Researchers use new hollow-core optical fiber to improve gyroscope performance

According to foreign media reports, researchers from Honeywell and the University of Southampton's Optoelectronics Research Centre in the UK have used a new type of hollow core optical fiber to overcome several factors that previously limited the performance of resonator fiber optic gyroscopes, greatly improving the performance of the gyroscopes, 500 times higher than previously published sensors involving hollow core fibers. Resonator fiber optic gyroscopes are a type of optical fiber sensor that uses only light to sense rotation. Since gyroscopes are the basis of most navigation systems, this new work is expected to bring significant improvements to these systems.

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"High-performance gyroscopes are used for navigation in a variety of air, land, sea and space applications," said Glen A. Sanders of Honeywell Research. "Although our gyroscopes are still in the early stages of development, if they can be used to their full potential, they will be part of the next generation of navigation technology that will be more accurate, smaller and lighter." Sanders added, "We expect to use these gyroscopes in navigation systems for the next generation of civil aviation, autonomous vehicles, and other applications. And as performance continues to improve, we expect to open up entirely new capabilities and applications."

Resonant fiber gyroscopes use two laser beams that travel in opposite directions through a fiber coil. The ends of the fibers are connected to form an optical resonator so that most of the light circulates around the coil. When the coil is stationary, the beams traveling in both directions have the same resonant frequency; but when the coil rotates, the resonant frequencies change relative to each other, which can be used to calculate the direction of motion or to locate the vehicle or device equipped with the gyroscope.

Honeywell has been developing resonant cavity fiber gyroscope technology for some time because it has the potential to provide high-precision navigation in smaller devices compared to current sensors. However, it is difficult to find an optical fiber that can consume very low laser power levels at the ultra-thin laser linewidth required for gyroscopes without generating nonlinear effects or even degrading sensor performance. Sanders said, "In 2006, we proposed the use of hollow-core fibers for resonant cavity fiber gyroscopes. Because these fibers confine light to a gas-filled void, sensors based on such fibers do not suffer from nonlinear effects like sensors based on solid fibers."

The researchers wanted to see if a new type of fiber, called node-free antiresonant fibers (NANFs), could offer further improvements. These fibers exhibit lower levels of nonlinear effects than other hollow-core fibers. NANFs also have lower optical attenuation, meaning that light maintains its intensity over longer propagation lengths through the fiber, which improves the quality of the resonator. In fact, these fibers have been shown to have the lowest optical losses of any hollow-core fiber and, in many parts of the optical spectrum, the lowest losses of any fiber.

For a resonant cavity fiber gyroscope, it is most important to ensure that light propagates along only one path in the optical fiber. NANF helps eliminate optical errors caused by backscattering, polarization coupling, and modal impurities, which are all potential sources of gyroscope errors or noise. Eliminating these errors eliminates the most important performance limiting factor of other fiber optic technologies. Sanders said, "While the most important component of this sensor is the new optical fiber, we are also working hard to significantly reduce noise while improving the accuracy of resonant frequency sensing. This is critical to improving sensor performance and reducing sensor size."

Honeywell researchers conducted laboratory studies to characterize the performance of this new fiber optic gyro sensor under stable rotation conditions (i.e., in the presence of only the Earth's rotation). To eliminate noise and interference in free-space optical setups, the gyro was mounted on a stable, static support post. Using NANF, the researchers achieved long-term bias stability of 0.05 degrees per hour, close to the level required for civil aircraft navigation.

"By demonstrating the high performance of NANF in this extremely demanding application, we hope to use these fibers in other precision scientific resonant cavities," said researcher Taranta. Currently, the researchers are working on creating a more compact and stable gyroscope prototype. In addition, they also plan to use the latest generation of NANF, which reduces optical losses by 4 times and greatly improves modal and polarization purity.