Wideband Gain Flattened Hybrid Erbium-doped Fiber Amplifier/Fiber Raman Amplifier

Transcription

1 Journal of the Optcal Socety of Korea Vol. 14, No. 4, December 2010, pp DOI: /JOSK Wdeband Gan Flattened Hybrd Erbum-doped Fber Amplfer/Fber Raman Amplfer Hossen Afkham Islamc Azad Unversty, Sepdan Branch, Sepdan, Shraz, Iran Alreza Mowla and Nosrat Granpayeh* Faculty of Electrcal and Computer Engneerng, K. N. Toos Unversty of Technology, Tehran , Iran Azadeh Rastegar Hormoz Islamc Azad Unversty, Arsanjan Branch, Arsanjan, Shraz, Iran (Receved September 29, 2010 : revsed November 29, 2010 : accepted November 29, 2010) An optmal wdeband gan flattened hybrd erbum-doped fber amplfer/fber Raman amplfer (EDFA/FRA) has been ntroduced. A new and effectve optmzaton method called partcle swarm optmzaton (PSO) s employed to fnd the optmzed parameters of the EDFA/FRA. Numerous parameters whch are the parameters of the erbum-doped fber amplfer (EDFA) and the fber Raman amplfer (FRA) defne the gan spectrum of a hybrd EDFA/FRA. Here, we optmze the length, Er 3+ concentraton, and pump power and wavelength of the EDFA and also pump powers and wavelengths of the FRA to obtan the flattest operatng gan spectrum. Hybrd EDFA/FRA wth 6-pumped- and 10-pumped-FRAs have been studed. Gan spectrum varatons are and db for the 6-pumped- and 10-pumped-FRAs, respectvely, n the km hybrd EDFA/FRAs, wth 1 mw of nput sgnal powers. Dense wavelength dvson multplexng (DWDM) system wth 60 sgnal channels n the wavelength range of nm,.e. the wde bandwdth of 98 nm, s studed. In ths work, we have added FRA s pump wavelengths to the optmzaton parameters to obtan better results n comparson wth the results presented n our prevous works. Keywords : Hybrd fber amplfer, Erbum-doped fber amplfer, Fber Raman amplfer, Optmzaton, Partcle swarm optmzaton OCIS codes : ( ) Fber optcs amplfers and oscllators; ( ) Optcal amplfers; ( ) Fber optcs and optcal communcatons; ( ) Fbers, erbum I. INTRODUCTION After the nventon of EDFAs, data transfer capactes of optcal communcaton systems exploded based on wavelength dvson multplexng (WDM) systems [1]. Then more demands for hgher capacty systems, n addton to the progress n optcal devce technology, encouraged the researchers to focus on the mult-wavelength pump FRAs whch have broader bandwdths [2]. Hybrd EDFA/FRAs could have the benefts of both EDFAs and FRAs, hence some efforts have been carred on to offer effcent constructons for hybrd EDFA/FRAs [3]. Hybrd EDFA/FRAs have long lengths and consequently lower numbers of lnk repeaters are needed. To buld these hybrd amplfers, low prce EDFAs are used. Ther costs are relatvely lower than FRAs and ther nose fgures (NFs) are lower than those of the EDFAs [2]. A WDM system wth hybrd EDFA/ FRAs has lots of elements and the combnaton of these elements and the value of ther parameters are of great mportance. The module combnaton s the art of engneerng work. But choosng the module parameters could be dffcult, because these parameters affect the characterstcs of the hybrd EDFA/FRAs. Here, a global optmzaton method (GOM) s qute necessary. GOMs help us to fnd the optmum *Correspondng author: granpayeh@eetd.kntu.ac.r Color versons of one or more of the fgures n ths paper are avalable onlne

