An EDFA based Optical Communication System Compensating the Effects of Jitter and Transients with maximum Gain flattening

Transcription

1 An EDFA based Optical Communication System Compensating the Effects of Jitter and Transients with maximum Gain flattening SHAFEENA P. K. 1,HARSHA THOMAS 2, MAHESH KUMAR 3, SINDHU N 4 Abstract This paper aims in developing an optimum communication system which uses EDFA and compensates the effects of Jitter and Transients and also to employ maximum gain flattening. Various transient reduction methods, timing jitter compensation techniques are explained and methods for obtaining flattened gain for the used optical amplifiers are explained. Index Terms Optical amplifiers, EDFA, Jitter, Transients,Gain flattening I. INTRODUCTION A communication system transmits information from one place to another. Optical communication systems use high carrier frequencies (~100 THz) in the visible or nearinfrared region of the electromagnetic spectrum. They are sometimes called light wave systems to distinguish them from microwave systems, whose carrier frequency is typically smaller by five orders of magnitude (~1 GHz). Fiber-optic communication systems are light wave systems that employ optical fibers for information transmission.[1] The optical communication systems are different from microwave communication systems in many aspects. In the case of optical systems, the carrier frequency is about 100 THz and the bit rate is about 1T bit/s. Further the spreading of optical beams is always in the forward direction due to the short wavelengths. Even though it is not suitable for broadcasting applications, it may be suitable for free space communications above the earth s atmosphere like inter satellite communications. Tyndall discovered that through optical fibers, light could be transmitted by the phenomenon of total internal reflection. Optical fibers can provide a much more reliable and versatile optical channel than the atmosphere. Each communication systems have its own advantages and disadvantages. In this work the focus is made more on optical amplifiers which are essential part in fibre optic communication systems. The optical amplifier of our interest in this work will be erbium doped fibre amplifier (EDFA) which is widely preferred in the area of fiber optic communication. In this paper an analysis of the performance of optical communication system consisting of single or a chain of EDFAs (depending upon the system performance) is made based on the parameters such as transients, gain and timing jitter. Section II explains about optical amplifiers in detail, Section III details about EDFA followed by spectral variations and power transient explanation in Section IV and V respectively. Section VI explains about gain flattening in EDFA followed by conclusion. II. OPTICAL AMPLIFIERS Transmission of signals in an optical fiber mostly takes place around 1310 nm and 1550 nm. The typical attenuation of silica fibers is on the order of 0.2 db/km at 1550 nm (0.35 db km at 1310 nm). Fibers designed for 1550 nm transmission have a dispersion coefficient close to zero with maximum tolerable values of -2-3 ps/(nm.km). It is more difficult to indicate a typical dispersion limit, since this not only depends on the fiber through the dispersion coefficient and the operating wavelength, but is also influenced by the optical source spectral width, bit rate, and transmission format. Practical limits range from just 20 km or less up to several hundred kilometers. For these reasons there is a need to install a number of suitably spaced repeaters in order to perform the functions of regenerating, reshaping, and retiming the signal along an optical link. These 3R repeaters act by means of a combined optical/electrical and electrical/optical conversion. They detect the optical signal arriving from the preceding transmission span, make the necessary corrections at the electronic level, and activate a transmitter diffusing the optical signal on the following transmission length. The availability of optical amplifiers really represents a key point regarding the attenuation limit of optical networks. Optical amplification allows system designers to increase network performance, and in the meantime lower the number of repeaters and simplify the network. This means lower installation and maintenance costs and higher link reliability. Moreover, the partial substitution of 3R repeaters with optical amplifiers enhances the optical transparency of the network. Roughly speaking, it can be said that optical amplification has overcome attenuation limits.[2] 190 SHAFEENA P. K.,HARSHA THOMAS, MAHESH KUMAR, SINDHU N

