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Spectral measurement of the film–substrate indexdifference in proton-exchanged LiNbO3 waveguides

Kacem El Hadi, Vipul Rastogi, Mangalpady R. Shenoy, Krishna Thyagarajan,Marc De Micheli, and Daniel B. Ostrowsky

We report the spectral characterization of proton-exchanged lithium niobate ~PE:LiNbO3! waveguides interms of the variation of the refractive-index difference between the waveguiding layer and the substrate.The dispersion of the extraordinary refractive-index increase ~dne! is measured from 405 to 1319 nm withseveral light sources. Two types of proton-exchanged waveguide, prepared under different conditions,are studied. These measurements should be of use in the optimization of PE:LiNbO3 waveguides fornonlinear optical applications, particularly in second-harmonic generation in the blue-green wavelengthregion. © 1998 Optical Society of America

OCIS codes: 160.3130, 160.3730, 160.4330, 190.4390, 230.7390, 310.2790.

1. Introduction

Lithium niobate ~LiNbO3! is one of the most widelyused substrates in the field of nonlinear integratedoptics,1,2 as it possesses large nonlinear optical coef-ficients, is amenable to periodic poling,3 supports low-oss optical waveguides, and is transparent in theptical window from 0.4 to 4.0 mm. Among the tech-iques for fabricating waveguides in this material,ne of the most convenient is the proton-exchangePE! method.4 Using this method, one can fabricate

waveguides with both step-index and graded-indexprofiles. Various studies have been made to charac-terize the PE process in LiNbO3 and to establish acorrelation between the fabrication conditions andthe optical properties of the waveguides.5–8

In the study of nonlinear optical interactions suchas second-harmonic generation and parametric am-plification it is important to know the refractive-index profile and particularly the index differencebetween the guiding layer and the substrate at theinteracting wavelengths. Because the efficiency of

V. Rastogi, M. R. Shenoy, and K. Thyagarajan are with theDepartment of Physics, Indian Institute of Technology—Delhi,New Delhi-110 016, India. K. El Hadi, M. De Micheli, and D. B.Ostrowsky are with the Laboratoire de Physique de la MatiereCondensee, Universite de Nice—Sophia Antipolis, Parc Valrose,06108 Nice Cedex 2, France. The e-mail address for K. Thyaga-rajan is ktrajan@physics.iitd.ernet.in.

Received 15 September 1997; revised manuscript received 14April 1998.

0003-6935y98y276463-05$15.00y0© 1998 Optical Society of America

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nonlinear interactions depends critically on phase-matching conditions, uncertainties in the wavelengthdependence of the index increase in the guiding layercould lead to deviations from phase matching andthus to drastically reduced conversion efficiencies.

There have been several studies of measurementsof the index difference in proton-exchanged and an-nealed proton-exchanged waveguides.9–12 Wave-guides formed by PE ~referred to as PEI waveguides!usually have a high refractive-index increase dne~;0.1! and a relatively large propagation loss that canbe reduced significantly by annealing.13 It has beenshown that both PE and annealed PE can lead to theerasure of domain inversion, thus reducing the effi-ciency of quasi-phase-matched nonlinear interactionsin such waveguides.1 In a recent study it was shownthat waveguides fabricated with a lithium concentra-tion that is $2.6% in the melt will permit fabrication ofproton-exchanged waveguides in periodically poledLiNbO3 without erasure of the periodic domain in-version.1 These waveguides, referred to as PEIIIwaveguides, have a relatively small value of dne~;0.03!. Because dne is small, the spot sizes of themodes at the fundamental and the second-harmonicwavelengths differ widely in these waveguides.Therefore the dispersion of dne is critical in the opti-mization of waveguide parameters for efficient nonlin-ear interactions in such waveguides.

We report measurement of the wavelength de-pendence of the index increase dne in proton-exchanged waveguides in the wavelength region405–1319 nm for several light sources. The resultsof measurements of two important kinds of proton-

0 September 1998 y Vol. 37, No. 27 y APPLIED OPTICS 6463

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exchanged waveguide, namely, PEI with high dne~;0.1! and PEIII with low dne ~;0.03!, are reported.

ur measurements are shown to be consistentith reported values of PEI waveguides. With re-

spect to PEIII waveguides, ours is the first report,to our knowledge, of spectral measurement ofsuch waveguides. We show that, for high dnewaveguides, dne can vary by a factor of 2 in theange of 400–1300 nm, whereas for low dne

waveguides it can vary by more than a factor of 5.It is also shown that a Sellmeier-type functionalvariation fits the measured values well. These re-sults should be of considerable importance in theoptimization of proton-exchanged waveguides innonlinear optical interactions.

