289 To investigate effects of extinction ratio on SOA based wavelength Converters for all Optical Networks Areet Aulakh 1, Kulwinder Singh Malhi 2 1 Student, M.Tech, ECE department, Punjabi University, Patiala, India 2 Associate Professor, ECE department, Punjabi University, Patiala, India ABSTRACT The advancements in the field of optical communication have revolutionized the flow of information. conversion employing semiconductor optical amplifiers (SOAs) has become an important component in WDM networks. This paper describes the effect of extinction ratio of probe signal is analyzed in terms of wavelength shifted for SOA based FWM wavelength converter. For this we used optimized SOA parameters to achieve adequate converted signal power and enhancement in four wave mixing effect. Simulations include converted signal power and OSNR measurements versus wavelength shifted that analysis with variation of extinction ratio of probe signal. Keywords: converter, extinction ratio, SOA, FWM. 1. INTRODUCTION Optical networks are high capacity telecommunication networks based on optical technologies and components that provide routing and wavelength based services. Optical wavelength converters have become the key components of the future broadcast optical networks. The use of wavelength division multiplexing in optical networks can increase system capacity, enhance network flexibility and provide for better management of current time division multiplexed systems. converter is final element in optical networks. The most important use of wavelength converters will be for avoidance of wavelength blocking in optical cross connects in wavelength division multiplexed networks. conversion is a technique to convert data signal from an original wavelength λ 1 to a different required wavelength λ 2 without changing the information of data [4].. In WDM systems the number of available wavelength is limited and probability that two data channels try to access the same routing path at same time with the same wavelength increases with the traffic. So in order to avoid data re transmission due to routing conflicts wavelength converters are used. conversion can be used in WDM networks to improve efficiency. The all-optical wavelength converters proposed would allow the network to fulfill the requirement of flexibility, maintaining the transparency of the optical communication. Many technologies exist for the implementation of wavelength conversion Optoelectronic, cross-gain modulation, and cross-phase modulation [3]. converters are candidate technologies that offer excellent performance in WDM systems [1]. In case of WDM systems, nonlinear effects become important even at moderate powers and bit rates. Four-wave mixing is a nonlinear effect arising from third-order optical nonlinearity. Among these different methods of wavelength conversion, FWM provides an interesting approach due to its simplicity and unique characteristics of providing the preservation of amplitude and phase, data format and bit rate transparencies[3]. Studies on FWM-based wavelength converters utilizing SOAs have been proposed in much previous works, due to the many advantages provided by the nonlinearities of SOA. Zhang Xinliang et al. [7] studied Extinction Ratio Degradation in XGM Converter. M. A. Summerfield [6] reported bit-error-rate measurements which show that there is a trade-off between maximizing the output SNR and minimizing intersymbol interference in the SOA. Consequently, the power penalty incurred in the frequency conversion can be minimized by careful selection of the input signal power. N.A. Awang et al. resulted the conversion efficiency 43 db with a novel configuration of a wavelength converter by using a dual wavelength bi-erbium-doped fiber laser that uses an Arrayed Waveguide Grating (AWG) [11]. Farah Diana Mahad et al. [10] investigates the performance of a 2.5 Gb/s SOA-based FWM wavelength converter system, in terms of its shifted wavelength conversion efficiency -11dB and optical signal-to-noise ratio (OSNR) 22dB, for both up and down conversions at 2nm. Earlier work on wavelength converters using FWM in SOA was limited to effects of input power, pump power and wavelength shifted [8, 10, 12]. In this paper effects of extinction ratio on wavelength converter is analyzed. In Section 2, the Theoretical analysis has been reported. In Section 3, the simulation setup for wavelength converter has been discussed. Section 4 represents the result and discussions. In section 5 conclusion is reported.
