Abstract:
An apparatus for detecting rotation and a method for constructing the apparatus are provided. The apparatus comprises an optical fiber having a hollow passageway therethrough, a laser medium within the hollow passageway and interconnecting the first and second portions of the hollow passageway, and first and second electrodes contacting the laser medium such that when a voltage is applied across the first and second electrodes, the laser medium is excited such that the laser medium emits laser light through the hollow passageway.

Description:
TECHNICAL FIELD 
   The present invention generally relates to gyroscope systems, and more particularly relates to optical gyroscopes, such as fiber optic gyroscopes and ring laser gyroscopes. 
   BACKGROUND 
   Ring laser gyroscopes (RLGs) and fiber optic gyroscopes (FOGs) have become widely used technologies in many systems to sense the rotation and angular orientation of various objects, such as aerospace vehicles. Both RLGs and FOGs work by directing light in opposite directions around a closed optical path enclosing an area whose normal is along an axis of rotation. If the device is rotated about the axis of rotation, the optical path length for the light traveling in one direction will be reduced, while the optical path length far the light traveling in the opposite direction will be increased. The change in path length causes a phase shiftbetween the two light waves that is proportional to the rate of rotation. 
   Generally speaking, the signal to noise sensitivity of such gyroscopes increases as the optical path lengths and diameters of the closed path are increased. In this sense, both RLGs and FOGs have an advantage in that light is directed around the axis of rotation multiple times. In RLGs, a series of mirrors is used to repeatedly reflect the light around the axis forming a high finesse resonator. In FOGs, the light travels around the axis through a coil (with numerous turns) of optical fiber, which often has a length of several kilometers. 
   In recent years, resonator fiber optic gyroscopes (RFOGs) have been developed which combine the above-described path length benefits of RLGs and. FOGs into a single device that uses both a recirculating element, such as a mirror or a fiber coupler, and a multi-turn optical fiber coil to form an optical resonator. The combination of the benefits of RLGs and FOGs allows RFOGs to use shorter optical fiber and to be very small. One difficulty associated with RFOGs is that phase shifts often occur that are not attributable to rotation, but rather errors due to the fact that monochromatic light is propagating in a glass medium provided by a conventional optical fiber. Additionally, besides the mirror and/or coupler and fiber coil, RFOGs typically rely on complicated optical and electronic systems to process and modulate the laser light that is directed into the resonator from an external laser or lasers, as well as process the light signal that comes out of the resonator. These electronic systems, including the external laser(s), can increase the overall size and costs of the RFOGs. 
   Accordingly, it is desirable to provide a gyroscope system with simplified electronic signal processing systems. In addition, it is desirable to provide a gyroscope system that does not require a separate, external laser source or multiple sources. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
   BRIEF SUMMARY 
   An apparatus is provided for detecting rotation The apparatus comprises an optical fiber having a hollow passageway therethrough, a laser medium within the hollow passageway, and first and second electrodes contacting the laser medium such that when a voltage is applied across the first and second electrodes, the laser medium is excited such that the laser medium emits laser light through the hollow passageway. 
   A method is provided for constructing a fiber optic gyroscope. The method comprises providing an optical fiber having an outer surface, first and second opposing ends, a central axis, and a hollow passageway therethrough, the hollow passageway having first and second portions and being symmetric about the central axis, forming first and second holes in the outer surface of the optical fiber to the hollow passageway, the first hole being adjacent to the first portion of the hollow passageway and the second hole being adjacent to the second portion of the hollow passageway, providing a laser medium in the hollow passageway, the laser medium interconnecting the first and second portions of the hollow passageway, and placing respective first and second electrodes into the first and second holes, the first electrode contacting the laser medium at the first portion of the hollow passageway and the second electrode contacting the laser medium at the second portion of the passageway such that when a voltage is applied across the first and second electrodes, the laser medium is excited such that the laser medium emits laser light through the hollow passageway. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is a schematic view of a fiber optic gyroscope system according to one embodiment of the present invention including an optical fiber assembly and an optics sub-system; 
       FIG. 2  is a cross-sectional schematic view of the optical fiber assembly illustrated in  FIG. 1 ; 
       FIG. 3  is a cross-sectional schematic view of a portion of the optical fiber assembly illustrated in  FIG. 2 ; 
       FIG. 4  is a cross-sectional view of the portion of the optical fiber assembly illustrated in  FIG. 3  taken along line  4 - 4 ; 
       FIG. 5  is a top plan view of a portion of the optical fiber assembly and the optics sub-system illustrated in  FIG. 1 ; and 
       FIG. 6  is a schematic view of a fiber optic gyroscope system according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. It should also be noted that  FIGS. 1-6  are merely illustrative and may not be drawn to scale. 
