BSTJ 50: 8. October 1971: Optical Modulation at High Information Rates. (White, Gerard)

17.1 MB·English

Optical Modulation at High
Information Rates

Py GERARD WHITE

aie popec devin an optical! million acer with «dewanatratad
Information ste af 1 Gh Tae nodule shee empl on laronir
Iutipleving fecha 10 obtain the hugh San vae fow fows 250-3
hanele Bilt stain betwee Lae aa aed the igor
Gactionic ens bs aebiged wing meingacare tyre medultor
ftrusare. General conaieretinns in ana sapdtaton syste are alo
Sretoe

(vr of the rani problems in ng onsing is that of inuerdueing be
Aoecon ino faving technoogiee: Tae prose is particularly sever
fe beer eommusieations viens share solid ste eleetrenies is Lo
Ire interface mich opie This esha, at this technalgien nteraer
tha the sary hogee of wopoasking she Shean Weaver mcormat
tates af lose? apstems huer Iwo ited, Whi wleeto-opbe erysals
hove Ieen set to have modulation zsachenisms operating mn exces:
of 10 GHe wr shotadezectore ich gin-buixinihoroduets of
{he over of 100 GH have bio ptezuacd? laser eomrneacions
fjstems, co far, have demazstroted is oabon rates significantly lest
“han the eapabilties of arse components, Sgaiieast iv the bande
Wilh misavon of demonetreted scr wommunications svsteris have
Ih the ounee's om sei eats ein perTarmance® apocet by Us
owes svete ed, i ystieular, the modulator, In th per en
bniesl modulation sehetbe is deserted iw wish eystem variations
tin! subeytara eowfguenons have been wleptel in en atm 22
Fes ube mover sii porturnene sccunrotzente sp thst eystem ine
oretion ral capabiies appmaeh the barat enpaiiies of the
fploveleelinis teansdacers ‘The ster deserbed, using an argon
Tose, Tex lemonstrated information wate of 1 Gl/e and some
2008 ur as, sNeEEW weUIeAT JouNRAL, eoxoLA 37

eehiiques for proving ps tis ste re pops. Tabontad ps
vem nie levels ave observa nsw wu la St may epi to
transmit w nulievel tspe code wich could at lest eriple the infor-
‘Tie paper, olfle desestbing state of the ert wile in side
boswidl optical coramnications, also provides emenial maiter a
fonneetion with gueral onsiderstions ithe ayetem design and eval=
rustian. The peper is divided bacioa!iy into four srtions, namely: (8)
ryatem eourierstione av: waivanetmngemete i which Tie sowa
Tose vavstione in somewhat standard opie configurations ean ses
ina high formatinn sate mde shes (3) Inertia, shiek
ie mainly soiceried wih the prebicing of elit terion eseen
he eln'zon ini cn the option egnal th tel agave we
Toadulaarn; (8) lene ete whieh serine how the Gy
Binary pulse code modulation (PCM) siguals exe generated by
gating and multplesing proces; and (01 system petformares, wilh
feral retcazks on eebasl and extrapolaced performer set,

Various snechanisme exist “er ipreesing ‘informacion onto. Hxht
Deus izclueng acousta-optic,* agheta-ontie! ana elect2o-opie," to
rmention but 2 fox. Applied to laser beams, electro-optic interaedions
have proved nos wisespvad, and in particular, the linear Pokal
‘Moe whirh is exhibited iy monn dielrvieeegstal, Thc effect es
Gov eleteraelésinlucad perbinlations of she natural biesRgenee of
Certain eet vesting chosen the point group dn se a ith
niobuie and lihiuss tanalate* Lithia uibate's edeceptibility to
voticaly ‘induced inkomogenstion ta the netural Siefrngecee™ at
relatively “ww ootical beam powers sakes i use ae 2 modulate
reeticedl ad, in Ui application, tba teatalato hes been used A
fun of mort for TATa0, when used ee em eletro-optie moduitor
isthe FalFwnve vlog this she valtage required to indice phase
retomdance of IS) degroes eure eumsonerie oF de bow ia the
frdinery anc exiewordivary divrGons. The values desived Sra Ue
‘element Fx and fy ofthe elctro-opsie censor

a fey
Lire — ral ad
where ag and x are the rofuctive indices expeieneed by ordinary
‘i extraardinary competent: ef the besza respectively, The felor ef

