2350 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 9, SEPTEMBER 2011

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1 2350 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 9, SEPTEMBER 2011 Tunable Carrier Generation and Broadband Data Upconversion for RoF Systems Based on Stimulated Brillouin Scattering Wei Li, Ning Hua Zhu, Member, IEEE, Li Xian Wang, Xiao Qiong Qi, and Liang Xie Abstract We present a frequency-tunable RF carrier generation and broadband data upconversion technique for radio-over-fiber (RoF) systems. A dual-parallel Mach Zehnder modulator is used to realize single-mode modulation (SMM). On the other hand, frequency-tunable RF carrier generation with single-sideband (SSB) modulation is performed using stimulated Brillouin scattering (SBS) with three types of configuration. The optical carrier-to-sideband ratio of the SMM-SSB modulated signal can be adjusted to achieve the best received sensitivity performance of the RoF system by simply modifying the pump power in the SBS process. Finally, the transmission performance of the RoF downlink system is examined. The power penalty is less than 1 db at the bit-error rate of 10 9 after 25-km single-mode fiber transmission. Index Terms Broadband data upconversion, radio-over-fiber (RoF), single-mode modulation (SMM), single-sideband (SSB) modulation, stimulated Brillouin scattering (SBS), tunable carrier generation. I. INTRODUCTION R F CARRIER generation and broadband data upconversion realized in optical domain are key functions for radio-over-fiber (RoF) systems, where RF signals are optically transmitted between central and base stations [1] [3]. Standard optical double-sideband (DSB) modulation generates two symmetric signal sidebands on both sides of the optical carrier. The fiber dispersion causes a walkoff in the relative phases of the sidebands, resulting in power penalty of the detected RF signal [4], [5]. Since the use of single-sideband (SSB) modulation can alleviate the dispersion induced power penalty (DIPP), various methods have been proposed to generate SSB modulated RF carrier for RoF systems [6] [11]. The SSB modulation can be obtained by filtering out one of the sidebands [6], using the SSB modulator [5], using two electro-absorption modulators [9], or using the strong optical injection-locked semiconductor lasers [10]. However, these methods suffered from a low level Manuscript received May 23, 2011; accepted May 27, Date of publication July 22, 2011; date of current version September 14, This work was supported by the Meteorology Industry Research Project of China under Grant GYHY and Grant GYHY , by the National Natural Science Foundation of China under Grant , Grant , Grant , Grant , and Grant , and by the National Basic Research Program of China under Grant 2009AA03Z409. The authors are with the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing , China ( liwei05@semi.ac.cn). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMTT of receiver sensitivity originating from a large difference in the power of the strong optical carrier and the weak sideband [11] [13]. On the other hand, data signal upconversion is conventionally carried out by simultaneous modulation of the SSB modulation modes [14], [15], namely, dual-mode modulation (DMM). However, it has been theoretically and experimentally proven that DMM also caused DIPP [14]. In [16], we have demonstrated that data upconversion with single-mode modulation (SMM) is more promising compared with DMM for overcoming the DIPP. Therefore, SMM-SSB modulation is an effective way to eliminate the DIPP for RoF systems [17], [18]. SBS results from the interaction among a pump wave, an acoustic wave, and a Stokes wave. The Stokes wave is either a signal internally generated as a result of spontaneous Brillouin scattering or an external optical signal injected into the fiber [19]. Shen et al. demonstrated the 11-GHz RoF system carrying the 10-Mb/s data using SBS [20], where the data bandwidth was restricted by the Brillouin gain profile. The key technique was based on the simultaneous use of Brillouin gain and loss of the pump wave, amplifying one of the DSB modulated sidebands while attenuating the other one [21]. In [22], an improved scheme with SMM-SSB modulation was proposed to achieve broadband data upconversion. Recently, we have demonstrated a harmonic RF carrier generation and broadband data upconversion technique with SMM-SSB modulation using SBS [23]. However, the main limitation of these methods is that the generated RF carriers were restricted to the Brillouin frequency GHz [22] or the multiple of the Brillouin frequency [16], [23]. In [15] and [24], all-optical SSB upconverters with a frequency-tunable RF carrier have been demonstrated using an optical interleaver, semiconductor optical