Abstract:
A fiber optic ring interferometer, such as an interferometric fiber optic gyro (IFOG), with a sensing loop doubler. The IFOG device includes a light source, a circulator in optical communication with the light source and a photo diode, an integrated optical chip (IOC) capable of splitting light emitted from the circulator into two paths each of which is capable of being modulated, the IOC further being capable of combining light from each of the two paths when light travels in a direction towards the circulator, a fiber coil, and a polarization maintaining (PM) combiner/splitter disposed in between the IOC and the fiber coil. The PM combiner/splitter operates to send light through the fiber coil a first time in a first polarization state and to subsequently send the same light a second time through the fiber coil in a second orthogonal polarization state, whereby light travels twice the distance compared to a single loop IFOG device and, as such, effectively doubles the LD product.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to interferometric fiber optic gyroscopes (IFOGs). More particularly, the present invention relates to an IFOG having double the sensitivity, reduced angle random walk (ARW), and increased bias stability compared to an IFOG having the same fiber coil length and loop diameter. The present invention also relates to an IFOG that has the same gyro performance of a conventional IFOG, but that uses only half the fiber coil length of the conventional device. 
   2. Background of the Invention 
   As is well-known, in an interferrometric fiber optic gyroscope (IFOG), an optical fiber is wound in a circular or looped shape and acts as a sensing device to detect a Sagnac phase difference for two counter-propagating waves in presence of rotation. The product of the length and diameter (LD product) of the wound fiber sensing-coil determines, to a large degree, the sensitivity of the gyroscope. Generally speaking, the greater the LD product, the greater the sensitivity and stability of the device, and the lower the random noise. However, large coil volume resulting from greater fiber length or larger loop diameter (to achieve a higher LD product) requires increasingly bulky packages for the gyroscope device, especially when compared to other technologies such as the ring laser gyroscope (RLG) or hemisphere resonant gyroscope (HRG). 
   BRIEF SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to improve the performance of an IFOG device without increasing its LD product by physically increasing dimensions, or, alternatively, to maintain the performance of a given IFOG device, but doing so with approximately only half the fiber coil length. The present invention is based on a new technique that routes light inside the fiber coil twice in two orthogonal polarization states, respectively. The polarization-state of the traveling light is preferably controlled by a polarization-maintaining (PM) combiner/splitter. This technique either doubles the sensitivity, and reduces the angle random walk (ARW) and bias instability with the same fiber coil length, or maintains the gyro performance of a given IFOG using only half of fiber coil length. 
   More specifically, polarization maintaining combiner/splitters are used to couple only one polarization state of light in and out of the fiber coil. The light traveling inside the fiber is rotated 90 degrees after one pass by using a 90 degree splice on the PM fiber coil. Light is then routed back to the fiber coil through the PM combiner/splitter for a second pass through the coil. After the second pass, the light is again rotated 90 degrees back to the original polarization state, and coupled out through the PM combiner/splitter to a phase modulator such as integrated optics chip (IOC). At least two embodiments using PM combiner/splitters can be implemented to achieve the functionality described. One embodiment uses only one PM combiner/splitter and one 90 degree splice. A second embodiment uses two PM combiner/splitters and one 90 degree splice. The first embodiment is relatively simple and requires minimal change to optical circuits. The second embodiment has more symmetry and is not subject to the drift of the PM combiner/splitter because the light travels along both the cross and through legs of the each PM combiner/splitter once. 
   A third embodiment relies not on PM combiner/splitters, but instead on a polarization beam splitter that can be implemented using, for example, bulk optics. 