2. Waveguide Fabrication

Planar waveguides were prepared by PE in Z-cutLiNbO3 substrates. We used the sealed ampouletechnique14 with a 300 °C melt consisting of a mix-ture of benzoic acid ~BA! and lithium benzoate ~LB!.

y varying the ratio r$ 5 100@mLBy~mBA 1 mLB!#%from 0 to 6% we obtain three kinds of index profile15

@Fig. 1~a!#:

Fig. 1. ~a! Maximum change in the extraordinary refractive indexof the proton-exchanged layer achievable by use of the sealed am-poule technique, as a function of percentage dilution of the melt, r.PEI, PEII, and PEIII refer to the three types of index profile, de-scribed in the text, which correspond to different ranges of r. ~b!Typical index profiles at 632.8 nm, corresponding to three fabrica-tion conditions. APE means annealed proton exchanged.

464 APPLIED OPTICS y Vol. 37, No. 27 y 20 September 1998

A step profile ~called PEI! is observed for 0 # r #%. This kind of exchange leads to the highest indexariation ~e.g., dne ' 0.11 at l 5 632.8 nm! that cane produced by the sealed ampoule technique.For 1 , r , rth ' 2.6%, the index profile is steplike

near the surface, followed by a graded tail into thesubstrate, and is designated a PEII profile.

For r . rth, an abrupt transition of dne is observed.The step-index region completely disappears, and oneobtains a pure graded-index profile ~PEIII! with amaximum index change dne ' 0.03 near the surface.

One can also anneal the first two types of waveguideto redistribute the protons deeper into the substrate.The resultant waveguides, called annealed proton ex-changed waveguides, have graded-index profiles witha maximum index increase of 0.1–0.01, depending onthe annealing time and temperature. Typical indexprofiles that correspond to the three types ofwaveguide are shown in Fig. 1~b!.

Inasmuch as PEI waveguides are characterized by alarge dne, they have the advantage of strong modeconfinement and hence increased overlap of the in-teracting fields in guided-wave nonlinear opticalprocesses. Such waveguides also exhibit high propa-gation loss and a negligible value of the nonlinear co-efficient in the guiding layer. In contrast, PEIIIwaveguides are characterized by low propagation lossand no degradation of the nonlinear property of thematerial and hence seem to have the best optical qual-ity,15 although with a decreased dne.

3. Spectral Characterization

For spectral characterization we chose two kinds ofwaveguide, PEI and PEIII, which cover the extremes.Multimoded planar waveguides were fabricated andwere characterized in terms of the modal effective in-dices by the prism-coupling technique.16 For eachkind of waveguide we measured the modal effectiveindex and estimated the variation of the extraordinaryrefractive-index increase dne in the wavelength range405–1319 nm, using several optical sources ~see Table!. When mercury–xenon and tungsten–halogen

lamps were used as the source, the waveguides werefirst end polished, and the light was coupled into thewaveguide by end-fire coupling and outcoupled by aprism. The wavelengths and the correspondinginput–output coupling conditions are given in Table 1.

We characterized three waveguides, one PEI andtwo PEIII, at all available wavelengths to measurethe dispersion of the index change ~dne!. The PEIwaveguide, prepared with a 1% diluted melt at300 °C for a duration of 30 min, had a depth of 0.7 mmnd had three modes at 543.5-nm wavelength. Be-ause PEI waveguides have steplike refractive-index

profiles, we calculated the values of guiding filmthickness d and film–substrate index difference dnefrom the measured effective indices by directly solv-ing the transcendental equation for a step profile.The substrate refractive indices at several wave-lengths were obtained from the Sellmeier equationfor LiNbO3.17 The wavelength dependence of index

F

dt

d

Table 1. Light Sources and Characterization Techniques Used To Measure the Spectral Dependence of the Index Variation in Proton-Exchanged

change dne is shown in Fig. 2. As can be seen, dneincreases by approximately a factor of 2 as one movesfrom 1300 to 400 nm. The curve shown in Fig. 2corresponds to the following Sellmeier-type equation:

dne 5 0.0847376 12862.93

l2 11.57563 3 109

l4 . (1)

As is evident from Fig. 2, Eq. ~1! fits the measureddata points well. This variation is also compared, inFig. 3, with the variation obtained in Ref. 10 for anX-cut PEI LiNbO3 waveguide. As can be seen from

ig. 3, our measured spectral variation of dne in PEIwaveguides is similar to that reported earlier.10 The

ifference between the two results can be attributedo the different cuts of the two waveguides and to

Fig. 2. Dispersion of dne for the PEI ~1%, 300 °C, 30 min!waveguide.

Fig. 3. Comparison of the spectral variation dne of PEI

waveguides reported in Ref. 10 and in the present study. Thecurves represent the Sellmeier-type functional variation deter-mined by the two studies.

LiNbO3

Light Source Wavelengths

Mercury–xenon lamp 405, 435

Tungsten–halogen lamp 420, 480, 500, 54620, 660, 680, 70

Argon laser 459, 488, 514He–Ne laser 543.5, 594.1, 611Ti:sapphire laser 730, 808Nd:YAG laser 1319

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different fabrication conditions. Our waveguidesare grown by the sealed ampoule technique14 and athigh temperature, in contrast to those reported inRefs. 9–11, in which the waveguides were grown at alower temperature and in the open atmosphere.

