Ultra-sensitive UV solar-blind optical wireless communications with an SiPM

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

A comparison of APD and SPAD based receivers for visible light communications

Visible light communications (VLC) is an alternative method of indoor wireless communications that requires sensitive receivers. Ideally, single photon avalanche detectors (SPADs) could be used to create more sensitive receivers. However, the dead-time, finite output pulse width and photon detection efficiency of existing SPAD arrays limits their sensitivity and bandwidth. In this paper an accurate equation for the impact of dead-time on the sensitivity of a SPAD array is presented. In addition the impact of the width of the output pulses on the on-off keying (OOK) data rate is investigated. Finally, a comparison between receivers containing an APD and a large array of SPADs shows that although the receiver containing the SPAD is more sensitive in the dark the APD-based receiver is more sensitive in normal operating condition. However, the models that predict the performance of both receivers suggest that newer SPAD arrays will enable significant improvements in receiver sensitivity

BODIPY star-shaped molecules as solid state colour converters for visible light communications

We thank EPSRC for financial support from the UP-VLC Programme Grant (EP/K00042X/1). I.D.W.S. and P.J.S. are Royal Society Wolfson Research Merit Award holders. The research data supporting this publication can be accessed at http://dx.doi.org/10.17630/20163d03-6cc2-43b6-915c-d271f5220454.In this paper we study a family of solid-state, organic semiconductors for visible light communications. The star-shaped molecules have a boron-dipyrromethene (BODIPY) core with a range of side arm lengths which control the photophysical properties. The molecules emit red light with photoluminescence quantum yields ranging from 22 - 56 %. Thin films of the most promising BODIPY molecules were used as a red colour converter for visible light communications. The film enabled colour conversion with a modulation bandwidth of 73 MHz, which is 16 times higher than of a typical phosphor used in LED lighting systems. A data rate of 370 Mbit/s was demonstrated using On-Off keying modulation in a free space link with a distance of ~15 cm.PostprintPublisher PDFPeer reviewe

A saturated red color converter for visible light communication using a blend of star-shaped organic semiconductors

Authors would like to acknowledge the EPSRC for financial support for the UP-VLC (EP/K00042X/1). PJS and IDWS also acknowledge Royal Society Wolfson Research Merit Awards.We report a study of blends of semiconducting polymers as saturated red color converters to replace commercial phosphors in hybrid LEDs for visible light communication (VLC). By blending two star-shaped organic semiconductor molecules, we found a near complete energy transfer (> 90% efficiency) from the green-emitting truxene-cored compound T4BT-B to the red-emitting boron dipyrromethene (BODIPY) cored materials. Furthermore, we have demonstrated the capability of these materials as fast color converters for VLC by measuring their intrinsic optical modulation bandwidth and data rate. The measured 3 dB modulation bandwidth of blends (~55 MHz) is more than 10 times higher than commercially available LED phosphors and also higher than the red-emitting BODIPY color converter alone in solution. The data rate achieved with this blend is 20 times higher than measured with a commercially available phosphor based color converter.PostprintPeer reviewe

Neural Network-Based Joint Spatial and Temporal Equalization for MIMO-VLC System

The limited bandwidth of white light-emitting diode (LED) limits the achievable data rate in a visible light communication (VLC) system. A number of techniques, including multiple-input-multiple-output (MIMO) system, are investigated to increase the data rate. The high-speed optical MIMO system suffers from both spatial and temporal cross talks. The spatial cross-talk is often compensated by the MIMO decoding algorithm, while the temporal cross talk is mitigated using an equalizer. However, the LEDs have a non-linear transfer function and the performance of linear equalizers are limited. In this letter, we propose a joint spatial and temporal equalization using an artificial neural network (ANN) for an MIMO-VLC system. We demonstrate using a practical imaging/non-imaging optical MIMO link that the ANN-based joint equalization outperforms the joint equalization using a traditional decision feedback as ANN is able to compensate the non-linear transfer function as well as cross talk

Optical antennas for wavelength division multiplexing in visible light communications beyond the étendue limit

Funding: UK EPSRC via “Ultra-parallel visible light communications (UP-VLC)” (Programme Grant EP/K00042X/1) and “Super Receivers for Visible Light Communications” (related grants EP/R00689X/1 and EP/R005281/1).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.PostprintPeer reviewe

