CA1265604A - Semiconductor lasers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The emission linewidth of a semiconductor laser can be reduced by operating it at a wavelength which is less than the wavelength of maximum gain at the threshold current, .lambda. max. An assembly for carrying out such operation comprises a ridge waveguide laser provided with a distributed feedback (DFB) grating. The period of the grating is selected to give a predetermined emission wavelength to the laser which is shorter than .lambda. max. The invention finds application in optical communications, particularly in coherent systems.

Description

I IJ\JU/ L~L`''~''/

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~ he pre~ent lnventlon relates to ~e~iconductor laser~
and finds ~pplic~tlon in optlc~l communication~, particularly in coherent systems.
The radiatlon u~ed in op~ical communic~Ion~ i~ not nece~6arlly in the vf~lble regioll, and the ~ord~ ~op~ical~
snd ~l~ght" when u~ed ln thi~ speciflc~tlon ~re not to be interpreted as lmplying any such llmltation. lndeed, lf ~llic~ opt~c~l flbres ~re used aB the tran~mission medium, infra-red radlation 19 of e~peclal usefulnes~ because the loss minlma occur ln ~uch flbres nt 1.3~m ~nd 1.55~m appro~imately.
Semlconductor la~er structures include ~ p-n Junctlon acro~s whlch current flows (the conventional current fro~
p to n) and an ~sctlve layer~ in whlcb electrons and holes combine ~lth the production of photon~ by ~timulated emlssion. The actlve ~ayer ha~ to relate sult~bly ln band gap and refractive inde~ to the other ~emiconductor regions of the structure in order to achleve a ~ultsble degrec of ~confinement" of these processe~ to the actlve layer. The layers of materi~l to elther ~ide of the ~ctive layer and in con~act ~ith the opposite f~ce~ of the ~ctlve layer are known as "conflnement l~yers".
A ma~or field of appllcat~on of se~iconductor optical device~ i~ ln Opeic~l fibre co~municstlon~ sy~tems.
Sillca optical fibres A~ produced in recent yearfi have 108s minlma at 1.3~m and 1.55~ ~pproxim~tely, the l~tter ~ini~um beinB the deeper. Accordlngly, there i~ an especial need for devlces operatin~ lo the range frGm l.l to 1.65~m, e~peclally from 1.3 to 1.6ym. (These wavelengths, llke all the wavelen~th~ hereln e~cept ~here the conte~t indicates ~therwl~e, ~re ln vacuo w~velengths~.
Semlconductor laser~ vperating in this region of the ~Z65~
infr~-red u~uAlly co~pri~e reg~on~ of lndlu~ phosph~de and of quaternnry ~aterial~, lndlu~ gallluo ar~enlde pho~phides (In~ ~al_~ A~y Pl_~). Br ~ultcble choice~ of ~ ~nd y ie i~ pos~lble to l~ttice-~atch the ~rio~ regions ~hlle varyin~ the baDd ~ap~ of the ~aterlal~. (Band gap~ cnn be determined experi~entally by~ ~or example, photol~ine~cence). Addltlonally, both lndlum pho~phide and the quaternary ~aterlals cun be doped to be p- or n- type ~8 de~ired.
Semlconductor lasers comprislng regions of galllu~
slu~inium arsenlde and galliu~ arsenlde are ~lso u~ed for co~municRtion6 purposes. These operate near to O.9~m.
The photons produced by stimulated eml~ion, wheo a laser i8 driven ae a current above a thre~hold current, are caused by the de~lgn of the l~ser to o~cillate tn n dlrection along it, in the actiYe layer, before belng emitted. ~uring each passage through the material oE the acti~e layer the number of photons ls lncreased to a degree deter~ined by the balance between galn and lo~ses iD the active layer. The ~ain sho~s a peaked 6pectru~
against emission uavelength and ehere 18 clearly ~n ~dvantage in ~orking st the galn peak of the ~aterisl of the ac~ive layer.
