SPECTRAL BEHAVIOR OF LONG WAVELENGTH VCSELs

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1 SPECTRAL BEHAVIOR OF LONG WAVELENGTH VCSELs Alexndre Bcou, Angélique Rissons nd Jen-Clude Mollier Université de Toulouse - ISAE, 10 Avenue Edourd Belin BP 54032, Toulouse Cedex 4, Frnce ABSTRACT For long time, only smll wvelength rnge of Verticl-Cvity Surfce-Emitting Lsers (VCSEL) ws vilble. The current evolution in process technology llows the fbriction of long wvelength VCSEL tht is interesting for Telecom systems becuse they offer higher integrtion level thn the existing opticl sources t lower costs since they re fbricted in rrys. We propose to focus our investigtion on the behvior of singlemode 1.55µm VCSEL. We im t precisely knowing their spectrl properties under direct modultion. We present study bout the linewidth mesurement nd the linewidth enhncement fctor, lso clled the Henry - or the lph - fctor. Mny studies hve been reported but only few of them re relly efficient. Two different set-ups re presented here to extrct lph fctor. The first one uses n interferometer bsed on the heterodyne technique nd the second uses the dispersive properties of n opticl fiber. We compre both results nd discuss bout ech set-up. Keywords: VCSEL, linewidth enhncement fctor, heterodyne detection 1. INTRODUCTION Since the lst ten yers, Verticl-Cvity Surfce-Emitting Lsers (VCSELs) hve been the most studied devices becuse of their importnt role in networks domin. Indeed, on-wfer testing, rry fbriction nd best opticl fiber coupling mke them key opticl sources. Nevertheless, long-wvelength VCSELs hve suffered for long time from the inexistence of n efficient p-doped distributed Brgg reflector (DBR), for the bottom mirror. Fortuntely, dielectric mirror technology hs been developed, which seems to be the best choice to fbricte commercilly vilble 1.55µm VCSELs. The VCSELs, in this wy, become excellent cndidtes for telecommuniction networks, spectroscopy or even free spce pplictions, thnks to their oculr sfety wvelength. Hence, it turns out to be importnt to precisely know their spectrl properties under direct modultion. Effectively, the linewidth nd the linewidth enhncement fctor re two importnt prmeters in telecomuniction systems tht finlly degrde the qulity of the trnsmission. Tht is why the spectrl behvior of such opticl source must be mesured. Severl studies hve been presented on this subject but it seems tht few chrcteristion methods re relly efficient s it hs been demonstrted. 1, 2 One of the most ccurte is the interferometric set-up. The mesurement of the linewidth llows, with fitting curve, the extrction of the linewidth enhncement fctor or Henry fctor α H. 3 But this kind of extrction requires intrinsic prmeters tht re not commonly known. This technique is compred to nother one tht uses dispersive properties of n opticl fiber by direct modultion of the source. The modultion response is influenced by the α H fctor which cn be esily clculted. We hence compre both resuslts. The im of the study is to present the extrction of the Henry fctor of long-wvelength VCSELs by using two different techniques: sttic nd dynmic ones. Further uthor informtion: lexndre.bcou@ise.fr, Telephone: +33 (0) ngelique.rissons@ise.fr, Telephone: +33 (0) jen-clude.mollier@ise.fr, Telephone: +33 (0)