   In an implementation of the present invention, an IFOG test bed with a single sensing loop was assembled first for establishing baseline performance. The proper frequency was measured at 24.6 kHz, and bias data were taken with ARW at about 500 μdeg/rt-hr. The test bed was then modified, in accordance with the first embodiment of the present invention, for achieving a doubled sensing loop by incorporating a Polarization Maintaining (PM) combiner/splitter and applying at least one 90° splice to redirect light in fast and slow axes. Fast and slow axes are the two principal axes of PM fiber, in which light travels faster (slower) in the fast (slow) axis with smaller (larger) index of refraction. The double pass through the sensing loop was verified by the proper frequency measured at about 12.3 kHz, half of that of the single loop configuration. In other words, the loop-transit time (the time for light to travel from one side of an Integrated Optical Chip (IOC) through fiber coil to the other side of the IOC) doubles in the double sensing loop configuration. The bias data was measured with ARW at about 300 μdeg/rt-hr. Thus, the ARW result shows about 40% improvement compared to the single loop ARW measurement. The difference from the theoretical 50% improvement is due to the increase of the thermal phase noise for twice the fiber path. 
   A more complete understanding of the present invention can be gleaned from the following detailed description in combination with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a first embodiment of an IFOG sensing loop doubler in accordance with the present invention. 
       FIG. 2  illustrates the polarization configuration of the PM combiner/splitter in accordance with the present invention. 
       FIG. 3  shows the polarization configuration of a 90 degree splice on PM fibers. 
       FIG. 4  shows the optical path and polarization state of light traveling through the doubler device according to the first embodiment of the present invention. 
       FIG. 5  shows a splicing configuration of a variation of the first embodiment on which experiments were conducted. 
       FIGS. 6A and 6B  show, respectively, bias measurement and cluster analysis for a single loop configuration with data taken at 8 second intervals. 
       FIGS. 7A and 7B  show, respectively, bias measurement and cluster analysis for the double loop configuration in accordance with the present invention with data taken at 1 second intervals. 
       FIG. 8  shows a second embodiment including two PM Combiner/Splitters in accordance with the present invention. 
       FIG. 9  shows the optical path and polarization state of light traveling through the doubler of the second embodiment of the present invention. 
       FIGS. 10A and 10B  illustrate a polarization beam splitter that can be used instead of a PM Combiner/Splitter, in accordance with a third embodiment of the present invention. 
       FIGS. 11 and 12  illustrate how the polarization beam splitter is employed in the same general architecture as that of the first embodiment, in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A detailed description of the present invention is set forth below in four sections. In section I, the structure of the sensing doubler is described. Section II shows measurements of a typical fused type of PM combiner/splitter. An optical test bed assembly is described in section III. Testing results of the sensing loop doubler and comparison with a single loop configuration are discussed in section IV. Finally, in section V, a second embodiment of the present invention is described. 
   I. Structure of the Sensing Loop Doubler 
   One way of effectively doubling the sensing loop (and thus the LD product), in accordance with the present invention, is to manage the polarization-state of the traveling light such that it will propagate inside PM fiber twice in two orthogonal states, respectively.  FIG. 1  depicts the configuration of an exemplary IFOG sensing loop doubler device. It differs from a single loop configuration by including an extra PM Combiner/Splitter  110  in addition to at least one 90° splice for redirecting light into fast or slow axes. The function of PM Combiner/Splitter  110  is to control the light polarization into specific output ports. Fiber light source (FLS)  102  provides light to circulator (or 50/50 coupler)  104 . An integrated optics chip (IOC) (or 50/50 coupler)  106  then splits the light into two counter-propagating waves that travel through the fiber coil  108 . After the counter-propagating waves re-emerge from fiber coil  108 , they interfere at the left end of IOC  106  and travel back again through circulator  104  and exit to a photodiode  112 . With the proper modulation applied via IOC electrodes  106   a–d , signals from photodiode  112  can be demodulated to detect the proper frequency and rotation rate, as is well known in the art. 
   In a single sensing loop configuration, there is no Polarization Maintaining (PM) Combiner/Splitter  110 , as there is in the preferred configuration of the present invention. In a conventional sensing loop, the counter-propagating waves pass through a fiber coil only once, and exit through an IOC. By adding PM Combiner/Splitter  110 , it is possible to control the cross and through exit of the light based on the polarization of the incident light. In a preferred implementation, PM Combiner/Splitter  110  can be made by fusing two PM fibers together. Alternatively, PM Combiner/Splitter  110  can be made by pressing two fibers together after polishing, or can be made using free-space micro-optics. Depending on the configuration of PM Combiner/Splitter  110 , incident light with 0° polarization direction will go across (through) PM Combiner/Splitter  110 , while 90° polarization incident light will go through (across) the PM Combiner/Splitter, as shown in  FIG. 2 . 