In a Fabry-Perot laser, oscillation ia cauæed by at least psrtially reflecting eDd-face~ of ~he la~er ~tructure, ly~ng at elther end of the sctlve layer. In a dlstributed feedback (D~B) la~er~ o~cillation 18 caused by corrugations which lle in the region of the actlve layer, e~tending generally perpendlcular to the len~th of the laser ~truc~ure, ~he corrugatioDs reflectin~ radistion in each direction along the la~er structure.
Structures e~ternal to the laser can 81BO contribu~e eO D~cillation b~ reflecting r~diation bsck into th~
la~er. Such structures include exter~al cav~ties aod distributed Brag~ reflector3 (DBR). ~ernal ca~i~ies ~ay for instance comprlse a ~irror pl~ced at a pre~elected d~t~nce ~long the emls~ion 9~ lternatlvely radlatlon mny be reflected b~ck into the la~er by ~e~na of corrugationa, ai~ilar to those of c DFB l~er but shlfted to 8 po~itlon outslde the laser, adJacent to the emis~ion a~is. La~ers having the latter e~ternal structure are known aæ dlstrlbuted Brag~ reflector (DBR) lasers.
In some Dpplications, particularly coherent optlc~l co~unicatlons, lt 18 i~portant that the emitted rndi~lon sho~s a narro~ linewidth. Thia allows coherent detectlon ~ys~em3, ~uch as heterodyne or ho~odyne detection, to be used ~nd much Breater nmounts of dflta to be tr~nsmltted as a consequence.
Fabry-Perot la~ers have bee~ found unsuitable, havlng linewldtha of ~ore than 100 M~z. It 18 kno~n thst DF~
lssers can be fabric~ted having narrower e~ission linewidths than those of un~odified Pabry-Perot laser6, and that additlonal structures ~uch as external cavities can result in narrowed emi~sion linewldths.
~ owever emlssion liDewldth has tended to remain an unpredictable characteristic of different laser ~tr~ctures.
Considerable work ha6 bee~ done ln trying to as~eas the factors which con~rol linewidth ln a laser. In the paper "Theory of the Line~idth of Se~iconductor La~ers~ by Charles ~ ~enry, IEEe Journal of Quantum Electronlcs, ll~b~ v Q~ (2), ~ebruflry l9B2 pp ~59-264, 9 theory 1~ pre~ented uhlch arrlves ~t ~ broadening ter~ Oc 2~, oc belng a fund~ment~l para~eter of the l~er ac~ive ~ster1al ~o~eti~e~ kno~n aa the linewidth enhaDce~ent factor.
As ~ell A8 linewldth, ~c has been aho~n to affect the de8ree of transle~t wavelength chirplng ln d~rectly ~odulated lasers.
There has been found, however, ~ubstan~lsl a~biguley in the magnltude of oe ~n long wavelength l~ers. Value6 of ~c rsnging fro~ 202 to ~.6 h~ve been mea~ured or inferred. Purther9 ~ ~ystematic dependence of PC on laser length ha~ been reported but une~pla~ned. Thi~ latter effect 18 descrlbed in ~Mea~urements of ~he Semiconduc~or Laser Line~idth Broadening F~ctor~ by aenning and Collins, ~lectronlc~ Letters 1~83 l9 pages 927-929.
In the paper ~On the Linewidth ~ohance~ent Factor ln Se~iconductor In~ection Lasers , by ~ Vnhala et al, Applled Phys~c~ Letteræ 42 (8), 15 Aprll 1983, i~ is predicted that in undoped Ga As, wlll decreaæe with increa~lng excitation frequency.
Nork has now been done, in making the present inveDeion~ by ~eaD6 of ~hich prAc~ical e~bodiments of lasers ~ay be des~gned whlch e~ploit a relationship bet~een e~sion line~idth snd operating ~svelength.
Signiflcan~ i~prove~ents in the emlssion line~idths of la~ers useful in optical com~unlcations for in6tance those havin8 e~ lon ~aveleDgth6 of 1.3 and 1.55~, can be achieved.
~ an obJect of the present inventio~ to provide semicoDductor la~er a~se~blies ~hich have reduced e~ission line~idths.

