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In many practical scenarios, single-photon avalanche diodes (SPADs) are good solutions to improve the performance of optical communication systems due to their high sensitivity to photon arrival. SPAD receivers can be implemented in large arrays to achieve higher data rates and additional protection against background light; however, they suffer from a significant intersymbol interference (ISI) if the SPAD dead time is comparable or larger than the symbol duration, i.e., sub-dead-time signal transmission. This work proposes a novel detection scheme designed for high-speed SPAD-based systems to effectively mitigate the degradation induced by ISI. Different from traditional receivers, in the proposed scheme, the information extracted from both the counts and arrival times of photons are utilised for the optimal symbol detection in the presence of the non-linear and random ISI effect due to dead time. Our extensive numerical and experimental results demonstrate the superiority of the proposed photon time information based detection (PTID) scheme in terms of both BER performance and background light tolerance of the communication link. In addition, a linear approximation of the SPAD-based channel is investigated, which illustrates that the traditional equalization methods are effective under some specific circumstances.

Integrated blue-violet and blue-green micro-LED arrays, fabricated via a transfer printing method, were employed to demonstrate wavelength division multiplexing underwater data transmission at 100 Mb/s over up to 9 attenuation lengths in a 1.5 m long water tank.

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The high internal gain of single-photon avalanche diodes (SPADs) operating in Geiger mode allows the quantum limit of detection to be approached. This offers a significantly improved sensitivity for optical communication over existing photodiodes. A fully integrated CMOS SPAD array receiver (RX) is presented which achieves 500 Mb s −1 four-level pulse amplitude modulation in a visible light communication link within 15.2 dB of the quantum limit. However, SPAD dead time induces around 5.7 dB of transient distortion which restricts error performance and data rate. We propose a model describing a discrete photon counting system which exhibits this nonlinear behaviour and compare it to practical measurements with the RX. A unipolar intensity modulated optical signal is considered, as opposed to bipolar electric fields in conventional radio frequency wireless systems. Intermodulation between the DC and harmonic components of the data-carrying waveform is investigated, and the resulting degradation of signal-to-noise-and-distortion ratio and bit error rate is evaluated. The model is developed as a tool for understanding distortion to ultimately allow rectification through RX architecture, modulation scheme, coding and equalization techniques. This article is part of the theme issue ‘Optical wireless communication’.