Analysis of the AO-FDPC optical heterodyne technique for microwave time delay and phased array beamsteering applications

This paper presents a theoretical analysis of the application of the acousto-optic frequency-dependent phase compensated (AO-FDPC) optical heterodyne technique for microwave time delay applications and phased array beamsteering. A primary goal of the paper is to resolve open questions that have been associated with this interesting and highly referenced technique. The work presented here quantifies, for the first time, the fundamental time delay performance bounds of this technique in terms of the parameters associated with the AO-FDPC acousto-optic (AO) cell and the signal bandwidth. The theory presented in this paper is used to interpret previously reported experimental results that have been subject to debate. Much of the theoretical approach is general and may, therefore, be modified to address the design of new FDPC approaches. Finally, the wide-band beamsteering performance that can be achieved with this technique is quantified in terms of the AO cell parameters and phased array specifications.

I. Introduction

There has been much interest in the use of microwave photonics for signal distribution in phased array antennas. Applications of microwave photonics have been proposed for various array signal-processing applications including signal distribution, phase shifting, and time delay generation. The utilization of photonics for time delay generation for wide-band phased array beamsteering is particularly appealing since RF delay lines are generally large, bulky, and heavy. An excellent review of optically generated true time delay (TTD) techniques is presented by Frigyes and Seeds [1]. While Frigyes and Seeds review the acousto-optic frequency-dependent phase compensated (AO-FDPC) time-delay technique in their work, they also point out that “questions remain still to be answered” regarding this technique. A primary purpose of this paper is to resolve these open questions. Therefore, it is appropriate to start with a brief discussion of how time delays may be generated.