Vol. 11, No. 5/May 1994/J. Opt. Soc. Am. B 835

Electro-optic measurements of tolane-basedpolymeric phase modulators

J. I. Thackara, M. Jurich, and J. D. Swalen

IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120

Received September 10, 1993

With a new nonlinear-optical polymer, electro-optic phase modulators have been designed, constructed, andoperated at a wavelength of 1.32 ,um. A large electro-optic coefficient, r33, of 26 pm/V was achieved with elec-trode poling. The polymer, poly(methyl methacrylate)-nitroamino tolane (PMMA-NAT), contained 43 mer. %of the nonlinear-optical chromophore attached as a side chain. A heterodyne detection system was used bothto determine the electro-optic coefficients of the poled polymer and to demonstrate the transmission of multipletelevision signals on an optical carrier. Other optical properties of PMMA-NAT, such as the dispersion inthe index of refraction and in the electro-optic coefficient, are also reported.

INTRODUCTION

The large bandwidth of the electro-optic effect in organicmaterials, as well as their low dielectric constants andthe ease with which polymeric thin films can be formed,has motivated considerable research on organic-polymer-based devices.' As the field matures, the need to evalu-ate the performance of these devices in practical applica-tions is becoming increasingly important. In this investi-gation we have used a heterodyne setup, which simulatedthe operation of a coherent data-transmission system, tocharacterize slab-waveguide phase modulators.

It is well known that the performance of opticaldata-transmission systems can be significantly improvedthrough the use of coherent detection techniques. Thisis primarily because of the practical advantages thatcoherent detection has over direct detection. These ad-vantages include increased sensitivity and wavelengthselectivity.2 Of the many coherent architectures avail-able, those based on phase modulation of the opticalcarrier provide the greatest receiver sensitivity. How-ever, because of the stability requirements imposed onthe optical source in such systems, it is advantageous togenerate the optical signals by using a cw optical sourceand an external phase modulator.2 3 We have used thisapplication to test our modulators further by transmit-ting and receiving a block of six television channels inour heterodyne system.

MODULATOR DESIGN AND FABRICATIONThe polymer poly(methyl methacrylate) (PMMA) with thechromophore nitroamino tolane (NAT) attached as a sidechain was chosen as the active material for the phasemodulators. The structure of PMMA-NAT is shownschematically in Fig. 1. PMMA-NAT has been foundto have good optical transmission at 1.32 and 1.55 Aum.1The NAT chromophore exhibits a large nonlinear molec-ular polarizability, j~o, of 37.5 10-30 esu, while havingan absorption maximum at 402 nm,4 far from the opti-mum fiber transmission windows in the near IR. The

relatively long length and stiffness of the NAT moleculeare expected to contribute to the thermal stability of theelectro-optic effect because of the anticipated increase inhindrance to chromophore rotation after poling.

Two different concentrations of the chromophore wereinvestigated. Expressed in terms of the percentage ofmonomers having an NAT side chain, the loading lev-els were 20 and 43 monomer % (mer. %). The glass-transition temperatures were determined from differen-tial scanning calorimetry measurements and found to beapproximately 117 and 121'C for the 20- and 43-mer. %polymers, respectively. The absorption and refractive-index data were taken on films spun from solution ontofused-quartz substrates. All films were vacuum bakedat 120'C for 14 h before any measurements were taken.The absorption spectra, measured on a Perkin-ElmerLambda 9 spectrophotometer, are shown in Fig. 2(a).Since the thicknesses of the 20- and 43-mer. % films,0.886 and 0.265 m, respectively, were not equal, theabsorption data for the 20-mer. % material have beenmultiplied by 0.30 to present the results on the samescale. Refractive-index data were determined on thickerfilms, between 1.7 and 1.9 m, with a Metricon PC-2000at wavelengths of 0.5435, 0.6328, 0.832, and 1.0642 ,um.These results are shown in Fig. 2(b) along with Sellmeier5

fits to the data.The structure of the phase modulator is shown in Fig. 3.

