I. ABSTRACT There is an increasing demand of transmission bandwidth for long haulwavelength division multiplexed optical fibre communication systems. Bandwidthis well used by using broadband and gain flattened amplifiers.
Broadbandamplification is done by combining many amplifiers having totally differentgain bandwidths. Totally different gain flattening techniques are offered forgain flattening functions to scale back gain variation like gain flatteningfilters as fiber Bragg gratings, improvement of fabric composition of fiberamplifiers, using rare earth doped ions, hybrid amplifiers, wavelengthsplitters.The hybrid optical amplifier composed of single erbium doped fiberamplifier and a Raman amplifier is proposed for wavelength division multiplexedsystem and also investigates the impact of reduced channel spacing. Theperformance has been evaluated in terms of gain and noise figure. Gain andnoise figure are important characteristics of the optical amplifier to evaluatethe performance. There are various parameters on which the gain spectrum ofEDFA and Raman amplifier depends.
The effort has been made to optimize theseparameters by simulating the system of 16 channels in Opti-system software. Inthis paper the length, pumping wavelength, pumping power of EDFA/Raman isoptimized. After the analysis the reduction in gain is about 2.7dB and thenoise figure is about6.
86dB. II. INTRODUCTION The gain flattening of Erbium-doped fibre amplifiers (EDFA) has been aresearch issue in recent years, with the development of high capabilitywavelength division multiplexing (WDM) optical communication systems. Forsingle channel systems, the gain variation isn’t a problem.
However, becausethe variety of channels will increase, the transmission downside arises as aresult of a traditional EDFA has intrinsic non-uniform gain. They usually giftgain peaking at concerning 1530 nm and therefore the helpful gain informationmeasure could also be reduced to but ten nm. The gain of EDFAs depends onseveral device parameters like erbium-ion concentration, amplifier length, andcore radius and pump power. To extend the gain-bandwidth of an amplified lightwave system many ways is used, however equalising optical filters operating asspectrally selective loss elements seem to be the most effective candidates.
This paper primarily focuses on totally different ways used for gain flatteningin EDFA. EDFAs are widelyused in comparison to other similar amplifiers and optical devices for largerwavelength optical communication system because of its advantages of high gain,large bandwidth, less noise figure (NF), polarization insensitivity, low pump powersand better performance. One difficulty in implementing a WDM system includingEDFA’s is that the EDFA gain spectrum is wavelength dependent and it doesn’tnecessarily amplify the wavelength of the channels equally. Bandwidth of EDFAcan be increased to more than 30nm with appropriate gain flattening in thethird transmission window for future development of WDM long-haul optical fibercommunication systems. Moreover, the noise performance of EDFA is characterizedby its NF which is signal to noise ratio reduction ratio of input to output ofthe amplifier and should have a low value.
Gain is the main characteristic toevaluate the performance of an optical amplifier. The gain spectrum must beuniform for the long distance transmission, but EDFA cannot amplify all thewavelengths equally 4. Hence the gain flatness is important for EDFA’swavelength division multiplexing (WDM) which is important technique for longhaul optical transmission link system. Different compensation methods werestudied in past and based on these methods efforts were made to increase thegain flatness of EDFA.
Methods such as hybrid Al-co doped &Al/P co dopedEDFAs 1, hybrid EDFA/RFA amplifiers, erbium doped waveguide amplifiers(EDWA) having gain flattened and gain clamped functions simultaneously 4, theusage of the gain flattening filters (GFF) such as, thin film dielectricfilters, sinusoidal filters 4, chirped fiber brag gratings 5, acousto-optictunable filter 6. The combination of distributed Raman amplifier and EDFApresent better performance than conventional EDFA 1. III.
LITERATURESURVEY Wideband and gain flattened optical amplifiers are crucial for long hauloptical communication systems. Gain flattening of EDFA is done by numeroustechniques. In initial approach, 2 stages of EDFA are used. a brief fiberlength C-band EDFA produces ASE that, in turn, with pump power enters thesecond stage to extend the output gain in long length L-band EDFA. 2 fiber braggratings (FBG) are placed within the second section at each ends of long fiberto form fabric part cavity. The gain in L-band is flattened by the compressedgain below the matched wavelength of FBG In another approach, hybrid amplifiersare used. Fiber to amplifier couplers will work severally of others and needed varietyof amplifiers is added in step with the demand. A double pass assembly of EDFAwith hybrid gain medium- silicon and oxide is employed in parallelconfiguration.
