,

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Unb n R£ d b -Sd e nt |f |q ue lnte rnq l io n£ !e

® ,--¯

I nj ection -locked Semiconductor L aser s for

W.-Y. C* d , Z -K . Seo, arun t -K . Sung

D epartment of Electr ical and Electronic Engineering, Y on- i Univ. siU , 134,Shinchon-D ong, Sudaemoon-K u, Seoul , 120-749, K orea

w choi @P In n a e-±

A B ST R A C T

W e have performed numer ical and experimentali nvestigations into character istics of ifl ect ion-locked

semiconductor lasers for radio-on-f iber appli cations. W e

show that the i fl ection-lock ing technique is very usef ul in

suppressing distort ions occur red by the intr insic

nonl inear it ies of semi conductor lasers and f iber chromati cdispersion along f iber-optic l ink f or the gi ga-hertz

appl ications. W e also show that the inj ection- locki ng of

semiconductor lasers can be empl oyed in generati ng highf requency beat signals w i th very low phase-noises.

1. INTRODUCTION

T he radio-on-f i ber system has been attracti ng m uchattent ion since h igh f requency carr iers and sub-car ri er

mul ti plexed data can be sim ul taneously transmi tted

through a single f iber f rom one cen- al station to manybase stations w i th lmÍ transmission loss. M any research

groups have proposed and im pl emented var iousconf igurations for the f i ber -on-radi o systems H -4] .

hn a tion-locked semiconductor lasers can prov ide several

f unctions requi red in such systems. I n this paper , w epresent tw o cases for the appl i cation of i nj ection-locked

semiconductor lasers for radio-on-f iber systems. Fi rst ,

ifl ection-locking can signi f icantly reduce the f requency

chirp in semiconductor laser and his can be uti l ized in

reducing nonl i near characteristics i n analog optical l inksusing d i rectly modulated lasers. Second , sideband

inj ect ion- lock ing can generate hi gh qual i ty opti calmi l l imeter -wav e signals. Research resul ts for each

appl ication are presented .

modulation of semiconductor lasers is a si mple and low -cost approach for transmi tting RF -range subcarr iers-

H ow ever , semiconductor laser nonl inear i t ies and f iber

chromatic dispersion cause harmonic and intermodulationdistorti ons, and these can severely degrade ov eral l system

performance [5] . A l though IM D 3 is much smal ler thansecond-order harmoni c (2HD ) and sum term

intermodulation distor tion (IM D 2) , the overal l systempeNorman- is mainly af f ected by IM D 3 in systems w here

2H D and IM D 2 f requenci es l ie outside the f requency bandof i nterest [Ì . H ence, i t is crucial to suppress IM D 3.

Several techniques have been proposed i n w hi ch f ibergrating equal izers Ì or ti l ted etalons [÷ are used toreduce the dispersion-induced d istor t ions in SCM fiber

opti c systems. I n addi tion, opti cal i fl ection lock ing ofsemiconductor lasers has been investigated in order to

suppress the semiconductor laser nonl i near i t ies and I M D 3

[9 , 1Ì .

W e have perf ormed the f i rst experi mental observationof £ e IM D 3 variation of the i fl ection-locked D FB éL D

over f i ber transm ission H H . Fig. 1 show s the measured RF

spectra at the fundamental and I M P3 frequenci es in thef ree-running (Fig. 1a, 1b) and i fl ection-locked states (F ig.

1c, 1d) at OK m (F ig. 1a, 1c) and 30K m f iber transmission

(F ig. 1b, 1d) . T he I M D 3 is den ned as the ratio of thepow er at I M P3 f requency (= 2.8 G Hz) to the pow er at the

fundamental modtdating frequency (= 2.7 GHz) . T heI M D 3 for the free-running state is - 2 15 dB c at OK m and

increases to - 14 0dB c at 301011. T he reduction of pow er at

the fundamental f requency after 30lm1 transmission is due

to f i ber los , but the pow er at I M P3 f requency is not asmuch reduced, result ing in 75 dB increase in IM D 3. Forthe i fl ection-locked state (F ig. 2c, 2d) , I M D 3 is 2 L I M Bc

at O k111 and - 26.0dBc at 30 km . T he I M D 3 increases only

by 1. l 7dB af ter tile transmission- T hese resul ts show that

inj ection lock ing suppresses the laser nonl ineari ty as can

be seen f rom the reduced 0K m IM D 3 by 5.67dB , and alsoreduces the f iber dispersi on-induced I M D 3 by 12.OdB as

can be seen f rom the much smal ler increase in IM D 3 af ter

transn11SS10n.

