363 Chirped Bragg Grating Dispersion Compensation in Dense Wavelength Division Multiplexing Optical Long-Haul Networks CHAOUI Fahd 3, HAJAJI Anas 1, AGHZOUT Otman 2,4, CHAKKOUR Mounia 3, EL YAKHLOUFI Mounir 3 1 Advanced Sciences and Technology Group, National School of Applied Sciences, UAE, Tetouan, Morocco ²Electronics & Microwaves Group, Faculty of Sciences, UAE, Tetouan, Morocco 3 Condensed Matter Physics Group, Faculty of Sciences, UAE,Tetouan, Morocco 4 National School of Applied Sciences, UAE, Tetouan, Morocco E-mail: fahdchaoui@outlook.com, hajaji.anas@gmail.com, otman.aghzout@gmail.com Abstract- In this paper, Chirped Fiber Bragg Grating (FBG) has been introduced as a dispersion compensator in dense Wavelength Division Multiplexing (WDM) for optical long-haul network. The new model has been compared with a previous work proposed by other authors having DCF as dispersion compensator. Both configurations have been modeled and simulated using the same initial setting so as to prove the efficiency of our model. For this purpose, different parameters of the Chirped FBG which include chirp parameter, apodization function and grating length, have been investigated in order to get the most suitable settings of the proposed model. The WDM transmission system is simulated using the advanced tools of Optisystem 7.0. The simulation results have been examined and validated by analyzing the eye diagram for short and long channel lengths, the Q-factor, the BER analysis and nonlinear effects at WDM network. Index Terms- Fiber Bragg Grating (FBG), Wavelength Division Multiplexing (WDM) I. INTRODUCTION In recent years, due to huge capacity demand for data transmission, the optical fiber communication technology has undergone an enormous growth, having big impact on the telecommunication industry by providing higher performance and more reliable telecommunication links with decreasing bandwidth cost. However, two main limitations confine the optical fiber transmission from utilizing the bandwidth efficiency, which are chromatic dispersion and signal loss. In order to face signal loss, the Erbium Doped Fiber Amplifier (EDFA) has been proposed by many authors [1, 2, 3]. Nevertheless, EDFA works correctly only in 1550 nm wave band, in which the average dispersion value of single mode fiber is very big (about 15-20ps/nm/km) [4]. Thus, EDFA has to be combined with a dispersion compensator to overcome chromatic dispersion effects. Chromatic dispersion occurs when different wavelengths of light pulses are launched into the optical fiber. These pulses travel at different speeds due to the variation of refractive index with the wavelength. So they tend to get spread out in time after traveling for some distance in the optical fiber. This phenomenon of broadening the pulse width causes distortion of the transmitted signals and leads to errors in data receivers [5]. To face this problem, Dispersion Compensation Fibers (DCF) has been widely used, with negative dispersion coefficient in order to disable the effect of positive dispersion in optical fibers [6]. Nonetheless, the DCF implementation increases the total losses, the nonlinear effects and the cost of the optical transmission systems [7]. Nowadays, Fiber Bragg Grating (FBG) is considered as one of the most advanced technologies suggested to compensate chromatic dispersion in optical fibers overcoming the challenges of the DCF technique. FBG is characterized by low internal loses, negligible nonlinear effect, cost efficiency and the high
364 capacity for working in WDM optical transmission systems [8]. In this paper, Chirped FBG has been studied as a dispersion compensator in 8-channels WDM long-haul optical network. The proposed model has been compared with a previous work using DCF as dispersion compensator [9]. The chirped FBG parameter values with chirp parameter (μm), apodization function and grating length (mm) have been examined and analyzed graphically in terms of the Q-factor, nonlinear effects at WDM network, the BER and the eye diagram for short and longer channel length. The main objective of this procedure is to find out the optimized values to be used in the proposed model. Both DCF and chirped FBG configurations have been modeled and simulated using the advanced tools of Optisystem 7.0 simulator. All results demonstrate high efficiency of the new model developed in this paper. II. DCF COMPENSATION MODEL In order to develop the new model based on the chirped FBG technology, firstly we present a detailed analysis of the DCF compensation technique studied and simulated in [9, 10]. Then, we highlight the main advantages and disadvantages of this technique to demonstrate how the proposed compensation approach with chirped FBG helps to get better results. Both configurations have been modeled and simulated using the same initial setting: Bit rate 2.5GBits/s, dispersion 16.75 ps/nm/km, dispersion slope 0.075 ps/nm 2 /km and attenuation coefficient at cable section 0.2dB/km. The model presented by [9] simulate WDM of 8-channels