   The polarization of the light traveling inside PM fiber can be changed by splicing techniques. For example, the light polarization can be changed by 180° by breaking the fiber and splicing it back together at 180°. The light can also be rotated 90° with a 90° splice of PM fibers, similar to a half wave plate to rotate light by 90°.  FIG. 3  shows the polarization configuration of a 90° splice following the right-hand rule. The left-hand rule can also be achieved by splicing the fibers to achieve a −90° orientation. 
     FIG. 4  shows the optical path and polarization state of light traveling through the doubler device. Assume the light coming out of the upper path IOC  106  has a polarization direction at 0° (counter-clockwise wave). At the 0° splice # 1 , the light is maintained at 0° polarization. It then goes cross PM Combiner/Splitter  110 , and exits through splice # 3 . The 90° splice at # 3  rotates the polarization 90°. It travels through fiber coil  108  and splice # 2  without changing the polarization direction at 90°. This time, because the polarization direction is 90°, light goes through the PM splitter and exits at splice # 3  again. It travels back through the fiber coil for a second time, but with polarization direction at 180°. When it exits the fiber coil, it goes across the PM Combiner/Splitter and exits at splice # 4  toward the lower path of IOC  106 . The light trace for the clockwise wave is similar to the counter-clockwise wave described above. These two waves meet together again at the left end of IOC  106 , but, in accordance with the present invention, each wave has traveled twice inside the fiber coil. 
   II. PM Combiner/Splitter Measurement 
   A PM Combiner/Splitter was obtained for evaluation of its polarization performance. Two device samples were obtained from SIFAM Fibre Optics Limited (Torquay, Devon, England). Data for the SIFAM PM Combiner/Splitters is shown in Tables 1 and 2. 
   Measurement of extinction ratio and isolation ratio were performed for each sample. The extinction ratio is a measure of the polarization error out of the same output port with maximum output. The isolation ratio is a measure of the light leakage on the other output port. For the extinction ratio measurement, polarized light is incident on an input port (e.g., A), and the output port (e.g., X) is measured through a polarizer. The incident light and polarizer are adjusted such that the maximum light output is obtained. The polarizer is then rotated by 90°. The ratio of the two results determines the extinction ratio. The same procedure is applied for different input ports and output ports to obtain all the extinction ratios. 
   Samples of the SIFAM device show good extinction with an average of about 30 dB. This high extinction ratio is sufficient to ignore the polarization error due to the PM Splitter/Combiner because the PM fiber coil has higher polarization error, or lower extinction ratio (17.8 dB in a measurement on 4 km of fiber). 
   The same procedure for measuring the extinction ratio is followed for measuring the isolation ratio. After the maximum output at the exit port is obtained, the other exit ports are measured for leakage light. The ratio of the two results determines the isolation ratio. In the design of a PM Combiner/Splitter suitable for use in the present invention, the slow axis is the pass through axis. If there is incident light on the slow axis, there is higher isolation ratio on the through exit port than the cross exit port. If the incident light is on the fast axis, there is maximum light output on the cross exit port. The smaller isolation ratio on the fast axis gives more leakage on the through exit port. In other words, the slow axis isolation ratio is better than that of the fast axis. 
   In the SIFAM PM Combiner/Splitter isolation ratio measurement, there is a larger difference on the slow and fast axes. This is believed to be due to the wavelength difference between the FLS employed and the optimized wavelength in the manufacturing of the PM Combiner/Splitter. A fused type of PM Combiner/Splitter typically shows better stability over temperature and humidity compared to the polished type. 
   It is noted that free-space micro-optics can also be used to fabricate PM combiner/splitter in combination with other optical components to obtain a the compact package. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               SIFAM PM Combiner/Splitter Measurement Data (Sample 1) 
             