amplifier, and the tunable optical filter. In this paper, to the best of our knowledge, we firstly demonstrate an SBS-based frequency-tunable RF carrier generation and broadband data upconversion technique for RoF systems without using any optical filter. In principle, the proposed technique works for arbitrarily high frequency of the RF carrier, which is only limited by the bandwidth of the optical transmitter and receiver employed. The SMM is achieved by using a dual-parallel Mach Zehnder modulator (DPMZM). Compared with our previous scheme using separate Mach Zehnder modulator (MZM) and injection-locked laser [23], the commercially integrated DPMZM has the advantages of compact size, easy control, and stable operation. On the other hand, frequency-tunable RF carrier generation with SSB modulation is performed /$ IEEE

2 LI et al.: TUNABLE CARRIER GENERATION AND BROADBAND DATA UPCONVERSION FOR RoF SYSTEMS 2351 Fig. 1. Principle of the tunable carrier generation and broadband data upconversion technique at central station, along with the optical spectra at different locations. Tunable laser source: TLS, dual-parallel Mach Zehnder modulator: DPMZM. using SBS with three types of configuration. The optical carrier-to-sideband ratio (CSR) of the SMM-SSB modulated signal can be adjusted to achieve the best received sensitivity performance of the RoF system by simply modifying the pump power in the SBS process. Finally, the transmission performance of the RoF downlink system is examined. The power penalty is less than 1 db at the BER of 10 after 25-km single-mode fiber transmission. This paper is organized into four sections. Following this introductory section, the principle of the proposed technique is described in Section II. Section III shows the experimental setup and results. Finally, conclusions and discussions are present in Section IV. II. PRINCIPLE Fig. 1 schematically depicts the principle of the tunable carrier generation and broadband data upconversion technique at central station, along with the optical spectra at different locations. In our scheme, the SMM is realized by a commercially -cut integrated DPMZM (Photline MXIQ-LN-40, also called the differential quadrature phase-shift-keying (DQPSK) modulator) structure shown in Fig. 1. The optical carrier at wavelength of from a tunable laser source (TLS) is sent to the DPMZM, which is structured as two MZMs (MZM1 and MZM2) set in parallel and forming a third MZM (MZM3). In the DPMZM, MZM1 is fed by an RF signal from a local oscillator (LO) and is biased at null point to implement DSB suppressed carrier (DSB-SC) modulation. MZM2 is driven by the broadband data signal and is biased at quadrature point to realize SMM. They are combined together in MZM3 to achieve the SMM-DSB modulation. In MZM1, the useless optical carrier is suppressed to avoid suffering from a low level of received sensitivity originating from a large optical CSR [11]. By controlling the dc bias of MZM3,, a phase difference between the DSB-SC and SMM signals is generated. In our case, the phase difference should be zero. This bias voltage of can be obtained by observing the maximum output power from the DPMZM. In such a modulation process, the broadband data signal is only modulated on the optical carrier. Notice, however, that the output from the DPMZM is a SMM-DSB modulated signal, which should be further processed using SBS to achieve SMM-SSB modulation. Fig. 2 shows the principle of the SMM-SSB modulation using SBS with three types of configuration. The SMM-SSB modulation is realized by counterpropagating an SMM-DSB Stokes wave and a DSB-SC pump wave in a length of dispersion shifted fiber: (a) Type I (,, (b), and (c). Dispersion shifted fiber instead of a standard single-mode fiber is employed to reduce the chromatic dispersion introduced in the SBS process. In the dispersion shifted fiber, SBS generates both Brillouin gain and loss, as described by Tanemura et al. [25], who implemented a narrowband passband and notch optical filter. The central frequencies of the Brillouin gain and loss profiles are and, respectively. As a result, one sideband of the Stokes wave suffers attenuation when locating at the Brillouin loss profile of the pump wave, while the other sideband of the Stokes wave is consequently amplified when locating at the Brillouin gain profile [21]. In such a process, we achieve SMM-SSB modulation. The Brillouin gain and loss spectra are also illustrated in Fig. 2. Since the SBS is only imparted on the two modulation sidebands, the optical carrier carrying the broadband data is not affected by the SBS. Therefore, the data bandwidth is not restricted by the Brillouin bandwidth. In principle, the proposed technique works for arbitrarily high frequency of the RF carrier, which is only limited by the bandwidth of the optical transmitter and receiver employed. Moreover, the RF carrier frequency can be easily tuned by adjusting the frequency of and,as described in Fig. 2. III. EXPERIMENTS AND RESULTS In order to demonstrate the feasibility of the proposed technique, we performed a proof-of-concept experiment using a

