Orbital Angular Momentum Multiplexing for Optical Fiber Communications

Space-division multiplexing (SDM) is a promising technique to further increase the transmission capacity in optical communication systems as the single-mode fiber (SMF)-based systems are approaching their capacity limit. SDM utilizes multiple spatial paths composed of different cores or/and modes as individual channels to transmit data parallelly, where orbital angular momentum (OAM) modes have a vast number of orthogonal eigenstates and can be transmitted in a specially designed ring-core fiber (RCF). This thesis explores devices that can generate and multiplex different OAM modes for SDM data transmission. A silicon photonic integrated OAM MUX chip has been designed, fabricated and characterized for OAM mode multiplexing. By utilizing the OAM MUX chip, two transmission experiments with 10 Gbit/s on-off-keying (OOK) signals have been demonstrated without multiple-input multiple-output (MIMO) processing: one with two- and three-mode OAM multiplexing and the other with two-mode multiplexing together with eight wavelength-division multiplexing (WDM) channels. All channels obtained bit-error-rates (BERs) lower than the 7% forward error correction (FEC) limit. Furthermore, another method of generating OAM optical vortices has been explored. Silicon metasurface q-plates that support OAM modes of |L| = 5, 6 and 7 have been designed and fabricated. The preliminary characterization results are presented where high mode purity has been achieved, indicating that it is a compact and promising device to generate OAM optical vortices. Finally, applications of OAM states to quantum communications have been explored. A quantum-key-distribution (QKD) transmission experiment with two and three OAM modes generated by the integrated OAM MUX chip has been demonstrated. A quantum randomness generator (QRNG) has also been realized by utilizing the
crosstalk among OAM modes in an RCF.