Power Transient Response of EDFA as a function of Wavelength in the scenario of Wavelength Division Multiplexed System

IJSRD - International Journal for Scientific Research & Development Vol. 1, Issue 9, 2013 ISSN (online): 2321-0613 Power Transient Response of EDFA as a function of Wavelength in the scenario of Wavelength Division Multiplexed System Prabhjyot Singh 1 Simranjit Singh 2 1 M. Tech. Student 2 Assistant Professor 1,2 Department of Electronics and Communication Engineering 1, 2 UCoE Patiala, Punjab, India Abstract In this paper power transient is investigated as function of add/drop wavelength and surviving channel wavelength. We have reported that power excursions varies with different wavelength allocations of the add/drop channels. Transient response is reduced by 73.39% in case when add/drop channels are taken in L band instead of C band. Also power transient response is calculated as a function of wavelengths of surviving channel. It has been observed that at higher wavelengths power excursions are less than at shorter wavelengths of C band. Key words: EDFA transient, Power excursions, WDM optical networks, add/drop (disturbing) channels. I. INTRODUCTION The transmission capacity of long-haul optical networks has evolved tremendously over the past decades by adding multiple wavelength channels through the use of wavelength division multiplexing (WDM) and dense WDM technology. Recently, there has been a rapidly growing interest in the study of all-optical wavelength-division multiplexed networks that provide both switching and transmission the optical domain. WDM system is an attractive means for large capacity transmission systems and flexible optical networks. Such WDM networks have become possible because of the availability of optical amplification, particularly erbium-doped fiber amplifiers (EDFA s). Although new amplification technologies, such as Raman and semiconductor optical amplifiers can provide remarkable performances in terms of gain bandwidth and flexibility, standard EDFAs are still the most attractive solution as the best tradeoff between cost end performances [30,31]. EDFA s provide low-cost and efficient amplification in wavelength-division multiplexed networks. The Performance of an Optical Communication system can be improved by the use of EDFAs as an Optical Amplifier. The EDFA is the most deployed fiber amplifier as its amplification window coincides with the third transmission window of silica-based optical fiber. EDFAs are reliable for transmitting data through long distance because of their wide bandwidth and optimum bit error rate. Although EDFA s are a key technology enabling the realization of transparent WDM communication, and despite their unsurpassed performance, a number of significant technological challenges remain. In multiwavelength light wave networks, the number of transmitted channels may vary due to, e.g., network reconfiguration, network growth to larger number of channels or failure of a channel that can cause one or more channel to drop out[1,2]. Because these amplifiers are operating near saturation, and since the total output power of a saturated EDFA is very nearly constant, independent of the number of channels, the gain experienced by each channel will, therefore, depend on the number of channels present. Rapidly changing gain, due to channel drop or addition, leads to cross-gain saturation in fiber amplifiers that in turn would induce power transients in the surviving channels which can seriously degrade system performance parameters such as bit-error rate (BER) and signal-to-noise ratio (SNR) during the transient events. They can result in the drastic deterioration of the surviving channels performance. Sudden changes in network configuration, fiber breaks, other failure mechanisms, and protection switching may cause abrupt changes in optical power which can also cause transience. The EDFA transients arise due to two factors: (i) they usually work in deep saturation and (ii) present long upper state lifetime of Erbium ions (~ 10 ms) [3]. Although this perturbation will generally be small in a single amplifier, it will grow rapidly along a cascade. To cater to the demand for an increasing number of channels, high signal powers, greater than 20 db m, cascaded/multistage amplifiers are used. So small transient perturbations in a single EDFA grow rapidly along a cascade [2], [4]. In general, system performance may be degraded by fiber nonlinearity when the channel powers are too high and by a small receiver signal-to-noise ratio (SNR) when the channel powers are too low. These transients occur on the time scale of microseconds to milliseconds, and could momentarily and significantly disrupt the system performance. When channels are dropped, the power of the surviving channel increases which severely degrades the performance of the surviving channel because of self-phase modulation. And when channels are added it degrades surviving channels due to cross-phase modulation (XPM) and four-wave mixing (FWM) for up to a few microseconds. To overcome these problems, the transient effects in the optical amplifiers must be controlled.. Also To maintain quality of service for surviving channels, it is necessary to limit the power excursions they experience. In the past several control strategies have been proposed to fix the EDFA gain at a given operating point. Low electronic cost control suggested in [5] is widely used. Also a combination of optical and electrical control schemes is possible [7]. Also proposed is gain clamping via the construction of ring lasers [6], [7] and insertion of a compensating signal [8]. Inserting a compensating signal into the first EDFA has been reported to be effective in stabilizing chains of six [9] and eight [10] EDFA s. Some solutions to mitigate optical amplifiers transients were successfully demonstrated and are based on a combination of techniques such as linear gain control, gain All rights reserved by www.ijsrd.com 1919