having channel spacing 0.5nm with DCF as dispersion compensator, see Fig. 1. This work has been based on one channel transmission system design developed in [10]. The idea behind this compensation technique is to make Single Mode Fibers (SMF) followed or preceded by DCFs with negative dispersion coefficient, in order to disable the effect of positive dispersion of SMFs. Therefore, the value of DCF dispersion coefficient β 2 must be logically calculated based on SMF dispersion coefficient β 1 to make null the total chromatic dispersion after a certain distance. In this configuration, the optical amplifier used is considered having gain and noise of 20 db and 4 db respectively. The SMF have the reference wavelength of 1550 nm located after the DCF. Fig. 1. Simulation model of DCF compensation with 8-optical channels
365 The loop control system has 2 loops with 100km each one. The total length of fiber channel in one loop is segmented in the ration of 1:5, i.e. 17 km DCF and 83 km SMF. The parameters for DCF are reference wavelength of 1550 nm, attenuation 0.6 db/km, dispersion value -80 ps/nm/km, dispersion slope 0.21 ps/nm 2 /km, β2= -20 ps 2 /km, differential group delay 3 ps/km and PMD coefficient 0.5 ps/km. Table 1 and Table 2 present the simulation results of Max Q- factor and Min BER at data receivers respectively. Table 1: Max Q-factor of DCF technique Max Q-factor Parameter Value Channel 1 6.46 Channel 2 28.90 Channel 3 35.37 Channel 4 61.15 Channel 5 75.99 Channel 6 41.41 Channel 7 69.54 Channel 8 46.60 As can be shown in Table 1, using DCF compensation technique, the quality factor values are not quite uniform. Good values are obtained in some channels, whereas bad values are obtained in others. For instance, the first channel had very low Max Q-factor (6.46), which means logically high signal distortion. Table 2: Min BER of DCF technique It can also be observed from Table 2, that the results in terms of Min BER are not uniform either. Thus, DCF can be considered as a good solution to compensate chromatic dispersion for reliable signal transmission, but only up to a certain level. III. CHIRPED FBG COMPENSATION MODEL In order to prove the technical feasibility of the new technique implemented in this paper, firstly we present the FBG technology to outline the working principle of the chirped FBG proposed for the WDM model. Secondly, a detailed study will be devoted to the design and the simulation of the proposed model with chirped FBG as dispersion compensator. A. FBG Working Principle FBG is a single mode fiber that exposes the core to the periodic pattern of intense ultraviolet light. The exposure increase the refractive index and the pattern will create a fixed index modulation that is called Grating [5] with wavelength selective mirror as can be shown in Fig. 2. Various methods were employed in order to map grating in optical fiber with extensive types of pulsed and continuous lasers to be used in visible and ultraviolet region. Resulted gratings reflect the propagated light in fiber according to Bragg wavelength which is given as follow: λ B = 2nΛ (1) Min BER Parameter Value Channel 1 4.9e -11 Channel 2 3.92e -184 Channel 3 1.86e -274 Channel 4 0 Channel 5 0 Channel 6 0 Channel 7 0 Channel 8 0 Where n and Λ are the core refractive index and the grating period in fiber respectively [8]. Broadband light Bragg wavelength Optical fiber Chirped FBG Fig. 2. Fiber Bragg Grating Transmitted light
366 Chirped FBG is one of the many types of FBG with some created changes in period of grating. This principle is presented in Fig. 3. Fig. 3. Chirped Fiber Bragg Grating As the period of grating changes along the axis, different wavelengths are reflected by different parts of the grating, and therefore are delayed with different time intervals. Thus, the final effect is compression in incident pulse and can be appropriate to compensate chromatic dispersion in communication links. B. Chirped FBG Design and Simulation The proposed model using chirped FBG for 8-channels WDM optical transmission system is operated with basic optical communication system which consists of a transmitter, transmission link and a receiver. The input signal contains electrical data represented by zeros and ones that have been generated by Pseudo-Random Binary Sequence (PRBS) through a non-return to zero (NRZ). Then the input signal is modulated through Mach-Zender Modulator with semiconductor laser that is represented by Continuous Wave (CW) laser. In dense WDM system, the number of channels can be increased up to any extent by using minimum channel spacing between 100GHZ and 12.5 GHZ (0.1nm to 0.8nm at 1550nm). So WDM Mux 8x1 is used as a multiplexer for 8 channels having channel spacing 100GHz from 193.1 to 193.8 THz frequencies. The system transmits information using optical carrier wave from transmitter to receiver via optical fiber. At the receiver part, WDM Demux 1x8 is required to demultiplex the signal. Afterwards, the Chirped FBG will be used as dispersion compensator. The use of Chirped FBG requires optical amplification to overcome the fiber loss and also to amplify the signal, so the signal will pass through EDFA before being received by Photo detector PIN. Fig. 4 represents the simulation setup for the proposed WDM transmission system and the Chirped FBGs using the advanced simulation tools of Optisystem 7.0. We have not changed the parameters of any other block discussed earlier in DCF compensation technique. However only the communication distance is increased approximately three times as compared to the previous model and tested that whether the proposed compensation technique holds the performance characteristics better to DCF compensation or not. Fig.4. Chirped FBG compensation simulation model with 8 optical channels