           
        
         
             
               Input 
               Output 
               Extinction Ratio 
               Input 
               Isolation Ratio 
             
             
               Port 
               Port 
               (dB) 
               Polarization 
               (dB) 
             
             
                 
             
           
        
         
             
               A 
               X 
               32.2 
               Fast Axis 
               10.7 
             
             
                 
               Y 
               32.5 
               Slow Axis 
               29.2 
             
             
               B 
               Y 
               31.5 
               Slow Axis 
               29.0 
             
             
                 
               X 
               34.1 
               Fast Axis 
               11.1 
             
             
               X 
               A 
               30.3 
               Fast Axis 
               10.9 
             
             
                 
               B 
               31.1 
               Slow Axis 
               30.3 
             
             
               Y 
               B 
               29.6 
               Slow Axis 
               30.2 
             
             
                 
               A 
               30.8 
               Fast Axis 
               11.0 
             
             
                 
             
           
        
       
     
   
                                                               TABLE 2                   SIFAM PM Combiner/Splitter Measurement Data (Sample 2)            Input   Output   Extinction Ratio   Input   Isolation Ratio       Port   Port   (dB)   Polarization   (dB)                    A   X   29.8   Fast Axis   14.2           Y   30.8   Slow Axis   27.4       B   Y   29.8   Slow Axis   26.7           X   29.2   Fast Axis   14.1       X   A   32.0   Fast Axis   14.0           B   30.3   Slow Axis   26.7       Y   B   29.7   Slow Axis   26.9           A   32.8   Fast Axis   14.3                    
III. Test Bed Assembly
 
   A test bed was assembled for demonstrating the sensing loop doubler illustrated schematically in  FIG. 1 . FLS  102  has an average wavelength of about 1550 nm and a bandwidth of 29.4 nm. IOC  106  has a channel length of 47 mm. PM fiber coil  108  comprised PM fiber of about 4 km in length. Underneath the coil was a rubber mat to prevent vibration, and also to serve as heat insulation from a metal plate below. All of the optical components were covered with a metal cover to prevent air flow, and thereby help to stabilize the temperature of the optical components. 
   More specifically, with reference to  FIG. 5  (a variation of the first embodiment in which the splices are all 90°), the procedure for splicing PM Combiner/Splitter  110  to the coil fiber and pigtail of IOC  106  is shown. Point # 1  is spliced first, followed by points # 2 , # 3 , and # 4  subsequently. The sequence can be chosen differently as long as good splices with the aid of the monitoring system can be made. The high isolation ratio (29.2 dB from Table 1) limits the light leakage back to IOC before entering PM fiber coil  108 . It reduces the interfered error caused by this straight pass-through leakage. It is noted that different combination of 0° or 90° could be used as long as the resulting configuration routes the light back into the sensing fiber for the second path. That is, the 90° splice (in  FIG. 1 , for example, need not necessarily be located at splice # 3 , but could be located elsewhere, as long as the result is that light is routed twice around the sensing fiber as a result of differences in polarization states. 
   More specifically, for the first embodiment, since 0 deg polarization light will go cross leg, and 90 deg polarization will go through the same leg (which is preferred because of fewer 90 degree splices), then the following splices are possible.
         Splice  1 , 0 deg;   Splice  2 , 0 deg;   Splice  3 , 90 deg;   Splice  4 , 0 deg   or   Splice  1 , 0 deg;   Splice  2 , 90 deg;   Splice  3 , 0 deg;   Splice  4 , 0 deg       

