Switching characteristics of GaAs directional coupler optical switches

Switching characteristics of GaAs directional coupleroptical switches

Hiroaki Inoue, Kenji Hiruma, Koji Ishida, Hitoshi Sato, and Hiroyoshi Matsumura

A directional coupler optical switch with good switching characteristics (extinction ratio of more than 22 dB at22.5-V applied voltage) and low-loss guiding properties (total insertion loss of 10.7 dB including Fresnelreflection loss of 4.0 dB) has been developed. The switch element was mounted on a ceramic stem and jointeddirectly with polarization-maintaining optical fibers. High speed operation of 1.6 GHz was achieved.

1. Introduction

Considerable attention has been given to monolithicoptical integrated circuitsl as key optical componentsfor future optical communication and optical process-ing systems. Optical integrated circuits may integratea number of different optical devices such as semicon-ductor lasers, modulators, switches, and photodetec-tors on the same substrate. To achieve this kind ofintegration, optical devices made of semiconductormaterials are required.2,3 Therefore, the developmentof low-loss semiconductor optical waveguides is a keyfactor in the development of optical integrated cir-cuits.4

Several attempts have been made to reduce propa-gation loss in semiconductor optical waveguides. 5 6

Nevertheless, the propagation loss was much higherthan that in nonsemiconductor waveguides such as Ti-diffused LiNbO 3 waveguides. 7

Recently, however, the authors have succeeded inreducing the propagation loss in a GaAs ridge wave-guide to 0.2 dB/cm.8 9 We were then able to proceed tothe development of directional coupler optical switch-es with low-loss GaAs ridge waveguides. The switch-ing operation of the optical switch is based mainly onthe refractive-index change from the electrooptic ef-fect in the depletion layer of the reverse-biasedSchottky contact between electrode and ridge wave-guide. As a result, much higher speed operation than

Hitoshi Sato is with Hitachi, Ltd., Fiber Optics Project Division,Totsuka, Yokohama 244, Japan. The other authors are with Hita-chi, Ltd., Central Research Laboratory, Kokubunji, Tokyo 185, Ja-pan.

Received 8 October 1985.0003-6935/86/091484-07$02.00/0.© 1986 Optical Society of America.

LiNbO 3 devices may be possible because of the lowerdielectric permittivity.10

Recently, extinction ratios2"1 larger than 20 dB andhigh speed operation' 2 faster than 1 GHz have beenreported in GaAs optical switches. However, integra-tion with other optical devices such as optical fibers,semiconductor lasers, and photodetectors has not beenreported. In this paper, the design of a directionalcoupler optical switch with guiding regions suitable forintegration is reported, and its switching and highspeed modulation characteristics are described.

II. Low-Loss Optical Switch Design

The schematic configuration of the directional cou-pler (DC) optical switch developed is shown in Fig. 1.There are two types of region in this device: oneswitching and two guiding. The switching region iscomposed of two identical parallel ridge waveguides;since light transfers from one ridge waveguide to theother, lightwaves should penetrate into the outside of aridge waveguide sufficiently to couple each with theother. In contrast, guiding regions contain S-bendingwaveguides to connect the switching region with otheroptical devices such as optical branches and opticalfibers. Therefore, the lightwave should focus on theridge waveguide. Since these two types play entirelydifferent roles in this device, we consider the propaga-tion characteristics in these regions separately fromnow on.

