Integrated optic devices on thin film lithium niobate for few-mode optical communications

Abstract

Mode division multiplexing is a remarkable strategy to meet the explosively increasing bandwidth requirements in optical communication systems and interconnects. This scheme harnesses the orthogonality of modes to scale the transmission capacity in communication systems. MDM technique necessitates the development of devices that support multimodes starting from waveguides to modulators and amplifiers, along with mode manipulating devices like mode multiplexers and mode converters. The primary objective of this thesis is to design and analyse multimode devices on the emerging lithium niobate on insulator (LNOI) platform. Firstly, horizontal and vertical multimode waveguides are designed in commonly used configurations of rib, buried and strip on both LNOI and SOI platform to compare the performance parameters like intermodal group delay, power confinement, and waveguide cross-sections. Characteristic equations for transverse electric (TE) and transverse magnetic (TM) modes are derived for different cuts and optic axis orientations of LN, starting from Maxwell’s equations. Improved coupled mode theory is applied to derive coupling coefficients between modes of a dual-mode directional coupler. TE-TM mode coupling resulting from offdiagonal elements in the permittivity tensor of LN is also evaluated. Then, mode hybridization phenomenon in both X -cut Y -propagating (birefringent) and Z -propagating (isotropic) LNOI multimode waveguides is theoretically investigated. Hybridization regions are utilized to design tapers that convert TM0 to TE1, and TM1 to TE2. A multimode rib waveguide exhibiting significantly large group velocity dispersions of ∼15000, 22000 and 49000 ps/nm.km at three different wavelengths, in the regions of mode hybridizations is proposed. Mode division multiplexers that exhibit ultra high bandwidth of 215 nm and a large fabrication tolerance of ±50 nm is designed on a rib asymmetric directional coupler. Zero birefringence (ZBR) dualmode waveguides are proposed on X-cut Y-propagating LN with birefringence zero to four decimal places, utilizing the interplay of intrinsic birefringence of LN and waveguide birefringence. Minimally birefringent dual-mode waveguides are designed using off-axis waveguide configuration. Finally, polarization-independent two-mode multiplexers/ demultiplexers are designed and analyzed