IM D3 Suppr ession in h M enon-L ocked

Semiconductor L asers2 .

Subcarr ier mul t i plexed (SCM ) f i ber optic systems hav e

many appl ications such as w ireless local loops, cabletelev ision distr i butions and f iber -radio systems. T he di rect

2 5 n

transmission, I M D 3 star ts to increase and reaches -9.3 l

dB c at 40lun resul t ing in 12dB increase compared to 0ion.On the contrary , for the inj ection-l ocked state, I M D 3

increases only sl ightly . T he IM D 3 degradation is 15 5dB

and i ts var iation is maintained w ithin about 1.5dB for the

enti re transmission length.

®3. Optical M il limeter -Wave Generation usingSideband In a tion-Locking

Fig. 1. Measured power specua of the DFB-LD directlymodulated by two-tone RF signals

a. free-running, 0km b. free-running, 30kmc. inj ection-|ocked, 0km d. inj ection-locked, 3Okm

-5 -

¤1 O¹ r

,C -1 5Ä£®¤¤̄m -2 0o=- -2 5

. . .r , . .3d B|¹ r .

= -é= |- - -¤-- --¤- - I

O 5 1O 15 20 25 30 35 40

Fiber Length (km)Fig . 2. |MD3 q ain- fiber transmission length for free-

running and ifl ection-locked statefree-running

- injection-lm ked

T he reducti on of the dispersion-induced I M D 3 for the

if l ection-locked laser can be ex pl ained as fo l low s. W hen

the semiconductor laser is d irectly modulated by RFsignals, i ts optical spectrum incl udes harmonic and

inM m odulation products due to the laser f requency chirp.

D uring the transmission , the harmonic and intermodulation

products can be aff ected by the f i ber d ispersi on, dependingon the modulation f requency and f i ber length [Ü , w hich

leads to IM D 3 increase as show n in our ex periments. T heinjection-locked sem iconductor lasers have the reduced

frequency chi rp M d thus, the modulated signal incl udes thesmal ler harmonic aml in ter -modulation products-

Conseq uently , i t is l ess af f ected by the f i ber chromaticdispersi on, and the f i ber dispersion- induced IM D 3 of the

inj ection-locked D FB -L D does not i ncrease m uch.

Fig. 2 show s the measured I M D 3 at var ioustransm ission lengths. For the f ree-running state, IM D 3

does not change much unti l l OK In- A f ter 10km

~

¤3 O

T he sideband i fl ection-loclu ng technique is w idely

used for optical - generating micro/ m i l l imeter -wave

signals. I t can easi ly produce very high frequency signals

w i th low phase noises. Fig. 3 show s the operationalpr inciple of the sideband inj ection-lock ing technique. A

D FB laser acting as M aster u ser (M L) is RFm odulated

and several sidebands are produced that have the

f requency separation of the modulation * equa lcy (Q Î .

T w o of the si debands hav ing the target f requencyseparation are selected and used to inj ecti on-lock tw o

separate D FB lasers acting as Slave L ases (Sb ) . T woinj ection-locked SL s have the target f requency separation,

and are synchronized to each other since they are bo th

locked to the same M L . W hen the output l ip s f rom tw oSL s beat each other in a photodiode P D) , the desiredmicro/m i l l imeter -w ave signal is generated that has thetarget f requency and very low phase-no ise.

| | |- - - - ¤*th opt--Frm¤Xcy

(m ÍEll-

~~

RF-sa me (QÎ

â®%I I ,

, | | | ! l l» ||l =Opt-¤Freq¤ ncy F opm ®JõFreíqéu¦ eìnq . ,±̄®mn

Fig . 3. B|0Ck diagram for opt ica| m icro-/m illinner-wave

9eneration with sideband inj ection |od lr.

I n t he si d eban d M ec u m - l oc k ing sch em e, i t i s o f t en thecase tha t th e M L m od u la t i on f r eq uency i s a su b -harm o n i c

o f t he d esi r ed bea t f r eq uency . T h i s can s ign i f i c an t ly reduce

the req u i r ed M L m o d u lat i o n bandw i d th and t he operat ing

f r eq u en cy f o r the R F so u rc e. F o r ex am p l e , B raun er d

used 3 .2 C H z i n o rder to g en erate 6 4 G H z beat s ig na ls [Ü .