2 Wdeband Gan Flattened Hybrd Erbum-doped Fber Amplfer/Fber Raman Amplfer - Hossen Afkham et al. 343 values for parameters of the modules. PSO s a GOM that s effectve and fast and can deal wth a hgh number of varables [4, 5]. Usng PSO, the parameters of the EDFA and FRA used n hybrd EDFA/FRA have been optmzed to obtan a wdeband gan flattened amplfer. In ths paper, hybrd EDFA/FRA confguratons are ntroduced, the method of smulaton of hybrd EDFA/FRA s dscussed and then PSO s presented and the results of optmzaton are gven for dfferent pumpng schemes and confguratons. In addton to optmzaton of the length, Er 3+ concentraton, and pump power and wavelength of the EDFA and also pump powers of the FRA all of whch have been carred on n our prevous work [6], the pump wavelengths of the FRA are optmzed n order to have a flatter-gan hybrd EDFA/FRA. Addng the pump wavelengths of the FRA to the other optmzaton varables ncreases the number of optmzaton varables to 24 n the 10 pumped FRA hybrd EDFA/FRA. Dealng wth ths hgh number of varables s qute a challenge. Here PSO shows ts strength to manpulate a large number of varables n a fast and straghtforward manner n comparson wth the other optmzaton methods such as genetc algorthm (GA) [7-14]. The results are dscussed n secton V. II. HYBRID EDFA/FRA CONFIGURATION FIG. 1 depcts the confguraton whch we have chosen for the hybrd EDFA/FRA. There are 60 sgnal channels wth frequences between and THz, placed equally n 200 GHz of frequency dstances launched to the sngle mode fber (SMF). SMF allows the propagaton of only one mode, retans the fdelty of each lght ray and has hgher bandwdths over longer lnks. TABLE 1 presents the characterstcs of the desgned WDM system. If we utlze communcaton systems wth the rate of 40 Gbt/s at each of these channels, the capacty of ths WDM lnk could reach 2.4 Tbt/s. Choosng the values of the nput sgnal powers s of great mportance and wll affect the gan spectrum varatons of the amplfer. We select the nput sgnal powers to be 1 or 10 mw to have a comparson between the results. Here, SMF acts as the transmsson medum. A dsperson compensatng fber (DCF) s located after the SMF to compensate the dsperson that took place n the SMF. DCF has a hgh negatve value of dsperson whch s supposed to neutralze the SMF lower postve value of dsperson. So, shorter length of DCF compared to the length of SMF s needed and the length of DCF must be chosen approprately. Afterward, a dsperson shfted fber (DSF) s placed whch has zero dsperson at 1550 nm wavelength and s pumped by 6 or 10 backward pump LD sources whch acts as an FRA. FRA pump powers are propagated counter to the drecton of the sgnal powers. In ths paper, we have chosen 2 dfferent sets of fber lengths. Accordngly, SMF, DCF, and DSF could be 50, 8.5, and 50 km n the frst confguraton and 80, 14, and 80 km n the second confguraton, respectvely. An solator (ISO) s lad between the DCF and DSF to suppress the resdue of the FRA backward pumps and nose, rased n the DSF. A WDM coupler s set up to couple the power of pumps to the DSF n the counter-propagatng drecton. Pump powers and wavelengths are chosen through the optmzaton process dscussed n secton IV. Next module s a C-band EDFA. Prncpal range of amplfcaton n the C-band EDFA s nm and ts gan falls at wavelengths over 1570 nm. On the other hand, the amplfcaton at the hgher range of takes place mostly through the DSF as an FRA. TABLE 2 lsts the characterstcs of dfferent modules used n our hybrd EDFA/FRA confguratons. Lots of varables exst n the desgn of EDFA. In ths paper we have chosen 4 of the most mportant varables to be optmzed, the length, the Er 3+ concentraton, the pump power and wavelength of the EDFA. Fnally, we have a detector at the end of the lnk span to receve the output powers. Gan spectrum varatons wll be computed at ths pont. Ths span can be repeated f longer lnks are needed. As ndcated, four dfferent confguratons have been proposed. These confguratons are or 174 km n length and each could adopt 6- or 10-pumped-FRA. Addtonally, the amplfer characterstcs are calculated n the cases of dfferent nput sgnal powers of 1 or 10 mw for each channel. As dscussed n secton V, gan spectrum varatons of db can be acheved for the 174 km lnk span, 10-pumped-FRA hybrd EDFA/FRA wth 1 mw of nput sgnal power. TABLE 1. Characterstcs of the WDM transmsson system FIG. 1. Confguratons of the hybrd EDFA/FRA wth 6- or 10-pumped-FRA n backward drecton. WDM characterstc Value Number of sgnal channels 60 Sgnal wavelength range nm Sgnal channels start frequency THz Sgnal channels end frequency THz Sgnal channels frequency space 0.2 THz Raman pumps wavelength range nm