Chirp fiber brag grating is employed in each stage to realizethe desired gain in Cband and L- band regions it causes the double propagationof the signal leading to overall enhanced gain however noise figureadditionally will increase that is that the major downside. Hybrid electronicequipment is EDFA–Raman amplifier. Raman amplifier is distributed and distinct.The distinct Raman electronic equipment is employed in spite of style oftransmission fiber and its main purpose is to expand the usable bandwidth ofthe fiber. Then again, the distributed Raman amplifier (DRA) improves the reachof fiber span. The impact of DRA is that it decreases the noise similarlybecause the non linearities. EDFA + DRA is wont to increase the gain similarlyas decrease the noise figure. EDFA includes a larger gain variation withrelevance the wavelength.
Raman amplifier includes a smaller gain variationcompared to EDFA. This property is wont to kind a hybrid optical amplifier(HOA). By increasing the quantity of pumps, the gain of HOA is enhanced .Butthere’s a limit to the rise in variety of pumps otherwise gain can decrease.
Byadjusting the pump wavelengths, pump power and length of fiber, amplificationand gain flattening is drained totally different wavelength regions. Erbium andytterbium co-doped phosphate (Er/ Yb–EDFA) and Raman phosphate fiber amplifieris used as hybrid amplifier and has the advantage of upper transmissioncapability than silicon based mostly fiber amplifiers. Phosphate glass haslarger phonon energy, greater solubility to rare earth ions, massive emissioncross section and better energy transfer efficiency. This hybrid amplifier isused for higher transmission capability on dense wavelength divisionmultiplexed systems.In another technique, Er doped wave guide Amplifier (EDWA) and Er dopedfiber amplifier is connected asynchronous to realize the gain flatness withinthe C- band. EDWA is a smaller amount economical than EDFA due to higher erbiumconcentration. Higher erbium concentration needs a lot of pumping power .Thereis loss of wave guide within the fiber.
EDWA provides high gain briefly opticalpath. High gain and gain flatness is obtained by this technique howeverbroadband amplification can’t be obtained. IV. PROPOSED WORK1.Current scenarioErbiumdoped fibre amplifiers have had a significant impact within the field of lightwave communications. Optical amplifiers have contributed to the expansion of afifth generation of optical communication systems. However because the demandson the networks accrued techniques like Dense Wavelength Division Multiplexing(DWDM) were developed. The performance of DWDM in ultra-long haul networksimproved as a result of amplifiers were accessible that might amplify thewavelengths utilized in the network while not requiring any conversion of theoptical signal to the electrical signal.
The importance of EDFA’s is because oftheir compatibility with the fibre network, low insertion loss, polarizationunfitness, high gain levels and close to quantum restricted noise performance.In DWDM transmission systems and their connected optical networks, one in allthe key technological problems is that the achievement of broad and flat gainbandwidth for erbium Doped Fiber Amplifiers (EDFA’s). Gain variations occurbetween optical channels having massive wavelength spacing (e.
g. ??> 1nm).In long electronic equipment chains, even little spectral gain variations (e.g.?G < 0.75 dB) may result in massive variations within the received signalpower, inflicting intolerably massive BER discrepancies between receivedsignals. for some optical channels, complete power extinction will occur at thesystem output, as a result of depleted gain compensation along the amplifierchain.
in addition, the ASE generated within the region of highest gain (i.e.,near ? = 1531 nm) in un equalised EDFAs causes solid gain saturation, thataffects WDM channels at longer wavelengths.2.Proposed Technology solutionTheincreasing demand for bandwidth in today’s backbone networks makes itprogressively necessary to increase the effective transmission bandwidth of thedeployed fibres on the far side the widely-used C-band (1530 nm to 1610nm).because the transmission bandwidth is especially restricted by today’samplifier-of alternative, the EDFA, new ways in which of extending theamplifier bandwidth should be pursued. As an example, hybrid combinations ofEDFA and Raman electronic equipment are wont to extend the seamless bandwidthof distinct amplifiers up to 80nm, a hundred nm and so on.