Two PEIII waveguide samples were fabricated.The first sample was prepared with a 3% dilutedmelt. The sample supported three modes at543.5-nm wavelength. In the case of the PEIIIwaveguides it has been shown that the refractive-index profile follows an exponential variation.1Therefore, in the case of PEIII waveguides, therefractive-index profile was first reconstructed fromthe measured effective indices by the inverse WKBmethod18 to yield rough estimations of d and dne.Using these values as approximate values for d andne, we solved the eigenvalue equation for the expo-

nential profile to obtain the mode effective indices.19

The values of d and dne were then varied until thecalculated effective indices matched the measuredeffective indices to five significant digits. Obviously,the depth of the index profile, d, should remain al-most the same at all wavelengths. Table 2 shows

guides

! Characterization Technique

Interference filters 1 end-fire inputcoupling 1 prism output coupling

0, Monochromator 1 end-fire inputcoupling 1 prism output couplingInput–output prism coupling

32.8 Input–output prism couplingInput–output prism couplingInput–output prism coupling

Table 2. Estimated Values of dne and Waveguide Depth d fromMeasured Effective Indices for Sample 1 ~3%, 300 °C, and 27 h 30 min!

of PEIII Waveguides

Wavelength ~nm! dne

WaveguideDepth d ~mm!

405 0.0450 5.8420 0.0400 5.7435 0.0370 5.8459 0.0290 5.9480 0.0260 5.7488 0.0240 5.7500 0.0220 5.8514 0.0210 5.8540 0.0180 5.7543 0.0170 5.8580 0.0150 5.7620 0.0130 5.7633 0.0110 5.7660 0.0105 5.8680 0.0100 5.7700 0.0095 5.8730 0.0090 5.8808 0.0080 5.7

1319 0.0070 5.8

Wave

~nm

0, 580

.9, 6

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the values of d and dne for the PEIII waveguide thatwe obtained by solving the TM-mode eigenvalueequation for an exponential profile. Note that,whereas dne varies with wavelength, d remains al-most a constant, as indeed it should. As for the PEIcase, the values of the substrate refractive indicesused in these calculations for different wavelengthswere obtained from the Sellmeier equation forLiNbO3.17 Indeed, we found that the measured val-ues of the effective indices fitted well the values cal-culated from the exponential profile and thusconfirmed the exponential nature of the refractive-index profile. The error in measuring the mode ef-fective indices by use of our prism coupling setup istypically 2 3 1024, which introduces an error of 2–4%in the estimated value of dne from the above calcula-tions.

The measured spectral variation of dne is shown inig. 4. The fitted curve corresponds to the followingquation:

dne 5 0.0065036 2827.123

l2 11.1944 3 109

l4 . (2)

We prepared the second sample of the PEIIIwaveguides by using 2.65% diluted melt to get ashigh as possible an index variation @Fig. 1~a!#. Thisample supported four modes at 543.5-nm wave-ength. The corresponding dispersion in dne is also

shown in Fig. 4 ~sample 2!. Because the Sellmeierype of functional dependence of dne has already been

established @see Eqs. ~1! and ~2!#, in this case mea-surements were carried out primarily at the availablelaser wavelengths. The fitted curve corresponds tothe following equation:

dne 5 0.0162485 21845.74

l2 11.82664 3 109

l4 . (3)

Again, we see that Eqs. ~2! and ~3! fit the measureddata points well.

Our measurements show an interesting and impor-tant result, i.e., that the dispersion in PEIII waveguidesis high compared with that in the PEI waveguide. In-deed, in the case of PEIII waveguides, dne increases by

Fig. 4. Dispersion of dne for the PEIII waveguides. Fabricationarameters for Sample 1: 3%, 300 °C, 27 h 30 min; for Sample 2:.65%, 300 °C, 30 h.

466 APPLIED OPTICS y Vol. 37, No. 27 y 20 September 1998

more than a factor of 5 as we go from ;1300 to ;400m. This datum should be important in the optimi-ation of waveguide parameters to achieve efficientonlinear interactions, particularly those that involvehorter wavelengths in the visible region.

4. Conclusion

We have reported measurements of the dispersion ofthe index change in proton-exchanged lithium niobatewaveguides in the 405–1319-nm spectral range usingseveral light sources. Both PEI and PEIII types ofwaveguide were characterized. A Sellmeier-typeequation was shown to fit the measured data well.Our measurements showed that the dispersion inPEIII waveguides is quite high. Measurement of thendex change in the short-wavelength range is impor-ant, particularly in the study of second-harmonic gen-ration in the blue–blue-green wavelength regions.

This study is partially supported by a collaborativeroject sponsored by the Indo-French Center for theromotion of Advanced Research, New Delhi. Theuthors thank Ruchi Vats for some data on PEI

waveguides and Ruchi Katyal for the help in endpolishing the waveguides.

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