3 344 Journal of the Optcal Socety of Korea, Vol. 14, No. 4, December 2010 TABLE 2. Characterstcs of hybrd EDFA/FRA fber modules Module Fber type Attenuaton (db/km) Dsperson (ps/(nm.km)) Length Transmsson SMF FIG. 2(c) [24] 16.5 [2] 50 and 80 km Compensatng DCF FIG. 2(c) [24] -95 [24] 8.5 and 14 km FRA DSF FIG. 2(c) [2] 0 50 and 80 km EDFA SMF FIG. 2(c) [24] 16.5 Varable (1 to 10 m) TABLE 3. EDFA and FRA parameters used n the hybrd amplfer modelng FRA parameter Value EDFA parameter Value Raylegh backscatterng (m -1 ) Upper level lfe tme 10 ms Temperature T ( K) 300 Claddng ndex Number of pumps 6 or 10 Fber core radus 6.5 µm FRA Pumps drecton Backward Excess fber loss 0.03 db/m III. HYBRID EDFA/FRA SIMULATION Smulaton of EDFA and FRA should be done separately. Also, attenuaton, dsperson and nose of the other components such as SMF, DCF, DSF, erbum-doped fber (EDF), and ISO must be taken nto account. Therefore, the smulaton of hybrd EDFA/FRA has to be a combnaton of smulatons of dfferent modules and components of the hybrd amplfer. In the followng sectons we dscuss the smulaton of EDFAs and FRAs separately. 1. Smulaton of the EDFA: Gles model [15] s the most common model used for the optcal amplfcaton that occurs n a slca fber doped by Er 3+. We assume a two-level, homogeneously broadened model for Er 3+ that s suffcent to have a smple and accurate EDFA model [15, 16]. In ths model 4 I 15/2 s the ground level and 4 I 13/2 s the excted level. We suppose that all the radatve transtons occur between these two levels. Computer analyss and smulaton s dscrete, so we should break the contnuous spectrum of the optcal waves nto dscrete optcal beams wth frequency bandwdth of Δν k and optcal power of P k centered at frequency ν k or wavelength λ k. We should solve an ordnary dfferental equaton for each one of the optcal beams to fnd the propagaton of optcal powers along the fber. Therefore, N ordnary dfferental equatons should be solved, where N s the number of the pump and sgnal channels. Rate equatons of the excted level of Er 3+ ons are computed as a summaton of the emsson and absorpton contrbutons of all the N optcal beams [17]. We have used Al/P-slca fber as the EDFA, the emsson and absorpton cross sectons of whch are depcted n FIG. 2(a) [18]. All fbers are assumed to be SMFs, hence the V number should be less than Also we have assumed the steady state condton of where N 2 s the upper level populaton, and also the weakly gudng approxmaton of << 1 where n 1 and n 2 are the refractve ndces of the fber core and claddng, respectvely. The flow of sgnal and pump power through EDFA s formulated by ordnary dfferental equatons [17]. Rate equatons n the steady-state condton have to be solved to acqure the metastable level populaton of Er 3+ ons [17]. Snce the EDFA pump s n the co-propagatng drecton, we need to solve an ntal value problem. We gnore the counter propagatng pumps and elmnate the backward amplfed spontaneous emsson (ASE) from our smulaton to avod solvng the boundary value problem whch s a bt more challengng. In ths paper, a one-step predctorcorrector method [19] s used to smulate the EDFA. Usng ths method, the propagated powers at channels are calculated step by step usng ther values at the prevous steps and ther predcted values at the next steps [20]. The pumpng wavelength s around 1480 nm. Step ndex fber wth unform dstrbuton of Er 3+ ons are used n ths smulaton. EDFA characterstcs whch have been used are lsted n TABLE Smulaton of the FRA: Coupled ordnary dfferental equatons govern the propagaton of the average optcal