Silicon photonic switch and coupler for wide-band operation

Abstract

In this dissertation, the new type of total-internal-reflection (TIR) switch and tunable grating coupler were introduced for the wide-band operation in silicon with overcoming limitations of previous Si-photonic devices. Those devices are newly proposed and demonstrated using CMOS-compatible fabrication process at National Nanofab Center (NNFC) in KAIST. At first, optical switch based on TIR effect is introduced composed with y-branched multimode wave-guide structure based on SOI wafer. By locating a pn diode using ion implantation process at the y-junction area, incident light can be normally reflected to the reflection output port based on free carrier plasma dis-persion effect by decreasing the refractive index of silicon, which can satisfy the TIR condition. For the switching operation of switch, thermo-optic effect based on reverse breakdown of pn diode is used for compensation of refractive index in pre-doped silicon area. The fabricated TIR switch shows the extinc-tion ratio of 10 dB and 20 dB with switching power of 138.56 mW at the reflection and transmission state respectively. This result shows a high performance comparison with previous silicon TIR switch. From the absence of Si light source due to indirect bandgap characteristic, the light coupling between the Si-photonic devices and external light source is regarded as major problems for the on-chip communi-cation. Grating coupler is mainly used as a component for the light coupling which can support wafer-level test with ease of fabrication. However, grating coupler shows a wavelength dependency due to its structur-al parameter, which shows a serious degradation of coupling efficiency when using the operation wave-length in out range of central wavelength. To solve this limitation, tunable grating coupler is proposed based on thermo-optic effect in silicon which can directly change the effective refractive index of silicon. By embedding of pn diode at the grating coupler region, central wavelength was tuned from 1548 nm to 1596nm with only 1-dB change of coupling efficiency. Using this tunable grating coupler, 1-dB bandwidth can be extended over 100nm suitable for wide-band operation. Proposed TIR switch and tunable grating coupler can be easily adopted in Si-photonics technology based on CMOS-compatible fabrication with the wide-band operation. We expect to those devices can be used for WDM communication which can increase the capacity of single channel.