Experimental Study on Variation of Signal Amplitude in Radio-Over-Fiber Transmission Induced by Harmonics of Modulation Sidebands Through Fiber Dispersion

Effects of fiber dispersion on the radio-over-fiber (RoF) transmission of a microwave signal are experimentally demonstrated when deep optical modulation is employed. In a previous paper (J. Lightwave Technol., vol. 30, pp. 2625-2632, Aug. 2012), the authors analytically predicted that harmonics of sidebands generated by the nonlinear response of optical modulators would contribute to the cyclic variation of the signal amplitude other than by dispersion-induced fading. To confirm this prediction, we set up an RoF link that transmitted a 16-quadrature amplitude modulation microwave signal, and we observed changes in the constellation map. Experiments were conducted both for optical double-sideband (ODSB) signals and optical single-sideband plus carrier (OSSB+C) signals. In the ODSB case, outer symbols in the constellation map moved slightly outwards as the signal propagated down the fiber. If we eliminated the harmonic sidebands by using an optical filter, the movement was reversed; this was probably due to the residual chirp in the LN modulator. In the OSSB+C case, we observed a similar movement but also a slight phase rotation that depended on the amplitude of the symbol. These observations qualitatively agree with our predictions.