Hollow Core Fiber Improves Performance of Fiber Optic Gyroscopes

The Optical Society journal “Optics Letters” news release revealed that the research team from Honeywell International and the University of Southampton’s Optoelectronics Research Centre in the UK have incorporated a new type of hollow core optical fiber to boost the performance of resonator fiber optic gyroscopes. These new fiber incorporated gyroscopes could form the basis of future navigation technologies that are more compact and more accurate than today’s systems. The hollow core optical fiber is known as a nodeless antiresonant fiber.

This is an important new step in advancing the performance of resonator fiber optic gyroscopes. Resonator fiber optic gyroscopes are a new type of fiber optic sensor that senses rotation using light. Gyroscopes are an integral part of most of the navigation systems and therefore the new work could one day bring important improvements to these systems.

Glen A. Sanders, who leads the research team at Honeywell International said that the high-performance gyroscopes are used for navigation in many types of air, ground, marine, and space applications. Although the newly developed 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.

In the article, the team described how they used a new type of hollow core optical fiber to overcome several factors that have limited previous resonator fiber optic gyroscopes. This allowed them to improve the most demanding performance requirement of the gyroscope stability by as much as 500 times over previously published work involving hollow core fibers

What is an Optical Resonator?

An optical resonator,  also known as an optical cavity or resonating cavity is a set of mirrors arranged to form a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. Light is amplified in an optical resonator in the laser. In other words, an Optical resonator is a cavity with walls that reflect light. This setup allows standing wave modes to exist with little loss. Optical resonators are used in optical parametric oscillators and some interferometers. Light confined in the cavity reflects multiple times, producing standing waves for certain resonance frequencies.

Standing waves were first noticed by Michael Faraday. A standing wave or a stationary wave is a wave that oscillates in time but whose peak amplitude profile does not move in space. Since it doesn’t move, it got the name ‘Standing wave’. The peak amplitude of the wave oscillations at any point in space is constant with time, and the oscillations at different points throughout the wave are in phase. The locations at which the absolute value of the amplitude is minimum are called nodes, and the locations where the absolute value of the amplitude is maximum are called antinodes.

A resonator is a device or system that exhibits resonance or resonant behavior. A resonator naturally oscillates with greater amplitude at some frequencies than at other frequencies. The oscillations in a resonator can be either electromagnetic (such as light) or mechanical (that includes acoustic). Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal.

How Rotations are Sensed Using Light

Resonator fiber optic gyroscopes use two lasers that travel through a coil of optical fiber in opposite directions. The ends of the fiber are connected to form an optical resonator so that most of the light will recirculate and take multiple trips around the coil. When the coil is at rest, the light beams traveling in both directions share the same resonance frequency, but when the coil is rotating, the resonance frequencies shift relative to each other in a way that can be used to calculate the direction of movement or orientation for the vehicle or device on which the gyroscope is mounted

Honeywell International has been developing resonator fiber optic gyroscope technology for some time because of its potential to deliver high accuracy navigation in a smaller size device compared to current sensors. However, it has been challenging to identify an optical fiber that can withstand the even modest laser power levels at the ultra-fine laser linewidths required by these gyroscopes without causing nonlinear effects that degrade the sensor’s performance.

Sanders said that Honeywell had proposed using a hollow core fiber for the resonator fiber optic gyroscope in 2016. Since hollow core fibers confine the light in a central air or gas-filled void, sensors based on hollow core fiber do not suffer from the nonlinear effects. Nonlinear effects cause troubles to the sensors based on solid fibers.

Use of Hollow Core Optical Fiber

In the new work, led by Austin Taranta at the University of Southampton, the researchers wanted to see if an entirely new type of hollow core fiber could bring even more improvements. Known as nodeless antiresonant fiber (NANF), this new class of fibers exhibits even lower levels of nonlinear effects than other hollow core fibers.

NANFs also have low optical attenuation, which improves the quality of the resonator because the light maintains its intensity over longer propagation lengths through the fiber. In fact, these fibers have been shown to have the lowest light loss of any hollow core fiber, and for many parts of the spectrum, the lowest loss of any optical fiber.

For resonator fiber optic gyroscopes, it is crucial that the light travels only in a single path through the fiber. The NANFs help make this possible by eliminating optical errors caused by backscattering, polarization coupling, and modal impurities, which are all potential sources of error or extra noise in the gyroscope. Their elimination removes the most significant performance limiters for other fiber technologies.

The Honeywell researchers performed laboratory studies to characterize the performance of the new fiber optic gyroscope sensor under stable rotation conditions, i.e., only in the presence of Earth’s rotation. This establishes the instrument’s “bias stability”. To eliminate noise and disturbances in the free-space optical setup, the gyroscope was mounted on a stable, static pier. By incorporating the NANFs, the researchers were able to demonstrate long-term bias stability of 0.05 degrees per hour, which is close to the levels required for civil aircraft navigation.

The researchers are now working to make a prototype gyroscope with a more compact and stable configuration. They also plan to incorporate the latest generation NANFs, which exhibit a four times improvement in optical losses, along with greatly improved modal and polarization purity.

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