Prof. Dr. Hartmut Bartelt: Innovations with modern fibre optics

faserlaser-ringresonator2The region surrounding the Optikzentrum Jena is a unique international ­company and research cluster for the manufacturing and application of special optical fibres. Apart from manufacturers of optical fibre materials and fibres there is a diverse range of further users and system integrators.

In the past decades, optical glass fibres have revolutionised our type of technological communication and made the Internet possible in the first place. Today, over 250 million km of glass fibres are produced every year for optical communication technology. The development of modern optical fibres has led to the creation of numerous other application fields with a high market potential and growth rate apart from telecommunication technology. New applications can be found in areas such as fibre optical sensors and measuring technology, lighting, medical technology and materials processing.

Fig. 1 Fibre preform manufacturing

Fig. 1 Fibre preform manufacturing

Such new ap­­pli­cations require highly specialised fibres with a tailored portfolio of prop­­erties. Traditional optical fibres are made of a light-transmitting core and a sheath around the core. In order to make highly efficient light-transmission possible, the core needs different optical properties (such as a higher optical refraction index) than the sheath. In addition, a coating is applied to the fibre, usually made of a polymer material, which serves as a mechanical and chemical protection of the fibre. Here, the fibre is usually manufactured from a so-called preform. This preform has the structure of the future fibre in a strongly magnified scale and can be made by separating differently doped glass layers on the inside of a glass pipe from a gas at high temperatures (Fig. 1). The preform is then drawn out into the actual fibre in a fibre drawing tower (Fig. 2). Typical fibre diameters equal about 125µm, which is slightly more than a normal hair.

Fig. 2 View inside a fibre drawing tower

Fig. 2 View inside a fibre drawing tower

The functionality of such optical fibres can then be changed for other fields by using special glass materials, varying the optical refraction index within the fibre, by structuring the fibre profile (e.g. with cavities as micro-structured fibre or photonic crystal fibre), by structuring locally and by coating fibres appropriately (Fig. 3). Such special fibres can open entirely new fields of application such as fibre optical sensors and measuring technology, endoscopic imaging and spectroscopic analysis, lighting in mi­­croscopy and medicine, materials processing and process control. Here, the fibre benefits from its specific properties such as miniature size, high durability in a hostile environment or resistance to high electric voltages and electro-magnetic fields.

Fig. 3 Examples of cross-sections of microstructured special fibres

Fig. 3 Examples of cross-sections of microstructured special fibres

Fig. 4 Gold-coated optical fibre with analyte

Fig. 4 Gold-coated optical fibre with analyte

Sensor functions can be achieved in optical fibres through appropriate coating or specific local structuring. Metallic layers in optical fibres can allow sensitive plasmonic measuring of layer reactions, which are needed, for example, for biological detection of antibody reactions in medicine (Fig. 4). By using modern structuring techniques, such as focussed ion beams, an optical fibre can be integrated in a microinterferometer for the detection of spectral or refractive properties in even the smallest sample volumes (Fig. 5). The optical photosensitivity of optical glass material can be used for imprinting of fibre grids in the optical core of a fibre. Periodic modifications of the optical refraction index then serve as a highly wave-selective mirror and allow for measuring temperature, expansion or vibration (Fig. 6). This is successfully used in structure monitoring and will make “intelligent” materials possible in the future in which such a special function is already integrated (Fig. 7).

Fig. 5 Locally structured optical fibres

Fig. 5 Locally structured optical fibres

Fig.6 Fibre grid recording with interferometer

Fig.6 Fibre grid recording with interferometer

Apart from transmission and processing of optical signals, optical fibres can also be used to generate light and thus be used as a light source. Fibre lasers and fibre supercontinuum sources are examples for this (Fig. 8). Fibre lasers are characterised by their high efficiency and stability as well as an excellent beam quality in connection with a high optical performance and their laser wave length can be tuned over larger ranges. For this reason, they are used successfully, particularly in material processing and marking technology. Fibre supercontinuum sources allow an immense spectral broadband light radiation within small fibre cores. Such light sources are of particular interest for spectroscopic measuring technology and analytics as well as in scanning microscopy. They make sensitive detection in connection with even the smallest sample volumes and high local resolution possible.

Fig. 7 Optical sensor fibre (core), embedded in a fibre-enforced composite material

Fig. 7 Optical sensor fibre (core), embedded in a fibre-enforced composite material

Fig. 8 Various fibre core cross-sections for fibre supercontinuum sources

Fig. 8 Various fibre core cross-sections for fibre supercontinuum sources

The regional industry already uses such options today and pursues a strategy of expanding these market fields with specifically modified special fibres while using a comprehensive value-added chain.

 

pProf. Dr. Hartmut Bartelt
The author studied Physics at the Universities of Karlsruhe and Erlangen-Nuremberg. After his doctorate and postdoctorate work, he was employed at Siemens AG’s research centre Erlangen. In 1994, he was appointed as Professor for Modern Optics and now works at the Leibniz Institute of Photonic Technology in the area of fibre optics. From 1999 to 2006, he headed the Institute as scientific director and has been the deputy director since 2006.