Single-Photon Avalanche Diodes in CMOS Technologies for Optical Communications

Abstract

As optical communications may soon supplement Wi-Fi technologies, a concept known as visible light communications (VLC), low-cost receivers must provide extreme sensitivity to alleviate attenuation factors and overall power usage within communications link budgets. We present circuits with an advantage over conventional optical receivers, in that gain can be applied within the photodiode thus reducing the need for amplification circuits. To achieve this, single-photon avalanche diodes (SPADs) can be implemented in complementary metal-oxide-semiconductor (CMOS) technologies and have already been investigated in several topologies for VLC. The digital nature of SPADs removes the design effort used for low-noise, high-gain but high-bandwidth analogue circuits. We therefore present one of these circuit topologies, along with some common design and performance metrics. SPAD receivers are however not yet mature prompting research to take low-level parameters up to the communications level.(UKFAT) showing severe restrictions on wireless bandwidth, especially as 4G mobile internet is expanded towards 5G. Ultimately, the radio spectrum is limited at the high-GHz and low-THz bands by atmospheric attenuation, principally the molecular resonances of water and oxygen. There can be no doubt that as a mature technology, optical communication has become ubiquitous and is growing as bandwidth demands rise [5]. However, with that ubiquity and growth, the total electrical power, and indeed the materials and complexity of systems, is also increasing at an extraordinary annual rate.The design of optical communication systems is highly application dependant [1,2,6]. The high-speed, long-distance intercontinental links can use complex, power-hungry and expensive transmitters (Tx) and receivers (Rx). This is principally as the energy per bit (J/b) and cost per bit ($/b) can be kept low through long-term installations with high throughput and multiple end-users. As a direct comparison, hardware for applications such as VLC to mobile phones and personal computers, must be simple, low-power and cheap [3,4]. This is especially true as smart-phones or personal computers imply a single end-user and a bandwidth low enough for simple or minimal forward error correction (FEC) schemes. It also implies battery operation with an expectation of long battery life and short product lifetimes (e.g. the 2-year average for mobiles). In terms of costs, VLC transmitters and receivers must: (i) be comparable to other system level components, i.e. memory, processor or display, and (ii) be suitable for high volume production, i.e. > 100,000 units. Basic receiver operations and research directionsTaking a high-level approach, all optical communication receivers must perform eight basic operations [1, 2, 6]:1. Convert incoming optical signals into an electrical signal, usually electrical current. 2.Amplify and convert the current signal into a form that can be processed easily. 3.Equalize and/or modify the signal to increase bandwidth or remove unwanted artefac...

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