290 2. THEORETICAL ANALYSIS The FWM scheme is inherently fast for both fiber and semiconductor nonlinear elements. The four-wave mixing phenomenon that occurs because of nonlinearities in the transmission medium can also be utilized to realize wavelength conversion. The four wave mixing process results from the nonlinear properties of the medium in which, more than one waves are injected. Generally, the new waves generated by the mixing process have intensities proportional to the interacting wave intensities and frequencies and phases which are linear combinations of those of the interacting waves. Figure 1: Four Wave Mixing (FWM) SOA Converter Conversion efficiency and optical signal-to-noise ratio are usually the two most important figures of merit for the converted wave, and both need to be highly considered in the design of any wavelength converter. FWM wavelength converter has less conversion efficiency. Typical value of conversion efficiency is -20 db, so optical power levels of ~10dBm have to be used for the pump of SOA converters while 10-20dBm is needed for fiber based converters. Because of the low conversion efficiencies the signal-to noise ratio for the converted signals needs attention, especially if converters have to be cascaded [5]. Both the converted signal power and converted OSNR decreased at large detuning wavelengths, due to the frequency response of the nonlinear process. The efficiency in the case of up wavelength conversion is smaller than the efficiency in the case of down wavelength conversion due to destructive interference among the several nonlinearities of the gain medium, which is responsible for the generation of the two sidebands. FWM process is polarization sensitive. This feature can cause significant degradation to the performance of the optical nodes in which the FWM based converter is going to be used. The dual pump scheme is used to overcome these drawbacks to some extent [2]. In telecommunications, extinction ratio (r e ) is the ratio of two optical power levels of a data signal generated by a laser source. P 1 is the logic 1 level and P 2 is logic 0 level of signal as shown in equation 1. Small changes in extinction ratio can make a relatively large difference in the power required to maintain a constant bit error rate (BER). The purpose of this paper is to demonstrate how variations in extinction ratio affect the performance of optical wavelength converters [7, 9] 3. SIMULATION SET UP This wavelength converter is based on four-wave mixing in an SOA. We have done a series of simulations by varying the extinction ratio of probe signal. A simplified schematic diagram of the simulation setup with different blocks is shown in Fig. 2. The pump and probe laser sources are used with input powers of 5dBm and 3 dbm resp. The wavelength of pump laser source is fixed at 1550 nm. For probe signal wavelength is varied according to up and down conversion. Starting a probe signal with a wavelength from 1549 to 1545, the converter is characterized as up converter describes wavelength shifted that is 2nm to 10nm. Beginning a probe signal with a wavelength from 1551 to 1555, the converter is characterized as down converter describes wavelength shifted that is 2nm to 10nm. The probe signal is modulated by a raised cosine NRZ electrical driver, at a bit rate of 20 Gb/s. The pump and probe waves each pass through individual polarization controllers (PC s) before being combined by 3 db coupler in order to match the state of polarization between them. After that erbium doped fiber amplifier (EDFA) is used to amplify the combined signal in order to saturate the SOA, having a fixed gain of 25 db and 4.2 db. Following amplification, the signals pass through a band pass filter (BPF) in order that
291 this filter increases the OSNR of the converted signal by subdue the amplified spontaneous emission (ASE) contributed by the high-power EDFA in the spectral region of the converted signal as described in [10] Figure 2: Schematic diagram of single stage up and down wavelength converter based on FWM TABLE 1: Optimized SOA Parameters Parameter Bias current Unit 500mA Cavity Length 300 µm Cavity Width 1.5 µm Thickness.15 µm Confinement factor.35 Linewidth enhancement factor 6 Later this, these signals are coupled into the SOA where FWM generates the converted signal. SOA parameters are optimized for the improvement of FWM signal. Following optimization salient parameters are shown in table 1. Just after SOA, BPF is used with center wavelength of converted signal wavelength, so that converted signal is captured by dismissing the probe and pump signal. 4. RESULTS AND DISCUSSIONS The simulation has been carried out by varying extinction ratio for the probe signal with the set-up shown in Figure 2. In this optimized SOA parameters are used to improve the converted signal power of wavelength converter. Table 2: Up conversion analysis Up conversion (converted signal power dbm) 2 6.227 5.177 4.368 3.590 3.383 4 1.29 -.001 -.98-1.64-2.1604 6-2.37-3.183-4.13-4.78-5.006 8-5.46-6.24-7.29-7.89-8.23 10-9.70-10.80-11.28-12.09-12.52 Down conversion (converted signal power dbm) 2 4.28 3.54 2.67 2.11 1.663 4 -.14 -.94-1.71-2.33-2.68 6-2.48-3.48-4.45-4.93-5.30 8-5.13-5.74-6.88-7.85-7.91 10-8.26-9.32-10.46-11.12-11.33