2 Optical amplifiers have really revolutionized the field of fiber optics communication. Optical amplifiers are in general bit rate transparent and can amplify signals at different wavelength simultaneously. Optical amplifiers are mainly of two types i.e. Semiconductor optical amplifiers and Fiber amplifiers.these are further classified into travelling wave semiconductor optical amplifier, Fabry-perot semiconductor optical amplifier, Erbium doped fiber amplifier, Raman & Brillouin fiber amplifiers. All optical amplifiers increase the power level of incident light through a stimulated emission to occur or an optical power transfer process. In SOAs and DFAs (doped fiber amplifiers),the mechanism for creating the population inversion that is needed for stimulated emission to occur is same as is used in laser diodes. Although the structure of such an optical amplifier is similar to that of laser, it does not have the optical feedback mechanism that is necessary for lasing to take place. Thus, an optical amplifier can boost incoming signal levels, but it cannot generate a coherent output by itself. The basic operation is as follows,the device absorbs energy supplied from an external source called the pump. The pump supplies the energy to electrons in an active medium, which raises them to higher energy levels to produce a population inversion. An incoming signal photon will trigger these excited electrons to drop to lower levels through a stimulated emission process. Since one in coming trigger photon stimulates many excited electrons to emit photons of equal energy as they drop to the ground state, the result is an amplified optical signal. To achieve optical amplification, the population of upper energy level has to be greater than that of lower energy level, i.e. N2> N1, where N1, N2 is population density of lower and upper state. This condition is known as population inversion. This can be achieved by exciting electron into higher energy level by external source called pumping. Stimulated emission occur, when incident photon having energy E= hc/λ interact with electron in upper energy state causing it return to lower state with creation of second photon, where h is Plank constant, c is velocity of light and λ is the wavelength of light. So light amplification occurs, when incident photon & emitted photon are in phase and release two more photon, continuation of this process effectively creates avalanche multiplication. Therefore amplified coherent emission is obtained. As shown in Fig1 optical amplifiers are divided into three as follows In-line Optical Amplifiers, Preamplifiers and Power amplifiers. III. ERBIUM DOPED FIBER AMPLIFIERS Erbium doped fiber amplifier (EDFA) is a device that boosts the signals in optical amplifier, i.e. it acts as a power amplifier. EDFA is designed for Dense Wavelength Division Multiplexing applications in which multiple optical signals are taken and are multiplexed into a single fiber. In doped fiber amplifiers (DFA), the fiber core is doped with rare earth erbium ions. The ions constitute the active medium through which optical gain is obtained. When optically pumped, these ions are excited to a higher energy state. When simulated by incoming photons the ions emit photons which results in optical gain. The EDFA has generated significant interest because of its high gain,large bandwidth and low noise.[3] Figure2:Erbium doped fiber amplifiers The active fiber is pumped with light from two lasers diodes. The pump light, having a wavelength of 980nm or 1450nm excites the erbium ions Er 3+, from where they can amplify light via. stimulated emission. The isolators present in the amplifier input stages will prevent spontaneously emitted light from affecting the previous stages and the one in the output suppresses the reflection of output back to amplifier. The EDFA consists of three basic components: length of erbium doped fiber, pump laser and wavelength selective coupler to combine the signal and pump wavelengths as shown in Figure 3. The optimum fiber length used depends upon the pump power, input signal power, amount of erbium doping and pumping wavelength. Erbium doped fiber amplifiers (EDFAs) can be extensively used in optical fiber communication systems due to their compatibility with optical fiber. An EDFA has a comparatively wide wavelength range of amplification making it useful as transmission amplifier in wavelength division multiplexing systems. Theoretically EDFA is capable of amplifying all the wavelengths ranging from 1500 to 1600 nm. However practically there are two windows of wavelength. Figure 1: optical amplifier form Figure 3: Components of EDFA 191 SHAFEENA P. K.,HARSHA THOMAS, MAHESH KUMAR, SINDHU N

