Broadband RF imaging by spatial Fourier beam-forming suffers from beam-squint. The compensation of this frequency dependent beam-steering requires true-time-delay multiple beam-forming or frequency-channelized beam-forming, substantially increasing system complexity. Real-time imaging using a wide bandwidth antenna array with a large number of elements is inevitably corrupted by beam-squint and is well beyond the capability of current or projected digital approaches. In this paper, we introduce a novel microwave imaging technique by use of the spectral selectivity of inhomogeneously broadened absorber (IBA) materials, which have tens of GHz bandwidth and sub-MHz spectral resolution, allowing real-time, high resolution, beam-squint compensated, broadband RF imaging. Our imager uses a self-calibrated optical Fourier processor for beam-forming, which allows rapid imaging without massive parallel digitization or RF receivers, and generates a squinted broadband image. We correct for the beam squint by capturing independent images at each resolvable spectral frequency in a cryogenically-cooled IBA crystal and then using a chirped laser to sequentially read out each spectral image with a synchronously scanned zoom lens to compensate for the frequency dependent magnification of beam squint. Preliminary experimental results for a 1-D broadband microwave imager are presented.
We present an optical approach to 1-D broadband microwave imaging. The imager uses a Fourier optical beamformer to generate a squinted broadband image which is then spectrally resolved by burning a spatial distribution (an image) of spectral signals into a spectral-hole burning material. This spatial-spectral image corresponds to the spectral content of the image at each resolveable spatial point. These narrowband images may be sequentially read out with a chirped laser, scaled to compensate for beam squint, and summed to form a broadband microwave image.
We present a proof-of-concept optical experiment that demonstrates the
ability to record squinted broadband RF images formed by a Fourier beamforming phased-array antenna and subsequent squint correction using spatial spectral holography. A cryogenically cooled inhomogeneously broadened absorber (Tm3+:YAG) acts as a spectrally selective holographic medium which records the squinted RF image, covering a wide RF bandwidth (approaching 20 GHz) with resolution of approximately 1 MHz. Subsequently, a frequency-swept laser can read out the squinted image while a magnification-compensating motorized zoom lens synchronously corrects the magnification due to beam squint. Time-integration the image on a CCD detector array produces a squint-compensated broadband RF image, while detection with a MHz bandwidth detector can produce spectral estimates for all sources recorded with this imaging system.