Replication of planar polymer micro-optical waveguides and components

Photonic integrated circuits represent a topic of increasing interest in the research community. Its attractiveness is linked to the potential wide range of applications in the fields of optical telecommunication, photonic computing and optical sensing. Parallel to semiconductor and silicon photonics, polymerbased optical integrated circuits are the focus of intense research due to the immense versatility in material properties and fabrication techniques of polymers compared to their semiconductor counterparts. This dissertation was conducted in the framework of the collaborative research center ”Planar Optronic Systems” (PlanOS), which aims at developing novel low-cost fabrication techniques and applications for planar polymer-foil integrated optical circuits and sensors. This thesis specifically investigates the use of the hot embossing process to create such micro-optical and photonic structures in thin polymer films.
To fabricate waveguide-based photonic elements on flexible thermoplastic polymer substrates, a thermal imprinting process suited for replication in thin polymer films was developed and transferred to a commercial hot embossing system. Various stamp materials and fabrication techniques were investigated. The replication quality was optimized through process parameter studies and integration of custom embossing machine parts. The resulting replicated foils were then used as waveguide cladding. For the waveguide core, various thermosetting and UV curing polymer materials were tested. To deposit core materials, a fabrication process based on two-step hot embossing, as well as a combination of hot embossing and doctor blading, were examined. The qualityof produced waveguides was investigated through the measurement of refractive index, propagation losses, crosstalk and bend losses. The experimental results demonstrate low propagation and bend losses and excellent signal confinement.
Coupling structures in the form of grating arrays were then integrated in the obtained low-loss optical waveguides through different approaches. First, couplers and waveguides were fabricated on different polymer sheets and later combined through thermal and adhesive bonding. Alternatively, a single-step integration process based on a silicon stamp having waveguide-integrated grating couplers was demonstrated. The obtained samples were used to abricate hybrid and full-polymer optical transmission links.
As an application for the waveguide manufacturing technique, optical beam splitters with different splitting properties were designed, fabricated and characterized with respect to their excess losses and power imbalance. The achieved components exhibit low excess losses and high output uniformity. Furthermore, optical strain sensors were successfully fabricated.
The fabrication of microresonators through hot embossing was also pursued in the course of this work. A novel two-step replication process was developed, which is based on the replication of micro-pillars and the flattening of their top surface to obtain disk shapes typical for resonator structures. A targeted modification of resonator dimensions and shape was demonstrated through an adequate parameter study.