TECHNICAL NOTE

Feasibility of potassium-exchangedwaveguides in BK7 glass for telecommunicationdevices

Ari Tervonen and Seppo Honkanen

Based on recent results for potassium-exchanged optical waveguides in BK7 glass @Appl. Opt. 34,455–458 ~1995!#, the feasibility of fiber-compatible waveguide fabrication for telecommunication wave-lengths is studied. Modeling of waveguides gives the result that such waveguides with good propertiescannot be achieved with this fabrication technique because of the observed saturation of diffusion depth.© 1996 Optical Society of America

Key words: Integrated optics, optical waveguides.

Recently a thorough study1 was published onpotassium-exchanged channel waveguide refractive-index profiles in BK7, a high-quality optical glassthat has been used for fabrication of waveguide de-vices. The characterization was carried out at theoptical wavelength of 0.633 mm, at which waveguideshad several guided modes, thus allowing the authorsof Ref. 1 to fit proper analytic refractive-index distri-butions into measured mode propagation constantsand making it possible for them to determine therelationship between waveguide profiles and fabrica-tion process parameters. The potassium-exchangedwaveguides are generally considered to have profilescompatible with single-mode fibers, since the indexincrease achieved is relatively low. The most impor-tant application area of typical devices utilizing suchwaveguides is in fiber-optic telecommunications atwavelength regions of between 1.3 and 1.55 mm.Thus it is important to take a further look at thewaveguide characteristics at these wavelengths. Inparticular, it was observed in Ref. 1 that the graded-index waveguide depth saturates during the diffusionprocess, which may cause serious problems. We

When this research was carried out A. Tervonen was with Op-tonex, Ltd., P.O. Box 128, FIN-02150 Espoo, Finland. He is nowwith Nokia Research Center, P.O. Box 45, FIN-00211 Helsinki,Finland. S. Honkanen is with the Optical Sciences Center, Uni-versity of Arizona, Tucson, Arizona 85721.Received 14 March 1996; revised manuscript received 6 August

1996.0003-6935y96y336435-03$10.00y0© 1996 Optical Society of America

used the results in Ref. 1 to examine this questiondirectly.The ion-exchanged waveguides in Ref. 1 were fab-

ricated by immersing BK7 substrates masked withaluminum thin films into a KNO3 melt, into whichstrip openings having various widths had been pat-terned. Weiss and Srivastava1 carried out ion ex-change for different lengths of time at a temperatureof 375 °C. The waveguide refractive-index distribu-tions were found to correspond well to analytic pro-files of the type

n~x, y! 5 ns 1 Dn erfc~xydx!exp~2y2ydy2! (1)

where coordinate x is the depth from the surface andy is the lateral distance from the waveguide symme-try axis. The substrate refractive index is ns and theactual parameters that can be used to define the pro-file are maximum refractive index increase Dn, pro-file depth dx, and profile half-width dy. For TE andTM polarizations, Dn was determined to have valuesof 0.0080 and 0.0092, respectively. Then dx and dywere derived as functions of mask opening width wand process duration t. In particular it was ob-served that dx saturates to a maximum value forgiven w. This is important since, in such diffusedsurface waveguides, a certain minimum depth is nec-essary for the waveguide to support any guidedmodes at all, and also the vertical mode confinementdepends on the waveguide depth.We used the same effective index technique, with

which the fit into distribution of Eq. ~1! was made, tosolve the mode propagation coefficients at telecom-munication wavelengths of 1.3 and 1.55 mm in

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waveguides with profile parameters from Ref. 1.Our calculation model was checked against the re-sults presented in Ref. 1. Because of dispersion, therefractive-index distributions have slightly differentvalues for Dn at these wavelengths, but this wouldhave an insignificant effect on general mode proper-ties. Also, since dx corresponds to the depth of theion-exchanged distribution producing the refractive-index increase, it should not depend significantly onwavelength. Table 1 lists normalized propagationconstants b 5 ~neff

2 2 ns2!y@~ns 1 Dn!2 2 ns

2# forwaveguides fabricated with given values of w and t.It was found that, with mask opening widths nar-rower than 5 mm, no guided modes exist at a wave-length of 1.3 mm, and at a wavelength of 1.55 mm, amask opening width of 8 mm is needed. It should beemphasized that even though the effective indexmethod is not considered accurate near cutoff, it is agood approximation to show the general mode behav-ior. In this case, for the fundamental mode given bythis method the cutoff condition exists when the pla-nar waveguide, which corresponds to a one-dimensional refractive-index distribution in themiddle of the waveguide ~y 5 0!, is under cutoff.This means that the more confined channelwaveguide does not have a guided mode.At l 5 1.55 mm, the modes are weakly confined to

waveguides having a relatively small index differ-ence. To examine this more closely, we solved theguided-mode field distributions with a scalar finitedifference method,2 since the effective index methodcannot be considered accurate when one calculatesthe mode field distributions. The use of a scalarmethod is valid for calculating mode fields in glasswaveguides with small refractive-index variations,and the use ofmore complicated vector solutionmeth-ods would make no significant difference in our esti-mations of fiber-to-waveguide coupling losses. FromTable 1 we chose the waveguide with w 5 10 mm, t 520 h, which had the highest mode confinement at l 51.55 mm. The mode field distributions are shown inFig. 1 for both TE and TM polarizations. Because ascalar solution method was used, the only differencebetween the two polarizations is the value of the

Table 1. Calculated Normalized Mode Propagation Constants forWaveguides with Various Fabrication Parameters

wymm tyh

Wavelength

1.3 mm 1.55 mm

b~TE00!

b~TE01!

b~TM00!

b~TM01!

b~TE00!

b~TM00!

