1995 IEEE A novel vertical Δκ directional coupler switch using liquid crystals

8/8/2019 1995 IEEE A novel vertical directional coupler switch using liquid crystals

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49JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO . I, JANUARY 1995

A Novel Vertical A K Directional CouplerSwitch Using Liquid Crystals

Wei Yu Lee, Jin Shin Lin, and Sung Yuen Wang

Abstract-To decrease the switching length of the directionalcoupler switch and to use the advantages of the electro-opticproperties of (LCs), we propose a novel vertical An directionalcoupler switch using LCs and present the calculated resultsrelevant to the design consideration s. Because of the large bire-fringence of LCs, a very short switching length less than 60 pmis possible. Besides, a N x N switch is also proposed for practicalapplications.

I. INTRODUCTIONHE FUNCTIONAL integrated optical device has at-T racted considerable attention from practical points of

view such as data processing and communication. One ofthe fundamentally necessary components is an electro-opticdirectional coupler, which is able to selectively switch thewaveguide path of the light. In conventional directional cou-plers, power switching is achieved by introducing a phasemismatch (AD) between the optical fields in two waveguides[l]. Most ones are made of inorganic materials such asGaAs and LiNbO3. Utilization of the electro-optic effectin these switches poses difficulties because of the smallelectro-optic coefficient in these materials and high-powerconsumption, along with rather long interaction lengths, allof these conditions are undesirable in a practical system [2].Either of these constraints results in high switching energy.Instead, the switching operation may be achieved by chang-ing the coupling strength between the coupled waveguides.Recently, a An vertical directional couplers based on themultiple quantum well (MQW) have been demonstrated insemiconductor structures [3]-[4]. Since the separation betweenthe waveguides can be much better controlled in the verticaldirection, the coupling coefficient will be designed moreaccurately. In particular, a much larger n can be obtained, witha corresponding reduction in the coupling lengths from severalmm to 150 pm [5]. However, MQW devices must be fab-ricated by molecular-beam epitaxy or metalorganic chemicalvapor deposition (MOCVD) [4]. Therefore, a long-time periodof fabrication and expensive processes are required. Liquidcrystals (LCs) are highly birefringent and thus coupling effectis much stronger than that of inorganic materials. So, somedevices using electro-optic switching in polymer waveguideswith LCs clad has been proposed recently [6]-[8]. However,their switching length are several mm and the long-term

Manuscript received April 4, 1994; revised Se ptember 13 , 1994.Th e authors are with the Department of Electrical Engineering, TatungInstitute of Technology, Taipei, Taiwan 10451, Republic of China.IEEE Log Number 9407209.

reliability is still in question since all the polymer waveguidesof these device are partially exposed in the air [4].In this paper, a novel vertical AIE irectional coupler switchusing liquid crystal is proposed. The F fact based on thebeam propagation method, which is proved to be available fordifferent distribution of refractive index in the switch, is usedto estimate the coupling length and some design considerationssuch as switching length, switching time, integration have beencalculated and discussed. It is important that a very shortswitching length about 60 pm or less can be achieved by thelarge birefringence of LCs in coupling region. Because of theadvantages of pixel electrodes, small size, easy integration, andsimple fabrication of the novel switches, it is expected to beeasily produced by the highly developed fabrication techniquesof liquid crystal displays (LCDs) and to be integrated on asmall chip to work as a N x N switch for applications in amulti-channel syetem.

11. PROPOSEDIRECTIONAL OUPLER WITCHGenerally, a directional coupler transfers power from oneguide to another in a coupling length Ld = 7r/2n, where

n is the coupling coefficient. Being unbiased, the bar stateoperation is established and the device length ( L d ) is(1)

As some biased voltage is applied, the coupling coefficient(n) ncreases to IE and the crossover state is operated. So, Ldmust also satisfy that

Combining the criteria for bar and crossover states, a minimumdevice length relating to the change of coupling coefficientcan be derived asmLd =A2( fd - E ) (3)

From the above expression, it is clear that the minimum devicelength can be achieved by designing a structure that maximizesthe change of coupling coefficient. So the optimization of thedesign focuses on increasing nand decreasing IE , which mustrely on the large refractive index change of LCs.Fig. 1 shows the device configuration we proposed here.It consists of two normal waveguides fabricated on twosubstrates respectively, a pair of electrode deposited on thetwo waveguides, and a thin LCs layer separating the two0733-8724/95$04.000 1995 IEEE

