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.

Demonstration of >1Tbit/s WDM OWC with wavelength-transparent beam tracking-and-steering capability.

Beam tracking-and-steering is crucial for the operation of high-speed, narrow beam, optical wireless communication (OWC) systems. Using a system based on two sets of low-cost cameras for continuous beam tracking and a set of mirrors for steering, we demonstrate here a high-capacity (>1Tbit/s) ten-channel wavelength-division multiplexed (WDM) OWC system based on discrete multitone transmission. The results, which are achieved over a 3.5-m perpendicular distance and across a lateral coverage up to 1.8 m, constitute to the best of our knowledge, the highest aggregate OWC capacity at this coverage.

Wide field-of-view optical broadcasting for bi-directional indoor optical wireless communications employing PAM-4 modulation.

We present wide field-of-view (FOV) bi-directional point-to-multipoint indoor optical wireless communications operating over a range of 4 m. The system is designed to integrate with fiber-to-the-home/building networks realized by a passive optical network. A phase-only spatial light modulator (SLM)-based beam steering base station with a ±30° FOV broadcasts downstream transmissions to two nomadic user terminals that use mirror-based beam steering to provide a ±50° FOV. At the base station, a composite phase mask is constructed on the SLM not only to perform optical broadcasting, but also to steer upstream optical transmissions from user terminals at a different wavelength. Successful upstream and downstream data transmission of 25 Gbit/s PAM4 is achieved.

A spectrally efficient equalization technique for optical sources with direct modulation.

In this Letter, a novel equalization method for directly modulated optical sources is introduced. Conventional source equalization methods balance the low- and high-frequency responses of the source by cutting down the low-frequency components in any drive signal. Typically, this ensures a flat frequency and linear signal response up to some predetermined upper-frequency limit. It is conventionally done under a fixed linear dynamic range. However, in this Letter, it is found that the source's dynamic range varies by frequency. We describe a novel method that determines the limit of signal linearity at each frequency and uses this to create the enhanced equalizer response. This leads to an improved source bandwidth and, in practice, allows greater transmitted signal energy. Experimental results for a resonant-cavity LED transmitter show date-rate improvement of ∼40% and, to the best of our knowledge, a record date rate of 8.76 Gb/s with a bit-error-rate less than 3.8×10-3.

Handheld free space quantum key distribution with dynamic motion compensation.

Mobile devices have become an inseparable part of our everyday life. They are used to transmit an ever-increasing amount of sensitive health, financial and personal information. This exposes us to the growing scale and sophistication of cyber-attacks. Quantum Key Distribution (QKD) can provide unconditional and future-proof data security but implementing it for handheld mobile devices comes with specific challenges. To establish security, secret keys of sufficient length need to be transmitted during the time of a handheld transaction (~1s) despite device misalignment, ambient light and user's inevitable hand movements. Transmitters and receivers should ideally be compact and low-cost, while avoiding security loopholes. Here we demonstrate the first QKD transmission from a handheld transmitter with a key-rate large enough to overcome finite key effects. Using dynamic beam-steering, reference-frame-independent encoding and fast indistinguishable pulse generation, we obtain a secret key rate above 30kb/s over a distance of 0.5m.

Zwitterion-doped liquid crystal speckle reducers for immersive displays and vectorial imaging.

Lasers possess many attractive features (e.g., high brightness, narrow linewidth, well-defined polarization) that make them the ideal illumination source for many different scientific and technological endeavors relating to imaging and the display of high-resolution information. However, their high-level of coherence can result in the formation of noise, referred to as speckle, that can corrupt and degrade images. Here, we demonstrate a new electro-optic technology for combatting laser speckle using a chiral nematic liquid crystal (LC) dispersed with zwitterionic dopants. Results are presented that demonstrate when driven at the optimum electric field conditions, the speckle noise can be reduced by >90% resulting in speckle contrast (C) values of C = 0.07, which is approaching that required to be imperceptible to the human eye. This LC technology is then showcased in an array of different display and imaging applications, including a demonstration of speckle reduction in modern vectorial laser-based imaging.

A Wide-Area Coverage 35 Gb/s Visible Light Communications Link for Indoor Wireless Applications.

Visible Light Communications (VLC) can provide both illumination and communications and offers a means to alleviate the predicted spectrum crunch for radio-frequency wireless communications. In this paper, we report a laser diode based white-light communications link that operates over a wide area and supports high data rates. The proposed system is a four-colour multiplexed high-speed VLC system that uses a microelectromechanical system (MEMS) mirror-based beam-steering. The system operates at record data-rates of more than 35 Gb/s (Bit Error Rate(BER) < 3.8 × 10-3) with a coverage area of 39 m2 at a link distance of 4 m. To the best of our knowledge this is the fastest VLC demonstration reported thus far. The paper also addresses issues of eye-safety, showing data rates of  more than 10 Gb/s are feasible.

Direct current balancing algorithm for FLCOS binary phase holograms.

The need for DC balancing phase pixels in ferroelectric liquid-crystal-on-silicon spatial light modulators leads to control schemes that limit their use in beam steering applications where a continuous display of a routing hologram is required. By analyzing the phase redundancy in binary phase holograms, a new DC balancing algorithm has been developed that allows more general beam splitting and multiple beam steering functions. The theoretical derivation of the algorithm and experimentally measured properties of the optical beams are presented and discussed.

Cross-talk analysis in a telecentric adaptive free-space optical relay based on a spatial light modulator.

We present an analysis of the performance limit of an adaptive multichannel free-space optical interconnect based on a spatial light modulator (SLM). The SLM function is to provide an active alignment of the signal beam in the detector plane. A thorough cross-talk analysis based on the diffractive properties of an ideal SLM in an isoplanatic optical system is shown. We analyze the performance in terms of the bit-error rate (BER) due to cross talk between different channels in the optical interconnect for different alignment states and for different phase-modulation schemes.

Design and analysis of an adaptive board-to-board dynamic holographic interconnect.

We describe the design and analysis of an adaptive free-space optical interconnect between two circuit boards in a standard electronic backplane. An array of vertical-cavity surface-emitting lasers is used as the transmitter, and this communicates with a detector array on the receiver circuit board. Routing is achieved with a holographic crossbar that has a ferroelectric liquid-crystal spatial light modulator to display binary phase computer-generated holograms. A detailed analysis of a 48-channel interconnect designed to operate at 1 (Gbytes/s)/channel indicates that such a switch will operate successfully given typical components and card misalignments.