292 Table 3: Down conversion analysis Up conversion (converted OSNR db) 2 46.149 46.13 45.74 45.38 45.14 4 44.64 44.43 44.27 43.96 43.76 6 44.54 44.41 44.25 43.90 43.7 8 43.20 43.10 42.70 42.43 42.09 10 38.30 38.17 37.88 37.63 37.47 Down conversion(converted OSNR db) 2 46 45.86 45.53 45.24 45.04 4 45.72 44.59 44.22 43.87 43.63 6 45.15 44.93 44.87 44.07 43.81 8 44.48 44 43.98 43.54 43.26 10 40.31 40 39.89 39.51 39.28 The details of the converted signal power and converted signal OSNR versus wavelength shifted at different extinction ratio are shown in table 2 and table 3 respectively. The extinction ratio is varied from 5dB to 25dB, to analyze its effect on the performance of wavelength converter. The results are observed for both up and down conversion. Figure 3: Converted signal power for up conversion Figure 4: Converted signal power for down conversion
293 Figure 3 & 4 shows the variation of up and down converted signal power respectively by varying the extinction ratio of probe signal. From figure 3& 4 it has been that that converted signal power decreases with increase in the wavelength shifted due to the due to the frequency response of the non-linear process. It has observed that at low value (5dBm) of extinction ratio maximum converted signal power is 6.227 and 4.28 dbm for up and down conversion respectively. As the extinction ratio (25dBm) increases the maximum converted signal power is 3.383 and 1.663 dbm for up and down conversion respectively. From these values it has noticed that with increase in extinction ratio of probe signal the converted signal power decreases. The reason behind this is that with increase in extinction ratio of probe signal, variation in carrier density leads to increases in gain fluctuations. Sufficient generation of maximum four-wave mixing signals occurs with minimum gain fluctuations. Figure 5&6 depict that converted OSNR decreases at large detuning wavelengths, due to the frequency response of the non-linear process. Figure 5: Converted signal OSNR for up conversion Figure 6: Converted signal OSNR for down conversion It can also be seen, for up conversion converted OSNR decrease from about 46.149 db to 45.14dB and for down conversion it decreases of about 46 db to 145.04dB, both for a 2nm wavelength shift at 5dB extinction ratio of probe signal. 5. CONCLUSION
294 In this paper we examined the effects of extinction ratio on SOA based FWM wavelength converter for data rate of 20 Gb/s. We decreased the gain unsteadiness by selecting suitable parameters of SOA to achieve sufficient enhancement in four-wave mixing signals. It has been demonstrated that the increase in extinction ratio leads to decrease in converted signal power and converted signal OSNR. It has evaluated that at 5dB extinction ratio converted signal power is 6.227dB and when extinction ratio is 25 db then power is 3.383 db, due to large gain variations at high extinction ratio. Furthermore down converter have better results as compared to up conversion at large wavelength shifted due to destructive interference among the several nonlinearities REFERENCES [1] G.P.Agrawal, Fiber Optic Communication Systems, John Wiley and Sons, New York [2] Rajiv Ramaswami Kumar, N. Sivarajan Galen H. Sasaki, Optical Networks, Morgan Kaufmann Publishers,2010. [3] T. Durhuus, B. Mikkelsen, C.G. Joergensen, and K.E.Stubkjaer, All optical wavelength conversion by semiconductor optical amplifiers," IEEE.J.Lightwave Technology., vol.14,pp.942-954, June 1996 [4] Biswanath Mukherjee, Optical WDM Networks, Springer, New York, 2006. [5] David F.Geraghty, Conversion for WDM Communication Systems Using Four-Wave Mixing in Semiconductor Optical Amplifiers, IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, no. 5, October 1997. [6] M. A. Summerfield, Optimization of Pump and Signal Powers for Converters Based on FWM in Semiconductor Optical Amplifiers, IEEE Photonics Technology Letters, vol. 8, no. 10, October 1996 [7] Zhang Xinliang, Sun Junqiang, Huang Dexiu, et al. Theoretical Study on Extinction Ratio Degradation in XGM Converter Journal of Huazhong University of Science and Technology (Natural Edition), 1999, 27(10) [8] Surinder Singh, 20-Gb/s and Higher Bit Rate Optical Conversion for RZDPSK Signal Based on Four- Wave Mixing in Semiconductor Optical Amplifier, Fiber and Integrated Optics, 26:5, 295-308. [9] DONG Jianji, ZHANG Xinliang, HUANG Dexiu, Theoretic analysis of all-optical wavelength converter based on single-port-coupled SOA Department of Optoelectronics Engineering, Huazhong University of Science and Technology [10] Farah Diana Mahad,Abu Sahmah M. Supa at, Analyses of semiconductor optical amplifier (SOA) four-wave mixing (FWM) for future all-optical wavelength conversion, International Journal for Light and Electron Optics (Optik), vol. 124,no. 1, February 2013. [11] N.A. Awang, H. Ahmad and A.A. Latif, Four-wave mixing in dual wavelength fiber laser utilizing SOA for wavelength conversion, International Journal for Light and Electron Optics (Optik), vol.122, no. 22, November 2011,pp. 754 757. [12] Surinder Singh, Wide-Band Optical Converter Based on Four-Wave Mixing Using Optimized Semiconductor Optical Amplifier, Fiber and Integrated Optics, 25:3, 213-230