Abstract:
Substantially symmetric RFOG configurations for rotation rate sensing using two input/output coupling components. Configurations are disclosed where optical coupling components handles both input and output lightwaves. Reducing the number of input/output coupling components while maintaining a substantially symmetric configuration for the CW and CCW beam reduces losses, prevents realization of bias errors due to asymmetric light paths in the resonator, and produces better signal to noise performance. In addition, the invention discloses systems integrating multiple functions into compact micro-optic devices that are easier to fabricate and package, leading to compact RFOGs with reduced cost and improved manufacturability.

Description:
BACKGROUND OF THE INVENTION 
     A resonator fiber optic gyroscope (RFOG) is a rotation rate measurement apparatus that uses a fiber ring resonant cavity to enhance a rotation-induced Sagnac effect. The basic principle of RFOG operation is that the effective resonator round-trip path length in a clockwise (CW) and counter-clockwise (CCW) direction is different when the rotation has a nonzero component in a resonator axis. By measuring the CW and CCW resonance frequency difference, which is proportional to Sagnac phase shift due to rotation, the RFOG can accurately measure the rotation rate. Several RFOG configurations are suggested by the prior art. Three specific prior art resonator configurations are shown in  FIGS. 1-3 . 
       FIG. 1  shows a resonator in reflection mode configuration, where light is introduced to a resonator formed by a resonator input mirror and a fiber optic resonator coil. The mirror has some reflectivity and a low, but non-zero, transmission coefficient. Thus, most of the light incident on the mirror is reflected, but a portion of the light is transmitted. In operation, light from a laser is introduced to the resonator after being transmitted through the resonator input mirror. Light recirculates within the resonator multiple times by means of the resonator input mirror, i.e. light emerging from one end of the fiber is repeatedly reflected back into the other end. Some of the recirculated light is transmitted out of the resonator (dotted line) to the detector, where the recirculated light is interfered with light that was originally reflected from the light source (solid line toward the detector). These interfering lightwaves from the resonator input mirror are used to measure the resonance signal. Specifically referring to the light propagating in the CW direction of the coil in  FIG. 1 , a small portion of light from the CW laser enters the resonator at the resonator input mirror. Most of the light from the CW laser is reflected by the resonator input mirror and continues towards the CCW laser. A portion of the light that recirculates within the resonator coil is coupled out of the resonator along the same path as the reflected light. A portion of the reflected light and the resonator output light are redirected by the CW tap mirror to a CW detector. The reflected light and the resonator output light interfere on the CW detector. The interference results in a lightwave having a resonance dip corresponding to zero light. The bottom of the dip occurs when the frequency of the light is at a resonance frequency of the coil. Similarly, light from the CCW laser is introduced into the resonator in the opposite direction and its resonance is detected by the CCW detector. 
     A problem associated with the architecture shown in  FIG. 1  is that large rotation sensing errors are caused by the interference between an undesirable portion of the reflected light and the resonator output light. The undesirable portion of the reflected wave could be slightly different in its spatial-mode or polarization-mode characteristics than that of the resonator output wave. Even after careful alignment, polarization dependent losses or spatial aperturing effects between the resonator input mirror and the detector can cause errors in the rotation rate measurement. For example, imperfections in the input polarization state will result in line-shape asymmetry, which in turn will result in a gyro rate bias error. Accordingly, there is a need to detect the resonance frequency of the resonator without interfering the reflected and resonator output waves. 