A. Switching Region Design

As the optical switch consists of two identical paral-lel ridge waveguides, even and odd modes with propa-gation constants fle and po, respectively, can propagatein these wavelengths. The length (L) of the switchingregion is usually designed by L = L so that the inputlight power at'the input end of a ridge waveguide (Pi inFig. 1) couples completely to the output end of the

1484 APPLIED OPTICS / Vol. 25, No. 9 / 1 May 1986

Guiding Switching Guiding -region region region (110) x

C*PO w

135pjm(001) ~ ~ BAuGeNi -ljA -Schottky(001) NAB A u ~e<ohmic electrode

electrod ri- GaAsn-GaAs woveguide layersubstrate

Top view of the optical switch Cross sectional view of AB

Fig. 1. Schematic configuration of the DC optical switch. Nota-tions and crystallographic orientations are also shown.

other ridge waveguide (Po, or P0 2). Here, the completecoupling length L, is defined by

Lc= Ar/IA3I,

A# =3 -10-

Therefore, by applying reverse-biased voltage betw(the electrodes on a ridge waveguide and on the bottsurface of the substrate, a refractive-index changeis induced through the electrooptic effect in the dertion layer of the Schottky contact, which resultsmodulations of 1Be and #%. When the proper appIvoltage is selected, the light couples back to the out]end of the same waveguide (Po, or P0 2).

To make this type of switching operation possilthe structural parameters listed in Fig. 1 (ridge wicw, ridge spacing s, ridge step height h, waveguide la,thickness a, and length of switching region L) havebe properly adjusted. To have low-loss GaAs ricwaveguides, w and a were set at 7 and 4.5 ,um, resptively. These values make it possible to hold the pr(agation loss to 1.0 dB/cm and the coupling loss wsingle-mode optical fiber to 1.5 dB.9

Theoretical calculations were carried out to clarthe dependence of the complete coupling length L oand h by using the conventional effective indexrefraction method.13"14 According to the method,genvalue equations for propagating TE-like modesthe switching region are expressed by

02 = 0k2- k ( = or fl,),

kxw = qr + arctan[(n/neq)2(k - kq - k)1/2/kx

+ arctan [(ni/neq)2(k2 - k2q k2)/2 H/kxJ,

H =coth[s/2(k - kq - k)1 /2] (even mode)

ltanh[s/2(k - kq - k)1/2] (odd mode),

kya = p-r + arctan[(k - k2- k)/ 2 /ky]

+ arctan[(k 2 - k2 -k2)1/kY1,

(1)

_y 10 1 Ridge step height:5 h=3.0 jim

at1-

C*! h =2.5jim

=10 h 2.0 jim

a.

E h= 1.050

0 1.0 20 30 4.0 5.0Ridg. spacing s (jim)

Fig. 2. Complete coupling length L, as a function of ridge spacing sfor various values of ridge step heights h.

modes in the slab waveguides13 with waveguide thick-nesses of a and a-h, respectively; and p and q arepositive integers denoting the mode number.

By solving the eigenvalue Eqs. (3)-(8) for /3e and fOnumerically and substituting the results into Eq. (2),the complete coupling length L, can be obtained. Fig-ure 2 shows L, as a function of s for various values of h.It can be seen from this figure that L, becomes largerwith increasing h and s and follows the exponential lawfor s.15

Next, the relationship between the extinction ratioand structural parameters, such as s and h, must bedetermined. The extinction ratio is defined as theratio of the light power (Po, and P 2) from the twooutput ends of the waveguides, i.e.,

extinction ratio = 10 log(PO/PO2)1 (9)

Here, Po, and P0 2 are given by using the normalizedelectric fields of the even and odd modes: E and E0.The results are as follows:

Po = | I AeEe exp[(i03L)]

el- + A0 E exp [(ifOlL)]I 2dxdy,in

P02 = - P01,

(3) J Eel'2 dxdy = I E.12dxdy = 1,-C -X Eo -G

(10)

(11)

(12)

where Pi is the input light power and Ae and A, are(4) constants which are normalized by

Ae'2+lAol 2=Pi. (13)

The values of A and A can be determined by theinitial boundary condition at the input end of theswitching region, i.e.,

(6)

ki = (27r/X)ni (i = 1,2,3,eq), (7)

n2 2 2+ n2 8flq = n - nleff 2eff (8)

where X is the wavelength in the vacuum; n, n2, and n 3are refractive indices of the GaAs waveguide layer, theGaAs substrate, and the air, respectively; neff and n2effare effective refractive indices of the fundamental TE

J J IAeEe + AEo2 dxdy - min. (14)

Next, the electric field distributions Ee and E0 weredetermined with the aid of eigenvalue Eqs. (3)-(8) andwere used to calculate the overlap integral of Eq. (10)for the extinction ratio.

