Abstract
The herein presented Ph.D. thesis finds its application in fibre-to-the-home (FTTH) optical access networks. FTTH networks have been widely deployed worldwide and are expected to evolve toward wavelength-division multiplexing (WDM) architectures. 
As the capacity and bandwidth per user requirements for broadband communication services continuously increase, technologies such as hybrid wireless-optical, ultra-wideband (UWB) radio, and millimetre-wave radio are being investigated as viable solutions to provide data rates exceeding Gigabit per second per user. Hybrid wireless-optical networks can provide simpler backhaul and are foreseen to play an important role in next-generation access networks which will require flexible deployment, high capacity, upgradeability, scalable to user number and demand, and economically feasible. Radio-over-fibre techniques combined with multigigabit wireless systems that provide capacities comparable to optical fibre communication systems is seen as a fast deployable and cost-effective solution for providing seamless integrated wired/wireless access to broadband services for the end-user. 
UWB and millimetre-wave wireless systems are capable of multigigabit communications. UWB permits an efficient use of the 3.1–10.6 GHz spectrum due to its unique coexistence characteristics and has market maturity. However, UWB is constrained by regulation worldwide. This regulation constraint makes millimetre-wave 60 GHz radio of great interest due to 7 GHz bandwidth consistently regulated worldwide, without coexistence restrictions. 
Two are the main technical objectives of this Ph.D. thesis: First, the proposal, analysis, and experimental demonstration of enabling techniques for flexible and cost-effective UWB radio-over-fibre systems. The work herein presented targets to extend the operation of conventional UWB radio-over-fibre systems in the 3.1-10.6 GHz band to the next-generation 60 GHz band. 60 GHz operation would reuse and extend UWB technology in terms of range and flexibility, and is the focus of this work. Second, the implementation of reconfigurable multiwavelength sources for flexible bandwidth aggregation in future WDM networks as an interesting solution for hybrid wireless-optical networks as herein developed. These two objectives are fulfilled by means of optical signal processing techniques. 
This Ph.D. thesis proposes and demonstrates experimentally several novel photonic techniques for UWB radio-over-fibre generation to remotely provide multigigabit wireless communications in different frequency bands, ranging from baseband to millimetre-wave, for a wide variety of applications. The operational limits of the proposed UWB-over-fibre systems are also analysed by simulation (VPItransmissionMaker™). 
The major achievements of the work herein presented can be summarized as follows: 
a) Proposal and proof-of-concept experimental demonstration of two techniques for remote UWB pulse shaping: (1) based on optical delay and balanced photodetection, and (2) based on differential photoreceiver and electrical delay. These techniques permit to adapt UWB spectrum to optical access transmission with varying fibre dispersion by adjusting delay. 
b) Proof-of-concept experimental demonstration of a 60 GHz UWB radio-over-fibre system at 1.25 Gb/s with 100 m of standard single-mode fibre (SSMF) transmission. The application of this system to interference-sensitive in-aircraft scenarios with a high number of users is proposed and analysed by simulation. A techno-economic comparison with a potential competitor solution based on baseband data over fibre and remote UWB generation and frequency up-conversion is also performed. 
c) Millimetre-wave UWB generation based on frequency shifting in the fibre of optical access networks is proposed and experimentally demonstrated. The technique employs optical carrier suppression in a Mach-Zehnder modulator combined with matched fibre chromatic dispersion targeting to overcome the bandwidth limitation of optical up-conversion, with the advantage of being easily reconfigurable generating simultaneously different RF bands. A comprehensive simulation analysis is performed with special focus on capabilities for dual 24 GHz/60 GHz operation paired with experimental demonstration at 1.25 Gb/s. Such an approach is suitable for integrating 60 GHz FTTH and 24 GHz UWB vehicular applications served from the same central unit. Practical implementation is addressed and competitive approaches are discussed.
d) UWB operation in the 60 GHz band is proposed and experimentally demonstrated to extend UWB capabilities in FTTH networks. 60 GHz UWB generation with combined 40 km of SSMF and 5 m of wireless transmission is experimentally demonstrated at 1.44 Gb/s. Transmission performance is evaluated for two major UWB implementations –dual-carrier modulation orthogonal frequency-division multiplexing (OFDM) reusing market-available devices, and binary phase-shift keying (BPSK) impulse radio- which directly modulate a cost-effective vertical-cavity surface-emitting laser (VCSEL). The results permit, from an application point-of-view, to select a given UWB implementation depending on network reach and system complexity desired. 
e) 60 GHz UWB generation, combined transmission over 6.5 km SSMF or 1 km of indoor bend-insensitive single-mode fibre and 2 m of wireless transmission, and RF power detection of on-off keying (OOK) impulse-radio UWB is proposed and experimentally demonstrated at 3.125 Gb/s. VCSEL is proposed for frequency up-conversion based on optical heterodyning and compared with conventional low-linewidth external-cavity laser (ECL). The results permit, from an application point-of-view, to select a given laser technology depending on network reach and system complexity desired. 
f) Finally, reconfigurable multiwavelength sources based on the spectral Talbot effect is proposed, numerically analysed, and experimentally demonstrated in this thesis. Twofold and fourfold multiplication of the number of wavelengths and wavelength shifting of a pulsed laser are demonstrated employing an electrooptic phase modulator. Potential applications of the reconfiguration technique are discussed.