- Accordin8 to the present in~entlon there ls provlded la~er ~sse~bly ~hlch comprlse~ a ~emlconductor la~er structure aDd ~ean0 for selecting the r~ ion ~avelength of the laser ~tructu ~, the selected ~avelength being ~h~rter thAn the ~avelength of maximu~ gain 0t the thre~hold current, ~ ~ax, by an a~ount ~uch ~hat the line~ldth enhancement f~ctor oc at ~ ma~, oc max, and oc ct the wavelengtb of the emltted radl~tlon, oC ~, are related in the ~anner cC e -~ 0.9 oC ~ax Preferably the ~elected ~avelengeh i~ shorter than by an nmount 0uch that ~c max and oc e sre related in the manner G~e ~ 0.8 ~C ~a~
and eveD more preferably, such that ~e ~ 0-7 oc ~a~
Advane~geously the means for selecting the eml~sl~n wavelength co~prises a structure which in it6elf will encourage a narrow linewldth e~ission from the laseT
~ssembly, such a8 a distributed feedback gratlng or an e~ternal cavity.
More advantageously, the ~eans for ~electing the emission wavelengeh co~prises a comblnation of ~tructures ~hich each ln the~selves encourage ~ narrow linewid~h e~ ion from the laser as~embly, such a8 a di~tributed feedback gratlng ln co~bination wlth an e~ternal caYit~.
Preferably the arrangement is such that the selected emission ~avelengeh lIes in one of the ran8es 1.2 to 1.35 ~m and 1.48 to 1.65 ~m. This iB important where the laser asse~bly ls to be used in generating radlaeion for transmis~ion by means o~ sillcR optical fibres.

Enbodlmento of the pre~ent lnventlon ~lll no~ be deocribed, by ~ay of e~ample only, ~i~h r~f~rence to the ~CCOmP~OY1nB P1gUreB 1D which:
P~BUre 1 8ho~ a three-qu~rter view of a laser sccordlng to ~n cmbodl~ent of the pre~ent i~ention; and Figure 2 ~ho~s ln graph for~ the relatlon~hip between the linPwldth enhancement factorsC snd photon energy ln a laser.
It ~hould ~e noted that Plgure I i8 Rche~atlc snd 1 not dra~n to scale.
In the followlng de~criptlon, and elsewhere ln thiA
specif~catlon, terms such a5 ~on top of~ ~nd ~underside"
are used. The~e terms are used for convenlenc~ only and should not be taken to denote a p~rtlcul~r orientation of ~ny device unle~s it i8 clesr from the conte~t that a particular orieDtation i8 intended.
Referring to Figure l, ~he laser is of the type de~cribed in our European patent appllcAtion number 85301599.8, le. a DFB rldge ~aveguide la~er.
The substrate 1 1~ a heavll7 S-doped InP ~n~-type) sub~trate approxi~stely lOO~m thick. ODto the (lOO) face i8 gro~n a flrst conflnement l~er 2, 0.15~ thick, of Te-doped ~n-type~ Ga~ Inl_~ Asy Pl_y~ ~ and y being ~elected such th~t the materisl ha8 a bsnd gap wa~elength equlvalent of l.lS~m a8 deter~ined by photolumine~cence. Onto the flrst conflnement layer 2 18 gro~n an ~ctiYe layer 3, 0.l5~m thlck, of undoped Ga~
Inl_x Asy Pl_y~ ~ sDd y belng selected such that ~he ~terial has a band gap ~quivalent of 1.667~m. Onto the active layer 3 i8 gro~n a second confi~ement layer 4, 0.2~m thick, of the sAme ~aterial as tbe first confinement layer 2.

