Low frequency noise of a 980 nm InGaAs/GaAs strained quantum well laser

Transcription

1 Low frequency nose of 980 nm InGAs/GAs strned quntum well lser B. Orsl J. Pernsn P. Sgnoret K. Dulsm To cte ths verson: B. Orsl J. Pernsn P. Sgnoret K. Dulsm. Low frequency nose of 980 nm InGAs/GAs strned quntum well lser. Journl de Physque III EDP Scences (9) pp < /jp3: >. <jp > HAL Id: jp Submtted on 1 Jn 1993 HAL s mult-dscplnry open ccess rchve for the depost nd dssemnton of scentfc reserch documents whether they re publshed or not. The documents my come from techng nd reserch nsttutons n Frnce or brod or from publc or prvte reserch centers. L rchve ouverte plurdscplnre HAL est destnée u dépôt et à l dffuson de documents scentfques de nveu recherche publés ou non émnnt des étblssements d ensegnement et de recherche frnçs ou étrngers des lbortores publcs ou prvés.

2 J. Phys Frnce 3 993) SEPTEMBER 1993 PAGE 1739 frequency nose of 980 nm InGAs/GAs strned Low well lser quntum hoppng hs Lorentzn dependence. The correlton wth the optcl nose s expermentlly mode shown n the low-medum frequency rnge. Mesurements of ths electrcl-optcl correlton gve hgh vlue of ths prmeter (0.8 my hoppng s present. chrcterstcs of crrer densty fluctutons. / f nose mode hoppng nose or mode response nose re troublesome these knds of nose mpede ttempts to mprove optcl prtton Mny studes hve been mde n order to ntroduce the termnl electrcl nose (TEN) s The devce used n ths work s 980 nm rdge structure Quntum well (QW) lser grown on durng 200 h. Current vlues were set n order to get n ntl output power of 30 mw. JOURNAL DE PHYSIQUE III -T No SEPTEMBER Q91 62 Clssfcton Phvtc..I Ab.f.c.t.I 42.55P B. Orsl J. M. Pernsn P. Sgnoret nd K. Dulsm Centre delectronque de Montpeller CEM (CNRS URA 391) Unverstd Montpeller II Scences et Technques du Lnguedoc Montpeller Cedex Frnce (Receed10 Noember 1992 reled I Aprl1993 <.<.opted 8 Aprl 1993j Abstrct. The longtudnl mode hoppng nd the relted termnl electrcl nose n InGAs/GAs rdge sngle quntum well (SQW) lsers re nvestgted. It s found tht electrcl w0.9) when the longtudnl mode Introducton. Severl knds of nose re generted n semconductor lsers becuse of the wdebnd coherence. of n stu chrcterzton of lser dodes n the hgh frequency rnge (10 Hz-10 Hz) men ]. Snce the mesurements re mde wthout ny optcl components undesred optcl II feedbcks re suppressed. The m of our work s to confrm the correlton beetween electrcl nose sgnture nd optcl power nose of strned quntum well lsers n order to control the n stu chrcterzton of these lser dodes by the low frequency TEN (I Hz-10 MHz). 1. Lser structure. GAs substrte (Fg. I). Ths lser elborted by Metl Orgnc Vpour Phse Eptxy method conssts of strned sngle quntum well GoInoAs s mterl n (MOVPE) double heterostructure. There re two confnng regons the gp of whch vres conventonl wth the composton n lumnum. The wdth of the rdge s 3 m. lnerly strned QW lser dode ws ged t 80 C n the utomtc current control (ACC) mode The

3 1740 JOURNAL DE PHYSIQUE III N 9 -) " f ). ;:( n.algas: clddng Fg I. Schemtc dgrm of 980 nm lngas/gas strned Quntum Well lser. photocurrent detected by the nternl control photodode. Ths photodode s not bsed n order dode : the lner zone where the optcl power ncreses very slowly wth the lser current re of spontneous emsson ; the «elbow» just below threshold (I I) 10 ma super-rdtve zone ; 0 lo p-gas:cp coln co ent (j$ O DA O.6..: n.gas: yo_24 butler 56h n.gas sub. 2. Sttc chrcterstcs. The I(I) chrcterstc recorded fter 200h of geng s shown n fgure 2. I s the to elmnte the I/f nose t genertes. I ndctes the lser current. On the I(I) curve we see the three usul regons of the sttc chrcterstc of lser nd the lner prt bove threshold where the photocurrent ncreses very quckly stmulted emsson. Iph (A) 1200 loco IL(mA) 2_ Frst order chrcterstc of the strned QW lser fter 200h geng: photocurrent F;g estts lser current IL- l