All devices consisted of a 2-p-tm-thick PMMA-NAT layersandwiched between two 5-Mum-thick V-cured acrylatebuffer layers, which had the device electrodes above andbelow. The ground electrode, deposited on the fused-quartz substrate, was composed of a 7.5-nm chromium ad-hesion layer and a 1-Mum-thick gold layer. After the spin-coating and the curing processes, the lower buffer layerwas vacuum baked at 120 0C for 4 h before the applicationof the PMMA-NAT layer. High-optical-quality PMMA-NAT films, 2 um thick, were formed from a 29 wt. %solids-in-diglyme solution by spin coating for 30 s. ThePMMA-NAT films were then baked at 120 0C for 14 h.The top buffer layer was composed of the same materialas the lower buffer layer and was applied in the same

0740-3224/94/050835-05$06.00 ©1994 Optical Society of America

Thackara et al.

836 J. Opt. Soc. Am. B/Vol. 11, No. 5/May 1994

CH3

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102Fig. 1. PMMA-NAT

Wavelength (in)

(a)

700 800 900 1000 1100 1200 1300

Wavelength (nm)

Electrode poling was used to orient the NAT chro-mophores. We found that poling with all three layerspresent, rather than just the PMMA-NAT and lowerbuffer layers, resulted in higher electro-optic coefficients.This is most likely caused by a combination of twofactors. First, the Tg of the UV-cured acrylate was sev-eral degrees lower than that of the PMMA-NAT material,resulting in the buffer layers' being more conductive thanthe PMMA-NAT core at the poling temperature. Conse-quently most of the applied poling voltage dropped acrossthe PMMA-NAT layer. Second, poling with both bufferlayers present permitted significantly higher voltagesto be applied before electrical breakdown of the poly-mer stack. Stronger electric fields in the PMMA-NATlayer could therefore have been achieved for the poling ofthree-layer stacks, accounting for the higher electro-opticcoefficients. Voltages as high as 850 V were appliedacross the three-layer stacks, and all modulators werepoled at 115'C for a period of 5 to 15 min before slowlycooling back to ambient temperature. After the poling,the top electrode was chemically etched off. An alu-minum microstrip electrode was then added and usedto drive the device. The microstrip electrodes, 500 ,umwide and 0.5-1.0,um thick, were deposited by thermalevaporation through a stencil mask. The active lengthwas 1 cm. The electrodes formed by this process were toowide to give the desired 50-0 impedance, but the devicefabrication was greatly simplified, and the modulatorsoperated well at over 200 MHz.

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(b)Fig. 2. (a) Absorption spectra and (b) refractive indices for 20-and 43-mer. % PMMA-NAT. Sellmeier fits have been drawnthrough the TE (circles) and TM (triangles) refractive-index dataat 543.5, 632.8, 832, and 1064 nm; the data were taken witha Metricon PC-2000. The refractive-index values at 1064 and1320 nm associated with the error bars are calculated fromprism-coupling angles measured in the heterodyne test-bed.

manner, except that portions of the underlying PMMA-NAT film were masked to facilitate prism coupling to theactive layer.

Fig. 3. Slab-waveguide phase-modulator geometry. Top, topview; bottom, side view (1 inch = 2.56 cm).

2.5

1.5C)

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1.8

1.75

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CA

a4

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Thackara et al.

Vol. 11, No. 5/May 1994/J. Opt. Soc. Am. B 837

TelevisionMonitor

Fig. 4. Heterodyne test-bed.

Since no lateral confinement was needed to keep the op-tical beam under the relatively wide modulating electrode,channel waveguides were not needed, and the slab wave-guide geometry could be used for all modulators. Mea-surements of the total insertion loss at 1.32 /im, throughboth coupling prisms and the modulator active region, in-dicated that the propagation loss in the poled regions wasbetween 1 and 3 dB/cm for the 20-mer. % devices andbetween 3 and 5 db/cm for the 43-mer. % devices. Thelowest total insertion loss value obtained was 6 dB in a20-mer. % device.

