Suppression of Intermodulation distortion in Radio-over-Fibre System

Fibre optics communication is considered one of the most innovative methods of communication, originated about 40 years ago. Radio-over-fibre (RoF) was introduced for the very first time in 1990, and since then it has been widely used due to its high bandwidth, low losses, low power consummation, being cost-effective and other fibre-optic signal propagation properties [1-3].

RoF as an analogue communications system includes RF and optical fibre components such as Photodiode, filters, optical amplifiers, optical fibre, optical modulator, and laser. So far, the optical communication system has resulted to be the best solution to the modern mode of communication due to the above-stated properties. However, optical communication has its limitations as well, including non-linearity as a feature that produces Intermodulation Distortions (IMDs), losses and dispersion of signal. One of the most challenging tasks is the suppressing of nonlinear distortions, which strongly affects the performance of the Microwave Photonic Links (MPL).

In this thesis, a background review of the RoF system is presented, with a focus on Analog Photonic Links (APL) linearization techniques. In chapters 1 and 2 an introduction to optical communications is presented, with a focus on how the optical communication system has emerged and the advantages and disadvantages of the optical communications system in comparison to other communications systems available. Later in this chapter, a background study of APL has been presented - which includes optical fibre and fibre dispersion, techniques to compensate for fibre dispersion, nonlinearization in the APL, modulation techniques, and balance detections. Later, research is done in various linearization techniques to eliminate Intermodulation and Harmonics which happen due to modulations.

In chapters 3, 4 and 5, different linearization methods are presented and demonstrated. A high linear analogue photonic link based on a Dual-drive Dual-parallel Mach-Zehnder Modulator (D-DPMZM) with two separate photodetectors is presented. Third-order Intermodulation Distortion and Second-Order Distortions products have been eliminated by controlling the phase of the input RF signal and driving voltage of D- DPMZM. Mathematical modelling and simulations, for the proposed configuration and the purity of the system, are developed and tested by introducing additional RF signals. In the proposed configuration, a high linear down-converted signal is transmitted by changing only the operating modulator biasing point from quadrature to maximum of free Dynamic Range (SFDR) of 58 dB and linearized signal and 77.84 dB down-converted signal.
A different Microwave Photonic Link (MPL) system configuration based on two D- DPMZMs and two Balanced Photo-Detectors (BPDs) is reported. The Intermodulation Distortions (IMDs), as well as the harmonic distortions, have been eliminated. The proposed linearization of RF signal configuration is double side banded in both D-DPMZM. A full mathematical model has also been developed and simulations have been performed for the proposed configuration. The proposed MPL system configuration exhibits significant performance, and it will have a great impact on aerospace, radar, and satellite-to-ground downlink communication system applications.

AMPL is proposed and experimentally demonstrated by unique ways of deploying Gallium Arsenide (GaAs), two Electro-optic Mach-Zehnder Modulators (MZMs), and BPD. All even and odd intermodulation distortions products are suppressed under the noise floor. Second-Order Harmonics (SOH) are also significantly suppressed by careful
arrangements of microwave shifters and MZMs. We have also developed and implemented a full mathematical model for the proposed configuration, and linearization of the system is tested by introducing an additional RF signal. Our experimental measurements exhibit the suppression under the noise floor of all even and odd IMD and significant suppression of SOH. Spurious Free Dynamic Range (SFDR) of the proposed analogue photonic link for a linearized signal is 119.5 dB.Hz2/3.