The most sensitive photodetectors that can be used in optical wireless communication (OWC) receivers are silicon photomultipliers (SiPMs). Unfortunately, in addition to the usual finite bandwidth, these devices have a non-linear response. Evidence is presented which shows that the finite bandwidth is the more important of these two non-ideal characteristics and that the use of decision feedback equalization (DFE) to compensate for the SiPM’s bandwidth can incur a significant power penalty. This power penalty is the motivation for an investigation into the potential benefits of zero-pole pre-equalization. The results of this investigation show that, despite the need to restrict the peak-to-peak signal applied to the transmitter, pre-equalization increases the achievable higher data rates.

This paper demonstrates that the use of FRET in fluorescent antennas significantly increases the modulation bandwidth and concentration gain, leading to a sevenfold increase in transmission rate when used in an OWC system.

The use of fluorescent antennas in optical wireless communications (OWC) has been demonstrated previously, and it has been shown that it is an efficient method for enhancing receiver performance, providing both signal gain and a wide field of view (FoV). To achieve a high concentration gain at the receiver output, the selected fluorophores should have a high photoluminescence quantum yield (PLQY), limited overlap between their absorption and emission spectra, and emit light that can be efficiently detected. In addition, to support a high modulation bandwidth, the photoluminescence (PL) lifetime of the fluorophore needs to be short. In this paper, we propose a new fluorescent antenna architecture based on Förster resonance energy transfer (FRET). Our results show that, due to the photophysical interactions between the energy donor and energy acceptor, the use of FRET simultaneously increases PLQY and reduces PL lifetime. Additionally, employing FRET leads to an increased Stokes shift, ensuring that the emitted light has longer wavelengths, thus reducing self-absorption. This shift can also increase the efficiency with which the fluorescence is detected by a typical silicon (Si) photodetector. Consequently, our OWC results show that a new FRET-based antenna can achieve a significantly higher concentration gain and a wider transmission bandwidth than a conventional non-FRET antenna, leading to much higher data rates.

Previous work on fluorescent antennas in visible light communications (VLC) has primarily focused on downlink receivers, which have to be compact. In contrast, uplink receivers can potentially occupy much larger areas. A convenient and inexpensive approach to increasing the antenna’s area is to use a large array of fluorescent optical fibers. The challenge is then to couple this array to a much smaller photodiode. To address this challenge an antenna that consists of two stages is demonstrated. The first of these stages is a large array of fluorescent fibers that can absorb light from the uplink transmitter. The fluorescence from this array is then coupled into the second stage, which is a single fluorescent fiber that can absorb the fluorescence from the first stage. The diameter of this fiber means that it can be effectively coupled to a photodiode. Results are presented which demonstrate this concept by measuring the system’s signal gain, field of view (FoV), bandwidth, and data transmission performance.

Silicon photomultipliers' relatively large areas and ability to detect single photons make them attractive as receivers for optical wireless communications. In this paper, the relative importance of the non-linearity and width of SiPMs' fast output in their performance in receivers is investigated using Monte Carlo simulations. Using these results, the performances of receivers containing different SiPMs are estimated. This is followed by a discussion of the potential performances of arrays of existing SiPMs. Finally, the possible dramatic improvements in performance that could be achieved by using two stacked integrated circuits are highlighted.

Previously, when selecting silicon photomultipliers (SiPMs) for use in visible light communications (VLC) systems the bandwidth of the SiPM has been a high priority. However, results of experiments on VLC receivers containing two different sizes of commercially available SiPMs show that, if equalization is used and the OOK bit time is less than one third of the duration of its output pulses, the SiPMs bandwidth doesn't significantly impact the receiver's performance. Consequently, for these data rates the criteria used to select which SiPM to incorporate in VLC receivers should put a higher priority on their area and photon detection efficiency than on their bandwidth. In addition, for these data rates, unless the SiPM becomes non-linear, the performance of a receiver containing a SiPM can be predicted based upon Poisson statistics.

Silicon photomultiplier’s relatively large area and ability to detect single photons makes them attractive as receivers for visible light communications. However, their non-linear response has a negative impact on the receiver performance, including making them particularly sensitive to ambient light. Experiments and Monte Carlo simulations have been used to study this non-linearity. The resulting detailed understanding of the origins of the non-linear response leads to concerns over the accuracy of some previous simulations of SiPMs. In addition, it leads to simple methods to determine the maximum rate at which an SiPM can count photons and of determining the impact of a SiPMs non-linearity on its performance of a receiver. Finally, a method of determining which filters should be used to protect an SiPM from ambient light is proposed.

