Abstract:
A compact fiber optic gyroscope including a first housing; a transceiver module disposed in the first housing, the transceiver module including a second housing; a non-coherent light source disposed in the second housing for producing a first beam of light; a single lens for focusing the first beam of light; an optical circulator disposed in the second housing and in the path of the first beam of light to produce polarized second and third beams respectively, with polarization orthogonal to each other; and first and second photodiodes disposed in the second housing and coupled to the optical circulator, wherein the first photodiode is a transmit monitor photodiode coupled to the second beam, and the second photodiode is a receiver photodiode. The first housing further includes a planar optical fiber loop having a first end and a second end; a phase modulator coupled to the third beam emitted from the transceiver module to produce fourth and fifth beams coupled to the first and the second end respectively of the optical fiber loop respectively, and for receiving the return sixth and seventh beams from the second and the first ends respectively of the optical fiber loop.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to fiber optic gyroscopes (FOGs) and in particular to integration techniques that implement a single-axis FOG transceiver subassembly having high accuracy and low noise in a small, compact cylindrical form factor. 
     2. Description of the Related Art 
     A FOG is a device that uses the propagation of light beams in an optical fiber coil to detect mechanical rotation of the fiber coil. The sensor is a coil of as much as 5 km or more of optical fiber. The typical implementation provides that two light beams be launched into the fiber in opposite directions. Due to an optical phenomenon known as the Sagnac effect, the beam traveling against the rotation experiences a slightly shorter path than the other beam resulting in a relative phase shift. The amount of the phase shift of the original two beams can be measured by determining how the beams interfere with each other when they are combined. The intensity of the combined beam then depends on the rotation rate of the fiber coil about its axis. 
     A FOG provides extremely precise rotational rate information, in view of its lack of cross-axis sensitivity to vibration, acceleration, and shock. Unlike the classic spinning-mass gyroscope, the FOG has virtually no moving parts and no inertial resistance to movement. The FOG also can provide higher resolution than a ring laser gyroscope and is utilized in inertial navigation systems requiring a very high degree of accuracy. 
     There are two types of FOG systems, closed loop and open loop. In a closed loop system, a feedback path is defined so as to maintain the phase difference between the light beams constant after the beams exit the ends of the fiber coil. The amount of feedback phase needed to maintain the fixed phase relation is therefore indicative of the rate of rotation of the coil about its axis. 
     Open loop FOG systems calculate the rotation rate by way of amplitude measurements taken along an interference curve that results when the two exiting light beams are recombined. 
     SUMMARY OF THE INVENTION 
     1. Objects of the Invention. 
     It is an object of the present invention to provide a fiber optic gyroscope in a small, highly integrated form factor. 
     It is another object of the present invention to provide a single-axis fiber optic gyroscope in a single integrated housing. 
     It is also another object of the present invention to provide a fiber optic gyroscope having high accuracy and low noise using an integrated opto-electronic transceiver subassembly. 
     It is also another object of the present invention to provide a fiber optic gyroscope having a rotational rate drift of less than 1 degree per hour. 
     It is also another object of the present invention to provide a fiber optic gyroscope having noise measured in angular degrees random walk per square root of hour of less than 0.02 degrees. 
     It is also another object of the present invention to provide a fiber optic gyroscope capable of recording angular rate changes of greater than 500 degrees per second. 
     It is also another object of the present invention to provide a fiber optic gyroscope having a closed loop feedback path so as to maintain the phase difference between the light beams constant after the beams exit the ends of the fiber coil. 
     It is still another object of the present invention to provide a fiber optic gyroscope that may be manufactured to a specified degree of accuracy by changing the fiber coil length and by programming an internal processor. 
     It is still another object of the present invention to provide a fiber optic gyroscope that is operable over a wide performance range that may be adapted by the user to a specified degree of accuracy or noise tolerance by changing certain parameters, such as fiber coil length, of the unit. 
     Some implementations may achieve fewer than all of the foregoing objects. 
     Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility. 
     2. Features of the Invention 
     Briefly, and in general terms, the present disclosure provides a compact fiber optic gyroscope including a first housing; a transceiver module disposed in the first housing, and including a second housing; a non-coherent light source disposed in the second housing for producing a first beam of light; an optical circulator disposed in the second housing and in the path of the first beam of light to produce polarized second and third beams respectively, with polarization orthogonal to each other; and first and second photodiodes disposed in the second housing and coupled to the optical circulator, wherein the first photodiode is a transmit monitor photodiode coupled to the second beam, and the second photodiode is a receiver photodiode; a planar optical fiber loop disposed in the first housing and having a first end and a second end; a phase modulator disposed in the first housing and coupled to the third beam from the transceiver module to produce fourth and fifth beams coupled to the first and the second end respectively of the optical fiber loop respectively, and receiving the return sixth and seventh beams from the second and the first ends respectively of the fiber loop, and coupling the return sixth and seventh beams to the receiver photodiode in the transceiver module; and a processor disposed in the first housing and coupled to the receiver photodiode for determining the phase relationship between the two counter propagating beams in the fiber loop and thereby determining the rotational rate of the planar optical fiber loop with respect to an axis. 
