The incorporation of phosphorus into silica soot material strongly changes during the multistep preparation process of the MCVD technology in combination with solution doping for Al and rare earths. We report on the influence of various process parameters on the phosphorus concentration, the bond types of phosphorus atoms and the relative density of the soot material. By optimization of the process the phosphorus concentration of the presintered soot could be increased by around 10% in comparison to the conventional treatment. The understanding of the interdependencies allows an improvement of the preparation process of phosphorus co-doped RE doped silica laser fibers with MCVD technology.

Fiber optic bending sensing has potential use in industrial and medical applications. Thus, so far, several configurations have been reported with that end, but state-of-the-art sensors are either complex, temperature dependent, or cannot be multiplexed easily. To circumvent these important limitations, we have developed a sensing platform based on an asymmetric coupled-core optical fiber that is combined with conventional Bragg gratings. The asymmetric fiber was designed with three cores arranged in an equilateral triangle. Said fiber supports supermodes that suffer drastic changes when it is bent. Consequently, the reflection of a Bragg grating inscribed close to the asymmetric fiber changes drastically, but its wavelength position is not altered. We demonstrate experimentally that our sensing platform allows the development of highly sensitive bending sensors that have important practical assets. The latter include compactness and simple fabrication, capability of distinguishing the direction of bending and simultaneous detection of temperature and bending. Moreover, the sensors can be multiplexed easily and can be interrogated with commercially available fiber optic sensor read out units.

During recent years, the optical-fiber-based simultaneous sensing of strain and temperature has attracted increased interest for different applications, e.g., in medicine, architecture, and aerospace. Specialized fiber layouts further enlarge the field of applications at much lower costs and with easier handling. Today, the performance of many sensors fabricated from conventional fibers suffers from cross-sensitivity (temperature and strain) and relatively high interrogation costs. In contrast, customized fiber architectures would make it possible to circumvent such sensor drawbacks. Here, we report on the development of a high-quality coupled-core fiber and its performance for sensors—from the initial fiber layout via elaboration of the preform and fiber up to the sensor evaluation. A compact, high-speed, and cost-effective interrogation unit using such a specialized coupled-core fiber has been designed to monitor reflectivity changes while even being able to distinguish the direction of the force or impact. Several fiber core material techniques and approaches were investigated, which made it possible to obtain a sufficient volume of material for the required fiber core number and a specialized fiber core geometry in terms of core distances and radial refractive index profile, whilst handling the non-symmetrical fiber architectures of such modeled, complex structures and balancing resources and efforts.