PhD Thesis Defenses

Temperature measurements on silicon core fibers during CO2 laser processing

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.