clamping and fast automatic gain control [13, 14]. Additionally, passive mitigation approaches have also been proposed, such as the use of EDF with a large active area of Er 3+ [15, 16]. In the optical domain, the idea of recirculating amplified spontaneous emission (ASE) as a clamping mechanism in a feedback loop was presented in the context of WDM ring networks [17]. The idea of recirculating ASE using a band pass filter to select the lasing wavelength, and a variable optical attenuator (VOA) to adjust the feedback cavity loss was presented in [18, 19]. The same principle using fiber Bragg grating (FBG) as band pass filters was demonstrated to flatten and clamp the gain of L-band EDFA for WDM applications [31]. Other approaches based on feedback techniques make use of a DWDM multiplexer [32]. Also novel techniques to minimize gain-transient time of WDM signals in EDFA in channel add/drop networks have been introduced. The newly proposed gain controller is composed of a disturbance observer and a PID controller. Another approach based on use of the per-band link control method in cascades of distributed fiber Raman amplifiers (DFRAs) for compensating power transients of WDM channels was introduced. By adding compensation channels the power excursion is reduced in the surviving channels. However many of these proposals are not practical because of high cost, or instability due to electronic control induced or chaotic behavior [20]. The all-optical scheme has a drawback, the frequency of channel add/drop should be less than that of the relaxation oscillation frequency of EDFA, which is several hundred Hz. Also inserting the extra channel is not economical in signal bandwidth. While transient suppression methods using PID controller, feedback electrical/optical are very complex and use additional circuitry, so is expensive. The transient response of EDF is a function of the wavelength and power of the survival channels, number of channels, pump etc. In order to gain a better understanding of the transient response and to reduce the power transient, analysis of the effect of the factors such as signal wavelength and power, pump wavelength and power, etc. is needed to be done. A true understanding of the EDFA gain dynamics will help in the design of protection and control schemes against deleterious nonlinear effects in transmission. So power transient response of surviving channels can be reduced by analyzing the factors on which transient depends and then by optimizing them. This method is very cost effective as it is done without adding any complex additional circuitry. Wang et al. [21] investigated Gain transients in both co-pumped and counter pumped distributed Raman amplifiers as a function of signal launch power, fiber type, surviving signal wavelength. All these parameters affect the magnitude of the pump-depletion level and, thus, determine the amplifier transients. It is found that gain transients are much more pronounced in a copumping scenario than in a counter pumping scenario for the same operating conditions. Transient suppression technique with better than 0.2-dB ripple capability was also demonstrated in this study. Chan et al. [22] simulated how the EDFA transient dynamics depend on different EDF lengths and erbium concentration. EDFA model was designed that used optimized length and erbium concentration to suppress the power transient. Gurkan et al. [23] presented a more extensive study of the differences between C and L-band EDFA dynamics for a 40-channel WDM system for varying numbers of cascaded EDFAs. Results have shown that the L-band EDFA transient response time is -5 times slower than for the C-band. Also longer transient time for L-band provides the necessary time to prevent the degrading transmission penalties. Kar asek et al. [2] analyzed and compared surviving channel power excursions resulting from switching ON OFF of one, three, and six out of eight WDM channels in a cascade of concatenated strongly inverted and standard two-stage EDFA s. It follows from the performed analysis that the rise time of the surviving channel power transients in the strongly inverted cascade is approximately twice as fast as in the cascade of standard amplifiers The effect of pump power, EDF length and span loss on the characteristics of the cascade of six strongly inverted EDFA s has been analyzed. Lee et al. [24] investigated steady state and transient behavior of a C-band EDFA has been for the various add/drop channel allocations. The channels are located at around short wavelengths, long wavelengths, and in the middle of C-band. The measured transient behavior in each case is calculated. Sugaya et al. [25] illustrated transient response of discrete Raman amplifier in case of channel add-drop through comparing co- and counter-propagating pump schemes by experiment and calculation. Karlsek et al. [26] investigated, both experimentally and theoretically, the effect of channel removal/addition on surviving channel power transients in distributed Raman fiber amplifier (RFA). The effect of pumping scheme, pump power, the length and type of Raman fiber, and number of added and/or dropped channels on the dynamics of surviving channel power fluctuations has been studied. Gest et al. [27] analyzed the dynamic response of nine different cascades of DFRAs. Three cases of cascades a cascade of all unclamped DFRAs, a cascade of all gain-clamped DFRAs, and a cascade of mixed unclamped and gain-clamped DFRAs is compared and analyzed in terms of gain excursions and overshoot & undershoot. The evolution of the rise and fall times in the surviving channel after each amplifier in the cascade is also monitored. Also investigation of the influence of the surviving channel location in the amplification band is done. Kaler [28] investigated Effect of channel adding/dropping on EDFA transients. Also further comparison of the transient response of Compact EDFAs and Transient EDFAs is done. Tian et al. [29] studied the transient dynamics of the EDFAs responding to channel adding/dropping events. The differences in the responses of the EDFAs pumped at 1480 and at 980 nm are compared and analyzed. It is also observed that EDFA pumped at 980 nm has much faster transient response than the one pumped at 1480 nm. Already many papers have been presented on the pump control method. Also effects of number of channels add/drop pump power, wavelength and different pumping scheme have been extensive studied. But the impact of the wavelength allocation of the add/drop channels in C and L bands on the survivor channels power transient have not clearly investigated. Since L-band transmission is becoming All rights reserved by www.ijsrd.com 1920

commercially available, optical ADM operations will be required for both the L and C bands. The differences between C- and L band dynamics have been recently studied. In this paper, we have measured the power transient of the surviving channel for different wavelength allocations of add/drop channels in C and L band. Also effect of surviving channel wavelength in terms of power excursions is observed. Q factor is also calculated for surviving and add/drop channel different wavelength allocations. And we have shown that the gain of the EDFA depends on the channel assignment in the steady state. This paper is divided into 4 sections. In Section 2, the optical simulation setup is described. In Section 3, simulation results have been reported and discussed for the different wavelengths of surviving and disturbing channels. And finally in Section 4, conclusions and future work are made. II. SIMULATION SETUP In this work, an analysis is presented regarding power transient excursions resulting from channel drop/add in the system of transmission using EDFA amplifier with as many as 25 WDM channels. In order to gain a better understanding of the transient response and, analyze the effect of the factors such as signal wavelength and add/drop channel wavelength we have made this system as shown in figure 1. Fig. 1 shows the simulation setup to analyze the EDFA transients. The simulation tool used in this work is the Optisystem (version 7.0), a software of Optiwave Corporation. A 25 channel WDM system was simulated in Optisystem. We consider twenty five WDM channels. This is sufficient to show the dynamics of an EDFA after channels dropping and adding. 24 channels out of 25 are added dropped. Powers of all 25 channels are kept constant with each channel given input power of -20 dbm. The add/drop of 24 channels was simulated by modulating the optical signal by a square wave at a low bit rate of 4000 bits/s. A 32-bit pseudorandom or user defined bit pattern is encoded on each add/drop channel using non-return-to zero pulse generator. Twenty four channels are added at t = 0.1 ms and dropped back at t =0.3ms and then is repeated with a period of 4ms. This 24 channel light source is modulated by an Optical Modulator (OM). Fig. 1: Simulation setup to calculate and analyze power transient of the surviving channel. (OTDV: Optical time domain visualizer; BER: Bit error rate) And these 24 channels are combined with a CWprobe (surviving) channel to simulate adding dropping twenty four out of twenty five channels (worst case condition) using a multiplexer. Add/drop channels (disturbing channels) are equally spaced with a uniform channel spacing of 1 nanometer (nm). The surviving channel is kept at constant wavelength of 1532nm; while we have given disturbing channels two different wavelength allocations in different set of measurements. In 1 st case disturbing channels are given wavelengths in C band from 1538 to 1561nm and in 2 nd case they are allocated wavelength in L band from 1566 to 1589nm. These channels are coupled into the dynamic EDFA. The amplification stage is composed by a WDM (coupler), that combines the input and pump powers, followed by a 5-m erbium doped fiber and isolator. The pump laser at 980 nm wavelength and with constant power of 20 dbm is used for forward (co) pumping the signal (channels). The pump laser with wavelength of 1480 nm can be used as well. We have used the dynamic erbium-doped fiber with the erbium ion density of 1000 ppm-wt. Its core radius is assumed to be 2.2 µm and numerical aperture is assumed to be 0.24. The metastable life time of 10 ms is used. The insertion loss at 1550 and 980 nm is taken as 0.01 and 0.015 db/m respectively. Power amplified signal is then passed through a fiber. For the sake of simplicity, only one section of 40 km standard fiber is used with one EDFA. At the receiver each channel passes through photo detectors and then through electrical low pass filter with a bandwidth of bit rate. System performance is evaluated by BER analyzer by calculating the factors like Q factor, eye diagram and bit error rate. III. SIMULATION RESULTS AND DISCUSSION We examine and analyze the relevant factors which affect the transient power excursion of the surviving channel in a single all-optically stabilized EDFA, such as add/drop channels wavelengths, surviving channel wavelengths. A. Power Transient as a Function of Add/Drop (Disturbing) Channels Wavelength Firstly, the system impact of the disturbing channels wavelengths on the power transients of EDFA is observed. To study and investigate the effect of disturbing channels wavelength all the other factors like surviving channel power & wavelength, pump power & wavelength, EDFA length are kept constant. 24 channels are added at 1 ms and then dropped at 3 ms. the surviving channel wavelength is taken at 1532 nm. The disturbing channels wavelengths are varied and two cases are made. When the wavelengths of add drop channels are changed, the output transient of surviving channel is affected In first case 24 disturbing channels are taken in C band from 1538 to 1561 nm. While in 2 nd case their wavelengths are taken in L band from 1566 to 1589 nm. Now corresponding to two cases power transient is calculated by simulation. The results are as shown in the fig. 2. It shows analysis of the results for the different disturbing channel wavelengths after the EDFA. All rights reserved by www.ijsrd.com 1921