4 not contnuously lner. Ths phenomenon cn be explned by the mode hoppng behvour : shll come bck on t n the next prt. we fgure 3 the P(I) chrcterstc s shown. The optcl power P emtted by the lser In s deduced from the photocurrent I by the usul relton : where «s the pprent senstvty of the photodetector. Here «Fg. 3. Frst-order chrcterstcs of one strned QW lser : optcl power er.ui lser current. N 9 LOW FREQUENCY NOISE OF QUANTUM WELL LASER 1741 But here we hve to notce dfferent brek-ponts n the thrd prt of ths chrcterstc t s " Ip P opt A/lV. spectr of the lser re gven n nserts n fgure 3 for three dfferent vlues of Optcl nd 50 ma. I. CN T/LAB/COD P m I stepl:0h # Step2:200h.08 I(R )

5 3. Low-medum frequency nose mesurements. performed on ths devce. We studed expermentlly for the frst tme n the low nd medum [2]. dodes one hnd the totl electrcl nose s due to the fluctutons of the lser voltge On V(t) nd s gven by the voltge nose spectrl densty S.(V/Hz). On the other hnd the densty St (A/Hz) [2] : 3.I EXPERIMENTAL SET-uP. In order to nlyse the nose behvour of the lser we developed the expermentl set-up shown n fgure 4. In the frst chnnel we mesure Fg. 4. Expermentl et-up. 2 Atnpl 1742 JOURNAL DE PHYSIQUE III N 9 In ddton to ths frst order chrcterzton low frequency nose mesurements hve been frequency rnge (I Hz 10 MHz ) the electrcl nose of strned QW lser durng < f < Ths ws ntended to provde ddtonl dt n order to specfy nd chrcterze the geng. lsers for nstnce usng electrcl nose to qulfy the nose behvour of lser semconductor nose s due to the fluctutons of the optcl power P of the lser. It ppers s optcl of the detected photocurrent I (t ) nd s gven by the photocurrent spectrl nose fluctutons S (A/Hz) «Sp (W2/Hz) (2) usng voltge mplfer connected n prllel wth the lser dode. Wth the second Sj we mesure Ih nd chnnel by mens of stndrd InGAs PIN photodode the DC St nd AC outputs respectvely. Possble optcl feedbck s suppressed by n optcl soltor. photod lser V F " 47n tenson V eltter T. TABLE ANALYSEUR CALCULATEUR

6 N 9 LOW FREQUENCY NOISE OF QUANTUM WELL LASER 1743 of one mode when n other mode strts lsng. The nsert shows tme dependence suppresson voltge nd optcl noses gven by V(t) nd I(t) respectvely. of -t4 to too tooo t toooo Furthermore longtudnl mode hoppng s ssocted wth output power fluctutons nd other mode. Ths phenomenon s controled by the rndomly generted spontneous the tht works s the trggerng force for brngng one mode to lse. Intensty fluctutons emsson follow the sttstcs of Posson process [3]. Ths fct ntroduces excess nose sources whch In fgures 6 nd 6b both electrcl nd optcl noses re gven ess the lser current Frst comprng these fgures wth I (I ) chrcterstc (Fg. 2) we cn notce tht the non n the sttc chrcterstc correspond wth the peks n both electrcl nd optcl lnertes curves. The sme behvour hs been observed by Yoshkun et l. on the optcl nose nose [12]. fgures 6 show tht the electrcl nose s sn.ongljcorelted to the optcl nose. Secondly 3.2 EXPERIMENTAL RESULTS. The shpe of the power spectrl densty of the electrcl nose t 20 C nd for lser current I equl to 30 ma cn be pproxmted s voltge [3] (Fg. 5). Ths fct s due to the hoppng nose whch s ttrbuted to the Lorentzn l to VWHz) SW d(t) l f) -t j Ph(t) ww 312ps t (&equency rnge : 50O khz) -t3 -t6 to t000 t Fmq-- llz) Fg. 5. Spectrl densty of the electrcl voltge nose S (f). n excess nose both n the optcl ntensty nose nd n the electrcl nose [3]. It s ttrbuted to the suppresson of one mode by the decrese n crrer densty becuse t s spent lsng by cn be detected by electrcl nd optcl nose mesurements II ]. I. The fne structure observed t hgh polrztons n the behvour of both nose sources cn be explned by the hoppng-nose phenomenon (see Fg. 5).

