USA As Represented by Secretary of the Navy

Lee T Nichols

Keith J Williams

Ronald D Esman

Paul D Biernacki

Allan Ward

An image rejecting microwave photonic downconverter uses a microwave sub-carrier modulation technique without concern for image frequency interference in the shifted signal, thereby allowing telecommunications systems to downconvert densely multiplexed communications channels into a low frequency band where conventionqal electronics can perform signal-processing functions. The image rejecting microwave photonic downconveter incoming microwave signals can be processed without ambiguity in direction finding applications, allowing remotable, multioctave microwave signal processing for frequency and phase determination. A first laser providing an optical carrier that is modulated by a first electro-optic modulator with a sinusoidal electrical signal generated by a first local oscillator and a second laser providing optical carrier that is modulated by a second electro-optic modulator with a sinusoidal electrical signal generated by the second local oscillator for a signal generated by a second local oscillator; are transmitted independently through two polarization-maintaining (PM) optical fibers of arbitrary length to a distant point. There the first modulated optical signal is converted to an electrical domain and mixed with an input from an ultra-broadband radio frequency (RF) antenna receive-array, shifting the entire RF band to a higher frequency band equal to the original RF signal plus the modulated optical frequency signal.

Image rejecting microwave photonic downconverter

The wideband single-sideband modulator generates single-sideband (SSB) modulation with suppressed carrier utilizing traveling-wave LiNbO,modulators in a Sagnac loop. In the wideband single-sideband modulator, optical light from a linearly-polarized source is injected into polarization-maintaining fiber (PolMF) where it is split equally via a polarization-maintaining (PolM) directional fiber coupler, resulting in counterpropagating lightwaves in a Sagnac loop. These counterpropagating waves are &pgr;/2 out of phase due to the action of the coupler and remain so when they return to the coupler because they travel equal distances around the loop. Because the coupler is a reciprocal device, if the waves return to the coupler with equal amplitudes, they will recombine and exit the original port toward the polarized light source. The technique achieves true carrier cancellation over a range of radio frequency (RF) power levels, thereby serving as an enabling architecture for photonically-remoted ultrawide-band single-sideband suppressed-carrier (SSB-SC) modulated links.

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Springfield, VA, United States of America

Lee T Nichols