Researchers Improve Performance of Light-Based Fiber Optic Sensor

Scientists have made a significant breakthrough by improving the performance of resonator fiber optic gyroscopes—a kind of fiber optic sensor that uses light to sense rotations.

Since gyroscopes form the basis of a majority of the navigation systems, the latest study could someday bring significant enhancements to such systems.

“High-performance gyroscopes are used for navigation in many types of air, ground, marine and space applications,” stated Glen A. Sanders, who headed the research group from Honeywell International.

Although our gyroscope is still in the early stages of development, if it reaches its full performance capabilities it will be poised to be among the next generation of guidance and navigation technologies that not only push the bounds of accuracy but do so at reduced size and weight.

Glen A. Sanders, Honeywell International

The research team, from International Honeywell and the Optoelectronics Research Centre at the University of Southampton in the United Kingdom, has demonstrated how they effectively applied a new kind of hollow-core optical fiber to resolve many factors that have restricted the performance of earlier resonator fiber optic gyroscopes. The study was published in Optics Letters, the journal of The Optical Society (OSA).

Through this method, the researchers were able to enhance even the most challenging performance requirement of the stability of the gyroscope by as much as 500 times over formerly published studies that involved hollow-core fibers.

We hope to see these gyroscopes used in the next-generation of civil aviation, autonomous vehicles and the many other applications in which navigation systems are employed. Indeed, as we enhance the performance of guidance and navigation systems, we hope to open entirely new capabilities and applications.

Glen A. Sanders, Honeywell International

Sensing Rotation with Light

Two lasers used by resonator fiber optic gyroscopes pass via a coil of optical fiber in reverse directions. The fiber ends are linked to create an optical resonator so that a large portion of the light recirculates and takes numerous trips around the coil of optical fiber.

When the coil is inactive, the beam of light that travels in both directions shares the same kind of resonance frequency. However, when the coil revolves, the resonance frequencies move relative to one another in a way that can be utilized to determine the direction of orientation or movement for the device or vehicle on which the gyroscope is placed.

For quite some time, Honeywell International has been designing resonator fiber optic gyroscope technology because this instrument is capable of delivering high-accuracy navigation in a more compact-sized device when compared to the present generation of sensors.

But scientists have found it difficult to detect an optical fiber that can tolerate even the modest levels of laser power at the ultra-fine laser linewidths needed by such gyroscopes without inducing any nonlinear effects that affect the performance of the sensor.

In 2006, we proposed using a hollow-core fiber for the resonator fiber optic gyroscope. Because these fibers confine the light in a central air or gas-filled void, sensors based on them don’t suffer from the nonlinear effects that plague sensors based on solid fibers.

Glen A. Sanders, Honeywell International

Using an Even Better Fiber

In the latest study, headed by Austin Taranta from the University of Southampton, the team set out to find out whether an entirely new kind of hollow-core fiber could offer even more enhancements. The new range of fibers, called nodeless antiresonant fiber (NANF), has nonlinear effects that are even lower than other kinds of hollow-core fibers.

Furthermore, NANFs exhibit low optical attenuation, which enhances the quality of the resonator because the light can sustain its intensity across extended propagation lengths via the fiber. As a matter of fact, such fibers have been demonstrated to have the lowest light loss when compared to all hollow-core fibers and, for many spectral components, have the lowest loss of all optical fibers.

In the case of resonator fiber optic gyroscopes, the light should pass only in a single direction via the fiber. The NANFs make this viable by removing the optical defects induced by modal impurities, polarization coupling, and backscattering, which are all known to be possible sources of additional noise or error in the gyroscope. The removal of such optical errors eliminates even the most major performance limiters for other types of fiber technologies.

“Although the backbone of this sensor is the new type of optical fiber, we also worked to greatly reduce noise when sensing the resonance frequency with unprecedented accuracy,” Sanders added. “This was crucial for enhancing the performance and moving toward miniaturizing the sensor.”

Achieving Long-Term Stability

The researchers from Honeywell International conducted laboratory studies to define the performance of the latest fiber optic gyroscope sensor under consistent rotation conditions, that is, only in the presence of the rotation of the Earth. This fact establishes the 'bias stability' of the instrument.

To remove disturbances and noise in the free-space optical setup, the researchers mounted the gyroscope on a stable, static pier. They subsequently integrated the NANFs and showed a long-term bias stability of 0.05° every hour, which is proximal to the levels needed to navigate civil aircraft.

“By demonstrating the high performance of NANFs in this extremely demanding application, we hope to show the exceptional promise of these fibers for use in other precision scientific resonant cavities,” Taranta stated.

At present, the team is looking for ways to create a prototype gyroscope that has a more stable and compact configuration. They have also planned to integrate the new generation of NANFs, which has a fourfold enhancement in optical losses, with considerably better polarization and modal purity.

Journal Reference:

Sanders, G. A., et al. (2020) A Hollow-Core Resonator Fiber Optic Gyroscope Using Nodeless Anti-Resonant Fiber. Optics Letters. doi.org/10.1364/OL.410387.