367 The initial setting of the chirped FBGs is shown in Table 3. Apodization function, chirp parameter (μm) and grating length (mm) have been investigated in order to get the most suitable settings of the chirped FBG to be used in the WDM transmission system. Grating period of each chirped FBG is calculated automatically based on the effective index n = 1.45 and Bragg wavelength of each channel. the case of WDM systems in which the wavelength channel spacing are very close to each other. The magnitude of this nonlinear effects depends on channel power, channel spacing and fiber dispersion. Table 3: Chirped FBG parameters CFBG parameters Chirp function Value Linear Chirp parameter 0.0001 Grating Length (mm) 15 Apodization function Uniform C. Results and Discussions Fig. 5 shows the effect of chromatic dispersion compensation on the eye diagrams of the received signals of both techniques at 300km WDM transmission system. By comparing the 2 eye diagrams, it can be obviously observed that the eye opening is higher and the signal quality is better when the Chirped FBG compensation is implemented in comparison with the DCF compensation. Fig. 6. Nonlinear effects at WDM network, (a): Transmitted spectrum, (b): Received spectrum Fig. 5. Eye diagram at the receiver of both techniques for WDM transmission system over 300km Fig. 6 shows the nonlinear interactions between the information signals and the fiber medium which get generated when light of different wavelengths are launched into a fiber in WDM transmission system. This type of nonlinear effects is called Four Wave Mixing and occurs in Fig. 7. Nonlinear effects at the received spectrum of the 1 st channel, (a): With DCF, (b): With Chirped FBG
368 By comparing transmitted spectrum plotted in Fig. 6 (a) with the received spectrum related to fiber output in Fig. 6 (b) before demultiplexing, it can be seen that additional pulses have been generated due to the nonlinear effects, which prevents correct reception of the transmitted signals. These additional pulses persist next to the main transmitted pulse with the use of DCF compensation as it can be seen in Fig. 7 (a). However, with the use of Chirped FBG, the additional pulses have been mostly eliminated as shown in Fig. 7 (b), Further, the results are compared in terms of Max Q-factor in order to optimize the parameter values of the Chirped FBG used in the proposed WDM transmission system. The Q-factor is an electrical domain measure of the ratio of separation between digital states to the noise associated with the state. Higher Q indicates a lower rate of energy loss. Obtained results of different profiles of apodization and different grating length have been compared at different chirp parameters and different transmission distances based on the obtained Q-factor as shown in Fig. 8 and Fig. 9 respectively. The 3 apodization functions are given as follow: Uniform: f(z) = 1 (2) Gaussian: f(z) = e ln 2.( z L 2 s. L ) 2 2 (3) distances higher than 200 km, and 15mm for transmission distances between 100km and 200km. Max Q-factor 60 55 50 45 40 35 30 25 20 0,0001 0,0002 0,0003 0,0004 0,0005 chirp parameter (um) Uniform Tanh Gaussian Fig. 8. Max Q-factor of different apodization functions for different chirp parameter Max Q-factor 100 90 80 70 60 50 40 30 10mm 15mm 20mm 25mm Tanh: f(z) = tanh 2(s 2 ) + tanh (s. z ). tanh (s. (1 z )) + 1 (4) L L 20 10 100 200 300 400 500 Distance (km) Where z is the coordinate of light propagation along the length of FBG, L is the grating length, and s is taper parameter used for fine tuning of reflection spectrum. Fig. 8 demonstrates that uniform function and Tanh are the most appropriate apodization function as compared to Gaussian apodization at the best value for chirp parameter 0.0001 μm. It is also shown in Fig. 9 that 20 mm is the most proper grating length for all transmission Fig. 9. Max Q-factor of different grating lengths for different transmission distances The optimized values of the chirped FBG are used in the WDM model to be compared with the state-of-art alternative. Table 4 shows the comparison results, and hence it demonstrates that the proposed method with Chirped FBG offers improved values of performance parameters such as Max Q-FACTOR and Min BER compared to DCF compensation technique.
