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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 22, NOVEMBER 15, 2007
Experimental Demonstration of 1.25-Gb/s Radio-Over-Fiber Downlink Using SBS-Based Photonic Upconversion Chul Soo Park, Member, IEEE, Chung Ghiu Lee, Member, IEEE, and Chang-Soo Park, Member, IEEE
Abstract—We experimentally demonstrate a radio-over-fiber downlink system using a stimulated Brillouin scattering (SBS)based photonic upconversion technique. The Brillouin selective amplification characteristic of SBS is incorporated to generate the 11-GHz band radio-frequency (RF) carrier. The dual-electrode Mach–Zehnder optical modulator, which is used to carry the broadband data in the optical carrier instead of the optical sideband, is adopted along with the SBS-based carrier generation setup. To vindicate the broadband capabilities of the proposed scheme, 1.25-Gb/s pseudorandom bit sequence data is carried in the optical carrier. Error-free operation of the 1.25-Gb/s downlink is achieved without critical power penalties after the 13-km fiber transmission. Index Terms—Brillouin selective amplification, photonic upconversion, radio-frequency (RF) carrier generation, radio-over-fiber (RoF) downlink system, stimulated Brillouin scattering (SBS).
I. INTRODUCTION
T
HE broadband data delivery in the wireless communication link requires microwave/millimeter-waves as a carrier wave. The higher the carrier frequency for the broadband data delivery, the smaller the cell size that eventually makes the system more expensive to cover the required area. The radio-over-fiber (RoF) technology is one of the methods that can carry the broadband wireless data while providing the cost-effectiveness in the system implementation by employing several merits such as the simplified base station structure, etc. [1]. Several researches have been reported that have adopted stimulated Brillouin scattering (SBS) as a technique of generating microwave signals [2]–[6]. The 5.5-GHz RoF system using the Brillouin selective amplification was first reported by Yao using 1320-nm pump source in a single-mode fiber (SMF) [2]. Harmonic frequency generation and frequency upconversion using the phase-to-amplitude modulation conversion incorporating phase modulators is reported by the same author [3]. Schneider et al. reported the method of the harmonic frequency generation using two pump sources to enhance the limited carrier frequency in the SBS-based carrier generation Manuscript received June 18, 2007; revised August 17, 2007. C. S. Park is with the RF and Optical Department, Institute for Infocomm Research (I2R), Singapore 119613, Singapore (e-mail:
[email protected]). C. G. Lee is with the Department of Electronic Engineering, Chosun University, Gwangju 501-759, Korea (e-mail:
[email protected]). C.-S. Park is with the Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2007.907595
method and analyzed its characteristics [4], [5]. Shen et al. demonstrated the 11-GHz RoF system carrying the 10-Mb/s data in the optical sideband, which is selectively amplified by SBS [6]. Recently, we have demonstrated the photonic upconversion method based on SBS that relieve the bandwidth limitations imposed in the previous SBS-based methods [7], [8]. In that report, the 1.0-GHz intermediate frequency (IF) signal was carried in the optical carrier instead of the optical sideband to alleviate the bandwidth limitation imposed by the narrow bandwidth of the Brillouin gain spectrum, by adopting the modulation technique using a dual-electrode Mach–Zehnder optical modulator (MZM) [9]. By combining the carrier generation using SBS with the different modulation technique, we have demonstrated the feasibility of increasing data capacity and characterized its analog characteristics as an upconverter. In this letter, we report the data carrying characteristics of the SBS-based photonic upconversion method. The 1.25-Gb/s downlink of the SBS-based RoF system is experimentally demonstrated to vindicate its broadband capability. Error-free operation is achieved from the proposed method without critical power penalties after the 13-km fiber transmission. II. EXPERIMENTAL SETUP Fig. 1 shows the experimental setup used for the 1.25-Gb/s RoF downlink, which adopts SBS for carrier generation and photonic upconversion. The commercial dual-electrode MZM is used. One of the electrodes is connected to the synthesized radio-frequency (RF) signal generator for the generation of the optical sidebands; one of the optical sidebands is to be selectively amplified by SBS. The other electrode is driven by the pulse pattern generator of the 1.25-Gb/s pseudorandom bit sequence (PRBS) data that is to be carried in the optical carrier as depicted in Fig 1. As explained in [7], a single optical source is used both for the pump and the signal wave that eliminates the relative drift between the pump and the selected sideband. A 20-km SMF is used as a nonlinear medium, which has the critical power of 6.5 dBm for the SBS generation. By using the optical circulator and the optical isolator, optical spectrum composed of the gigabit data and the 11-GHz band RF-carrier is devised to direct toward the photodiode. Two polarization controllers were used to optimize the polarization state for the MZM and the pump wave inducing SBS, respectively. A microwave amplifier with an operating bandwidth of 2–20 GHz is used to boost the detected signal and to remove the undesired baseband signal after the upconversion. The double-balanced mixer (DBM) that has a broadband IF port
1041-1135/$25.00 © 2007 IEEE
PARK et al.: DEMONSTRATION OF 1.25-Gb/s RoF DOWNLINK USING SBS-BASED PHOTONIC UPCONVERSION
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Fig. 3. Electrical spectrum after the photonic upconversion of the 1.25-Gb/s PRBS data. (a) Full RF spectrum measured after the RF amplifier, and (b) its zoom-in view.