       FIG. 2  shows prior art of a resonator in transmission mode. This resonator architecture overcomes the problem with the previous reflection-mode resonator by placing another mirror within the cavity to tap off a portion of the light recirculating in the coil. Specifically, the light that is reflected by the resonator is removed by an optical isolator placed in front of the opposing laser. Thus, only the light recirculating in the resonator in the CW direction reaches the CW detector, i.e. no interference occurs between the recirculating light and the reflected light. However, this architecture is not symmetric in the CW and CCW directions because only some light in the CW direction propagates through the fiber coil prior to reaching the CW detector, while all of light in the CCW direction propagates through the fiber coil prior to reaching the CW detector. This asymmetry poses a problem. If some light propagating in the CW direction is not in the correct spatial-mode or polarization mode, it can leak through to the CW detector and may be different from the light that is recirculating within the resonator. Thus, detected light could still be mismatched to the resonator output light and reach the detector without first passing through the resonator coil. It is known that this asymmetry combined with polarization and spatial mode imperfections can lead to significant rotation sensing errors. Accordingly, there is a need for a symmetric, transmission mode resonator. 
       FIG. 3  shows a prior art illustration of a symmetric, transmission mode resonator. This resonator is formed by adding a third mirror to the resonator cavity. This resonator is both in transmission-mode configuration and symmetrical for the CW and CCW light propagation. In this configuration, the light reaching the detector always passes through the resonator coil fiber at least once, thus eliminating the issues discussed for the configurations shown in  FIGS. 1 and 2 . However, adding a third mirror adds significant complexity to the optics within the resonator cavity. To achieve high performance, the round trip optical loss within the cavity must be very low. Achieving low optical loss in the cavity is far more important than achieving low loss outside the cavity. Specifically, a round trip loss of under 1 dB is usually acceptable within the cavity, and up to three dB of loss is acceptable outside the cavity. The angular alignments of each cavity mirror are critical to achieving low loss in the cavity. Including the third mirror within the cavity increases the difficulty in obtaining a low cavity loss with a low cost device that is capable of being manufactured with a high degree of automation. Additionally, environmental changes may exacerbate the cavity loss because temperature changes can disrupt the alignments of the cavity mirror. Accordingly, there is a need for a simplified, symmetric, transmission mode resonator with only two mirrors. 
     SUMMARY OF THE INVENTION 
     The present invention provides for improved compact resonator fiber optic gyroscope systems that are symmetric, in transmission mode, and/or use only two coupling devices (e.g. mirrors, partial reflectors or devices that partially transmit and partially reflect light) within the resonator cavity. 
     A fiber optic gyro system includes a coil having a first end and a second end and a device in optical communication with the coil. The device is configured to receive light from a source and to also receive light from each end of the coil. A first component of the device is configured to direct at least a portion of light from the source to the first end of the coil so that light propagates through the coil in a first direction. A second component of the device is configured to direct at least a portion of light from the source to the second end of the coil so that light propagates through the coil in a second direction. The first component is also configured to direct at least a portion of light from the first end of the coil to the second component, direct at least a portion of light from the second component to the first end of the coil, and direct at least a portion of light propagating in the second direction to a first detector. The second component is configured to direct at least a portion of light from the second end of the coil to the first component, direct at least a portion of light from the first component to the second end of the coil, and direct at least a portion of light propagating in the first direction to a second detector. 
     In accordance with further aspects of the invention, the device may also include a third component and a fourth component. The third component is configured to direct light received from the first component to the first detector, direct light from the source to the first component, and prevent light received from the source from entering the first detector without circulating in the coil. The fourth component is configured to direct light received from the second component to the second detector, direct light from the source to the second component, and prevent light received from the source from entering the second detector without first circulating in the coil. 
     In accordance with other aspects of the invention, the third component is configured to direct at least a portion of the light from the source to a first RIN monitoring component, and the fourth component is further configured to direct at least a portion of the light from the source to a second RIN monitoring component. 
     In accordance with still further aspects of the invention, the third and fourth component each comprise a directional coupling device. 
     In accordance with yet other aspects of the invention, the device includes at least one polarizing component. 
     In accordance with still further aspects of the invention, the single polarizing component is configured to polarize light received from the third component, the fourth component, the first end of the coil, and the second end of the coil. 
     In accordance with still another aspect of the invention, the device and the coil are configured so that all light directed to the detector has been passed through at least one polarizer and the coil. 
