This thesis presents temperature measurements on silicon core optical fibers during CO2 laser processing. Silicon core fibers are a new type of fiber offering a unique platform to combine the optoelectronic properties of silicon and the possibilities of the optical fiber platform. This makes them a promising candidate for many applications, such as mid-IR detection and transmission, studies of nonlinear optical devices, or fiber amplifiers. Today, two hurdles limit their usage: high optical transmission losses and complicated coupling into the core due to its high refractive index. The first task of this thesis work was to find suitable postprocessing of the as-drawn fibers in order to decrease optical transmission losses. The goal was to improve the fibers by the liquid-phase recrystalliza[1]tion method. In this method, the core of the fiber is heated to a temperature above its melting point by a laser beam. By scanning the beam along the fiber, a melt zone is moved through the fiber. When the silicon solidifies, it recrystallizes into a single crystal with lower optical losses. Successively, a fully computer-controlled setup was developed for fiber processing. Furthermore, a lab-size fiber draw tower was built to fabricate specialty fibers, especially silicon core fibers. Here, a CO laser acts as the heat source. The developed draw tower is very flexible and can be used to manufacture ample amounts of many fiber types quickly. It is known that the cooling rate at which the silicon core solidifies is a crucial parameter for the final transmission losses. Yet, it has so far only been estimated from black-body radiation. Here, an interferometric method was developed, allowing for in-situ temperature measurements in silicon core optical fibers. The method relies on probing the fiber with a laser beam during processing and observing the interference pattern caused by glass reflections. A suitable calibration of the interference pattern with temperature allowed to remotely monitor the fiber temperature in real-time during processing with high precision.

This thesis describes the development of a new prototyping technique for specialty optical fibers and covers all the fabrication steps from preform to fiber. The technique allows to produce fibers of a custom core structure and material composition, mainly focusing on semiconductor core fibers. By combining the optoelectronic properties of semiconductors with the advantages of the glass optical fiber platform, such fibers become a promising candidate in applications that require a wider infrared transmission window or stronger non-linear response. In contrast to traditional optical fibers, semiconductor core fibers are not standard off-the-shelf components. They exist as research samples, typically short in length, limited in core size, and exhibit a high loss due to the challenging and expensive fabrication process. 

The proposed preform fabrication method utilizes a carbon monoxide laser as a heat source. Employing this laser ensures extremely effective heat transfer to the preform with low surface silica vaporization, a minimal thermal gradient across the preform cross-section, and short exposure time of the preform to high temperatures. This allows to reduce the manufacturing time of the preforms and improve their optical properties. 

The aim of this thesis work was to design and build a system to fabricate fiber preforms made of semiconductors or other crystalline core materials. The work was primarily focused on preforms with silicon cores, but germanium and sapphire cores were also demonstrated. The ability to achieve preform tapering was a vital part of the preform fabrication. The process was developed using silicon as a test core material due to its abundance and widespread applications. As a typical representative of hybrid core materials, the properties of silicon imposed some common challenges that had to be addressed during the preform fabrication process. This includes a drastic difference in thermal expansion and thermal conductivity of the core compared to the cladding. Combined with a rod-in-tube approach, this system was used throughout the project to create silicon core fiber preforms in a wide range of core-to-cladding ratios, covering the core sizes from 17 μm up to 1 mm for preforms of 6 mm in diameter. The silicon core fibers produced from these preforms showed record minimal loss values of 0.1 dB/cm.

Additionally, glass additive manufacturing was applied for the first time in combination with the laser-based preform manufacturing technique to prototype specialty optical fibers of custom core composition and structure. In particular, the Laser Powder Deposition method was used to prototype fiber preforms with alumina, titania, and erbium-aluminum doped cores in concentrations not achievable by standard techniques. The drawn fibers showed losses as low as 3.2 dB/m, which is the best result achieved for glass fibers produced using 3D printing. Furthermore, multicore fiber preforms made of multi-component glass using a filament-based glass 3D printer have been demonstrated, showing the potential of using additive manufacturing for specialty fiber fabrication.

These silicon core and glass-doped preforms were pulled into hundreds of meter-long fibers of a standard 125 μm diameter and a core size in the range of 1 to 20 μm. This was achieved in a specially designed lab-sized fiber draw tower. To further utilize the benefits provided by the laser heating, the tower was also retrofitted with a carbon monoxide laser-based furnace. This allowed a very flexible operation of the tower, suitable for on-demand fiber prototyping of different types and experimental compositions.