2 ( Em -LS! J! O p t i c l P o w er m W ) V o l t g e ( V ) C u r r e n t ( m A ) Figure 1. Sttic mesurements 0. 5 m m misso. 654 nm 0.80 nmid m mnm Figure 2. Spectrl behvior of the VCSEL 2. VCSEL CHARACTERISTICS The device tested here is 1.55 µ m VCSEL fbricted by VERTILAS GmbH. The top mirror is Brgg reflector mde of 35 lyer-pirs of InGAlAs/InAlAs while the bottom mirror is 2.5-period hybrid CF2/-Si/Au lyer stck. The ctive region consists of five 8nm quntum wells. Electrons re injected through the top n-doped mirror wheres holes re generted by buried tunnel junction (BTJ) whose dimensions nd geometry llow the device to operte in single longitudinl mode. The VCSEL from VERTILAS ws pckged in TO-46 cse nd then pigtiled with single mode fiber. Fig. nd present the sttic behvior of the VCSEL. In fig. 1, the light-current chrcteristic shows threshold current of 1.8mA t room temperture (close to 25C) nd mximum opticl power of 1.34mW. The voltge-current curve gives the series resistnce, which is bout 45Ω. Fig.2 shows tht the device opertes in single mode emission throughout the bis rnge with Side-Mode Suppression-Rtio (SMSR) of 35dB. The wvelength tuning s function of bis current is 0.25nm/mA. 6 i s 3. THE HETERODYNE METHOD The first set-up presented in this pper is n opticl heterodyne interferometer for the mesurement of the lser linewidth. 5, The heterodyne nlysis is well-known technique bs ed on interference between two lightwves. The first one is emitted by the VCSEL nd the second is delivered by tunble single mode light source used s locl oscilltor (LO). The linewidth of the VCSEL is obtined by mesuring the bet-note between both lsers. However, there is condition to respect bsolutly when p erforming this kind of mesu rements: the theoreticl linewidth of the LO must be less thn tht of the VCSEL. The experimentl set-up described in figure 3. The VCSEL is followed by dul stge isoltor (more thn 40dB) to protect the cvity from ny feedbck, nd both lsers re connected using F C/APC connector becuse of their very low return loss. Then the two opticl fields re mixed into coupl er where interference is loclized As the VCSEL is pckged in TO-46 cse nd pigtiled, therml control is difficult to chieve efficiently. Therefore the mesurements were done t room temperture while polristion controler ws dded to improve the fringe pttern contrst. cl Spectrum Anlyser (OSA) to control the super- The first output of the coupler is connected to n Opti position of both lightwves through the visul inspection of their respective spectr.

3 LI Oscilltor Isoltor VOSEL Isoltor H t (z SbUed ention Figure 3. Opticl Heterodyne set-up Opticl Spectrum Anlyzer h Electricl Spectrum Anlyzer The second output is linked to p-i-n photodiode followed by trnsimpednce mplifier nd then connected to n Electricl Spectrum Anlyser (ESA) to mesure the VCSEL linewidth. The linewidth is mesured t -3dB but lso t other different levels like -10 nd -20dB with proportionlity coefficient tht re 5 99 ν respectively. 7 The lser linewidth is given by the following reltionship: 9 ν nd ν = (Γ.v g.g th ) 2 η 0 4 πp 0.n sp.h.ν (1 + α H ) (1) v g g 0 η 0 η 0 η d i, η d η i ν where Γ is the opticl confinement fctor, the group velocity, th the threshold gin, P the output opticl power, the efficiency coefficient with = /η the differentil quntum efficiency nd the internl quntum efficiency, n sp the spontneous emission fctor, h the Plnck constnt, the lsing frequency nd α H the Henry fctor. The first term of this reltion cn be written s function of the photon lifetime: Γ.v g.g th = 1 τ p (2) Generlly Γ nd v g re known nd with vlues of τ p selected from the litterture, tht llows us to found g th esily. All vlues used in this rticle re summrised in tble 1. As it hs been demonstrted in the reltion (1), the linewidth vries s function of the inverse of the opticl power. Linewidth mesurements nd fitted curve re shown in fig.4. We found minimum lser linewidth of 20MHz nd mximum lser linewidth of 112MHz. Some similr results re described by Shu et l. In fig.4, we cn see two different regions. The first one describes the decrese of the linewidth when the output power incre ses wheres in the second, there is rebrodening of the linewidth. This effect could be due to the presence of side-mode on the right-hnd side of the min mode in the fig.2. 9, 10 To confirm this effect, n nlysis of the linewidth fter hving filtered the side mode could be done. But it is not the focus of this pper. The extrction of the linewidth enhncement fctor (LEF) is found from fitting linewidth with inverse output power in the liner regime. Th best fit corresponds to LEF α H = 7. This vlue grees well with results found in the litterture 11 for the linewidth nd Henry fctor. 8, 12 This kind of mesurement is importnt in chrcterizing VCSELs becuse the α H fctor depends on the shpe of the gin profile nd is function of the wvelength. 13

