                     
                     = 
                     
                       
                         1 
                         2 
                       
                        
                       
                         E 
                         
                           0 
                            
                           x 
                         
                       
                        
                       
                         
                            
                           
                             
                               
                                 - 
                                  
                               
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 φ 
                                 B 
                               
                             
                             - 
                             
                               φ 
                               R 
                             
                           
                         
                          
                         
                           ( 
                           
                             A 
                             - 
                             
                               B 
                                
                               
                                   
                               
                                
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     2 
                                   
                                 
                               
                             
                             + 
                             
                               C 
                                
                               
                                   
                               
                                
                               
                                  
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   
                                     t 
                                     3 
                                   
                                 
                               
                             
                             - 
                             
                               D 
                                
                               
                                   
                               
                                
                               
                                  
                                 
                                   
                                     
                                       - 
                                        
                                     
                                      
                                     
                                         
                                     
                                      
                                     
                                       t 
                                       2 
                                     
                                   
                                   - 
                                   
                                      
                                      
                                     
                                         
                                     
                                      
                                     
                                       t 
                                       3 
                                     
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0019]    The total field that reaches the rate detector  260  is 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           E 
                           rate 
                         
                         = 
                           
                          
                         
                           
                             1 
                             2 
                           
                            
                           β 
                            
                           
                               
                           
                            
                           
                             
                               E 
                               
                                 0 
                                  
                                 x 
                               
                             
                              
                             
                               ( 
                               
                                 A 
                                 - 
                                 
                                   B 
                                    
                                   
                                       
                                   
                                    
                                   
                                      
                                     
                                       
                                         - 
                                          
                                       
                                        
                                       
                                           
                                       
                                        
                                       
                                         t 
                                         2 
                                       
                                     
                                   
                                 
                                 + 
                                 
                                   C 
                                    
                                   
                                       
                                   
                                    
                                   
                                      
                                     
                                       
                                         - 
                                          
                                       
                                        
                                       
                                           
                                       
                                        
                                       
                                         t 
                                         3 
                                       
                                     
                                   
                                 
                                 - 
                                 
                                   D 
                                    
                                   
                                       
                                   
                                    
                                   
                                      
                                     
                                       
                                         
                                           - 
                                            
                                         
                                          
                                         
                                             
                                         
                                          
                                         
                                           t 
                                           2 
                                         
                                       
                                       - 
                                       
                                          
                                          
                                         
                                             
                                         
                                          
                                         
                                           t 
                                           3 
                                         
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                           
                          
                         
                           ( 
                           
                             
                                
                               
                                 
                                   
                                     - 
                                      
                                   
                                    
                                   
                                       
                                   
                                    
                                   2 
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                                 - 
                                 
                                   φ 
                                   R 
                                 
                               
                             
                             + 
                             
                                
                               
                                 
                                    
                                    
                                   
                                       
                                   
                                    
                                   2 
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                                 + 
                                 
                                    
                                    
                                   
                                       
                                   
                                    
                                   
                                     φ 
                                     R 
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           β 
                            
                           
                               
                           
                            
                           
                             
                               E 
                               
                                 0 
                                  
                                 x 
                               
                             
                              
                             
                               ( 
                               
                                 A 
                                 - 
                                 
                                   B 
                                    
                                   
                                       
                                   
                                    
                                   
                                      
                                     
                                       
                                         - 
                                          
                                       
                                        
                                       
                                           
                                       
                                        
                                       
                                         t 
                                         2 
                                       
                                     
                                   
                                 
                                 + 
                                 
                                   C 
                                    
                                   
                                       
                                   
                                    
                                   
                                      
                                     
                                       
                                         - 
                                          
                                       
                                        
                                       
                                           
                                       
                                        
                                       
                                         t 
                                         3 
                                       
                                     
                                   
                                 
                                 - 
                                 
                                   D 
                                    
                                   
                                       
                                   
                                    
                                   
                                      
                                     
                                       
                                         
                                           - 
                                            
                                         
                                          
                                         
                                             
                                         
                                          
                                         
                                           t 
                                           2 
                                         
                                       
                                       - 
                                       
                                          
                                          
                                         
                                             
                                         
                                          
                                         
                                           t 
                                           3 
                                         
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                           
                            
                           
                             cos 
                              
                             
                               ( 
                               
                                 
                                   2 
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                                 + 
                                 
                                   φ 
                                   R 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           β 
                            
                           
                               
                           
                            
                           
                             E 
                             
                               0 
                                
                               x 
                             
                           
                            
                           U 
                            
                           
                               
                           
                            