power of pumps, sgnals, ASEs, and Raylegh backscatterng beams n a FRA [19, 21]. For co-propagatng pumps, an ntal value problem must be solved and for the case of counter-propagatng or bdrectonal pumps, two-pont boundary value problems must be solved. Here, we have counterpropagatng pumps, hence a two-pont boundary value problem has to be solved. Four-step predctor-corrector method based on the Adams-Bashforth-Moulton formula [19] and shootng algorthm [22] are used to solve ths problem numercally [23]. FIG. 2(b) shows the Raman gan effcency of the DSF [2] and FIG. 2(c) shows attenuaton spectra of the DSF [2], SMF, and DCF [24] used n our smulaton. We have used C ++ programmng to model the hybrd EDFA/ FRA. The tme needed to accomplsh the smulaton, depends

4 Wdeband Gan Flattened Hybrd Erbum-doped Fber Amplfer/Fber Raman Amplfer - Hossen Afkham et al. 345 (a) whch are bunches of varables that present canddate solutons, wll be selected randomly from the possble soluton space and these partcles wll move accordng to ther socal learnng from each other toward the optmum soluton. The rules and constrants whch lead the flyng partcles n the search space toward the global optmzed soluton are orgnated from the behavoral models of brd flocks [4, 5]. Ths method was ntroduced and developed by Kennedy and Eberhart [25, 26]. Let us assume that F s the ftness functon. There are n partcles P where = 1,, n, x s the current poston of P, Gb s the global best whch s the best ftness value that has ever happened, Pb s the personal best of P whch s the best prevous ftness value of P, x Gb and x Pb are the postons of the Gb and Pb, respectvely, v s the velocty of P and k s the tme nterval. We can present PSO method n the followng steps: Step 1. P s are randomly chosen from the search space to form the swarm P(k). Step 2. Calculate the ftness values F(x (k)) for each P. Step 3. If F(x (k))>pb, then Pb =F(x (k)) and x Pb=x (k). Step 4. If F(x (k)) max>gb, then Gb=F(x (k)) max and x Gb=x (k). Step 5. Set the new veloctes (b) υ ( k) = ϕυ ( k 1) + ρ1c ( x f Pb x ( k)) + ρ 2C ( x x ( k)) f Gb (1) 1 k ϕ = (2) 1 k max (c) FIG. 2. Characterstcs of the fbers (a) emsson and absorpton cross sectons of Er +3 doped n Al/P-slca SMF (b) Raman gan effcency of DSF (c) DCF, SMF, and DSF attenuaton spectrum. [2, 23, 24] on the confguraton of the setup. It could be 0.2 to 1 second when run by an AMD Athlon 2.71 GHz CPU wth 1.00 Gb of RAM. IV. PARTICLE SWARM OPTIMIZATION IMPLEMENTATION PSO s a populaton-based stochastc method for solvng optmzaton problems whch s classfed as a swarm ntellgence technque. In ths method, a populaton of partcles where φ s the nerta weght, k s the teraton number and k max=300 s the total number of teratons, C f =2 s the accelerator constant. ρ 1 and ρ 2 are unform random values between 0 and 1. Step 6. Set the postons x ( k) = x ( k 1) + υ ( k) ( k = k + 1) (3) Step 7. Check the convergence crteron, f fulflled, the optmzaton s over, f not go to step 2. To perform the optmzaton, PSO acts as the man program and the smulaton of the hybrd EDFA/FRA wll be the subprogram. Ftness functon s the maxmum sgnal network gan devaton from zero whch s supposed to be mnmum, formulated n the followng: ( L) ( 0) k Ps F = 10 log ft Max k (4) Ps where P s k (0) and P s k (L) are the k-th nput and output sgnal powers, respectvely. Takng nto account the desgn

