Graded-index polymer optical fiberfor high-speed data communication

Takaaki Ishigure, Eisuke Nihei, and Yasuhiro Koike

We successfully obtained a high-bandwidth (1 GHz km) and low-loss (90 dB/km at 0.572 Plm ofwavelength) graded-index polymer optical fiber by using the interfacial-gel polymerization technique, inwhich we used an unreactive component to obtain the quadratic refractive-index distribution. Thishigh-bandwidth graded-index polymer optical fiber makes it possible to transmit a high-speed opticalsignal in a short-range network, which is not possible when we use the step-index type of polymer opticalfiber commercially available.

Key words: Graded index, polymer optical fiber, interfacial-gel polymerization technique, highbandwidth, low loss.

Introduction

For long-range optical communication a single-modeglass optical fiber has been widely used, because of itshigh transparency and high bandwidth. In contrast,for short-range communication, recently there hasbeen considerable interest",2 in the development ofpolymer optical fibers (POF's). In short-range com-munications (such as local area network systems,interconnections, the termination area of fiber to thehome, and domestic passive optical network con-cepts3), many junctions and connections of two opti-cal fibers would be necessary. In a single-mode fiberthe core diameter is approximately 5-10 pLm, so whenone connects two fibers, a slight amount of displace-ment such as a few micrometers causes a significantcoupling loss. The POF is one of the promisingpossible solutions to this problem, because commer-cially available POF usually has a large diameter suchas 1 mm. Therefore, low transmission loss and highbandwidth has been required for POF's to be used asa short-distance communication media.

All commercially available POF's, however, havebeen of the step-index (SI) type. Therefore, even inshort-range optical communication, the SI POF willnot be able to cover the whole bandwidth of the orderof hundreds of megahertz that will be necessary in

The authors are with the Faculty of Science and Technology,Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223,Japan.

Received 15 June 1993; revised manuscript received 19 October1993.

0003-6935/94/194261-06$06.00/0.© 1994 Optical Society of America.

fast datalink or local area network systems in thenear future, because the bandwidth of the SI POF isonly approximately 5 MHz km.

In contrast, graded-index (GI) POF is expected tohave a much higher bandwidth than SI POF, whilemaintaining a large diameter. We have proposedsome preparation techniques of GI POF that usecopolymerization. 7 Here we obtain a low-loss andhigh-bandwidth GI POF with good flexibility by usinga different technique. The formation of the GI andthe optical properties of such a GI POF are describedhere.

Preparation of Graded-index

Materials

The preform with a 10-mm diameter, in which therefractive index gradually decreases from the centeraxis to the periphery, was prepared by an interfacial-gel polymerization technique. In the previous inter-facial-gel copolymerization technique,67 two mono-mers, Ml and M2, with different refractive indices andreactivities were copolymerized. For example, methylmethacrylate (MMA) and vinyl benzoate (VB) wereselected as Ml and M2 monomers, respectively. Inthe interfacial-gel polymerization technique, we se-lect the Ml monomer with a low refractive index aswell as an unreactive component with a high refrac-tive index (instead of the M2 monomer) to form the GIpreform. Here we select MMA and bromobenzene(BB) as the Ml monomer and the unreactive compo-nent, respectively, whose properties are shown inTable 1.

The monomer MMA (Wako Chemical Company)

1 July 1994 / Vol. 33, No. 19 / APPLIED OPTICS 4261

Table 1. Properties of the Used Monomer, Homopolymer, andUnreactive Component

Parameter MMA PMMA BB

Molecular weight 100.1 1.57Density 0.94 1.19 1.5Refractive index 1.42 1.49 1.56

was purified as follows: we removed the inhibitorsfrom the monomer by rinsing with a 0.5N NaOHaqueous solution, and we then washed out the remain-ing NaOH with distilled water several times. Themonomer was dried with calcium hydride, filteredthrough a membrane filter whose pore size was 0.2pm, and distilled with reduced pressure.

