Evaluation of Distributed Feedback (DFB) Laser Operating Parameters for Direct Modulation Schemes in IM-DD RoF Links.

Transcription

1 Evaluatio of Distributed Feedback (DFB) Laser Operatig Parameters for Direct Modulatio Schemes i IM-DD RoF Liks. S. Mikroulis, A. Chipouras, S. H. Karabetsos ad A. A. assiopoulos RDTL Laboratory, Departmet of Electroics, Techological Educatioal Istitutio of Athes, Egaleo - Greece Abstract. The icreasig demad for high data rates i wireless etworks idicates Radio-over-Fiber (RoF) systems as a good cadidate for physical layer ifrastructure i the developmet of future high-speed wireless commuicatio systems. I this work, the performace of a distributed feedback (DFB) laser is evaluated i order to exploit its operatioal parameters ad idetify its limitatios for the purpose of utilizatio i broadbad RoF systems. It is show that the laser s itrisic o-liearity restrictios ca be tolerated by appropriate adjustmet of the operatig parameters ad choice of proper modulatio schemes. Idex Terms Radio over Fiber (RoF), IM-DD RoF systems, itermodulatio distortio, Distributed feedback (DFB) laser. W I. ITRODUCTIO IRELESS local area etworks (WLAs) are attractive sice they combie high capacity ad icreased flexibility[]. With fiber-optic radio liks (RoF), oce the radio sigal is modulated oto light ad multiplexed, it is trasmitted over a log distace through optical fiber. A RoF system cosists of a Cetral Site (CS) ad a Remote Site (RS) coected by a optical fiber lik or etwork. If the applicatio area is i a Global Service Mobile (GSM) etwork, the the CS could be the Mobile Switchig Cetre (MSC) ad the RS the Base Statio (BS). For WLAs, the CS would be the headed while the Radio Access Poit (RAP) would act as the RS. The frequecies of the radio sigals distributed by RoF systems spa a wide rage (usually i the GHz regio) ad deped o the ature of the applicatios. The added value i usig such a system lay i the capability to dyamically allocate capacity based o traffic demads. RoF systems of owadays, are desiged to perform added radiosystem fuctioalities besides trasportatio ad mobility fuctios. Hece, the electroic sigal processig could be miimized as most of the sigal processig fuctios could be performed at the cetral statio (CS) resultig i the reductio of total etwork cost []. Better coverage ad icreased capacity, cetralised upgradig ad adaptatio, higher reliability ad lower maiteace costs, support for future broadbad applicatios, ad ecoomic access to mobile broadbad are amog the most importat advatages of RoF [3]. Amog Several techiques for distributig ad geeratig microwave sigals [4], the Itesity Modulatio Direct Detectio (IM-DD) is the most usable. The laser is directly modulated by usig the appropriate RF sigal. The advatage of the IM-DD method is its simplicity ad low cost. Secodly, if low dispersio fiber is used i cojuctio with a appropriate choice of the Laser operatig parameters the system becomes quasi liear. Cosequetly, the optical lik acts oly as a amplifier or atteuator ad is therefore trasparet to the modulatio format of the RF sigal. That is to say that both Amplitude Modulatio (AM) based schemes ad costat evelop based modulatio schemes such as Phase Modulatio (PM / QPSK) ca be used. Such a system eeds little or o upgrade wheever chages i the modulatio format of the RF sigal occur. Sub-Carrier Multiplexig (SCM) ca also be used o such a system. Furthermore, ulike direct laser bias modulatio, exteral modulators such as the Mach Zehder Modulator (MZM) ca be modulated with mm-wave sigals approachig 00 GHz, though this comes with a huge cost regardig power requiremets. Moreover, they require high drive voltages, which i tur lead to very costly drive amplifiers. Distributed Feedback (DFB) lasers are the proper light sources for aalog trasmissio due to their sigle logitudial mode, with side-mode suppressio ratio (SMSR) i the order of 40dB, ad stable operatio with a arrow spectral width. However, they iheretly have a serious problem of the modulatio distortio caused by several factors; spatial-hole burig (SHB), leakage curret, oliear iteractio of carriers ad photos, gai compressio, oliear I-V characteristics i a p heterojuctio, ad power depedet absorptio. The SHB takes place due to ouiform photo distributio alog the cavity, which depeds o both the couplig costat ad the gratig phase at laser facets, ad ca be elimiated by proper device desig. The leakage curret, which flows outside of a active layer, also causes the degradatio of the L-I liearity. The distortio iduced by the leakage curret becomes remarkable at a highpower operatio because the leakage curret relatively icreases i high-bias coditio irrespective of the modulatio frequecy. Fially, the itrisic oliear iteractio drastically icreases the harmoic or itermodulatio distortio as the modulatio frequecy approaches the laser s resoace frequecy. Multi-Quatum Well (MQW) DFB lasers eable high differetial gai values thus icreasig sigificatly the laser s resoace frequecy. I this work, the performace of a high-speed distributed feedback (DFB) laser is evaluated i order to exploit its operatioal parameters ad idetify its limitatios for the purpose of utilizatio i future broadbad RoF systems. It is show that the laser s itrisic o-liearity restrictios ca

