Optical diffraction effects in a photonic delay line

Abstract

Accurate, low-noise, wideband phased array radar antennas depend on the use of true time delays between microwave elements. A photonic means of generating the required element-to-element phase shifts was developed by Toughlian and Zmuda for systems operating in the VHF frequency range, and extended by Monsay and Caccuitto for operation in the radar C band. The photonic beamformer is based on a heterodyne optical system, in which the reference beam acts as a probe of the acoustic wave in the acousto-optic (AO) cell. It is apparent that the benefits of this photonic system rely greatly on the ability of the probe beam to pick out just the right phase shift/time delay from the acoustic waveform in the AO cell. A basic limitation to system performance could come from intrinsic diffraction effects due to the finite beamwidth of the probe in the AO cell. In addition, the finite size of the photodetector in the system will have an effect on the final SNR. These effects have been studied in detail and their implications for design and performance of the photonic beamformer are discussed. © 1997 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(97)01312-3]

Analysis of a radar beamformer based on a photonic delay line

“…The nonlinearities appearing in the nominally linear time delay versus frequency relationship ͑the ''wiggles''͒, which were seen experimentally and modeled previously 6,9 do not appear to destroy the capabilities of the delay line.…”

“…Detailed calculations 8,9 show that the SNR of the optical system must be multiplied by a factor of ␣ 2 , where ␣ is a function of true-time delay, and is calculated by considering the diffracted beam size at the photodetector. As mentioned earlier in this section, an integral of the received beam at the photodetector must be carried out based on the diffraction-limited probe beam also considered in the calculation leading to Equation ͑6͒.…”