4 Mesure α H = 3.7 Linewidth MHz Inverse Power 1/mW Figure 4. VCSEL Linewidth versus inverse opticl power Prmeter Symbol Vlue Confinement fctor Γ Group velocity v g m.s 1 Threshold gin g th 2410cm 1 Externl differentil efficiencies η d 0.26 Internl differentil efficiencies η i 0.8 Spontneus emission fctor n sp 1.6 Plnck constnt h J.s Lsing frequency ν Hz Linewidth Enhncement Fctor H 3.7 Photon Lifetime τ p 3 12 s Tble 1. Intrinsic prmeters of the VCSEL This kind of mesurement is cumbersome nd mny conditions must be known in dvnce to obtin good vlue of the LEF. Moreover, some intrinsic prmeters must be known to extrct representtive vlue of the LEF nd there is degree of uncertinty on these prmeters. Finlly the extrction of the LEF gives n pproximted vlue, 3.7 ± 0.3, tht is not very ccurte but relistic, tht is not the cse from other methods. 13 Tht is why this method is compred to the following one becuse it llows direct extrction of the α H fctor. 4. OPTICAL FIBER DISPERSION SET-UP The second set-up relised to extrct the LEF uses the dispersion properties of n opticl fiber. 14 Indeed, chromtic dispersion of opticl fibers is responsible for deformtion of n opticl signl when it propgtes through the fiber, inducing the pprition of extinction frequencies. The set-up implemented to mesure the extinction frequencies of n opticl fiber link employs Vector Network Anlyser (VNA) HP8510C coupled with n opto-electronic module HP83420A including clibrted lser nd photodetector. Indeed, before ech mesurement, ll the components tht do not contribute to the test re tken into ccount in the clibrtion step to compenste for their effects. Figure 5 presents the extinction frequencies of the tested fiber.

5 0 5 Module (db) Frequency (GHz) Figure 5. experimentl extinction frequencies of singlemode fiber The response mesurement is crried out with exctly km of opticl fiber in the rnge of GHz. The longer the fiber, more is the number of significnt extinction frequencies. The reltion between extinction 14, 15 frequencies nd fiber length is given by: ( fu 2.L = c 2.D.λ u 2 ) π rctn(α H) (3) where f u is the frequency of the u th order, L is the fiber length, c the light velocity, D the fiber dispersion, λ the wvelength of the VCSEL nd α H the linewidth enhncement fctor. We see tht extinction frequencies re directly ffected by the α H fctor of the emitter. So, s the LEF increses, we see the extinction frequencies shifted towrds lower vlues. We limit the study to the first extinction frequency, u = 0, nd thus the reltion (3) cn be written s: ( π α H = tn 2 π.f2 ex1.l.d.λ 2 ) (4) c This mesurement ws relised with the set-up presented in figure 6. The VCSEL is mounted on 50Ω microstrip line nd is directly modulted by smll-signl from the VNA. In order to relise the VNA clibrtion, we used stndrd clibrtion kit (Short, Open nd Lod) mounted on TO-46 pckge. It llowed us to void mximum of prsitics from the pckge module. Moreover, leds of the TO-46 cse hve been shortened to void resonnt pek frequencies. 16 The opticl signl then trvels long the fiber nd is detected by the clibrted photodetector of the VNA. When it is modulted, the opticl field of the VCSEL is composed of the fundmentl bnd surrounded by sidebnds. Extinction frequencies expresses interference between the crrier nd the sidebnds of the signl. By precisely clculting fiber properties, the α H fctor cn be exctly found. The operting wvelength of the VCSEL is 1561nm, so we find dispersion prmeter of D =17.93ps/(nm.km). An Erbium Doped Fiber Amplifier (EDFA) is connected to the output of the VCSEL to hve better dynmic rnge. The result is presented in figure 4.