2 be tolerated by appropriate adjustmet of the operatig parameters ad choice of proper modulatio schemes. II. MODEL The variatio of the electric field E ad the ijected electro cocetratio are described by a set of rate equatios, III. RESULTS AD DISCUSSIO de dt ( + iα) G E + F () t τ () p d dt I ev τ s G E () where, V is the volume of the active regio, G is the modal gai, G A ( s E ) (3) With A the liear gai coefficiet ad s the self gai saturatio coefficiet. A v g( ) g 0 (4) where g is the differetial gai coefficiet, 0 is the carrier cocetratio at trasparecy, τ p is the photo lifetime, e is the electro charge, τ s is the carrier lifetime, v g is the group velocity, ad α is the liewidth ehacemet factor. Equatios () (4) are umerically itegrated to simulate the laser operatio. I the computatio of laser dyamics, usual Lagevi oise source is used represeted by the term F( i Eq.. Values for various parameters of a high-speed MQW DFB laser used i the simulatios appear i Table. Apart from the itrisic oliear iteractio betwee carriers ad photos, beig the major oliearity, the model accouts for oliear gai compressio, also. Fig. Dyamic Light Curret (L-I) characteristic of the DFB Laser at bias curret I b 60 ma, fghz Fig. Dyamic Light Curret (L-I) characteristic of the DFB Laser at bias curret I b 60 ma, f.4ghz Other oliear mechaisms like leakage curret or SHB assumed to be of mior importace for the curret levels ad DFB structure cosidered throughout this ivestigatio. A. Liearity of the DFB Laser This sectio describes the simulatios performed by a umerical solutio of the rate equatios. The laser oliearity may cosiderably degrade the performace of the RoF system. As it is show i Figs., the laser exhibits a usual o-liear multistable behavior. The dyamic light curret characteristic is obtaied solvig umerically the rate equatios (Eqs.-4) usig as a iput excitatio a triagular pulse at frequecies ad.4 GHz, respectively. The degradatio of the liearity of the L-I curve is ehaced by icreasig the pulse amplitude ad the frequecy, as it is evidet i Figs.,. The small sigal modulatio respose is calculated usig the rate equatios (-4). For each frequecy poit, the rate equatios are solved with a iput siusoid at that frequecy. The small-sigal amplitude (p-p) is 0.5 times the threshold curret. I this example, a evaluatio of differet DFB lasers commercially/research available is preseted with resoace frequecies ragig from 5 to 5 GHz which results i a equivalet itrisic modulatio badwidth from 9 to 5GHz, at a curret bias approx..5i th, where I th is the laser s

3 threshold curret. The calculated frequecy respose for g values from.0 to 5.0 x 0-6 cm is show i Fig 3. the modulated RF sigal or equivaletly of the modulatio depth will be the result of a compromise betwee a acceptable sigal distortio ad the miimum itesity oise. Moreover, oise figure (Fig.4) ca be used to defie appropriate levels for M-ASK/M-QAM modulatio schemes. B. Harmoic distortio aalysis Distortio i the laser ca be aalyzed by a umerical solutio of the rate equatios (Eqs. -4). A sigle toe iputs as a curret modulatio. Fig.3 Frequecy respose of the DFB Laser for various differetial gais Fig 5 Frequecy spectra of the optical output of directly modulated DFB laser with a sigle toe at,, 5.78 ad 7 GHz. Fig.4 Dyamic Light Curret (L-I) characteristic of the trasmitter Laser at bias curret I b 60 ma, f.4ghz, Ip-pmA The simulatio results show that the DFB laser is well behaved (almost liearly) for bias currets up to 3 times the threshold curret ad sigal amplitudes from about 0. to 0.9 times the threshold curret as well. Thus, the RF sigal will ot experiece sigificat distortio i the typical trasmissio bad (well below the resoace frequecy). As expected for lower sigal amplitudes the liearity of the L-I curve is improved but the relative oise of the laser is icreased (Fig.4). oise i lasers is maily due to the amplified spotaeous emissio (ASE) oise. The chage i the photo populatio (or electric field, E) is usually associated with a oise term. This oise results i radom fluctuatios i the optical output sigal ad may sigificatly degrade the system s carrier-to-oise ratio (CR). However the laser