     In accordance with yet another aspect of the invention, the device includes a first pathway configured to facilitate optical communication in both directions between the first component and the third component, and the device further comprises a second pathway configured to facilitate optical communication in both directions between the second component and the fourth component. 
     In accordance with further aspects of the invention, the device includes a third pathway configured to facilitate optical communication in both directions between the first component and the second component. 
     In accordance with still further aspects of the invention, the device and the coil are configured such that light propagating in the first direction and the light propagating in the second direction travel in substantially symmetrical paths. 
     In accordance with additional aspects of the invention, the first component and the second component include a coupling device configured to transmit and reflect light. 
     In accordance with yet other aspects of the invention, each coupling device includes a mirror. 
     In accordance with other aspects of the invention, the first component and the second component include no more than two mirrors total. 
     In accordance with still other aspects of the invention, the first component, the second component, the third component, and the fourth component are integrated into a single micro-optic device. 
     In accordance with still further aspects of the invention, the first component and the second component are integrated into a single micro-optic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is a schematic view of a prior art RFOG resonator in reflection mode; 
         FIG. 2  is a schematic view of a prior art asymmetric two-mirror RFOG in transmission mode; 
         FIG. 3  is a schematic view of a prior art symmetric three-mirror RFOG; 
         FIG. 4  is a schematic view of a symmetric RFOG resonator with two directional components and two coupling optical components formed in accordance with an embodiment of the present invention; 
         FIG. 5  is a schematic view of an RFOG coupling optical component used in the resonator shown in  FIG. 4 ; 
         FIG. 6  is a schematic view of a symmetric RFOG resonator including two directional and one coupling optical components formed in accordance with an embodiment of the present invention; 
         FIG. 7  is a schematic view of an embodiment of an RFOG directional optical component including four ports formed in accordance with an embodiment of the present invention; 
         FIG. 8  is a schematic view of an embodiment of an RFOG coupling optical component including four ports in accordance with an embodiment of the present invention; 
         FIG. 9  is a schematic view of an alternative embodiment of an RFOG coupling optical component including four ports; 
         FIG. 10  is a schematic view of a symmetric RFOG resonator including one directional and a coupling optical component in accordance with an embodiment of the present invention; 
         FIG. 11  is a schematic view of an embodiment of an RFOG directional optical component including eight ports; and 
         FIG. 12  is a schematic view of a symmetric RFOG resonator with one eight-port optical component in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows a resonator fiber optic gyroscope (RFOG)  100  formed in accordance with an exemplary embodiment of the present invention. The RFOG  100  includes light source  8 , directional optical components  20  and  30 , resonator input/output coupling optical components  40  and  50 , detectors  11  and  13 , and a resonator  10  including a fiber loop  80 . The light source  8  emits at least two beams of light: a beam of light for a CCW direction and another beam of light for the CW direction. Alternatively, light source  8  could include two independent light sources one providing light for the CCW direction and the other for the CW direction. Each of the directional optical components  20  and  30  includes an input port  21  and  31 , an output port  22  and  32 , and a detector port  23  and  33 . A substantial portion of light received by the input port  21  and  31  is directed to the output port  22  and  32  but prevented from entering the detector port  23  and  33 . In a reverse direction (indicated by an up-arrow below the output port  22  and  32 ), a substantial portion of light inputted into the directional optical component  20  and  30  at the output port  22  and  32  is directed to the detector port  23  and  33 , which directs light to the detector  11  and  13 . 
     The directional optical components  20  and  30  may include, but are not limited to, a fiber optic circulator, a fiber coupler, a beam splitter, or any optical components that provide desired functions described above. The ports  21 ,  22 ,  23 ,  31 ,  32 ,  33  of directional optical components  20  and  30  could be connected to optical fibers or to free space optical beams. Most, if not all, of the foregoing directional optical components are commercially available. 
