Experimental Demonstration of 38 Gbps over 2.5 m OWC Systems with Eye-safe 850 nm SM-VCSELs

With a directly modulated 850 nm single-modevertical cavity surface emitting laser (SM-VCSEL), we experimentally achieve a gross data rate of ∼38 Gbps over a 2.5 moptical wireless communication (OWC) link at the 7% ReedSolomon forward error correction (RS-FEC) limit. The OWClink is demonstrated using an eye-safe transmitted optical powerof -1.47 dBm and discrete multi-tone (DMT) modulation withadaptive bit-and-power loading. The SM-VCSEL has a relativeintensity noise (RIN) of ∼-137 dB/Hz, which is lower than that ofa typical commercial 850 nm multimode VCSEL (∼-129 dB/Hz).Therefore, under almost identical OWC link operating conditions, the SM-VCSEL provides a gross data-rate increase of∼19 Gbps and an optical signal-to-noise-ratio (SNR) gain of∼5 dB compared to its multimode counterpart having a similarmodulation bandwidth. Furthermore, we demonstrate an errorfree net data rate of ∼17 Gbps at a received optical power of-7 dBm, which suggests the feasibility of utilising the SM-VCSELto realise indoor gigabit OWC applications

Experimental demonstration of 38 Gbps over 2.5 m OWC systems with eye-safe 850 nm SM-VCSELs

With a directly modulated 850 nm single-mode vertical cavity surface emitting laser (SM-VCSEL), we experimentally achieve a gross data rate of ∼38 Gbps over a 2.5 m optical wireless communication (OWC) link at the 7% Reed-Solomon forward error correction (RS-FEC) limit. The OWC link is demonstrated using an eye-safe transmitted optical power of -1.47 dBm and discrete multi-tone (DMT) modulation with adaptive bit-and-power loading. The SM-VCSEL has a relative intensity noise (RIN) of ∼-137 dB/Hz, which is lower than that of a typical commercial 850 nm multimode VCSEL (∼-129 dB/Hz). Therefore, under almost identical OWC link operating conditions, the SM-VCSEL provides a gross data-rate increase of ∼19 Gbps and an optical signal-to-noise-ratio (SNR) gain of ∼5 dB compared to its multimode counterpart having a similar modulation bandwidth. Furthermore, we demonstrate an error-free net data rate of ∼17 Gbps at a received optical power of -7 dBm, which suggests the feasibility of utilising the SM-VCSEL to realise indoor gigabit OWC applications

Non-linear equalisation techniques for high-speed step-index plastic optical fibre communication

Step-index plastic optical fibres (SI-POF) have become a promising candidate as the media for short-range in-home and automotive networks due to their low cost and their ease of installation. However, they have the smallest bandwidth compared to the other optical fibres. Therefore, high-speed communication over SI-POF results in inter-symbol interference (ISI) that linearly distorts the signal. Moreover, there are non-linearities from the optical front-end that further degrade the SI-POF performance. A straightforward solution is to use non-linear equalisers (NLE) with the SI-POF system as they compensate for the non-linear distortions while mitigating the channel ISI. Three NLEs – transversal decision feedback equaliser (DFE), Volterra equaliser/DFE, and the multi-layer perceptron-based equaliser/DFE (MLPDFE) – have been introduced in the literature. High-order modulation formats – like pulse amplitude modulation (PAM), carrier-less amplitude and phase modulation (CAP), and discrete multi-tone (DMT) – can be used in combination with the NLE to overcome the bandwidth limitation further. Thus, the thesis deals with the performance of these NLEs for PAM, CAP, and DMT transmission in order to achieve high data rates (from several hundreds of megabits-per-second (Mbps) to gigabits-per-second (Gbps)) in SI-POF. The contributions of this research work are in threefold: firstly, a simulation model is used to evaluate and compare the performance of the NLEs for PAM and CAP schemes. The study shows that for a highly non-linear SI-POF with higher PAM (or CAP) modulation order, the MLPDFE offers higher data rates than the Volterra DFE followed by the transversal DFE. This simulation study is further verified with various experiments. For instance, the MLPDFE offers an error-free bit rate of about 6.2 Gbps over a 30 m SI-POF while the transversal DFE offers about 5 Gbps at similar SI-POF length. A computational complexity comparison of each NLE shows that the transversal DFE requires the least computing requirement, and the VOLT2DFE has higher computational order than the MLPDFE. Secondly, the work investigates a recently introduced frequency domain NLE (FD-NLE) for DMT transmission over SI-POF. It explores the performance of the FD-NLE for DMT with clipping distortion in a highly non-linear SI-POF system. The FD-NLE is shown in this case as the better choice than the conventional frequency domain equaliser. With insight from the FD-NLE for DMT, both Volterra and the MLP equalisers are translated to the frequency domain for PAM and CAP transmission over SI-POF. A computational complexity analysis shows that implementing the NLEs (with PAM and CAP) in the frequency domain reduces their complexity by at least 60% if there are more than 16 feedforward taps for the equaliser. Finally, extensive experiments are carried out to evaluate and compare the bit error rate (BER) performances and the computational complexity of the modulation schemes with their respective NLEs. The comparisons show that for a short-length SI-POF of up to 30 m, representing benign channel conditions, bit-loaded DMT with FD-NLE offers the best performance requiring the least complexity and the least transmitted electrical power. However, at longer lengths, PAM with MLPDFE gives the best performance. CAP with the MLPDFE demands the highest computational complexity and the transmitted electrical power