     FIG. 1  to  FIG. 6  illustrate a gyroscope system. The gyroscope system includes an optical fiber with a hollow core that contains a laser medium, such as a gaseous compound or mixture. Holes are formed in the optical fiber, which extend to the hollow core to expose the laser medium within. Electrodes are inserted into the holes to seal the hollow core and contact the laser medium. A power supply is connected to the electrodes to supply a voltage across the electrodes to thereby cause a discharge that excites the laser gain medium and therefore creates laser light within the optical fiber. 
     FIG. 1  illustrates a gyroscope system  10  according to one embodiment of the present invention. The gyroscope system  10  includes an optical fiber assembly  12 , an, optics sub-system  14 , a photodetector  16 , a processor/controller  18 , and a power supply  20 . As will be described in greater detail below, the system  10  may be implemented as either a type of ring laser gyro (RLG), a type of resonator fiber optic gyro (RFOG), or a combination or hybrid of both an RLG and a RFOG, as will be appreciated by one skilled in the art. 
   The optical fiber assembly  12  includes an optical fiber  22  and three electrodes  24 ,  26 , and  28 . As illustrated in  FIG. 1 , the optical fiber  22  has a first end  30  and a second end  32 , each being directed toward the optics subsystem  14 . As is discussed in greater detail below, a highly reflective mirror  66  in the optical subsystem is positioned to receive the light from fiber end  30  and reflect a large majority of the light emitted from fiber end  30  into fiber end  32 , thus forming a resonant cavity for light traveling in the clockwise (CW) direction. Likewise, the mirror  66  is positioned to reflect a large majority of the light exiting fiber end  32  into fiber  30 , thus forming a resonant cavity in the counterclockwise (CCW) direction. Lenses  64  may be used to collimate or spatially condition the light to minimize fiber-end to fiber-end optical losses. The fiber  22 , the mirror  66 , and the lenses form an optical ring-resonator which has resonant frequencies in the CW and CCW directions determined by the roundtrip optical pathlength inside the resonator path in each direction, respectively. In the presence of rotation about an axis perpendicular to the plane of the resonator path, the pathlengths will not be the same, and the resonance frequency of the CW and CCW directions will be different, with the difference proportional to the rotation rate magnitude. When the gain medium is excited and lasing occurs, the laser frequencies in the CW and CCW directions are shifted from each other, proportionally to the rotation rate. 
   Referring again to  FIG. 1 , a central portion of the optical fiber  22  is wound, or arranged, in a coil  34  having an outer diameter  36  of, for example, less than 3 cm with between 20 and 40 turns. In one embodiment, the outer diameter  36  is approximately 1 cm. Although not illustrated, the coil  34  is formed around an axis of rotation, as is commonly understood. 
     FIG. 2  schematically illustrates the optical fiber assembly  12  in greater detail with the optical fiber  22  being shown un-wound. The optical fiber  22  may have a length  38  of greater than 1 m and be substantially symmetric about a mid-point thereof. In one embodiment, the length  38  of the optical fiber  22  is approximately 1.25 m. The optical fiber  22  is, in a preferred embodiment, a glass-based, hollow core, band-gap, optical fiber with an extremely low bend loss. Referring to  FIGS. 3 and 4 , the optical fiber  22  includes a region  40  of periodic photonic crystal cells, having a thickness  42  of, for example, between 30 and 40 microns, around a central passageway  44  (i.e., hollow core). The central passageway  44  has a radius  46  of, for example, between 2.5 and 10 microns and extends in a direction parallel to, and is symmetric about, a central axis  48  of the optical fiber  22 . The optical fiber  22  also has an outer glass layer  50  which forms an outer surface  52  of the optical fiber  22 . The optical fiber  22 , in one embodiment, has an overall diameter  54  of between 100 and 125 microns. 
   Referring to  FIG. 2  in combination with  FIGS. 3 and 4 , the electrodes  24 ,  26 , and  28  are inserted into holes  56  which lie respectively near the first end  30 , a central portion (e.g., the mid-point), and the second end  32  of the optical fiber  22 . Although  FIGS. 3 and 4  only specifically illustrated electrode  26 , it should be understood that electrodes  24  and  28  may be connected to the optical fiber  22  in a similar fashion. In the embodiment shown, a distance between adjacent electrodes, as measured along the optical fiber  22 , may be approximately 0.6125 m (i.e., half the length of the optical fiber  22 ). The holes  56  extend through the glass layer  50  and the periodic photonic crystal cell region  40  to expose the central passageway  44 . The holes  56  may have widths  58  of, for example, between 5 and 20 microns or larger and may be formed using a carbon dioxide laser, as is commonly understood. In the embodiment shown, the electrodes  24 ,  26 , and  28  are pins with widths similar to the widths  58  of the holes to form a seal over the central passageway  44 . Referring specifically to  FIG. 3 , a lower surface  60  of the electrodes  24 ,  26 , and  28  lies a distance away from the central axis  48  of the optical fiber  22  that is greater than the radius  46  of the central passageway  44  so that the electrodes  24 ,  26 , and  28  do not protrude into the central passageway  44 . As shown in  FIG. 4 , the holes  56 , and thus the electrodes  24 ,  26 , and  28 , extend into the optical fiber  22  in a direction that is substantially perpendicular to the central axis  48 . 