     The optical circulator can include a polarizing beam splitter. 
     The optical circulator can further include a Faraday rotator coupled to an output of the polarizing beam splitter. 
     The phase modulator may be a lithium niobate modulator. 
     The phase modulator may have an output pair of optical fibers which may be spliced to the first end and the second end respectively of the planar optical fiber loop. 
     The planar optical fiber loop can be circumferentially disposed around the interior periphery of the first housing. 
     The first housing may be cylindrically shaped and the planar optical fiber loop may be circumferentially disposed around the interior periphery of the first housing. 
     The first housing may be cylindrically shaped having dimensions of approximately 80 mm in diameter, and 20 mm in height. 
     The weight of the gyroscope may be 160 grams or less. 
     The second housing may be a hermetically sealed butterfly package. 
     The rotational rate of the gyroscope may have a drift of between 0.005 and 1.0 degrees per hour. 
     The rotational rate of the gyroscope may have a drift that is adjustable by the user to a desired operational range. 
     The rotational rate of the gyroscope may have a drift that is operable over an operational range of up to 1.0 degree per hour. 
     The noise measured in angular degrees random walk per square root of hour of the gyroscope may be less than 0.02 degrees. 
     The noise tolerance of the gyroscope may be selected by the user to a desired angular degrees random walk per square root of hour. 
     The length of the fiber optic loop may be greater than 180 meters. 
     The length of the fiber optical loop of the gyroscope may be selected by the user to a desired length. 
     In another aspect of the invention, the present disclosure provides a transceiver subassembly for a fiber optic gyroscope including a housing; a non-coherent light source disposed in the housing for producing a first beam of light; a single focusing lens disposed in the housing directly adjacent to the light source; a polarizing beam splitter disposed in the housing and disposed in the path of the first beam of light for producing a second and a third beam respectively, with polarization orthogonal to each other; a monitor photodiode disposed in the housing and disposed in the path of the second beam to monitor the intensity of the second beam; a Faraday rotator disposed in the housing and disposed in the path of the third beam; a single input/output optical fiber directly adjacent to and optically coupled to the Faraday rotator; and a receiver photodiode disposed in the housing and coupled to the Faraday rotator for receiving an input optical signal from the input/output optical fiber. 
     The polarizing beam splitter can direct the S polarization of the initial beam of light from the incoherent source through and into the Faraday rotator. 
     The polarizing beam splitter can direct the P polarization of the initial beam of light from the incoherent source onto a power monitoring photodiode. 
     The polarizing beam splitter may direct the return light of the P polarization to the receiver photodiode. 
     The optical circulator can include a Faraday rotator, and the polarizing beam splitter can couple the light of S polarization to the Faraday rotator. 
     A monitor photodiode can be coupled to the polarizing beam splitter for monitoring the light of P polarization from the light source. 
     The transceiver module may include a single input/output optical fiber coupled to the optical circulator. 
     The single input/output optical fiber may terminate in a pig tail. 
     The single input/output optical fiber may be coupled to a lithium niobate modulator having an input optical fiber which may be spliced to the end of the pig tail input/output optical fiber extending from the transceiver. 
     The transceiver module may include a single lens disposed adjacent to the non-coherent light source for coupling the first beam from the light source through the optical circulator to focus on the input/output optical fiber. 
     The Faraday rotator may change the polarization of the S polarized return sixth and seventh beams to a P polarized eighth beam, and the polarizing beam splitter may then direct the P polarized eighth beam to the receiver photodiode. 
     Some implementations of the present invention may incorporate or implement fewer of the aspects and features noted in the foregoing summaries. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of this invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a highly simplified block diagram of a prior art single-axis fiber optic gyroscope; 
         FIG. 2  is a detailed block diagram of a transceiver subassembly in a prior art single-axis fiber optic gyroscope; 
         FIG. 3  is a top plan view of the transceiver subassembly in a prior art single-axis fiber optic gyroscope; 
         FIG. 4  is a detailed block diagram of the transceiver subassembly in a single-axis fiber optic gyroscope according to the present disclosure; 
         FIG. 5  is a top plan view of the transceiver subassembly of  FIG. 4 ; and 
         FIG. 6  is an exploded perspective view of an embodiment of a compact single-axis fiber optic gyroscope according to the present disclosure. 
     
