Photonic devices based on engineered silicon structures for optical guidance and modulation

Abstract

The development of electronic photonic integrated circuits (EPIC) has revolutionized inter/intra chip short-haul communication by replacing bandwidth-limited metal interconnects with high-speed optical interconnects. Silicon photonics is playing a major role in the growth of EPIC that enables the fabrication of various on-chip photonic components on matured CMOS platform. This dissertation investigates the guidance and electrical control of light in photonic structure-based engineered silicon for application in optical modulation. Here, mainly four issues are identified for an optical modulator working in depletion and injection modes of operation, those are: (i) RC time constant limited bandwidth in lumped electrode-based electro-optic modulator, (ii) weak coupling between RF and optical modes at higher microwave frequencies in traveling-wave electrode-based configuration, (iii) FSR limited and highly thermal sensitive optical characteristics in resonant based structure, and (iv) shifting of transmission dip/peak in resonance-based optical structure with applying an electric field. A high-speed electro-optic modulator based on CMOS compatible silicon waveguide is proposed to improve coupling between RF and optical mode. The optical waveguide in silicon is formed by creating a slot in a rib structure that results in strong optical confinement with an acceptably low propagation loss. The proposed design of the slotted-rib waveguide with a traveling-wave electrode facilitates an efficient optical modulation in silicon. Taking the advantage of velocity matched between optical and microwave modes, a highspeed operation up to 70 Gbps with an extinction ratio of ≈ 4.9 dB is reported at a peak to peak voltage of 3 Vpp under a reverse bias of 4 V. An insertion loss of 3.1 dB is obtained for a 4 mm long device. However, moderate modulation efficiency remains a major issue with the proposed device structure. The given issue of moderate modulation efficiency is considered in the next work, where optical modulation in silicon with high data rate and high modulation efficiency is proposed by a laterally separated vertical p-n junction. Two independent but synchronized p-n junctions that support the common 6 optical mode are created by forming a slot waveguide structure. It provides a prominent way to enhance the light-material interaction necessary to achieve low VπL along with the low RC time constant which is crucial for the high-speed operation. The proposed device shows a high modulation efficiency of 0.74 V-cm for a 1.2 mm long device. The calculated intrinsic 3-dB bandwidth reaches up to ≈ 58 GHz at a reverse bias of 6 V. We show high-speed operation up to 25 Gbit/s for the device length of 600 µm with a simple lumped electrode configuration. The Traveling-wave electrodes as a coplanar waveguide are employed to further improve the speed performance of the device. By taking advantage of excellent velocity matching between optical group index and RF effective index, high data rate performance up to 100 Gbit/s is obtained with an extinction ratio of 2.4 dB. The proposed device opens new avenues for high speed optical interconnect on an SOI platform. In order to achieve a compact on-chip device with a novel guiding mechanism, a silicon-based compact comb-like asymmetric grating is proposed as a thermally stable optical filter for multi-functional applications such as bio-sensor, electro-optic modulator, DWDM, etc. The device is designed and fabricated with a cavity section introduced between the two grating regions which are partially etched in the lateral direction to ensure the nonzero coupling of the fundamental and first-order modes in the propagation and counter-propagation direction. To demonstrate efficient optical guidance in the proposed device, refractive index sensing based on resonance shift in the spectrum is demonstrated with sodium chloride (NaCl) dissolved in DI water. In contrast to conventional Bragg grating where stopband lies in the transmission spectrum, the proposed device allows a single narrow passband transmission peak with a large Free Spectral Range (FSR) attributed to the engineered photonic bandgap of two modes present in the waveguide region. The device is deliberately designed such that slightly wide resonance peak is obtained that makes device operation thermally stable for a large temperature variation of ±15 K. A higher-order leaky mode with strong light-analyte interaction (due to long corrugation width) in the gratings governs high sensitivity of ≈ 352 nm/RIU for different 7 concentrations of NaCl from 0% to 10% in the Deionized (DI) water with a small footprint area of 18 µm2 only. Proposed filter characteristics are well suited for multifunctional applications in integrated photonic devices. However, fabrication of the proposed device is complex due to the presence of two grating periods of nearly equal dimensions. Further, to ease the fabrication complexity in the dual perturbed structure, next we proposed a tapered cavity coupled comb-like asymmetrical gratingbased optical filter where both set of gratings are having an equal grating period and duty cycle. The narrow FWHM of 1.2 nm with a high extinction ratio of 14 dB is demonstrated in the fabricated device. In addition to the measured optical guidance mechanism, we theoretically investigate electrically tunable optical characteristics of the device by considering the two p-n junctions formed in both sets of gratings. Optical signal tunability is reported in two different ways: (i) resonance peak shift with an applied bias to one set of gratings, (ii) tuning of peak resonance intensity at a fixed wavelength using a programmable set of bias voltages at both the junctions. A high reduction of nearly 79% is achieved with the given multi-bias approach.