Photonic signal processing is used to implement common mode signal cancellation across a very wide bandwidth utilising phase modulation of radio frequency (RF) signals onto a narrow linewidth laser carrier. RF spectra were observed using narrow-band, tunable optical filtering using a scanning Fabry Perot etalon. Thus functions conventionally performed using digital signal processing techniques in the electronic domain have been replaced by analog techniques in the photonic domain. This technique was able to observe simultaneous cancellation of signals across a bandwidth of 1400？MHz, limited only by the free spectral range of the etalon. 1. Introduction Photonic signal processing offers a new paradigm for processing high bandwidth signals and opens up possibilities for directly processing signals that are modulated onto an optical carrier. The ability to process and manipulate ultrawideband signals enables high-resolution, reconfigurable processing of potentially the entire RF spectrum of signals. It is this potential that has received a lot of attention over the past few decades [1, 2] and is suitably demonstrated through devices such as wideband adaptive microwave filters enabled through coherence and phase control [3]. Photonic technologies are a natural source for delivering increased bandwidth, but also the low loss of fibre optic transmission systems has seen developments in transmitting Radio over Fibre (RoF) and antenna remoting [4, 5]. As requirements for increased data bandwidth develop and as spectral densities increase, with congestion becoming an issue, the need for ever more powerful digital signal processing (DSP) is a constant driver. This has inevitable consequences for size, weight, and power (SWaP) considerations, especially in RF signal monitoring and characterisation equipment. With the RF signal in photonic form and the inherent bandwidth benefits that ensue, there is a natural opportunity for implementing analog signal processing (ASP) techniques and making the processing an inherent part of the optical link [6]. Signal cancellation is conventionally implemented by engineering a version of the target waveform that is matched in amplitude but 180° out of phase (negative feedback) such that the interference of the target and engineered waveforms result in cancellation. This approach can be performed with analog or digital signals but is likely to be limited in its operational bandwidth by frequency dependent phase shifts. Photonic carriers modulated by RF signals are not limited by these frequency dependent phase shifts in circuitry
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