PERFORMANCE ANALYSIS OF 4 CHANNEL WDM_EDFA SYSTEM WITH GAIN EQUALISATION

Transcription

1 PERFORMANCE ANALYSIS OF 4 CHANNEL WDM_EDFA SYSTEM WITH GAIN EQUALISATION S.Hemalatha 1, M.Methini 2 M.E.Student, Department Of ECE, Sri Sairam Engineering College,Chennai,India1 Assistant professsor,department of ECE, Sri Sairam Engineering College,Chennai,India2 hemalathasekarb@gmail.com methini.ece@sairam.edu.in Abstract Wavelength-Division-Multiplexing (WDM) is generally regarded as the most promising technology for the backbone of future next-generation Internet. Optical fibers are more secure, compared to copper cables, from tapping and also immune to interference and crosstalk. Optical networks employing wavelength division multiplexing (WDM), is the technology of the future. WDM can be used in submarine cables, to extend the lifetime of fiber cables, and reduce the cost of all land-based long distance communications links. The predefined compound component for EDFA with 10dB fixed gain using manufacturing parameters. A 32 channel transmitter generates 32 NRZ modulated optical signals with total power 3dBm with 100GHz channel spacing covering 25nm bandwidth. This optical signal propagates into four spans where each span consists of a fiber span and an EDFA. The EDFA provide a gain across the bandwidth with 10dB average gain and a gain shape variation peak to peak about 1dB.Hence the accumulated gain variance after 4 spans will reach about 4dB and may cause power penalty at the receiver. To reduce the power penalties, an optical filter is used after the last amplifier. This filter is called gain equalization filter (GEF) and is based on user defined data file with spectral shape equivalent to the inverted shape of 4 cascaded amplifiers. Comparing the optical spectrum after GEF with one after the last amplifier, one can see that the gain shape variation reduced from 4dB to less than 1dB. Selected 3 out of 32 channels are de multiplexed and sent to receivers to demonstrate the performance of these channels with gain equalization. Keywords NRZ,WDM,EDFA,Gain Equalisation Filter,Power Penalty,Gain I. INTRODUCTION The increasing demand for new telecommunications services is creating an increase in network capacity requirements. System capacity can be increased by deploying new optical fiber, increasing transmission bit rate and multiplexing more channels on to the existing fiber. Deployment of new fiber is time and cost-prohibitive because it involves equipment burial/installation, while increasing transmission bit rate is problematic due to the cost of replacing transmission equipment. Wavelength division multiplexed (WDM) technology employing erbium-doped fiber amplifiers (EDFA s), however, provides an immediate cost effective alternative for increasing network capacity. In a multichannel environment optical amplifiers should provide a flat gain spectrum, independent of input parameters; however, this is not the case with erbium-doped fiber amplifiers. Indeed, the EDFA exhibits a non-uniform and dynamic gain spectrum, so that each channel input to the amplifier experiences a different gain. Therefore, it is needed that the gain of EDFA be flat in the range of signals for getting an adequate signal to noise ratio at each wavelength. One of the methods to flatten gain of EDFA is using an optical gain-flattening filter. This is the method to flatten the gain of EDFA by using a filter with reverse loss spectrum against the gain spectrum. This filter is called the optical gain-flattening filter. The fiber optic communication also have a number of applications in the area of telecommunication. This paper focus on optical amplifier will be Erbium doped fiber amplifiers (EDFA) which are essential part in fiber optic communications. II.EDFA CHARACTERISTICS At the heart of EDFA technology is the Erbium Doped Fiber (EDF), which is a conventional Silica fiber doped with Erbium. When the Erbium is illuminated with light energy at a suitable wavelength it is excited to a long lifetime intermediate state following which it decays back to the ground state by emitting light within the nm band. Thus, if a pump wavelength and a signal wavelength are simultaneously propagating through an EDF, energy transfer will occur via the Erbium from the pump wavelength to the signal wavelength, resulting in signal amplification. The Erbium can be either pumped by 980nm light, in which case it pass through an unstable short lifetime state before rapidly decaying to a quasi-stable state, or by 1480nm light in which case it is directly excited to the quasi-stable state. Once in the quasi-stable state, it decays to the ground state by emitting 77