BB (also from Wako Chemical Co.) was used with-out further purification. 1,1-bis(t-butylperoxy)3,5,5-trimethylcyclohexane (from Nippon Oil and Fats Co.)was used as an initiator and n-butylmercaptan (fromWako Chemical Co.) was used as a chain transferagent, both without further purification.

Formation of Graded Index

As we stated above, we used BB, instead of the M2monomer, as the unreactive component with a highrefractive index. MA and BB monomer mixtureswith specified amounts of 1,1-bis(t-butylperoxy)3,5,5-trimethylcyclohexane and n-butylmercaptan wereplaced in a poly(methyl methacrylate) (PMMA) tubeprepared in our laboratory, the outer and innerdiameters of the tube were 10 and 6 mm, respectively.The PMMA tube filled with the monomer mixturewas placed in a furnace at 90-95 C for 24 h.

First the inner wall of the PMMA tube swellsslightly toward to the monomer mixture; then a gelphase is formed on the inner wall of the tube. Thisstate is shown schematically in Fig. 1(A). Here,because the rate of the polymerization reaction insidethe gel is much faster than that in the monomerliquid phase because of the gel effect, the polymeriza-tion occurs from the inner wall of the tube.

As the polymer phase thickened, the concentrationof the M 2 molecules in the monomer liquid phasegradually increased at the center region of the tubebecause of the dissolution of the PMMA homopoly-mer into the monomer liquid phase from the tube andbecause of the gradual diffusion of the MMA mol-ecules and BB molecules into the gel phase. Here,because the molecular volume of BB is slightly largerthan that of MMA, MMA molecules can more easilydiffuse into the gel phase than BB molecules.Therefore, the concentration of BB in MMA mono-mer at the center region increased. Also note thatthe polymer phase gradually thickened, while thepolymer content of the monomer liquid phase at thecenter region slightly increased. This is shown sche-matically in Fig. 1(B).

Next, as shown in Fig. 1(C), the polymer phasereached the center axis of the PMMA tube and the GIpreform rod was obtained. We heat treated the GIpreform at 110 'C under 0.2 Torr for 24 h to completethe polymerization. We obtained the GI optical fiberby heat drawing the preform at 160-190 C. Weobtained a GI POF with the desired diameter bycontrolling the velocity at which the fiber is taken up.The diameter of the GI POF was controlled to 0.5 mmin this experiment.

Results and Discussion

Characteristics of the Graded-Index Preform

Refractive-Index DistributionWe measured the refractive index distribution of thepreform by using the longitudinal interferometrictechnique,8 shown as curves (A), (B), and (C) of Fig. 2,in which no and n are the refractive indices at thecenter axis and at distance r from the center axis,respectively, and in which Rp is the radius of thepreform. All the preform rods have cladding corre-sponding to the PMMA tube and have an almostquadratic index profile at the core region. The refrac-tive-index difference between no and n, (refractiveindex of cladding) decreases with an increase in theratio of BB to MMA (MMA/BB, wt/wt), because the

0

Mome UqAd Phase

~elPhaseA

I I #A t

Rp

(A) (B) (C)

Fig. 1. Schematic representation of the interfacial-gel polymeriza-tion technique.

-0.005

-0.01

-0.0150 0.5

(rIRp)1

Fig. 2. Refractive-index distribution of the GI preform: (A)MMA/ BB 10/1, (B) MMA/BB 7/1, (C) MMA/BB 5 6/1.

4262 APPLIED OPTICS / Vol. 33, No. 19 / 1 July 1994

Mm phase I

F.I91

2

BB concentration at the core center becomes higheras the ratio of BB to MMA increases.

