Squint-Free Fourier-Optical RF Beamforming Using a SHB Crystal as an Imaging Detector

We describe and demonstrate a Fourier-optical beamformer that modulates RF energy from an antenna array onto a corresponding array of coherent optical beams and forms RF-frequency-scaled (or beam-squinted), far-field images through a single-lens optical Fourier transform. The squinted image is formed into a cryogenically cooled spectral hole burning (SHB) crystal, where it writes spatially-localized spectral images as holes in the crystal's spatial-spectral absorption profile. The spectral-hole images at each resolvable spectral bin (10 MHz at 4.2 K) integrate during the population lifetime ( 1 ms) for frequencies within the inhomogeneous crystal bandwidth ( 20 GHz). A chirped laser diffracts from the holes, producing a sequence of spectrally resolved RF images. The diffracted light passes through a swept-zoom lens synchronized to the chirped laser readout. This zoom lens compensates for beam squint during readout and forms the squint-free images onto a charge-coupled device (CCD), where they are coherently detected. We demonstrate experimental squinted images from a 5 element array across a 350 MHz bandwidth that are recorded into and read from a Er:Eu:YSiO SHB crystal before a digital zoom compensates for beam squint. This system can be scaled up to form images from 2-D arrays with 1000s of elements across 20 GHz bandwidths.