Micro-ring cavity gyroscope with magnetic field lock-in minimization

#Ring laser gyrocopes based on the Sagnac effect are commonly used in many modem navigational and aerospace guidance systems. In summary, two light beams are caused to propagate in opposite directions around a closed-loop path. Rotation of this closed loop path causes the effective path length in one direction to become shorter while lengthening the effective path length in the other direction. This change in path length is a direct measure of inertial rotation. Further details regarding the general operation of a ring laser gyroscope may be found in U.S. Pat. No. 3,373,650, by J. E. Killpatrick.

#The two most widely used versions for both civilian and military applications are the Ring Laser Gyroscope (RLG), and the Interferometric Fiber Optic Gyroscope (IFOG). While these are proven devices, there are limitations to the reduction in size and cost achievable with these devices. Both systems are based on the construction of relatively long closed paths that are difficult to manufacture and require a relatively large area (see for instance, the review by D. Z. Anderson in Scientific American, vol 254, #4, p. 94, April 1986). A gyroscope based on integrated solid state designs would be an attractive alternative to fiber-optic systems in terms of cost and manufacturablility, and ruggedness of design. One example is an integrated planar optical waveguide approach to a compact gyroscope as described by Ford, et al. in IEEE 2000, p. 285-290.

#In recent years integrated semiconductor ring lasers of very small size have been developed. Micro-ring cavity lasers and various means to construct them are discussed by Ho, et al. in U.S. Pat. No. 5,790,583 entitled “Photonic-Well Microcavity Light Emitting Devices” and U.S. Pat. No. 5,825,799 entitled “Microcavity Semiconductor Laser” and by Jezierski, et al. in IEE Proceedings, Vol. 135, Pt. J, No. 1, p. 17-24, February 1988. More recently a semiconductor ring laser in combination with a fiber-optic loop has been characterized in the context of an optical gyroscope by Taguchi, et al. in Optical and Quantum Electronics vol. 31: 1219-1226, 1999. In this study it was verified that lock-in phenomenon was one of the most dominant noise sources.

#FIG. 1 illustrates a perspective view of a micro-ring-cavity laser and standing wave detection means.

#FIG. 2 illustrates a top-down view of a micro-ring cavity laser and standing wave detection means.

#FIG. 3 illustrates a cutaway view of the micro-ring cavity gyroscope of the invention.

#The objective of the present invention is to provide a rotational sensor of microscopic dimension whose principle of operation is based on the effect rotational motion has on the propagation of electromagnetic wave along a closed path, commonly referred to as the Sagnac effect.

#Herein “micro-ring cavity laser” means any closed path that can support counter-propagating electromagnetic waves, is capable of generating light by stimulated emission of radiation, and comprises a magneto-optical material. The light amplifying medium may make up a portion of, or the entire closed path. The term “micro” means that the effective radius of such closed path is less than about 1000 microns. Preferably, the closed path has a radius of between about 20 microns and about 300 microns. In the case where a noncircular closed path is used, preferably the path length is between about 100 microns and about 2000 microns in length. This provides a preferred balance of sensitivity and manufacturability for the gyros of the invention. The device has to be sufficiently large in order to have sensitivity of practical value. As the sensitivity scales with size, the smaller the device, the lower the sensitivity; see for example equation (4) in W. W. Chow, et al, IEEE Journal of Quantum Electronics, vol. QE-16, No 9, September 1980. In particular, a device 1000 times smaller exhibits 1000 times lower sensitivity. Given that ring gyros of several meters across can sense rotation with an accuracy of 10−7 degrees per hour, (see for instance G. E. Stedman, et al, Phys. Rev. A, vol. 51, June 1995, p 4944.) a 50 micron radius device should be capable of sensing rotational speeds with an accuracy of 0.01 degrees per hour. This sensitivity would still be of practical value. On the other hand, in order to be lightweight, unobtrusive, and integrable into a variety of systems, the device has to be small. Hence the preferred upper limit on the size of the device.

#Preferred micro-ring cavity lasers for the invention are integrated solid state lasers. Lasers that are circular, triangular, square and polygons in geometry are preferred. Other preferred lasers take the form of a disc on a pedestal and a ring on a pedestal. Micro-ring cavity lasers and various means to construct them are discussed by Ho, et al. and Jezierski, et al. described above.