Light-Scattering LossIt is well known that the attenuation of opticaltransmission through fiber is caused by absorptionand scattering. To measure the light-scattering lossand investigate the heterogeneities inside the pre-form rod we measured polarized (Vv) and depolarized(Hv) light-scattering intensities from a He-Ne laser(0.633-Lm wavelength) against the scattering angle,0. Although the Hv intensity was independent ofscattering angle and was approximately 10-6-10-7cm-', the Vv intensity strongly increased with adecrease of the scattering-angle forward scattering inthe range of 10-5-10-6 cm-1. In randomly orientedpolymer bulks, the isotropic part, Vviso, of Vv is givenby8

Vvis = Vv - (4/3)Hv. (1)

Therefore, the observed Vv scattering was dividedinto three therms, Vvjiso, V 2iso, and (4/3)Hv, asfollows:

Vv = (Vvliso + VV2iso) + (4/3)Hv, (2)

where Vvjiso denotes the isotropic background scatter-ing independent of the scattering angle, and Vv2iso isthe isotropic scattering that depends on the scatter-ing angle caused by large-size heterogeneities. Fi-nally, the total scattering loss at (decibels per kilome-ter) is obtained by

a, = 1.346 x 106 f [(1 + cos2 0\viso + Vv2 iso)

+ (13 + cos2 0) v sin OdO. (3)

Here At is divided into three terms, aiso, U-2 iso, andaaniso That is,

t= ciSO + 2iso + aniso (4)

To investigate the heterogeneous structure by us-ing the angular dependence of the V 2iso, we usedDebye's fluctuation theory,9 in which the correlationfunction y(r) was approximated by exp(-r/a). Here,the heterogeneous structure with the mean-squareaverage of the fluctuation of all dielectric constants isdescribed by (q 2). Details of these estimation meth-ods from the Vv2iso are described elsewhere.5

Table 2 shows the results of these scattering datafor the GI preform rods, compared with those of theMMA-VB GI preform rod previously reported.' 0

It is confirmed that the Vv2iso is caused by the

'I-

.

Table 2. Scattering Parameters of MMA-BBand MMA-VB GI POF's at 0.633 R±m

a (rq2 ) aliso Oa2 iso aniso at

Ratio (A) (x 10-8) (dB/km) (dB/km) (dB/km) (dB/km)

MMA/BB5/1 722 0.45 8.2 19.1 7.3 34.67/1 774 0.16 19.6 7.5 4.3 31.4

MMA/VB5/1 762 0.74 6.1 33.9 10.1 50.1

heterogeneities with dimensions of 500-1200 A of theorder of (f2) = 10-8. It is noteworthy that the totalscattering losses at for the GI preform rods ofMMA/BB = 5.0 (wt/wt) are only approximately 30dB/km and are comparable with those of PMMAhomopolymer." In the case of the MMA-VB systempreform, the isotropic scattering loss with angularinterdependence (liso) is almost the same as that ofthe MMA-BB system preform, whereas the isotropicscattering loss with angular dependence (0-2 iso) is twiceas high.

Because these heterogeneities originate inside thecopolymer glasses, we should also investigate theeffect of the distribution of the copolymer composition.In the MMA-VB preform, we were able to calculatethe distribution of the copolymer composition fromthe monomer reactivity ratio, details of which aredescribed elsewhere.'2 The calculated result is shownin Fig. 3, in which the ratio of BB to MMA ofMMA-VB is 4/1 (wt/wt). Figure 3 indicates thatthe distribution of the poly(MMA-co-VB) composi-tion, where co stands for copolymer, is divided intomain two components: one is the copolymer formedin the initial stage of polymerization and whosecomposition is quite similar to PMMA, and the otheris the copolymer that is formed in the final stage ofpolymerization and is almost a VB homopolymer.From this result we see that the copolymer is virtu-ally similar to a polymer blend of MMA and VBhomopolymer. Assuming that the heterogeneitiesof these preform rods consist of two phases withvolume fraction +i corresponding to PMMA andvolume fraction (b2 corresponding to poly(vinyl benzo-ate), and with refractive indices nj and n2, respec-

40

30

20

10

0 0.0 0.5 1.0

Weight fraction of MMA unit yFig. 3. Distribution of copolymer composition after 100% polymer-ization when MMA/VB = 4/1 (wt/wt).