3 2352 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 9, SEPTEMBER 2011 Fig. 2. Principle of the SMM-SSB modulation using SBS with three types of configuration. (a) Type I (f <f, (f = f + f ). (b) f >f (f = f 0 f ). (c) f <f (f = f 0 f ). Fig. 3. Experimental setup (tunable laser source: TLS; polarization controller: PC; dual-parallel Mach Zehnder modulator: DPMZM; pulse pattern generator: PPG; optical isolator: OI; dispersion shifted fiber: DSF; erbium-doped fiber amplifier: EDFA; optical circulator: OC; single-mode optical fiber: SMF; photodetector: PD; low-noise amplifier: LNA; dc-block capacitor: dc-block; electrical low-pass filter: LPF; error detector: ED). Type I scheme. Fig. 3 shows the experimental setup. The optical carrier from a TLS emitting at nm, with the linewidth of narrower than 100 khz, was divided into two branches by a 3-dB optical coupler. In the upper branch, the optical carrier was sent to the DPMZM. In the DPMZM, MZM1 was driven by the RF signal of 20 GHz and was biased at null point with V to perform DSB-SC modulation. MZM2 was fed by a 2.5-Gb/s 2 1 pseudorandom bit sequence (PRBS), nonreturn-to-zero (NRZ) data from a pulse pattern generator (PPG) and was biased at quadrature point with V to realize SMM. In addition, was set at 9.8 V to achieve zero phase difference between the DSB-SC and SMM signals. For clear comparison, Fig. 4(a) shows the optical spectra of the SMM (the RF port of MZM1 was shorted), DSB modulation (the RF port of MZM2 was shorted), and SMM-DSB modulation signals. It can be clearly seen that the 2.5-Gb/s data signal was only modulated on the optical carrier and the two sidebands were not affected. On the other hand, the lower branch went to an MZM driven by the RF signal and was biased at null point to generate a DSB-SC modulated signal, which implemented the pump wave. The pump wave was then amplified by an erbium-doped fiber amplifier (EDFA) to increase the optical power beyond the Brillouin threshold. For the nm pump wave, the Brillouin frequency was measured to be GHz for the 4.1-km dispersion shifted fiber used in the experiment. The zero-dispersion wavelength of the dispersion shifted fiber is around 1550 nm and its dispersion slope is 0.06 ps nm km. For the Type I scheme, the driven signal of the MZM should be GHz. Fig. 4(b) shows the optical spectrum of the DSB-SC modulated pump signal. The residual optical carrier is shown to be 21 db below the sidebands. This suppression ratio was sufficient to avoid having the residual optical carrier induce any significant Brillouin gain or loss. An optical circulator was used to counter-propagate the DSB-SC modulated pump wave and the SMM-DSB modulated Stokes wave in the 4.1-km dispersion shifted fiber. The optical powers of the pump and Stokes waves injected into the dispersion shifted fiber were 12.5 and 4.2 dbm, respectively.

4 LI et al.: TUNABLE CARRIER GENERATION AND BROADBAND DATA UPCONVERSION FOR RoF SYSTEMS 2353 Fig. 5. Measured USSR versus f for the fixed pump power of 12.5 dbm. Different types of SMM-SSB modulation configuration were used in the measurement. Fig. 4. Measured optical spectra of: (a) SMM (the RF port of MZM1 was shorted), DSB modulation (the RF port of MZM2 was shorted), and SMM-DSB modulation signals, (b) DSB-SC modulated pump wave, and (c) SMM-SSB modulation signal. The optical circulator and the optical fibers connected to it were all angle polished connectors (APCs), which were used to minimize the back-reflection light. An optical isolator was added to ensure unidirectional transmission. Three polarization controllers (PC1-PC3) were added to optimize the polarization state. As can be seen in Fig. 4(c), the upper frequency sideband of the Stokes wave is amplified by the upper frequency sideband of the pump wave, while the lower frequency sideband of the Stokes wave is consequently attenuated by the lower frequency sideband of the pump wave in the SBS interaction. The SMM-SSB modulation can be clearly seen with 25-dB undesired sideband suppression ratio (USSR) (i.e., the power ratio of the amplified sideband and the attenuated sideband). The SMM-SSB modulated signal generated from the central station was then transmitted to the base station through 25-km single-mode fiber. According to [11], the optical CSR (the power ratio of the optical carrier and the amplified sideband) can affect the received sensitivity performance of the RoF system with optimum performance occurring when the CSR is equal to 0 db. In our