HETERODYNE MEASUREMENTS

The heterodyne test-bed is shown schematically in Fig. 4.The half-wave plate and the prism angle were adjustedfor optimum coupling to either the TEO or TMo mode.Several TE and TM modes were supported by the PMMA-NAT layers, but little intermode coupling was observed.The sidebands impressed on the 1.32-,um optical carrier,generated by rf signals applied to the modulating elec-trode, were downshifted by the heterodyne detection.6

The acousto-optic modulator was driven at 120 MHz toproduce the frequency-shifted optical reference beam.This arrangement simulates the operation of a practi-cal coherent detection system, which would employ afrequency-locked local laser oscillator. The referenceand the phase-modulated beams were combined at thebeam splitter and interfered at the detector. The de-tected signal had a center beat at v = 120 MHz andupper and lower sidebands at va ± n Vn rf

If the modulator is driven with a single frequency,the heterodyne detection system provides a convenientmethod for determining the electro-optic coefficients ofthe poled PMMA-NAT layers.6 This type of measure-ment technique has been used effectively to measure pol-ing transient effects in organic nonlinear-optical films.7The high sensitivity inherent in heterodyne detection per-mits accurate device characterization with only a smallfraction of the drive voltage that is needed to produce a phase shift. The electric fields in the phase-modulated

(or signal) and optical reference beams are6

Esig = EO exp(iwct)

+ 2 {exp[i(w, + &W)t] - expi(wc - Wm)t]},

(la)

Eref = Ea exp[i(w + aj)t], (lb)

where EO and Ea are the field amplitudes, eoj,, cm, andca are the angular frequencies of the carrier, the phasemodulator, and the acousto-optic modulator, respectively,and M is the modulation index. The modulation index isrelated to the device parameters by

M rV/LV(n,') 3 ra3 (1Atc O3m + 2 ectb) 1 (2)

Eb tc

where A is the carrier wavelength, V, is the rms appliedvoltage, t and e are the thickness and the dielectric con-stant of the PMMA-NAT in the poled region, tb and eb arethe thickness and the dielectric constant of each buffer(assumed to be the same) in the poled region, L is thelength of the active region, nTm is the TM refractive in-dex of the poled PMMA-NAT layer, and r33 is the electro-optic coefficient. The modulation index for the TE modeis obtained from Eq. (2) by replacement of n,' with thecorresponding nTE and replacement of the electro-opticcoefficient r33 with r 3 . E®m is a correction factor account-ing for the difference between the change in the effectiveindex neff resulting from a change in the PMMA-NAT in-dex induced by the electro-optic effect. We used the ra-tio of the change in the core material index to that ofneff for m. The largest values of m (1.07) were for20-mer. %-based devices operated in the TMo mode at1.32 ,um. The determinations of n and e are difficultin modulators poled with both lower and upper bufferlayers present. Based on measurements of PMMA-NATlayers poled at the above levels but with only the lowerbuffer layer present, the poling induced an increase in therefractive index for 1.32-,um TM radiation between 0.01

Thackara et al.

838 J. Opt. Soc. Am. B/Vol. 11, No. 5/May 1994

i'1

I-

i).2

U

C.)

0

18-

16_

14-

12 -

10 -

8-

6-

4-

280800 900 1000 1100 1200 1300

Wavelength (nm)

Fig. 5. Dispersion of r1,33 and n3 rl,33 for 20-mer. % PMMA-NAT.

and 0.03 for 20 mer. % films and between 0.03 and 0.08for 43 mer. % films. The value of eC/eb was estimatedwith the assumption that the ratio of dielectric constantsfor methacrylate-based materials at frequencies above afew hundred kilohertz is nearly equal to the ratio of thesquares of the refractive indices in the near IR.