6 000 p. ore0 VCSEL Figure 6. Set-up configurtion for VCSEL extinction frequencies mesurement Vectoril Network Anlyzer Opticl Fiber Roll X: Y: Module (db) Frequency (GHz) Figure 7. Extinction frequencies of 1.55 µ mvcsel

7 The first extinction frequency is then mesured t 3.061GHz, s shown in figure 7. We clculte hence the LEF of the VCSEL: α H =3.72. This vlue is in good greement with vlues reported in the previous 11, 12 experimentl investigtion nd in the literture. Finlly this method gives very ccurte vlue of the LEF of 3.72 ± 0.02 nd is esier to implement. The dvntges of such method cn be stted s follows: the set-up is not difficult to implement so the mesurement is fst. mesurements cn be done for ech bis current: the VCSEL wvelength vries s function of the bis current. The fiber dispersion prmeter cn be modified nd the vlue of α H cn be found. the extrction of α H fctor does not need preliminry mesurement of ν. the evolution of α H fctor s function of temperture cn be esily found. 5. CONCLUSION The linewidth enhncement fctor of 1.55µm singlemode BTJ VCSEL hs been extrcted from mesurements bsed on two different experimentl techniques: the first one is bsed on heterodyne interferometer nd the second uses the dispersion properties of opticl fibers. It hs been shown tht, if the second method gives the Henry fctor by simple reding of extinction frequencies, the first method lthough more complicted, llows us to tke into ccount the level of opticl power delivered by the VCSEL. Experimentl results from ech set-up re in good greement but the first one is vlid only s long s the opticl output power is less thn 0.5mW, before the rebrodening. This rebrodening cn be cused by the side mode tht ppers on the VCSEL spectr even if the side-mode suppression-rtio is 35dB. Moreover, it could be interesting in future work to mesure the linewidth of only the min mode fter hving filtered the side mode. The threshold behvior could be investigted too 13, 17 using the Fokker-Plnck model to fit the linewidth behvior. 6. ACKNOWLEDGMENT The utors would like to thnk VERTILAS GmbH for providing us TO-46 cse for the VNA clibrtion nd for vluble technicl informtion on the device structure. REFERENCES 1. T. Fordell nd A. Lindberg, Experiments on the Linewidth-Enhncement Fctor of Verticl-Cvity Surfce-Emitting Lser, IEEE Journl of Quntum Electronics. 43(1), pp. 6 15, A. Villfrnc, A. Villfrnc, J. Lzro, I. Slins, nd I. Grces, Mesurement of the linewidth enhncement fctor in DFB lsers using high-resolution opticl spectrum nlyzer, IEEE Photonics Technology Letters. 17(11), pp , C. Henry, Theory of the linewidth of semiconductor lsers, IEEE Journl of Quntum Electronics. 18(2), pp , M. Ortsiefer, M. Furfnger, J. Rosskopf, G. Bohm, F. Kohler, C. Luer, M. Mute, W. Hofmnn, nd M.-C. Amnn, Singlemode 1.55 µm VCSELs with low threshold nd high output power, Electronics Letters 39(24), pp , H.-P. Compny, Fiber Optic Test nd Mesurement, Prentice Hll PTR, P. Signoret, F. Mrin, S. Vicini, G. Belleville, M. Myr, J. Tourrenc, B. Orsl, A. Plis, F. Gborit, nd J. Jcquet, 3.6-MHz linewidth 1.55µm monomode verticl-cvity surfce-emitting lser, IEEE, Photonics Technology Letters 13(4), pp , L. A. C. S. W. Corzine, Diode Lsers nd Photonic Integrted Circuits, John Wiley & Sons, Inc., R. Shu, H. Hlbritter, F. Riemenschneider, M. Ortsiefer, J. Rosskopf, G. Bohm, M. Mute, P. Meissner, nd M.-C. Amnn, Linewidth of InP-bsed 1.55µm VCSELs with buried tunnel junction, Electronics Letters 39(24), pp , 2003.

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