Optical power transfer in a planar waveguide directional coupler is dependent on the geometry and refractive indices of the two waveguides. A method for fabricating optical directional couplers using ion-exchange Ag+/K+/Ca+ processes are presented, where the time of ion-exchange process is used to control the ratio of output power at the two output ports at a fixed temperature of 300 C. As far as the device geometry is still in single-mode regime, this method eliminates the need for high-resolution lithography for producing exact geometry of the device

Kok, Swee Leong

Shaari, Sahbudin

English

Universiti Teknikal Malaysia Melaka (UTeM) Repository

ICSE2004 Proc. 2004, Kuala Lumpur, MalaysiaOptical Waveguide Coupler Fabrication Based On TimeVariation Ion-Exchange TechniqueKok Swee Leong' and Sahbudin Shaari2 Member, IEEE, Member, SPIEiFaculty of Electronics and Computer Engineering,Kolej Universiti Teknikal Kebangsaan Malaysia,75450 Ayer Keroh, Melaka, MALAYSIA2Photonics Technology Laboratory,Institute of Micro-Engineering and NanoelectronicsUniversiti Kebangsaan Malaysia,43600 UKM Bangi, Selangor, MALAYSIAAbstract - Optical power transfer in a planarwaveguide directional coupler is dependent onthe geometry and refractive indices of the twowaveguides. A method for fabricating opticaldirectional couplers using ion-exchangeAg+/K+/Ca+ processes is presented, where thetime of ion-exchange process is used to controlthe ratio of output power at the two outputports at a fixed temperature of 3000C. As far asthe device geometry is still in single-moderegime, this method eliminates the need forhigh-resolution lithography for producingexact geometry of the device.1. INTRODUCTIONDirectional couplers are used to divide the guidedlight at almost any desired ratio. The main drawback of this device which is based on the fieldcoupling between closed waveguides is their highsensitivity to the fabrication process and devicegeometry, especially the spacing of thewaveguides in the coupling region [1]. Due to thisfact access to high resolution lithography isinevitable for successful fabrication of thesedevices based on conventional designs.Optical planar and channel waveguidesfabrication using ion-exchange process is verycommon due to low cost and simple technique.The process involves the exchange of sodium ionsoriginally in the glass with other alkali ions oflarger polarizability.In this study, 5 samples of buriedwaveguides were fabricated using thermal ionexchange process, immersed in moltenmixture of AgNO3, KNO3 and NaNO3, atfixed temperature of 3000C. As the ionexchange time increases, the embeddedgraded index waveguide formed by thediffusion of silver ions is getting deeper andwider. When the two waveguides in thecoupling region become closer, the couplingbecomes more easy and faster, and this allowsthe power ratio at output ports to vary. As thetime is kept longer, larger size of the channelwaveguides are obtained and effectivelyreducing the spacing. This effectively changesthe coupling coefficients between the twowaveguides and eventually their splittingratio.Therefore this method offers a simple way ofgetting directional couplers with different outputor coupling ratio by using only a single set oflithography photomask.II. DIRECTIONAL COUPLER DESIGNA directional coupler is usually formed bytwo identical parallel dielectric waveguidesseparated from each other by a gap on the order ofa wavelength. Consider when two waveguides arefar enough from each other, two normal modespropagate independently on each waveguide withpropagation constants Pa and Pb , and the fielddistributions are ya and Yb respectively. When theseparation of the waveguides is reduced, the twowaveguide fields interact with each other and thisallows the exchange of energy. Coupled modetheory can be used as a method to analyze thisbehavior [2]. When the separation decreases thecoupling between the two waveguides becomeeasier and the energy periodically exchangesalong the coupling region. At a certain length L,the maximum transfer of power from onewaveguide to another will take place. For a simple0-7803-8658-2/04/$20.00(c)2004 IEEE 279ICSE2004 Proc. 2004, Kuala Lumpur, Malaysiastructure of two identical coupled waveguides, Lis given by:where,L = g/2fi#/A = VK +A2 ;Af=a A; and2K = CJY*VYbA6dxCdyThe schematic of the designed couplers are shownin Fig. 1 Each of them consists of two identicalwaveguides with width 5 ,um. The designdifference is on its separation parameter; one with3 pm and the other is 5 jim.III. ION-EXCHANGEUnder certain conditions, it is possible toreplace some of the network modifier ions of glassby others with the same valence and chemicalproperties. In the so called exchanged region,thus it modifies the glass property. The new ionshave different polarizabilities, sizes, andmobilities and thus increasing the refractive indexin selected areas of glass substrate.By exposing the glass surface to the meltnitrates solution, the Ag3+ ion is diffused into theglass and exchanges with Na+ ions. This is calledthermal ion exchange process, where theintroduction of the ions