Optical EngineeringNovel guided-wave electro-optic processor for synthetic aperture radar imaging
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We present the theoretical investigation of a novel architecture of a LiNbO3 guided-wave optical processor suitable for real-time microwave imaging in on-board synthetic aperture radar (SAR) applications, using a moving platform on either an aircraft or a spacecraft. The new configuration, which is basically interferometnic, includes four Mach-Zehnder (MZ) modulators, each characterized by an additional aperiodic phase-reversal traveling wave modulator. The electrode configuration is appropriately designed to reproduce the product of the base-band SAR received echoes and two reference signals and, then, to perform both the range and azimuth compression. The two product signals are separately time integrated by two fast photodetectors. The sampled signals coming out of the detectors include both the required product signals and intermodulation products and are electronically filtered for reproducing the correct product with no amplitude distortion. At this stage, the signals are multiplexed and registered on a two-dimensional buffer to reproduce the final correlation functions for the range and azimuth directions. Because of the serial multiplication between the received echoes and reference signals, the circuit can operate with very long data streams. Our proposed processor can reproduce the images coming from ground features using a conventional chirp pulse SAR system with the antenna beam looking at 90 deg from the direction of platform motion. The circuit can operate at a free-space wavelength of 1.3 μm with a radiofrequency (rf) of 410 MHz, a response time/bandwidth ratio of ≈ 10-5 μs/MHz, a linear dynamic range of ≈50 dB, and an SNR ≈8 dB, assuming chirp pulses as reference signals. The circuit performance is evaluated by considering lithium niobate as the substrate material and two different waveguide fabrication techniques: titanium indiffusion (Ti:L1NbO3) and proton exchange with annealing (APE:LiNbO3). The best results in terms of linear dynamic range and optical losses are obtained by the titanium indiffused channel waveguides. We calculated in any fabrication condition a swath range resolution of dσ ≈ 10 m in airborne simulation and about 60 m in the satellite simulation, whereas the azimuth resolution is about dρ = 1.5 and 6 m, respectively. Significant improvements in terms of SNR (90%) and SAR resolution (50%) are obtained by using binary pseudorandom sequences as reference signals, i.e., ≈15 dB, Δσ = 2 and < 58 m, Δρ ≈ 1.5 and 6 m, respectively.