2 light in the nm band. This decay process can be stimulated by pre-existing light, thus resulting in amplification. Fig.3. Gain Vs Wavelength IV.GAIN EQUALISATION IN EDFA Fig.1. Principle Of EDFA III. GAIN EQUALISATION FILTER In general, the gain profile of an EDFA can be equalised by modifying the material composition in the erbium-doped fiber or by using optical filters to compensate for the variations in the gain spectrum Various kinds of optical filters have been demonstrated for this application, including long-period fiber gratings, fiber Bragg gratings, fiber acousto optic tunable filters Mach Zehnder filters, and a split-beam Fourier filter. The principles of optical gain-flattening filter are shown in fig.2 after flattening of gain by using the optical gain-flattening filter, it is shown that dependence of EDFA on wavelength is flattened and deviation of signal power is improved where the filter is inserted in the middle stage of EDFA between two erbium doped fibers (EDFs). Erbium doped fiber amplifiers have had a major impact in the field of light wave communications. Optical amplifiers have contributed to the growth of a fifth generation of optical communication systems. The importance of EDFA s is due to their compatibility with the fiber network, low insertion loss, polarization insensitivity, high gain levels and near quantum limited noise performance. Gain differences occur between optical channels having large wavelength spacing (e.g. Δλ> 1nm). In long amplifier chains, even small spectral gain variations (e.g. ΔG < 0.75 db) can result in large differences in the received signal power, causing unacceptably large BER discrepancies between received signals. Uniformity of gain involves two aspects, namely gain equalization and gain flattening. Gain equalization means achieving identical gains for a discrete number (two or three) of optical channels. Gain flattening means achieving a spectrally uniform gain bandwidth. Standard EDFA gain flattening techniques were based on six different principles: (1) Gain clamping with enhanced inhomogeneous saturation, (2) Use of passive internal/external filters, (3) Use of external active filters, (4) Cascading EDFAs with different gain spectra, (5) spatial hole burning in twin core fibers and (6) Adjustment of input signal powers. Fig.2. Gain Equalization Filter (GEF) to achieve a flat gain spectrum Fig.4. Gain Equalisation Filter 78

3 V. BLOCK DIAGRAM OF A SINGLE CHANNEL EDFA SYSTEM In this simulation model we optimise the gain of the single WDM channel. EDFA has high gain, large bandwidth, and low noise but has uneven gain. This unevenness reduces the transmission bandwidth and degrades the performance of system. The gain flattening is done with the help of the Gain Equalisation Filter (GEFs). We approach to the external modulation in the proposed model. Numerical simulation result shows that this technique can minimise unevenness up to 1dB gain. Our proposed model is shown in the Figure5 which consists of the transmitter having the Wavelength of about 1550nm at -20 dbm power. The signal is then routed to the MUX whose properties are almost same to that of the transmitter having the bandwidth of 13 GHz. The signal is than passed through the optical fiber through the EDFA. The MUX receives the signal after passing through the GEF. Finally the signal is retrieved by the photo detector. The photo detector convert the optical signal to electrical signals which is processed to get the exact information. The Gain Equalisation Filter component is placed after the EDFA and it will equalize the amplifier gain. The user can change the filter parameters manually or use the Gain Equalisation Filter Optimization of Opt System. The optimization engine is built specifically for the Gain Flattening Filter component. The filter can be placed anywhere in a layout, for example, between two stages of an optical amplifier. Fig.5. Block diagram of a single channel EDFA System with gain equalization VI. BLOCK DIAGRAM OF 4 CHANNEL WDM_EDFA SYSTEMS The basic configuration consists of 4 channels with 50GHz channel spacing, the system has a WDM transmitter with first channel is at 193.5THz and increases as the number of channels increases, the input power of WDM transmitter is 20dBm. All the channels are transmitted into the WDM multiplexer with zero insertion loss, here all the light signals are combined and transmitted over the erbium doped fiber of length 9m.The erbium doped fiber amplifier is counter pumped at 1480nm.The counter pumping scheme gives more gain than that of co-propagating pumping scheme. The pump power at which the EDFA pumped is 130Mw.The output of EDFA is then passed through the non linear fiber. The output after the optical amplifier then fed into the amplifier of length 11m which is counter pumped at 1450nm with constant pump power of 450mW.The over all amplified signal is fed into the optical spectrum analyzer to analyze the optical spectrum. The gain variations and the noise figure variations are noted down for different frequencies. At the end, it passes through the photo detector to convert the optical signal into electrical signal and the bit error rate value can be noted down. The bit error value should be less than 10 6 db. Fig.5. Block diagram of a single channel WDM_EDFA system 79