is diffusion driven. In ourwork a mixture ofNaNO3, KNO3, and AgNO3 at aratio of 50:50:1 mole % are used. The meltingpoint is at 2200C and the exchange process is at2500C, the maximum obtainable refractive indexcontrast is 0.078 (at the surface) [3,4,5].V. EXPERIMENTAL ANDSIMULATION RESULTSThe ion exchange process was done onCorning glass with refractive index of 1.515, andthe surface index difference of 0.078 wasobtained. Figure 2 shows the waveguide depthincreases with time. It should be noted that whilethe ion diffuses perpendicular to the maskopening, it also diffuses laterally.Five samples are fabricated based on the twodifferent designs. Sample 1 and 2 have thewaveguide separation of 3 jim. Exchange timesfor these samples are I minute and 5 minutesrespectively. Whereas samples 3, 4, and 5 havethe separation of 5 gm, with the exchange time of1 minute, 5 minutes, and 8 minutes respectively.The width all waveguides is fixed at 5 ,um, and theinteraction or coupling length is 2000 gm.25mmFig. 1 Schematic diagram of two directional couplersdesign.4SW 3,2-1 ~ 3 5 7Exchange Time (Minutes)Fig.2 Waveguide depth increases as ion exchangetime increases.Table I summaries the specification of thetwo directional couplers. The output ratio of bothchannels for sample 1, 2, 3, 4, and 5 are shown inFig. 3a, 3b, 3c, 3d, and 3e respectively.Table 1. Specification ofDC in FiguLre IFigure 3(a(c(d(eSample 1 2 3 4 5Time (min) _1 5 1 5 8Guide spacing (pm) 3 3 5 5 5Guide width (pLm) 5 _ 5__ 5 5Sample 1 2 3 4 5m (nm-1) 6E-4 1E-4 5E-4 1OE-4 6E-4The power transfer between the two armschanges with the wavelength and is measured0-7803-8658-2/04/$20.00(c)2004 IEEE 280ICSE2004 Proc. 2004, Kuala Lumpur, Malaysiaby the slope m of the graph for each sample. Itis difficult to discuss and draw therelationship of the slope obtained related withthe device design data. As the ion exchangetime is made longer, the spacing in thecoupling region becomes smaller and at thesame time the depth of the embedded guide isdeeper. The variation of the width and thedepth of the waveguides will changes thecoupling behavior between the two guides.Figures 3(a) to 3(e) show how the powerratios between the two output ports changewith the wavelength for all five samplesrespectively.VI. CONCLUSIONSTime for ion exchange process plays veryimportant role where we have shownexperimentally that the coupling behavior indirectional couplers changes dramatically. Byfabricating many more samples with variousparameters a more clear trend can be observed andmay be a some conclusions can be suggested. Thismay help to determine the exact manufacturingsteps process if ion-exchange is adopted as thefabrication method. However from this work wediscover that a number of planar waveguidedirectional couplers with different coupling ratioscan be made from a single design only, ie. byallowing different time duration during thefabrication.[5] Stewart. Laybourn, "Optical Waveguide from DiluteSilver Nitrate Melts," IEEE J. Quantum Elec., Vol. QE-14, 1978.0.70(U0~0.3 -14800(UcoI-U).:01500 1520 1540 1560 1580Wavelength (nm)(a)0.7 -0.6. I mT Oa -IJ TerminalTerminal 2AA5 Terminal 10-4 I__ V .,w0.30.21480 1500 1520 1540 1560 1580Wavelength (nm)(b)ACKNOWLEDGMENT 1This work is supported by Malaysian Governmentresearch grant IRPA 020202T001 for a top downproject on the development of WDM-FTTHsystem.REFERENCES0.8_0.62) 0.40=0.60.2[1] Hamid Reza Azarinia, "Optical Coupler FabricationBased On Burried Field-Assited Ion-Exchange Method",IEEE, ISIE'99, Bled, Slovenia, 1999.[2] H. Nishihara, M. Haruna, T. Subara, Optical IntegratedCircuits, Newyork: McGraww-Hill, 1993"[31 S.I. Najafi, "Optical Behavior of K+ Exchange GlassWaveguides. Appl. Opt., Vol. 27, 1988.[4] Janet L., Jacket, "Glass Waveguide Made Using LowMelting Point Nitrate Mixtures". Appl. Opt., Vol. 27No. 3, 1988.01480 1500 1520 1540 1560Wavelength (nm)(c)0-7803-8658-2/04/$20.00(c)2004 IEEE--qp- V4a A^ A A* Ilwlw wTerminal I- Terminal.1 58Cn~~~~~~~~~~~~~~~~~~~~~n arm rro-"' um tro-a &V EI-0 tru-M281ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia0.82°X0.6 006- TmTerrninal 1aC 04 _- nTerminal2 °0a- 0.4 n0.2 0.2V . .--- T ---, --1480 1500 1520 1540 1560 1580Wavelength (nm)(d)0.8 13 a aato Terminal 1x lO0n.4 (-) Sample3,5x0nm,(d)Sample4,1Terminal20.2 X1525 1535 1545 1555 1565 1575Wavelength (nm)(e)Fig. 3 Output power ratio dependence on wavelength at both channel of the five fabricated samples correspondingto wavelength of 1480 nlm to 1580 nm. The gradient for the graph in (a) Sample 1, 6.0 x104X nm-', (b) Sample 2, 1x 10-4nm-1, (c) Sample 3, 5 x 104 nm-', (d) Sample 4, 1 x 10-3 nm- 1, (e) Sample 5, 6 x I10-4nm-1.0-7803-8658-2/04/$20.00(c)2004 IEEE 282

Optical waveguide coupler fabrication based on time variation ion-exchange technique

http://eprints.utem.edu.my/id/eprint/4615/1/Optical_waveguide_coupler_fabrication_based_on_time_variation_ion-exchange_technique.pdf

Optical waveguide coupler fabrication based on time variation ion-exchange technique

Abstract

Similar works

Full text

Available Versions

Universiti Teknikal Malaysia Melaka (UTeM) Repository