    
    
     The novel features and characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to a detailed description of a specific embodiment, when read in conjunction with the accompanying drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments or the relative dimensions of the depicted elements, and are not drawn to scale. 
       FIG. 1  depicts a highly simplified diagram of a single-axis fiber optic gyroscope (FOG) transceiver subassembly as is known in the prior art. The diagram shows a fiber-coupled broadband source, e.g. a super luminescent diode (SLD)  101  for producing a non-coherent beam of light, and a directional coupler  102  in the path of the beam. A lithium niob ate (LiNbO 3 ) phase modulator or FOG chip  105  is provided in a first path  104  from the output of the directional coupler, and a power monitoring photodiode  103  is provided in a second path  110  from the output of the directional coupler. A fiber loop or coil  108  with a fiber having a first  106  and second  107  end is coupled to the output of the phase modulator  105 . 
     Light from the SLD  101  is split in the Y-junction of the phase modulator  105  and each path through the phase modulator is modulated before being applied to the first  106  and second  107  ends of the fiber loop  108  and counter-propagated through the coil. The optical signals then pass back through the phase modulator  105 , are recombined in the Y-junction in the phase modulator and propagate back along path  104  to the directional coupler  102 , whereby the return optical beam is then focused onto a receiving photodiode  109  where the intensity produces an electrical signal. The electrical signal is processed externally of the transceiver subassembly to compute the rotation rate of the coil  108  to provide inertial guidancy information. 
     Rotation in the plane of the fiber coil induces a change in the phase relationship of the two counter propagating beams, known as the Sagnac effect. The phase change may be measured as an intensity fluctuation on the receiving photodiode and further processing of the electrical signal may be used directly to determine the rotational rate of the coil. Since there is only one fiber loop and one plane, such measurement is a one-axis inertial measurement. 
       FIG. 2  depicts a more detailed block diagram of a prior art single-axis FOG transceiver subassembly  200 . In an effort to decrease cost, size, and parts count, the SLD  201 , a collimating lens  202 , an optical circulator (implemented with a polarizing beam-splitter, or PBS  203 , and a Faraday rotator  207 ), a power monitoring photodiode  205 , a receiver photodiode  214 , and a trans-impedance amplifier (TIA)  215  are integrated into an extremely small housing or form factor package  200 . One optical output beam of the PBS  203  is coupled through a Faraday rotator  207 , which is in turn coupled to a focusing GRIN lens  208  which couples the beam emanating from the Faraday rotator  207  to the I/O optical fiber  209  extending from the housing  200 . Another optical output beam  204  of the PBS  203  is coupled to a monitor photodiode  205 . Outputs  216  of the trans-impedance amplifier (TIA)  215  are coupled the external processing circuitry. 
     The subassembly  200  further includes a thermal control unit  217  including a thermal electric cooler (TEC)  218  and a thermistor  219 . The TEC  218  extends over the length of the unit incorporating the SLD  201 , collimating lens  202 , PBS  203 , and Faraday rotator  207 , and GRIN lens  208  so as to maintain a uniform temperature over all subcomponents. 
     One such embodiment of such a single-axis FOG transceiver subassembly  200  is a commercial product known as the FOG PB3010 transceiver module manufactured by Emcore Corporation in Alhambra, Calif. 
       FIG. 3  is a top plan view of the prior art FOG shown in  FIG. 2  with the lid of the housing removed to show the internal components. The Figure depicts the SLD  201 , and a collimating lens  202  adjacent thereto. A polarizing beam-splitter or PBS  203  is disposed in the path of the beam from lens  202 , and the transmitter monitor photodiode  205  located on one side of the PBS  203 , and the receiver photodiode  214  located on a printed circuit board on the other side of the PBS  203 . A trans-impedance amplifier (TIA) is also mounted on the printed circuit board, and is coupled to the photodiode  214  to produce the output electrical signal. The optical output beam of the PBS  203  is coupled through a Faraday rotator  207 , which is in turn coupled to a focusing GRIN lens  208  which couples the beam emanating from the Faraday rotator  207  to the I/O optical fiber extending from the housing  200 . 
     In the device illustrated in  FIGS. 2 and 3 , 96% of the optical output from the SLD  201  is in the out-of-plane polarization (S polarization) while the remaining 4% is in the in-plane polarization (P polarization). The polarizing beam splitter  203  reflects the P polarization from the SLD  201  onto the power monitoring photodiode  205  but passes the S polarization. The light then passes through the Faraday rotator  207  unaltered and is coupled at  211  into the fiber  213  with a lens  208 . From this point the S polarized light travels through the rest of the fiber loop  213  until it returns at the second end  212  in the same polarization state as when it left. When the light passes through the Faraday rotator  207  in this direction (i.e. back towards the SLD  201 ) the polarization is rotated into the P state. The polarizing beam splitter  203  then reflects the optical return signal onto the receiving photodiode  214  that produces an electrical output directly connected to an internal TIA  215  and provides a typical gain of 10,000. 