C o nseq uen tly , a caref u l co n tro l i n the S L l ash - f r eq ueu es

an d th e am o un t o f M L l i gh t i I l a ted to SL is req ui red i n

o r d er to hav e eac h S L l o ck ed to the targ et si d eban d .

R ec en t ly , w e hav e show n ex p er i m en ta l ly that , w hen th e

SL is in j ec t io n - l oc k ed to a cer ta i n tar g et si deband , the

p r esense o f a£ ac em M L si debands that a r e no t used fo r

i n j ec t i o n - l oc k ing p roc ess can i nf l uenc e the spect ral

charac ter is t i cs o f the resu l t i n g beat sig nal s [ 12] . T he

p resen ce o f un d esi red beat s ig n al s b g rap h i cal - i l l us tr ated

~~

2 5 1

¤%°Ï

ý Õ mas --

in Fi8. 3. Since these unselected sidebands can have

signi f icant inth ence on the overal l sy stem performance, i t

is very impor tant to be able to analyze the characteristics

of the undesired beat signals.

W e have f i rst performed the inv estigation into the

eff ects of unselected M L si debands on the character istics

of the undesired beat signals. W e hav e also considered the

infl uence of chromati c dispersion on the characteristics of

those beat signals.

¢£ ,cu£

£ ¤

®¢k Í * æ &5 þ

/ / ¬®,z¤

( N¾ £) ô ½î

. / V̄

(Bs--o¤s ¤-£¤¹ ¤E£

Ä( a )

Ý 15¢ - - -

s - -

/, ´ u¢º/ ¬ ¢1 O | CD

| O äu£-Ä -1o ¤

R (dB)

(b) Measurement

Fig. 5. Dependence of Ys (cHd es) and stable-lockingbandwidth (triangles) on R s̄.

-48 -32 4 6 O 16 32 48

OpticaI Frequency Offset (GHz)

H-2 5 -1 5

- - ®-

X = 23 . 1 dB

-

0 8 ( µ´¢£ £ ON) »O*£a ¤¹q

x = 37.5dB

X = 9 dB£ Ï

(ZeBu o¤s ¤-£® ,¹g

0( c )

O 16 Z4 32 4U 48RF-Frequency (GHz)

Fig . 4. Back-- -back optica| and RF-spectra by two slave

lasers when Qr--=8G Hz and n=k=2 in Fig . 3. Calculatedopt ica| (a) and RF-spectra (b) fo r R = -222 dB, andmeasured RF-spectra (c ) for estim ated R = -16 dB.

F i g . 3 sho w s t he ca l c u la ted o p t i ca l o u tp u t sp ec tr u m o f

tw o l oc k ed S L s i n F ig . 3 an d t he resu l t i ng bac k - to -back

R F -sp ec tr um fo r R = -2 2 2 d B , w h ere th e c en ter o f the

o p t ica l f r eq u en cy ax i s i n t he f ig u re r ep resen ts t he M L

c en ter lasi ng f req uency (e.g ¤- h = 15 50 n ItU - R i s d en ned

here as th e rat i o betw een t he av e rag e M L in j ec t ion andf ree-r un n in g S L p ow ers . M o st o f S L p o w er s ar e sh i f ted m

{he la s et s id eband - B ut . SOIl - o f t i le u nd esi r ed s id ebands.

can hav e summ o n g a i ns f r o m SL s, si n ce they c o u ld be i nIhe unstab l e- lo c k i ng r eg i m e- C o nseq uen t ly , the u nw an ted

si debands a re no t su f f i c ien t ly sup p ressed (F ig . 4 - (a) ) and

sig n i f i can t un d esi r ed bea t s ig na ls ar e p resen t i n the bac k -

to -back R F spec t ra (F i g . 4 - (b) ) . F ig . 4 - (c) sho w s a

m easured back - to -bac k R F -sp ec tr um f o r est i m ated R = - 16

d B .