3 These are C and L band. This allows the data signal to stimulate the excited atoms to release photons. Most erbium-doped fiber amplifiers (EDFAs) are pumped by lasers with a wavelength of either 980 nm or 1480 nm. Typical gains are on the order of 25 db.typically noise figure lies between 4-5 db with forward pumping and equivalent figures for backward pumping are 6-7 db assuming 1480 nm pumping light was used. can use an eye diagram visualizer to obtain our eye diagram plot IV. SPECTRAL VARIATIONS IN EDFA A. TIMING JITTER One of the important factor that affects the performance of EDFA are its spectral variation effects. These variations results in the performance degradation of these amplifiers. The main source for these spectral variations are amplifier spontaneous emission noise(ase noise) and propogation effects. Improving spectral efficiency is one of the challenging efforts in optical communication for efficient transmission. So far many studies were done on the reduction of spectral variations. The effect of time jitter is found to be more critical in the optical communication system. Here in this work, the focus is mainly on time jitter associated with EDFA based optical fiber communication. Basically jitter results in shift of optical pulses from their original time slot i.e. it results in short term timing variations of optical pulses from their ideal positions. As a result of jitter the performance of the transmission system gets degraded and causes bit errors in the transmission and it causes the decision point to get shifted away from the optimized position[4] B.SIMULATION SETUP AND WORKS The aim is to find the optimum encoding scheme for a predictable length of optical link with or without cascade of EDFAs for a particular value of jitter.the performance measure is done by obtaining its corresponding eyediagram. From the eye diagram plot we can obtain the value of quality factor and biterror rate for different length of fiber and for different modulation formats. For a particular value of jitter we can obtain an optimum modulation format for a particular length of the fiber. The set up contains a number of components models represented by block which can be categorized into three categories: transmitter, channel and receiver. In this set up we have used Mach-Zehnder type modulator in which the sine function is used instead of the cosine function so that the modulated signal will have the same polarity as the original binary sequence The Optical WDM Multiplexer accepts multiple optical signals at its input ports and produces a WDM optical signal at its output port which includes all the input WDM optical signals. In the receiver section we have the Optical filter which implements a Gaussian transfer function filter having band pass filter synthesis.then we have Compound Optical Receiver which receives the optical input and generates the electrical output signal. From the received output we Figure 4: block diagram setup(with jitter).results Figure 6 and Figure 7 gives the results of the works so far done, in which it is found that for a particular value of jitter included NRZ format have enhanced performance at lower frequencies while at higher frequencies RZ has good performance Figure 6: Eyediagram plot for NRZ format Figure 7: Comparison of RZ and NRZ for 10Gbps and 20Gbps 192 SHAFEENA P. K.,HARSHA THOMAS, MAHESH KUMAR, SINDHU N

4 V. CHANNEL ADDING/DROPPING EFFECTS IN EDFA TRANSIENTS In DWDM optical networks based on wavelength routing, amplifiers may be exposed to transport a variable number of channels, either due to network dynamic reconfigurations or due to the increase of the network capacity. In this scenario, optical amplifiers can present transients which represent a major limitation to the performance of DWDM networks. If some channels are dropped, the power of the surviving channels may surpass the threshold above which the fiber nonlinearities cannot be neglected any longer. If channels are added, the power of the surviving ones diminishes and may fall below the receiver sensitivity. In both cases the performance of these networks can be significantly degraded.[5] The input powers to erbium-doped fibre amplifiers (EDFAs) may vary as slowly as their gain relaxation time in wavelength-division-multiplexed (WDM) networks because wavelength channels which pass through the EDFA can change as a result of network configurations or any other partial failures. Due to this power transients or fluctuations are introduced in surviving channels which results in power transients or fluctuations. All-optical gain-control technique is used to prevent transients in which an EDFA is made to lase at a wavelength different from signal wavelengths, and thus regardless of input signal power level the gain is clamped. Multi-wavelength optical networking (MONET) is a method for communicating digital information using lasers over optical fiber. It also uses EDFAs extensively to limit the effect of attenuation and power splitting in fibers. It provides the next level of communication networks after SONET optical networks with even greater bandwidth capacity. However as we move from the optical networking to the burst and packet switching the EDFA s aren t that steady in operation. Optical burst switching (OBS) is a