/ U~ &5;~;~4 The second confineoent lsyer 4 iB corrug~ted to provide a dl~tr~buted ~eedback grating 9 by che~lcal etching through an electroD-bea~-e~posed ~k ln the ~snner de~cribed b~ ~ec~brook e~ al, Elec~ronlc~ ~etter~
1982 lô pa~e~ 863 to 865. The corrugatlon~ run ln the (110) direction, co~prising triamgular grooves ~lth (111) A aide vall~. The period of the gratlng 9 i~ 475 n~ and the grooves are approxi~a~ely 170n~ deep.
On top of the grating 9 lles the ridge of ~he rldge wavegulde ~tructure, comprlsln~ ~ layer 5 ~pproxlmately 1.5ym thlck of Zn-doped (p-type) indIum pho~phlde grown by aemospheric pre~sure, metal organic chemlcal vapour depo~ition (MOCVD) while ~a~ntalniog the integrlty of the grating~ a~ prevlously de~crlbed (European Patent Applicntion 84 300240.3 and al~o Nelson et al, ~lectronlcs Letter 1983 19 pa~ea 34 to 36). ~o top of the latter lndium phosphlde layer 5 i~ grown, al80 by MOCVD3 a lager 6 appro~imately O.l~m thick of heavlly ~n-doped ~p~-type) ternary material of nomlnal co~pofiitlon InO 53 Gao 47 ~8. Lsstly electrical eootAct layers 79 8 of titanlum and gold respecti~ely, each about O.lym thick, are provlded on the layer 6 of ternar~ materi~l. A further contact layer 10 i~ provided on the underside of the substr~te 1, by evaporation of tin and gold follo~ed by alloy~ng.
The ridge ic about 6~m ~ide and the laser a~ a whole i6 300~ lo~g, havlng one cleaved end fscet and one end facet damaged to reduce reflect~oD. I
To either side of the ridBe lie further rai~ed portion6 11, 12 of ~emiconductor ~aeerial, esch ~eparated fro~ the ridBe by a channel. These ralsed pvrtlons 11, 12 have a ~ilar layer construction to the ridge bue each hsA ~n extra layer 13 of ~ilica below the cont~ct layer~
7, 8.

- B

In u~e, the DFB rid~e ~aveguld~ er described abo~e ~ill e~lt r~distlon hsvlng a uavelength centred cn 1.5S~, and a linewidth of about 10 H~z ~t 10 ~W outpnt pouer.
The bsDd 8ap ~qal~Alent of the materl~l of the activ~
lJyer, a~ stated, 1~ 1.66t~m. In a Pabry-Perot la~er ~ithout A gratin~, this ~erlal uould ~how a wavelength of ~axl~u~ gaio at the threshold current ( ~ ma~) of 1.61~m. Therefore the DFB ridge wavegulde ~tructure e~it~
radiQtion hsvlng ~ wavelength ~llch 18 00 nm le~ thno the ~ a~ of the ~aterlnl of the a,ctlve lsyer.
Other la~er ~tructures may give a narrower e~is~ioo llnewidth than the one descrlbed above. For in~tance If ~he lengeb of the laser a8 a whole ~ere increa~ed to 8~0~m, the lloewldth ~hould be reduced to sbout 1 MHz ~t lOmW
output power. ~owever in each csse the ls~er will be benefitting to the ~ame degree from operatlng at an eml3slon ~avelength of 1.55~m, 60 nm belo~ ~ ma~ at 1.61ym, in ~ccordance with the pre~ent invention.
The la~er de~cribed above is designed to emlt at ooe of the opti~al wsvelen~th~ 1.55~m~ for u~e wlth ~illc~
optical fibres. A la~er de~igned to emit at ~he other optimal ~avelength, 1.3~m, ha~ the following ~odiflc~tion~:
~ i) the quaeernary materisl of the sctlYe layer has a composltlon selected such that lt~ band gap equlvalent ~avelen~th ls 1.452~m (equivaleot to 855 meV) ~ ~a~ bei~g 1.36ym; ~Dd (il) ~be grating oolDpri6es corru~atlos~8 ~hich hnve 8 period of 398 nm and are about 142 Dm deep.
, Thl8 laser structure, at a lensth of 300~1m, will agalo be operating at 60 nm belo~ ~ ~ax at 1.36~m, nnd its llnewIdth vIll agaln be of the order of 10 MHz at un output pow~r of l~mW. If the length of ~he laser ie locrea~ed t~ 800~m, its e~l~sion liQe~ldth ~ill drop to sbou~ z at ao output potPer of l~t~.

