Dominic O’Brien

Control of a free-space adaptive optical interconnect using a liquid-crystal spatial light modulator for beam steering

This paper is a revision of a paper presented at the SPIE Conference on Micro-Optics, VCSELs, and Photonic Interconnects, Apr. 2004, Strasbourg, France. The paper appears (unrefereed) in SPIE Proceedings Vol. 5453. An adaptive beam position control system is reported, for use in alignment correction for free-space parallel optical interconnects at the board-to-board level. A liquid-crystal spatial light modulator (SLM) displaying binary phase gratings is used as a diffractive optical element to steer the beam. The design and implementation of a feedback controller for the SLM are presented, and shown in experimental results to completely correct for step disturbances within 1 to 2 sample periods. Extension of the system to tolerate random vibrations is considered, in terms of both the required SLM technology and the design of the control algorithms. The discussion highlights some of the trade-offs in choosing components when using this technology to correct for different levels of vibration.

Challenges in Gbps Wireless Optical Transmission

In this paper, the link budget of Gbps wireless infrared indoor communication is analysed. We particularly focus on the receiver sensitivity and identify the most suitable wavelengths range. We show that an optical receiver operating at 1 Gbps will hardly achieve the shot noise limit, which is determined by the received amount of background light. Regarding the link budget, we present two case studies. One deals with (very) short range communication, the other one with a wireless personal area network. We reveal that a network demands for avalanche photodiodes as well as beam steering. This clearly causes major challenges regarding compact and inexpensive components.

Wide field-of-view fluorescent antenna for visible light communications beyond the étendue limit

Visible light communications (VLC) is an important emerging field aiming to use optical communications to supplement Wi-Fi. This will greatly increase the available bandwidth so that demands for ever-higher data rates can be met. In this vision, solid-state lighting will provide illumination while being modulated to transmit data. An important obstacle to realizing this vision are receivers, which need to be inexpensive, sensitive, fast, and have a large field of view (FoV). One approach to increasing the sensitivity of a VLC receiver is to increase the area of the receiver’s photodetector, but this makes them expensive and slow. An alternative approach is to use optical elements to concentrate light, but conservation of etendue in these elements limits their FoV. In this paper, we demonstrate novel antennas that overcome these limitations, giving fast receivers with large collection areas and large FoV. Our results exceed the limit of etendue, giving an enhancement of light collection by a factor of 12, with FoV semi-angle of 60°, and we show a threefold increase in data rate.

Hybrid Radio and Optical Communications for Energy-efficient Wireless Sensor Networks

Abstract A wireless sensor network (WSN) consists of a large number of networked sensor nodes deployed to sense and report a particular phenomenon to a base station. One of the main design considerations for WSNs is the energy consumption of the individual sensor nodes. Currently, most WSNs are radio frequency (RF) based, and communications account for a significant portion of the energy expended. The hybrid radio frequency/free space optical (RF/FSO) WSN is proposed to lower energy consumption. The RF and FSO link models used for the RF/FSO WSN are first discussed. Next, the energy expended by the sensor nodes for sensing, data processing and communications is modeled. These models are used to simulate the lifetime of the WSN. To investigate the energy efficiency of the RF/FSO WSN, an RF-only WSN is used for comparison. Simulations show that the average energy consumed by the nodes in the RF/FSO network is significantly lower than that of the RF-only network, thus increasing the lifetime of traditional RF-based WSNs.

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 users 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.

Radio Frequency/Free Space Optical and Radio Frequency-only Wireless Sensor Networks: A Comparative Study of Performance

Abstract Wireless sensor networks (WSNs) consist of networked sensor nodes, deployed to sense a phenomenon and report it to a base station. Sensor nodes are small and usually equipped with small batteries with limited capacity, and therefore, one of the most important design considerations for WSNs is power consumption. WSNs expend energy for communications, data processing and sensing; for commonly used RF communications this task accounts for the largest portion of power expended. Hybrid radio frequency/free space optical (RF/FSO) communications has been proposed to reduce power consumption by the sensor node, and in this paper, the performance of the RF/FSO WSN is compared against an RF-only WSN in terms of network lifetime and coverage. Results show that for the wide range of scenarios considered, the RF/FSO WSN lasts at least twice as long as its RF-only counterpart, despite providing the same level of network coverage. This paper also investigates network parameter selection for optimum RF/FSO network coverage.

Polymer colour converter with very high modulation bandwidth for visible light communications

For white light data communications, broadband light emitting materials are required, whose emission can be rapidly modulated in intensity. We report the synthesis, photophysics and application of a novel semiconducting polymer for use as a high bandwidth colour converter, to replace commercial phosphors in white LEDs. The high modulation bandwidth (470 MHz) is 140 times higher than that measured using a conventional LED phosphor.

All-optical mode-group multiplexed transmission over a graded-index ring-core fiber with single radial mode

We present a design of graded-index ring-core fiber (GI-RCF) supporting 3 linearly polarized (LP) mode-groups (i.e. LP01, LP11 and LP21) with a single radial index of one for mode-division multiplexed (MDM) transmission. Reconfigurable spatial light modulator (SLM) based spatial (mode) (de)multiplexers are used to systematically characterize spatial/temporal modal properties of the GI-RCF. We also demonstrate all-optical mode-group multiplexed transmissions over a 360m fabricated GI-RCF without using multiple-input multiple-output digital signal processing (MIMO DSP).