8 ( µÝ¢.. oS ¤ m¤ -£¤H¹X

*URF-Frequency (GHz)

(a) After 30 km of nber,,40

35

m ê

b25 PT

2o

V |..o vo a 30 40 M a --

Fi¼ r Length {km]

(b)

Fig . 6 . Calculated dispersion effect on the beat signals for R= -222 dB. The target frequency is 32 GHz (squares),

and two a£ acent frequenc ies are 24 G Hz (c irc les) and40 G Hz (triangles). The solid hne represents thesuppression m no of und esired beat s igna ls (%).

W i t h i nc rpash - R the undesi red b eat s ig na l po w er s

i nc rease an d can b eco m e co m parab l e to the d esi red b eat

sig na l p ow er i n the R F sp ec tr um . T he d ec r ease o f R can

su pp ress th e un d esi red b eat si g na ls, bu t r ed uces the stab l e-

lo c k i ng ran g e. T h e t rad e-o ff i n R be tw een th e sup p ressi o n

o f a£ ac en t u nd es i red bea t sig na ls (P 24 @ 2 4 G H z , õ o @

40 C H z) aga i nst ta rg e1 bea t s ig nal (õ 2 @ 3 2 G H z) and

~

- 252 -

Ä

(b) ×

¢¢¤®* &A0 f ¢®

stable-lock ing banw id th is show n in F ig. 5 for the back -to-

back case. Fig. 5 show s that the numer ical resul ts are

qual i tati ve- in good agreement wi th the experimental results.

Fig. 6 show s the calculated dispersion eff ect on the RFspectrum of the resul t i ng beat signals. In order to consider

n ber chromatic dispersion along the fi ber -optic l ink

between SLs and PD in Fig. 3, the optical f iber is si mplymodeled w i th a l ow -pas equivalent transfer functi on g i ven

by

H¼Î (n =UPI- * Dõ 2L/c)l

w here |D | = 16 pm utt-kIn, and L is the f iber -optic l ength.

0 ptia l f i ber loss is not incl uded in the simulati on in order

to observe only the infh ence of fi ber dispersion on the

characteristi cs of t111desi red beat signals.

A n RF-spectr um af ter 3O km of f i ber (F ig . 6- (a))

clearly show s that the a£ acent undesired beat signals are

greatly suppressed by more than 14 dB com pared to theback-to-back case (F ig . 4- (b)) under the M me sim ulation

condi tions- h is because the f i ber chromatic dispersion

changes the opti cal phases of sidebands, and the sidebandscause the per iodic n ud ualions of beat signals af ter photo-detection as can be ex pected from [13] . Thus, i t i s possi ble

that tw o a£ acent t111desired beat signals af ter a certainlength of f i ber can be both greatly decreased due to thedestructive in ter ference betw een mul tip le sidebands at

lheir beat f requencies (24 GH z and 40 GH z, here) asshow n in F ig . 6- (b) . I t occurs f irst at around 30 km of f i ber

ln our simulati on.

4. St1111111aq

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° W iMd* ee -B BÐ aì nm d hMM 1± iH¹¦ lH¹¹ lU± ihú ¼mmnm tò¦ ew. rF - W aè vw e° /£ O pmm tnÈ ik®CÄd a£a lL -N ed tMw oîÆß fr±Ì .»k l ´ A p p l i c a t i o n s

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a n d d a t a t r a n s m i s s i o n u s i n g o p t i c a l s i d e b a n d i ¦ e c l i o n

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1 9 9 8 .

, { 5 l C . S . 1 h , a n d W . G u , ± F i b e r i n d u e d d i s t o r t i o n s i n a

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A r e a s f n C o m m - n . , v o l - 8 , p p . l 2 9 6 1 3 0 3 , 1 9 9 0 .

¹ D . B . C r o s b y , a n d G . J . L a m a l -d , ° D i s p e r s i o n - I n d u c e d l i m i t

o n t h e r a n g e o f o c t a v e c o n f i n e d o p t i c a l S C M t r a n s m i s s i o n

s y s t e m - ¯ f E E F P * o f o n . r e d n o f . Z e f L , v o l - 6 , p p . l 0 4 3 4 0 4 5 ,

1 9 9 4 .

m D . P a s t o r , J . C a p m a n y , a n d J . M a n i , ° R e d u c t i o n o f

d i s p e r s i o n i n d u c e d c o m p o s i t e t r i p l e b e a t a n d s e c o n d - o r d e r

m t e r m o d u l a t i o n i n s u b c a r r i e r m u l t i p l e x e d s y s t e m s u s i n g

n b e r g r a t i n g e q u a l i z e r V ¿ ì ¹ o m n T e c h m Z Z e r r - - v £ r

9 , p p . l 2 8 0 4 2 8 2 , 1 9 9 7 .

