Hardware Demonstration of Extremely Compact Optical True Time Delay Device for Wideband Electronically Steered Antennas

Abstract

An optical true time delay device is demonstrated that is capable of supporting 112 antennas with 81 different delays (>; 6 bits) in a volume 16" × 5" × 4" including the box with electronics. It uses a free-space design based on the White cell, and alignment is made simple, fast, and robust by the use of slow-tool diamond turning of many optics on a single substrate. Pointing accuracy of the 12 objective mirrors is better than 10 μrad, and surface roughness is ≈ 45 nm RMS. Delays vary from 0 to 25 ns in 312.5 ps increments. Short delays are implemented using delay rods of high refractive index, and long delays using folded mirror trains. Total insertion loss from fiber to detector was 7.82 dB for the no-delay path, and 10.22 dB for the longest lens train. A three-state tip-style MEMS micromirror array is used to select among the delays, with tilt angles ±1.4° plus flat, and switching time <; 100 μs for the entire array. An InP wideband optical combiner photodetector array converts the optical signal to RF with 20 GHz bandwidth. The unit survived temperature cycling 0 to 50 C and random vibration on three axes (9.84 g RMS) with no degradation of signal.