   If the PM Combiner/Splitter legs  1  and  2  are swapped (or legs  3  and  4  are swapped) then the following is also possible:
         Splice  1 , 90 deg;   Splice  2 , 0 deg;   Splice  3 , 90 deg;   Splice  4 , 90 deg   or   Splice  1 , 90 deg;   Splice  2 , 90 deg;   Splice  3 , 0 deg;   Splice  4 , 90 deg       

   Also, it is noted that the PM Combiner/Splitter can be anywhere inside the fiber coil (although the described location between the coil and IOC is relatively simple to implement). Likewise, the 90 degree splice on either splice  2  or splice  3  can also be anywhere inside the fiber coil. 
   The measurement of splice loss and extinction (isolation) ratio was measured upon splicing PM Combiner/Splitter  110  with coil fiber  108  and IOC  106 . Measurement indicates that coil fiber  108  has a loss of 5.12 dB and an extinction ratio of 17.8 dB. Polarization error causes light leakage to cross PM splitter before entering coil fiber  108  for the second pass. Single loop leakage has its own interferrogram, which is different from the one in the double loop configuration. The light leakage along the single loop is larger with lower extinction ratio of PM combiner/splitter. The higher loss on the coil fiber reduces the ratio between the double loop and single loop. In the configuration in accordance with the present invention, there is about 5% of single loop leakage compared to the double loop. Fortunately, the single loop leakage cancels out in the demodulation process. Table 3 shows data reflecting splice loss and extinction (isolation) ratio for the configuration shown in  FIG. 5 . 
                                     TABLE 3                   Measurement on splice loss and extinction (isolation) ratio.                    Optical           Splice   Port   Power   Remark               Splice #1   P1 (Pre-Splice)   −4.51 dBm                   (354 μW)           Y (Post-Splice)   −28.8 dBm   Isolation Ratio =               (1.32 μW)   24.1 dB           X (Post-Splice)   −4.73 dBm   Excess Loss =               (336.5 μW)   0.22 dB       Splice #2   P2 (Pre-Splice)   −4.37 dBm           B (Post-Splice)    −4.6 dBm           X (Post-Splice)   −4.74 dBm       Splice #3   X (Pre-Splice)    −4.7 dBm   Coil +                   Splice Loss = 5.12 dB           Coil (Pre-Splice)   −11.3 dBm,   Extinction Ratio =           90°   −29.14 dBm   17.8 dB                    
IV. Testing
 
   In a preferred testing set up, a hot plate is used to control the temperature of optical components. An light source controller is used for controlling FLS  102  with constant pump current and constant temperature controller. A function generator is used to generate the modulation signal into IOC  106 . Output from photo detector  112  is sent to a lock-in amplifier (LIA) for the demodulation processing. The data from the lock-in amplifier is then acquired by a data acquisition system (DAS) and saved with the rate information. 
   For a single loop configuration, a pump current of 67 mA is required to obtain 2V un-modulated signal at photo detector  112 . After initial set up, the proper frequency was measured first. An oscilloscope was used to measure the output of photo detector  112 . IOC  106  was driven with π/2 square wave modulation. In addition to the bias modulation, a sinusoidal wave with 64-times the proper frequency measurement was applied as the eigen-frequency modulation (EFM) signal. The bias frequency (and EFM frequency, accordingly) was adjusted to obtain the minimum side lobes on the spectrum analyzer at the EFM frequency. The proper frequency was measured at about 24.6 kHz. 
   After the proper frequency measurement, the gyro was stabilized in a temperature-controlled oven for about two days, in preparation for the bias measurement. The bias data was taken when square-wave modulated at π/2. The bias data is shown in  FIG. 6A .  FIG. 6B  shows the cluster analysis with ARW calculated at about 500 μdeg/rt-hr. Bias stability was better than 0.001 deg/hr. 
   For the double loop configuration, the pump current required to reach 2V un-modulated signal was 174 mA.  FIG. 7A  shows bias measurement again square-wave modulated at π/2 and  FIG. 7B  shows the cluster analysis for the double loop configuration. The ARW was calculated to be about 300 μdeg/rt-hr, which is about a 40% improvement compared to the single loop configuration. Lower than expected bias stability, about 0.003 deg/hr, was obtained. It is believed that the instability could be due to single loop leakage because of the low extinction ratio of PM coil fiber  108 . It could also be due to the extra length of PM Combiner/Splitter  110  arranged before the sensing coil. 
   V. Second Embodiment 
     FIGS. 8 and 9  illustrate a second embodiment of the present invention in which a pair of PM Combiner/Splitters are employed to achieve a sensing loop doubler. Here, a symmetric PM Coupler/Splitter configuration is achieved and light travels through and cross legs twice for each of the PM Coupler/Splitters. This provides a more balanced configuration. 
   Possible splices configuration for the symmetric configuration according to the second embodiment include the following: 
   Since 0 deg polarization light will go cross leg, and 90 deg polarization will go through the same leg (which is preferred because of fewer 90 degree splices), the following are possible splice arrangements:
         Splice  1 , 0 deg;   Splice  2 , 0 deg;   Splice  3 , 90 deg;   Splice  4 , 0 deg;   Splice  5 , 0 deg   or   Splice  1 , 0 deg;   Splice  2 , 90 deg;   Splice  3 , 0 deg;   Splice  4 , 0 deg;   Splice  5 , 0 deg       

