29.7 A 500Mb/s -46.1dBm CMOS SPAD Receiver for Laser Diode Visible-Light Communications

Abstract

III-nitride laser diodes (LDs) are promising sources for light fidelity (LiFi) networks, underwater wireless optical communications (UWOC), and plastic optical fiber (POF) communications [1] due to their high modulation bandwidths ($\gt 5$GHz) compared to LEDs ($\lt 20$ MHz). Their narrow linewidths also enable robust free-space Gb/s LiFi links in the presence of high intensities of sunlight through selective spectral filtering. Fully integrated CMOS visible light communications (VLC) receivers have recently been developed to enable miniaturised and low cost Gb/s LD-based links [2]. Sensitivity of these devices is constrained by electrical noise sources such as thermal, shot or excess noise related to the employment of PIN photodiodes (PDs) or linear avalanche photodiodes (APDs) and their amplification circuits. The extremely high gain of SPADs operating in Geiger mode allows quantum sensitivity limits to be approached [3], [4]. A SPAD receiver (RX) for fiber optic applications achieved 200Mb/s at $6.5\times 10 ^{-3}$ BER within 24dB of the quantum limit [3]. In this paper, we demonstrate a fully integrated CMOS SPAD RX SoC extending this data rate by $2.5 \times $to 500Mb/s, whilst improving sensitivity to -46.1dBm, reducing the margin to the quantum limit to only 15dB. Our RX architecture permits massively parallel (4096) photon event summation to be achieved at a high fill-factor (43%) and sample rate (800MHz). Detector redundancy obviates the requirement on current RX implementations that the SPAD dead time be matched to the symbol period to achieve the maximum data rate [3, 4, 5]. Another advantage is that complex modulation schemes such as OFDM or PAM can be applied for high spectral efficiency and multipath interference mitigation. We demonstrate the RX in a practical, background insensitive VLC link at 1m in 1klx ambient conditions using a 450nm LD. The higher power consumption and dead time pile-up nonlinearity of the SPADs at high signal levels, leads us to conclude that the practical use of this device to be in assistance (rather than replacement) of existing APD or PIN RXs for LiFi, UWOC and POF applications towards extended link range or maintaining a low rate link in highly scattering environments.