Frequency Quadrupling and Upconversion in a Radio Over Fiber Link

In this paper, a novel technique to realize frequency quadrupling and upconversion in a radio over fiber (RoF) link is proposed and experimentally demonstrated. The frequency quadrupling is achieved by using two cascaded Mach-Zehnder modulators (MZMs) that are biased at the minimum transmission point, with a tunable optical delay line placed between the MZMs. By properly adjusting the time delay between the two MZMs, a pair of optical wavelengths with a wavelength spacing corresponding to four times the frequency of the microwave drive signal is generated. The two wavelengths are then sent to a third MZM to which an intermediate-frequency (IF) signal is applied. At the output of the third MZM, a frequency-upconverted signal at the millimeter-wave (mm-wave) band is obtained. The advantages of the technique are that a relatively low-frequency local oscillator (LO) signal is used to generate a high-frequency LO signal and the upconverted signal is more tolerant to the dispersion-induced power fading compared with a conventional RoF link based on double-sideband (DSB) modulation. Experiments are performed to verify the technique.