1 July 1994 / Vol. 33, No. 19 / APPLIED OPTICS 4263

tively, we see that (2

) is given by

('q 2 ) (41 2 (nl -n2)2,

4Va = - +1(6

where S is the total surface area of the boundarybetween the two phases, Vis the total volume, n is theaverage refractive index, An is (n1 - n2), and 4) +(42 = 1. From Eq. (5), we find that if 4), = 0.1-0.9,the fluctuation of the refractive index is of the orderof 10-s.

In contrast, in the case of the MMA-BB systempreform, PMMA and BB molecules are not phaseseparated but completely miscible. In Table 2 theexperimental results of the light-scattering measure-ment of the MMA-BB system GI preform are shown.It should be noted that the isotropic scattering loss ofthe MMA-BB preform becomes lower than that of theMMA-VB preform. Therefore, it is considered thatthe BB molecules are not aggregated but randomlylocated and that there are no heterogeneous struc-tures large enough in the MMA-BB preform to causea high scattering loss.

Optical Properties of Graded-Index Polymer Optical Fiber

Refractive-Index DistributionWe measured the refractive-index distribution of theGI POF by using the transverse interferometrictechnique.8 The refractive-index distribution of theMMA/BB = 5 (wt/wt) fiber is shown as curve F5 ofFig. 4 compared with that of the preform rod, curvePS, where no and n are the refractive indices at thecenter axis and at distance r from the center axis,respectively. The index distribution of the fiber isalmost quadratic against the normalized radius (r/Rp)and is almost the same as that of the preform rod.The numerical aperture estimated from the indexdifference is approximately 0.21.

Attenuation of Light TransmissionWe measured the total attenuation spectrum of thelight transmission through the GI optical fiber byusing a spectrum analyzer (Advantest Model TQ8345).The result for the MMA/BB = 5(wt/wt) fiber isshown in Fig. 5. The minimum total attenuationwas 90 dB/km at 0.572 pim of wavelength for theMMA/BB = 5 fiber. It should be noted that theattenuation of this MMA-BB system fiber is almostthe same or is superior to that of SI POF's commer-cially available (100-300 dB/km). The dramaticdecrease of the attenuation compared with our previ-ous results5-7 concerning MMA-VB (134 dB/km at a0.652-[im wavelength) and MMA-vinyl phenylacetate(143 dB/km at a 0.651-[im wavelength) copolymer GIPOF is attributed to the reduction of the scatteringloss described above. It is noteworthy that the opti-cal window of the MMA-BB GI POF is located near0.57 jim of wavelength, whereas the optical windowof the copolymer system GI POF such as MMA-co-VBor MMA-co-VPAc was located near 0.65 [im of wave-length.

BandwidthWe measured the bandwidth of the GI and SI POF'sby determining the impulse response function of thefiber. The experimental setup of this measurementsystem is shown in Fig. 6. A pulse of 10 MHz froman InGaAlP laser diode (wavelength, 0.67 pim) wasinjected (N.A., 0.5) into a 55-m-long fiber. We de-tected the output pulse with a sampling head (Hama-matsu Photonics Model OOS-01). The result for theMMA-BB GI POF, compared with that of the SIPOF, is shown in Fig. 7. It is quite noteworthy that,although the output pulse through the SI POF isspread quite widely, the pulse through the GI POF isalmost the same as the input pulse even after a 55-msignal transmission. The bandwidth of the SI POFestimated at the 3-dB level in the impulse responsefunction is 5 MHz km, whereas the bandwidth of theMMA-BB GI POF in Fig. 7 is 1.0 GHz km, which is200 times larger than that of the SI POF.

1000

0

0CC

-0.005

-0.010

-0.015 0.5

800

co

00

600

400

200

1.00

(r/R,)Fig. 4. Refractive-index distribution of the MMA-BB GI preformand the GI POF: P5, preform rod (10mm 4); F5, fiber (0.5mm +).

0.55 0.6 0.65 0.7 0.75 0.8 0.85

Wavelength (m)

Fig. 5. Total attenuation spectrum of the GI POF.

4264 APPLIED OPTICS / Vol. 33, No. 19 / 1 July 1994

A

I V

i

I

0

-0.005C

C

-0.010

Fiber

TriggerI OOS-0 ISampling Optical Oscilloscope

Fig. 6. Experimental setup of the impulse response functionmeasurement; LD, laser diode.