The process used to detect individual photons in passively quenched Silicon Photomultipliers (SiPMs) creates a nonlinear response. A model is presented to show this nonlinearity is an unavoidable consequence of microcells recharging after a detection. However, results are presented which show the nonlinearity is increased by the inclusion of an anode readout resistor. Removal of this resistor improves ambient light performance of communication links by a factor of 1.9 under 300 $\mathrm{mWm}^{-2}$ of total 405 nm irradiance.

The range of transmitters, detectors and other components that can be used in optical wireless links means that they can have a wide variety of frequency responses. Both pre- and post- equalization have been used in optical wireless systems to increase data rates. Pre-equalization has the advantage that noise impairments due to equalization can be minimised. However, the variety of pre-equalizer transfer functions, which can be easily created using analog circuits, is limited by the frequency responses of these circuits. Consequently, it may not be possible to equalize some optical wireless links with analog circuit-based pre-equalizers. In this paper a flexible digital pre-equalization scheme that can equalize complex frequency responses is described. The flexibility and the applicability of the scheme is also demonstrated using two example systems, including one that incorporates a fluorescent optical fiber to create a wide-field-of-view receiver for handheld devices. In particular, the bandwidths of the two example systems have been increased to over 500 MHz and to 350 MHz, resulting in at least a three-fold increase in data rate without any post-equalization. The authors believe that this is the first time such significant improvements in on-off-keying data rates have been demonstrated in systems with a relatively complex frequency response and a limited dynamic range.

A silicon photomultiplier (SiPM) contains an array of microcells that can each detect individual photons. Consequently, it can arguably result in the most sensitive receiver in visible light communication (VLC). However, each microcell needs a period of several nanoseconds to recover after detecting a photon. This creates a non-linear response and introduces a unique form of inter-symbol interference. In this paper, we first show that this interference splits each element of the received signal constellation into multiple clusters. This observation motivates the investigation into the use of a Radial Basis Function Neural Network (RBFNN) to deal with the impact of the nonlinearity. Both the training procedures and the performance of the RBFNN are explained and discussed in detail. The influence of the number of the RBFNN centers, the widths of the centers, the constellation size and the period of the transmitted signal samples on the system performance are investigated. In addition, two different RBFNN-based data demodulation methods are introduced. The simulation results suggest that the new RBFNN-aided receivers reduce the negative impacts of the SiPM nonlinearity and can result in lower bit error rates (BERs) for a wide range of irradiances on the SiPM.

In this paper we demonstrate a novel optical receiver for visible light communications that combines a fluorescent antenna and a silicon photomultiplier (SiPM). The fluorescent antenna is configured in a slab geometry to collect and absorb an incoming optical data signal, and to waveguide the resulting fluorescence to an edge where it is detected by the SiPM. The antenna incorporates the fluorophore, pentafluorene, selected for its very high bandwidth of 245 MHz, high photoluminescence quantum yield of 90%, and emission spectrum that matches the wavelengths most efficiently detected by the SiPM. The performance of a receiver, comprising the fluorescent antenna and a J-series 30020 SiPM manufactured by ON Semiconductor, was assessed in a 30 cm data link with a 405 nm GaN laser diode transmitter, both in the dark and under 500 lux of ambient white light. The fluorescent antenna successfully rejects ambient light by a factor of 20, limited by extrinsic scattering in the thin film. Using on-off-keying with decision feedback equalization, a maximum data rate of 1.4 Gbps was demonstrated in 500 lux of ambient light. Using the pentafluorene antenna plus a BG3 filter, provides a 200 fold suppression of ambient light, and can increase the data rate of the receiver at low signal powers by up to a factor of 2. When operating in ambient light the composite receiver requires only 2.5 times more signal power than when it operates in the dark, with this power penalty reducing to a factor of 1.2 at 1 Gbps.

In this article, the performance of a new time domain signal pre-equalization method for use with optical orthogonal frequency division multiplexing (OFDM) and a silicon photomultiplier (SiPM) based receiver is studied.A SiPM contains a large array of microcells and each microcell is able to detect single photons.Therefore, a SiPM can be used to create arguably the most sensitive optical receiver, which can detect light intensity signals by counting the number of arriving photons within each signal sampling period.However, each photon detection triggers an avalanche-and-quenching process and the related microcell becomes inactive for a recovery time of several nanoseconds.Consequently, any photons arriving during this period cannot be detected.This effect can cause a non-linear distortion of the received signal and, when the OFDM sampling period is short, also introduces interference between signal samples.In this article, a new signal pre-equalization method is specifically designed to compensate for the impact of the finite recovery time.In this method, the number of active microcells during the transmission of each OFDM signal sample is first estimated.Then, the amplitude of the time domain signal sample is pre-adjusted based on the predicted fraction of microcells that are active.Using this approach, the negative impacts of the recovery time of the microcells are significantly reduced.The results that are presented show that when this new form of preequalization is used the bit error rate (BER) performance of the system is improved for a wide range of irradiance levels.