   Although not specifically illustrated, a laser (or gain) medium may be formed in, or injected into, the central passageway  44 . The laser medium may have an index of refraction such that light propagates in nearly free space. Therefore, environmental changes will have virtually no effect on the way light propagates through the gain medium. The laser medium may be a low pressure gaseous compound, such as helium neon (HeNe) or a mixture of gases. Referring again to  FIGS. 2 and 3 , the electrodes  24 ,  26 , and  28  contact the laser medium at different portions of the central passageway and prevent the laser medium from leaking from the optical fiber  22  through the holes  56 . Additionally, referring to  FIG. 2 , end caps  62  may be added to the first end  30  and the second end  32  of the optical fiber  22  to prevent the laser medium from leaking from the ends  30  and  32 . Although not specifically illustrated, the end caps  62  may be angled at Brewster&#39;s Angle to eliminate losses and reflections, as is commonly understood, or there may be a tapered region in the endcap to prevent Fresnel-reflection losses. 
   Referring again to  FIG. 1 , the optics sub-system  14  includes lenses  64 , a recirculator  66 , mirrors  68 , and a beam splitter  70 . Although not specifically illustrated, the lenses  64  are each positioned near a respective end of the optical fiber  22  with a central axis thereof congruent to the central axis  48  of the optical fiber  22  shown in  FIGS. 3 and 4 . The recirculator  66  is a mirror with a very high reflectivity (e.g., above 95%) and a non-zero transmittance. As is commonly understood, the recirculator  66  may have a reflectivity for a desired state of polarization of light that is significantly higher than the reflectivity for the state of polarization of light that is orthogonal to the desired state of polarization of light. The recirculator  66  positioned on a side of both lenses  64  directly opposing the ends  30  and  32  of the optical fiber  22  and oriented at an angle to both ends  30  and  32 . This angle may be chosen, in combination with a design of the mirror, to provide high loss for one polarization state, while providing low loss for the other. This enhances the ability for the laser to emit at a single frequency in each direction. As is commonly understood, the optical fiber  22  and the recirculator  66  may jointly form an optical resonator. 
   The mirrors  68  are positioned to receive light that is transmitted from the resonator in each direction respectively. Light from the ring laser resonator is transmitted through the recirculator  66  for the purposes of reading out the rotation-signal information. The mirrors  68  are positioned to receive light from the resonator output in the two directions respectively direct the two light beams to be recombined, or interfered via beam splitter  70 , which in this case, acts as a beam combiner. 
   Still referring to  FIG. 1 , the photodetector  16  is positioned on a side of the beam splitter  70  to receive combined light waves from the beam splitter  70  and the photodetector  16  includes a photodiode, as in commonly understood. The processor/controller  18  is in operable communication with the photodetector  16  and the power supply  20  and may include electronic components, including various circuitry and integrated circuits, such as an Application Specific Integration Circuit (ASIC) and/or instructions stored on a computer readable medium to be carried out by a computing system and perform the methods and processes described below. As shown, the power supply  20  is electrically connected to the electrodes  24 ,  26 , and  28  and although illustrated as a separate component, may be implemented as part of the processor/controller  18 . Although not depicted explicitly in  FIG. 1  the power supply  20  also supplies energy to the photodetector  16 , either directly, or via the processor/controller  18 . 
   During operation, referring to  FIGS. 1 and 2 , the power supply  20  supplies a voltage across electrode  26  and electrodes  24  and  28  so that a plasma discharge current runs from electrode  26  (i.e., anode), through the laser medium within the central passageway  44 , and into electrodes  24  and  28  (i.e., cathodes). As will be appreciated by one skilled in the art, a plasma discharge in the laser gain medium excites atomic and molecular transitions in the gain medium of the laser, and thus stimulates release of photons. Thus, laser light is generated within the central passageway  44  and propagates in both directions through the optical fiber  22 . That is, a first portion of the light propagates in the CW direction through the coil  34  towards the first end  30  of the optical fiber  22 , and a second portion of the light propagates in the CCW direction through the coil towards the second end  32  of the optical fiber  22 . The electrodes are arranged to ensure that particle flow due to the plasma discharge from electrode  26  to electrode  28  is equal and opposite to that from electrode  24  to electrode  28 , thus reducing or eliminating Fresnel-drag effects, as are well-known in the art. 