       FIG. 4  is a highly simplified diagram of a single-axis FOG transceiver subassembly  400  with a single focusing lens  401  for focusing the light beam from the SLD  201  onto the optical circulator (implemented with a polarizing beam-splitter, or PBS, and a Faraday rotator  207 ), according to an embodiment of the present disclosure. The focusing lens  401  is positioned in the optical path between the SLD  201  and the optical circulator instead of using a collimating lens, and focuses the light beam over the optical path from the SLD  201  through the optical circulator including the Faraday rotator  207  to an optical focus point at the end  403  of the polarization-maintaining input/output optical fiber  402  for optically coupling the light beam to exterior of the FOG assembly  400 . This way, the focusing GRIN lens conventionally placed in the light path between the Faraday rotator  207  and the internal fiber  402  (e.g. the focusing GRIN lens  208  in  FIG. 2 ) and the collimating lens conventionally placed in the optical path between the SLD  201  and the optical circulator (e.g. lens  202  in  FIG. 2 ) need not be utilized in the subassembly. Eliminating the additional lens may permit easier alignment and be able to permit a smaller form factor package. The input/output optical fiber  402  typically terminates in a “pig-tail” exterior of the FOG transceiver subassembly  400 , whereby in some embodiments it may be spliced to the modulator  210  or other optical components. 
     The subassembly  400  further includes a thermal control unit  217  including a thermal electric cooler (TEC)  218  and a thermistor  219 . The TEC  218  extends over the length of the unit incorporating the SLD  201 , focusing lens  401 , PBS  203 , and Faraday rotator  207  so as to maintain a uniform temperature over all subcomponents. 
       FIG. 5  is a top plan view of the FOG assembly  400  shown in  FIG. 4  with the lid of the housing  404  removed to show the internal components. The focusing lens  401  is positioned between the SLD  201  and the optical circulator as described above. The PBS  203  component of the optical circulator is disposed in the path of the light beam from the focusing lens  401 , and the transmitter monitor photodiode  205  located on one side of the PBS  203 , and the receiver photodiode  214  located on a printed circuit board on the other side of the PBS  203 . A trans-impedance amplifier (TIA)  215  is mounted on the printed circuit board and coupled to the photodiode  214  for producing the output electrical signal. The optical output beam from the PBS  203  is coupled through a Faraday rotator  207 , which is in turn directly coupled to internal fiber  402  which couples to the external I/O optical fiber  402  extending from the housing  404 . 
       FIG. 6  is an exploded perspective view of an embodiment of a single-axis fiber optic gyroscope according to the present disclosure. The unit includes a metallic housing formed from a base plate  601  and a top cover  602 . In one embodiment, the housing may be cylindrically shaped having dimensions of 80 mm in diameter and 20 mm in height. In one embodiment, the weight of the entire gyroscope is 160 grams. The housing may be composed of a suitable magnetically shielding material. 
     The base plate  601  includes an annular cavity  603  around the periphery thereof for securing a portion of the fiber coil  604 . In one embodiment, the fiber coil  604  may be 200 meters in length, in which case the gyroscope may be considered a low accuracy model. In another embodiment, the fiber coil  604  may be 500 to 1000 meters in length, in which case the gyroscope may be considered a medium accuracy model. In another embodiment, the fiber coil  604  may be over 2 kilometers in length, in which case the gyroscope may be considered a high accuracy model. 
     The base plate  601  further includes a support or mount  605  for mounting the phase modulator  210 . Above the base plate  601  is a first printed circuit board  606  on which the transceiver and other electronic components may be mounted. Above the first printed circuit board  606  is a second printed circuit board  607  which includes the processor  608  and other digital components, and an external input/output connector  609  which extends through an opening  610  in the top cover  602 . The first printed circuit board  606  and the second printed circuit board  607  may be connected by an inter-board electrical connector  611 . In one embodiment the processor may be a digital signal processor having a sufficiently high clock rate or processing speed to allow rate changes in the angular orientation of the fiber coil  213  of greater than 2000 degrees, and up to 3000 degrees per second, or more, to be processed and reported. 
     Depending upon the length of the fiber coil the housing may be suitably sized in both diameter and height, and appropriate programming of the software associated with the processor to account for the fiber coil length, the fiber optic gyroscope as described above may be specified to have the following performance parameters: 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 
               