369 Table 4: Comparison parameters of DCF and Chirped FBG models Parameters DCF model Proposed model N of channels 8 8 Distance (km) 200 200 Channel 1 6.46 66.17 Channel 2 28.90 61.79 Channel 3 35.37 59.63 Max Q- factor Channel 4 61.15 51.14 Channel 5 75.99 60.97 Channel 6 41.41 50.29 Channel 7 69.54 62.22 Channel 8 46.60 66.04 Channel 1 4.9e -11 0 Channel 2 3.92e -184 0 Channel 3 1.86e -274 0 Min BER Channel 4 0 0 Channel 5 0 0 Channel 6 0 0 Channel 7 0 0 Channel 8 0 0 Power (dbm) 13 10 It can be seen that in the proposed model we obtained Max Q-factor higher than 50 in the 8 channels, however fin the DCF model only 3 channels out of 8 were above 50. Moreover, we obtained more uniform Max Q-factor values than the previous model where the first channel for instance had very low Max Q-factor (6.46) which means high signal distortion. It is also observed that Min BER values are all null, which means that the probability that any received bit is in error equal to zero in all channels, unlike DCF dispersion compensation technique. To demonstrate the validity of the proposed method in this paper, the eye diagram simulation results at the received signal of both techniques Fig. 10. Eye diagrams for different transmission distances for CFBG and DCF WDM systems
370 are also compared for three times longer channel length of 600km as shown in Fig. 10. It can be obviously observed that for all channel lengths, the eye opening is higher and the signal quality is better when the Chirped FBG compensation is implemented, especially for transmission distances longer than 200km. Note that, the system developed in this paper offers reduced signal distortion and improved eye opening compared to DCF compensation, which proves that Chirped FBG offers better dispersion compensation and improved synchronization in WDM long haul networks. IV- CONCLUSION In this paper, Dense WDM transmission system of eight channels is simulated and analyzed for chromatic dispersion compensation. For this purpose, Chirped FBG compensation has been compared with a previous model having DCF as dispersion compensator based on Eye diagrams and nonlinear effects at WDM network. The proposed optical transmission system has been modeled by using OptiSystem simulator in order to investigate different parameters of the system. During the analysis of simulation results, we concluded that the most proper grating length for the WDM long haul optical network equal to 20 mm for all transmission distances higher than 200 km and 15mm for transmission distances between 100km and 200km. It is also observed that Uniform function and Tanh are the most appropriate apodization function as compared to Gaussian apodization at the best value for chirp parameter 0.0001 μm. Finally, the results have been compared with the previous model in terms of Max Q-factor and Min BER. The simulation results prove that the Chirped FBG method offers improved values of performance parameters compared to DCF compensation technique. The efficiency of the proposed method is also verified based on Eye diagram patterns for three times longer transmission distance (600km). It is also observed that even for such long distance, Chirped FBG compensation offers negligible dispersion and improved synchronization. REFERENCES [1] M. Ismail and M. Othman, «EDFA-WDM Optical Network Design System», ELSEVIER, Procedia Engineering 53, pp. 294-302, 2013. [2] J. Gujral and M. Singh, «Performance Analysis of 4- Channel WDM System with and without EDFA», IJECT, vol. 4, Issue 3, April-June 2013. [3] B.H. Choi, H.H. Park and M.J. Chu, «New Pump Wavelength of 1540-nm Band for Long- Wavelength-Band Erbium-Doped Fiber Amplifier (L-Band EDFA)», IEEEJournal of Quantum Electronics, vol. 39, No 10, October 2003. [4] P. Singh and R. Chahar, «Performance Analysis of Dispersion Compensation in Long Haul Optical Fiber using DCF», The International Journal Of Engineering And Science, vol. 3, Issue 8, pp. 18-22, 2014. [5] M. Chakkour, A. Hajaji and O. Aghzout «Design and Study of EDFA-WDM Optical Transmission System using FBG at 10 Gbits/s Chromatic Despersion Compensation Effects», Mediterranean Conference on Information & Communication technologies, May 2015. [6] R. Shukla a. M. Kumar, «Performance Analysis of Dispersion in Optical Communication link using Different Dispersion Compensation Fiber (DCF) models», IJRTE, vol. 1, Issue 2, June 2012. [7] G. Agrawal, «Fiber-Optic Communication Systems», Third edition Wiley-India edition, 2002. [8] S. O. Mohammadi, S. Mozzaffari and M. Shahidi., «Simulation of a transmission system to compensate dispersion in an optical fiber by chirp gratings», International Journal of the Physical Sciences, vol. 6(32), pp. 7354-7360,, December 2011. [9] C. Shekhar and P. Vind «An Improved Methodology for Dispersion compensation and Design of Dense WDM System in Optical Fiber Communication Networks», International Journal of Emerging Technology and Advanced Engineering, vol. 4, Issue 1, January 2014. [10] A. S. Verma and A. K. Jaiswal, «An Improved Methodology for Dispersion Compensation and Synchronization in Optical Fiber Communication Networks,» IJETAE, vol. 3, Issue 5, May 2013.