In general, thecombination of quite one optical amplifier in any configuration is termedhybrid optical amplifier (HOA). EDFA amplifies the signal however the gainspectrum isn’t uniform. to scale back the gain variations hybrid combination ofEDFA and Raman amplifier is the most suitable option.V.METHODOLOGYVarioussoftwares needed for implementation of project are as follows:OpticalSpectrum analyzer- An Optical Spectrum analyzer (or OSA) could be a precisioninstrument designed to measure and displays the distribution of power of anoptical supply over a mere wavelength span. Associate degree OSA tracesdisplays power within the vertical scale and therefore the wavelength withinthe horizontal scale. The increasing field of optics connected applications hascreated a massive style of industries and organizations that need advancedoptical spectral measurements for both R and producing. These industriesembrace telecommunications, consumer electronics, healthcare, lifescience/medical analysis, security, sensing, microscopy, and gas/chemicalanalysis, and environmental monitoring.
Simulation Model-The basic configuration consists of 16 channels with 50GHz channel spacing, thesystem has a WDM transmitter with first channel is at 193.5THz and increases asthe number of channels increases, the input power of WDM transmitter is 20dBm.All the channels are transmitted into the WDM multiplexer with zero insertionloss, here all the light signals are combined and transmitted over the erbiumdoped fiber of length 9m.The erbium doped fiber amplifier is counter pumped at1480nm.The counter pumping scheme gives more gain than that of co-propagatingpumping scheme.
The pump power at which the EDFA pumped is 130Mw.The output ofEDFA is then passed through the optical isolator. Optical Isolators are used toprotect a source from back reflections or signals that may occur after theisolator. The output after the isolator then fed into the Raman amplifier oflength 11m which is counter pumped at 1450, 1452, 1454, 1456nm with constantpump power of 450mW.The over all amplified signal is fed into the opticalspectrum analyzer to analyze the optical spectrum. The dual port WDM analyzeris placed after the Raman amplifier which gives the values of gain and noisefigure. The gain variations and the noise figure variations are noted down fordifferent frequencies. At the end, it passes through the photo detector toconvert the optical signal into electrical signal to note down the bit errorrate value.
The bit error value should be less than 10?6 dB. The simulationsetup of hybrid optical is shown in the given figure 1.Fig.1 Schematic ofHybrid optical amplifier of 16 channelsFigure2 shows the results viewed from spectrum analyzer in the Opti-system software.These are power spectrum of Erbium doped fiber amplifier and the Ramanamplifier when observed as inline amplifier.Fig.2 Outputpower spectrum (red) noise spectrum (green) forEDFAFig.
3 Outputpower spectrum (red) noise spectrum (green) forRamanAfterthe analysis, RFA as an inline amplifier at pump power of 200mW provides lessergain variations than the EDFA. Moreover the gain response of RFA shows oppositecharacteristics than EDFA and hence this property of RFA can be exploited forgain flattening. As the HOA is placed in the network system with reducedchannel spacing, the different gain and noise figure is recorded at every step.
It is found that the gain of HOA is almost flat after the observation. Themaximum gain is achieved after the hybrid optical amplifier. It means that thecombination of EDFA and Raman is responsible to increase the gain.
Toillustrate the performance of this hybrid optical amplifier, gain and noisefigure is recorded after HOA incorporated. Figure 6 shows gain as a function offrequency. The overall gain for given frequency band is increased from 12.41dBto 16.21dB. Figure 7 shows the characteristic in terms of noise figure as afunction of input signal frequency.
The overall noise figure is maintainedbelow 6dB.MATLAB7.1- The hybrid configuration has properties of each the amplifiers thewavelength used for optimizing ranges from 1569 nm -1577nm that lies within thec band of optical communication to get uniform gain with less noise figure.There are varied parameters on that gain of optical amplifier depends, byoptimizing these parameters in prefer system software the maximum value of gainwill be obtained.