               
                   
               
               
                 Coil length  
                 Rotational Rate Drift 
                 Noise 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 200  
                 meters 
                 0.5  
                 degrees/hour 
                 0.0200 degrees/square root-hour 
               
               
                 1200  
                 meters 
                 0.01  
                 degrees/hour 
                 0.0017 degrees/square root-hour 
               
               
                   
               
             
          
         
       
     
     Thus, one embodiment of a fiber optic gyroscope as described above has a rotational rate drift of less than 0.01 degrees per hour, and noise, measured in angular degrees random walk per square root of an hour, of less than 0.0017 degrees per square-root-hour. 
     In some embodiments, the rotational rate has a drift of between 0.005 and 1.0 degrees per hour. 
     In some embodiments, the noise measured in angular degrees random walk per square root of hour is between 0.001 and 0.02 degrees. 
     In some embodiments, the length of the fiber coil is between 100 m and 2 km. 
     In some embodiments, the drift of the rotational rate, the noise measured in angular degrees random walk per square root of hour, and the length of the fiber coil, may be adjusted or selected by the user to achieve a desired performance or operational capability. 
     The present disclosure contemplates that the fiber coil length selected, and the software appropriately programmed for the fiber coil length, so the fiber optic gyroscope as described above may be operationally specified to meet the customer&#39;s or application&#39;s desired performance parameters substantially within the ranges suggested above using currently available components. 
     While the present disclosure illustrates and describes a fiber optic gyroscope, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present disclosure. 
     It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. In particular, certain configurations presented according to particular aspects of the present invention have been shown and described as discrete elements, i.e., lasers, splitters, combiners, mirrors, lenses, shifters, fiber optical cable, etc. Those skilled in the art will readily appreciate that many or all of these individual, discrete components may be fabricated and/or packaged into integrated elements. By way of particular example, the use of integrated waveguides and associated structures is envisioned for the described structures and arrangements. Alternatively, the discrete elements, i.e., lasers, splitters, combiners, mirrors, lenses, shifters, etc. may also be individually-packaged in modules with optical fiber interconnects to achieve the same topology and functionality. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The foregoing described embodiments depict different components contained within, or connected with, different other components. It is to be understood that such depicted arrangements or architectures are merely exemplary, and that in fact many other arrangements or architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of specific structures, architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected” or “operably coupled” to each other to achieve the desired functionality. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). 
     Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc. and are also not intended to be limiting. 
     Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the disclosed technology for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.