Ring laser gyroscope that does not require mirrors

[0001] This invention relates in general to gyroscopes and more specifically to ring laser gyroscopes.

[0002] Navigational systems represent an important use of gyroscopes since it is important to know and/or maintain a specific orientation of a vehicle or movable device relative to another location or object. A gyroscope outputs information related its change in orientation and hence can determine the orientation of the vehicle to which it is mounted. This output information can be processed and displayed to provide a measurement of the orientation of the vehicle and can also be used as a feedback signal coupled to a propulsion system to maintain a specified orientation of the vehicle.

[0003] Conventional gyroscopes with a spinning wheel make use of the effect of precession in which the spinning wheel maintains its orientation relative to the plane in which the wheel spends. Although spinning wheel gyroscopes have been effective, such gyroscopes have limitations in accuracy and durability due to friction and mechanical aspects associated with the spinning wheel.

[0004] The ring laser gyroscope represents an improvement over the spinning wheel gyroscope by eliminating the need for moving parts. The Sagnac effect is a relativistic effect related to the rotation of a closed optical path. The Sagnac effect in a ring laser results in a frequency difference between counter rotating laser beams (modes) when the ring is rotated such frequency difference being proportional to the angular rate about an axis perpendicular to the plane of the beam path. A conventional ring laser gyroscope uses a series of mirrors to reflect two free-space laser beams in opposite directions around a “ring”. The ring consists of a triangle, rectangle, or other closed multiple-sided geometric configuration in which mirrors are disposed at the corners to reflect the laser beams along the adjacent straight legs in the configuration. The two laser beams create a virtual interference pattern or standing wave around the ring which remains stationary in inertial space. The movement of a ring laser gyroscope about the plane of the ring causes a shift of the position of the body of the gyroscope relative to the standing wave depending on the direction of the movement of the gyroscope. A detector coupled to the ring measures the position of the detector/ring relative to the standing wave and provides an output signal related to the change in orientation of the ring.

[0005] Extremely high-quality mirrors are required for conventional ring laser gyroscopes and must be made to exacting standards of flatness, high reflectivity, and low dispersion. Producing the mirrors to such exacting standards results in high manufacturing costs that must be reflected in the total cost of the gyroscope. Thus, there exists a need for an improved gyroscope that can substantially overcome the difficulties and costs associated with requiring multiple mirrors.

[0006] In accordance with an embodiment of a ring laser gyroscope pursuant to the present invention, a hollow core photonic fiber is filled with a material, e.g. a gas, that will generate and sustain laser beams within the fiber upon being excited by an energy source. A detector coupled to the fiber detects a standing wave pattern within the fiber, wherein changes in the detected standing wave pattern indicates a corresponding change in the orientation of the fiber.

[0010] FIG. 1 shows an embodiment 10 of a ring laser gyroscope in accordance with the present invention. The ring 12 is preferably constructed of a hollow photonic bandgap fiber 14 and is shown in FIG. 1 as if it had been severed along its longitudinal centerline to better show that the interior is hollow. It will be understood that fiber 14 is actually a closed cylindrical elongated fiber. The ends 16 and 18 of the fiber 14 are coupled together by a splice 20 to create a closed configuration that defines a continuous, hollow passage 22. The purpose of the splice 20 is to connect the ends 16 and 18 of the fiber 14 together to form a continuous loop with a minimum of discontinuities being introduced by the splice. Before the ends 16 and 18 are coupled together, the passage 22 is filled, preferably completely, with a material, such as a gas, that will lase when excited by externally provided energy. For example, helium neon gas can be utilized and when excited produces a laser beam of 633 nanometer wavelength. If helium neon gas is used, the fiber is preferably designed to pass this wavelength with a minimum of attenuation. The ring 12 may be mounted to a substantially planar base 24 that provides a mechanically stable environment for the ring. The plane of the ring 12 is preferably parallel to the plane of the base 24.

[0011] Additional elements are associated with the ring 12 to comprise the gyroscope 10. An energy source 26 is used to supply a sufficient amount of energy to the gas in the fiber in order to cause the gas to lase. In this example, energy source 26 can be a source of radio frequency energy that is coupled to the fiber 14 via a coil 28 that encircles the fiber.

[0012] A coupler/combiner 32 is used to extract a small portion of the light from each of the counter-propagating beams in fiber 14. Coupler/combiner 32 also couples the beams in order to create an observable interference pattern. A detector module 30 detects the movement of the peaks and valleys of the standing wave created by the interference of the two counter-propagating beams in coupler/combiner 32. The detector module 30 may, for example, be a photo-detector capable of discerning changes in light intensity representing the peaks and valleys in the standing wave.