   If the PM Combiner/Splitter legs  1  and  2  are swapped (or  3  and  4  are swapped), both splice  1  and  4  are preferably 90 degree splices, splice  5  is 0 deg, splice  3  or splice  4  are 90 degrees. 
   If splice  2  (or  3 ) goes to the other leg of PM Combiner/Splitter, Splice  2  and  3  are 0 deg, Splice  5  is 90 deg, Splice  1  (or  4 ) are 90 deg. 
   VI. Third Embodiment 
     FIGS. 10–12  illustrate a third embodiment of the present invention. Here, instead of employing one or more PM Combiner/Splitters, sensing loop travel doubling can be achieved using a polarization beam splitter that can be implemented using, for example, bulk optics. Alternatively, polarization beam splitter could be implemented using micro optics technique, which could integrate other components such as the circulator, coupler or IOC, among others. 
   Referring to  FIGS. 10A and 10B , the function of the polarization beam splitter is similar to the PM Combiner/Splitter. It allows one polarization state to pass through the splitter, and the other perpendicular polarization state to be reflected back at the interface via total internal reflection. 
   The polarization beam splitter can be made using two optical materials with different indexes of refraction. The refraction index is chosen such that one polarization of the 45° incident light on the interface passes through, and the other reflects back at the interface. 
     FIGS. 11 and 12  illustrate how the polarization beam splitter can be employed in the same general architecture as that of the first embodiment of the present invention that comprises a single PM Combiner/Splitter, as shown in  FIGS. 1 and 4 . 
   In view of the foregoing, those skilled in the art will appreciate that it is possible, in accordance with the present invention, to increase the sensitivity of an IFOG without increasing its LD product in a physical sense. Alternatively, it is possible, in accordance with the present invention, to halve the LD product of a conventional IFOG device, yet still maintain the same performance sensitivity. 
   Moreover, the present invention can be used on both low cost, low accuracy gyros and higher cost, higher accuracy gyros. Therefore, in some implementations a circulator and/or IOC may be eliminated. Also, the present invention should be considered applicable to any type of sensor, that uses a ring interferometer. That is, the invention is not limited to IFOG devices. 
   Also, as mentioned previously, circulator  104  can be replaced with any device that combines or splits light waves, such as a fiber coupler or a free-space micro-optic beam splitter that is part of a highly integrated, multi-function, optic-optic device. Similarly, IOC  106  could also be replaced by a fiber optic coupler or possibly a micro-optic device. 
   The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
   Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.