At the detector the total intensity is given by

lEsig + Eref 2 = 2EoEa cos(wt) - MEoEa sin(cwa - m)t

+ MEoEa fsin(Wa + cjm)t, (3)

neglecting terms of order M2 or higher. The ratio of theoptical powers in the central beat at 1a and the side peaksat J', + vm is M/2, and therefore the ratio of the electricalpowers is M2 /4. Expressed in terms of the difference Abetween the electrical powers in the central beat and theside peaks measured in decibels, M is

M = 2 X 10-/20 (4)

The electro-optic coefficient r33 can then be written as

/2 At 10-/ 2 0 ( 2e tb r3= iTLVs(ncTM)3

6b tc 5

All electro-optic measurements were carried out with a2-MHz drive signal at 1-2 Vrns. The 2-MHz frequencywas high enough to avoid any contribution to the electro-optic signal from chromophore motional effects (the fre-quency responses of all modulators were found to be flatover the measured range from 10 kHz to 2 MHz), whilestill being low enough to permit precise determination ofthe modulating voltage. The drive voltage was, however,limited to 2 Vrm, to ensure the validity of the assumptionthat M2 << M. The values of r13 and r33, calculated frommeasurements of A at 0.832, 0.884, 1.064, and 1.32 Aumfor a 20-mer. % PMMA-NAT device are shown in Fig. 5.At each wavelength the absolute level of the side peakswas more than 30 dB above the noise floor. The dropin r3 3 from 0.832 to 1.32 um is small, only 25%, whichis expected because Amass is far from the wavelengths atwhich the modulator was operated. The largest r3 3 valuewas achieved in a 43-mer. % device. At 1.32 Am, r33 was26 pm/V.

The transmission of multiple television channels on a1.32-/gLm carrier was demonstrated with a block of sixchannels (channels 7 through 12, occupying the rangefrom 175.25 to 205.25 MHz) as the rf drive to a 20-mer. %phase modulator. A Lightwave Electronics 122 1.32-,umlaser was used as the narrow-band source. The total rfpower applied to the modulator drive electrode was 1 W,and the optical powers in the modulated and the refer-ence beams were 4 and 10 mW, respectively. With theacousto-optic modulator driven at 120 MHz, the lowersideband occupied the range from 55.25 to 85.25 MHz(channels 1 through 6). Owing to the wrap-around zerofrequency, the lower sideband was correctly oriented forreception with a standard television receiver. The pic-ture color and audio of the input channels were correctlyreproduced in the received channels, and no cross talkbetween channels was observed.

CONCLUSIONSWe have demonstrated the use of a heterodyne detec-tion system for both the characterization of polymeric slabwaveguide phase modulators and for the transmission ofmultiple television channels on a 1.32-,um carrier. Thesystem's high sensitivity and the modulator fabricationprocedure permit many materials with wide-ranging opti-cal and electro-optical properties to be evaluated quickly.While the temporal stability of the electro-optic effect inPMMA-NAT is not sufficient for practical applications 8

(because of the low value of Tg), the large electro-opticcoefficients obtainable with NAT make this type of chro-mophore a strong candidate for further study. The lowdispersion of r33 in the near IR indicates that the electro-optic coefficients for NAT-containing compounds will alsobe high at 1.55 Am.

ACKNOWLEDGMENTSWe thank R. D. Miller and V. Y. Lee for the preparationof the PMMA-NAT polymers and solutions and for pro-viding the differential scanning calorimetry data. Thecontributions of W. W. Fleming to the modulator fabri-cation process and of B. A. Smith to the development ofthe heterodyne test bed are appreciated. We also thank

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G. C. Bjorklund for his support and help. This work waspartially supported under a National Institute of Stan-dards and Technology-Advanced Technology Program co-operative agreement.

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6. B. A. Smith, M. Jurich, W. E. Moerner, W. Volksen, M.E. Best, W. Fleming, J. D. Swalen, and G. C. Bjorklund,"Lightwave transmission of multiple television signals us-ing an organic polymer electro-optic phase modulator," inNonlinear Optical Properties of Organic Material VI, G. R.Mohlmann, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 2025,499-506 (1993).

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