In this paper, the performance of a wide field of view VLC receiver that includes a silicon photomultiplier (SiPM) is reported. In particular, a receiver field of view of ±45° and On-Off keying data rates of 1.8 Gbps are demonstrated in 500 lux of ambient light. These results are achieved by combining optical absorption filters with a 6 mm by 6 mm SiPM. By absorbing ambient light between 450 nm and 750 nm these filters ensure that the SiPM is not saturated and reduces the transmitter power needed to support the required data rate. Unlike other optical filters the performance of absorption filters is not sensitive to angle of incidence. Consequently, the FOV is explained by a combination of the changes to the receiver’s projected area, the path length of light in the filters and reflections from the filter surfaces. In addition, the results of the calculations, described by the IEC 62471:2006 safety standard, needed to determine the eye safety of a transmitter are reported. These calculations and the resulting irradiance levels available in a representative office scenario are used to show that data rates of more than 1 Gbps could be achieved with eye safe transmitters.

Fluorophore doped plastic optical fibers can be used to create optical concentrators in receivers for visible light communications, that also act as wide field of view filters. Increasing the length of these fibers allows them to collect more of the signal from the transmitter, however, it can also reduce the bandwidth of the fiber. Results are presented from 3 different lengths of fiber which show that the best length of a fluorescent fiber depends upon the data rate. However, a simple calculation results in a length that is a good choice for a range of OOK data rates.

Orthogonal Frequency-Division Multiplexing (OFDM) is a popular modulation scheme, which requires a linear channel. Unfortunately, the most sensitive receivers for visible light communications, silicon photomultipliers (SiPMs), have a non-linear response. Despite this incompatibility, results from an easily implemented method of combining OFDM and SiPMs are shown to successfully limit the bit error rate to below the limit required by forward error correction.

A relationship between irradiance and the current needed to maintain the bias voltage applied to a silicon photomultiplier (SiPM) is shown to agree with experimental data. In addition to showing the saturation of the SiPMs response this relationship can be used to determine the power consumed by an SiPM. In addition, results are presented which show that, because of its higher maximum photon count rate, a 30020 SiPM can achieve a bit error rate (BER) of 10 -3 at a data rate of 3.45 Gbits/s.

Results of an investigation into the performance of a wide field of view silicon photomultiplier (SiPM) receiver under ambient light are reported. During these investigations a new form of inter-symbol interference (ISI) was discovered, which can be attributed to saturation of the SiPM. This new form of ISI means that using decision feedback equalization (DFE) improves the receiver's performance at low data rates.

This paper presents a novel fluorescent receiver for visible light communications that combines a broad field of view (FoV) with a significant gain, and simultaneously enables wavelength division multiplexing (WDM) for data communications. These characteristics are achieved by creating a receiver that includes a stack of two fluorescent optical antennas, each designed to receive a different part of the spectrum and create two separable communication channels. The device outperforms the theoretical étendue limit in terms of the combination of FoV and gain it can achieve. Furthermore, the demonstrated de‐multiplexing of blue and green spectral components enables the parallel streaming of data by WDM. Since these devices are inexpensive, simple, and compact, they can easily be integrated into portable electronic devices such as phones, tablets, and laptops.

A simple approach to digital pre-equalization is described whose performance depends upon the number of time points needed to represent each pre-equalized bit. This parameter is investigated using a white LED as an example system which highlights the flexibility of the proposed approach.

The directionality of visible light transceivers can be exploited to pack visible light communication links close together. However, this makes these links vulnerable to the random orientation of smartphones that occur when users are moving. Previously, it has been suggested that several transceivers can be used to increase the robustness of the link to a smartphone. However, this solution increases both costs and power consumption. In this paper, a receiver that incorporates a fluorescent fibre acting as an optical concentrator is described. Results from experiments with this receiver are presented which show that it is possible to transmit 1.1 Gbps with a BER of less than $3.8\times 10^{-3}$ to the receiver. More importantly, it has a field of view (FoV) of 240° in one direction.