                           
                             cos 
                              
                             
                               ( 
                               
                                 
                                   2 
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                                 + 
                                 
                                   φ 
                                   R 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0020]    Where β is a coefficient that takes into account the amplitude loss of light propagating from the combiner  232  to the rate detector  260 . U is a simplifying symbol that stands for the expression in the first parentheses of the above equation. The intensity at the rate detector  260  is 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     rate 
                   
                   = 
                   
                     
                       1 
                       2 
                     
                      
                     
                       β 
                       2 
                     
                      
                     
                       
                          
                         
                           E 
                           
                             0 
                              
                             x 
                           
                         
                          
                       
                       2 
                     
                      
                     
                       
                         
                            
                           U 
                            
                         
                         2 
                       
                        
                       
                         [ 
                         
                           1 
                           + 
                           
                             cos 
                              
                             
                               ( 
                               
                                 
                                   4 
                                    
                                   
                                     φ 
                                     B 
                                   
                                 
                                 + 
                                 
                                   2 
                                    
                                   
                                     φ 
                                     R 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0021]    Since the A, B, C, and D matrix elements of the SM fiber coil  250  depend on wavelength, |U| 2  is a function of wavelength and describes the light power spectral distribution at the rate detector  260 . The rate signal at the rate detector  260  contains the Sagnac phase that can be demodulated for rotation rate sensing. 
         [0022]    The light that reaches the RIN detector  270  does not combine with its counter-propagating part and is not bias modulated. The intensity of the light that reaches the RIN detector  270  is 
         [0000]        E   RIN   =αE   0x   e   iφ     B   ( A−Be   −it     2     +Ce   −it     3     −De   −it     2     −it     3   )=α E   0x   e   iφ     B     U    (5) 
         [0000]        I   RIN =α 2   |E   0x | 2   |U|   2    (6) 
         [0000]    where α is the amplitude loss incurred by the RIN coupler and path to the RIN detector  270 . When comparing Equation 4 with Equation 6, the lightwave spectrum reaching the RIN detector  270  is the same as that at the rate detector  260 , both described by |U| 2 . However, the signal produced by the RIN detector  270  does not contain any intensity variation from Sagnac phase and bias modulation. This is ideal for RIN subtraction because the Sagnac phase induced intensity variation shall not be removed during RIN subtraction and are useful in the demodulation process for rate sensing. 
         [0023]    The coupler  281  can also be placed between the IOC  230  waveguide  234  and the 45° splice  242  in the upper section of the depolarizer  240 . The above theory showed that the spectrum of CW and CCW light tapped between the splitter/combiner  232  and the 45° splices  245  and  242  after transiting through the sensing loop is identical. The two configurations are equivalent and are considered covered by the same embodiment shown in  FIG. 3 . 
         [0024]    To reduce the polarization induced bias errors that the PM coupler in the sensing loop might introduce, PM couplers with as small as possible polarization cross-couplings are used, e.g. smaller than −25 dB. The coupler shall be located as close as possible to the 45° splice so that even in cases where polarization cross-couplings cannot be avoided in the coupler, their location can be substantially close to that of the 45° splice and introduce negligible bias errors. This provides easier design for the depolarizer. 
         [0025]      FIG. 4  shows example of a gyro  300  formed according to another embodiment of the present invention. According to this embodiment, a RIN waveguide coupler  381  is incorporated into the IOC  230  so that it will not affect packaging of the depolarizer fiber. The RIN waveguide coupler  381  in an IOC  330  couples a small fraction of light into a waveguide  383  also in the IOC  330 . The waveguide  383  directs the light to a RIN detector  370 . The waveguide  383  is a polarizing waveguide that polarizes the light in the same way as other waveguides in the IOC  330 , so the spectrum of light reaching the RIN detector  370  is substantially identical to that which reaches a rate detector  360 . To prevent back-reflected light going into the sensing loop, the IOC interface  384  to the output coupling fiber has to be angle polished and an isolator  386  is inserted between the RIN detector  370  and the IOC  330 . Another end  382  of the RIN waveguide coupler  381  is properly terminated to prevent any light reflections from re-entering the IOC  330 . 
         [0026]    Essentially in both embodiments, the RIN light is tapped inside the sensing loop before the light recombines with the counter-propagating lightwave but after the light transits through the coil and the depolarizers. The two embodiments shown in  FIG. 3  and  FIG. 4  are exemplary and by no means to limit realization of this concept to gyro having two depolarizers on each end of the coil fiber. Other gyros with different numbers of depolarizers placed at different locations of the optical circuits may use this invention as long as the light spectrum at the RIN detector are substantially identical to that at the rate detector. 
         [0027]    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.