     The coupling optical component  40  and  50  includes an in/out port  41  and  51 , a loop port  42  and  52 , and a relay port  43  and  53 . A portion of light directed into the in/out port  41  and  51  is received by the loop port  42  and  52  but does not enter the relay port  43  and  53 . A substantial portion (typically 80˜99%) of light directed into the coupling optical component  40  and  50  at the relay port  43  and  53  is directed to the loop port  42  and  52  but does not enter in/out port  41  and  51 . In the reverse direction, a substantial portion (typically 80˜99%) of light directed into the coupling optical component  40  and  50  at the loop port  42  and  52  is received by the relay port  43  and  53 , and a portion (typically 1˜20%) of it is directed to the in/out port  41  and  51 . 
     The coupling optical components  40  and/or  50  may include, but are not limited to, a fiber coupler, a fiber splitter, a beam splitter, a waveguide device, and/or a combination of the above. The coupling optical component  40  and/or  50  may also include optical components such as polarizers. Most, if not all, of the foregoing coupling optical components are commercially available. 
     The fiber loop  80  includes ends  81  and  82 . The first end  81  is connected to the loop port  42  of the coupling optical component  40 , and the second end  82  is connected to the loop port  52  of the coupling optical component  50 . The ports  43  and  53  are connected by a connector  45  such as, but not limited to, a piece of fiber, a waveguide or simply a free space optical path for light beam propagation. 
     To direct light into the resonator  10  in the CW direction, light is directed from the source  8  into the input port  21  of the directional optical component  20 . At least a portion of the light passes through the directional optical component  20  and the output port  22  to the in/out port  41  of the coupling optical component  40 . The directional optical component  40  directs at least a portion of light through the loop port  42  and into the fiber loop  80 . The light circulates in the resonator  10  by transit through the loop port  52 , the relay port  53 , the connector  45 , the relay port  43  and the loop port  42  in the CW direction. 
     A portion (typically 1˜20%) of the CW circulating light entering the loop port  52  is directed out of the resonator  10  at the in/out port  51  by the coupling optical component  50 . This light propagates to the output port  32  of the directional optical component  30 . The directional optical component  30  directs the light to the detector port  33 . The CW detector  13  receives light from the port  33  for CW resonance detection. 
     To direct light into the resonator  10  in the CCW direction, light is directed from the source  8  into the input port  31  of the directional optical component  30 . The light coming out of the port  32  is directed into the in/out port  51  of the coupling optical component  50 . The coupling optical component  50  directs the light from the in/out port  51  of the coupling optical component  50  to the loop port  52  where it is coupled into the fiber loop  80 . The light circulates in the resonator  10  by transit through the loop port  42 , the relay port  43 , the connector  45 , the relay port  53  and the loop port  52  in the CCW direction. A portion (typically 1˜20%) of this CCW circulating light is directed out of the resonator  10  at the coupling optical component  40 . Specifically, a portion of light from the loop port  42  is directed to the in/out port  41  by the coupling optical component  40 . This light propagates to the output port  22  of the directional optical component  20  and is directed to the detector port  23 . The CCW detector  11  receives light from the port  23  for CCW resonance detection. 
     The directional optical components  20 ,  30  and the coupling optical components  40 ,  50  are substantially similar (i.e. mirror images of each other). Accordingly, the resonator  10  configuration is substantially symmetric in the CW and CCW directions. CW and CCW light travel along the exact same optical path in the resonator  10  (but in the reverse direction). This high level of symmetry and reciprocity of the CW and CCW optical path helps cancel the phase delays and asymmetrical beam paths that could cause bias errors by making them common to light traveling in both CW and CCW, thereby improving the gyro bias stability. 
       FIG. 5  shows an exemplary implementation of the coupling optical component  40 . The coupling optical component  40  includes a beam splitting mirror  104 , which reflects a portion (typically 1˜20%) of input light from a port  101  to a port  102  and from the port  102  to the port  101 . The mirror  104  transmits a portion (typically 80˜99%) of light that propagates from a port  103  to a port  102  and from the port  102  to the port  103 . A polarizing component  105  may be inserted between the mirror  104  and the port  102  to reduce/remove unwanted polarization modes. In this device, the port  101  and the port  102  may include fiber adaptors. The port  103  may include an optical opening for passing light beams through free space. 