7 Ill1 o 10 j j 6. ) Electrcl voltge nose of the strned QW lser vestts lser current l T 20 C Fg. b) Optcl nose of the strned QW lser vestts lser current l T20 C Ih11.5mA. 1 l1.5 ma. s ws ponted out by Ohtsu et l. nd the spectrl densty of power fluctutons process Lorentzn profle [3]. Ths model s bsed on the rte equtons for the mpltudes exhbts from the densty mtrx formulton nd detls of these equtons re clculted by derved the mterls constnts of the sem-conductor lser. usng 1744 JOURNAL DE PHYSIQUE III N 9 S (V2/Hz) 30mA j0 IV l I I 10 j l _ johz 10 -H 00fZ - fuohz - loom 10 I lo 100 IL (ma) ) s (A2/llz) lph 30 ma _j4 lo lo _j jjj _j6 lo o-? x o - Hz o-9-8» o _j _ oonw lo o o _25 lo loo IL (ma) 3.3 MODE HOPPING PHENOMENON. Mode hoppng follows the stochstcs of Posson of the electrc feld of the two neghbour modes (E ; I 1 2) nd for the crrer densty n (t n the ctve lyer of the lser cvty. As computed by Ohtsu et l. [3] these equtons re

8 N 9 LOW FREQUENCY NOISE OF QUANTUM WELL LASER 1745 emsson fluctutons whch work s rndom trgerrng forces for mode hoppng. spontneous the opposte of the Ohtsus ssumpton we cn not ssumed the crrer densty fluctutons In 6b). our cse f we consder therml nd I/f noses of seres resstnce S cn be wrtten s : In 4 KTRS + $ II R( + M l 2 fn q n( A f the I/ f nose source relted to men vlue of lser current I s the Hooge prmeter gves the men number of crrers whch flow trought the ohmc lyers. N on the lser current I. It s tghtly correlted to the optcl nose (Fg. 6b). depends the equtons to be solved for the followng dscussons re the rte equtons for Thus nd n(t) wth the nose terms due to spontneous emsson. E ths formulton t s nterpreted tht the occurrences of mode hoppng re cused by In nduced by ntrbnd relxton of the crrer n the conducton bnd [3]. bumng men vlue of ths relxton tme r hs been reported s bout: The 10 s w r w 4 x 10 s. normlzng the rte equtons for E set of non lner Lngevn equtons cn be By whch s expressed s [3 8] derved ( E f E E + q (r (I j normlzed to the rms vlue of the power fluctutons of spontneous emsson. power the pump prmeter s proportonl to the smll sgnl gn nd depends on the Therefore modes nd s expressed s j vlue of the mode couplng constnt f of 1.08 whch s lrger thn unty nd represents A Nose terms re dded to the rte equtons for E to represent the contrbutons from re neglgble becuse the Termnl Electrcl Nose (T.E.N.) due to fluctutons (An of the crrer number n(t) s very strong nd s correlted to the optcl nose (see Fgs. 5 nd 6 [1 2 5] S (F (3) vol Iv /c where Nc Nv re the effectve conducton nd vlence bnd denstes k s the Boltzmnns constnt T s the bsolute temperture no s the stedy stte crrer number An f) s the fluctuton of crrer densty Vol s the volume of the ctve lyer nd Rs s the seres resstnce. The frst term gves the therml nose due to the seres resstnce Rs the second We cn see tht the electrcl nose s not neglgble t s not constnt (see Fg. 6). It fluctuton of spontneous emsson nd by non lner mode couplng due to the spectrl hole de 1 2 ; I # j (4) where E s the normlzed mpltude of the electrc feld of the I-th mode nd r s the normlzed tme. The quntty s the pump prmeter correspondng to the sttonry mode bs level nd temperture. The quntty f represents the mode couplng constnt between the _ AA 2 where c s the speed of lght A s the wvelength nd AA s the wvelength seprton between the two modes [9]. nm 783 nm nd r 0.2 ps nto ths equton gves 0.79 Substtuton of AA strong couplng between the two modes [ ].

