Optical ASK and FSK Modulation By Using Quantum Well Transistor Lasers

Transcription

1 Iteratioal Joural of Optics ad Photoics (IJOP) Vol. 6, No., Summer-Fall 01 Optical ASK ad FSK Modulatio y Usig Quatum ell Trasistor Lasers A. Horri a ad R. Faez b a Youg Researchersa ad Elite Club, Arak rach of Islamic Azad Uiversity, Arak, Ira b epartmet Of Electrical Egieerig, Sharif Uiversity of Techology, Tehra, Ira Correspodig Author: ashka_horri@yahoo.com ASTRACT I this paper, trasistor lasers (TLs) are used as a optical modulator for geeratio of ASK(Amplitude Shift Keyig) ad FSK (Frequecy Shift Keyig) optical sigals. Our aalysis is based o cotiuity equatio, rate equatios, ad the theory of discotiuity of quasi-fermi level at the abrupt juctio. Our simulatio results idicate that, the specificatio of ASK ad FSK optical sigals, are affected by dyamical behavior of TL. Also our simulatio results idicate that, the collector-emitter voltage amplitude should be small eough that the oliear properties of TLs do ot destroy the modulated optical sigals. KEYORS: Quatum well trasistor laser (QTL), ASK (Amplitude shift keyig), FSK (Frequecy Shift Keyig), Optical Modulatio. I. INTROUCTION I digital optical commuicatios, the ASK(Amplitude Shift Keyig) ad FSK (Frequecy Shift Keyig) modulatio formats are ofte used. The optical commuicatio systems based o ASK ad FSK modulatios, are attractive because like all coheret systems they show a improved receiver sesitivity over direct detectio ad the iheret frequecy selectivity of the optical receiver eables a close chael wavelegth spacig[1-3].i ASK system, the carrier amplitude is chaged i respose to bit iformatio, oe particular amplitude for bit 1 ad aother amplitude for bit 0 [3]. I FSK system, the carrier frequecy is chaged i respose to bit iformatio, oe particular frequecy for bit 1 ad aother frequecy for bit 0 [3]. The geeratio of FSK optical sigals i semicoductor laser by employig a electrical equalizig circuit, has bee reported[3]. Also a optical FSK trasmitter based o a itegrated distributed feedback (F) laser was proposed [4]. Recetly, it is foud that, oliear mixig i a twi-base-cotact trasistor laser (TL) is able to geerate output mixig frequecies up to 8 GHz i the laser threshold regio[5]. Also it is foud that, the tuel juctio -TL eables a ew oliear sigal processig (addig ad mixig) device operatig above laser threshold for improved optical output power[6].the purpose of this paper is to preset a theoretical aalysis of quatum well trasistor lasers used as modulator for geeratio of ASK ad FSK optical sigals. To our kowledge, for the first time, this paper describes the trasistor laser as a digital modulator. Our aalysis is based o cotiuity equatio, rate equatios [7],[8], ad the theory of discotiuity of quasi-fermi level at the abrupt juctio [9]. The orgaizatio of the paper is as follows: i sectio II, we develop the mai formulatio for the ASK ad FSK modulatio, i sectio III, results are discussed, i sectio IV, coclusios are preseted. II. METHO AN THEORY A schematic of quatum well TL is show i Fig. 1. The device, cosist of -AlGaAs emitter, followed by p-gaas layer as base. There is oe quatum well (Q) that is 105

