Post-Fabrication Trimming of Silicon-Photonic Devices Using a Photo-Sensitive Upper Cladding

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Ran Califa
Engineering Building 1103, Room 329
Faculty of Engineering, Bar-Ilan University

Recent years have witnessed intensive efforts to promote the technology of photonic integrated circuits (PICs), trying to bring complete optical communication systems down to a single silicon on insulator (SOI) chip. PICs can be incorporated in future high bandwidth optical communication networks, all-optical signal processing or photonic sensors for various quantities. The maturity of the nano-electronic industry in the SOI platform and the material transparency in the telecom wavelength range make it the material of choice for the implementation of PICs. The main drawback of the material platform is the absence of numerous electro-optic and nonlinear optical effects, due to the crystalline symmetry of silicon.  
The design of the transfer function of PICs requires control of the optical path lengths along individual cm-long waveguides, with nanometer precision. Such accuracy can seldom be reached. Instead, post-fabrication trimming of waveguides lengths is often mandatory. However, tuning of the refractive index of silicon is limited to temporary changes through local heating, or the injection of carriers into semiconductor junctions.  
Our research focuses on a new hybrid platform for the fabrication of PIC devices. The platform incorporates the family of chalcogenide glasses, such as As2Se3 or As2S3, as upper cladding layers on top of SOI structures. Chalcogenide glasses are highly photo-sensitive, and provide the hybrid structures with a new set of potential effects for post-fabrication trimming. Our results show the feasibility of tuning the basic photonic components properties such as the relative phase delay, group delay and coupling ratio of each element in the circuit using illumination with a focused laser beam. The method was demonstrated on various selected devices such as hybrid chalcogenide-on-SOI Mach-Zehnder interferometers, ring resonators and complex Mux/Demux MZIs cascaded filters.
* Research was carried out towards the Ph.D. degree at the Faculty of Engineering, with supervision of Prof. Avi Zadok