‘Soon we will fly more safely thanks to optical fibers’

Airplanes that can feel when they are damaged, just as we can feel when we are in pain. It sounds like something out of a science fiction movie, but it will soon be possible thanks to optical fibers and ‘lighting technology’. Sidney Goossens will discuss the possibilities for fiber optic sensors for the University of Flanders.

2022: The International Glass Year

Glass and light have an ancient relationship. Just think of stained glass windows that have allowed light to penetrate beautiful cathedrals for centuries, the glass in lenses used to focus light in cameras, microscopes and telescopes, or the glass in the screen of the device you are reading this article with. is. Even the super-fast internet, which could mean that someone on the other side of the world is reading this article right now, is only possible because of fiber optic connections that can transport digital information at the speed of light.

Soon we will fly more safely thanks to optical fibers.

Precisely because glass has such important properties, the UN has declared the year 2022 as ‘the international year of glass’. This year we will celebrate the vital role of glass in society: from glass in art and architecture to glass in technology and science. An ideal moment to put the possibilities of optical glass in the spotlight.

Fiber optic technology: more than super-fast internet

Although we can not see them as the glass panes in cathedrals, we use all optical fibers every day. They are the backbone of the super fast internet. A typical fiberglass is as thin as a human hair and made of ultra-pure glass. A laser beam emitting on one side of the fiber remains trapped in the glass and can travel hundreds of kilometers through the fiber. Digital signals are encoded in these laser beams and transmitted through fiber optic connections across the ocean floor at the speed of light. Optical fibers today offer the most reliable and fastest Internet connections with extremely high data speeds. However, Internet connections are not the only use of optical fibers.

Inside these glass fibers, where the laser beams move, a small ‘micro mirror’ can be engraved. This micro mirror will only reflect one particular color or wavelength of the laser light. All other colors are just let through. The laser light typically used in these fiber optics is infrared light, so it is not visible to our eyes. Such a micro-mirror in the glass fiber then reflects only one wavelength of the infrared light, for example infrared-dark red. What happens if we stretch the fiberglass at the location of this micromirror: then the reflected wavelength of that micromirror will change (for example, from infra-light red to infra-dark red). In this way, a fiberglass with micro mirrors can be used as a sensor for measuring load and temperature.

The great advantage of this type of fiber optic sensors is that several micro mirrors can be engraved in a single fiber optic. All of these micro mirrors are so slightly different that they each reflect a different wavelength of infrared light. Imagine a fiberglass thread as thin as a human hair, containing hundreds of sensors a few inches apart. For a hundred electronic sensors, you would need at least two copper wires for each sensor, which would be a nightmare with cables and scales. These fiber optic sensors are already widely used in industries requiring long-distance measurements, from monitoring the structural condition of long bridges to monitoring pressure and temperature in gas or oil pipelines. The light signals that these fiber optic sensors work with are also completely harmless in situations where, for example, an electric spark or an overheated electronic sensor can pose a danger.

Aircraft with fiberglass nervous system

A much more recent field of application where these fiber optic sensors are used is in the aerospace industry to detect damage to aircraft. This is especially true in the latest aircraft made from composite materials. These new materials are susceptible to shock damage: cracks that occur as a result of a collision on the aircraft, such as a bird impact, a lightning strike, or even a wrench that is accidentally dropped on the aircraft during maintenance. These cracks can be further torn during use of the aircraft once the composite is loaded. This can eventually cause the composite to break.

Aircraft manufacturers are therefore actively looking for a sensor network that is permanently present in the composite material in order to detect and locate this shock damage as soon as possible. For example, if the aircraft collides with a bird during a flight, the sensor network can immediately after the flight tell if and where the aircraft needs to be repaired, long before real danger arises. Fiber optic sensors are the ideal candidates for this sensor network: they are thin and light, so they can be placed even inside the composite material without adding extra weight to the aircraft. In addition, they use glass and light signals that do not interfere with other communication signals.

In our research at the Brussels Photonics Group of the Vrije Universiteit Brussel, we examined what exactly is needed to make these fiber optic sensors work under the demanding aviation conditions. For example, you want the sensors to still work properly after the plane has flown from Brussels to Seville, and you do not want to confuse a temperature difference there with the presence of shock damage. This research was part of the European Commission’s Clean Aviation initiative, which brings together the aviation industry and research institutions across Europe with the ultimate goal of reducing emissions from future aircraft by 30%.

This is made possible, among other things, by fiber-optic sensors, which in combination with new materials make aircraft safer, lighter and more environmentally friendly. Just as we can feel pain with the nervous system in our body, aircraft will soon be able to feel if they are damaged with their own fiber optic nervous system.

dr. ir. Sidney Goossens is affiliated with the Brussels Photonics research group at the Vrije Universiteit Brussel (VUB).

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