Fig. 1. Experimental setup (DC-block: DC-block capacitor; DBM: Doublebalanced mixer; ED: Error detector; EDFA: Erbium-doped fiber amplifier; Iso: Optical Isolator; LPF: Low-pass filter; OC: Optical circulator; PC: Polarization controller; PD: Photodiode; RF-SA: RF-spectrum analyzer; PPG: Pulse pattern generator; PRBS: Pseudorandom bit sequence, SMF: Single-mode optical fiber; T: Bias tee).
Fig. 2. Optical spectrum after the selective amplification by SBS (a) without, and (b) with carrying 1.25-Gb/s PRBS data in the optical carrier.
operating from DC to 10 GHz is used to bring the optically upconverted 1.25-Gb/s data down into the baseband. The electrical phase shifter is used to adjust the phase of the local oscillation (LO) signal for the DBM. Two low-pass filters (LPFs), which have a cutoff frequency of 4.5 and 3.0 GHz, are used for the transmitting and receiving sides to limit the high-frequency noise of the PRBS data, respectively. Finally, the downconverted baseband signal is detected by both the RF-spectrum analyzer and the error detector. III. RESULTS AND DISCUSSION The characteristics of the carrier generation and broadband upconversion of the proposed scheme were already reported [7]. The frequency of the RF signal generator is tuned to match with that of the Stokes frequency shift for the given optical conditions. Thereby, the lower-frequency optical sideband generated by the RF signal generator is selectively amplified by the Brillouin gain. Eventually, two optical signals composed of the optical carrier and the selectively amplified sideband are contributed to the carrier generation after photodetection. This method dispenses with the requirement of the chromatic dispersion management for the optically carrying microwave signals
since it removes the possibilities of the destructive interference by selectively amplifying only one of the sidebands [2]. For the 1553.82-nm pump source, the Stokes frequency shift is measured to be 10.831 GHz for the SMF used in the setup. The optical pump power of 8.0 dBm is injected into the SMF to generate the Brillouin gain spectrum. The optical power of the signal wave after the modulation is set to 5.4 dBm to compensate the power losses of the data signal after the nonlinear medium. As shown in Fig. 2(a), measured at the port 3 of the optical circulator, the intensity of the lower-frequency sideband is magnified after the selective amplification by the Brillouin gain spectrum. By separately driving each arm of the dual-electrode MZM with the RF signal of 10.831 GHz and the 1.25-Gb/s PRBS data, respectively, the optical spectrum composed of the optical sideband selected by the Brillouin gain spectrum and the optical carrier that contains the 1.25-Gb/s data is generated as shown in Fig. 2(b). As can be seen in Fig. 2(b), the linewidth of the optical carrier becomes broadened after the modulation of the 1.25-Gb/s PRBS data (pattern length of 2 1) while having no effect on the selectively amplified optical sideband. Once the optical spectrum of Fig. 2(b) is detected by the photodiode, it generates the upconverted RF spectrum of the amplitude modulated signal as shown in Fig. 3. There clearly show the strong peaks separated about 1.25 GHz from the 10.831-GHz carrier. To downconvert the optically upconverted 1.25-Gb/s data into the baseband, the broadband-capable DBM is used with the phase shifter controlling the phase of the LO signal. Electrical power injected into the LO port of the DBM is set to 10 dBm to have a normal mixing operation. The LPF of the 3.0-GHz cutoff frequency is attached to the IF port of the DBM to limit the high-frequency noise after the downconversion. Fig. 4 shows the electrical spectrum measured at the IF port of the DBM under the optical power of 6.0 dBm, downconverted 1.25-Gb/s data. After the downconversion, there clearly shows the baseband signal of the 1.25-Gb/s PRBS data that has the peaks of the 1.25-GHz separation. The sidelobes of the 1.25-Gb/s data were remained since the LPFs used for the transmitter and the receiver have higher cutoff frequency than 1.25 GHz and those sidebands do not hamper the system operations. Since the low-frequency edge of the RF amplifier used after the photodiode is 2.0 GHz, which amplifies the upconverted 1.25-Gb/s data component and attenuates other lower frequency components, the data is easily recovered after the amplification. There are possibilities of the signal leakage
