German-Polish group makes optical gyroscopes less susceptible to interference

19 Aug 2022

Use of hollow core fiber makes scopes more resilient – therefore better for driving and aerospace applications.

The persistent narrative of cars that drive or planes that fly themselves will only come true if the on board systems can precisely determine where the vehicle is located at any time. In the aerospace sector, this function depends on optical gyroscopes that measure light to check and stabilize the aircraft’s course in flight.

Now a German-Polish research consortium has come together to develop a reliable means to transmit light to make such gyroscopes less susceptible to interference. Their secret: hollow-core fibers that can channel light with minimal loss.

Optical fibers form what the group calls “an elementary part of gyroscopes, that is, highly precise rotation sensors”. If only one axis of movement is relevant, acceleration sensors would suffice, but when an autonomous object’s movement through all three dimensions of space needs to be tracked, the measuring system has to be more complicated and include three accelerometers and gyroscopes.

Optical gyroscopes at the limit

One may consider an optical gyroscope measuring rotation being like taking a trip around the world: depending on the direction of travel, one loses time or gains time. A fiber gyroscope includes a fiber wound around a coil to form a ring resonator. In that resonator, light can travel with or against the clock.

When the object turns, the path passed by the light wave changes imperceptibly, either shrinking or expanding by a tiny margin. It is this minute change that a detector can pick up and use to calculate the rotation.

But this is where optical fibers come up against the limits of their capabilities. Magnetic and electrical fields can interfere with the sensor’s interpretation work, and the material itself can interact with the light and cause a change in its optical properties. These so-called nonlinear effects directly impact how the light travels.

The interference is so minimal that it poses no problem for telecommunications, but it can prove critical for navigating autonomous objects, as the tiny deviation from the expected direction will soon mean a measurable deviation from the chosen course.

In their work to avoid these effects, researchers at the Fraunhofer Institute for Reliability and Microintegration (IZM) have come across a promising new candidate in the market: hollow-core fiber. For photonics experts Wojciech Lewoczko-Adamczyk and Stefan Lenzky, the challenge was to exploit the disruption-resilient properties of these fibers to construct highly precise gyroscopes while minimizing production costs.

They needed to find an interconnection technology that would work with the hollow fiber. One major challenge was the means to split the light signal for several channels. Typically, individual waveguides would be coupled by simply fusing them together, but this was impossible for the hollow-core fibers, as their unique structure would be lost when exposed to heat.

To counter this effect, the researchers constructed miniature collimators: highly precise lenses that capture the light from one fiber and emit it before any diffraction can happen. With this crucial step passed, the light can be split by half-reflective mirrors and fed into the ring resonator. After one trip around the ring, it is measured and fed back into the fiber through a second collimator.

Assembly platform for SMEs

When coupling light with two collimators, extreme precision is of the essence. In laboratory environments, the components can be placed and aligned with precise positioning tools, but these are unlikely to be available in industrial production sites.

This means that small to medium enterprises have, to date, been unable to offer this process. This is why the German-Polish consortium is developing a passive coupling platform that allows the technology to be integrated in individual applications. Its layout allows the precise fitting of the finished collimators, removing the need for additional alignment.

Even with the project still scheduled to run to the end of 2022, the researchers have already made substantial progress: Collimators are still needed to bend the beams, the optical components produced by Fraunhofer IZM already outperform current solutions in the market with tenfold precision, at a maximum angle of refraction of 0.04 degrees. This means that pairs of collimators can be used for the passive coupling platform without requiring additional alignment, while achieving coupling efficiency of more than 85 percent.

The mission for the third and final year of the project is to test how reliable the platform will be, add more optical and mechanical components, and fit everything in a gyroscope. Once the rotation sensor has been constructed, everything is ready to field-test the technology under real-life conditions.

All of these developments were made possible by the ERDF funding for the ROYCE (Resonant hOllow-core fiber gYrosCopE) project with the involvement of the Nicolaus Copernicus University in Torun, Piktime Systems, and Eagleyard Photonics. The ROYCE project was initiated during the fourth joint call for proposals by Berlin and Brandenburg together with Poland as part of the PHOENIX internationalisation network of the Photonics Cluster Berlin Brandenburg.