ITh3C.4.pdf Advanced Photonics Congress (IPR, Networks, NOMA, PVLED, SPPCom) © OSA 2019 Ultra-Compact Bragg-Assisted Silicon Photonics Orbital Angular Momentum Emitter F. Gambini (1) , Y. Liu (1) , B. Song (1) , H. Zhao (1) , V. Rosborough (1) , F. Sang (1) , P. Velha (2) , S. Faralli (2) , J. Klamkin (1) (1) Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106, USA (2) Scuola Superiore Sant’Anna, via G. Moruzzi, 1, 56124, Pisa, Italy fgambini@ucsb.edu Abstract: An ultra-compact microring-based Orbital Angular Momentum (OAM) emitter is demonstrated for free-space optical communications. The device relaxes the fabrication constraints and enables fast OAM switching and multiple OAM modes at the same wavelength. OCIS codes: 130.0130, 250.5300. 1. Introduction The spectral efficiency [1-4] and security [5], of the future reconfigurable high-speed [2] free-space and wireless optical communication systems, could be strongly improved by the properties of mutually OAM modes [1]. OAM provides additional data carrier dimension with reduced crosstalk, high multiplexing and de-multiplexing efficiency and theoretically infinite number of OAM states supported by a single light beam [3]. Compact, fast, reconfigurable and high efficiency devices are required for large scale integration wavelength division multiplexing (WDM) systems. The viability of integrated reconfigurable ring-based OAM emitters has been demonstrated by Strain et al. [6]. Although it exhibits high switching speed and thermal efficiency, fabrication constraints are introduced by the use of a 300-nm thick silicon dioxide layer. This work, based on previous results [7] and theoretical analysis [8], experimentally demonstrates the performance of fast thermally tunable OAM emitter, based on a 10-μm radius ring resonator in silicon-on-insulator (SOI) platform, for vertical emission of radially-polarized OAM modes. The device supports multiple OAM states at the same operating wavelength exploiting an alternative design using holes instead of angular gratings [8] and an integrated conducting path. The fabrication constraints for the SOI platform are relaxed despite the ultra-compact footprint and the limited optical loss. Fig. 1. Device microscope image of the realized OAM emitter a), scheme b), experimental setup c) and resonance tuning of the ring for different applied voltage d). 2. Device design The device consists of an integrated heater, based on a high-doped silicon circular ridge, which surrounds a single 10-μm radius silicon microring. Finite-difference time-domain [9] and multiphysics simulations [10] were used for electromagnetic and thermal analysis, respectively. The microring resonator is a single-mode 500x220 nm 2 transverse-electric (TE) silicon waveguide with 69 fully-etched cylindrical holes periodically placed along its central path. This distributed Bragg grating is composed of 50-nm radius holes and filled with silica. The ring resonator has been designed in order to emit around 1555 nm an OAM with topological charge [1] l=1. The inter-waveguide gap between the bus and the ring is limited, due to technological constraints, to 100 nm. In order to optimize the coupling coefficient between the bus and the ring, the waveguide width of the bus is set to 300 nm and the bus is bent by an optimized angle of 38 degrees in the coupling region. The 220-nm thick and 8.5-μm wide integrated resistive heater is composed of highly doped silicon and it is located 1 μm far from the silicon ring, in order to minimize the optical loss. The resonant wavelength is changed exploiting the Joule effect by controlling the injected electrical current. The footprint of the device is about 30.4x59.5 μm 2 and the photonic integrate circuit (PIC) was manufactured in a multi-project wafer run. In Fig. 1 a) the microscope image of the realized device is reported while in Fig. 1 b) the schematic of the device is shown. 3. Experimental results The setup shown in Fig. 1 c) was used to test the performance of the PIC. A homodyne interferometer was built in order to detect the emitted OAM state. A continuous wave signal with 5 dBm of optical power was generated by an external cavity tunable laser