Optical waveguides for control of antenna arrays
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Many communication and radar applications require antennas with high directionality and narrow beam width. Phased array antennas (PAA) are able to meet these requirements and have the added benefit of agile beam steering without physical movement of the antenna structure. It is generally expected that future PAA systems will be designed to operate across ultra-wide bandwidths. It will be necessary to use true time delay (TTD) steering techniques, rather than the phase shifters, in order to meet these large bandwidth requirements and avoid beam squint. In contrast to optical fiber delay lines, waveguide optical delay lines, defined by photolithography, are able to deliver precise delays for PAA systems. A procedure for fabricating low loss polymer optical waveguides has been developed. Single mode waveguides with propagation losses as low as 0.38 dB/cm were fabricated with UV curable polyacrylate materials. A propagation loss measurement technique was invented that decouples the waveguide coupling and bend losses from the propagation loss. Optical switch technology was evaluated for use in an integrated polymer optical TTD device. Polymer digital optical switch (DOS) and total internal reflection (TIR) switch devices were characterized. An integrated TTD device structure was described. The n-bit delay device was composed of optical waveguide delay lines and 2x2 optical switches. Waveguide trench and offset structures were introduced to reduce the bend loss of the waveguide delay lines. Offsets reduced the mode coupling loss associated with straight to curved waveguide transitions while trenches confined the mode in curved waveguides to prevent radiation losses. Optimization of the trench and offset structures enabled a 50% reduction of the polymer waveguide delay line bend radius. Fully integrated polymer TTD devices were fabricated and packaged. The 4-bit TTD device insertion loss was 14.5 dB with a 2.9 dB variation in the loss dependant on the activated delay state. Two optical phased array system structures were presented that used the proposed TTD devices to steer the radiation pattern of an array antenna. A wavelength multiplexed optical delay system was implemented to demonstrate the performance of the fabricated 4-bit TTD devices. The far field radiation patterns of 1-dimensional (1D) and 2- dimensional (2D) X-band array antennas were measured and compared to simulations. Transmitting frequencies of 10.54 and 10.66 Ghz were steered to angles of -15, 0, and 15 degrees with no beam squint effect.