Millimetre-wave optically injection-locked oscillators for radio-over-fibre systems

Theoretical analysis and experimental results for millimetre-wave optically injection-locked oscillators are presented in this thesis. Such oscillators can be employed to replace conventional photodiode plus amplifier receivers for local oscillator signal reception in millimetre-wave radio-over-fibre systems.

The theories for electrical injection-locked oscillators are reviewed in detail. Three differences between Adler’s and Kurokawa’s equations for locking bandwidth are highlighted for the first time. These differences are the absence of l/cos# factor in Adler’s equation, larger bandwidth predicted by Kurokawa’s equation, and a difference in definition of Q factors. Locking bandwidth equations for optically injection-locked oscillators are developed based on the theories of electrical injection-locked oscillators and are then used to design optically injection-locked oscillators.

A novel millimetre-wave indirect optically injection-locked oscillator is presented. An edge-coupled photodiode is used to detect the optical signal. Negative resistance and computer simulation techniques were used for predicting the free running oscillation frequency. The maximum output power of the oscillator is 5.3 dBm, and the maximum locking bandwidth is measured to be 2.6 MHz with an output power o f-12 dBm. Results from a comparison with conventional optical receivers show that the gain of the optically injection-locked oscillator is more than 10 dB higher than that of a photodiode plus amplifier receiver, that the oscillator output power remains constant with input signal power variations whereas the output power of the photodiode plus amplifier receiver changes (linearly) with the input signal power, and that, at high-offset frequencies, the phase noise of the optically injection-locked oscillator is much lower than that of the photodiode plus amplifier receiver. These advantages make the optically injection-locked oscillator an ideal replacement for the photodiode plus amplifier receiver in radio-over-fibre systems. An improved wide-band design for millimetre-wave optically injection-locked oscillators is presented for future work.