Liquid-crystal-based planar waveguide devices for optical interconnects

Abstract

Optical interconnect supports data transport in optical form and has recently attracted renewed research interest as motivated by the rapid development of Big Data and Internet of Thing (IoT), which rely heavily on large-capacity data mesh network. However, the key components for advanced optical interconnect such as optical cross-connect (OXC) switches and optical buffers still need further development. In view of such an urgent need, this thesis proposes three new planar waveguide devices based on nematic liquid crystal (NLC), including (1) the OXC switch based on electro-optic (EO) routing in NLC (EO-OXC), (2) the OXC switch based on polarization dependent (PD) routing in NLC (P-OXC), and (3) the variable ODL using waveguide lattices (QWL-ODL). As the first device of this research, the EO-OXC is in the cross-bar fabric, and each unit cell consists of two portions: (1) the NLC EO waveguides for 1 × 2 switching using the EO effect in NLC and the spatial control of local electric field; and (2) the fixed NLC-core waveguides to support subsequent beam paths of the lead-in/lead-out and the bending process. Numerical simulations using an 8 × 8 EO-OXC sample device show a low dispersion over the whole C-band, a maximum total insertion loss of 14 dB, an average cross-talk of -32.8 dB, a switching time of 16 ms, and a footprint of 2 mm × 2 mm. Preliminary experiments have demonstrated the electric-controllable waveguiding effect and the polarization-dependent light confinement. Compared with the reported OXCs in literature, this EO-OXC is new in the working principle since it makes use of the electric-field induced NLC-core waveguide. It is also the first compact planar OXC using the NLC waveguides The second device, the P-OXC, is also in the cross-bar fabric. Each unit cell consists of two functional parts, including (1) the planar polarization rotator in an NLC-core waveguide to control the polarization state and (2) the PD waveguides to support passive beam path selection based on the polarization state of input light. Numerical simulations of a 32 × 32 P-OXC sample device show a mild dispersion over the O/C/L bands, a maximum total insertion loss of 14 dB, an average cross-talk of -33.7 dB, a switching time down to 1 ms, and a footprint of 8 mm × 8 mm. Preliminary experiments have demonstrated the electric-controllable planar polarization rotation using the simplified device based on the PDMS structure. This P-OXC design is original and novel since this is the first time to present the electric-controllable planar polarization rotator. It is also the first integrated planar OXC using the polarization control for beam routing. The third device in this work is the QWL-ODL, a variable ODL based on the discrete harmonic oscillation (DHO) in the waveguide lattice with a quadratic distribution of coupling coefficients. It utilizes the non-resonant mechanism to retain the broad bandwidth and uses the DHO effects to shrink the total footprint. With theoretical analysis and numerical simulation, a sample design is demonstrated, which can achieve a time delay of up to 10 ns with the resolution of 0.5 ns for broad bandwidth (3-dB bandwidth over almost the whole C-band for 5-ns delay). It allows a data packet of up to 17 bits at the rate of 100 Gb/s. In this design, the propagation path is folded by up to five times (or ten times for the one-port design) to allow a small footprint (about 10 mm × 0.5 mm). A group of them can be paralleled and cascaded within a small area (e.g., 20 copies into 1 cm2) using the microfabrication. The QWL-ODL is the first variable ODL using waveguide lattices as the delay line structure, and provides broadband compact ODL for high bit-rate ns-scale true time delay. Among the three devices of this work, the EO-OXC and the P-OXC both use the layered structure and may leverage the process like the LCoS fabrication. The EO-OXC is suitable for small-scale optical circuit switching (OCS) such as protective switching and non-frequent switching. In contrast, the P-OXC has better performance and may be used for medium-scale OCS applications such as fast optical switching. Moreover, the QWL-ODL can be used to obtain the ns-scale time delay for all optical switching. In a word, the three new devices are highly potential for the advanced optical interconnect in data centers.