5 11 0.0116 0.02865 20 0.0274 0.04548 5 0.00318 11 0.0794 0.0999 0.0117 0.02848 20 0.0970 0.1175 0.0263 0.044110 5 0.0087 0.023610 11 0.1195 0.1404 0.0094 0.0456 0.064810 20 0.1462 0.0138 0.1672 0.0357 0.0705 0.0904

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maximum index increase. The mode field sizes arerather large in comparison with those in single-modefibers. We calculated the coupling loss as a modeoverlap integral with a Gaussian mode having a 1yefield diameter of 10 mm: for the TE mode this losswas 0.78 dB, for the TMmode it was 0.65 dB. Thesevalues are inconveniently high, particularly sincethere are also other contributions to coupling loss inpractice. Also, it is better for one to compromise themode size fit by using waveguides with smaller modesizes than those used in optical fibers, because thensmaller bending radii may be used in waveguide de-vices. The calculated mode sizes would make nec-essary large bending radii, leading to impracticalwaveguide device lengths.At l 5 1.3 mm, this waveguide was not single mode,

so from Table 1 we chose a waveguide with w 5 10mmand t5 11 h. Actually the results in Table 1 alsoshow this waveguide to have a TM01 mode, so it is notstrictly single mode, but this TM01 mode has lowconfinement and is quite close to cutoff. So thewaveguide is a good representation of the highestfundamental mode confinement achievable at thiswavelength for single-mode operation. The modefield distributions are shown in Fig. 2. The modeoverlap integral with a Gaussian mode having a 1yefield diameter of 8 mm was 0.78 dB for the TE modeand 0.67 dB for the TMmode. The situation is prac-tically the same as that for l 5 1.55 mm.It is possible that deeper waveguides with better

mode confinement could be achieved with widermaskopening widths, although Ref. 1 gives no data beyondw 5 10 mm. However, this is already a rather largemask opening size for a single-mode waveguide, so it

Fig. 1. Calculated guided-mode field distributions for TE and TMpolarizations at l 5 1.55 mm and withw 5 10 mm, t 5 20 h. Fieldcontours drawn at values 0.1, 0.2, 0.3, . . . , 0.9 of maximum.

is quite doubtful that small mode sizes with single-mode operation could be achieved at large values forw.We have also carried out several experiments to

fabricate potassium-exchanged channel waveguidesinto BK7 glass using a variety of temperatures andlengths of time. We have managed to achievesingle-mode waveguides only at l 5 1.3 mm, whichhave large mode field distributions and fiber couplinglosses around 1 dB or higher. Although a detailedcharacterization similar to that in Ref. 1 is not pos-sible at these longer wavelengths, this result con-firms our findings.In the literature there are reports on potassium-

exchanged channel waveguides in BK7 at l 5 1.3 mm.Directional couplers for polarization splitting werereported in Ref. 3 to have refractive-index distribu-

Fig. 2. Calculated guided-mode field distributions for TE and TMpolarizations at l 5 1.3 mm and with w 5 10 mm, t 5 11 h. Fieldcontours drawn at values 0.1, 0.2, 0.3, . . . , 0.9 of maximum.

tions similar to those in Ref. 1 but with a Dn approx-imately 10% lower at both polarizations, which wouldgive even weaker confinement of modes than in ouranalysis. No fiber-to-waveguide coupling losseswere given. In Ref. 4 saturation of diffusion depthwas found in both BK7 and soda-lime silicate glass.Miliou et al.4 also reported a surface channelwaveguide at l 5 1.3 mm with a fiber-to-waveguideloss of 0.34 dB, but they did not provide details of thefabrication process parameters. This seems to showthat it is feasible to use these waveguides, but beforefurther confirmation of a practical fabrication pro-cess, the effect of waveguide depth saturation shouldbe carefully considered in an attempt to realizepotassium-exchanged channel waveguides in BK7,for which, as far as we know, there are no reports forBK7 at l 5 1.55 mm.To conclude, we have shown that the refractive-

index profile characterization of potassium-exchanged channel waveguides in BK7 glass carriedout in Ref. 1 and particularly the observed saturationof waveguide depths casts serious doubts on the fea-sibility of using such waveguides in devices compat-ible with optical fibers at telecommunicationwavelengths. Although this is not expected to be thefinal word on the subject, there is clear evidence thatthe implications cannot be ignored when one tries touse this fabrication technology, particularly for a tele-communication wavelength window around 1.55 mm,which is becoming more important with the use oferbium-doped fiber amplifiers.

References1. N. Weiss and R. Srivastava, “Determination of ion-exchanged

channel waveguide profile parameters by mode-index measure-ments,” Appl. Opt. 34, 455–458 ~1995!.

2. A. Tervonen, “Theoretical analysis of ion-exchanged glasswaveguides,” in Introduction to Glass Integrated Optics, S. I.Najafi, ed. ~Artech House, Boston, Mass., 1992!, pp. 73–105.

3. A. N. Miliou, R. Srivastava, and R. V. Ramaswamy, “A 1.3-mmdirectional coupler polarization splitter by ion exchange,” J.Lightwave Technol. 11, 220–225 ~1993!.

4. A. Miliou, H. Zhenguang, H. C. Cheng, R. Srivastava, and R. V.Ramaswamy, “Fiber-compatible K1–Na1 ion-exchanged chan-nel waveguides: fabrication and characterization,” IEEE J.Quantum Electron. 25, 1889–1897 ~1989!.

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