Integrated Photonics for High Speed Optical Interconnects and Biosensing Applications


Intensive research efforts and investments from both academia and industry have resulted, during the last decade, in rapid progress of Photonic Integrated Circuits (PICs) based on Silicon technology (“Silicon Photonics”). As key benefits, this technology enables the integration of electronics and optics on a single chip and leverages mature microelectronics fabrication processes. Compatibility with existing CMOS manufacturing infrastructure enables a cost-effective mass production of optoelectronic circuits. However, a number of challenges are also associated with the technology that are being addressed herein:
First, the lack of efficient silicon-based light sources is typically addressed by the hybrid integration of prefabricated III-V laser diodes. The stringent laser alignment and attachment accuracy required for maintaining efficient coupling of light into the PIC has a crucial impact on yield, on the complexity of the required machinery and thus on final cost. In this research work, novel coupling devices that facilitate the passive assembly of laser diodes are conceived and experimentally demonstrated. The proposed devices allow a three-fold increase of required placement tolerances as compared to conventional couplers and put the laser attachment processes in reach of high throughput pick-and-place tools. Second, silicon based electro-optic modulators are often enhanced by resonant effects in order to reduce required drive voltages. These resonant effects however also restrict the wavelength range of operation and require the implementation of power hungry thermal stabilization and complex control systems. A modulator architecture combining a sizeable resonant enhancement (with a 6X reduction in power consumption relative to a conventional travelling wave architecture) with an enhanced optical passband of 3 nm compatible with a free running laser and modulator temperature is designed and validated with first experiments. Third, the implementation of optical filters and wavelength division multiplexers relying onmultiple resonant structures in Silicon Photonics is hindered in practice by the resolution, accuracy and uniformity of the fabrication processes. Uniformity in particular limits the performance in terms of achievable insertion losses and/or extinction. A higher order filter is designed relying on a combination of exactly identical unit cells (matched layout) resulting in maximized performance without active tuning or trimming. Together these innovations create the basis for enhanced Silicon Photonics transceivers aimed at low cost data center interconnects. Finally, the range of applications of CMOS compatible photonics is further expanded to visible wavelengths with the experimental demonstration of a low loss silicon nitride technology platform (SiN/SiO2 core/cladding waveguides). Several building blocks for the implementation of an evanescent field fluorescence biosensor have been fabricated in a standard CMOS pilot line and optically characterized. As an outstanding contribution, the light coupling problem has been successfully addressed by means of grating couplers with a tailored diffracted off-chip beam profile in which the reduced refractive index contrast of the material system has been compensated by means of a multi-beam interference effect.


Integrated Photonics for High Speed Optical Interconnects and Biosensing Applications. Dissertation (2016), RWTH Aachen University