Thin, flexible waveplate of fluorinated
polyimide
V
cutting
SAndo, T Sawada and Y Inoue
f_ )
o
dr)'lng SL
solvent'v7
poly (amlc aCid)
[ndnmg lellln Optlwl polallsers, Oplllal pol)n/elS Lzght
polalZsatlO1l
film
A thm optical half-waveplate at 1 55 f1l11, only 14 5 f1l11 thIck,
whIch IS 63 tImes as thm as a zeroth order quartz waveplate, IS
prepared usmg fluonnated polYlmlde ThIs matenal has good
fleXlblhty, thIckness controllablhty, and hIgh thermal stablhty
The retardatIOn of the waveplate was retamed after annealmg at
350°C for I h
Il/tloduellOn The most commonly used blrefnngent matenals for
optIcal components are calcite and quart,l Such morgalllc crystals
have high optical transparency and good thermal and environmental stdb!llty However, their blrefrmgence cannot be vaned and
they are difficult to make mto thm plates or small components
The thickness of a zeroth order calcite half-waveplate for the long
distance optical commUlllcatlOn wavelength (1 55!ill1) IS -5!ill1,
makmg the waveplate almost Impossible to gnnd and polish
becduse of ItS fragility On the other hand, the low blrefnngence of
quartz makes the half-waveplate thick (92!ill1) This results m conSiderable excess loss when the waveplates are mserted mto
wavegUides [I) There has been a strong demand for new optical
matenals whose blrefnngence can prOVide a half-waveplate 1020!ill1 thick while havmg good process ability and tractability
UlllaxIally drawn Kapton®-type poly(amlc aCid) film [2) can
gIVe a highly onented polYlmlde film WIth a large m-plane blrefnngence of 0 18, which IS larger than that of calcIte In thiS study, we
mvestlgated fluonnated polYlmldes as a novel blrefnngent matenal
whose thickness and retardatIOn can be precisely controlled
Fluonnated polYlmldes [3, 4) exhibit high transparency m the VISIble near-mfra-red regIOn, and have high thermal stability (above
300°C), and low water absorptIOn (02-0 7wt%) These characterIStiCS are supenor to those of conventIOnal nonfluonnated polYlmIdes, such as Kapton®, which have high water absorptIOn of -23wt%, which causes optical loss at optical commUlllcatlOn wavelengths
Sample p,epa/atlOn and measurement The fluonnated polYlmlde,
PMDAlTFDB (Fig I), used m thiS study has a rod-like structure
accompallled by a large alllsotropy m electromc polansabllity
ThiS IS advantageous for generatmg large m-plane blrefnngence by
umaxIaI drawmg The preparatIOn of poly(amlc aCid) of PMDAI
o
0
/c:(Xc\
I
II
N
'c
II
o
\I
IN
C
\I
0
PMDAITFDB
n
Fig. 1 Fillolillated pol} zlllzde fOi optzw!lI'avep!ate
Tl"DB has been descnbed m detail elsewhere l4J The Elms of
poly(amlc aCld)s were prepared by spm-coatmg N,N-dlmethylacetamlde solutIOn onto a 4 mch Silicon wafer, followed by drymg
m mtrogen at 70°C for I h The film was then peeled from the substrate, two of ItS Sides were cut off, and It was fixed m a metal
frame by the other Sides (Fig 2) The film was 55mm long (m the
fixed directIOn) and 60mm Wide The film was then heated to
350°C at 4°C/mm to convert the poly(amlc aCid) mto polYlmlde
As the temperatUie mcreases, the polymer chams spontaneously
onent along the fixed directIOn as a result of the tenSile stress ansmg from the shnnkage of the polymer film and the evaporatIOn of
the solvent Hence the onented polymer chams exhibit m-plane
blrefnngence
Fig 2 Method of jabllwtlllg ill-plane bllejiillgellt pO/l'll1lzde jzbm
The retardatIOn of the films was directly measured by the parallel Nicole rotatIOn method at I 55!ill1, and thickness was measured
from the mterference fnnge obselved m near-mfra-red absorptIOn
spectra (wavelengths I 6 - 2 6!ill1) The retardatIOn and the thickness wele measUied at the centre of the films In-plane blrefnngence (/).n) was calculated by dlvldmg the retardatIOn by the
thickness The largest m-plane refractive mdex (n m ) and the
smallest m-plane refractive mdex (nTd at I 523!ill1 were measured
usmg a Metncon PC-2000 refractometer
1 0,----------------,
o9
E
o6
o 5 L...-_ _-'-_~_~--_:_':_:__~~
500
1665 131
550
600
spinning speed, rpm