case, the CSR of the SMM-SSB modulated signal can be adjusted to achieve the best received sensitivity performance of the RoF system by simply modifying the pump power in the SBS process. Fig. 5 shows USSR versus for the fixed pump power of 12.5 dbm. More than 24 db of USSR is achieved from 5 to 20 GHz. The variations in the USSR are mainly attributed to the frequency response of the DPMZM. Different types of SMM-SSB modulation configuration were used in the measurement. For, the electrical source of was saved.for, the MZM in the lower branch was saved since the pump signal was directly provided by the optical carrier. The measurement from 5 to 20 GHz was limited by the 0.01-nm resolution bandwidth of the optical spectrum analyzer and the bandwidth of the DPMZM. In principle, arbitrarily high frequency of the RF carrier with SMM-SSB modulation could be achieved by the proposed technique if high-frequency devices could be available. Once the optical power of the SMM-SSB modulated signal, as shown in Fig. 4(c), was detected by the photodetector (PD, dc 40 GHz), it generated the upconverted RF signal. A lownoise amplifier (LNA, 38 khz 40 GHz) with a 26-dB RF gain was used to boost the detected RF signal. The upconverted RF signal is shown in Fig. 6(a). It can be clearly seen that the strong

5 2354 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 9, SEPTEMBER 2011 Fig. 7. Measured BER of the RoF downlink system. Insets are the eye diagram of: (a) the 25-km transmission case with the optimized polarization and (b) the B-T-B case with the pump polarization mismatch. IV. CONCLUSIONS AND DISCUSSIONS Fig. 6. Electrical spectra of: (a) upconverted RF signal and (b) downconverted 2.5-Gb/s data signal. peaks separated 2.5 GHz from the 20-GHz RF carrier. To downconvert the optically upconverted 2.5-Gb/s data to the baseband, a mixer was used with a phase shifter controlling the phase of the LO signal, as shown in Fig. 3. An electrical low-pass filter (LPF) of the 5-GHz cutoff frequency was attached to the IF port of the mixer to remove the high-frequency noise after the downconversion. Fig. 6(b) shows the downconverted 2.5-Gb/s data, which has the peaks of the 2.5-GHz separation. Note that the sidelobes of the 2.5-Gb/s data do not hamper the system performance. Fig. 7 shows the measured bit-error rate (BER) of the RoF downlink system. The power penalty is less than 1 db at the BER of 10 after 25-km single-mode fiber transmission compared with the back-to-back (B-T-B) case. Fig. 7(a) and (b) shows the eye diagram of the 25-km transmission case with the optimized polarization and the B-T-B case with the pump polarization mismatch. In the SBS process, the Brillouin gain or loss can be affected by the polarization state of the pump wave [22], [26]. As shown in Fig. 7(a) and (b), the polarization state of the pump wave should be optimized to reduce the intensity noise in the SBS interaction [22], [27]. We have demonstrated an SBS-based frequency-tunable filterless RF carrier generation and broadband data upconversion technique for RoF systems. The SMM has been achieved by using the DPMZM. On the other hand, frequency-tunable RF carrier generation with SSB modulation has been performed using SBS with three types of configuration. The optical CSR of the SMM-SSB modulated signal could be adjusted to achieve the best received sensitivity performance of the RoF system by simply modifying the pump power in the SBS process. The transmission performance of the RoF downlink system has also been examined. The power penalty for the RoF downlink system is less than 1 db at the BER of 10 after 25-km single-mode fiber transmission. Notice that the optical power reflection of the pump signal from port 2 to port 3 of the optical circulator may affect the system performance when GHz because the beating signals between the reflected pump signal and the 2.5-Gb/s data signal, which is carried on the optical carrier, may fall into the dc 2.5-GHz frequency range and result in noise [28]. This may happen in Type II and III configurations. In this paper, we have emphasized the idea of frequency-tunable filterless RF carrier generation and SMM-SSB modulation using SBS. In principle, the proposed technique should be feasible for even high data rate and arbitrarily high frequency of the RF carrier, which is only limited by the bandwidth of the optical transmitter and receiver employed. Moreover, the RF carrier frequency can be easily tuned by adjusting the frequency of and, as described in Fig. 2. However, the use of two tunable electrical sources is expensive for the practical application. An alternative way is using a tunable LO, a fixed frequency electrical source, and a proper electrical mixer. The upconverted or downconverted electrical signal at can be used to drive the MZM.