     FIG. 5  illustrates the ends  30  and  32  as the light exits the optical fiber  22 . As the first portion of light exits the first end  30  of the optical fiber  22 , the light wave spatially diverges in transitioning from propagation in the fiber to free space, and thus “fans out,” as shown. However, as the light passes through the lens  64 , it becomes collimated. The collimated light strikes the recirculator  66 , and because of the high reflectivity of the recirculator  66 , the majority of the light is reflected towards the second end  32  of the optical fiber  22 . As the collimated light passes through the lens  64 , it becomes re-focused before entering the second end  32  of the optical fiber  22 . Likewise, the second portion of light exits the second end  32  of the optical fiber  22  and is collimated by the first lens  64  through which it passes. As with the first portion of light, the majority of the second portion of light is reflected by the recirculator  66  towards the first end  30  and refocused by the second lens  64  through which it passes before entering the first end  30  of the optical fiber  22 . This process is continually repeated as the light circulating through the optical fiber  22  resonates within the resonator path, which is comprised of path inside the optical fiber  22  and the optical path from fiber end  30  to the recirculator  66  to fiber end  32 . 
   Still referring to  FIG. 5 , as previously suggested, not all of the first and second portions of light are reflected by the recirculator  66 , as a relatively small portion of each passes through (i.e., is transmitted) the recirculator  66 . As shown in  FIG. 1 , the light that passes through the recirculator  66  is directed by the mirrors  68  and the beam splitter  70  onto the photodetector  16 . As will be appreciated by one skilled in the art, the photodetector  16  is capable of detecting any relative phase shifts or frequency differences in the two light beams as caused by any rotation of the system  10  about the axis of rotation of the coil  34 . The photodetector  16  sends an electrical signal to the processor/controller  18  which processes the signal and determines the rate of rotation of the system  10 . It is further noted that, while not shown in  FIG. 1 , that errors in rotation rate measurement may arise because of light that is backscattered inside the resonator from one beam into the other, and vice versa. One solution that is commonly used in conventional ring laser gyros (not using hollow core fibers) is to mechanically dither the gyro about its axis of rotation, and to add a degree of randomization of the amplitude of the dither. In this way, errors, sometimes known as lockin, may be greatly reduced or substantially eliminated. This may be similarly employed in embodiments of the present invention. 
   One advantage of the gyroscope system described above is that because the light is generated within the optical fiber, the optical and electronic systems external to the resonator may be simplified. Another advantage is that it eliminates rotation rate errors that may stem from signal processing steps that might otherwise be used to derive the rotation signal if the laser light was not generated in resonator, i.e., if it was a passive resonator. 
     FIG. 6  illustrates a gyroscope system  72  according to another embodiment of the present invention. The gyroscope system  72  includes an optical fiber assembly  74 , an optics sub-system  76 , a photodetector  78 , a processor/controller  80 , and a power supply  82 . The system  72  may be in many respects similar to the system  10  shown in  FIG. 1 . However, in the embodiment illustrated in  FIG. 1 , the optical fiber assembly  74  includes only two electrodes  84  connected to an optical fiber  86 . Additionally, the distance between the electrodes  84  (as measured along the optical fiber  86 ) has been reduced to facilitate in the lasing of the laser medium within the optical fiber  86 . As will be appreciated by one skilled in the art, this distance may be varied, along with the voltage applied across the electrodes  84 , depending on the amount (i.e., concentration or density) and type of the laser medium within the optical fiber  86 . Further, a third electrode could be added at an equal distance to the middle one as in  FIG. 1 , so that the flow induced by the plasma discharge may be counterbalanced, as it is in  FIG. 1 . 
   Additionally, in the embodiment shown in  FIG. 6 , the recirculator  70  and the lenses  64 , shown in  FIG. 1 , have been replaced by a concave recirculator  88 . The concave recirculator  88  may the same high reflectivity as the recirculator  66  shown in  FIG. 1 . However, because of the shape of the concave recirculator  88 , light that is reflected by the concave recirculator  88  is re-focused before re-entering the optical fiber  86 . Thus, a further advantage of the embodiment illustrated in  FIG. 6  is that the light can be recirculated without the use of lens, which further simplifies and reduces the cost of the system  72 , and reduces errors that may be caused by environment influences and non-linearities in the glass medium of the lenses. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in anyway. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.