It is well known that one can maximize the band-width by optimizing the shape of the refractive-indexdistribution of the fiber core. When the index distri-bution is expressed by a power law of the form

n(r) = no[l - (r/Rp)YA], (7)

the bandwidth is maximized for

g = 2 - (12/5)A. (8)

In this formula A is a parameter that measures therelative index difference, A = (no - n,)/no, where noand n, are the index values at the core center and inthe fiber cladding, respectively, and Rp is the radius of

-0.015 0.5 1.0

(r/Rp)Fig. 8. Refractive-index distribution of the MMA-BB GI POF(solid curve) in which the bandwidth is 1 GHz km and the indexprofile (dashed curve) is approximated by Eq. (7).

the core. The parameter g is the exponent of thepower law. Because A = 0.01-0.02 for the GI POF,the maximum bandwidth is achieved when g isapproximately 2. The refractive-index distributionof this high-bandwidth GI POF is shown by the solidcurve in Fig. 8. The index profile approximated byEq. (7) through the use of a least-squares technique isshown by the dashed curve in Fig. 8. Figure 9summarizes the bandwidth of the various GI POF'sagainst the g in Eq. (7), along with that of the SI POE.The bandwidth is maximized near g = 2 for theMMA-BB fiber and is 1 GHZ km, which is 200 timeslarger than that of the SI POF.

The high bandwidth of the GI POF compared withthat of the conventional SI POF was experimentallyconfirmed, even for a short-distance communicationas small as 50 m. We measured the tensile strengthof the GI POF by elongating a 100-mm length of POF

1000

N

1-4

X

3-1;

500

0

1.5 2 2.5 3

Index Exponent gFig. 9. Normalized bandwidth of GI POF's against index expo-nent g: 0, MMA-benzyl methacrylate system; A, MMA-VBsystem; *, MMA-VPAc system; 0, MMA-BB System.

1 July 1994 / Vol. 33, No. 19 / APPLIED OPTICS 4265

........... ........... ........... ........... ........... ........... ............... ...................... ...................

i ~~~Si POF

...... ..... .. ...... ... .. . .. ... ..... .. ...

.. . .. . ......... ........... .. .. . ..,. . .. . .. .. .. . .. .

. .. .. . .. ............. ........... .. .. . .

........... . . .. ... ...... .. ........ .... . . . .. .. . ........... . , , , A .GI.P.F

.. ... .. .. . .... .. .. . .. .. .. . .. .. .. . . .. ... . . .. .. .. . ... .. .. . . .. . .. ............................ ....... .. ... ..................................

- {i ~~Input pulse............ ... ......

.......... ........... ........... ........... ........... 1........... . .. .. .. ..,. .. . .. . .'''' .. .'' ''' .V '''' ..... .. .. . ........... .. . .. . .. .. .. . ''''''.''..'.'._. .. .. .. .. . . . . . . .. . .. ... . . . .. .. . .. .. .. .. . .. .. . .. .

101I I

I II|

I I

I0 0

0 A 0

..0 Ao*A\

-'"I... Y.... .. I..t.. ......I.... .A..

H K 500psFig. 7. Output pulse spread through both GI and SI POF's.

at a rate of 100 mm/min. The strength of theMMA-BB GI POF is 2000 kgf/cm2, which is compa-rable with that of commercially available SI POF(900-1200 kgf/cm2 ).

Conclusion

We successfully obtained a low-loss and high-band-width GI POF with good mechanical properties byutilizing the swelling of the polymer and the diffusionof the monomer molecules. This GI POF essentiallyconsists of PMMA homopolymer, so the scatteringloss is lower than that of the copolymer GI POF wepreviously prepared. Furthermore, as the refractiveindex of this GI POF monotonically decreases fromthe center axis to the periphery without any fluctua-tion, the bandwidth becomes three times higher thanthat of our previous MMA-VB copolymer GI POE.

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