2 A. Horri ad R. Faez Optical ASK ad FSK Modulatio y Usig Quatum ell Trasistor Lasers located i the middle of the base regio. Also the itrisic GaAs layer acts as a collector. δn t δn δnn x τ (1) Eq. (1) is solved coditios[7]: subject to the boudary Fig. 1. Schematic of HTL. The structure parameters of TL used i this paper are defied i Table 1. Table 1. Material Specificatio of HTL used i simulatio [7], [10]. Regio opig Material idth ase P + -GaAs 100m Q - i-igaas 10m Emitter AlGaAs 500m Collector - i-gaas 105m Fig. Schematic of carrier diffusio ad quatum capture i the Q, ad the coductio bad eergy of the base regio. Fig. shows the coductio eergy bad of the base ad dc excess miority carrier distributio δn(x), i the base regio. The carriers ijected from the emitter, diffuse across the base ad reach the quatum well (Q). The ubouded carriers at x=0 are located at the virtual boud states. e use the diffusio equatio for the excess carriers i the base regio[ [7],[8]: δn J E q x δn JC q δx δn 0 δn 0 δn δn - JV.S q J E J C J I above equatios, is the diffusio coefficiet i the base, τ is the carrier recombiatio lifetime i the base regio, J E is the emitter curret, J C is the collector curret, J is the base curret, is the base width, N V.S is the virtual states carrier cocetratio, ad J V.S is the curret to the virtual states due to diffusio. N / KT KT N qve / J Ee 0 e 1 evr N w qv KT N e C / 0 1 Nw N δ 0 δx at x at x N N V.S q From the cocept of discotiuity of quasi- we Fermi level at the base-emitter juctio, have[9]: where N 0, is the iitial electro desity at equilibrium, is the coductio bad discotiuity, v R is the richardso velocity give by v R KT / m, with m as the effective mass of electros i the base. V E, ad VC are the base-emitter ad collector- ad base voltages, respectively. Usig Eq. (1) δ 0 x () (3) (4) (5) (6) (7) (8) (9) (10)

3 Iteratioal Joural of Optics ad Photoics (IJOP) Vol. 6, No., Summer-Fall 01 above boudary coditios, the carrier cocetratios ca be foud: N.e L /q J δn1 cosh /L N e L /q J e cosh /L δn /L V.S E x/l /L V.S E x/l N N exp /L w V.S sih /L NV.Sexp /L Nw exp -x/l sih /L e exp x/l (11) (1) where δn 1 ad δn are the dc carrier cocetratios i the regios before the Q ad after the Q, respectively, ad L is the diffusio legth defied as L τ. Table. Parameters of trasistor laser [7]-[10]. Symbol escriptio Value Spotaeous emissio τ S lifetime 00ps recombiatio lifetime τ i base 00ps τ Carrier capture time i cap Q 1ps Carrier escape time τ esc from Q 10ps τ Photo lifetime i the p cavity 4ps Spotaeous emissio s lifetime 00ps G 0 ifferetial optical gai 10-5 cm 3 s -1 Optical cofiemet factor 0.05 Area Area of the TL 16μm Spotaeous emissio factor 10-5 L Cavity legth 00µm R 1, R Facet reflectivity 0.6 i itrisic absorptio coefficiet 4cm -1 c Speed of light cm/s r Reflective idex 3.5 Solutios for J E ad J are obtaied as: cosh q L E V.S L L sih L J N sih JV.Scosh w coth L q N L Ld (13) q J N V.S sih J V.Scosh (14) L L L The Eq. (14) states that the base curret has two compoet. The first term of Eq. (14), is the (radiative or oradiative) recombiatio of carriers. The secod term is the additioal (radiative) recombiatio due to laser operatio. The rate equatios describig the Q curret, virtual states, Q boud states, ad photo desities are [7]: JQ qd dn V.S dt dnq dt ds dt N V.S τcap NQ τesc (15) J V.S JQ N V.S qd qd τs (16) JQ NQ G 0 N Q N tr S (17) qd τs ΓG 0 N Q N tr 1 ΓβNQ S τ p τs (18) where J Q is the curret from the virtual states to the boud states withi the Q, N Q is the Q carrier desity, S is the photo cocetratio, is the optical cofiemet factor, d is the Q width, N tr is the carrier desity at optical trasparecy, d is the Q width, τ cap is the capture lifetime for the carriers fallig from the virtual states to the Q states, τ esc is the escape lifetime from the Q to the virtual states, τ s is the spotaeous emissio lifetime, G 0 is the optical gai.the photo lifetime i cavity τ p is writte as : 107