The results are shown in Fig. 3, from which it can beseen that the extinction ratio becomes larger with in-

1 May 1986 / Vol. 25, No. 9 / APPLIED OPTICS 1485

mz00.2a

C0t;UCKw

0 1.0 2.0 3.0 4.0Ridge spacing s (m)

1 30

. 20a

0r_ 2 0CKw

5.0 Fig. 4. Extinction ratio vs complete coupling length L, for variousvalues of ridge step heights h as a parameter of ridge spacing s.

Fig. 3. Extinction ratio as a function of ridge spacing s for variousvalues of ridge step heights h.

creasing h and s. This tendency is the same for thecomplete coupling length. Judging from these results,it can be assumed that the extinction ratio changeslinearly with the complete coupling length. The cal-culated results are shown in Fig. 4. It can be seen thatthe extinction ratio decreases as the complete couplinglength decreases. These results can be understoodqualitatively if we take the field matching betweeneven and odd modes into consideration. Figure 5(a)shows electric field distributions of fundamental evenand odd modes for s = 1,3,5 Am. The sum of thesedistributions, i.e., the output field, is illustrated in Fig.5(b). It is clear that for large ridge spacing s, the evenand odd modes have similar field distributions. As aresult, the output electric field is confined in one wave-guide, resulting in a good extinction ratio. However,as the ridge spacing s decreases, the mode fields do notmatch well and therefore power is distributed over thetwo waveguides.

From these calculations, we can fix desirable struc-tural parameters at this stage from the manufacturingprocess point of view. In practice, holding the wave-guide spacing s within 1 or 2 Am and fabricating a large-scale optical switch are both very difficult. Therefore,in our experiments s = 3 ,um, LC = 9 mm, h = 2.2 Amwere chosen, and an extinction ratio larger than 20 dBcan be expected.

B. Guiding Region Design

In optical integrated circuits it is essential to havecurved optical waveguides. Guiding regions in thisDC optical switch, which consist mainly of S-bendingwaveguides, serve to connect the switching region andother optical devices such as optical fibers. In ourexperiments, the lateral displacement of the twostraight waveguides comprising the S-bending wave-guide was set at 135 Am to make it easy to adjust theoptical fiber positions.

s=1pm

s=3pm

X, EI-- I I_ 1 5y

V w s ' =,~

(a) (b)

Fig. 5. Electric field distributions in the switching region for vari-ous values of ridge spacings s: (a) fundamental even and odd modesfor s = 1,3,5 gim; (b) sum of fundamental even and odd modes for s =

1,3,5 Mm.

The bending loss caused by the curved waveguideincreases with a decrease in its curvature radius (R)and with the weakness of the mode confinement intothe waveguide. As mentioned in Ref. 9, the modeconfinement becomes stronger with an increase in theridge step height h. However, the ridge step heightmust satisfy the condition for the quasi-single modeoperation mentioned in Ref. 9. When w = 7.0 Am, h =2 .7 gm was chosen as one of the optimum conditions forthe mode confinement, and the length of the guidingregion was chosen to be 2 mm because the S-bendingwaveguide loss was kept below 0.5 dB for more than R= 10 mm.9 This latter figure corresponds to a curva-ture radius of 17 mm.


