Testing a Real Time Monitoring System for Passive Optical Networks using an Array of Fiber Bragg Gratings

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1 Testing a Real Time Monitoring System for Passive Optical Networks using an Array of Fiber Bragg Gratings a Abdosllam M. Abobaker, a Issa Eldbib, a Abdulaziz Hasen Daw, and b P. Ramesh Babu a Department of Communications Engineering, College of Electronic Technology, BaniWalid, Libya. b Photonics, Nuclear and Medical Physics Division, School of Advanced Sciences, VIT University, Vellore , India. ABSTRACT In this paper, we propose and design a passive optical network using an array of fiber Bragg gratings (FBGs) for centralized monitoring of the branched optical networks. Further, we exploit the linear properties, namely, reflection and transmission spectra of FBGs for identifying the fiber fault. Besides, we also verify the quality of the downstream signals (1490 nm and 1550 nm) through the well-known eye-diagrams through the spectrum analyzer. We find that the quality of downstream signals remains same even in the presence of monitoring signal and the passive element FBG. Thus, these numerical results corroborate that the monitoring signal and passive element (FBG) do not affect the quality of the downstream signals. Thus, we envisage that the results presented in this paper would turn to be an asset in the area of passive optical network (PON). KEYWORDS Passive optical network, fiber fault monitoring, optical network unit, optical spectrum analyzer, eye-diagram, bit-error rate, fiber Bragg grating. 1 INTRODUCTION The search for a new communication technology has always been the quest of human society for the betterment of our living conditions. The communication technology is always at the forefront of all other technologies. Ever since the invention of telephone in the end of 19 th century, the dissemination of information has become faster and the volume of information being transferred has grown exponentially [1]. Nowadays, in addition to the telephone, people exclusively rely mainly on internet for their everyday activities such as e- shopping, e-business transaction, playing games, downloading (music and scientific articles) etc. Besides, they also communicate with other people anywhere on the globe by means of various social networks such as , voice chatting, facebook, twitter, research gate, etc. These services ultimately demand high bandwidth information transmission networks. Undoubtedly, optical fiber communication (OFC) system is the only answer to cope with such a phenomenal growth in the bandwidth requirement [1]. At this juncture, it is to be emphasized that the quality of the signals must be monitored at different locations in the optical network for ensuring the delivery of good quality signals. Thus, we need to monitor the system to efficiently administer, maintain and provide the network to assure that each customer receives a reasonable predetermined quality of service [2, 3]. Having realized the importance of monitoring the signals for providing the good quality services to the users, this paper is devoted to investigate the fiber fault monitoring scheme in the optical network unit (ONT). In general, optical time-domain reflectometer (OTDR) and zigbee wireless sensor (ZWS) are used for fiber fault monitoring. It is known that the OTDR is ineffective for a point-to-midpoint network and similarly the implementation of ZWS is highly challenging due to the presence of active components [4-8]. Recently, a real-time, centralized and cost efficient monitoring and fault localization system has been reported using FBGs by Naim et al [9]. Using correlation technique, Zhao et al has experimentally demonstrated a method for fault location in optical communication networks [10]. Very recently, Li et al has proposed a method based on optical code division multiple access (OCDMA) for monitoring PON systems [11]. As the existing methods do not provide the complete solution, in this paper, we propose a new 146

