I. Introduction
Radio over fiber (ROF) technology has attracted much attentions in which the microwave signal is carried on an optical wave that is transmitted through low-loss and large bandwidth optical fibers [1], [2]. Among the many commonly-used modulation schemes for ROF systems, such as optical double sideband (DSB) modulation, optical carrier suppressed (OCS) modulation and optical single sideband (OSSB) modulation [3]–[5], OSSB is the most promising modulation scheme because it is free of fiber dispersion induced power fading [5] and has a high spectral efficiency. Extensive efforts have been devoted to implementing the OSSB modulation. One typical approach is based on DSB modulation and optical filtering. In this method, a DSB modulated signal is first generated by an electro-optic modulator. Then, a fiber Bragg grating (FBG) [5] or a stimulated Brillouin scattering (SBS) [6] based notch filter is followed to remove one sideband of the DSB signal. OSSB modulation is thus obtained. The main limitation of this approach is the use of the wavelength dependent device, i.e., the optical filter. If a wideband optical filter is used, it is difficult to eliminate one first-order sideband without affecting the optical carrier and the other sideband, and if a narrow-bandwidth optical filter is applied, the OSSB modulation can only realized in a small frequency range. OSSB modulation can also be achieved using electro-optic modulators without any wavelength-dependent optical filter. A dual-electrode Mach–Zehnder modulator (MZM) [7], a dual-parallel MZM [8] or an MZM cascaded with a phase modulator [9], which are driven by two orthogonal RF signals, have been demonstrated to realize the OSSB modulation. The key advantage of these systems is the wavelength independent operation, which leads to a large operation bandwidth and a good wavelength tunability. However, for most of the previously reported schemes with or without an optical filter, the optical carrier-to-sideband ratio (OCSR) of the OSSB modulation can not be tuned. Recently, OSSB modulation with tunable OCSR is found to have many applications in ROF and microwave photonic signal processing systems [10], [11], e.g., by optimizing the OCSR, the transmission performance of an ROF system can be significantly improved. Up to now, several schemes have been proposed to realize the OCSR-tunable OSSB modulation [10]–[12], such as using an OSSB with carrier modulation followed by a notch filter [10], [12] or using a dual-parallel MZM [11]. Those systems are simple since only one modulator is employed, but the use of the optical filter would make the system wavelength dependent, and the high requirement of complex DC bias controlling may increase the complexity of the systems.