A novel wavelength division multiplexed system architecture for high resolution sampling and digitisation of broadband microwave signals

Military applications such as electronic surveillance (ESM), intelligence (ELINT) and countermeasures (ECM) require real-time analysis of broadband RF signals, often with very high frequency resolution. Direct sampling and digitisation, followed by Fourier analysis and signal processing, will enable such signals to be analysed with much higher resolution than can be achieved with conventional microwave techniques. Direct sampling and digitisation of signals at frequencies above a few GHz is difficult to achieve electronically because the rise and fall times of electrical sampling gates are too long. This can be overcome by using a low-jitter optical sampling pulse train to measure the voltage on an electro-optic modulator. Optical sampling pulses can be very much shorter than electrical sampling pulses because of the large optical carrier frequency. In this paper we describe a novel multiple wavelength optical sampling system architecture for the real-time digitising of microwave signals between 1 and 20 GHz with a target resolution of more than 10 effective bits (>60 dB spurious free dynamic range). We also describe our work on making highly linear electro-optic modulators for this and other microwave-photonic applications.