11~bL)/U~ LI~U
_ 9 _ In ~n alternatlve con~trucelon to tbe DPB lDserD
descrlbed ~bove, an e~tern~l c~vity or a distrlbuted Bragg reflector msy be Uhed to reduce GC , ~nd ~herefore osrro~
the e~ls~ion linewidth of a laser, ~ccording to the preRent Invention. Botb ~tructure~ can be u~ed to select the e~ission wavelength of n laser. By u8ing it to ~elect an e~ission w~velength ~hort~r thAn ~ ~ax, an externsl cavity or di6tributed Bragg refllector can be used to achieve the obJect of the inventlon.
By u~ing ~ combination of structures which i~
them~elve6 narro~ the emission liDewidth of a laser, and ~or~ing at an emis5io~ ~avelength les6 than ~ ma~
narrower linewidths c~n be achieYed than those quoted above. For in~tance, ~ DFB ridge ~avegulde laser equipped wlth A suitable external cavity ~ay ~ho~n an e~issioD
linewidth At 10~ output power of le3s than 100 ~z.
It ~hould be noted that structures whlch can be used to select aD emis~ion ~avelength for a lsser can ~ffect the emission llnewidth independently ofoC For ~nstance, if an external cav~ty ~ith a ~irror i~ u6ed, the lioeu~dth ~8y be si~nificaDtly broadened lf the rouod trip p~th for radlAtion iD the ca~ity cau~es i~ to be out of phase with radiatioD ~scillsting ~ith~n the laser.
It i8 po~ible to esti~ate oc ~t different operatlng ~avelengths for the same structure. ~eferring to Pigure

2, oC has been deter~ined fro~ the Pabry-Perot re~o~ances 1D the laser eml~sion 6pectrum below thre~hold current by ~ea~uring the change in the ~ode gain G and the resonant Yavelength ~ ~ith curre~t I. G ~a5 deter~ined by the method of Hakki and P~oli di~closed in ~Gain Spectra in GaAB Double ~eterostructure In~ectlon Laser~n, Journ~l of Applied Phy~ic~ 1975 46 page~ 1299-1306.
~ riting the rafrscti~e index of the la~er active regio~ a~ n' + JD~ tSen ~c i~ defined a~

OC dD ' dn~ ( U / U L O ~ f 1 L~ U _ 10 Since bOeh r~ nd lm~g1nsry p~rt~ of the refractlve lndex ~re ~unctlon~ o~ the lnJected ~arriLer den~lty N,C
re usuall~ 8~n ~

c~ . 4~r d _ N (2) ~ dgldN

where the naterial galn g 18 related eo n~ ~hrough n~ - ~ g/4 1r dG/dI and d~ /dI sre related Ito dg/dN snd dn'/dN through dG - C dg dN (3) dI dN dI

(4) d~ = C.~ dn' dN
dI n dN dI

~here C 1B the ~ode confinement Eactor and n i~ the group refractive inde~ given by n ~ n~ dn' ) ~5 n' d ~

n ~as experlmeDtall~ determined in the u~ual way fro~ the ~abry-Perot mode spacing ~ ~ s uslng the rela~ion n 8 ~ (6) 2L ~fi ~here L 1~ the device length. Substltueion of equations ~3) and 14~ lnto equation (2) yields oc - 4~n d~ /dI (73 dG/dI

