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Optical Single Sideband Modulation of 9-GHz

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    918 PIERS Proceedings, Suzhou, China, September 12–16, 2011 Optical Single Sideband Modulation of 9-GHz RoF System Based on FWM Effects of SOA P. H. Hsie1, W. S. Tsai1, C. C Weng1, and H. H. Lu2 1Department of Electrical Engineering, Ming Chi University of Technology 84 Gungjuan Rd., Taishan, Taipei 24301, Taiwan 2Department of Electro-Optical Engineering, National Taipei University of Technology 1, Sec.3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, R.O.C. Abstract— We propose an optical single sideband (OSSB) modulation scheme using self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM) effects of SOA to achieve wavelength conversion. Drive with the 9 GHz RF signal into electro-absorption modulator laser (EML) and IM modulator. By properly adjust the phase shifter for phase difference between the two paths of electrical signal. FWM signal is OSSB format for a 25 km SMF transmission. Finally, we use an optical spectrum analyzer (OSA) to observe optical spectrum. 1. INTRODUCTION In traditional intensity modulation, the optical carrier is modulated to generate an optical signal with double sideband (DSB) format. DSB modulation produces two sidebands on both sides of the carrier, and the beating components between the carrier and two sidebands induce interference at the receiver. Over a long haul fiber transmission, RF signal will cause severe power degradation due to chromatic dispersion [1]. This phenomenon will degrade systems’ performance. In order to overcome RF power degradation due to fiber dispersion, optical single sideband (OSSB) modulation technique must be implemented [2]. The fact that OSSB modulation can remove a half of the optical spectrum is expected to attain a dispersion benefit because the optical spectrum has been reduced by a factor of two. Several OSSB generation methods [3–6] have been proposed. By eliminating one of the sidebands, OSSB modulation not only immunizes to fiber dispersion, but it also increases the spectral efficiency twice. One way for generating OSSB signals is to exploit the Hilbert transform [7]. Using a dual electrode Mach-Zehnder modulator can be reality to generate OSSB signal [8]. Another way to generate OSSB signals is to utilize narrow optical filter. The narrow optical filter at the end of the ODSB transmitter output can eliminate one of the sidebands to achieve OSSB modulation [9]. Optical amplifier plays an important role in a long haul fiber transmission system, the use of semiconductor optical amplifier (SOA) as an optical amplifier is very attractive since it can potentially be used to help upgrade fiber penetration, easily integrated and small compact. However, SOA also has several nonlinearity effects such as self-phase modulation (SPM), cross-phase modulation (XPM), self-gain modulation (SGM), cross-gain modulation (XGM) and four-wave mixing (FWM) that will degrade systems’ performances. In this letter, we propose an OSSB modulation scheme using SPM, XPM and FWM effects of SOA to achieve wavelength conversion. The translation wavelength possessing OSSB modulation format can prevent fiber dispersion induced RF signal fading. Using the phase modulation effects in SOA to generate OSSB format is relatively simple to implement. It only needs SOA and electrical phase shifter to construct the OSSB system instead of a complex circuit or device to exploit the Hilbert transform of the conventional OSSB systems. 2. EXPERIMENTAL SETUP The experimental system configuration of our proposed OSSB system to operate wavelength conversion based on SPM, XPM and FWM effects of SOA associated with electrical phase shifter is shown in Fig. 1. The main parts of transmitter consists of a tunable laser source as local oscillation light, a polarization controller (PC), a Mach-Zehnder modulator (MZM), a microwave signal generator with 9 GHz, a RF power splitter, a phase shifter, a RF power attenuator and an electro-absorption modulator laser (EML). The MZM input light source is 1549.6 nm provided by a tunable laser source, because the optical modulator has the sensitive characteristic regarding the input photo source’s polarization state. A PC needs to be added on in front of the MZM input section. This function is to maintain the input light at the specific polarization state when the light launches into the optical modulator. Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept. 12–16, 2011 919 9 GHz RF signal is generated by a microwave signal generator and fed into a RF power splitter to split two copies. One copy of RF signal passes through a phase shifter and adjusts optimal power with tunable RF power attenuator, then feeds into optical modulator. Another one copy of the RF source supplies to EML as modulating signal. Two paths of optical light are combined by an optical coupler. Passing through an optical power attenuator to adjust the suitable input power avoids the SOA output power saturation. Output signal launches into the band-pass filter to filter out the noise which provided by SOA. For a 25 km SMF transmission, we use an optical spectrum analyzer (OSA) to measure optical spectrum. On the other hand, PD transforms optical signal to electrical one, and measures the output frequency spectrum by using an electrical spectrum analyzer (ESA). Figure 1: Experimental setup of OSSB modulation based on SPM, XPM and FWM effects of SOA. (a) (b) (c) (d) Figure 2: Conceptual diagram of (a) OSSB modulation system for wavelength conversion technique, (b) optical spectrum before passing through SOA, (c) optical spectrum after passing through SOA, (d) filtering out λFWM and adjusting parameters of system achieved OSSB signal. 