[0013] Outputs 34 of detector module 30 are coupled to signal processor 36. The analog signal representing the sensed light intensity generated by detector module 30 is converted by the detector module 30 or signal processor 36 into a digital signal for further processing by the signal processor 36. An exemplary signal processor 36 may include a microprocessor, read-only memory (ROM), and random access memory (RAM). It may also include an analog to digital (A/D) converter if the signal processor is required to convert an analog output from detector 30 into digital form. Stored program control instructions may be stored in the ROM to control the processing of signal information from the detector 30 by the microprocessor. Depending upon the specific application, the signal processor 36 may also include a visual display showing the sensed orientation of the ring 12 relative to an initial point of reference, and may include a digital output 38 that carries a digital output signal representative of the sensed orientation. The digital output signal may be used to track the orientation of the body, or may be used to stabilize a body. For example, the digital output signal may be utilized as a feedback signal coupled to a propulsion system (not shown) that can be utilized to correct or adjust the position of a vehicle in order to maintain a desired orientation as sensed by the gyroscope 10. Although the energy source 26 and signal processor 36 is shown in FIG. 1 as not being supported by base 24, these elements may be supported by the base depending upon the specific application.

[0014] FIG. 2 shows a cross-section of fiber 14. The fiber preferably has a generally circular wall with an outer surface 40 and an inner surface 42. The inner surface 42 defines a hollow interior chamber 44 that is filled with the helium neon gas in the illustrative example. As will be known to those skilled in the art, hollow-core photonic bandgap filters of various designs are known. In the illustrative example, a single hollow chamber is enclosed by the wall of fiber 14. For this construction, multiple layers of alternating high and low index of refraction materials are sandwiched together to form the wall, with the materials being selected to provide low losses for the laser wavelength to be utilized with the fiber. Alternatively, photonic bandgap fibers have been constructed from arrays of parallel air holes in a solid dielectric. As used herein, a photonic bandgap fiber means all types of such fibers that contain one or more hollow holes that can be filled with a material that will lase as well as photonic bandgap fibers may be filled or constructed with a solid or liquid material that can provide optical gain and sustain the counter-propagating laser beams.

[0015] FIG. 3 is a longitudinal cross-sectional view of a portion of the fiber 14. It shows a virtual pattern of standing waves disposed about the longitudinal centerline 46 of the fiber. It is understood that this interference pattern is not actually realized or observable unless the beams are coupled together. Thus FIG. 3 shows what the “virtual” interference would be if the beams were interfered at every point along the path. In reality, the beams are typically only interfered at one given location defined by the coupler/combiner 32. Referring to FIG. 3, a plurality of peaks 48 and valleys 50 extend along the path defined by fiber core 44. The peak intensity of the interference pattern is indicated by the level 49 while the minimum intensity is indicated by level 51. The virtual standing waves are the result of constructive and destructive interference between two laser beams (modes) traveling in opposite directions in the hollow chamber 44 in the closed ring of fiber 14. Two laser beams occur in this environment whenever sufficient energy is coupled to the gas within chamber 44 as the excitation of this gas causes amplified stimulated emission resulting in lasing within the closed ring 14.

[0016] The property of a ring laser that makes its suitable for use as a component in a gyroscope is that the virtual standing wave pattern remains stationary relative to an external fixed point of reference even if the ring itself is rotated about its axis. Thus, as the ring laser is rotated about its center 52, the wall of the fiber 14 will move relative to the standing wave pattern causing the detector 30 to sense relative movement of the peaks and valleys of the standing wave pattern relative to the detector which is fixed at a location on the fiber. Thus, the number of peaks and/or valleys that is sensed passing by the detector 30 can be utilized to measure the amount of rotation of the ring about its center.

[0017] As will be apparent for the above explanation of the illustrative embodiment, a ring laser gyroscope in accordance with the present invention is provided which does not require the use of mirrors. Conventional ring laser gyroscopes that utilize mirrors require the precise positioning and alignment of such mirrors. Additionally, the mirrors are expensive to manufacture due to the high exacting standards required to yield mirrors with suitable characteristics. Thus, the difficulties associated with the use of such mirrors in conventional ring laser gyroscopes are avoided.

[0018] Although an embodiment of the invention has been described above and shown in the figures, it will be apparent to those skilled in the art that various modifications and variations can be made. For example, various types of other energy sources can be utilized including, but not limited to, a source of light energy such as another laser as well as other types of sources capable of coupling sufficient energy to the gas/material within the fiber to cause it to lase. Other gases or materials capable of sustaining a laser beam could be used. The fiber can be designed to support the wavelength of operation with minimal losses. The shape of the fiber could be other types of closed geometric configurations as well as a circle.

[0019] The scope of the present invention is defined by the claims that follow, and is not to be limited to the specific embodiment disclosed above.