4 A. Horri ad R. Faez Optical ASK ad FSK Modulatio y Usig Quatum ell Trasistor Lasers c/ r i l 1/ R R L ca (19) where R 1 ad R are the cavity reflectivities, L is the cavity legth, ad i is the iteral loss of cavity[7]. For our aalysis, we cosider a typical TL with material ad geometrical parameters as give i Table [7]-[10]. 1 p 1 III. RESULTS A. Electrical resposes of TL The C-V (curret-voltage) aalysis of trasistor laser is doe by solvig Eqs. (9)-(18) umerically. The collector curret of TL versus collector-emitter voltage, for differet base curret, is show i Fig.3. The plot idicates the three regios of operatio: The cut off, active, ad saturatio regios. S0 P / qdj Q 1/ s N th (1) The simulatio results idicated that threshold base curret is 1mA approximately [7], [8]. C. Optical resposes of TL y solvig Eqs. (9)-(18), the output photo umbers ca be foud. The output photo umber of TL versuss collector-emitter voltage, for differet base emitter voltages is show i Fig. 4. Fig. 4. Output photo umbers versus collector- emitter voltage for differet base-emitter voltage Fig. 3 Collector curret versus collector-emitter voltage for differet base curret. I active regio, collector curret is costat with icreasig collector-emitter voltage. Ideed, it oly depeds o base curret. I saturatio regio, the collector curret depeds o both the base curret ad collector-emitter voltage. From the Fig.3, it ca be foud that, the dc curret gai dc IC / I is approximately 50. This tedecy aggrees with previous report [7].. Threshold Curret At threshold coditio, S 0, ad NQ Usig Eq. (18) : G 0 N th N tr 1 p N th. (0) Usig Eq. (0), ad Eqs. (15)-(18) at steady state coditio, output photo umbers is: As we ca see, by icreasig base-emitter voltage, the output photo umbers become higher. This icremet of output photo umbers, occurs due to the icrease i base curret for larger V E. It ca be foud that, i saturatio regio, ad for small collector- betwee output photo umber ad collector- emitter emitter sigals, the liear relatioship exists voltage. Fig. 5. Modulatio respose of TL for differet base curret The Modulatio respose of TL i the commo base (C) cofiguratio is doe by calculatig the C modulatio trasfer fuctio s( jw) / j ( jw. e )

5 Iteratioal Joural of Optics ad Photoics (IJOP) Vol. 6, No., Summer-Fall 01 The small sigal relatioship betwee photo desity ad emitter curret is foud by liearizaitio of Eqs. (13)-(18) ad applyig appropriate maipulatios. The effect of base curret o the small sigal frequecy respose of TL i C cofiguratio is show i Fig. 5. As show i the figure, lower base curret results i a degraded frequecy respose.. ASK modulatio by usig TL The optical ASK modulator is show i Fig.6. As we ca see, the digital bits are applied to the base-emitter juctio. Also the siusoidal sigal is applied to collector-emitter juctio. cotiuous pulse (5s width) varig betwee 0 ad 60mV, is applied to base emitter juctio. The 0mV C bias voltage is applied to collector-emitter juctio. Also a 6GHz siusoidal sigal with 10mV amplitude is applied to collector-emitter juctio. y solvig Eqs. (9)-(18), the ASK optical sigal ca be foud. The output photo umbers versus time, is show i Fig. 7. It ca be foud that, the dyamical properties of TL, affect the output photo umbers, whe the base-emitter voltage chages. E. FSK modulatio by usig TL The optical FSK modulator is show i Fig.8 As we ca see, the digital bits are applied to the base-emitter juctio of first TL. Fig. 6. Schematic of ASK modulator usig TL. Fig. 8. Schematic of FSK modulator usig TL Fig. 7. ASK optical sigal versus time. he the digital bit is zero, the trasistor laser is off, ad TL caot be able to emit photos. he the digital bit is oe, the trasistor laser is o. Accordig to Fig.4, i saturatio regio, the output photo umbers have liear relatioship with collector emitter voltage. Therefore the frequecy of output photo umbers is equal to frequecy of collector emitter voltage whe the digital bits are oe. It is oteworthy, that the collector emitter voltage amplitude should be small eough so that the oliear effects do ot appear. I our simulatio for ASK modulatio, the The digital bits are iverted by usig the NOT gate. The iverted digital bits are applied to the base-emitter juctio of secod TL. Also the siusoidal sigals are applied to the collector-emitter juctio of trasistors. It is oteworthy that the frequecy of siusoidal sigals should be differet. Fially the output power of first ad secod TLs are added together ad make the optical FSK sigal. he the digital bits are zero, the first TL is off, ad the secod TL is o. I this case, the frequecy of output power is equal to frequecy of collector-emitter voltage of secod trasistor. he the digital bits are oe, the first TL is o, ad the secod TL is 109