VI. BLOCK DIAGRAM Fig.4 Schematic Diagramof EDFAVII. CONCLUSIONTheerbium doped fibre amplifier has high gain and may be made appropriate foroptical transmission systems for long haul communications by using acceptablegain flattening technique. The system of16-channels are going to be designed within the range of 1532-1547nm, that liesin C-band. Within the hybrid configuration of EDFA/Raman, it’s doable to scaleback gain variations by optimizing erbium fibre length that is 8m within theprojected model and correct selecting of pump wavelengths and injected pumppowers to Raman fibre amplifier. By applying this gain equalization technique,the hybrid optical amplifier has gain worth of 15 dB. The gain variations arereduced to two.
7dB and noise figure is obtained below 6 dB. There are numeroustechniques used for gain flattening of EDFA. Each technique has its own meritsand demerits. EDFA in C-band (1520-1570 nm) incorporates a higher gain thanEDFA in L-band (1570- 1620 nm). Raman amplifier encompasses a lower noisefigure, wide gain bandwidth and flexibility on choice of gain medium. A C+ Lband EDFA and Raman amplifier (hybrid amplifier) are often wont to increase thegeneral gain and scale back the noise figure at the same time.
Hybrid amplifieris that the best technique because it reduces the non linearities, willincrease the general gain, reduces the noise figure, price effective anddoesn’t have any flexibility problems. By using hybrid amplifiers, broadbandamplification will be done in specific wavelength regions.VIII. RESULT The finest amplification span length is one thatfacilitates the best trade-off between the low-cost requirements and stringentsystem performance. Long amplifier spans result in high input powers tomaintain a good optical signal-to-noise ratio (OSNR), leading to increasedeffects of nonlinearities. In such situation a best balance between the highoptical signal-to-noise ratio and nonlinear impairments is necessary. Thesolution of this problem is use of distributed Raman amplification (DRA). Ascompared to Erbium doped fiber amplification scheme, DRA improves significantlythe link’s OSNR.
Distributed Raman amplifier in combination with Erbium-dopedfiber amplifier termed as, hybrid fiber amplifier, can be used for bettercontrol of nonlinear effects. In our system reverse dispersion fiber (RDF) isused instead of dispersion compensating fiber (DCF). InDCF based Raman amplification systems pump powerefficiency is very low and a significant amount of pump power is unused andwasted. This can be attributed to strong nonlinear effects in dispersioncompensating fibers. A distributed hybrid Raman/Erbium-doped fiber amplifier issimulated and optimized in 1530 nm to 1565 nm wavelength range.
A 60 km (30 kmSMF+30 km RDF) transmission fiber is pumped by a backwardRaman pumping unit (BRPU). This unit consists of pumpsat wavelengths 1440 nm and 1450 nm with pump powers 120 mW and 60 mWrespectively.An Erbium-doped fiber of length 8 m is forward pumpedby 980 nm laser diode of pump power 12mW. Forward pumping in Erbium gain medium and backwardpumping in Raman amplifiers give better conversion efficiency and noise figure.Almost flat gain of 21 dB with gain tilt ±0.
4 dB is obtained over entirewavelength range. The noise figure is well below 7 dB. The Noise figure isslightly high in lower signal wavelength region because of allocation of higherpump energy in this range. Thermal instabilities, pump-to-pump Ramaninteractions and power fluctuation in pumps result in more noise figure inlower signal wavelength region.
Fig.5 Gain and noise figure of hybridRaman/Erbium-doped fiber amplifier (HFA) for different signal wavelengths. Fig.
6 Flattened gainresponse of gain v/s frequency Fig.7Flattened gain response of frequency v/s noise figure The erbium doped fiberamplifier has very high gain and can be made suitable for optical transmissionsystems for long haul communications by employing appropriate gain flatteningtechnique. In this paper a system of 16-channels was designed in the range of1532-1547nm, which lies in C-band. In the hybrid configuration of EDFA/Raman,it is possible to reduce gain variations by optimizing erbium fiber lengthwhich is 8m in the proposed model and proper choosing of pump wavelengths andinjected pump powers to Raman fiber amplifier. By applying this gainequalization technique, the hybrid optical amplifier has gain value of 15 dB.
The gain variations are reduced to 2.7dB and noise figure is obtained below 6dB. In future more the 16 channels can be transmitted with reduced channelspacing by incorporating different co doping concentration, multiple pumpingschemes, where the pump wavelengths and pump powers can be chosen carefully toensure a good performance.