Evidence of modulation-dependent fading of signal amplitude due to fiber dispersion in radio-over-fiber transmission

Effects of fiber dispersion on Radio-over-Fiber (RoF) transmission of a single-carrier microwave signal employing 16-quadrature amplitude modulation (16-QAM) have been experimentally demonstrated. In addition to the signal fading due to fiber dispersion, another cyclic variation of signal amplitude that depends on the modulation depth has been theoretically predicted in the literature (Reference [2]). To confirm the prediction experimentally, we set up a RoF link transmitting a 16-QAM microwave signal, and observed the change in the constellation map of the QAM signal. The optical signal was prepared by modulating the output of a distributed feedback-laser diode using a lithium-niobate intensity modulator. When the modulation depth was small, the constellation maps after the transmission experienced degradation in their signal-to-noise ratio, but any other significant change was not observed. If we employed a large modulation depth, however, outer symbols in the constellation map moved slightly outwards as the signal propagated down the fiber. If we eliminated harmonic sidebands using an optical filter, the movement of the outer symbols is relaxed compared with the unfiltered case. From these results, we could confirm the prediction in the literature that the movement of the outer symbols was due to the beat between the fundamental and the second-harmonic sidebands, the relative phases between which were shifted due to fiber dispersion.