5 346 Journal of the Optcal Socety of Korea, Vol. 14, No. 4, December 2010 TABLE 4. Varables of the optmzaton of the EDFA/FRAs and ther ranges Varable to be optmzed Range EDFA pumpng wavelength p nm EDFA Er 3+ concentraton n t (1-30) EDFA fber length L EDFA pump power FRA pump wavelengths FRA pump powers 1-10 m mw nm mw crtera, varables of optmzaton and ther ranges n ths work have been lsted n TABLE 4. As t was mentoned, we used C ++ programmng to smulate the hybrd EDFA/ FRA and execute the optmzaton process. The necessary tme to accomplsh one round of optmzaton depends on the confguraton whch s to be optmzed and could be between 2 and 3 hours, by an AMD Athlon 2.71 GHz CPU wth 1.00 Gb of RAM. The number of partcles and teratons n our PSO model are 40 and 300, respectvely. V. RESULTS AND DISCUSSIONS In ths paper we have smulated 4 dfferent confguratons for hybrd EDFA/FRA. Two confguratons for km and two for 174 km lnk wth 6- and 10-pumped-FRA. Also, we survey the results for both nput sgnal powers of 1 or 10 mw. A group of optmzaton varables or the group of smulaton nputs whch are ntroduced n TABLE 4 are known as a partcle P n the PSO. We have n=40 partcles n our optmzaton process and t means that we have 40 groups of optmzaton varables whch are randomly spread n our optmzaton space. Each one of the optmzaton varables makes a dmenson of the optmzaton space. Partcle postons x are arrays whch are composed of the optmzaton varable values. We also have n=40 partcle postons x n our optmzaton process that need to be optmzed. At each teraton, we nsert the nputs whch are the optmzaton varable values or the partcle postons to the man hybrd EDFA/FRA smulaton computer program or ftness functon. Then the smulaton wll result n the output or the ftness value F(x ) whch s the maxmum sgnal network gan devaton from zero. We consder the devaton from zero to take nto account both the negatve and postve gan values. Accordng to the PSO steps whch are ntroduced n the prevous secton, new values for optmzaton varables are selected at the end of each teraton. We have 24 and 16 optmzaton varables for 10- and 6- pumped FRA hybrd amplfers. So, the partcle postons are composed of 24 or 16 elements. To reach the flattest gan spectrum for the hybrd EDFA/FRA, t s better to set the EDFA pump wavelength to have a C-band EDFA. Because of the shape of the EDFA gan spectrum, t s preferred to have amplfcaton at lower wavelengths by EDFA and at hgher wavelengths by FRA. In the optmzaton process, the EDFA pump power s nclned to attan hgh values. So, t s better to choose 1480 nm pumpng wavelength for the EDFA, although t has hgher nose compared to 980 nm ones. Also, n hgh power schemes t s more effcent to use 1480 nm pumpng wavelength. The radus of the Er 3+ doped area at EDFA s equal to the radus of the fber used as EDFA that s 6.5 µm. An EDFA wth a small core radus wll result n better gan spectrum characterstcs, but ts fabrcaton s hard and t s more susceptble to damage n facng hgh EDFA pump power. Also a small core radus EDFA s rare n the market. Er 3+ on concentraton has to be chosen n a confned area, snce low Er 3+ concentraton would elongate the EDFA and consequently create hgher varaton of the gan spectrum. Also, hgh Er 3+ concentraton wll ncrease unfavorable ntra- on nteractons [1]. The man source of nose of EDFAs s ASE. The level of nose n FRAs s lower than that n EDFAs. In ths paper, we have assumed an acceptably low level for nose, and to speed up the optmzaton process, we have not smulated noses. Lower nput sgnal power n EDFAs wll result n hgher and flatter gan spectra, but nput sgnal power should not be so low that t could not be retreved from the nose. The desgned amplfer s a dstrbuted amplfer, and t s better to defne net gan for t. We prefer the sgnal powers at the end of the lnk to be the same as ther values at the begnnng of the lnk. So, the deal net gan of our hybrd EDFA/FRA s zero. We have assumed the EDFA to be polarzaton ndependent as t almost s, but we consder the polarzaton dependency of FRA by ncludng a factor n our smulatons. Amplfcaton n the FRA s due to the nonlnear phenomenon of stmulated Raman scatterng (SRS). By usng a hgh number of pumps n FRAs wth dfferent wavelengths and powers, the gan spectrum s talored and the EDFA gan varaton s