Optik 121 (2010) 1280 1284 Optik Optics www.elsevier.de/ijleo Simulative investigation of the impact of EDFA and SOA over BER of a single-tone RoF system Vishal Sharma a,, Amarpal Singh b, Ajay K. Sharma c a College of Engineering & Management, Kapurthala, Punjab, India b Beant College of Engineering and Technology, Gurdaspur, Punjab, India c NIT, Jallandhar, Punjab, India Received 5 September 2008; accepted 10 January 2009 Abstract In this paper, we investigate the impact of EDFA and SOA amplifiers over BER and Q-parameter of a Radio-over- Fiber (Rof) system consisting of two different system set-ups using direct- and external-laser modulation techniques. In this work, we also measured and compared the electric Rf power at receiver at different modulating Rf frequencies up to 20 GHz using two different optical amplifiers, i.e. EDFA and SOA. Further, we also compared the received electric Rf power at different optical powers without and with different optical amplifiers. An improvement of 11 db (approx.) of received Rf power was observed using EDFA with the external modulation technique, on comparing with direct modulation. r 2009 Elsevier GmbH. All rights reserved. Keywords: Radio over frequency (RoF); EDFA; SOA and BER 1. Introduction Radio-over-Fiber (RoF) techniques are attractive for realizing high-performance integrated networks. The growth of mobile and wireless communications fuels increasing demand for multimedia services with a guaranteed quality of service. This requires realization of broadband distribution and access networks. Within this framework, RoF schemes can be applied for realizing seamless wireless networks since they allow for the easy distribution of microwaves and millimeter waves over long distances along optical fibers [1,2]. Several techniques for distributing and generating microwave signals via optical fiber exist. The techniques may be classified into two main categories, namely Corresponding author. E-mail address: s_amarpal@yahoo.com (V. Sharma). Intensity Modulation Direct Detection (IM-DD) and Remote Heterodyne Detection (RHD) techniques [3]. In such systems, it is desirable to achieve better receiver sensitivities, higher dynamic ranges, and lower nonlinear distortions. Techniques to reduce nonlinear distortions have been investigated extensively. A method for reducing nonlinear HDs and IMDs is to use the predistortion method [4], counteracting the nonlinear effects of the optical modulation characteristics. However, if EAMs are used, it was found that EAM modulation characteristics are dependent not only on wavelength but also on input optical power [5]. Thus, different pre-distortions for different wavelengths and/ or different power levels have to be used, making the radio-over-fiber system design very complicated. Another technique is to use two wavelengths for each RoF system, in which one wavelength is tunable so that EAM transfer function nonlinearities at the two 0030-4026/$ - see front matter r 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2009.01.031
V. Sharma et al. / Optik 121 (2010) 1280 1284 1281 wavelengths will be matched. Recently, two balanced systems with one wavelength and two fibers and two wavelengths and one fiber were demonstrated to suppress second-order HD (HD2) and second-order inter-modulation distortion (IMD2) and improve dynamic range in an RoF system [6,7]. The balanced system that utilizes one wavelength and two fibers, in which two EAMs have mirrored transfer functions, is very difficult to maintain balance between two fiber transmissions, resulting in less suppression of HD2 and IMD2. Also, a feedback control system is required to maintain balance [6]. Later, this technique was improved by using two wavelengths and one fiber for each RoF system [7], which is referred to as the conventional balanced system in this paper. However, spectral efficiency is considerably reduced because two wavelengths for each RoF system are utilized. Also, the two EAMs must have very similar modulation characteristics at the two wavelengths, which may not be easy to obtain because EAM modulation characteristics depend on wavelength [4]. Alternatively, in order to reduce nonlinear distortions, low optical modulation indexes or depths (the ratio of optical signal subcarrier to optical carrier) were usually preferred. Unfortunately, in this case the optical carrier is dominant compared to the optical signal subcarrier, which leads to reduced receiver sensitivity. So, higher modulation indexes are preferred, which leads to significant increases of nonlinear distortion. In this paper, we investigate the impact of EDFA and SOA amplifiers over BER and Q-parameter of a RoF system consisting of two different system set-ups using direct- and external-laser modulation techniques. We will show by simulation that the EDFA with external modulation technique performs better than other techniques discussed in this paper in improving the BER and the Q-parameter of a single-tone RoF system. 2. Simulation set-up The simulation set-up, schematically shown in Fig. 1 containing a single-tone Rf signal of varying frequency from 1 to 20 GHz, is modulated either by using external modulation technique over a continuous wave (CW) laser at 1550.5 nm biased at 4 a.u of laser line width 10 MHz with a CW power of 10 mw or by using direct modulation technique over an LD at 1549.5 nm of a modulation index of 0.04 biased at 0.07 a.u. The propagation is modeled with an attenuator since RoF systems are usually employed over short distances. At the receiver section, the two channels are splitted, amplified with either an EDFA or an SOA and detected by connecting Electric Spectrum Analyzer (ESA) and two narrow bandwidth electric power meters. 3. Result and discussion We first consider our simulation set-up to investigate the impact of EDFA and SOA over BER at the received power of a single-tone RoF system as shown graphically in Figs. 2 and 3. It is observed by comparing Figs. 2(a, Fig. 1. Simulation set-up to calculate BER of a single-tone RoF system.
