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Coherent Optics for Squint-Free RF Antenna Array Beamforming

Event Details

Event Dates: 

Monday, October 28, 2013 - 4:00pm

Seminar Location: 

  • Engineering Building Room ECCR 105

Speaker Name(s): 

Kelvin Wagner

Speaker Affiliation(s): 

University of Colorado Boulder
Seminar Type/Subject

Scientific Seminar Type: 

  • COSI Seminar

Event Details & Abstract: 

The historical highlights of Optical Signal Processing will be briefly reviewed and compared to the dramatic progress but inevitable limitations of DSP. This will motivate our work applying coherent Fourier-optical processing to the problems of array signal processing for true-time-delay beamforming and squint-free RF antenna array imaging. Modern antennas often utilize phased arrays with 1000s of elements whose output can be combined to synthesize the desired beam, but this can require a huge amount of RF hardware. Optical modulators and hair thin optical fiber allows much of this RF hardware to be shrunk down to a small cable which doesn’t interfere with the radiation pattern of the antenna, and whose outputs can be optically combined to steer the antenna beams. Such antenna arrays are susceptible to unwanted interference or jamming signals leaking through on the antenna sidelobes, and the elimination of these signals requires adaptive processing, however the computational load for the wideband adaptive processing of antenna arrays can be enormous. We will discuss the BEAMTAP adaptive phased-array radar optical processors that use a photorefractive crystal to time-integrate the adaptive weights, null out jammers, and beam steer the main beam towards desired signals.Then two broadband, sparse-array, multiple-beamforming, optical processors will be presented. These novel optical systems enable squint-free, true-time-delay beamforming of large, randomly-spaced antenna arrays by compensating the frequency-dependent scale factor of the RF images produced by a Fourier optical imager. The first uses novel tunable-filter optical modulators and the broad optical bandwidth of octave-spanning femtosecond lasers to enable a wavelength-based squint-compensated RF beamforming system to produce the time-domain waveforms from each RF source. The second uses the optically-modulated signals from a fiber-remoted, coherently-fed, randomly-spaced, RF antenna array and a Fourier lens to form a squinted RF image on a spectrally-selective spectral-hole burning material. The frequency resolved accumulated RF scene is read out with a frequency-scanned and synchronously-magnified system to produce the squint-free output image, allowing the spectral analysis of every source in the field of view simultaneously.