Fig. 2: Transient response comparison of surviving channel for disturbing channels wavelengths in C and L band. Now, power excursion, P is given by: P = P (0) P ( ). Where P (0) is power before the channel add or drop and P ( ) is power after the channel add/drop. Simulation shows a different power transient behavior under different disturbing channels wavelengths although the output gain of the surviving channel is observed to be almost same. As it can be seen that power excursion when disturbing channels are in C band is 43.5388 mw. While when we take disturbing channels in L band the power excursion is dropped to 11.583 mw, that is 31.9558 mw less than what it was in case of C band disturbing channels. So we can see that power transient is reduced by 73.396% when disturbing channels are taken in L band instead of C band. The reason behind this transient reduction can be attributed to the difference in wavelengths between the surviving and disturbing channel. A power transient in the surviving channels is mainly induced by cross gain saturation and four wave mixing when channels are added or dropped. The cross gain saturation and four wave mixing in WDM system depends directly on the how closely channels are spaced. Channels with small channel spacing or gap are found to be more affected by cross gain saturation & four wave mixing than the channels which have more channel spacing. Thus when disturbing channels are taken in C band their difference in wavelength with surviving channel at 1532 nm is less so more is the power transient. As we have seen just by optimizing the disturbing channel wavelength we can reduce the unwanted power transient effect in the amplifier. Optimizing basic parameter like add/drop channel wavelength can reduce the transient behavior which can be used in future reconfigurable networks. Moreover reduction in transient behavior is not obtained at the expense of the gain or output power. Output power remains at same level but transient effect is reduced. figure 3. It can be seen that Q factor is higher when the disturbing channels are in L band. As power transient effects in case of channel add/drop in L band is less as compared to when they are in C band. So Q factor is better when channel add/drop takes place in L band rather than in C band. This confirms our experimental analysis that when disturbing channels are allocated wavelengths in L band, while surviving channel is in C band, it reduces the power transient and Q factor is improved. B. Power Transient as a Function of Surviving Channel Wavelength We have also calculated the effect of surviving channel wavelengths on the power transient and power excursions. The wavelengths of add/drop channels are kept. They are allocated wavelengths from 1557 to 1564 nm. For sake of simplicity 8 channels are added or dropped. While we change the wavelength of the survival channel across the C band and get the wavelength dependence. This is shown in the figure 4. Fig. 4: Power excursions as a function of surviving channel wavelength. The power excursions depend on the amplifier gain. And amplifier gain is wavelength dependent. Thus transient response of EDF is a function of the wavelength. Higher gain or power leads to higher power transient. Power excursions behavior is closely related to the shape of the EDFA gain response. The longer wavelengths receive less power gain than the shorter ones. The shorter wavelength channels exhibits faster and stronger transient as compared to longer wavelengths. It is observed that the channels centered around 1550 nm exhibit almost constant power excursions. Fig. 3: Q-factor comparison for the wavelength allocation in C and L band of disturbing channels. The Q factor at the receiver side with the corresponding system is also calculated. It is shown in the Fig. 5: Comparison of power excursions of surviving channel for channel dropping and adding. Also power excursions are calculated for channel dropping and channel adding separately. As can be seen from the figure, amplitude of the transient power excursions of the surviving channel are lower in the case of adding channels versus that of dropping channels. These simulation results are in good qualitative agreement with the experimental results reported in [11], [12]. Both simulation and experiment indicate that dropping of channels results in much more severe effects on the surviving channels. All rights reserved by www.ijsrd.com 1922

IV. CONCLUSION Surviving channel power excursion resulting from 24 channels add/drop out of 25 total WDM channels is investigated and simulated for various wavelength allocation of disturbing (add/drop) channels in C and L band. It is found that power transient is reduced by 32.9558 mw or 73.39% when disturbing channels wavelengths are taken in the L band instead of C band. Also Q factor is found to be better and higher when wavelength of adding/dropping channels is in L band. Power transient of EDFA is also calculated as a function of surviving channel wavelength. It is observed that power excursions are less in case of higher wavelengths of C band. Power excursions are observed to be almost constant for surviving channels wavelengths near the 1550 nm region (from 1442-1458 nm). Thus by just optimizing the disturbing and surviving channels wavelengths the power excursions or power transient can be reduced by a large factor. 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