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 22, NOVEMBER 15, 2007
Fig. 4. Electrical spectrum after the down-mixing of the optically upconverted = 6:0 dBm. 1.25-Gb/s PRBS data. P
0
The -factors for additional cases of 622.08 and 155.52 Mb/s data rate are measured to verify the system performance for different data rate. It is measured to be about 7 at the optical power of 4.0 dBm, which means BER values 10 . From the measurement, it is shown that the system performance is more critically dependent on the optical conditions rather than data rate. Since the polarization state of the pump wave affects the system performance in a way to change SBS gain and noise, it eventually affects related -factors of the system. By locating the modulated data, the data rate is limited by the spectral distance between the optical carrier and the selectively amplified sideband. Also, the flexibility of the carrier frequency is lost when compared to the one in the previous method [3]. However, there are possibilities of overcoming this limitation through the frequency tripling method that was reported recently in [8]. Additionally, the frequency tuning within a few hundreds of megahertz ranges can be obtained by adopting different fiber medium and pump wavelength since the Stokes frequency shift is dependent on them. IV. CONCLUSION We experimentally demonstrated the 1.25-Gb/s RoF downlink using the SBS-based photonic upconversion technique. By integrating the carrier generation using SBS and the dual-electrode MZM modulation technique, the upconversion with broadband data is achieved. Error-free operation of the 1.25-Gb/s RoF downlink is demonstrated. Also, no severe power penalties are measured after the 13-km additional fiber transmission.
Fig. 5. Bit-error rate of the system. Insets are the eye diagram of (a) the back to back, (b) 13-km transmission case with the optimized polarization, and (c) the = 4:0 dBm. back to back with the pump polarization mismatch. P
0
that affect the system performance due to the low-frequency edge of RF amplifier as shown in Fig. 3(a). However, it does not deteriorate the system performance since the main lobe of the directly detected baseband signal has been removed. Furthermore, even if the remaining sidelobe around 2.0 GHz could leak out to the RF mixer, the signal might be additionally attenuated due to the high isolation of 25 dB among the ports of the RF mixer. Fig. 5 shows the measured bit-error rate (BER) for the back-to-back and the 13-km additional fiber transmission case. As shown in Fig. 5, error-free operation of the system is achieved. A small amount of noise is added after the 13-km fiber transmission [Fig. 5(b)] compared to the back-to-back case [Fig. 5(a)], though, no critical power penalties were observed. Fig. 5(c) shows the measured eye-diagram when the polarization of the pump is not optimized for the back-to-back case. The polarization of the pump wave has the effect on the Brillouin gain [10]. Also, as can be compared in Fig. 5(a) and (c), the polarization state of the pump wave should be optimized to reduce the intensity noise involved in the recovered signal since the degree of the signal deterioration by the intensity noise and the overall signal intensity are changed depending on the polarization state of the pump wave during the experiment [10], [11].
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