9 of spontneous emsson. They re supposed to be delt-correlted Gussn rndom processes (q*(t).qj(t)) where* mens the complex conjugte 6 functon. If f < (j 0) nd the two mode powers fluctute round these vlues fluctutons of spontneous emsson. by power fluctutons correspond to the mode prtton phenomenon. These f rndomly between zero nd non zero vlues whch corresponds to the mode hoppng jump Both of the phenomen cn therefore be descrbed by these equtons f the phenomenon. j dt q r g s + f (t ) w k w m (7) $ r r " s the spontneous emsson fctor n the k mode. y nose source term due to emsson nd bsorpton of photons s f(t ) nd Lngevn Lngevn s f(t). emssons) mode contrbutes prllel brnch to the crcut wth current lo correspondng to the Ech The gn of the k-th mode s tken s [7 8] : sgnfcntly reduces the sgnl to nose rto n communcton systems JOURNAL DE PHYSIQUE III N 9 The qunttes q (r) (I 1 2 ) re Lngevn nose terms whch represents the fluctutons wth zero men nd 4 &. &(r r) (I j 1 2) (6) s the Kronecker delt nd 3 (r r) s the delt set of Lngevn equtons gven by (4) hs set of sttonry stble solutons of ( (Ej (E ) If < < f on the other hnd these equtons hve two sets of sttonry stble solutons I-e- (j 0 ) nd (0 ). Fluctutons of spontneous emsson drve the two mode powers from one of the solutons to the other t rndom ponts n tme. Ech mode power therefore tends to vlues of the pump prmeters re pproprtely justed. In the experments these djustments re performed by chngng the bs level nd the temperture. 4. Interpretton. All these results could be explned by usng the nose equvlent crcut of sem-conductor mono- or mult-mode lser dode derved by the rte equton tkng nto ccount Mc Cumbers Hrders nd Andreksons theores nd lso by usng the coherence functon between photocurrent nd electrcl noses [ ]. y/_j rte equtons for lser emttng nto m longtudnl modes re [5 7 9] The : m dn ds n s here n s the crrer number n the gn medum nd s s the photon number n the lsng mode. k. s the pump term. r s the crrer lfetme nd s the photon lfetme. T q nose source term due to the crrer generton nd recombnton (spontneous nd stmulted k mode [ ] (see Fg. 7). g go + A nj (9) The ntensty fluctutons of dfferent modes re negtvely correlted becuse the dfferent modes re competng for gn from the electron «tnk» [9]. Ths so-clled competton nose

10 % L lph Lj._j j l t lonkudtnl Optcl modes Fg. 7. Nose equvlent crcut of multmode ler. nose prtculrly t the onset of stmulted emsson. Sf) n (W/Hz) s gven by s the spectrl densty of lght output power noe S(VW/Hz j s the cross spectrl S(W/Hz between power nose nd electrcl noe. If we consder the relton () equton (10) densty s f s f ph 1 ph y decre±;e; becuse optcl nd electrcl nose ource re decorrelted. j N 9 LOW FREQUENCY NOISE OF QUANTUM WELL LASER 1747 hoppng hoppn% nose nose L f ml se L v V t <Lm Our results clerly show the expected correlton between the electrcl nose nd the optcl 5. Electrcl-optcl coherence functon of strned QW lsers. The coherence yjj) between the voltge nose Sj(f) (V/Hz) nd optcl nose lsp(t)[ 2. (0) lf) Spt) where Sj(V Hz) s the pectrum denty o totl voltge nose of the lser dode cn be lso wrtten s S (1 j where Sj_ (f) the cros spectrum between photocurrent nose nd electrcl noe. In g/res 8 we gve the coherence uncton o the strned QW ler >e.i the y/_ normlzed ler current Illjj t low nd medull recluency (f 10 Hz. 100 LHz). Under threhold electrcl nd optcl nose re trongly correlted bout 80 %. We cn thnk tht the I/f no;e luctutons hghly depend on the spont±neou emsson (10(. Durng the trn;ton rom the ;pontneou ellson to the stllulted em;on we ob;erve decrese o the coherence whch s due to the f;t decresng of the electrcl nose Sj (I t the threshold. Just bove the threhold the y coherence reches t mxmum (round 90 %) whch cn be by the very I±st cre±ng o electrcl ±n(i optcl nor;es. Th ct I due to ode explned phenomenon whch generte; very hgh noe-level (Fg;. 6 nd 6b). When the hoppng current becolle hgher thn 50 ma hoppng proce; d;±ppe±r nd the coherence functon