Fig 3 RelmdallOlI ofpohmllde filllls agalllst
~peed
ot
~Plll
watlllg
Waveplate jahllcatlOlI The /).11, n TEl , and n TE2 of the film prepared
by the above method are 0 053, I 638, and I 585, respectively The
film was very pale yellow, tough, and fleXible Because retardatIOn
IS the product of the thickness and m-plane blrefnngence, preCl;e
control of retardatIOn reqUires film thickness control It IS well
known that the thickness of polYlmlde film can be controlled by
changmg the spmnmg speed of the poly(amlc aCid) solutIOns As
shown m Fig 3 the retardatIOn of the polYlmlde films IS lmearly
proportIOnal to the spmlllng speed From Fig 3, the spmnmg
speed for ptoducmg a half wavelength letardatlOn at I 55!ill1
should be 570rpm The letardatlOn of the film thus obtamed was
o 772 ThiS mdlcates that the retardatIOn of fluonnated polYlmlde
films can be controlled to have an error of less than I % In addition, the half-waveplate was only 14 5!ill1 thiCk, which IS 63 times
as thm as a quartz waveplate ThiS w!ll decrease the excess loss
caused by msertmg waveplates mto wavegUides An attempt to use
thiS polYlmlde waveplate as a TE/TM mode convertor m sllicabased planar lightwave CirCUits IS under way [5) An Important feature of retardatIOn IS thermal stab!llty, m order to proVide compatibility With high performance optical CirCUits We exammed the
thermal treatment effect by annealing half waveplates prepared by
the above method at 250, 300, 350, 380, and 400°C for I h As
shown m Fig 4, the retardatIOn of the waveplates does not change
below 350°C, the preparatIOn temperature of polYlmldes Above
350°C, the retardatIOn mcreases With annealmg, suggestmg that
self-onentatlOn of polYlmlde molecules takes place
Reprinted from ELECTRONICS LETTERS 25th November 1993 Vol. 29 No 24 pp.2143-2144
© lEE 1993
Electrolllcs Lettels Onlme No 19931378
10
o
E 08
:::1.
c
Q
0-
.... 0. ..... .0.
.0
4 Octobel 1993
SAndo and T Sawada (NTT Interdls(lplmaty Research LabOialolles,
Mldoll-cho 3-9-11, Musashmo-shz Tokyo, 180 Japan)
Y Inoue (NTT Opla-elecirolllcs Labaratolles, Taka/-mUla
319-11 Japan)
06
-0
D
Ibaragl,
004
~
02
O~--~~~~~~~~~--~--~
none
1665/41
250
300
350
References
400
anneall ng temperature, '(
Fig. 4 RetatdatlOn af palVIl111de films afte/ annealmg}OI 1 h
2
ConclusIOn FlUOrInated polYlmlde IS a novel m-plane bIrefrIngent
optical materIal that can be used to make a half-waveplate at
I 55[ID1 that IS only 14 5[ID1 thIck ThIS waveplate IS flexIble and
tough and has a large m-plane bIrefrIngence The optical retardatIOn of the polYlmlde could be controlled wlthm I % by adJustmg
the spm-coatmg speed The waveplate was 6 3 times as thm as a
quartz waveplate. whIch wIll decrease the excess loss due to the
msertlOn of waveplates mto wavegUIdes The retardatIOn of the
waveplate was retamed after annealmg at 350°C for I h
Acknowledgments We wIsh to thank T Matsuura, S SasakI, and
F Yamamoto for theIr useful comments We also wIsh to thank
K Nakagawa and S Suzuki for theIr helpful comments and dIScussIons
3
4
5
TAKAHASHI, H, HIBINO Y, and NISHI I 'Po1anzatlOn-msensltlve
arrayed-wavegUIde gratmg wavelength multiplexer on SIlIcon', Opt
Lett, 1992, 17, pp 499-501
NAKAGAWA K 'Optical amsotrophy of polYlmlde'. J Appl Polym
SCI, 1990,41, P 2049
MATSUURA T, ANDO, S, SAS<\.KI s, and YAMAMOTO F 'PolYlmldes
denved from 2,2'-bls(tnfluoromethyl)-4,4'-dJammoblphenyl IV,
Optical properlies of fluonnated polYlmlde for opto-electromc
applIcatIOns', to be publIshed m Macromolecules
MATSUURA, T, HASUDA Y, NISHI, s, and YAMADA, N 'PolYlmldes
denved from 2,2'-bls(tnfluoromethyl)-4,4 -dJammoblphenyl I,
syntheSIS and charactenzatlOn of polYlmldes prepared WIth 2,2dJanhydnde
or
bls(3,4-dlcarboxyphenyl)hexafluoropropane
pyromellItlc dlanhydnde', Macromolecules, 1992,25 p 3540
INOUE Y, OHMORI Y, KAWACHI, M, ANDO S, SAWADA T, and
TAKAHASHI, H 'PolanzatlOn-msensltlve arrayed-wavegUIde gratmg
wavelength multiplexer WIth polYlmlde waveplate as TE/TM mode
converter' Submitted to Proc Integrated Photomcs Research
Conf '94, 1994, (San FranCISco)