920 PIERS Proceedings, Suzhou, China, September 12–16, 2011 3. RESULTS AND DISCUSSION Figure 2(a) shows a conceptual diagram of OSSB modulation system for wavelength conversion technique using SOA and electrical phase shifter. Two optical signals, λEML and λLO are launched into SOA as shown in Fig. 2(b). After passing through SOA, the FWM effect causes new optical signal (λFWM) to appear shown in Fig. 2(c). In general situation, FWM is a bad noise disturbance item. However, this kind of non-linear effect can easily perform wavelength conversion. Filter out FWM and adjust parameters of system can realize OSSB system shown in Fig. 2(d). In the case of a single RF tone modulation, the OSSB optical field can be written as [10] E(t) = E0 + K m1 2 sin(ωmt) × exp i ω0t + m1α1 2 sin(ωmt + β1) + K m1α2 2 sin(ωmt + β2) + m2α3 2 sin(ωmt + γ) (1) where ω0 denotes the optical carrier frequency, ωm is the subcarrier frequency, and m is modulation index. k, α1, α2, β1, and β2 denote modulation decrease factor, prechirp parameter, SPM chirp parameter, phase difference of prechirp from AM modulating term and phase difference of SPM of SOA from AM modulating term, respectively. α3 is a chirp parameter which is relevant to XPM, γ can be any value between 0 and 2π. In our experiment, we launched electrical source at 9 GHz to EML and IM modulator. The wavelength of optical wave from EML was 1550.8 nm. EML was biased to −0.1 V, and the operation current was 75 mA. SOA operation current was 320 mA. Fig. 3 shows the optical spectrum in back of the optical coupler. Because 9 GHz RF signal is fed into EML and IM modulator, we can observe two optical carriers with DSB modulation format. By the nonlinear effects of SOA, the optical spectrum can be observed in Fig. 4. In the Fig. 4, λFWM appears in the right side and it also has DSB modulation format. The FWM efficiency is defined as the FWM signal power at the SOA output divided by signal power at EML. When the input signal power is too much, the converted signal power decreased, which is due to the gain saturation of SOA. The conversion efficiency in our experiment is about −20 dB. Figures 5 and 6 show the λFWM from DSB format transformed to SSB format, when we tune the phase shifter and the operation current of SOA. We tune the phase shifter to adjust the phase difference between the two ways of electrical signal. When two orthogonal waves are modulated at the same single tone RF carrier, propagate in SOA. Due to SPM and XPM effects of SOA, the λFWM can reveal SSB modulation. In this way, we can observe the transformation of modulation format by tuning phase shifter. Figure 3: Measured optical spectrum in back of the optical coupler. Figure 4: Measured optical spectra based on nonlinear effects of SOA. Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept. 12–16, 2011 921 Figure 5: λFWM from DSB format transformed to Figure 6: λFWM from DSB format transformed to SSB one at the upper sideband. SSB one at the lower sideband. 4. CONCLUSION We propose an OSSB modulation scheme using SPM, XPM and FWM effects of SOA to achieve wavelength conversion. Using this method to construct the OSSB system can substitute a complex circuit or device to exploit the Hilbert transform of the conventional OSSB system. In the future work, we can enhance the wavelength conversion efficiency and transmit digital signal in our proposed OSSB system. REFERENCES 1. Chi, H. and J. P. Yao, “Frequency quadrupling and upconversion in a radio over fiber link,” J. Lightwave Technol., Vol. 26, 2706–2711, 2008. 2. Hong, C., C. Zhang, M. J. Li, L. X. Zhu, L. Li, W. W. Hu, A. S. Xu, and Z. Y. Chen, “Singlesideband modulation based on an injection-locked dfb laser in radio-over-fiber systems,” IEEE Photon. Technol. Lett., Vol. 22, 462–464, 2010. 3. Blais, S. R. and J. P. Yao, “Optical single sideband modulation using an ultranarrow dualtransmission-band fiber bragg grating,” IEEE Photon. Technol. Lett., Vol. 18, 2230–2232, 2006. 4. Fonseca, D., A. V. T. Cartaxo, and P. Monteiro, “Adaptive optoelectronic filter for improved optical single sideband generation,” IEEE Photon. Technol. Lett., Vol. 18, 415–417, 2006. 5. Sung, H. K., E. K. Lau, and M. C. Wu, “Optical single sideband modulation using strong optical injection-locked semiconductor lasers,” IEEE Photon. Technol. Lett., Vol. 19, 1005– 1007, 2007. 6. Sun, X. Q., K. Xu, S. N. Fu, J. Q. Li, X. B. Hong, J. Wu, J. T. Lin, and P. Shum, “All-optical WDM subcarrier modulator for binary phaseshift keying (BPSK) with optical ssb format using aphase modulator loop mirror filter,” Conference on Opt. Fiber Communication, 2009. 7. Takano, K., N. Hanzawa, S. Tanji, and K. Nakagawa, “Experimental demonstration of optically phase-shifted SSB modulation with fiber-based optical hilbert transformers,” Conference on Opt. Fiber Communication, 1–3, 2007. 8. Hou, C., Y. Shao, X. Liu, X. Zheng, X. Li, S. Zou, and N. Chi, “Dual-level optical single side band modulation scheme for 0.1 tera Hz radio-over-fiber systems,” Communications and Photonics Conference, 1–6, 2009. 9. Fonseca, D. D., A. V. T., Cartaxo, and P. P. Monteiro, “Opto-electrical filter for 40 Gb/s optical single sideband signal generation,” Conference on Opt. Fiber Communication, 2006. 10. Lee, U.-S., H.-D. Jung, and S.-Kook, “Optical single sideband signal generation using phase modulation of semiconductor optical amplifier,” IEEE Photon. Technol. Lett., Vol. 16, 1373– 1375, 2004.

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