compensated. The amplfcaton band can be chosen smply by adjustng the pump wavelengths at approprate dstances. The nose transton from pumps to sgnals occurs less n the counter-propagatng pump scheme, hence t s more advantageous to place the pumps at the end of the FRA and ncrease the length of lnk span. We have to appont the nput sgnal powers to somehow avod harmful nterference wth nose due to the low level of the nput sgnal power, and unfavorable nonlnear effects due to the hgh level of the nput sgnal powers. All the optmzaton results, obtaned by PSO, for dfferent schemes and confguratons are gven n TABLES 5-8. It s obvous that gan spectrum varatons wll be lower f we choose 10-pumped confguratons nstead of 6-pumped ones, or f we choose km lnks nstead of 174 km ones, and also f we choose 1mW of nput sgnal power nstead of 10 mw. As lsted n TABLE 5, gan spectrum varatons of and db are attaned for 108.5

6 Wdeband Gan Flattened Hybrd Erbum-doped Fber Amplfer/Fber Raman Amplfer - Hossen Afkham et al. 347 TABLE 5. Values of the optmzed parameters of 6- & 10-pumped-FRA, km hybrd EDFA/FRA wth 1mW of nput sgnal power, obtaned by PSO Varable Value for 10 pumps FRA HA Value for 6 pumps FRA HA EDFA pump wavelength p nm nm EDFA Er 3+ concentraton n t EDFA fber length L m m EDFA pump power mw mw FRA pump power P 1 and wavelength mw and nm mw and nm FRA pump power P 2 and wavelength mw and nm mw and nm FRA pump power P 3 and wavelength mw and nm 18.6 mw and nm FRA pump power P 4 and wavelength mw and nm mw and nm FRA pump power P 5 and wavelength mw and nm 94.0 mw and nm FRA pump power P 6 and wavelength mw and nm 74.2 mw and nm FRA pump power P 7 and wavelength mw and nm FRA pump power P 8 and wavelength mw and nm FRA pump power P 9 and wavelength mw and nm FRA pump power P 10 and wavelength mw and nm Ftness value db db Gan spectrum varatons db db TABLE 6. Values of the optmzed parameters of 6- & 10-pumped-FRA, 174 km hybrd EDFA/FRA wth 1mW of nput sgnal power, obtaned by PSO Varable Value for 10 pumps FRA HA Value for 6 pumps FRA HA EDFA pump wavelength p nm nm EDFA Er3+concentraton n t EDFA fber length L m m EDFA pump power mw mw FRA pump power P 1 and wavelength mw and nm mw and nm FRA pump power P 2 and wavelength mw and nm mw and nm FRA pump power P 3 and wavelength mw and nm mw and nm FRA pump power P 4 and wavelength mw and nm mw and nm FRA pump power P 5 and wavelength mw and nm 50.2 mw and nm FRA pump power P 6 and wavelength mw and nm 30.6 mw and nm FRA pump power P 7 and wavelength mw and nm FRA pump power P 8 and wavelength mw and nm FRA pump power P 9 and wavelength mw and nm FRA pump power P 10 and wavelength mw and nm Ftness value db db Gan spectrum varatons db db km, 6- and 10-pumped-FRA hybrd EDFA/FRA, respectvely wth 1mW nput sgnal powers. Also, as lsted n TABLE 6, gan spectrum varatons of and db are attaned for 174 km, 6- and 10-pumped-FRA hybrd EDFA/FRA, respectvely wth 1mW nput sgnal powers. Usng the results lsted n the TABLES 5 and 6, optmzed sgnal gan spectra of and 174 km 10-pumped-FRA hybrd EDFA/FRA are shown n FIG. 3(a), and 174 km 6-pumped-FRA hybrd EDFA/FRA are depcted n FIG. 3(b), 6- and 10-pumped-FRA km hybrd EDFA/FRA

7 348 Journal of the Optcal Socety of Korea, Vol. 14, No. 4, December 2010 (a) (a) (b) FIG. 3. Optmum sgnal gan spectra of (a) & 174 km 10-pumped-FRA hybrd EDFA/FRA and (b) & 174 km 6-pumped-FRA hybrd EDFA/FRA (both for nput sgnal power of 1mW). (b) FIG. 4. Optmum sgnal gan spectra of (a) 6- &10-pumped- FRA km hybrd EDFA/FRA and (b) 6- &10-pumped-FRA 174 km hybrd EDFA/FRA (both for nput sgnal power of 1mW). TABLE 7. Values of the optmzed parameters of 6- & 10-pumped-FRA, km hybrd EDFA/FRA wth 10mW of nput sgnal power, obtaned by PSO Varable Value for 10 pumps FRA HA Value for 6 pumps FRA HA EDFA pump wavelength p nm nm EDFA Er 3+ concentraton