{ 8 l S . K a n a o , A : A d a m i , a n d J . Y a m a s h i t a , ¤¤A c o m p e n s a t i o n

m e t h o d f o r d 1 S p e r s o n - i n d u c e d t h i r d - o r d e r i n t e r m o d u l a t i o n

d i s t o r t i o n u s i n g a n e t a l o n , ± Z Z î ô ¹ ¦ Õ T e c h n o Z , v o l - 1 4 ,

p p . 2 7 8 6 2 7 9 2 , 1 9 9 6 .

[ Ì G . Y . Y a b l -e , a n d J . L . B i h a r L ° R e d u c t i o n o f n o n l i n e a r

d i g o n i o n i n d i r e c t l y m 0 ì l a t e d s e m i c o n d u c t o r l a s e r s ¢

c o h e r e n t l i g h t i f l e c t i o n - ± f E E E . f . Q u a n t u m H e c f r o m v o l -

3 3 , p p . l 1 3 2 - 1 1 4 0 , l 9 9 7 .

{ 1 Ì X . J . M e n s T . C h a t 1 , a n d M . C . W u , ° I m p f -o v a l i n t r i n s i c

d y n a m i c d i s t o r t i o n s i n d i r e c t l y m o d d a t a 1 s e m i c o n d u c t o r

l a s e r s b y o p t i c a l i n j e c t i o n l o c k i n g , ± E H T r a n s . 4 a e r o m - -

7 ¬ e o r y T e d - , v o l - 4 7 , p p . l 1 7 2 - 1 1 7 6 , 1 9 9 9 .

{ 1 H H . K . S u n g . Y . - K . S e o . a n d W . - Y . S e o . ° D e p e n d e n c e o f

S e m i c o n d u c t o r L a s e r I n t e r m o d u l a t i o n D i s t o r t i o n s o n F i b e r

L e n g h a n d i t s R e d u c t i o n b y O p t i c a l I f l e c t i o n L o c k i n g - ¯ i n

T e c h . D r y - M H P 2 0 0 0 , W E 2 . 1 0 , p p . 1 8 6 4 8 9 , 2 0 0 0 .

{ 1 2 | Y . K . S e o , W . - Y . C h o i , a n d A ¹ . K i m , ° O p t i c a l g e n e r a t i o n

o f 3 2 G H z m i l l i m e t e r - w a v e s u s i n g s i d e - m o d e i n j e c t i o n -

l o c k i n g o f s e m i c o n d u c t o r l a s e r s , ± i n D Ò . C o n * r e n

c

0 p r o d ¤ c f r o m c s a n d 0 p r * a f C o m m u n * a f M n s , H 1 C 2 - 5 , p p .

2 3 9 2 4 0 , 2 0 0 0 .

H 3 l U . G h e e - S . N o r s k o v , a n d T . N . N i e l s e n ° C I w o m a n c

D i s p e r s i o n i n F i b e r - O p t i c M i c r o w a v e a n d M i l l i m e t e r - w a g s

L i n k s , ± f E E E D a n s - M c r o w a v e n - o a n d T e d n f q u a s i

o l - 4 4 , n o . 1 O , p p . 1 7 1 6 4 7 2 4 , 1 9 9 6 .

W e have investigated the character istics of i nj ecti on-Å kd semicond uctor lasers that m vm useful i n¢uppressmg intermodulati on distort ions due to theCOMbined eff ect of the intr insic semiconductor laser

?on Ma rines and f i ber chromatic dispersi on. W e havedRO lnvest- ated the optical m i l l i meter-w ave generationu ing sideband i fl ection locking scheme and show ed that

¦ unselected M L si debands produce undesired beatlgnals whose pow er can be comparabl e to the desired beat

Ü M power T he decrease in me i fl ection pom r ratiom ween the M L and SL can suppress the undesired beatHRnals, but reduces the lock ing bandw id th. I n addi t ion,®h0W lhat the undpsired beat signals can be sU nm eant-

u pmessed dependmg on the transmissi on leng th.

m ô n# |? ve m r i nvestigations can prov ide use- lh ÕES ln u mg i f l ection-locked semiconductor lasers

ra too n-f i ber appl ications.

- 253 é