The polarisation dependence of Bragg gratings photowritten in ion-exchanged glass waveguides is characterised for waveguides with different mask-opening widths and burial depths. It is found that polarisation-independent gratings can be written in waveguides with a wide variation in fabrication parameters

Drapp, B.

Geraghty, D. F.

Honkanen, S.

Ingenhoff, J.

Morrell, M. M.

Peyghambarian, N.

Provenzano, D.

Yariv, A.

English

Caltech Authors - Main

Talilc 1 chows (lie iniiiiniurii transmiltarrcc or the sol-gel iiiatc- 
rial within a broad spcctrum riingiiig fkoiii IJV to sliort-\v;tve 
illrl.amil. Williin tlic iiarrowci. banclpass of 400 .I I ODnin, the mini- 
iniirii 1riiiisinitt;ince of Ihc 27,51uii thick aiwl 17.Xp1ii hick Filius 
WBS 92 wnd 97%1, i*espcctivcly. 
The mean RMS s i i r h x  roiighiiess valiies aftcr tIcvclopmeii( 
listed in Table I corrcspoiid to ;I lord inlograted scatter of 10 wnd 
I1 %J at 587mn [4]. 
‘[’he miximuin p;itlwiiecl thickness of Llie hybrid glass iiinterial 
described in Tahlc I c m  [E inte.rprctcrl in t e r m  of‘ a rdractive iir 
a rcllectivc micro-optical elemcul. With a I .2riim clcar ;tpclure, a 
refrnctive letislcl can be hbricated will1 an L-ircctivc i k w l  length 
K,) of 21 intii aut1 a wnicricai apcrturc (Ni l )  of 0.03. With w 
hOOwn clear qxrhire, a dractivc lenslet ciiii he fabricakcl with ,f;, 
= 5.25” ancl N A  .- 0.06. With il 6OOpn clear :rpcriure, a con- 
c w e  inicrominor can be hhriciiled wilh ./;, = 1.3iUiii anrl NA = 
0.23. 
The miixirnuiii patterned tleptlis lislal in l’;tble I wci-e utii- 
fnriuly iichievd within widc reatures iind within the narrowest fca- 
tnres on the pholomask uscd to create tlic eleineril sliown in 
Fig. I .  Ttic n;~rroowest patterned zoiie of the m i c  plak in Fig. 1 
1x1s a radial width of 27.Spm rcsulling in mi aspect ratio of0.6. 
C’unclu,~in17.~: ‘I’he hybrid glass malerial dcscrihcd in lliis Lctkr rep- 
rcscnts :I stcp rowiirds obtaining itre nialurial properties rcquired 
by the siiiiultaricoiis rihhricalioii of oplicai and opto-mccliiiriical 
fcalures i n  i x i cm-oph l  systcms. hi ;idditinti, I l l i s  hybrid glass 
inatel.iii1 cxliihits superior scnsihvily to UV light allowing the hi)- 
ric:ition of 1 7 p  deep slriicrurcs after only a 03s  exposiirc. 
Firlure cffvrls will bc rljrectcd towards patierning [his hybrid glass 
iiiatcrial witti ii grey-sciilc phutoniiisk to rcdise morc coniplcx 
micro~opto-iiicclianiwl slriic(iires. 
nckrioil;ktigrrrtnts. R. Lcvy is supported by i l ie  0ptic;il Dat;i Slor- 
agc Ccntcr i i  t the Opticd Scicnccs Ccntcr, Univcrsi1.y of Arixwnw. 
Rcfcrenccs 
ELECTRONKS LETTERS ‘16th March 2000 Vol. 36 
Polarisation-inde endent Bragg gratings in 
ion-exchanged g P ass channel waveguides 
D.F. Geraghty, D. I’rovenzano, M.M. Morrcll, 
J. Ingenhorf, U. Drapp, S. I.Iotikill~cn, A. Yariv and 
N I  Peyghambarian 
‘I’lic polarisation dclmrlcncc of t h g g  graliiigs pliciiowritteii in 
ion-cxcliimgDd gliiss wjivcguidm is cilnwclerisctl for runvcguidta 
with iiilTeer.ctit miisk-oywiiig widths oncl burial depths. It is roiind 
ttiiil p0larisntici i l - i i idc[~i i~~i~l giatiiigk can bc written in 
wweguidcs witk a widc viiriation in fiibricalion panmrcteis. 
Hragg rcflcctinn griitings in fibre optic mintnunicdon systems 
atid SCIISINS liavc ilciiionstrrilrd i~umcroirs appli&ons, such iis 
~nulliplexing, dispersion compciis;i(ion and laser fccdhack [ I ] .  
Thcir widesprciirl use is ;I clear intlicahti of their importancc iri 
thcsc fields. Most iipplicatioiis require p01;iri~iition-inde~iciidei~t 
operation tlrrc to thc specitkaiiuiis of thc other components i i l  thc 
systclns. 
L’holowrittcn 131,agg grstiiigs h;ivc lxceiitly been dcmoristratctl in 
ion-exchaiigud clienncl wavcgiiidcs, ai1 iiitcgrtiled o p h s  tcchnol- 
