Active phased-array antenna systems offer highly accurate beam pointing, increased beam scan flexibility in three dimensions, and beam steering without physical movement, making them attractive for applications such as cellular communications, satellite communications, and air-traffic-control radars. The biggest disadvantage of the technology currently lies in the electronic time-delay control systems associated with most phased-array antenna systems—they are expensive, hardware intensive, and consume significant amounts of power.
Optical-control and signal-processing systems, which potentially offer an economical alternative to electronic controls, have garnered significant attention in the past few years. Nicholas Madamopoulos and other researchers at the University of Central Florida's Center for Research in Electro-Optics and Lasers (UCF/CREOL; Orlando, FL) have demonstrated a compact, lightweight reflective-geometry photonic delay line (PDL) design for antenna control that incorporates nonpolarization-maintaining fiber to achieve high signal-to-noise ratios.
A switching device controls the sequence of signals that drive a subarray in a phased-array antenna. The PDL system demonstrated by the CREOL group consists of a two-dimensional pixelated optical array that acts as a compact polarization switch. This two-bit device consists of one module containing free-space optics and a second based on nonpolarization-maintaining fiber. The design can, however, be generalized to an N-bit system by adding multiple fiber modules.
Input to the switch passes through a polarizer, resulting in vertically polarized (S-polarized) output. A subsequent liquid-crystal switch either switches the polarization to horizontal polarization (P-polarization) or leaves it unaltered. Depending on the polarization of the beam exiting the liquid-crystal switch, the polarization beamsplitting cube next in the optical path either allows the beam to propagate straight through the beamsplitter and the output components and into the fiber-based module (P-polarization) or deflects it by 90° into the delay path (S-polarization). With the aid of quarter-wave plates, light in the delay path is directed to two mirrors before reflecting off the beamsplitter and exiting to the fiber-based module.
In the second module, optical fiber replaces the open path in the delay line, inducing greater delay over the same distance. Previous polarization-based switching designs have incorporated polarization-maintaining fiber, but the CREOL device uses telecommunications-grade, nonpolarization-maintaining fiber as a more economical alternative. A Faraday rotator replaces the quarter-wave plate in the delay path, compensating for random induced birefringence of the fiber caused by source spectral drift or environmental conditions. A gradient index (GRIN) lens couples the beam into the fiber, and the exiting beam passes through a collimator and beam expander before a prism redirects it into the beamsplitter cube.
In the demonstration system, an acousto-optical modulator operating at 1 GHz modulates the output from a 10-mW helium-neon laser to provide input to the free-space module. A high-speed photodetector (Model 1601, New Focus, Santa Clara, CA) detects the L-band output from the fiber delay module, which is the signal that drives each subarray in the phased-array antenna.
Lowest signal-to-noise ratio for the system was 50 dB, driven primarily by high loss from the optical components. Antireflection coatings and increased coupling efficiency in future systems should increase this figure, and ferroelectric liquid-crystal arrays as optical switches will yield increasingly compact devices.