6 A. Horri ad R. Faez Optical ASK ad FSK Modulatio y Usig Quatum ell Trasistor Lasers off. I this case, the frequecy of output power is equal to frequecy of collector-emitter voltage of first TL IV. CONCLUSION The optical ASK ad FSK modulator are desiged by usig quatum well trasistor laser. These modulators ca be used i optical commuicatio systems. The results show that dyamical behavior of TL affects the specificatio of modulated optical sigals. Also we foud that, for liearity of respose, carrier sigal amplitude should be limit to small value. Fig. 9. FSK optical sigals versus time Ideed, the frequecy of output power is chaged, whe the digital bits chage from zero to oe or from oe to zero. I our simulatio for FSK modulatio, the cotiuous pulse (5s width) varig betwee 0 ad 60mV, is applied to base emitter juctio of first TL. Also the 6Ghz ad 0Ghz siusoidal sigals with 10mV amplitude are applied to collector-emitter juctio of first ad secod TLs respectively. The C voltage bias for collector-emitter juctio is 0mV. y solvig Eqs. (9)-(18), the FSK optical sigal ca be foud. The output photo umbers versus time, is show i Fig. 9. It ca be foud that, the dyamical properties of TL, affect the output photo umbers, whe the base-emitter voltage chage. The most importat parameter, that limits the maximum bit rate, is trasiet respose of TL. Ideed, the pulse width must be greater tha settlig time of TL. From Fig.7, it ca be foud that settlig time is 0.5s. The settlig time must be less tha 5% of pulse width. Therefore the maximum pulse width should be greater tha s ad bit rate should be lower tha 500 MHz. It is oteworthy that, this settlig time amout, obtaied from our simulatio results with parameters defied i Table. Ideed, settlig time depeds o ijected curret level, ad laser parameters. REFERENCES [1] L.A Coldre, iode lasers ad Photoic itegrated Circuits, iley, New York, [] P. Spai, M. Tamburrii, M. Pizzolla, Optical FSK modulatio usig ijectio locked laser diodes, IEEE/OSA J. Lightw. Techol. Vol.7, pp , [3] S. Saito, Y. Yamamoto, ad T. Kimura, Semicoductor laser FSK modulatio ad optical direct discrimiatio detectio, Electro. Lett. Vol. 18, pp , 198. [4] C.A. Park ad P.J. illiams, Sigle mode laser source for FSK system, IEE Proceedig, Vol. 136, pp. 18-1, [5] M. Feg, N. Holoyak, N. Cha, A. James, ad G. alter, Sigal mixig i a multiple iput trasistor laser ear threshold, Appl. Phys. Lett. Vol. 8, pp (1-3), 006. [6] H.. The, C.H. u, G. alter, M. Feg, ad N. Holoyak. Electrical optical sigal mixig ad multiplicatio ( Ghz) with a tuel juctio trasistor laser, Appl. Phys. Lett. Vol. 94, pp , 009. [7] A. Horri, S.Z mirmoeii, ad R.Faez, Aalysis of carrier dyamic effects i trasistor laser, Opt. Eg. Vol. 51, pp , 01. [8] A. Horri ad R. Faez, Large sigal aalysis of double quatum well trasistor laser, Opt. Quatum Electro. Vol. 45, pp , 013. [9] R. asu, Modelig of curret gai compressio i commo emitter mode of a trasistor laser above threshold base curret, J. Appl. Phys. Vol. 111, pp ,

7 Iteratioal Joural of Optics ad Photoics (IJOP) Vol. 6, No., Summer-Fall 01 [10] I. Taghavi, H. Kaatuzia, ad J.P. Leburto Performace optimizatio of multiple quatum well trasistor laser, IEEE J. Quatum Electro. Vol. 49, pp , 013. Egieerig, Sciece ad Research rach, Islamic Azad Uiversity, Tehra, Ira. Ashka Horri received Sc ad Msc degrees from Islamic Azad Uiversity, i 008, ad 009 respectively. He is curretly workig toward the Ph degree i the field of computatioal aoelectroics, ad trasistor lasers from epartmet of Electrical Rahim Faez received his S degree from Sharif Uiversity of Techology i 1977 ad the MS ad Ph degrees from UCLA i 1979 ad 1985, respectively. He joied Sharif Uiversity of Techology ad he is curretly a associate professor. His research iterests iclude desig ad simulatio of advaced semicoductor ao ad quatum devices. 111

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