The use of optically coherent detection techniques for true-time delay phased array and systems

We analyze a true-time-delay, optically controlled phased array antenna system whereby beam forming is accomplished using a large number of antenna elements that can receive any of several different microwave true-time-delays via the use of coherent optical carriers transmitted through a single fiber. The transmitter and receiver architectures are described in detail. We present calculations for the signal-to-noise ratio (SNR), the signal-to-interchannel interference ratio (SIR) and dynamic range (DR) of one channel for various modulation-demodulation schemes (i.e., AM-heterodyne, FM, PM). We show for an example system that can accommodate 128 antenna elements and provide 128 different delays (7 bits), assuming AM modulation with a bandwidth of 1 MHz, a transmitter power of -5 dBm and a laser linewidth of 10 MHz, we can obtain DR/spl sime/50 dB for one channel using a channel separation twelve times of that of the microwave frequency (f/sub m/). For the FM scheme, DR/spl sime/60 dB can be obtained for one channel with a laser linewidth of 1 MHz, a FM modulation index of one, and a channel spacing of about 13f/sub m/. With a local oscillator laser tuning range of 100 /spl Aring/, the system can provide seven-bit resolution for a 128 element antenna array. It is found that coherent optically controlled phased array antennas provide improved controllability over direct detection method, and can have the SNR, SIR and DR that meet the stringent requirements of modern high resolution microwave antenna systems.<
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