Patent Application
20100027022
Gyros have been used to measure rotation rates or changes in angular velocity about an axis.
In a resonator fiber optic gyro (RFOG), the counter-propagating light beams are monochromatic and recirculate through multiple turns of the coil and for multiple passes through the coil using an external recirculator such as a reflective device. The beam generating device typically modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on a constructive interference of successively recirculated beams in each optical path. A rotation of the coil produces a shift in the respective resonance frequencies of the resonant coil and the frequency difference associated with tuning the CW beam and CCW beam frequencies to match the coil's resonance frequency shift due to rotation indicates the rotation rate. A reflective mirror may be used to recirculate the counter-propagating light beams in the coil but this typically reduces the signal-to-noise ratio from losses generated at the transition from the mirror to the coil.
Accordingly, it is desirable to provide a fiber optic gyro capable of measuring rotational rates with an accuracy sufficient for navigation systems. In addition, it is desirable to provide a high accuracy fiber optic gyro for integration with relatively small platforms and made relatively inexpensively. The RFOG's key to getting good performance for a given coil diameter is to have low fiber-to-fiber coupling loss so that the light makes many trips through the fiber. The prior art in this field, shown in
Another coupling design is shown in
The present invention provides an improved resonator fiber-optic gyro (RFOG). An example RFOG includes a closed-coil resonator where counter-propagating laser beams are done by fiber couplers. Signals are extracted from the ring resonator using other fiber couplers.
The fiber couplers may be fiber spliced couplers, free-space fiber-to-fiber coupling elements. A silicon structure may be used to align components of the gyro or just the coupling elements.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
Examples of the optical couplers 110, 112, 114 may include fiber splicing optical couplers and very short free-space fiber-to-fiber coupling elements. The coupling element provides a spacing of a few microns.
The ends of the hollow core fiber resonator 24 are joined in any of a number of different manners. For example, an optically compliant epoxy is used to bond the ends together. In another example, the ends are aligned in a micro-machined silicon structure. The silicon structure may include precision etched v-grooves sized to receive the ends of the fiber coil, splice together, laser welding, and/or use optical fiber connectors. The ends of the resonator 24 may also be bonded in a manner that allows a gap to exist between the fiber ends.
In one embodiment, the couplers are non-mirror/beam splitter input/output optical couplers that allow for evanescent coupling between the resonator 24 and the fiber leads connected to the light sources 30, 40 or the sensors 50, 60. The evanescent coupling can occur at or near a gap formed between ends of the resonator 24 (i.e., a break in the fiber loop) and the fiber leads. In this embodiment, there is not a need to couple the ends of the resonator 24 together as described above.
In another embodiment, evanescent coupling occurs when the resonator 24 is located adjacent to the fiber lead. This can be accomplished by removing a section of cladding of the resonator 24 and/or the fiber lead and placing the partially exposed fibers in close proximity to each other. A significant fraction of the cladding thickness is removed to accomplish this coupling.
In one embodiment, the resonator 24 is integrated into a prefabricated structured silicon chip/substrate. The chip is formed to precisely receive the resonator 24 and the couplers 110, 112 and 114. This would improve the accuracy of aligning the components, thus reducing alignment errors and improving sensitivity of the RFOG.
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. 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.