A Multi-Gigabit/sec Integrated Multiple Input Multiple Output VLC Demonstrator

In this paper, we report the performance of an imaging multiple input multiple output (MIMO) visible light communication (VLC) system. The VLC transmitter consists of a two-dimensional, individually addressable Gallium Nitride micro light emitting diode (µLED) array. The receiver uses a two-dimensional avalanche photodiode (APD) array fabricated using complementary metal oxide semiconductor (CMOS). Using integrated CMOS-based LED drivers, a data rate greater than 1 Gbps was obtained at a link distance of 1 m with the system field of view (FOV) of 3.45 degree using four channels. At a reduced link distance of 0.5 m, a data rate of 7.48 Gbps was obtained using a nine channel MIMO system. This demonstrates the feasibility of compact MIMO systems which offer substantial data rates

Design, fabrication and application of GaN-based micro-LED arrays with individual addressing by n-electrodes

We demonstrate the development, performance and application of a GaN-based micro-light emitting diode (μLED) array sharing a common p-electrode (anode), and with individually addressable nelectrodes (cathodes). Compared to conventional GaN-based LED arrays, this array design employs a reversed structure of common and individual electrodes, which makes it innovative and compatible with n-type metal-oxide-semiconductor (NMOS) transistor-based drivers for faster modulation. Excellent performance characteristics are illustrated by an example array emitting at 450 nm. At a current density of 17.7 kA/cm2 in direct-current operation, the optical power and small signal electrical-to-optical modulation bandwidth of a single LED element with 24 μm diameter are over 2.0 mW and 440 MHz, respectively. The optimized fabrication process also ensures a high yield of working μLED elements per array, and excellent element-to-element uniformity of electrical/optical characteristics. Results on visible light communication are presented as an application of an array integrated with an NMOS driver. Data transmission at several hundred Mbps without bit error is achieved for single and multiple-μLED-element operations, under an on-off-keying modulation scheme. Transmission of stepped sawtooth waveforms is also demonstrated to confirm that the μLED elements can transmit discrete multi-level signals

Design, fabrication and application of GaN-based micro-LED arrays with individual addressing by n-electrodes

We demonstrate the development, performance and application of a GaN-based micro-light emitting diode (μLED) array sharing a common p-electrode (anode), and with individually addressable nelectrodes (cathodes). Compared to conventional GaN-based LED arrays, this array design employs a reversed structure of common and individual electrodes, which makes it innovative and compatible with n-type metal-oxide-semiconductor (NMOS) transistor-based drivers for faster modulation. Excellent performance characteristics are illustrated by an example array emitting at 450 nm. At a current density of 17.7 kA/cm2 in direct-current operation, the optical power and small signal electrical-to-optical modulation bandwidth of a single LED element with 24 μm diameter are over 2.0 mW and 440 MHz, respectively. The optimized fabrication process also ensures a high yield of working μLED elements per array, and excellent element-to-element uniformity of electrical/optical characteristics. Results on visible light communication are presented as an application of an array integrated with an NMOS driver. Data transmission at several hundred Mbps without bit error is achieved for single and multiple-μLED-element operations, under an on-off-keying modulation scheme. Transmission of stepped sawtooth waveforms is also demonstrated to confirm that the μLED elements can transmit discrete multi-level signals

Flexible Glass Hybridized Colloidal Quantum Dots for Gb/s Visible Light Communications

Color converting films of colloidal quantum dots (CQDs) encapsulated with flexible glass are integrated with microsize GaN LEDs (μLEDs) in order to form optical sources for high-speed visible light communications (VLC). VLC is an emerging technology that uses white and/or colored light from LEDs to combine illumination and display functions with the transmission of data. The flexible glass/CQD format addresses the issue of limited modulation speed of typical phosphor-converted LEDs while enhancing the photostability of the color converters and facilitating their integration with the μLEDs. These structures are less than 70 μm in total thickness and are directly placed in contact with the polished sapphire substrate of 450-nm-emitting μLEDs. Blue-to-green, blue-to-orange and blue-to-red conversion with respective forward optical power conversion efficiencies of 13%, 12% and 5.5% are reported. In turn, free-space optical communications up to 1.4 Gb/s VLC is demonstrated. Results show that CQD-converted LEDs pave the way for practical digital lighting/displays with multi-Gb/s capability