5 10 50 100Complete coupling length Lc(mm)

Y�I

ARCqL �- I I' I I l

C. Discussion

So far, we have examined the optimum conditionsfor both the switching region and the two guidingregions in the DC optical switch. The results showedthat different ridge step heights h should be chosen inthese regions to maintain good switching characteris-tics and low-loss guiding properties at the same time.However, the abruptness of the boundary betweenthese regions causes coupling loss. To hold this lossdown as much as possible, smooth step structures werefabricated by both dry and wet etching techniques.First, the dry etching technique using Ar+ ion beams atAr pressure of 8.0 X 10-5 Torr and ion current densityof 0.4 mA/cm2 was applied to etch the ridge waveguidesfinely until the optimum switching region conditionwas satisfied. The wet etching technique using H2-P0 4:H202:C2H4(OH)2 = 1:1:3 solution was then ap-plied, not only to further etch the ridge waveguides in theguiding regions, but also to make the step structuresat the boundaries smoother to prevent additional loss.

Using these fabrication processes, the coupling lossbetween the switching and guiding regions was re-duced to <1.5 dB. Therefore, the total insertion lossof the DC optical switch, which included the Fresnelreflection loss at both ends, was held to 10.7 dB. Byway of comparison, the loss value of a conventional DCoptical switch of a = 3.0,um and h = 1.7 Am without thestep structure was 24.2 dB.

Ill. Switching Characteristics

In the previous section, DC optical switches de-signed to achieve low-loss guiding and low insertionloss properties were described. In this section, theswitching characteristics are discussed.

Theoretical analysis was carried out to clarify theexpression of the refractive-index change An, inducedby the applied voltage at the Schottky barrier for theTE-like mode. The assumptions used in our theoryare as follows: (1) the depletion layer arising from theSchottky barrier extends over the ridge waveguide and(2) the applied electric field in the depletion layer isuniform along the direction normal to the junctionplane. In fact, when a GaAs waveguide layer and aGaAs substrate have carrier concentrations of 1014cm-3 and 1018 cm-3, respectively, the above assump-tions can be supported. In this case, n, can be ap-proximated in the crystallographic orientation shownin Fig. 1 by

An, = -n3r 4 lV/2a, (15)

where r4 l is the electrooptic coefficient of GaAs at 1.3Am and V is the applied voltage. The induced effec-tive refractive-index change e of the propagating TEmode in a three-layer slab waveguide is given by

1 2an-2n r41 adni (16)

Using the eigenvalue equations of the layered slabwaveguide, (ne 2 )/(ani2 ) can be expressed briefly bythe fraction of the mode's power in the waveguidelayer, i.e.,

1.0 20 so 40 soWaveguide layer thickness (pm)

Fig. 6. Relationship between the switching voltage for the com-plete switching and the waveguide layer thickness for w = 7 ,um and s= 3 ,um. Experimental results are also shown by the open circles.

On2 la(-n= f EO ' (17)

where Ex is the electric field in the x direction of the TEmode normalized by

JE IE2dy = 1. (18)

Substituting Eq. (17) into Eq. (16), we finally find that

n =-2 1 r41V 1 I E.12dy. (19)

Based on this result, the switching voltage Vs of the DCoptical switch can be estimated. As mentioned in theprevious section, the two ridge waveguides in theswitching region are separated from each other toachieve a high extinction ratio. Consequently, thecoupled mode theory'6 can be used to ascertain thecomplete switching condition. This condition is givenby

I 6ne(Vs) A L = @T-X

(20)

From Eqs. (19) and (20), the switching voltage V canbe calculated as

(21)

The switching voltage V. was calculated from Eq. (21)as a function of the waveguide layer thickness a. It canbe clearly seen from this figure that V, becomes mini-mum (14.2 V) at a = 2.0 Am. This tendency can beunderstood as follows: As the waveguide layer is madethinner, the applied electric field in the depletion layerbecomes stronger, but the electric field confinement inthe propagating TE mode becomes weaker. As a re-sult the compensation of these two opposite effectsresults in a minimum value V at a certain value of a.

It is also shown in Fig. 6 that the switching voltage V,when a = 4.5 ,4m, which is the value of greatest interestto us, is 1.68 times larger than the minimum value.