ogy cturcntly in wminercinl use. Single step ion-exchangcd 
waveguiilca havc an inlrinsic birehiiigciicc due to lhc nsymineiiy 
UT the wavcguidc slructurc ; ind stress. ‘L‘his rcsults i t i  i i  strong 
polarisalioii dzpcndcnce of the R~agg rcflcctian, with thc irkinsinis- 
siclii dip for TE i d  TM xpwi’ared by 0.56nm [2]. Rirckingeiicc 
control in ioii-cxcliangccd waveguides has hccn deliionstrated by 
inciiiis ol‘ a two step ion-excliangc process. Ilowcvcr, the study 
sliowed ;I ucarly linear dcpcndei~cc of t l ic birefringciice on the f ib -  
rictilioii paranicLcrs. This results i n  w crossiiig point for zero t i i re-  
fringencr, which ciiii only be acliicved by cxercisitig strict coiitral 
or the bbrication piirimicters [3J. 
I r i  this Letter, we clcmons(ratc pol;irissti~n-iiidepcri~Ieiit 13ragg 
rctlcclion gfiitinga pliotowrilkn in ion-excliangcri wavcgiiidcs. ‘I’he 
bircfriiigeticc ul“ tlic w;ivegnidcs is eliiniii;itctl through an addi- 
tional step, a f ie ld assislctl lmriiil [4]. This sccond stcp twies thc 
w;tvegiiiilc bclow ttic str~lbce of Ihc glass, dcciuising the lisymme- 
t iy of both the wiiveguidc slrtictirrc m r l  ilie slrcsscs. Aa: a i.esiilt, it 
is possible to wrik n siinplc h g g  grating with identical perforin- 
a l i a  for both I’F, wud TM polarisulioiis. 
The wnvegiiidc fiibricatioii is siiiiiliir to tha l  dwcribcd in [SI. 
Five smiples IVC~CTC hbric;itcrl by siivcr ion-edimgc i n  RGG31 
glass [6], crrch sample liaving slixiglit wavegnicics htiricakcl from 
mask opnings uf 1 ,  I .5, 2 and 2.5)IIii. ‘[’lie simples wcrc theti bur- 
ied hy iioltl-assisicd iun-e?cch;ingc lo rliffcrcnt depths: 2.8, 5 6 ,  8.4, 
11.2, and 13.8pn. Gi,titings w r c  photuwrittcn into e x h  sainplc 
using i i  24811~1 cxcimer l i i w .  With ~III cxpowrc area or-2 x Zmm, 
50401 of pulsed exposure wrotc gratings of 535nm periodicity. 
OBJ POL 
sample 
The tr;nismisaion of each saiiiple w i s  cliarxlcrised iisiiig (hc 
setup shown in !Jig. 1. Tlic output or a tunable lascr was fibre 
hitl-couplcd lo tlie sample. Thc waveguide output was collcctcd 
with ii 20x objcctivc lens and pnsscrl thruugli ii polnriscr 10 allo\v 
scpit1’ijlIon of the TE ancl TM inodes hefnrc being rlclcctcd. 
Tlic rransmission 01’ ilic siiinplcs through thc wtivcguidc with 
thc 2yn  miisk opening is shown in Fig. 2 for I-mri;il depths a T  2.8, 
5.6, 8.4 and 133pi i .  ‘lhe flal bolrom evident an the dip i s  thc 
rzsult of thc liinitcil dynamic range of oiir iiic;isurcmenl setup. A 
different sclup iiixible to disliiiguisti pulaiisatioiis incasurcrt a 
MdU dip in the liwtisiiiissioii 011 one of our Ywmplcs. ‘Thc auynz- 
metric shape rcsdls finin imperfections in ihe grating writing 
p r o ~ s s .  A sliglilly GXISS~~II sliiipc 10 tlic beam profilc in thc 
trmisverse dircclion will rcsiilt i n  ii slight Gaus4an apodisariori of 
the giating in t l ie loiigitudinal dircction wlitii the bcain is not pcr- 
rectly aligned to 1111: wtivcguide. This: comtincd with w negative 
induced refractive index cIi;wgc, woiild yicld the asymiiieiric: s h q x  
observed [7]. 
No. 6 531 
Fig. 2u shows a small yol;iris;ition dependcncc rcniaiiiing for 
the 2.Xprn tlecp burial. The trmsinission clip is shiftcd by -0.1 niii 
from I'E lo TM. This is much less than pi+cviously reporlcd polar- 
isation de~~endcnce [2], which is surprising roar siicti a short burial 
rtcpth, altliougli it is still uriacccptdde for niosl systcm npplicn- 
tions. This polarisation depeiidencc is quickly rcduccd ns the 
waveguides xrc further buried to ;I depth or 5 . 6 p .  Fig. 2h shows 
that this sample has a riegligiblc polnrisation dcpcndence of 
-0.01 iiin. Lvcn this i s  eliminatcd with fiirther burial. The 8.4pn 
dccp burial, sliown in Fig. 2(., revcels no measuriiblc polarisation 
dependcnw 