2 scheme by exploiting the linear properties, namely, reflection and transmission spectra of fiber Bragg gratings (FBGs). The unique reflection spectrum of FBG which is located in each ONT has been manipulated for detecting the fault in the network. Further, we also verify the quality of the downstream signals (1490 nm and 1550 nm) in terms of the well-known eye-diagram. The paper is arranged as follows. In section II, we discuss the important design parameters and scenarios of simulation in detail. We examine the behavior of the links of optical fiber when the signal goes through all the elements such as optical fiber, splitters, multiplexers, etc. Further, in section III, we propose a new PON design for identifying the fault in the optical networks using FBGs. Then, we also investigate the quality of downstream signals in terms of eye-diagram to ensure that the users do receive good quality signals. We present the summary of the research findings in section IV. samples to make sure if the network of this design works. 2 PASSIVE OPTICAL NETWORK: DESIGN In this section, we describe the design, display of various network scenarios and analysis of the most important parameters. Hence, we begin with description of the network. Finally, we will demonstrate the results of each ONT which would prove that the network is viable and therefore its practical implementation would work. Here, the design starts by designing the fiber to the home (FTTH) network with a bit rate of 10 Gb/sec for fifteen users (ONT) with different distances away from the splitter. It is to be noted that the maximum distance is 4 km and the shortest distance is 0.6 km. The two transmitters located at the central office (CO) transmit data with a default bit rate. The wavelengths 1490 nm and 1550 nm are assigned for voice data and downstream video, respectively. In addition, we use 1:16 power splitter, WDM and optical fiber cables as shown in Fig.1. There is an optical spectrum analyzer (OSA1) connected at the output of WDM for calculating and displaying optical signals. The next step is to assign an FBG for each user (ONT) as in Fig.1. As discussed earlier, there are fifteen users being designated as ONT1 ONT15. For brevity, we consider odd numbered users, namely, ONT1 (first user), ONT3, ONT5, ONT7, ONT9 and ONT13 as Figure 1: The design of proposed passive optical network. Now, the next usual analysis is to check the quality of the downstream signals (1490 nm &1550 nm) at ONTs by means of eye-diagram analyzer as shown in Fig.2. Now, we present the quality of downstream signal. Here, the quality of these signals is characterized by the wellestablished eye diagram, the quality factor and the probability of error (BER). At ONT, the signal will pass through the PIN photodetector where it will be converted into electrical signal. This signal will be regenerated again for the display purpose on the eye diagram analyzer. The Fig.2 represents the above mentioned signals at the output of 147

3 WDM. Here, the signals are observed by using OSA 1. present design without FBGs operates in an acceptable state. Figure 2: Downstream spectrum after WDM. Having explained the design aspects, it is essential to investigate if the proposed design works or not. Based on the results, we will be able to judge whether it is feasible to implement the proposed PON design practically. As has been discussed in the previous section, there are fifteen users being designated as ONT1 ONT15. For illustration purpose, we consider few odd numbered users, namely, ONT1 (first user), ONT7 and ONT13 as samples to make sure if the network of this design works. Now, the next usual analysis is to check the quality of the downstream signals (1490 nm & 1550 nm) at ONTs by means of eye-diagram analyzer as shown in Fig.3. Figure 3: Checking the quality of the signal at the user end. ONT 1 fiber length = 1.5 km: Now, we present the quality of downstream signal. Here, the quality of these signals is characterized by the well-established eye diagram, the quality factor and the probability of error (BER). From the Fig.4, it is obvious that the results are in the accepted level as all of them have good eye diagrams, high quality factor and a less probability of error of the order of For the illustration purpose, we measure the quality of downstream signals at various locations, i.e., at 0.6 km, 1 km, 1.5 km and 4 km. As expected, the results of ONTs placed at shortest distance are better than the results of the ONTs which are placed at longest distance. It should be noted that the network designed in Fig.3 does not have fiber Bragg gratings. However, it is interesting to note that the 148

4 ONT 3 fiber length = 0.6 km: ONT 7 fiber length = 1 km: Figure 4: Quality analysis of downstream signals through eye-diagrams at various locations. 3 FIBER FAULT REAL TIME MONITOR- ING USING FBGs in PON ONT 9 fiber length = 4 km: ONT13 fiber length = 1.3 km: In the previous section, we have discussed the quality of the downstream signals in the PON without FBGs and we have found that the results are in the acceptable level. Now, in this subsection, we would like to monitor the quality of the downstream signals under the influence of FBGs. Therefore, the next step is to design FBG for each user in order to design a monitoring system for this PON network. Before designing the new PON using FBGs, first we design the required FBGs with different periods using OptiGrating 4.4 simulation software. For each network at each user (ONT), a unique FBG is designed. The reflection and transmission spectra of FBGs are presented in Fig.5. For further insight into the reflection and transmission spectra of FBGs, a separate graph is presented for a particular wavelength of B = nm. It signifies the pass spectral characteristics and a unique reflective spectrum (but the other spectrum is passed normally). The same explanation holds good for all FBGs designed using Optigrating 4.4. The optical coding unit is actually the unique FBG that has been designed by OptiGrating 4.4 software. This FBG will be exported to Optisystem 8.0 simulation software in order to implement the overall system to learn the influence of FBGs over PON. The FBG will be imported in the form of files from the OptiGrating 149

