RF Nonlinearities in an Analog Optical Link and Their Effect on Radars Carrying Linear and Nonlinear Frequency Modulated Pulses

Optical links are afflicted with RF nonlinearities, originating from various opto-electronic devices, like intensity modulators. The effects of such nonlinearities on nonlinear frequency modulated (NLFM) pulses are studied in detail for both clutter and discrete returns and compared to the linear frequency modulated case. While for discrete returns, NLFM coding generates weaker ghosts, this advantage is lost for clutter, where both codings exhibit similar performance. When such an optical link in a photonic beamformer processes the signal returned from a high contrast scene, third and higher order nonlinearities severely limit the obtainable contrast of the imaging radar. For third-order nonlinearities and high RF input powers, an inverse fourth-order relationship exists between the contrast and the input voltage. For lower inputs, the contrast is initially limited by system noise and then by the skirts of the compressed impulse response. These results for the two frequency codings are experimentally confirmed in a high frequency (10 GHz), wideband (1 GHz) photonic link, where the nonlinearities originate from the behavior of a Mach-Zehnder intensity modulator. Specifically, to achieve a contrast better than 25 dB, the RF input to the Mach-Zehnder modulator should not exceed 1/8 of the link input third-order intercept voltage.