Ultra-compact optical true time delay device for wideband phased array radars

An ultra-compact optical true time delay device is demonstrated that can support 112 antenna elements with better than six bits of delay in a volume 16"×5"×4" including the box and electronics. Free-space beams circulate in a White cell, overlapping in space to minimize volume. The 18 mirrors are slow-tool diamond turned on two substrates, one at each end, to streamline alignment. Pointing accuracy of better than 10μrad is achieved, with surface roughness ~45 nm rms. A MEMS tip-style mirror array selects among the paths for each beam independently, requiring ~100 μs to switch the whole array. The micromirrors have 1.4° tip angle and three stable states (east, west, and flat). The input is a fiber-and-microlens array, whose output spots are re-imaged multiple times in the White cell, striking a different area of the single MEMS chip in each of 10 bounces. The output is converted to RF by an integrated InP wideband optical combiner detector array. Delays were accurate to within 4% (shortest delay) to 0.03% (longest mirror train). The fiber-to-detector insertion loss is 7.82 dB for the shortest delay path.