4 Fig 6.Block diagram of a 4 channel WDM_EDFA systems with gain equalization VII. IMPLEMENTATION In this design, twelve spans of 81-km optical fiber are pre-amplified by 100-mW 980-nm pumped EDFA. The photo receiver is also pre amplified by an EDFA. After loading the topology, run a simulation by clicking on OPTSIM Simulate Link button and hitting OK. At the conclusion of the simulation, double-click the Property Map block, which displays a plot of the signal power along the link. As can be seen, at the end of the last EDFA-fiber span, the signal power is roughly equal to its value at the start of the link. As an exercise, try adjusting the EDFA and fiber parameters to achieve uniform gain compensation along the entire link (i.e., the power output of each fiber should be equal). A single channel transmitter generates NRZ modulated optical signals with total power 3dBm with 100GHz channel spacing covering 25nm bandwidth. This optical signal propagates into two spans where each span consists of a fiber span and an EDFA. The EDFA provide a gain across the bandwidth with 10dB average gain and a gain shape variation peak to peak about 1dB. Hence the accumulated gain variance after 2 spans will reach about 2dB and may cause power penalty at the receiver. To reduce the power penalties, an optical filter is used after the last amplifier. This filter is called gain equalization and is based on user defined data file with spectral shape equivalent to the inverted shape of 2 cascaded amplifiers. Comparing the optical spectrum after GEF with one after the last amplifier, one can see that the gain shape variation reduced from 2dB to less than 1dB. A single channels are de multiplexed and sent to receivers to demonstrate the performance of these channels with gain equalization. VIII. RESULTS AND DISCUSSION Gain equalization Filter (GEF) provides a simple and compact solution to equalize the gain, when placed at transmission channel in a WDM network, along with low power consumption. In addition, the low cost and simple circuit make GEF very attractive for WDM applications. This component provides a very large dynamic range to equalize the gain of channels passing through it. As a result high bitrates can be achieved with minimal device and management cost. In this result analysis GEF is evaluated in detail. The issue with gain equalization due to various non-linear effects and their managements are addressed. The un-even gain of the conventional amplifier due to the non linear effect, which is responsible for the reducing the performance of the leading devices connected in the network. So in order to enhance the performance its important to equalize the gain of the amplifier. This gain equalization is possible with the help of the GEF (Gain Equalization Filter). In this results, single channel EDFA system with gain equalization is demonstrated. A similar algorithm should be also applicable to an multiple channel EDFA. The plan is to achieve gain equalization for 4 channel using WDM_EDFA system. As the number of channels increases, the transmission problem arises because a conventional system has intrinsic non-uniform gain. To increase the gain-bandwidth of an amplified light wave system several methods can be used, but equalizing optical filters operating as best method. This paper mainly focuses on gain equalization in EDFA for 4 channel. We input the 4 channel wavelengths as nm, nm, nm and nm. These wavelengths are multiplexed. The multiplexed outputs are amplified and the wavelengths are equalised using cascading of amplifiers. Hence the equalized wavelengths are de multiplexed, and displayed in the Eye diagram analyzer. IX. OUTPUT OF A SINGLE CHANNEL EDFA SYSTEM A. Output of The PRBS Pattern Generator The output from a pseudo binary sequence generator is a bit stream binary pulses ie., a sequence of 1 s or 0 s a known reproducible pattern. Fig.7. Output of the PRBS Pattern Generator 80

5 A. Output of the Electrical Signal Generator In electrical signal generator, which converts binary sequence into electrical signal. Here binary sequence are converted to 2V. Fig.11. Output of the Physical EDFA D. Output of the SECOND EDFA Fig.8. Output of the Electrical Signal Generator The performance of an EDFA within an optical link, a variety of reference plots can be generated by the model in order to study internal power evolution, signal gain, noise figure, and the population densities. B. Output of the Optical Power Normalizer Normalizer attenuates all input optical signals to the same average output power regardless of their different average input powers, or it may be used to attenuate all input optical signals by the same amount such that the signal with the largest average input power has the specified average output power. Fig.12. Output of the SECOND EDFA E. Output of the Eye Diagram Analyzer distance at the wavelength of nm shown in figure. The eye opening for distance 40 km is Fig.9. Output of the Optical Power Normalizer C. Output of the Physical EDFA Optical signals propagating along the EDFA interact with the local population densities, resulting in power gain or loss via stimulated emission and absorption. Spontaneous emission and its subsequent amplification also occur. The optical signals being amplified by the EDFA usually have wavelengths ranging from nm. Fig.13. output of the Eye Diagram Analyzer X. OUTPUT OF THE 4 CHANNEL EDFA SYSTEMS A. Output of the Eye Diagram Analyzer at the wavelength nm 81

