Fibre-optic sensors

*Updated version of the 2023 article

Definition

Fibre-optic sensors measure physical variables such as temperature, pressure or strain using light signals in glass or plastic fibres. They are extremely precise and are chemically very resistant, while also being barely affected by electromagnetic disturbances. Due to these properties and their small diameter (on the scale of a human hair), they are suitable for use in confined spaces, in demanding environments or where other sensors cannot be used. Hollow core fibres that transmit light in an air-filled core are a groundbreaking innovation. This technology reduces losses and optical nonlinearities, which in turn allows for longer transmission paths and faster response times. Hollow core fibres open up new applications in the fields of telecommunications, medical technology and industrial monitoring. 

Current applications and opportunities

Fibre-optic sensors are already in use in high-voltage systems because of their high measurement accuracy over long distances and their insensitivity to electromagnetic interference. Very small, biocompatible fibre-optic sensors are used in medicine for precise measurements in the body, such as measuring blood pressure in coronary vessels or tissue temperature during tumour treatment in magnetic resonance imaging. 

In addition, a single fibre can control up to 100 sensors at the same time using the multiplex method. This involves several signals or information streams being sent simultaneously as a single complex signal via a communication link, making it ideal for aerospace or building monitoring. In addition, the fibres can provide continuous measurement data along their entire length, for example for leakage and temperature monitoring in pipelines and power grids. 

Optical glass fibres are now reaching their physical limits, as the light loss in quartz glass can barely be reduced further and optical nonlinearities lead to overlaps and thus to signal jams. This obstacle can be overcome with the introduction of “anti-resonant” hollow core fibres. This entails the light being guided through an air core with almost no loss, which reduces the loss below the level of conventional glass fibres and minimises disruptive nonlinearities. As a result, hollow core fibres enable even faster and more powerful data connections than fibre-optic sensors – ideal for data centres, big data applications and artificial intelligence. In addition, they can be filled with gases or liquids and adapted specifically for sensor technology or for the transmission of laser output in industrial and medical applications.  

Up to this point, optical glass fibre research has been carried out by telecommunications companies and not by national research programmes. Since innovations mainly occur in start-ups, it might be worthwhile to focus economic development on the area of fibre-optic sensors. 

The development of innovative hollow core fibres is driven by research groups at universities and research institutes, but the first few companies are starting to include these fibres in their range. However, their products are not yet able to keep up with conventional glass fibre in terms of transmission losses. 

Challenges

Measuring and readout devices for fibre-optic sensors are expensive. This limits their use in the construction industry in particular, where a large number of measuring points are required to monitor buildings, for example. Photonically integrated circuits (see PICs) such as those developed by Ligentec in Lausanne offer a promising approach for more compact systems that enable cost reductions. 

Another challenge is producing hollow core fibres, as the complex microstructure of thin-walled capillaries, which are layered with a high degree of precision, is time-consuming and expensive to produce. The availability of particularly long fibres is therefore limited. In addition, hollow core fibres can only achieve their theoretically low losses under certain conditions, meaning that, under standard conditions, they cannot yet compete with conventional glass fibres. Their lower mechanical stability also requires additional measures to protect the fibres. Connecting them to other optical components is also technically complex, especially as consistent standards are still lacking. 

Focus on industry and international perspective

The technology is a good fit for Swiss industry, with its tradition of precision manufacturing. There are various companies that manufacture fibre-optic sensor systems or corresponding components, including Diamond SA (Losone (Ticino)), Baumer Group (Frauenfeld), Solifos AG (Windisch) and Volpi AG (Schlieren). These companies are still leaders in niche markets, but competition from Europe and China is quickly catching up. 

In Switzerland, production of glass fibres has stopped over the last ten years, meaning it is unlikely that a Swiss production facility will be able to become competitive again. Switzerland is significantly better positioned in terms of developing the corresponding accessories (plugs, cables, couplers, etc.). 

Companies that will use hollow core fibres can benefit from higher data rates – a decisive advantage for applications with big data and artificial intelligence. The fibres are specifically developed to meet the increasing demands of AI workloads. One potential niche is the development of specialised connectors and the Swiss company Diamond SA (Losone (Ticino)) is a leader in this area. 

Future applications

The future applications of fibre-optic sensors are varied. In infrastructure, they enable the early detection of cracks, stresses, strain or subsidence in bridges, tunnels, roads, tracks and buildings. In personalised medicine, they are used as in vivo sensors, for example to monitor medical values, during operations or for long-term diagnostics. Intelligent energy systems benefit from the capabilities of fibre-optic sensors for real-time monitoring of faults and overloads in power grids and hydrogen pipelines.  

They also play a role in ensuring environmental and climate protection by being able to record methane, CO₂ and pollutant emissions, even in hard-to-reach areas. In aerospace, they ensure the continuous condition monitoring of aircraft fuselages, engines and structures in space. In automated industry, they enable precise process control in real time, for example in chemical, pharmaceutical or food production. They also play a role in quantum technology applications, such as atomic clocks and quantum computers – applications which require extremely high precision.