¥
formes seuuLanow 2600

el ¢ and a sing uke width and
Tenth of the erst exportvely. orn value watength wal to
40,6 a, the hellowave vole for LiTWOy is 1120 vale for the wits
fwpact ret ess, Whie lange aajuet niu anllator vos ean be
Prepared, eo as to reduce the voltage required for 8 ceradiac phase
rlewlance, he vale of voltage is stul High enough to erento severe
petformaressemande on moduler driving situ in eymetae whic
Fely on an Monenf” type aiedalater owt, Chay, a pata! modula
tior selene wil tein these standards but such w exstom ie aot
Implemen-eé. Tt iy pe-ine at this ooint 2p coneidor some of the
probleme in roalixny patio! modulation seheme> and at the seme
tise dieeuee wane o Ue ages,

Conair faz bisie open! nistew rho iw Fig, 1. Ite easy ta chow
that che modulated optical power, Tq. at the detntor, ie related to
the apse smocalarar vote aed ube ative paw eetalance
bf che bous vompments ince: az whe modulator by

ei

t

Fig cto Ress atin on ats

2 Spa nde ear
Y6I0 me mt aver eeemeKt TouRMSAD, SonORER 271

whore 1, is dhe toit. bexia yowor al Ue saaulntor fosuming 260
Insets) snd «isthe retaationsntrduced by Ue uatorl bneringence
a the e-petel. Clearly, mexinim har romdalaion per nt orale
tion vetage is ebraineé when cho # ead che y cempasente af the beam
dae eq i ample nv aljustt Tor =/2 eer ab the ane
lyger. At thie otiva Vas and a ervte) aspee ato uf 60 «sal
gual beem mo:ulatos depth ot & perent per elt cen be ebsanee.
With such an arrangement satifgctory modalstion dep of sppeesi=
rately 20 to 30 percent ean he ebssiaed wich just e for valte of
alton potential

Tn en setualeystem ip whieh Ue modulated beam i detect via
8 photedesretor whic: ia eoup'ed into @ taxemission structure of
haruolarinto isin Zy th nylon (safer “anion iene a
the aiferentel rato of montion potential uw» potential apronting,
esos the ctypattrinerussion ine! for quae sual sign mode 0
operasion is denied ‘rom equation (2) as

Boy Le
We 7 Fete a

where ¢ ie the electron chongo, iis Pluck's constant, and ¢ the
ssloticy of hight, The azove relsionehip, haeod on the fenertion of
cone hole-leetrs pair per photon, seeumee rro optical Joeees antl a
10-percent quart Teeny mn the Rotadeceter ane works av £0
fe appnamately 0. yz wall of Tedca power, Cleat, Ligh Ina
‘ones will enhance thr agent teense function eosEeins, equation
0; Bosmrver nt x imam? ey pray duced inonsogeaeiies

lo the fiveitagnee of une eyslel In ae exjerimemal eyete an
Upper limis of appeaximacey 80 mW is waste which gover transfer
tein voeficiest of M013 (vet allowing for tennsmission loses)
hush Jor «modulation nal of several vl's resus ia eabfuetory
uipat ampitude, Savefactory gpl amplitude levee within the

ster, of eos, end a Tn ale sige Levels in hia eect,
the proseace of tle eonpesitun in a Tre-uming Iserr aseten.
results ina vor noisy ehatnel. Two salutions to thie problem are
Evailablo; wove king! aut elalon turing.” Mode lacing efeo=
fEvely“oeks che longue] sales af te laer yen a ta of

‘optical ples et he Crequeney scparation of the longicudinal mode,
2) where {ia the length of Che bus eas, Cex! vom
Yong salons iseorparating modelorked “sere offer sore acvan:age

tued have been soeesallydemonateatd Medelacket Taser e-
ent somenion aan

teams offer the anally fs aeear: of very narrow pales of wth

ively sroportiora” to Es osrilaton outed uf he Taser which
far eeelt 2 pubas of il few piaseernds

A disulvaniag of the mods-locked alow ie ube? optimam modal
ion éaptis and sttisilary ee ane deGnision n> nol compat ble
‘vith lns-voltege mocilaton sine, im genera, tie sirable it chose
fystame to supply a mzdnataanpotencal whieh is a eubeiaate) Feae=
Ton of she halowave reteadation paved. One futher dicadvanage
of mo-Tocked negime inane lasers is the seatively low repetition
rates of he aptiea plats: Ibis repeilinn weve i elated to the fee
favency separa of the longitudinal voles, r/2l and. at shot
Fesnnatar spacings (rine! for high pulse repetition rates the gain
OT jasevus lees i ieiclent 9 stain ezilations. Whila lovh
igus exist for efleetively Hnerewing Whe petition xate by, for
Seample, mulivleing® aF harmon pping of the modeooked fr
faveney" neither ieluique ix a well cetabliced Taboratoey practice
find the japlomenesion af araronably tig rales (ne, 1 GFR) is ot,
fe prvsent, conenien:y reali

