Theory and optimisation of double conversion heterodyne photoparametric amplifier

An optical wireless transmission technique represents an attractive choice for many indoor and outdoors applications within fixed and mobile networks. It has the advantage of providing a wide bandwidth that is unregulated worldwide, with availability to use it in a very dense fashion, and potentially very low cost. Due to the high attenuation suffered by Infrared radiation through the air, operating low power transmission sources, and generally adverse signal to the noise environment found by ambient background light, where the optical signal is typically at it is minimum power level when detected. A high sensitivity and high selectivity receiver will be imperative for such applications as subcarrier multiplex systems, millimetre-wave radio over fibre and other wireless optical system applications. The thesis details the research, design, and optimisation of a novel, low-noise frontend optical receiver concept using a photoparametric amplifier (PPA) technique, in which the detected optical baseband signal is electrically amplified and up-converted to upperside frequency, based on the nonlinear characteristic of the pin photodiode junction; the desired signal passes through a further signal processing stage, and the original baseband signal is recovered again, using the concept of the superheterodyne principle. The designed DCHPPA receiver acts in a parallel manner to a conventional double superheterodyne detector system, but without the noise penalty normally incurred in the first stage. The PPA is used instead of a resistive/transistor based mixer at the first stage. DCHPPAs have the properties to provide very high gain, with high selectivity, combined with a very low noise operation. The research is conducted from three aspects: theoretical analysis, modelling and simulation, and practical implementation and result analysis. The three approaches followed the same trend shown, and the results correspond closely with each other. Theoretically, a new non-degenerate PPA mode of operation is discussed, in which the applied dc bias to the pin photodetector is replaced by the applied ac pump signal. This is shown to be advantageous in terms of the desirable characteristics for PPA operation, leading to improved conversion efficiency and the potential for low noise operation. PPA was shown to behave more optimally with load resistance which was much lower than normally used in the common optical wireless receiver-amplifiers. A new PPA gain theory was derived and optimised accordance with the original gain theory, PPA input/output admittance power was analysed for optimum power transfer. More accurate DCHPPA circuit configurations were modelled and simulated using nonlinear simulator tools (AWR) which help to understand and optimise system performance, particularly device parameters and characteristics. The full DCHPPA system was implemented practically, and tested in VHF and UHF as a sequel to the simulation configuration, which subsequently exhibited a 34.9dB baseband signal over the modulated optical signal; by employing a chain gain DCHPPA cascaded configuration, 56.3 dB baseband signal gain was achieved. The PPA noise was also measured and analysed, which satisfied the tough front-end optical system requirements