       FIG. 6  illustrates an alternate embodiment of an RFOG  300 . The RFOG  300  includes relative intensity noise (RIN) detectors  12  and  14 , directional optical components  120  and  130  are substantially similar to the directional optical components  20  and  30  but with an additional RIN monitoring port  124  and  134 . Ports  121 ,  131 ,  122 ,  132 ,  123 ,  133  have substantially the same light directional function as the input port  21  and  31 , the output port  22  and  32  and the detector port  23  and  33 . In addition to these functions described above, a portion of input light entering the port  121  and  131  is directed to the RIN monitoring port  124  and  134 . The RIN detector  12  and  14  receives light from the RIN monitoring port  124  and  134  for measurement of the intensity noise of the input beam. These RIN signals may provide signal feedback to a typical RIN servo electronic system for reduction of the intensity noise or unwanted intensity or amplitude modulation of the input light. Integrating the RIN monitoring port  124  into the directional optical component  120  and  130  reduces the number of total optical components of the gyro. Using fewer optical components reduces packaging constraints. In addition, a cleaner input light beam with less intensity noise is expected because the RIN monitoring point is located proximately to the resonator  10 . 
     A coupling device  60  shown in  FIG. 6  is a variation of the coupling optical components  40  and  50 . The coupling device  60  combines the functions of the coupling optical components  40  and  50  into one device, thereby making the gyro more compact and easier to package. Generally speaking, the coupling device  60  includes four ports, a CW input port  61 , a CCW input port  63 , a loop port  62  and another loop port  64 . A portion (typically 1˜20%) of light directed into the CW input port  61  is directed to the loop port  62  (but prevented from directly entering the loop port  64 ) and propagates in the CW direction in the fiber loop  80 . The CW light exiting the fiber loop  80  at the end  82  is directed into the loop port  64 . A substantial portion (typically 80˜99%) of the CW light is directed to the loop port  62  and circulated in the resonator  10  in the CW direction. A portion (typically 1˜20%) of the CW light circulating inside the resonator  10  is directed out of the resonator  10  by propagating from the loop port  64  to the CCW input port  63 . This light is then directed to the CW detector  13  through the detector port  133  of the directional optical component  130  for CW resonance signal detection. 
     A portion of light (typically 1˜20%) directed into the CCW input port  63  is directed to the loop port  64  and propagates in the CCW direction in the fiber loop  80 . The CCW light exiting the fiber loop  80  at the end  81  is directed into the loop port  62 . A substantial portion (typically 80˜99%) of this light is directed to the loop port  64  and circulated in the resonator  10  in the CCW direction. A portion (typically 1˜20%) of the CCW light circulating inside the resonator  10  is directed out of the resonator  10  by propagating from the loop port  62  to the CW input port  61 . From the CW input port  61 , the light is directed to the CCW detector  11  through the detector port  123  of the directional optical component  120  for CCW resonance signal detection. 
       FIG. 7  shows an exemplary embodiment of a directional optical component  400  that could be utilized as the directional optical component  120 . The directional optical component  400  includes a partially reflecting mirror  405  that transmits a portion of input light from a port  401  to a port  404  for RIN monitoring. A substantial portion of input light (assumed here to be polarized horizontally without lost generality) is reflected by the mirror  405  and passes through a polarization beam splitter  406  which transmits horizontally polarized light but reflects vertically polarized light. A Faraday rotator  407  rotates the input light by 45°. A following half wave plate  408  changes the input light polarization state back to the horizontal plane, allowing it to pass a polarizer  409  whose polarization pass-axis is oriented along horizontally. In the reverse direction, a horizontal polarized light directed into the port  402  passes through the polarizer  409  without significant losses. The half wave plate  408  rotates the polarization state of the beam by 45° with respect to the horizontal plane. The Faraday rotator  407  is a nonreciprocal device which rotates the polarization of the reverse propagating light to the vertical plane. The beam splitter  406  reflects most of the vertically polarized light to a detector port  403 . The beam splitter  406 , the rotator  407 , the plate  408  and the polarizer  409  is an exemplary implementation of a typical circulator device. Other implementations are possible that integrate the mirror  405  (for RIN signal tap) with a circulator (for separating the reverse propagating beam from the input beam) using micro-optic components. The ports  401 ,  402 ,  403  and  404  may be either fiber or free space coupled ports. 
