Photonic microwave generator utilizing narrowband Brillouin amplification and a fiber-based oscillator

A photonic microwave signal generator is assembled utilizing self-oscillation and bidirectional modulation in a LiNbO3-phase modulator. The generated frequency is determined by the round trip of an outer and an inner fiber cavity, in addition to a narrow bandpass electrical filter in the feedback loop. A Faraday mirror reflects the light in the outer fiber cavity, and the polarization shift enhances the stability of the oscillation. Light is detected in the inner cavity, using a photodetector followed by a filter and a high power electrical amplifier. The modulator is thereby driven at several Vπ by the detected output signal. In our experiment, the oscillation is locked on 10 GHz, but generate harmonic frequencies which are extracted through narrowband Brillouin amplification. The output of the system is thus only dominated by two frequencies in the optical domain, separated by arbitrary harmonic of the oscillator frequency. This technique is shown for generation of 60 GHz microwaves, limited mainly by the amount of phase-shift that can be achieved in the modulator. It is a simple technique that does not require an electrical signal generator, or any electronics faster than 10 GHz.