n t EDFA fber length L m m EDFA pump power mw mw FRA pump power P 1 and wavelength mw and nm mw and nm FRA pump power P 2 and wavelength mw and nm mw and nm FRA pump power P 3 and wavelength mw and nm 6.1 mw and nm FRA pump power P 4 and wavelength mw and nm 98.4 mw and nm FRA pump power P 5 and wavelength mw and nm 51.7 mw and nm FRA pump power P 6 and wavelength mw and nm 43.3 mw and nm FRA pump power P 7 and wavelength mw and nm FRA pump power P 8 and wavelength mw and nm FRA pump power P 9 and wavelength mw and nm FRA pump power P 10 and wavelength mw and nm Ftness value db db Gan spectrum varatons db db s pctured n FIG. 4(a), and 6- and 10-pumped-FRA 174 km hybrd EDFA/FRA s pctured n FIG. 4(b), all for 1 mw nput sgnal powers. These fgures present good vsual comparson of the optmzed gan spectra of dfferent schemes. It s common to select the FRA pump wavelengths n an equdstant form, but here we search the FRA pump

8 Wdeband Gan Flattened Hybrd Erbum-doped Fber Amplfer/Fber Raman Amplfer - Hossen Afkham et al. 349 TABLE 8. Values of the optmzed parameters of 6- & 10-pumped-FRA, 174 km hybrd EDFA/FRA wth 10mW of nput sgnal power, obtaned by PSO Varable Value for 10 pumps FRA HA Value for 6 pumps FRA HA EDFA pump wavelength p nm nm EDFA Er3+concentraton n t EDFA fber length L m m EDFA pump power mw mw FRA pump power P 1 and wavelength mw and nm mw and nm FRA pump power P 2 and wavelength mw and nm mw and nm FRA pump power P 3 and wavelength mw and nm 26.5 mw and nm FRA pump power P 4 and wavelength mw and nm mw and nm FRA pump power P 5 and wavelength mw and nm 54.4 mw and nm FRA pump power P 6 and wavelength mw and nm 14.9 mw and nm FRA pump power P 7 and wavelength mw and nm FRA pump power P 8 and wavelength mw and nm FRA pump power P 9 and wavelength mw and nm FRA pump power P 10 and wavelength mw and nm Ftness value db db Gan spectrum varatons db db FIG. 5. Comparson of the convergence speeds of the PSO and GA toward the optmum values versus the normalzed executng tme. wavelengths through an optmzaton process whch wll result n better outcomes for gan spectrum varatons. For example, gan spectrum varatons of 2.91 db for a km, 6-pumped-FRA hybrd EDFA/FRA, equdstant FRA pump wavelengths [6] has become db for 174 km, 6-pumped-FRA hybrd EDFA/FRA, by optmzaton of FRA pump wavelengths and also gan spectrum varatons of 2.03 db for a km, 10-pumped-FRA hybrd EDFA/ FRA, by equdstant FRA pump wavelengths [6] has become db for 174 km, 10-pumped-FRA hybrd EDFA/FRA, by optmzaton of FRA pump wavelengths, whch are notable mprovements. For most cases, the wavelength spectral dstance between the FRA pump wavelengths, presented n TABLES 5-8, are longer than 8 nm. At the closest pont t wll reach to 2.3 nm. By usng narrow spectral wdth laser dodes at the places where the pump wavelengths are near the adjacent pump wavelengths, the pump powers wll not nterfere wth each other and sound operaton s guaranteed. Typcal spectral wdth of a laser dode s about nm. FIG. 5 shows a comparson between the convergence speeds of PSO and GA optmzaton processes whch are carred on the optmzaton of one of the confguratons of our WDM systems. It s apparent that PSO acts faster and more effcently n searchng for the optmum values for a system under test. VI. CONCLUSION In ths paper, we have used an effectve optmzaton method, called partcle swarm optmzaton (PSO) to offer optmum constructons for hybrd erbum-doped fber amplfer/ fber Raman amplfers (EDFA/FRAs). PSO allowed us to nclude a hgh number of parameters n the optmzaton process, so that we could optmze as much as 24 parameters smultaneously. Usng the effcent PSO algorthm, pump wavelengths of FRA are optmzed n addton to the FRA pump powers, EDFA pump power and wavelength, Er 3+ concentraton, and EDFA length. Ths results n obtanng hybrd amplfers wth gan spectrum varatons of as low