       FIG. 8  shows an exemplary implementation of a coupling device  500  that could be utilized as the coupling device  60  in  FIG. 6 . The coupling device  500  includes a partial reflecting mirror  505  and  507  that reflects a portion (typically 1˜20%) of input light directed at a port  501  and  503  to a port  502  and  504 , and reversely, light directed at the port  502  and  504  to the port  501  and  503 . The mirrors  505  and  507  transmit a substantial portion (typically 80˜99%) of light from the port  502  to the port  504 , and reversely, from the port  504  to the port  502 . A polarizer  506  and  508  may be inserted between the mirrors  505  and  507  and port  502  and  504  for controlling the polarization state of the light beams. It should be noted that coupling device  500  is an exemplary embodiment that highlights the basic idea of using micro-optic components to implement the functions of the four-port coupling device. 
       FIG. 9  illustrates another embodiment of a coupling device  600 . In this embodiment, input ports  601  and  603  (analogous to  501  and  503  in  FIG. 8 ) may be arranged on the same side of loop ports  602  and  604  for easier packaging. In addition to partial reflection mirrors  605  and  607 , directional mirrors  609  and  610  are also used for directing beams from the input ports  601  and  603  to the loop ports  602  and  604 . 
     Device  500  and  600  are advantageously configured so that the free-space optics can be arranged inline with the light beams recirculating through the coil. By doing so, the alignments of optical components  505 ,  506 ,  508 ,  508  and  605 ,  606 ,  607 ,  608  are less critical. Resonator sensitivity to environment changes due to misalignment of these components can be reduced. Furthermore, those devices along with  502 ,  504 ,  602  and  604  can be self aligned using techniques such mounting the fibers in v-grooves on a miniature substrate which can significantly reduce production costs. 
       FIG. 10  shows another RFOG system  700  in accordance with the present invention. The RFOG system  700  includes a directional optical component  200 , a coupling optical component  70 , and the fiber loop  80 . In this embodiment, the directional optical component  200  is an eight-port device that combines the functions of the directional optical components  120 ,  130  into a single device. Ports  221 ,  222 ,  223 ,  224 ,  231 ,  232 ,  233 , and  234  correspond to the ports  121 ,  122 ,  123 ,  124 ,  131 ,  132 ,  133 , and  134 , respectively. Their directional functions are substantially the same. The major advantages integrating CW and CCW directional optical component includes more compact design, reduced number of components in the package, and increased level of symmetry for CW and CCW propagating beams. 
     The RFOG system  700  includes a coupling optical component  70  that has substantially the same functions of the coupling device  60 . A partial reflecting mirror  75  and  77  transmits a portion of input light directed into a port  71  and  73  to a loop port  72  and  74 . A substantial portion (typically 80˜99%) of light directed into the loop port  72  and  74  is reflected by the partially reflecting mirror  75  and  77  and  77  and  75  successively and directly to the loop port  74  and  72 . A portion (typically 1˜20%) of circulating CW (CCW) light is directed out of the resonator  10  by transmitting through the partially reflecting mirror  77  and  75  to the port  73  and  71 . A polarizer  76  is shared by both CW and CCW light for polarization control. Sharing components in the coupling optical component  70  is advantageous because a reduced number of components allows lower manufacturing costs and higher degree of resonator  10  symmetry. 
     The embodiment in  FIG. 10  is a variation of the embodiments in  FIG. 4  and  FIG. 6 . 