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50 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 1, JANUARY 1995

1 electrodes tz )1Z

substrate In34 L , + ?+ ; I

Fig. 1. The device configuration we proposed is shown here. It consists oftwo normal waveguides fabricated on two substrates respectively, a pair ofelectrode on the two waveguides, and a thin LCs layer separating the twoguides.

guides. Hence, the electric field can be applied entirely to theL2 region in the LCs layer. In our design, L1 and LJ , whichare weakly coupling regions, work as a separator to degradecross-talk and to increase extinction ratio and La , a activeregion, selects the bar state or crossover state.In the case of no voltage applied, n2 in the region becomesno and L Z is also a weak coupling region (bar state); as aexternal voltage is applied, 712 in L z region changes to ne andL2 is switched to be a strong coupling region (crossover state).The larger index of the coupling layer at bias is expectedto yield strong coupling coefficient because of large K . Theelectro-optic effect of LCs is much effective because itsrefractive index change is orders of magnitude larger than thatof the electro-optic solid material [lo]. So a very small 6 anda large n may achieved (6


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LEE et al.: A NOVEL VERTICAL A6 DIRECTIONAL COUPLER SWITCH USING LIQUID CRYSTALS 51

1 n l = l 4 8n3=1J7

0 9 - n 2 = 1 4 8 4, a=lurn s=2urn ,0 7

guide 1:-guide 2 : -

0.3 I0 2 - I' I

Fig. 3.distance z along the coupler region Lz .At crmsover slate, the power exch'ange bctween waveguidc I (solid line) and waveguide 2 (dash line)is shown as a function of propagation

(a ) (b)Fig. 4.another one, where L1 = Lz = LJ = 60 p.m.(a) The bar state: power is confined in waveguide I and no coupling occum; (b) lh e crossover state: power in waveguide 1 is coupled to

t

r = h nBPM:--C M T -11 = 1.48n3 = 1.47;I = luin

10 '- I1.45 1.455 1.46 1.465 1.47 1.475 1.45 1.485 1.49112

Fig. 5.the case of nl = 1.48, z = 1.484,and a = 2 pm .The switching length as function of coupling layer thicknesses (s) for two widely used wavelength (0.6328 pr n and 1.32 pm) s compared in

The coupling layer thicknesses (s ) also enter the figuresof merit by determining the length of the device. Narrowingthe coupling layer thickness can either lower bias voltage or

shorten the device length, as desired. However, it is worthy ofnoting that the influence of the coupling thickness (s) to thecoupling length (L2 )depends on the difference between n1 and



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5 2

130--5 120---2 110-$ 100-to-v,

90

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JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL. 13, NO. I , JANUARY 1995

0.6328 : +n 1= 1.480n2=1.454n3=1.470a= 2um

1.32 : o +0

JI

+ -1. 1I

- .-

TABLE ITHE REFRACTIVENDEX n~ SE E N B Y TE OR TM WAVE IN Lz REGION ND THE STATES OF THE SWITCH FOR DIFFERENT CONDITIONSA > 0 Ae


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LE E et al.: A NOVEL VERTICAL A6 DIRECTIONAL COUPLER SWITCH USING LIQUID CRYSTALS

~

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state) and 56.77 dB (the bar state). The smaller value of ratioat the crossover state is due to the radiation loss increasingwhen the mode in waveguide 1 crosses the coupling layer towaveguide 2 at L2 region.top :0 bottom : ~~~~~~~~~

Fig. 7. Fig. 7 shows the top view of a 4 x 4 switch we proposed.