5 software package to the optisystem software package. technique known as wavelength division multiplexing (WDM). The Fig.6 clearly illustrates that the monitoring signal at 1625nm is connected to PON using WDM. Figure 5: Reflected spectrum when B = and Transmitted spectrum when B = We choose the monitoring signal as 1625nm wavelength from U-band in order to avoid interference with traffic at 1310 and around 1550 nm. Further, the chosen wavelength is commonly referred to as in-service testing and usually used for remote monitoring system or in fiber to the home and passive optical network applications and troubleshooting networks with live traffic. As we know before, the simultaneous transmission of separate service types on the same fiber in the optical distribution network (ODN) is enabled by using different wavelengths for each direction. For downstream transmissions, a PON uses a 1490 nm wavelength for combined voice and data traffic and a 1550 nm wavelength for video distribution. Upstream voice and data traffic use 1310 nm wavelength. Further, we note that each optical line termination (OLT) is used to avoid interference between the contents of downlink and uplink channels using two different wavelengths superimposed. Thus, we can add another wavelength (1625 nm) from U-band as we proposed before by using a Figure 6: Monitoring signal is connected to PON using WDM. The typical output of WDM is displayed as a power spectrum by OSA1 as shown in Fig.7. Besides, this figure clearly indicates the presence of downstream of signals in addition to the monitoring signal. By using a power splitter, the signals are separated to each optical network terminal (ONT). Each ONT possesses a unique FBG reflection signal to differentiate the each user. All the signals reflected from all FBGs will be combined together by a device called power combiner. As a result, the power combiner combines all optical input signals as shown in Fig

6 each ONT will distinguish it. In real application, the optical spectrum analyzer (OSA) is required to analyze the reflected signal from the FBG as shown in Fig.8. Thus, all the reflected signals from each network will be analyzed in optical spectrum analyzer 2 (OSA2). Fig.9 shows the optical power spectrum from OSA4 when there is no cut in the optical network. This is the accumulated reflected spectrum from all the FBGs i.e., from each network. Figure 7: Optical power spectrum after adding the monitoring signal. Figure 9: Reflected spectrum of all FBGs shown by OSA4 In order to detect the cut, we compare the result presented in Fig.9 with that of Fig.10. It can be seen that there is no signal against the wavelength nm and hence the optical spectrum suffers a marked drop, which shows that the source of this spectrum is not connected with the combiner and this ultimately indicates that there is a cut in ONU 1. Figure 8: Combining all reflected signals from FBGs by a power combiner. The above discussed argument holds good for all ONTs. Thus, if there is any cut in any of the ONTs, it will be identified with the corresponding display of OSAs by comparing it with the combined reflected spectrum presented in Fig.9. Here, we have obtained the reflected spectra of all the users starting from ONU1 to ONT13 and they are presented in Fig.10 when there is a cut. In reality, the FBG consists of one input and one output. The reflected signal from the FBG will pass through the fiber optic line which carries the input signal. The unique reflected signal from 151

7 When there is a cut at ONU 8 When there is a cut at ONU 9 When there is a cut at ONU1 When there is a cut at ONU 2 When there is a cut at ONU 3 When there is a cut at ONU10 When there is a cut at ONU1 When there is a cut at ONU 4 When there is a cut at ONU 5 When there is a cut at ONU12 When there is a cut at ONU13 Figure 10: Reflected spectrum of FBGs when the is a cut When there is a cut at ONU 6 When there is a cut at ONU 7 As we have previously seen, there are 15 FBGs corresponding to the 15 homes or users (ONT) that will receive data with a default bit rate with the wavelengths of 1490 nm and 1550 nm. It should be noted that the sample ONTs (ONT1, ONT3, ONT5, ONT7, ONT9 and ONT13) will be tested as discussed above. We emphasize that the presence of FBGs in all ONTs in the second PON distinguishes it from the first design of PON. Therefore, it is mandatory to 152

