Using highly dispersive fiber gratings as wavelength-tunable true-time-delay in phased array antenna

Abstract

We have designed, assembled and tested a phase-array antenna using fiber Bragg gratings in the highly dispersive transmission region as our tunable true-time delay (TTD) generators. The TTD generator is designed by cascading 29 identical fiber gratings and 1×2 fiber splitter pairs. Tapping from each fiber splitter allows us to steer our RF microwave beam from a 30×4-element antenna by tuning the wavelength of a laser. The 10Ghz RF signal is superimposed upon a laser beam by means of a LiNbO₃ modulator. However, with a conventional modulator, the optical frequency spectrum of the modulated beam consists of two sidebands on opposite sides of the optical carrier; all three of which may experience very different time delays due to dispersion. This may have detrimental effects on time-delay sensitive processes such as antenna beamforming. Therefore, we studied the use of single-sideband versus double-sideband modulators in the transmitters of photonic phased array antenna. We focus in particular on the effect of the different spectral profiles of single and double sideband modulators on beamforming when using the fiber Bragg gratings as TTD generators. With very high dispersion in our fiber Bragg gratings close to the band edge, the absolute propagation times are different for each sideband and the optical carrier. Therefore double sideband modulated signals will generate two sets of separate delays in the same microwave signal which causes beam deterioration and increased side-lobes. We demonstrated this theoretically and verified experimentally by comparing the antenna patterns generated by single-side-band and by double sideband modulators.