6 distance at the wavelength of nm shown in figure 14. The eye opening for distance 40 km is Fig.16. Output of the Eye Diagram Analyzer at the wavelength of nm D. Output Of The Eye Diagram Analyzer At The Wavelength Eye diagram of signal for 4 channel of EDFA at 40km distance at the wavelength of nm shown in figure 17. The eye opening for distance 40 km is Fig.14. Output of the Eye Diagram analyzer at the Wavelength nm B. Output Of The Eye Diagram Analyzer At The Wavelength nm distance at the wavelength of nm shown in figure 15. The eye opening for distance 40 km is Fig.17. Output of the Eye Diagram Analyzer at the wavelength nm E. Map Between Power And Distance For Gain Equalisation The power versus distance for 4 different wavelength is shown in figure 18. Power increases with distance from.57 X to.134 X db for 4 channels. The acceptable power for optical transmission is 10db. It is observed that by increasing distance from 60 to 3000 km, power is also increasing. Fig.15. Output of the Eye Diagram analyzer at the Wavelength nm C. Output of the Eye Diagram Analyzer at the wavelength nm distance at the wavelength of nm shown in figure 16. The eye opening for distance 40 km is Fig.18. Output of the Power map between Power and Distance for Gain Equalization VIII. CONCLUSION AND FUTURE WORK This paper shows that the fiber amplifier is to be used to amplify the signal. The project proposed a simple means of the gain equalization of the single channel EDFA, and applied this concept for the control of gain equalization. Using this 82

7 technique, maintain the gain flatness variation can be maintained below 0.03 db/10 nm for the input power variation from -40 dbm. Operation with an even wider input dynamic range or wavelength should be possible with higher pump power or properly designed gain equalizing filters. Further studies are in progress to apply this equalization scheme to control gain variations. A similar algorithm should be also applicable to an multiple channel EDFA, to relax strict requirements on the transmitter wavelength and input power. IX. SCOPE FOR FUTURE WORK The successful introduction of commercial WDM systems, enabled by practical EDFAs, has in turn fueled the development of high-power, wide-bandwidth, low-noise, gainflattened optical amplifiers. The availability of such highperformance optical amplifiers and other advanced optical technologies, as well as the market demand of more bandwidth at lower costs, have made optical networking an attractive solution for advanced networks. Optical networking utilizes the WDM wavelengths not only to transport large capacity but also to route and switch different channels. Compared to point-to-point systems, optical networking applications are more demanding of optical amplifier requirements such as gain flatness, wide bandwidth, and dynamic gain control. Considerable progress has been made in optical amplifier technology in recent years. The bandwidth of amplifiers has increased nearly 7 times and flat gain amplifiers with 84 nm of bandwidth have been demonstrated, made possible by addition of the L-band branch. With the advent of these amplifiers, commercial terabit light wave systems will be realized. Progress has also been made in the understanding of amplifier gain dynamics. Several control schemes have been successfully demonstrated to mitigate the signal impairments due to fast power transients in a chain of amplifiers and will be implemented in light wave network design. Terrestrial light wave systems have been increasing in transmission capacity. To meet the enormous capacity demand, the currently available 400-Gb/s capacity system with 80 channels will soon be followed by systems having terabit and higher capacity on a single optical fiber. and Sensors Faculty of Engineering, Rand Afrikaans University, Auckland Park 2006 [6] Ezra Ip Gain Equalization for Few-Mode Fiber Amplifiers Beyond Two Propagating Mode Groups IEEE PHOTONICS TECHNOLOGY LETTERS, Volume. 24, NO. 21, November 1, 2012 [7] Shivani Radha Sharma, Gain Flattening of EDFA using Hybrid EDFA/Raman Amplifier with Reduced Channel Spacing Volume 3, Issue 3,2015 [8] Dr. Neena Gupta A Novel Approach For Performance Improvement of DWDM Systems Using TDFA-EDFA Configuration IJECT Vol. 1, Issue 1, December 2010 [9] Ramandeep Kaur, Analysis of Dense Wavelength Division Multiplexing Using Different Optical Amplifiers Volume 2 Issue X, October 2014 [10] A. W. Naji, Review of Erbium-doped fiber amplifier International Journal of the Physical Sciences Vol. 6(20), September, 2011 REFERENCES [1] Dr. T. K. Bandhopadhya Gain Equalization of EDFA International Journal of Computer Technology and Electronics Engineering (IJCTEE) Volume 3, Issue 1, February 2013 [2] Faroze Ahmad Gain Equalisation of Hybrid Fiber Amplifiers International Journal of Advanced Research in Computer Science and Software Engineering Volume 4, Issue 3, March 2014 [3] X. S. Cheng Wide-Band Bismuth Based Erbium-Doped Fiber Amplifier With a Flat-Gain Characteristic Volume 1, Number 5, November 2009 [4] Prateeksha Sharma Study Of Single and Multi Wavelength (WDM) EDFA Gain Control Methods International Journal of Engineering Trend and Technology (IJETT) - Volume4 Issue5- May 2013 [5] Thabiso J. Nhlapo, Gain Equalization of Erbium Doped Fiber Amplifiers with Tuneable Long-Period Gratings Centre for Optical Communications 83