8 analyze the influence of the passive components on the signal by using the eye diagram, the quality factor and the probability of error (BER). signal (1625 nm to 1675 nm) and passive element FBG. Thus, it is a good indication that the chosen monitoring signal and passive element (FBG) do not affect the quality of the downstream signals. We do envisage that the results presented in this work would turn to be an asset in the area of PON. ONT 1 fiber length = 1.5 km Figure 11: Checking the quality of the signal in presence of FBGs at the user end. The next step is to check the quality of the downstream signals in the presence of FBGs. As usual, the quality is ensured by monitoring the eye diagram, the quality factors and the probability of error (BER) by connecting the analyzer of eye diagram after FBG as shown in Fig.11. The eyediagrams and the corresponding quality factors for the downstream signals with different distances are presented in Fig.12. As has been discussed in section 2, here also, for the illustration purpose, the quality of the downstream signals has been measured at various lengths (0.6 km, 1 km, 1.5 km and 4 km). We find that all the eye diagrams possess good eye-opening in both vertical and horizontal directions, high quality factor and a less probability of error. ONT 3 fiber length = 0.6 km: ONT 7 fiber length = 1 km It is interesting to note that the results of the ONTs with the passive elements (FBGs) are very similar to ONTs without FBGs. Thus, we infer that the FBGs do not have any effect on the signal of end user (ONT) because the FBG reflects particular wavelength of light and transmits the rest. In addition, we find that the quality of downstream signals (1460 nm to 1530 nm) remains same even in the presence of monitoring 153

9 ONT 9 fiber length = 4 km: ONT 13 fiber length = 1.3 km Figure 12: Quality analysis of downstream signals through eye-diagrams with FBGs at various locations. 4 CONCLUSIONS In this work, we have designed a new passive optical network using an array of fiber Bragg gratings for centralized monitoring of a branched optical networks. As FBGs are deployed for monitoring, it is essential to check if the FBGs affect the quality of the downstream signals. Therefore, first we have designed the PON without FBGs and recorded the quality of the downstream signals that include BER and quality factor through their respective eye-diagrams. Secondly, we have designed the proposed PON using an array of FBGs for fiber fault monitoring. In order to confirm that the usage of FBGs in PONs does not result in detrimental effects, we have reexamined the quality of the downstream signals after incorporating the FBGs in networks. As a first step, in the absence of FBGs, we have tested the quality of the downstream signals at various lengths such as 0.6 km, 1 km, 1.5 km and 4 km. It has been found that the results are in the accepted level as all of them have good eye diagrams, high quality factor and a less probability of error (BER) which is of the order of Thus, these results turn out to be the ultimate figure of merit for the proposed network. Obviously, the next step is to monitor for the fiber faults using FBGs. However, it is of paramount importance to reassure the quality of the downstream signals. In this study, we exploit the reflection spectrum of FBGs for fiber fault monitoring. Here, the reflection spectrum of FBGs with different spectrum shape, frequencies and amplitude is used to differentiate each optical network. The simulation results reveal that the unique characteristics of FBGs have been able to distinguish each optical network terminal (ONT). This means that the unique spectrum shape of any ONT distinguishes itself with other ONTs. To understand the influence of FBG, we have once again tested the quality of the signals at various locations. In this case also, we have found that all the eye diagrams possess good eyeopening in both vertical and horizontal directions, high quality factor and a less probability of error. It is interesting to note that the results of the ONTs with the passive elements (FBGs) are very similar to ONTs without FBGs. Thus, we infer that the FBGs do not have any effect on the signal of end user (ONT) because the FBG reflects particular wavelengths of light and transmits all others. In addition, it has been found that the quality of downstream signals (1460 nm to 1530 nm) remains the same even in the presence of monitoring signal (1625 nm to 1675 nm) and passive element FBG. Thus, it is a good indication that the chosen monitoring signal and passive element (FBG) do not affect the quality of the downstream signals. Thus the proposed system undoubtedly has very unique advantages such as being cost effective, real time monitoring, automatic restoration, and less detection time. We are enormously optimistic that the proposed passive optical network based on fiber Bragg gratings would turn out to be a right future candidate for monitoring fiber faults. 154

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