on a device has been studied to increase the density andachieve high performance of integrated optical devices in thefuture [4]-[6].Fig. 7 shows the top view of a 4 x 4 switch we proposed.It is organized by five 2 x 2 switches consisting of twochannel waveguides (bottom and top). The pattern definingthe waveguides can be symmetrically fabricated on the topand bottom substrate at one time. It is worthy noting that thewaveguide pattern on top or button substrates are the sameso only one mask is required in the fabrication process. Thecoupling layer thickness of the LC directional coupler switchcan be extremely well controlled by spacers, which have beenwidely used in LCDs. Since the fabrication process of LCDshas been highly developed, the concept can be easily extendedto a N x N switching array. Besides, all the waveguides arebond in a cell so the vibration from the environments can bedegraded largely.The F-fact based on BPM As an example, we have simulateda vertical directional coupler switch of dimensions s =2 pm and a = 1 pm, and of index profile 721 = 1.48and 723 = 1.47 at X = 0.6328 pm. Consider the nematicliquid crystals (NLCs, 14627 BDH Ltd., 71, = 1.4874 andno = 1.4531), which is allowed the construction of activeoverlayer devices on common waveguides [16], is chosenas the coupling layer and the propagation loss in the layeris assumed to be -28 dB/cm [17], [18]. Since the NLCspossess a negative anisotropy of susceptibility, the moleculestend to align themselves perpendicular to the electric field.The normal alignment of NLCs to the substrate is assumed.If the pretilted angle is the effective refractive index n, seenby the TE wave can be estimated by [19]. We give onlythe analysis for TE modes because a similar analysis can beadopted for TM mode. The lowest mode corresponding to thewaveguide boundary conditions, is launched into waveguide1. Here 72; = 1.484 and infinitely thin electrodes are assumed.At crossover state, the power exchanges between waveguide1 (solid line) and waveguide 2 (dash line) as a function ofpropagation distance z along the coupler region L 2 ,as shownin Fig. 2. We note that a switching length as short as 60 pmis possible. To our knowledge, it is the shortest switchinglength in all kinds of optical directional couplers [ 2 ] , [4].Furthermore, the power exchanges in the novel directionalcoupler switch with L1 = L2 = L3 = 60 pm is simulated,shown in Fig. 3(a) and Fig. 3(b) respectively. At zero bias,power is confined in waveguide 1 and no coupling occursthrough the whole device (the bar state); while some biasapplied, power in waveguide 1 is coupled to another one(the crossover state). In the example, the extinction ratios ofwaveguides at the output is as high as 44.14 dB (the crossover

D . Comparing ConventionallyThe coupling phenomena between waveguides is analyzedby the couple-mode theory [ l] , [20], which is based onthe perturbation of the refractive index in an electro-optical

waveguide by an applied voltage. Recently, some verticaldirectional couplers, which is based on the change of refractiveindex in coupling layer, has been proposed. A theoreticalanalysis of distributive coupling along dielectric waveguidesis presented and often resolved by finding elementary solu-tions to Maxwells equations specified by the geometry ofthe system. The result is an eigen-equation from which thepropagation constants can be numerically computed as rootsof this equation [2]-[5]. However, the root of eigen-equationfails to be extracted if the index of coupling layer is close toor larger than that of adjacent waveguides. There have beenvery few reports refemng to solve the problem, although thecoupling phenomena still occurs in the structure. The F-factbased on the BPM method is useful to solve the problem.It is that the coupling coefficient and length can be solvedby finding the z position where the local maximum of the Ffact along waveguide 2 appears. Comparing with the modemethod [3], our method is suitable for various refractive indexdistribution of a device, although numerical integration ofF using a step-by-step propagation through a model devicerequires a larger computation. Besides, the power exchangebetween waveguides is observable and scattering or radiationloss are also included for every step in our simulation. It ishelpful for us to realize the device practical performance indetail. Fig. 5illustrates the switching length ( L 2 )as function ofrefractive index of coupling layer (722) calculated for differentguide separation (s = 1 and s = 2 pm) by the two method.The numerical results are very similar. But the mode methodis out of use when 71 2 is near or larger than n1 (continues).

V. CONCLUSIONTo improve the long switching length of conventionaldirectional coupler switch and to use the advantages of theelectro-optic properties of LCs, we propose a novel verticalAK.directional coupler switch using LCs and presented thecalculated results relevant to the design considerations. Aintegration F fact based on the BPM method is used toanalyzed the power coupling between waveguides for var-ious conditions. Some characteristics of the switch such asswitching length, switching time, capacitance, and integration

are discussed in detail and compared with that of otherswitches. Because of the large birefringence of LCs, a veryshort switching length less than 60 pm is possible. To ourknowledge, it is the shortest switching length in all kindsof optical directional couplers. Besides, the switching regiondefined by the electrode pair of the switch is designed asthe pixel of a LCD panel, which is easy to be produced bythe highly developed techniques in LCDs. For the present



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level of technical competence in integrated optics and LCDs,it is possible that a large number of these elements can befabricated together to constitute a N x N high density switchfor practical application.

