Drawing tower at Institut für Strahlwerkzeuge at the University of Stuttgart (Source: University of Stuttgart)

The radiation from high-performance lasers is delivered to the processing location using fiber optic cables. Performances of up to several kilowatts can only be transferred 1- 3 m using fibers without significant loss of power. Users like the automobile industry require 100 m transport lengths. Non-linear effects and scatter destroy the fibers and limit the amount of radiant power that can be transferred. New fibers are therefore needed.

Solid-state lasers, disc lasers and fiber-optic lasers are becoming increasingly powerful. The radiant power generated is transferred along fiber-optic cables from the laser unit to the processing station, which is often located in a robust industrial environment. The light transfer in fiber cables occurs via total reflection. Within the limit area between two transparent media, the nucleus and the mantle with differing refractive index the light is reflected virtually free of loss and is transferred along the fibers. Inside (nucleus), the refractive index of a fiber along the axis is large and reduces as it gets further away from the nucleus either in abrupt stages (stage index fiber) or gradually (gradient index fiber), in that a cladding is mounted with a lower refractive index. When the high-energy radiation field of a high-performance laser is transferred there is an interaction with the material of the nucleus through elastic and non-elastic scattering, which together with the non-linear effects such as multi photon absorption sharply reduces the transfer efficiency and results in the destruction of the fibers. The transport length achieved to date for the transfer of 1 kW in basic mode is 5 m in the near infrared range, while the company IPJ Photonics Corp. specifies a length of 3 m for a 5 kW basic mode. Laser performances are constantly being increased. At the LASER´ 2005 trade fair, IPG presented an 18 kW fiber-optic laser. Thanks to the modular construction and the associated scalability of performance it was already possible to build a 36 kW fiber-optic laser by the time that LASER´2007 was held. In order to exploit these levels of performance and use them for industrial applications, the automobile industry for example, is demanding transport lengths of 100 m for pulsed laser sources for welding in the time-sharing process. New types of fibers need to be urgently developed to satisfy these requirements.

The starting material for fiber-optic manufacture is special quartz glass, which is molded into a perform using a number of cutting processes. In order to draw out the fibers the preform is heated to its melting point of 2150 °C in a furnace. During the subsequent procedure of drawing out the fibers from the tower which is almost ten meters high, the geometric conditions of the preform remain intact, so that the cross-section of the fibers contains a smaller version of the preform. During the drawing-out procedure the fibers are covered with further layers in order to protect them. By varying the shape of the nucleus and the surrounding layers it is possible to influence the loss-free supply of light to the fibers.

At the Institut für Strahlwerkzeuge [Institute of radiation tools] at the University of Stuttgart (IFSW) a new system is being introduced for the research and development of new types of transport fibers for the transfer of high-performance laser radiation with maximum brilliance (photo). Here, a number of interesting projects are planned for overcoming the above-mentioned limitations. Firstly, the concept of the "Solid core bragg fibers" is to be investigated. In this concept the nucleus of the fibers is surrounded by 3 - 5 layers with precisely coordinated refraction indices. As a result, an interference field stabilizes the light supply, which avoids losses. A further project is entitled "Hollow Bragg Fibers". In this project the nucleus of the fibers is surrounded by concentric ring structures with alternating refraction indices. According to calculations that have already been made, these fibers are designed to maintain polarization and reduce losses by a factor of 1000. The new system will greatly help to increase the range of potential applications of high-performance laser radiation sources with optimum radiation quality.

In this connection the systematic investigation of special fibers for high-performance transfer is also important. As an example, the company Fibertech will investigate fibers as part of a joint study with Bundesanstalt für Materialforschung und -prüfung [Federal Institute of materials research and testing] in connection with the transfer of higher levels of performance. for this a series of analyses will be carried out in respect of the technological parameters of the fibers. This means that a systematic classification of special fibers will be carried out for the first time. This joint project is being financed by the EU as part of European Regional Development Fund (ERDF) and the Berlin Senate.