TRelaive noice immcusity www be ebtained wath ax etalon tuser by
Uiectivey solsting one pure upiesl Frauenes from the Gaussian
Tine etsivctign, ‘hie obtsel oe snturning the ersanant poles of
the lesen ap teat only one vmjupate past bee ntain the Ineo i
tunve." Tae conse teen of “> typ of operation ie that ome ene
fettion je eerily litnated at: foun meio matte ie observe to
be less thn 3 perveat of she total besos wir. With sth le noise
posers low mud degths ean be utite aeepessle and x parti!
hslation sexeme rulitySmperpente

"There 2 no eles oplinins 49 the ehuler vf a parvicular optical
frojiemey an a eystem euerier While edrantagzone neduetone nthe
imovllator lloweve volte ea be saved with shoror wavelengths
Teauaivon {1)], the qvsatn evieney of the detector is in general
redtee uh lise alongs Ge, ene ROD asa for Si). Poeusing
Fequirntets me tlaxed seme wil shorter wavelengths sinee
fharoner ditacionlimitd six ane obtained for a given fnrfel
acosny angie Tiere is oto yoy saint evidence for mn aptirnare
svavelenety from the etangpoint uf wpieally inducee ehangrs 9 the
Tirviringenee, Tie absogpin nes w)TiTaDy ate spsced sve that,
for gaseove veer spe frequen, Hie etal is que transparen:
ari ho cuits af faueney is ass deterined hy available powere
Troy sh veto nee eystens, To ie afer under @iseusion wn ion
2612 ee REEL sraMM AMT RICAK JKR, BCTONSH 18H

luser (argon) epeating a= 486 ne: Eas heen fourd convenient. This
vwavelznsth ie somewhst iwlow the eharscleriaio peck in quantond
oie or silison phhtcltetoes (0 an), Ink verte quate
lon eficiences af 08 AF ew sil be obtained

Cling for oqiew) cummaniesions systems has so far recived
Jie attention. Tn the past the uae of moce-ockes lasers ae given
prefernee to 8 binary BCML coding technique. Reoent studs!” tea-
fate that polaratton multiplexed, phase shift eying, or pul potion
rmodvieton may be more appropriate to epee! communications wys-
tome from e tronmssion standpoint. Probleme sll remain in mple-
menting theee mocilntion seenes. woven, feueneymodultion
scheme fer space applivaions fus ceceatly been decribed" which
hows promise for potentislly win humdveiths. Beene of eke ace
‘eentagte of regenerion, PEM systems ape lunaetive end in lowe
hoise channels qultevel avstens eam be eassidened. ‘The present
systeat uses a binary type PCM eong schemo byt 3 rohly adapt
ie m9 anulsleve with Ie ineease in complescy.

‘The ateal anrargemen: weed i of a fei conventional marae
employing a type at Senarinnt cemngermatan an aber in Pig. 2
‘The argon Tacer provides a ingle frequency ousput af approximately
‘a0 wn plane polaziseel in the wertion!circeton, Tt x easy to abo
Tala nesters eandiion for seeing optim: modulation is tha
the beam be rtewanly polasard at se analyaer plate. The conse-
‘quoute ofa et oF Fightchard polaization stao is meuly to indice
2 180-dogree phase inversion of the devectes eletncal cial. Tae
Greulrly polaieed stale can be obtained Uy storing that the opie
fry stl extmerdinary beer eampanese arsplitades are equal With
tee meessare eelative phase roterdance. This is uebioved by a 45-
Alege sting of the quarterwnve plne with indent bear plane
polaiaed at an angle 0/2 which ie meld Wy dhe halt-wave read
formes sumamon 2a

ion plete. This pasticular arrange en be esily checked tye
reference to lie Possuat-spbese® Under these conditions, the and
femaponente uf he san are ivan By

#.- asin (t—4)
ne wasn(t +l

* “

‘Tae exes angle @/2, 3 sels ls vompenaate for the rane
birofringence of the LiT0) eval, The clvri-Fel@-indueed per.
trations ofthe natura hinfingeaee of the moduler ra canslae
to perurbations of the relative pha reaction ofthe x and y som
nents of che exiting beam 10 preduce a mulation of te beam
tlptisy state, Colimabion of the Tans is provided to permit distant,
franeaission, The anlvaer rie plaed bafore or aiter collimation
Viepending on whetho ite desir to cranemit amplitude or polaron:
tion state tanduation, at Uae movie the beam modulation is eon
‘ert Uo an eletres! signal vig he silicon alate photedeteetor
"The avelazchig mela wien posies a signal gain of ony 2 oF 3 de
1 the sigh optiee! ewer let Th a detect operating at low Joel,
Teeuling From, son exuttle, gazeaphere attomsasion of the hear,
Ich hig gaine sinald Loe pesab.e, This ip « desirable fotre of
the ava the deterter in shat i provivts w ling ar sutomatio
{in vcs the sigral

