Publications

In this letter, we present a non-mechanical 2D beam-steering system suitable for optical wireless communication. Steering is achieved using polarization gratings combined with nematic liquid crystal cells operating as voltage-controlled polarization shifters. These steering elements are combined with a holographic diffuser matched to the discrete steering angles of the polarization grating, enabling continuous angular coverage. Beam-steering is also used at the receiver, allowing a large collection area receiver with a relatively narrow field of view to be used. The approach presented here could, in principle, be applied to a broad range of wavelengths (including visible light and near-infrared wavelengths). Furthermore, the technique does not inherently limit the transmission data rate. Besides, it improves the link margin and offers the potential for a bidirectional steered link using the same beam-steering elements. Details of the approach are set out in the letter, followed by experimental results from a 50 Mbit/s optical link operating over one meter. Future directions are then discussed.

In this Letter, an SiPM with a dedicated cooling system suitable for receiving ultra-low-power solar-blind wavelengths is reported. This is designed to decrease the temperature of the detector from 21°C to –10°C, and the corresponding dark count rate (DCR) is reduced by approximately 10 dB. A 275 nm optical wireless communication (OWC) system is established using on–off-keying (OOK) modulation. Transmission rates ranging from 100 kbit/s to 2 Mbit/s are demonstrated with this cooled SiPM. The received power is as low as 30 pW (corresponding to 41.5 photons per bit) at a data rate of 1 Mbit/s and a bit error rate of 2.4×10^{-3}.

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

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 a 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 a 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.

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.

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.

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.

The performance of a Silicon photomultiplier (SiPM) receiver when used with a 405 nm transmitter is discussed. A comparison of the irradiance available from a low-power transmitter that covers 20 m2 and the performance of the SiPM shows that in an indoor application this combination of transmitter and receiver can support OOK data rates of 1 Gbps. A comparison of the performance of the SiPM and an OEIC highlights the benefits that arise from the relatively large area of the SiPMs. Finally, the potential impact of using SiPMs on future VLC systems is discussed.

Results are presented which show that by using a SiPM with a higher maximum output pulse rate it is possible to increase the highest maximum OOK data rate ever achieved at a BER of 10-3 from 2.4 Gbits/s to 3.45 Gbits/s.