<title>Fiber optic Bragg grating true-time-delay generator for broadband rf applications</title>

Abstract

We discuss the development of a fiber-optic approach for low-loss true time delay of wideband RF signals for phased-array-antenna beamsteering and general analog signal-processing applications. The device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber to reflect light of different optical wavelengths. An optical carrier modulated by the RF signal of interest is launched into this delay-line fiber. The desired RF time delay is realized by runing the optical carrier wavelength for reflection from the appropriate grating. Unlike conventional switching schemes, this time- delay element has an RF insertion loss that is independent of the number of time delays (bit resolution) provided. The wavelength selectable nature of the TTD device can simplify beamsteering control and channel multiplexing in phased-array antennas. Experimental results are reported for optical and RF characterization of the Bragg-grating element. Both time and frequency-domain RF data are shown that demonstrate the wavelength-selectable time delay concept.