       FIG. 11  illustrates an exemplary eight-port device  800  that could be used as the eight-port device  200 . This embodiment shows the possibility of sharing the same optical components for the CW and CCW directional optical component in order to reduce the number of individual components. The eight-port device  800  includes a partial reflecting mirror  841  and  842  that transmits a portion of input light directed into a port  821  and  831  to a RIN monitoring port  824  and  834 . A substantial portion of input light (assumed horizontally polarized without lost of generality) is reflected by the partially reflecting mirror  841  and  842  to a polarization beam splitter  843  and  844 . The polarization beam splitter  843  and  844  transmits horizontally polarized light but reflects vertically polarized light. A Faraday rotator  845  rotates the input light by approximately 45°. A half wave plate  846  changes the input light polarization state back to the horizontal plane, allowing it to pass a polarizer  847  whose polarization pass-axis is oriented along horizontally. The light is then directed out of a port  822  and  832 . Propagation of light in the reverse direction (i.e. from ports  822  and  832  to ports  823  and  833 , respectively) has been similarly described above for  FIG. 7 . It should be noted that the Faraday rotator, the half wave plate and the polarizer  845 ,  846  and  847  are common to both CW and CCW beams in this design. Reducing components saves space and reduces device cost. Reducing components is also advantageous from the performance point of view because sharing the same optical components for the CW and CCW beam increases the symmetry of the device, leading to more effective cancellation of bias errors. 
       FIG. 12  is another embodiment of an RFOG system  900  in accordance with the present invention. The RFOG system  900  includes an eight-port optical device  300  that combines the functions of the directional and coupling optical components of the previously mentioned embodiments. More specifically, a substantial portion of the input light directed at port  321  and  331  is directed to loop port  322  and  332  and a portion of the light is directed to a RIN monitoring port  324  and  334 . A portion of light circulating in the CW/CCW direction in the fiber loop  80  is directed out of the resonator  10  through polarizing component  308  and  306 , beam directing component  307  and  306 , and beam splitting device  312  and  311  to a detector port  333  and  323 . Beam splitting devices  311  and  312  separate a substantial portion of light directed out of the resonator  10  from light coming into the resonator  10 . It is preferable that the beam splitting devices  311  and  312  are implemented to have low loss for both incoming and outgoing beams which may require non-reciprocal polarization rotation device such as Faraday rotators. Polarizing components  306  and  308  are used to reduce the unwanted polarization modes circulating inside the resonator  10 . It is preferable to share the optical components with CW and CCW beams because if the system using fewer components reduces costs. Also, sharing optical components leads to a higher degree of symmetry in the RFOG, which improves performance. 
     The foregoing resonator architectures are symmetric, in transmission mode, and/or use only two coupling devices (e.g. mirrors, partial reflectors or devices that partially transmit and partially reflect light) within the resonator cavity. The resonator is symmetrical because the optical paths of CW and CCW light are substantially identical even though the direction of lightwave propagation is opposite. A symmetrical resonator architecture helps to cancel bias errors that would otherwise erroneously appear as rotation rate and reduce gyro-bias instability. The symmetrical architecture also allows the resonator cavity optics to be in-line with each other, which reduces difficulties in resonator alignment and sensitivity to environmental changes. Furthermore, the symmetrical architecture improves gyro performance and manufacturability and also reduces the number of components, costs and form factor (i.e. size) of the device. 
     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. The corresponding directional and coupling optical components are considered interchangeable between the embodiments to form new embodiments. For example, an embodiment could be proposed using the 8-port directional optical component  200  with the coupling device  60 . Another embodiment can be formed using the coupling optical component  70  with the directional optical components  120  and  130 . Still another embodiment could utilize the directional optical components  20  and  30  with the coupling device  60  or the coupling optical components  40  and  50  could be used with the directional optical components  120  and  130 . Also, two light sources could be utilized, each source emitting at least one beam of light. 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.