technique proposed to overcome the shortcomings of deploying conventional wavelengthdivision-multiplexing(wdm) deployment which includes lack of fine bandwidth granularity in wavelength routing and electronic speed bottlenecks in SONET/SDH. In burst and packet switching the data packets are transported to the destination in the form of the bursts.due to this there is great possibility of the occurrence of long inter burst idle intervals which results in transients in erbium doped fiber amplifiers. These transients can to great extent deteriorate the overall network performance.the transient speed in a cascade of EDFAs is proportional to the number of EDFAs in the cascade, making such effects more difficult to suppress, and the transients caused by channel dropping are more severe than those by channel adding. The main application of EDFAs is in wavelength-division multiplexing (WDM) systems, where different independent users transmit data over a single fiber using different wavelengths It is observed that as we increase the number of EDFAs cascaded in the optical link, the transients are significantly suppressed when using a chain of ten EDFAs as compared to chain of six EDFAs. In EDFA, a lasing wavelength filtered from the ASE (i.e., amplified spontaneous emission) spectrum at the amplifier output is feed back into the EDFA. As we change the input power, the power of the lasing wavelength will change correspondingly. When a channel is dropped, the power of the lasing wavelength will increase and when a channel is added, its power will decrease.the total input power into the EDFA is kept almost constant. However, it is provided by simulation results that the EDFA can t completely eliminate transients. On varying number of channels to find the optimized value for the number of channels when the input frequency is 1546 nm, it is found that maximum number of channels with minimum gain tilt of around 0.32 db is 16. As the number is increased the gain tilt increases almost linearly and it is found to be around db when the number of channels. As the number of channels is increased the gain tilt is found to be minimum upto 16 number of channels beyond which the gain tilt increases. The maximum number of channels with minimum gain deviation is found to be 16. Several methods have been proposed through the years for transient control in EDFA. It includes cascading of EDFA, Link Control, Pump Control, etc. A. POWER TRANSIENT IN A SINGLE EDFA The function response of a C-band EDFA to a step excitation at a signal wavelength is simulated. The step function has a period of 1.2 ms, longer enough to enable the amplifier to achieve the steady-state solution. The pump power is constant over the time and is counter propagating with relation to the signal. The basic block diagram is shown in Figure 8. B.RESULTS Figure 8: Block schematic 193 SHAFEENA P. K.,HARSHA THOMAS, MAHESH KUMAR, SINDHU N

5 unacceptably large BER discrepancies between received signals. For some optical channels, complete power extinction can occur at the system output, due to insufficient gain compensation along the amplifier chain. Additionally, the ASE generated in the region of highest gain (i.e., near λ = 1530 nm) in unequalized EDFAs causes homogenous gain saturation, which affects WDM channels at longer wavelengths.[6] Figure 9: Transient effects RESULTS The output power increases as the pump power increases. For a given pump power,the output power increases in initial stage and tends to decrease after the fiber lengthwas optimized and remain almost constant. It is observed that the optimum value of fiber length is between 4m to 6m due to the minimum losses. Figure11: shows the results viewed from a visualizer in the OptiSystem software.it displayed a clear view of the gain flatness for different pump powers (10, 20, 30 and 40mW) when the power (dbm) versus the wavelength (m). The best case for the maximum gain flatness is at 20mW while the power of 40mW represent the worst case as it yield the most unequalized gain. Figure 10: Transient reduction From Figure 9(a) it is clear that when the output of the AM modulator is considered there is no fluctuation in the output power. From Figure 9(b) it is clear that when the output of the EDFA is considered there is fluctuation in the output power with respect to time, called transients. By using cascades of EDFAs, different configuration using AGC/APC and also by using different lengths these transients can be reduced. VI. GAIN FLATTENING IN EDFA In DWDM transmission systems and their related optical networks, one of the key technological issues is the achievement of broad and flat gain bandwidth for Erbium Doped Fiber Amplifiers (EDFA s). 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 Figure 10: Gain flattening in EDFA VII. CONCLUSIONS Here we have presented a reliable optical communication system which considers the effects of power transients and timing jitter that affects in this communication system and also on the gain of this system. Various methods for compensating the effects of timing jitter and transients are proposed and also a method to obtain maximum flattened gain is also proposed. As a future work, we can develop this into an optimal communication system. 194 SHAFEENA P. K.,HARSHA THOMAS, MAHESH KUMAR, SINDHU N