itesity oise (Relative Itesity oise-ri) is reduced for a higher optical power (higher ijectio levels). The laser s RI has its spectral maximum ear the relaxatio oscillatio frequecy (i the order of GHz for our laser). The RI is calculated to be i the order of - 50dB/Hz, which is i agreemet with published values for similar lasers [5]. Evetually the choice of the amplitude of The respose of the laser is the Fourier-trasformed, to yield the frequecy iformatio of the output light. The harmoic distortio power ca be aalyzed as a fuctio of modulatio frequecy. Because of the itrisic o liearity of the trasmitter the appearace of higher order harmoics is ievitable ad as it is show i Fig. 5 is more proouced as the frequecy icreases. Usig the simulatio model, the ratio of high order distortio harmoics to carrier power, HMD/C, is calculated. The ratios HMD/C, where,3, for the electric equivalet power spectra are i the order of 30dB icreased with frequecies over 0 GHz. However, these harmoics are far eough over the cocered frequecy bad ad ca easily be removed usig the appropriate filters. Cosequetly their existece is ot crucial for the systems with operatig frequecies i the order of 0 GHz). C. Multichael distortio oise The performace of a direct IM-DD scheme is strogly iflueced by the oliearity of the laser. Whe may RF subcarriers are multiplexed together ad modulate the light source, harmoic distortios ad itermodulatio products are geerated that ifluece egatively the quality of the sigal at the receiver. The origi of oliear distortio ca be the carrier leakage, spatial hole burig ad the oliear iteractio betwee photos ad electros iside the laser cavity. Whe the laser is operated i multi-ghz rage, it is the latter that domiates the oliear distortio. The distortio is especially promiet whe the modulatio frequecy is i the proximity of the relaxatio oscillatio peak.

4 The impact of the multichael distortio ca by uderstood by cosiderig a direct modulatio of the laser with a set of geeric siusoidal toes. The iput curret i( to the direct modulated laser trasmitter ca be expressed as a beats fall withi the operatioal bad where the origial toes are allocated, thus yieldig itrabad distortio. It is importat to ote that i those RoF-systems, where a sigle wavelegth is used to allocate several services, there ca be a iterbad egative ifluece too, due to the harmoics or itermodulatios that, geerated i a certai bad, fall withi that dedicated to aother service. I our case to esure that a large umber of mixig products were icluded i the 80MHz width bad of iterest, four-toe tests were performed for frequecy arragemets of the type: f c ± f mi where f c 5787 MHz ad 7000 MHz while f m f m 0 MHz ad 60 MHz respectively. I fig. 6 distortio products are preseted i the regios aroud the operatig frequecies f c ad aroud f c as well. As we ca see, the dyamic oliearity of the laser results i the geeratio of sidebads aroud the origial sigal frequecies due to IMD3. I additio ito the sidebads that are visible i the received spectrum, there are also IM products sittig o the f c regio that will affect the performace of these chaels. The ratios IMD/C,,3, for the electric equivalet power spectra are i the order of 30dB, icreased with frequecies Fig.6 Frequecy spectra of the optical output of directly modulated DFB laser with subcarrier multiplexig sum of sigle toes ω i with a modulatio idex m i respectively, it () Ibias + mi cosπ fit (5a) i where I bias is the applied bias curret. The the output optical sigal expaded as a Taylor series is obtaied as P(: P( [ i( ] P ( a0i( + i( (5b)! [ i( ] t It is obvious that the output cosists of the origial siusoidal sigals at frequecies ω i, with the additio of the followig frequecy compoets mf i ± f j, ± pf k where m, ad p are iteger umbers that ca assume the values 0,, If two of m, ad p are simultaeously ull, the harmoic compoets of a toe are obtaied; if at least two of them are differet from zero, we talk of itermodulatio products. The sum m + + p defies the order of the harmoics ad of the itermodulatio products; So, the secod-order harmoic distortio (HMD) is idetified as f i, f j, f k; the third order harmoic distortio (HMD3) as 3f i, 3f j, 3f k ; secod-order itermodulatio (IMD) as f i ± f j ; third order itermodulatio (IMD3) as f j ±f i, f i ±f j, f i ± f j ± f k ad so o. Some of these Fig.7 Iput (electrical) ad output (optical) sigal for the case of f5.787ghz carrier frequecy with DSB-SC modulatio. over 0 GHz ad modulatio depth. It is oted that duplicatig the modulatio depth the ratios IMD/C ad IMD3/C, for the electric equivalet power spectra, are Fig.8 Iput (electrical) ad output (optical) sigal for the case of f5.787ghz carrier frequecy with stadard AM modulatio.

