We designed an 8×8 double-layer photonic network (DLPN) using ring-assisted directional coupler (RADC) switches to minimize crosstalk and accommodate fabrication errors. The RADC photonic switch was realized in a silicon-on-insulator (SOI) platform with enhanced optical confinement using the finite difference time domain method. Electro-optic tuning was achieved by injecting carriers across the P-i-N junction. Our proposed RADC switches have a 5% fabrication tolerance on waveguide dimensions, resulting in optimized crosstalk performance. A comparative study of ring-assisted Mach Zehnder interferometer switches and the proposed RADC switches was conducted in the DLPN architecture, considering various performance metrics such as extinction ratio, free spectral range, quality factor, finesse, crosstalk, and insertion loss. The proposed RADC switches achieved a 35% footprint reduction and crosstalk reduction (−20.71 to −35.07 dB) in waveguide crossings within the double-layer architecture.
Single-photon detectors record photon-detection events with high efficiency and picosecond-timing resolution allowing for lidar (ranging) with millimetre depth resolution and low light levels. However, only silicon SPADs offer high quantum efficiency without cryogenic cooling. InGaAs SPADs are limited to below a wavelength of 1800 nm with low efficiencies. We have developed an approach for high single-photon detection efficiencies in the mid-infrared (2-3.5um) using upconversion in a PPLN waveguide in combination with a silicon SPAD. Using a waveguide we confine the mode over extended lengths to increase conversion efficiency and reduce the required pump power. Using a precision-machining waveguide fabrication process we have fabricated waveguides that support modes in the mid-infrared with low loss. We show overall photon detection efficiency in excess of InGaAs SPADs. We use our new detection system for imaging and ranging applications.
We demonstrate a source of polarisation-entangled photons that produces pairs of entangled photons at a rate of 1.25 gigahertz. Our system is enabled by Periodically Poled Lithium Niobate waveguides that produce degenerate photon-pairs with a centre wavelength of 1560 nm and a 0.1 nm bandwidth. We measured the degree of entanglement and obtained a CHSH parameter of 2.73. From measurements of polarisation discrimination in a BB84 protocol with two mutually unbiased bases, we obtained a measurement fidelity of 98% and estimated the maximum secure key rate to be 0.633 gigabits per second.
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