6 LI et al.: TUNABLE CARRIER GENERATION AND BROADBAND DATA UPCONVERSION FOR RoF SYSTEMS 2355 REFERENCES [1] K. H. Lee and W. Y. Choi, Harmonic signal generation and frequency upconversion using selective sideband Brillouin amplification in single-mode fiber, Opt. Lett., vol. 32, no. 12, pp , Jun [2] Z. Xu, X. Zhang, and J. Yu, Frequency upconversion of multiple RF signals using optical carrier suppression for radio over fiber downlinks, Opt. Exp., vol. 15, no. 25, pp , Dec [3] P. T. Shih, C. T. Lin, W. J. Jiang, J. Chen, H. S. Huang, Y. H. Chen, P. C. Peng, and S. Chi, WDM up-conversion employing frequency quadrupling in optical modulator, Opt. Exp., vol. 17, no. 3, pp , Jan [4] R. Hofstetter, H. Schmuck, and R. Heidemann, Dispersion effects in optical millimeter-wave systems using self-heterodyne method for transport and generation, IEEE Trans. Microw. Theory Tech., vol. 43, no. 9, pp , Sep [5] G. H. Smith, D. Novak, and Z. 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Theory Tech., vol. 58, no. 11, pp , Nov Wei Li received the Ph.D. degree in microelectronics and solid state electronics from the Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing, China, in He is currently an Assistant Professor with the Institute of Semiconductors, CAS. His research interests include high-frequency characteristics of microwave opto-electronic devices, optical injection techniques, and microwave photonics. Ning Hua Zhu (M 92) was born in Guizhou, China, on December 16, He received the B.S., M.S., and Ph.D. degrees in electronic engineering from the University of Electronic Science and Technology of China, Chengdu, China, in 1982, 1986, and 1990, respectively. From 1990 to 1994, he was with the Electronics Department, Zhongshan University, Guangzhou, China, initially as a Post-Doctoral Fellow, an Associate Professor in 1992, and a Full Professor in From 1994 to 1995, he was a Research Fellow with the Department of Electronic Engineering, City University of Hong Kong. From 1996 to 1998, he was with the Siemens Corporate Technology, Munich, Germany, as a Guest Scientist (Humboldt Research Fellow), where he was involved with the microwave design and testing of external waveguide modulators and laser modules. He is currently a Professor with the Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing, China. In 1998, he founded a Joint Photonics Research Laboratory between the Institute of Semiconductors, CAS, and the City University of Hong Kong, where he served as Deputy Director. He has authored over 100 journal papers and one book. His research interests are in modeling and characterization of integrated optical waveguides and coplanar transmission lines, and optimal design and testing of opto-electronics devices. Dr. Zhu was involved in the Hundred-Talent Program, CAS, and selected by the National Natural Science Foundation as a Distinguished Young Scientist in 1998.

7 2356 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 9, SEPTEMBER 2011 coherent optics. Li Xian Wang was born in Jiangsu, China, on February 21, He received the B.S. degree in microelectronics from Jilin University, Changchun, China, in 2006, and is currently working toward the Ph.D. degree in physical electronics at the Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing, China. In 2006, he joined the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, CAS. His current research interests are high-frequency measurement of opto-electronic devices and Liang Xie was born in Lanzhou, China, in He received the B.Sc. degree in condensed matter physics and M.S. and Ph.D. degrees in physics from Lanzhou University, Lanzhou, China, in 1992, 1995, and 1998, respectively. Since 1999, he has been with the Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing, China, initially as a Post-Doctoral Fellow, then an Associate Professor in 2002, and since 2007, as a Full Professor. His research interests include high-frequency characteristics of microwave opto-electronic devices and design of high-speed optical transceiver modules. Xiao Qiong Qi was born in Gansu, China, in She received the B.S. degree from Lanzhou University, Lanzhou, Gansu, China, in She is currently a Post-Doctoral Fellow with the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing, China. From 2007 to 2009, she conducted research with the University of California, at Los Angeles (UCLA) under a joint-training Ph.D. Program. Her research interests include ultra-high-speed fiber-optic transmission, fiber nonlinearity compensation, and RoF systems.