Monolithic and hybrid silicon-on-insulator integrated optical devices

Aydınlı, Atilla
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Bilkent University
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Silicon, the basic material of electronics industry is rediscovered nowadays for its potential use in photonics and integrated optics. The research activity in silicon integrated optics have been speeding up during the last decade and even attracting interest of leading industrial companies. As a contribution to this world wide effort, we have designed, fabricated and characterized a class of monolithic and hybrid silicon integrated optical devices. These devices were realized on high-quality silicon-on-insulator (SOI) wafers. Beam propagation method (BPM) based simulations and analytical calculations were employed for the design. We have demonstrated for the first time an SOI device that splits light into its TE and TM components. An SOI rib waveguide becomes birefringent as its size reduced. This idea is used to design and fabricate a directional coupler polarization splitter based on geometrical birefringence. The device uses 1 µm sized SOI waveguides. This compact device (only 110 µm in length) shows extinction ratios larger than 20 dB. SOI waveguides with the same geometry was used to realize a batch of single and double bus racetrack resonators having radii in the range of 20 to 500 µm. Design of these racetrack resonators are presented in detail. The bending loss and coupling factor calculations were performed using BPM. During the design and analysis of waveguide resonators, we proposed a novel displacement sensor that can be used for scanning probe microscopies. The sensor operates by means of monitoring the changes in transmission spectrum of a high finesse micro-ring resonator due to stress induced by displacement. Operation principles and sensitivity calculations are discussed in detail. SOI resonators with quality factors (Q) as high as 119000 have been achieved. This is the highest Q value for resonators based on SOI rib waveguides to date. Finesse values as large as 43 and modulation depths of 15 dB were observed. Free spectral ranges increased from 0.2 nm to 3.0 nm when radius was decreased from 500 to 20 µm. The thermo-optical tunability of these resonators were also studied. A high-Q racetrack resonator is used to develop a wavelength selective optical switch. The resonator was thermo-optically scanned over its full free spectral range applying only 57 mW of electrical power. A low power of 17 mW was enough to tune from resonance to off-resonance state. The device functioned as a wavelength selective optical switch with a 3 dB cutoff frequency of 210 kHz. We have also demonstrated wavelength add/drop filters using the same racetrack resonators with double bus. Asymmetric lateral coupling was used in order to get better filter characteristics. Filters with crosstalks as low as -10.0 dB and Q-factors of as high as 51000 were achieved. Finally, we introduce the use of a layer transfer method for SOI wafers. Such a layer transfer results in the possibility of using the back side of the silicon layer in SOI structure for further processing. With this method, previously fabricated SOI waveguides were transferred to form hybrid silicon-polymer waveguides. Benzocyclobutene (BCB) polymer was used as the bonding agent. The method is also applied to SOI M-Z interferometers to explore the possibilities of the technology. We additionally studied asymmetric vertical couplers (AVC) based on polymer and silicon waveguides and fabricated them using a hybrid technology.

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