11 Z I l/lj The expermentl results dsplyed bove confrm tht t s possble by mens of TEN to ccompnyng lgnments [1 2] JOURNAL DE PHYSIQUE III N 9 Coherence Hz khz 0 ) Coherence o -- j0 Hz khz 0 oz ll/lh b) 8. ) Coherence of the strned QW lser fter 200 h geng 1 Fg. I.5 ma. b) Coherence of strned QW lser fter h geng 1 the 15 ma. Concluson. The mesurements of electrcl nd optcl noses n the low-medum frequency rnge (10 Hzl00 Hz) hve shown the presence of mode hoppng nose n the strned QW lsers studed. Ths presence s confrmed by the mesurements of the electrcl-optcl coherence functon whch s very hgh when the mode hoppng phenomenon s present. the nose performnces of lsers: TEN s used for n stu mesurement nd pprse of lser dodes prtculrly durng geng process wthout ny optcs nd chrcterzton

12 The uthors wsh to thnk Ms Dr I. Jondot nd Mr Dr P. Devolddre from Frnce Telecom Lnnon) Frnce nd Mr Dr M. Fukud from N.T.T. Opto Electroncs Lbortores (CNET Ths work ws supported by Frnce Telecom (CNET Lnnon) under contrct Jpn. Andrekson P. A. Andersson P. Alpng A. nd Eng S. T. In stu chrcterzton of lser dodes I wdebnd electrcl nose mesurements IEEE J. Lghtwe Technol. LT4 N 7 (1986). from [2] Orsl B. Albedr R. Sgnoret P. nd Leteller H. InGASP/lnP Dstrbuted Feedbck Lsers for study Proceedng of S-P-I-E- E.C.O.4 «vol. 1512A (Mrch 1991). Netherlnds Ohtsu M. nd Termch Y. Anlyses of mode prtton nd mode hoppng n sem-conductor [3] Sgnoret P. DEA Report Study of semconductor DFB lers emttng t A 1.5m: [4] nd Electrcl Nose Mesurements Unversty of Montpeller II (June 1990). Optcl Mc Cumber Intensty fluctutons n the output of C.W. lser osclltons Phys. Ret. 141 II 966) [6] Orsl B. Rnrhrnosy K. Albedr R. Jondot I. Proc. lth Int. Conf. on Nose n Physcl [8] Budpet. Hungry (1989) Systems Jondot I. Ph. D Thess Reltve ntensty Nose of Semconductor lser C-N-E-T- Lnnon [9] of Montpeller II Scences et Technques du Lnguedoc II Unverty I Andrekson P. A. Alpng A. Electrcl mode hoppng nose n externl cvty semconductor lsers N 9 LOW FREQUENCY NOISE OF QUANTUM WELL LASER 1749 Acknowledgments LAB/OCM/TOH. References long wvelength optcl communcton systems A 1.55 m : Electrcl nd Optcl noses Infrred Mterl nd Devces» The Hgue The lsers IEEE J. Quntum Elecf.ol. 25 (1989) [5) Hrder C. Ktz J. Mrglt S. Schchm J nd Yrv A. Nose equvlent crcut of semconductor lser dode IEEE J. Quntum Ele<.f.on. 18 (1982) Rnrhrnosy K. Ph. D Thess Anlyss of electrcl nd optcl nose of semconductor lsers [7 t the wvelength o.8 m Unversty of Montpeller li Scences et Technques emttng du Lnguedoc (1988). Fronen R. J.. Wvelength dependence of I/f Nose n the lght output of lser dodes : n [lo] study IEEE J. QuntuJ1 Ele<.fr)n. 26 (1990) expermentl nd mode hoppng estmton by non optcl control loop IEEE.I. Quntum Elecf.ol. 23 n 12 (1987). Yoshkun Y. Kwgush H.. Tetsuhko I. ntensty Fluctuton of 1.5 m ngas/inp D-F-Bnvolvng [12] the optcl feedbck effect TEE Pf<.eedqI 132Pt J n (1985).