Abstract
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.
© 2012 IEEE
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