‘ifiient focusing of the or for low-loss transmission though
the mecilator este is nuporant. Ta problem eum be dealt wt
fin number of way aad ote vib is given By Reel wel T5*
in tonne of the conforal paraoelere ioe optimum clearance. Another
proach is (o cousnler the prevent trsnamision ef power :hrough
the ro! Consider a od of eielar cvs serton, as shown in Fig. do,
vith an iit Gastssan boson ih eld distsbnron given by

Ba Re a ©
lhe a inthe wid of the beam at the 1/e points. The intensity
Aisebtion, 8 cherefore given hy

a
‘The total ges i the eo

©
Tei comptes fom

o
»u cove ewe ene, SoHE
LO >
. oo
“ aa
tawe wdewk-

ideas bese i

eS) Motaatsrcoosganeze,

wih reduces to

Los
=* ®

4-39

Non, forthe eaw of the cee ens section to we ean compte the
su dimen for Breen. pseor asin Tom

oe( 2")
hie, efter a poraretie substivcion, eduees to

eh ier de, ®

fo tte am

Th siue nner, re sare puoi ense uf equ ess eeetio
sed {Fig ap eez be decit 9th,
serie ue ats 2816

Wale, again after a pararetieecbettatien, reduces ta

anf [eer macan an

Law 0%

Clearly, the reuley struc i sone sent from the standpoint
of power transmission; Howeern, the penetioal problem of epplving
‘igor coansvoreeeletse Held probs ts ws

Tn matehing the optical brsm to the madulator ul, most power
Joss, de to ditensiooalltitations, will ake plnce at the feos of the
thon eto best espenson, Kogeiik hiss analyve che expe
fon of focused Gian Iwate in sone detail Figure 3b shows &
ypieal beara wis t the focus puinl. The equations governing the
Doom parsnotas ins Wiratin fila ease ee

wok a3
of 25) ae
wo [1 (22). as

fron which the confocs!povomior, fs iene a the dstanse ‘rom
the phese frome zeit nine

oe,

Paes

‘The bam exjaninn ease completa epesiid, fora given wave

lengt, in ferme af far fd iteartion angle, ®, wie is, af eoure

the forssing eae hall pls Por the ease ofa collimated heat inci
hen nthe les, #8 sen be

8,

= ati,

ero isthe Boos midine t the lene aa #4 the lens foc eng

Tie enti “otal” win the apt of Geen atin tee
see's hana er mace ea tte fe ath Be
2818 roe mu exon mucuueal souANAL, VorouM 871

aroens onsnustegsby mnehes

ae SY ae
rf is
a tH
ail sii. Sea hE
pe, era 8
x Sa

rik tat

Fle Bes gt of mudlator a 8 teesion of eng conn ang
TS aa 22 aes

‘enueian (14; tay oe wged 2p cleulae che eam raciue at fuses of
9 nod of lng L for ¢ given 0. Tais haw Leen deve “ar the este of
{Sai rom xe 4 — 682.8 rm an the res ar presen in Figs da,
td resrsively- For rach rasta rs it, x ler apa
torte ange is in-eaee. Thi optim angle irs the sutze eoufoeal
pararmcer con the uyper alien} ux Ropes analy Theanine
fon the right gives che neoresary eed ere svtioa Jor @eperent opie
Powor transmission in accordance with equation (12). ven for tho ese
Df the Linch od with an sepect ratio 0: 10, 9 wid range of tuewing
fone angles are permit for high opties! power teananission. In
Penoical eases, clea Tor High-ficnsy Lnaamision Hough the
Tod are easily met and other erteria asume importance, Ti incerest=
ing to note thatthe ears fur eearamse fn Figs ta aad apalesee
for all tod length a tone angle Iw thar 1.5 alia, To, In
thie case, the tranamissan of eptca! power becemes relatively inde
pn of theo Teg

One other eitrior:uporkanh iu bea mulching is cat of son-
nsiquenear inthe Seem planation face acrore the width of the
‘beam, Thi eres ce net of the dering path lengths experoer
Ly the various portions of the Sci i ube na, Coir the wavefront