1282 ARTICLE IN PRESS V. Sharma et al. / Optik 121 (2010) 1280 1284 Fig. 2. Simulated BER at received power versus modulating Rf frequency with sin 2 modulator using (a) external modulation and (b) direct modulation. b) and 3(a d) that the BER is improved from the singletone RoF system by using EDFA with external modulation with chirp ¼ 0 at an Rf modulating frequency of 20 GHz. Fig. 4 calculates the Q-parameter at various Rf frequencies with direct- and external modulation techniques with either SOA or EDFA for a single-tone RoF system. The Q-parameter with chirp ¼ 0 at an Rf modulating frequency of 20 GHz using EDFA with the external modulation technique is calculated as 14.1 and reduced to 10.2 using SOA, which is further reduced to 9.8 using the external modulation technique without using any optical amplifier. An improvement in Q- parameter with the external modulation technique on Fig. 3. Simulated BER at received power versus Rf modulating frequency with (a) EDFA with external modulation, (b) EDFA with direct modulation, (c) SOA with external modulation and (d) SOA with direct modulation.
V. Sharma et al. / Optik 121 (2010) 1280 1284 1283 Fig. 5. Received Rf power versus Rf modulating frequency with (a) external- and direct-modulation without optical amplifier (b) EDFA with external- and direct-modulation. Fig. 4. Simulated Q-parameter at received power versus Rf modulating frequency with (a) external- and direct-modulation without optical amplifier, (b) EDFA with external- and directmodulation and (c) SOA with external- and direct-modulation. comparing with direct modulation technique is also observed as shown in Fig. 4(a) (c), which improves BER of the single-tone RoF system. Fig. 5 determines the received Rf power with sin 2 modulator at various Rf frequencies using the external modulation technique with and without EDFA for a single-tone RoF system. By comparing Fig. 5(a) and (b), the received Rf power with chirp ¼ 0 at an Rf modulating frequency of 20 GHz is calculated as 27 db using EDFA with external modulation and reduced to 51 db using external modulation without using any optical amplifier. An improvement of 11 db (approx.) of received Rf power using EDFA with the external modulation technique, on comparing with direct modulation, is also achieved as shown in Fig. 5(a) and (b). Fig. 6 investigates the impact of EDFA over the frequency response and second-order harmonic distortion (HD2) of a single-tone RoF system. It is observed that the second-order harmonic distortion is almost suppressed by using EDFA with external modulation in a single-tone Rof system with chirp ¼ 0. By comparing
1284 ARTICLE IN PRESS V. Sharma et al. / Optik 121 (2010) 1280 1284 Fig. 6(a) and (b), an improvement of 25 db of electrical Rf power is achieved. It is also observed that the Rf power increases at the receiver as we increase the optical power at the transmitter and is improved by 25 db with EDFA using external modulation on comparing with SOA and without using any optical amplifier of a single-tone Rof system as shown graphically in Fig. 7. 4. Conclusion In this paper, we have investigated the impact of EDFA and SOA amplifiers over BER and Q-parameter of a Rof system using direct- and external-laser modulation techniques. Using our simulations, we have shown that the BER is improved from a single-tone Rof system using EDFA with external modulation with chirp ¼ 0 at an Rf modulating frequency of 20 GHz. An improvement of 3.9 in Q-parameter using EDFA on comparing with SOA and of 4.3 on comparing with no optical amplifier using external modulation is also observed. We have also achieved an improvement of 11 db (approx.) of the received Rf power using EDFA with the external modulation technique, on comparing with direct modulation. References Fig. 6. Electrical power measurement of single-tone Rof system at chirp ¼ 0 with (a) external- and direct-modulation without optical amplifier (b) EDFA with external- and directmodulation. Fig. 7. Received Rf power versus optical power at Rf modulating frequency of 20 GHz with chirp ¼ 0. [1] A. Vilcot, B. Cabon, J. Chazelas (Eds.), Microwave Photonics, Kluwer Academic Publications, Dordrecht, 2003. [2] Chi H. Lee (Ed.), Microwave Photonics, CRC Press, Boca Raton, FL, 2007. [3] U. Gliese, T.N. Nielsen, S. Norskov, K.E. Stubkjaer, Multifunction fibre optic microwave links based on remote heterodyne detection, IEEE Trans. Microw. Theory Tech. 46 (5) (1998) 458 468. [4] L. Roselli, V. Borgioni, F. Zepparelli, F. Ambrosi, M. Comez, P. Faccin, A. Casini, Analog laser pre-distortion for multiservice radio over fiber system, J. Lightwave Technol. 21 (5) (2003) 1211 1223. [5] B. Liu, J. Shim, Y. Chiu, A. Keating, J. Piprek, J.E. Bowers, Analog characterization of low-voltage MQW traveling-wave electro-absorption modulators, J. Lightwave Technol. 21 (12) (2003) 3011 3019. [6] S. Mathai, F. Cappelluti, T. Jung, D. Novak, R. Waterhouse, D. Sivco, A. Cho, G. Ghione, M. Wu, Experimental demonstration of a balanced electro-absorption modulated microwave photonic link, IEEE Trans. Microw. Theory Tech. 49 (10) (2001) 1956 1961. [7] Y. Wu, Optical heterodyned radio over fiber link design using electroabsorption and electro-optic modulators, Ph.D. Dissertation, University of California, San Diego, 2004.