6 REFERENCES [1]P. K. Shafeena and N. Sindhu, Gain flattening in Erbium Doped Fiber Amplifier Based Optical Communication-AReview,IIEEE J Technol, vol. 1,issue 3, pp , March 2013 [2] T. Harsha and N. Sindhu, Study of Techniques to Control Power Transients in Optical WDM Networks- A Review,IIEEE J.Technol, vol. 1, pp , March 2013 [3]R. Maheshkumar and N. Sindhu. " Compensation of spectral loss variations in erbium doped fiber amplifier based optical communication,". IIEEE J.Technol.,vol. 1,issue 3, pp , March [4] Xiang Liu Chris Xu and Xing We. "Comparison of return-to-zero differential phase shift keying and on-off keying in long-haul dispersion managed transmission". IEEE Photonics Technology Letters, vol. 15, no. 4, pp , April [5] Dimitry Gorinevsky, et al.: System Analysis of Power Transients in Advanced WDM Networks, IEEE/OSA Journal of Lightwave Technology, Vol. 22, No. 10, pp , 2004 [6] M. Yamada, T. Kanamori, Y. Terunuma, K. Oikawa, M. Shimizu, S.Sudo, and K. Sagawa, Fluoride-based erbium-doped fiber amplifier with inherently flat gain spectrum, IEEE Photon Technol Letter, vol 8, page , June Harsha Thomas (harshatheresa@gmail.com) received her B.Tech degree in electronics and communication engineering in 2011 from AWH college of Engg. and Technology,Calicut,,Kerala. She did her Post graduation degree in Communication Engineering and Signal Processing from Government Engineering college, Wayanad Kerala in Currently she is working as Assistant professor in department of ECE in Sahrdaya Engineering College, Trissur, Kerala. She has authored a paper which has been accepted fornational conference (CISP 2013) and also authored a paper on EDFA which was published in an international journal.her current areas of interest include Electronic circuit analysis, Signal processing and Optical Communication.. MaheshKumar R (trismahesh@gmail.com) received his B.Tech degree in electronics and communication engineering in 2010 from College of Engineering, Attingal, Thiruvananthapuram,Kerala He did his Post graduation degree in Communication Engineering and Signal Processing from Government Engineering college, Wayanad Kerala in Currently he is working as Assistant professor in department of ECE in Government Engineering College, Trivandrum, Kerala. He has authored a paper which has been accepted for national conference (CISP 2013) and also authored a paper on EDFA which was published in an international journal.his current areas of interest include Electronics and Electronics network analysis,signal processing and Optical Communication. Shafeena P. K. ( shafeenapk@gmail.com) received her B.Tech degree in electronics and communication engineering in 2011 from Government Engg. College, Wayanad Kerala. She did her Post graduation degree in Communication Engineering and Signal Processing from Government Engineering college, Wayanad Kerala in Currently she is working as Assistant professor in department of ECE in Calicut University Engineering College,Calicut, Kerala. She has authored a paper which has been accepted for national conference (CISP 2013) and also authored a paper on EDFA which was published in an international journal.her current areas of interest include Electronics and Electrical Devices analysis,,signal processing, Communication systems and Optical Communication. N. SINDHU (sindhun@gecwyd.ac.in) received BTech from NSS College of Engg.,Palakkad in 1996,MTech with honours from National Institute of Technology Karnataka in 2004, and currently working towards PhD at Kerala University. After graduation she joined LBS College of Engineering as Lecturer. In 2004 she joined Govt.Engineering College Sreekrishnapuram, Palakkad. She was Assistant Professor from 2006 and joined at Govt.Engineering College Wayanad in Currently she is working as assistant professor in Govt.Engineering College,Bartonhill Trivandrum.She was,incharge of the Innovation centre under Centre for Engineering Research &Development Satellite Centre at GECW,holds the charge as Placement Officer, she is Life member of ISTE. She is the principal Investigator of two projects sponsored by Centre for Research &Development,Thiruvananthapuram,Kerala She has authored and coauthored several papers in National Conferences and International Journals. Her Research interests include Optical Amplifiers especially, Gain Flattening, Controlling Power transients,compensating spectral loss variations etc. in EDFA s. 195 SHAFEENA P. K.,HARSHA THOMAS, MAHESH KUMAR, SINDHU N