Wavelength-controlled hologram-waveguide modules for continuous beam-scanning in a phased-array antenna system

Abstract Wavelength-controlled true-time delay modules based on the dispersive hologram-waveguide are presented here to provide continuous beam-scanning for a X-band phased-array antenna system. The true-time delay modules operating in the 1550nm region were fabricated with continuously tunable time delays from 5ps to 64ps. All-optical wavelength conversion in the semiconductor optical amplifiers was proposed in the system to extend the beam-scanning scope from one dimension to two dimensions. The wavelength-controlled time delays were measured across the X-band (8-12GHz) in the experiment. Keywords : Dispersion, wavelength conversion, semiconductor optical amplifier, true- time delay, phased-array antenna 1. Introduction The phased-array antenna offers various advantages such as accurate and quick beam-scanning without physical movement. It has numerous applications in both military and civilian radar systems and in wireless communication systems. Optical true-time delay techniques have been extensively researched in the past few years because of their promising potential applications in phased-array antenna systems. The main advantage of using the optical true-time delay technique in phased-array antenna systems is freedom from the frequency squint effect. This effect can cause change in the beam-scanning angle, an undesirable feature for the phased-array systems. The application of the optical true-time delay technique in phased-array antenna systems offers better performance compared with the traditional electrical true-time delay technique. Optical true-time delay technique has much larger bandwidth and is freedom from the electromagnetic interference, which is often a serious problem that needs to be considered in the design of an electrical scheme. Many different optical true-time delay schemes have been proposed and demonstrated in the past few years [1,2,3]. The acoustic-optic-based optical true-time delay scheme is compact and easy to integrate. However, its bandwidth is quite limited. Chirped Bragg grating written into the fiber was used to achieve true-time delay for the antenna system. The problem associated with chirped Bragg grating is that the time delay ripple is difficult to overcome. The tradeoff between the bandwidth and the length of the grating is also a problem. Chromatic dispersion in single-mode fiber was used to produce the desired time delay for the antenna systems. But the approach requires fiber length up to several kilometers to get several picoseconds of time delay. The hologram-waveguide-based true-time-delay technique was researched because of its advantages of low cost, high packaging density, and simple fabrication process. The digitalized true-time-delay modules using hologram-waveguide-based technique were demonstrated [4, 5], in which time delay modules were designed for working in both the 850 nm and the 1550 nm wavelength regions. A quasi-analog true-time-delay scheme was proposed and demonstrated [6], an improvement compared with digitalized schemes, but continuous time delays are available only in a narrow range. It is desirable for true-time-delay modules to provide continuously tunable time delays with operating wavelengths in the 1550 nm region, considering their potential real applications. In this paper we present a wavelength-controlled scheme in hologram-waveguide true-time-delay modules to provide continuous beam scanning for a X-band phased-array antenna system. Time delays can be continuously tuned by varying the wavelengths in the various modules. The true-time-delay modules reported herein can provide continuous time delays from 5ps to 64ps. All-optical wavelength conversion in the