a 1. nm b 1. nm 
1 , .  . . , . I  
11559.0 1580.0 1561.0 1562.0 
C 
1, om U >., nm 
!:il-1J( % -20 
-25 
-3 0 
1560.5 1561.5 1562.5 1563.5 
shapc of thc wnveguidc and tlie s w s s c s  bccome iiiorc symmetric. 
It was conccrn that lllo wrivcguide slriicturc asymnictry InighL 
rliiniiiish inorc rapidly, resulting in an oppositc hirerringc~iw: (the 
traiisn~issio~~ dip for 'I'M :it longer wavclcngths) hor rlccpcr Iwials. 
Sucli a11 effect would rcsiilt in a crossing point for polarisntion- 
inrlcpeiideiil prfonnaiice, rctliiiring stric( control or the fabrica- 
lion piraarneters and liiniting t l ie poicntiaily achicvable waveguide 
struchrrcs. However., Fig. 2d slinws thal continuing tlic burial to a 
dcptli or 13.8piii still resulted iu ii polarisaliwn-independeiil grat- 
ing. 
Additionally i t  is Iinpoclwiit to note that the polarisalion 
depcridcncc u'as the sainc for all wavcguides of itlcntical hiirid 
tiine, i~egiirdlcss of the mask-opening width. Fig. 3 shows the 
characterised transmission thlaugli waveguides with I .O, 1.5, 2.0 
and 2.5pn wide mask openings b r  a burial depth of 1 l.2pii. 
Nnrie or Lhc wwegiiides sliowcd m y  sigiiificant poladsatinn 
dcpandcnce. Although not s l io~i i  Iicm, tlie polarisatiori sensitivity 
or the gratings slio~vetl 110 depeiidcnrrc on the wwvcgiiidc tnask- 
opming width for a11 of the biirial times clianicterised. 
In coidusioti, we hwvc dcinoiistt-ii tcd polarisstion-inrlcpctideiit 
gratings pliotowritten iii ion-excliniigetl glass waveguidcs. 
Wavcgtiide bireikingcncc is quickly cliriiinatctl by a second ioii- 
excliiiiigc step if field-assisted hi1rki1, with alinosl polarisatioii- 
independent gratings resulting t1fte.i- just B 5 . h p 1  dmp burial. 
Mditiotially, tlie bircfringemx rcimins negligiblc with further bur- 
ial, resulting in a widc range of burid litnes with polarisation- 
indepeiitlcnt gratings. This allows for greater flexihilily in thc faah- 
ricatioii parariieters ;ind resulting w;iveguides. 
Ackiiowledgtwnt: 'The authors would likc to xckiiowlcdge TOT for 
their partid support OL this work through thc COEIXP center. 
a b, nm b 
Rcfcrcricfi 
It is impoilant tlwt furtlier burial docs riot rcsult in increaser! 
polar isah dependence. As previously mcntioned, thc bircfrin- 
gcncc is eliniinatcd in two ways by thc burial process. Both the 
GILES, C 1 1 . :  'Lighlwiivc applici~tivns of fitm nragg gratings', J. 
/ ig/ imxw Techno/., 1997, IS, (E), p p  1391- 1404 
YARIV, A.,  and PliYCitlAMRARTAM, hi,: 'Gratings phoiowrittcn in ion- 
cxcbangcil glass clianricl wnwcguidcs', Elecrroii. I.i!tl., I999, 35, (7), 
CrF.RAGIlTY, D.P., I 'KOVIAZANO, D., MAI<SIIALL, W . K . ,  IIONKANBN. L . ,  
pp. 585-  587 
AYRAS, P., NIJNA CONTI, G., IIONKANEN', S , and I'liYCT~IAMIIRlll~\N. N.: 
'Birdringeiicc co~iirol for iori-cxcIi;uiged cliaiii~cl glass w;ivcg~iirles', 
Appt. Opf., 1998, 37, (3h), pp. 8400 8405 
I~UCHI]I.D. II., ant1 vmi's,  I:.: 'Pol;u.isaIion inscnsirive wraycd- 
wavcyuidc grating mtiltiplcxcrc w i ~ h  ion-cxcliwligcd wavoguides in 
glass', Rkctrmi. Lcfr.. 11196, 32, (24), pp. 2248 2250 
I~AJAII .  S.I. (Ed.): 'I~ii~odnction to glass iniegratcrl uptic,%' 
(Norwood, MA: Artech I-Iourc, 19'12) 
FA~LICIIIS. N,. ofisr?., I[ . ,  WTTMANN. ILJ., QUAST. I I . ,  iind ROSS. I , . :  
'BGG 31: A licw ghss for iniihiinodc wnvcguidc fabrication'. 
Ul:OC/LAN Hi, Anisknlim, 1988 
lil(l)OliAN, T.: 'I;ihcr gr i l l ing  spcctrn', J. L$trwrrw Tr.chnl., 1997, 
IS, (8). pp. 1277-1294 
532 ELECTRONICS LETTERS 16th March 2000 Vol. 36 No. 6 


Polarisation-independent Bragg gratings in ion-exchanged glass channel waveguides

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Polarisation-independent Bragg gratings in ion-exchanged glass channel waveguides

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