Transmission capacity improvement for high speed multimode waveguide links using advanced optical launch and multilevel modulation schemes
This dissertation investigates the potential technologies in both optical and electrical domains for transmission capacity improvement in multimode waveguide links. The first half of the dissertation focuses on the multimode waveguide made from glass, the multimode glass fibre. A new optical launch scheme is developed, namely low-loss Hermite-Gaussian launch, to achieve single mode group excitation in the multimode glass fibre. Novel implementations using elliptical Gaussian beam and square-shaped launching profiles are developed, achieving a coupling loss at least 2.5 dB lower than the previously reported line launch scheme. Theoretical calculations show that these launches provide 50% bandwidth-distance product improvement over the dual launch scheme for a 99% yield of the entire OM1 fibre installed base. It is also found experimentally that the low-loss Hermite-Gaussian launches outperform dual launch for fibres favouring either centre launch or offset launch. Misalignment tolerance measurements reveal that the bandwidth improvements over a perfectly aligned centre launch using these launches are maintained within a radial offset range of ≤ 8 μm. Error free transmissions at 10 Gbit/s are demonstrated for different orders of low-loss Hermite-Gaussian launches over 250 m worst case OM1 fibre without the use of equalisation. The second half of the dissertation concentrates on another type of multimode waveguide which is made from plastic, namely step-index plastic optical fibre (SI-POF). Both baseband and passband multilevel modulation schemes are investigated to provide over gigabit/s transmissions using LEDs. For the first time, fractionally-spaced equalisers are thoroughly examined in multilevel modulation systems for LED-based SI-POF links. Based on the link budget analysis, it is found that PAM-8 and CAP-64 are the best baseband and passband solutions respectively. For 25 m links, calculations show that PAM-8 and CAP-64 can achieve data rate up to 3 Gbit/s with system margins of 2.2 dB and 1.3 dB. Meanwhile for 50 m links, they achieve data rate up to 1.5 Gbit/s with system margins of 1.9 dB and 1.2 dB respectively. A new FPGA system is developed to experimentally evaluate the PAM-8 and CAP-64 schemes. Error free transmissions are achieved at 3 Gbit/s using PAM-8 for 25 m SI-POF and 1.5 Gbit/s using CAP-64 for 50 m SI-POF. These results record the highest bit-rate-distance-product achieved in LED-based SI-POF links without the use of forward error correction.