9 350 Journal of the Optcal Socety of Korea, Vol. 14, No. 4, December 2010 as db for a km and db for a 174 km, 10-pumped-FRA hybrd EDFA/FRA, respectvely. A comparson between the convergence speeds of PSO and genetc algorthm s done that shows that PSO s faster than genetc algorthm n convergng toward the best values of amplfer parameters. ACKNOWLEDGMENT The frst author would lke to thank the Sepdan Branch of the Islamc Azad Unversty, Shraz, Iran, for the fnancal support of ths project. REFERENCES 1. P. C. Becker, N. A. Olsson, and J. R. Smpson, Erbumdoped Fber Amplfers: Fundamentals and Technology (Academc Press, San Dego, USA, 1999), Chapter M. N. Islam, Raman Amplfers for Telecommuncatons I & II (Sprnger, New York, USA, 2004). 3. H. Masuda and S. Kawa, Wde-band and gan-flattened hybrd fber amplfer consstng of an EDFA and a multwavelength pumped Raman amplfer, IEEE Photon. Technol. Lett. 11, (1999). 4. J. Kennedy and R. C. Eberhart, Swarm Intellgence (Academc Press, San Dego, USA, 2001), Chapter A. P. Engelbrecht, Fundamental of Computatonal Swarm Intellgence (John Wley & Sons, New York, USA, 2005), Chapter A. Mowla and N. Granpayeh, Optmum desgn of a hybrd erbum-doped fber amplfer/fber Raman amplfer usng partcle swarm optmzaton, Appl. Opt. 48, (2009). 7. C. Cheng and M. Xao, Optmzaton of an erbum-doped fber amplfer wth radal effects, Opt. Comm. 254, (2005). 8. C. Cheng, Z. Xu, and C. Su, A novel desgn method: a genetc algorthm appled to an erbum-doped fber amplfer, Opt. Comm. 227, (2003). 9. C. Cheng, A global desgn of an erbum-doped fber and an erbum-doped fber amplfer, Opt. Laser Technol. 36, (2004). 10. B. Neto, A. L. J. Texera, N. Wada, and P. S. Andre, Effcent use of hybrd genetc algorthms n the gan optmzaton of dstrbuted Raman amplfers, Opt. Express 15, (2007). 11. X. Lu, Optmzaton for varous schemes of dstrbuted fber Raman amplfers, J. Opt. A: Pure Appl. Opt. 6, (2004). 12. X. Lu and Y. L, Optmzng the bandwdth and nose performance of dstrbuted mult-pump Raman amplfers, Opt. Comm. 230, (2004). 13. J. S. Yoon and N. Km, Optmzaton of dffractve optcal elements by genetc algorthm, J. Opt. Soc. Korea 4, (2000). 14. J. H. Jung, Optmal desgn of delectrc-loaded surface plasmon polarton wavegude wth genetc algorthm, J. Opt. Soc. Korea 14, (2010). 15. C. R. Gles and E. Desurvre, Modelng erbum-doped fber amplfers, IEEE J. Lghtwave Technol. 9, (1991). 16. A. A. M. Saleh, R. M. Jopson, J. D. Evankow, and J. Aspell, Modelng of gan n erbum-doped fber amplfers, IEEE Photon. Technol. Lett. 2, (1990). 17. C. Cheng and M. Xao, Optmzaton of a dual pumped L-band erbum-doped fber amplfer by genetc algorthm, IEEE J. Lghtwave Technol. 24, (2006). 18. W. J. Mnscalco, Erbum-doped glasses for fber amplfers at 1500 nm, IEEE J. Lghtwave Technol. 9, (1991). 19. X. Lu and B. Lee, A fast and stable method for Raman amplfer propagaton equatons, Opt. Express 11, (2003). 20. A. Mowla and N. Granpayeh, A novel desgn approach for erbum-doped fber amplfers by partcle swarm optmzaton, Prog. In Electromagn. Res. M 3, (2008). 21. J. Bromage, Raman amplfcaton for fber communcatons systems, IEEE J. Lghtwave Technol. 22, (2004). 22. X. Lu and B. Lee, Effectve shootng algorthm and ts applcaton to fber amplfers, Opt. Express 11, (2003). 23. A. Mowla and N. Granpayeh, Desgn of a flat gan multpumped dstrbuted fber Raman amplfer by partcle swarm optmzaton, J. Opt. Soc. Am. A 25, (2008). 24. C. Headley and G. P. Agrawal, Raman Amplfcaton n Fber Optcal Communcaton Systems (Elsever, Burlngton, USA, 2005), pp J. Kennedy and R. C. Eberhart, Partcle swarm optmzaton, n Proc. IEEE Int. Conf. on Neural Networks (Perth, WA, Australa, 1995), pp R. C. Eberhart and J. Kennedy, A new optmzer usng partcle swarm theory, n Proc. 6th Int. Symposum on Mcro Machne and Human Scence (Nagoya, Japan, 1995), pp