~ea~urement~ were performed on two noninally ldentical 190~m long In Ga A~ P rldge ~avegulde la~ers. The la~iog wavelength for both was 1.53~ (0.811 eV) ~nd the threshold curreoe ~a~ 3~mA. The laser temperature ~as malntained at 20C ~ 0.05C u~ing a ther~o-electric cooler. ~Ll measurements ~ere ~ade u~ing pulsed condltlon~ t500 n~ec pulses~ 0.12 duty c~clo~ thu8 a~oidlng effect~ due to de~lce heating.
The ~e~red varlatloD ln oC orer the v~velength r~nge ~ - l.b9 to I.57y~ ~0.7~1 eo 0.833 cV) i~ shovn ln Figure 2. The left hand vertical ~xiL3 i8 OC ~hile the rlght hand axls i8 the linewidth bro~deniog ter~
(1 + ~c 2) dlscloaed by Chsrles ~ ~enry. The ~æasured var~ution In oc over the 42 ~eV range in photon energles is approslmately oc ~ 3 to ll ~-ich enco~passes nearly ~11 the previously rcported me2surement~.
oC can be 3een to incrense rapidly a6 the photon energy approaches the band g8p energy, 0.~91 eY. The curve lllustrates the ad~an~sge that can be gained by operating a lsser at a wavelen~th belo~ ~ ~ax. At ~ a~, ie. at 0.811 eV, oc ma~ ~ 5.1. By operating the laser structure at a wavelength equivalec~ to0.819 eV
oC ~ 0.9 cc ~a~. By further decrea6ing the uavelength at which the la~er structure operates, for iostance tD
~avelengths equlvaleDt to 0.824 and 0.834 eV, oC can be reduced to value~ of 0.8 oc ma~ aod 0.7 oC max respectively.
As mentioned above, the emI3sloD li~e~idth of a lacer depends on many fsctors sDd the effect of a reduc~lon iD C can be heavily obscured, or even re ~han cancelled out, by other factors in the desig~ or operation of 8 laaer. F~r in6tance, operating ~emperature ha6 a stron~
influen~e on line~idth, increaslng te~perature re6ultlng in increased linewldth. In the description above, ~he figure~ for liDewidth given relate to laser ~tructure~
which are heat-stablli~ed in operation, by known techniques.
It ls ~ptimal, if the emission r~dlatloo ha~ a waveleDgth which lie6 iD the range 1.1 t~ 1.65~m, to operate at a wavelength 40 to 80nm, 8na particulsrly 60 n~, shorter than ~ ma~ for a partlcular la~er structureO Thi~ give~ a signiflcant reductio~ in emi~sioD
linewidth ~'Lthout iDcreasing the threshold current of the laser ~oo hiLgh.

- l2 -The reductlon in lln~ldtb ~hlch cun u~efullg b~
obtslned by oper~ting a ls~er ~tructure ot A ~avelen8th below ~ ~n~ ener~ tbe orde~ of 50~.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A laser assembly comprising a semiconductor laser structure and means for selecting the emission wavelength of the laser structure, the selected wavelength being shorter than the wavelength of maximum gain at the threshold current, .lambda. max, by an amount of such that the linewidth enhancement factor a at .lambda. max, .alpha. max, and .alpha. at: the wave-length of the emitted radiation, .alpha.e, are related in the manner .alpha.e ? 0.9 .alpha. max

2. A laser assembly according to claim 1 wherein .alpha. max and .alpha.e are related in the manner .alpha.e ? 0.8 .alpha. max

3. A laser assembly according to claim 1 wherein .alpha. max and .alpha.e are related in the manner .alpha.e ? 0.7 .alpha. max

4. A laser assembly according to claim 1, 2 or 3, wherein the means for selecting the emission wavelength of the laser structure comprises a distributed feedback grating or an external cavity.

5. A laser assembly according to claim 1, 2 or 3, wherein the means for selecting the emission wavelength of the laser structure comprises a distributed feedback grating and an external cavity.

6. A laser assembly according to claim 1, 2 or 3, wherein the selected emission wavelength of the laser structure lies in one of the ranges 1.2 to 1.35µm and 1.48 to 1.65µm.