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Cryst. , vol. 202, pp. 163-169, 1991.[8] R. S . Moshrefzadeh, M. D. Radciliffe, T. C. Lee, and S . K. Moha-patra, Temperature dependence of index of refraction of polymericwaveguides, J. Lightwuve Technol., vol. 10, pp. 420-425, 1992.[9] M. Ozaki, Y. Sadohara, T. Hatai, and K. Yoshino, Fast optical switchingin polymer waveguide using ferroelectric liquid crystal, Jup. J. Appl.[ I O ] S . Y. Wang, W. Y. Lee, and J. S. Lin, A vectorial Galerkin methodbased on H fields, to be published in J. Opt. Commun.[l 11 M. M. Howerton, C. H. Bulmer, and W. K. Bums, Effect of intrinsicphase mismatch on linear modulator performance of the 1x 2 directionalcoupler and machzehnder interferometer, J. Lightwave Technol., vol. 8,pp. 1177-1 185, 1990.[I21 H. J. Lee and S. Y. Shin, Fabrication-tolerant An: irectoonal couplerswitch using annealing in proton exchange, Opt. Commun., vol. 102,[I31 L. ThylEn, The beam propa gation method: An an alysis of its applica-

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[141 M. D. Feit and J. A. Fleck, Mode properties and dispersion for twooptical fiber-index profiles by the propagating beam method. Appl. Opt. ,vol. 19, pp. 3140-3150, 1980.[15] A. Neyer, W. Mevenkamp, L. Thylen, and B. Lagerstrom, A mearnpropagation method analysis of active and passive waveguide cross-ings, J. Lightwuve Technol., vol. LT-3, pp. 635-642, 1985.[16] I. Sage and D. Chaplin, Low RI liquid crystals for integrated optics,Electron. Lett., vol. 23, pp. 1192-1193, 1987.[17] T. G. Giallorenzi and J. P. Sheridan, Light scattering from nematicliquid crystal waveguides, J. Appl. Phys. , vol. 46, pp. 1271-1282, 1975.[ l e ] J. R. whinnery, C. Hu , and Y. S . Kwon, Liquid-crystal waveguides forintegrated optics, IEEEJ. Quun. Elec. , vol. QE-13, pp. 262-267, 1977.[19] L. C. Khoo and F. Simoni, Physics of Liquid Crystalline Muteriuls.New York: Gordon and Breach Sc ience Publishers, 199 1, pp. 234-249.[20] K. Yasumoto, Coupled mode formulation of parallel dielectric waveg-uides, Opt. Lett., vol. 18, pp. 503-504, 1993.[21] M. Hikita, Y. Shuto, M . Amano, R. Yeshimura, S . Tomaru, and J.Kozawaguchi, Optical intensity modulation in a vertically stackedcoupler incororation EO polymer, Appl. Phys. Lett., vol. 63, pp.1161-1 162, 1993.[22] R. Kashyap, C. S . Winter, and B. K. Nayar, Polarization-desensitizedliquid-crystal overlay optical-fiber modulator, Opt. Lett., vol. 13, pp.401403 , 1988 .[23] J. P. Sheridan and T. G . Giallorenzi, Electro-optically induced de-flection in liquid-crystal waveguides, J. Appl. Phys., vol. 45, pp.5160-5163, 1974.[24] J. S . Lin, S . Y. Wang, and W. Y. Lee, Capacitance analysis of periodicpixel of LCD using discrete Fourier transform, Electron. Lett., to bepublished.

[25] J. S . Lin and W. Y. Lee, An efficient two-dimensional discrete Fouriertransform analysis of periodic pixels of TFTL CD s, Microwave andOpt. Technol. Lett., to be published.

W. Y. Lee, photograph and biography not available at the time of publication.

J. S.Lin, photograph and biography not available at the time of publication.

S. Y. Wang, photograph and biography n ot available at the time of publication.

1995 IEEE A novel vertical Δκ directional coupler switch using liquid crystals

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