5 icreased about 3 db ad 9 db respectively. Besides the asymmetry of the itermodulatio products as they ot have the same phase ad magitude due to the memory of the trasmittig laser, must be oted. The IMD products are Fig.9 Frequecy spectra of the optical output of directly modulated DFB laser with subcarrier multiplexig for the secod harmoic. reduced with icrease i resoace frequecy, because the resoace peak shifts to the higher frequecy. Therefore, a high resoace frequecy, which ca be achieved by MQW-DFB laser structure is idispesable to obtai low distortio. I additio, IMD3 decreases with icreasig bias curret, which is attributed to the icrease of resoace frequecy. I additio IMDs are oticeably decreased by icreasig the bias curret above 3.5 times the threshold curret. However, at large bias levels the oliearity due to the leakage curret limits the device s performace. Besides, low bias curret is ecessary to realize log-term reliability. Furthermore, otice that Eq. 5a also expresses a Double Side Bad Suppressed Carrier (DSB-SC) modulatio scheme havig a carrier frequecy of f c. A example of the time domai iput-output sigal is depicted i Fig. 7. I cotrast to this, the case of stadard amplitude modulatio (AM) usig the same parameters is depicted i Fig. 8 while the frequecy spectra of the optical output of the DFB laser for the secod harmoic of the carrier frequecy, is illustrated i Fig. 9. It is observed that the AM sigal is more resiliet to itermodulatio distortio sice itermodulatio harmoics are suppressed compared to the DSB case (Fig. 6). E. Calculatio of the C/ for typical RoF lik A RoF system coects a Cetral Site (CS) to Remote Sites (RS), supportig radio chaels coected by a optical fiber lik. The curret applied to the trasmitter is the sum of idividuals chaels with sigal s(: it ( ) Ibias + stm ( ) i si( π ft i + φi ) (6a) i For a large umber of chaels, by the cetral limit theorem, the iput sigal ca be modeled as a Gaussia radom variable with a stadard variatio of m eff I bias, where m eff is the RMS modulatio idex: mi i m eff (6b) The the received CR ratio to the most distat (RS) is give by [7]: ( al+ L ) /0 c C ( meff RP0 0 ) (7) e Pthermal + Pshot + PRI + PIMD3 Where R is the detector resposivity, P 0 is the average trasmitted optical power, L is the fiber legth, L c the coupler losses, ad P shot, P Thermal, P RI, ad P IM3 are the rms values of the oise currets associated with the shot oise, thermal oise, laser itesity oise, ad distortio sources, respectively. The total IM power of the ceter chael o which the largest umber of 3 rd order itermodulatio products will is [7]: 3( + ) P IM 3 ( IMD3) (8) 8 By calculatig IM3 usig the preseted rate equatios, the relatio (7), ad typical values of the relevat parameters, the CR ratio ca be calculated. F. Coclusios Through simulatios, the performace of a distributed feedback (DFB) laser is evaluated i order to exploit its operatioal parameters ad idetify its limitatios for the purpose of utilizatio i broadbad RoF systems. The results showed a depedece of a possible RoF system s degradatio, due to the DFB laser s oliearity, o bias curret level, modulatio depth, carrier frequecy ad modulatio scheme. evertheless, a proper adjustmet of these parameters is crucial for feasibility of IM-DD RoF systems with a acceptable performace. REFERECES [] G. Baghersalimi, T. O Farrell, V. Postoyalko, Modellig Laser-Diode o-liearity i a Radio-over-Fibre Lik Proceedig of Lodo Commuicatio Symposium, Pages: 3-34, September 003 [] C. Harder, J. Katz, S. Margalit, J. Shacham, ad A. Yariv, "oise equivalet circuit of a semicoductor laser diode," IEEE Joural of Quatum Electro., vol. QE-8, pp , 98. [3] I. Frigyes Basic Microwave Properties of Optical Liks: Isertio Loss, oise Figure, ad Modulatio Trasfer i Radio over Fiber Techologies for Mobile Commuicatios etworks, H. Al Raweshidy, ad S. Komaki, ed. Artech House, Ic, USA, 00 [4] P. Vakwikelberge et al., Aalysis of the carrier-iduced FM respose of DFB lasers: Theoretical ad experimetal case studies, IEEE J. Quatum Electro., vol. 5, pp , ov. 989 [5] O. K. Toguz ad H. Jug, Persoal commuicatios access etworks usig subcarrier multiplexed optical liks, IEEE J. Lightwave Techol., vol. 4, pp , Jue 996 [6] A.R. Shah ad B. Jalali, Adaptive equalisatio for broadbad predistortio liearizatio of optical trasmitters IEE Proc.- Optoelectro., Vol. 5, o., pp.6-34, February 005 [7] H. Jug ad O.K. Toguz, Compariso of fibre ad coaxial lik for access etwork i microcellular PCS, Electroc Letters, Vol. 3, o 5, pp , Feb. 996