JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 5, MARCH 1, 2011 775 Optical Single-Sideband Modulation With Tunable Optical Carrier to Sideband Ratio in Radio Over Fiber Systems Bouchaib Hraimel, Xiupu Zhang, Senior Member, IEEE, OSA, Yinqing Pei, Ke Wu, Taijun Liu, Senior Member, IEEE, Tiefeng Xu, and Qiuhua Nie Abstract—We show that an optical modulator that consists of an integrated dual parallel Mach–Zehnder modulator (dMZM) can be used for obtaining not only optical single-side band modulation but also tunability of optical carrier to sideband ratio (OCSR) simultaneously. Such a modulator will be vital for optimizing the performance of radio over fiber links by improving modulation efficiency and receiver sensitivity and by removing fiber chromatic dispersion induced RF power fading. It is shown that a wide range of OCSR tunability can be obtained by altering bias voltage of dMZM, and optimum OCSR, to maximize the output RF power, depends on RF modulation index and extinction ratio of the integrated dMZM. Good agreement between theory, simulation, and experiment is obtained. For typical extinction ratio and low modulation index, it is found that an OCSR of 0 dB is optimum to maximize RF carrier power. However, for multiband orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) radio, the best error vector magnitude (EVM) of 21.8 dB is obtained experimentally at an OCSR of 5.4 dB due to avoidance of clipping induced nonlinear distortion. Index Terms—Fiber optic communications, multiband OFDM ultra-wideband wireless, radio over fiber, RF photonics. I. INTRODUCTION R ADIO over fiber has been considered a promising tech- nology for the distribution of future high capacity wire- less signals such as ultra-wideband [1] and millimeter-wave [2] wireless signals. Manuscript received June 12, 2010; revised December 09, 2010; accepted January 17, 2011. Date of publication January 24, 2011; date of current version March 02, 2011. This work was supported in part by the Quebec FQRNT Team Research Project and by the National Natural Science Foundation of China under Project 60972064. B. Hraimel is with the Advanced Photonic Systems Lab, Department of Elec- trical and Computer Engineering, Concordia University, Montreal, QC H3G 1M8 Canada, and also with the PolyGrames Research Center, Department of Electrical Engineering, Ecole Polytechnique de Montreal, Montreal, QC H3T 1J4 Canada (e-mail: hraimel@encs.concordia.ca). X. Zhang is with the Advanced Photonic Systems Lab, Department of Elec- trical and Computer Engineering, Concordia University, Montreal, QC H3G 1M8 Canada (e-mail: hraimel@encs.concordia.ca; xzhang@ece.concordia.ca). Y. Pei is with the Advanced Photonic Systems Lab, Department of Elec- trical and Computer Engineering, Concordia University, Montreal, QC H3G 1M8 Canada, and also with the Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China. K. Wu is with the PolyGrames Research Center, Department of Electrical Engineering, Ecole Polytechnique de Montreal, Montreal, QC H3T 1J4 Canada. T. Liu, T. Xu, and Q. Nie are with the College of Information Science and Engineering, Ningbo University, Ningbo, Zhejiang 315211, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2011.2108261 It was found that radio over fiber links can be improved in performance by optimizing optical carrier to sideband power ratio (OCSR). For an optical carrier used for distributing an op- tical subcarrier or sideband signal, it was found experimentally that the optimum OCSR is 0 dB [3], [4]. One approach to alter the OCSR is to use an optical filter [3], [5] for reducing optical carrier power. However the carrier filtering needs tuning when the laser or carrier wavelength drifts. An optical carrier-sup- pressed single sideband modulation using a hyperfine blocking filter based on a virtually imaged phased-array was reported [6]. In this scheme, the blocking filter can be tuned for obtaining si- multaneous single sideband and strong carrier suppression of up to 30 dB below the unsuppressed single sideband. How- ever, this technique suffers from high insertion loss of more than 13 dB and high power penalty caused by the blocking fil- tering effects such as sideband shaping and residual chromatic dispersion. Stimulated Brillouin scattering (SBS) is also used intentionally as a carrier filter [7]. An injected power above the SBS threshold into a length of optical fiber is used to reduce the carrier power while the modulated sidebands are allowed to pass with low loss. However the system suffers from signal dis- tortion and instabilities in the transmitted power, attributed to noise in the spontaneous scattering process that initiates SBS [8]. Later, a technique, in which a Mach–Zehnder modulator (MZM) is used to tune OCSR with the help of bias voltage, was proposed [9]. However, this technique only enables to obtain optical double sideband (ODSB) modulation and optical single sideband (OSSB) modulation cannot be obtained without op- tical filtering. In [10], an x-cut Lithium Niobate dual parallel Mach–Zhender modulator (dMZM) was designed for OSSB modulation with carrier-suppression ratio over 25 dB. The dMZM comprises a main MZM with an MZM inserted in each arm. Several applications are obtained with this device: generation of millimeter-wave (mm-wave) signals using frequency doubling [11], quadrupling [12] and octupling [13], and optical carrier-suppressed optical differential quadrature and phase-shift keying signals in radio over fiber systems [14]. Using the same modulator as in [10]–[14], this paper will show by theory, simulation and experiment that not only OSSB modulation but also tunability of the OCSR can be obtained simultaneously. By changing bias voltage, the OCSR can be tuned continuously. Theoretical analysis and numerical simula- tion are presented for the analysis of OCSR tunability, RF power dependence on and impact of extinction ratio and RF modu- lation index of the two integrated MZMs. We also experimen- tally investigate the distortion effects using this modulator and 0733-8724/$26.00 © 2011 IEEE