�_3nX a -1Vs = 4 (' f JE ,12 dy) .n1r41L a "

7jpmIS

Ia5

5 10 15Applied voltage (V)

Fig. 7. SEM of cleaved cross section in thesample 1 in Table I.

switching region of

Fig. 8. Measured output powers Po, and P02 as a function of theapplied voltage. Extinction ratios of 24.0 dB for the crossover state

(0 V) and 22.0 dB for the straight-through state (22.5 V) are ob-tained.

With these results in mind, switching samples werefabricated. An n--GaAs waveguide layer with carrierconcentration of <1014 cm- 3 was grown on an n+-GaAssubstrate by the metal-organic chemical vapor deposi-tion (MOCVD) method. 8 After Al was evaporated asSchottky electrodes, ridge waveguides were formed byetching the waveguide layer in the manner mentionedin the previous section. Two samples, the structuralparameters of which are shown in Table I, were pre-pared. Figure 7 shows a SEM photograph of thecleaved cross section in the switching region of sample2 in Table I.

For the measurements, a linearly polarized 1.3-Mmsemiconductor laser beam was focused into the clevedface of one of the ridge waveguides by a 20X micro-scope objective to excite the fundamental TE-likemode. Reverse-biased voltage was applied betweenan electrode on a ridge waveguide and that on thebottom of the substrate. The output powers (Po, andP0 2) were then monitored as a function of the appliedvoltage. The results for sample 1 in Table I are tracedin Fig. 8. As can be seen from the figure, a completepower exchange occurs at an applied voltage level of22.5 V, and the extinction ratios of 24.0 and 22.0 dBwere obtained for the crossover state (0 V) and for thestraight-through state (22.5 V), respectively. Thenear-field intensity distributions of the outputs forthese two states were measured by an IR video systemand are shown in Fig. 9. It is clearly seen that thefundamental mode propagates and almost perfectswitching is performed at 22.5 V. By way of theoreti-cal comparison, the switching voltage V, values for thecomplete power exchange obtained from several sam-ples are plotted in Fig. 6 by the open circles. Theexperimental results are in good agreement with theo-retical predictions. In Fig. 8 it can also be seen that

Table 1. Structures Used In Experiments; Sample I has a Step Structure,Sample 2 has No Step Structure.

Cc

Pol Po2 Pol Po20V 22.5V

Fig. 9. Near-field intensity distributions of outputs for the cross-over and straight-through states monitored by an IR video system.

the extinction ratio obtained in unbiased conditionswas higher than that obtained in biased conditions.These results are well understood from the interfer-ence effect between the fundamental even and oddmodes propagating in the directional coupler. By ap-plying the reverse-biased voltage to the depletion layerin a ridge waveguide, the refractive index in the ridgewaveguide changes. This produces asymmetry in therefractive-index distribution across ridge waveguides,with the result that the electric field distributions foreven and odd modes lose their field symmetry as shownin Fig. 10. In the figure, electric field distributions inthe x direction for 22.5 V of applied voltage are illus-trated. By comparing Figs. 5 and 10, it becomes clearthat the interference of these two asymmetric electricfields cannot provide any perfect compensation, andthe extinction ratio deteriorates.

IV. High Speed Operation

To achieve high speed operation in the DC opticalswitches, the switch element was mounted on a ceram-ic stem with strip lines specially designed for highspeed operation, as shown in Fig. 11. The opticalswitch element was connected directly to polarization-maintaining optical fibers.'7 The mode spot size ofoptical fibers was 4.5 ,um, and a 1.3-,um semiconductorlaser beam with polarization parallel to the plane of theoptical switch was injected into the optical switch.

Here, switching characteristics for small-signal andhigh-frequency modulations as well as for step-pulsefeeding are discussed for the above-mentioned opticalswitch module (OSM). The experimental setup usedis shown in Fig. 12. Frequency characteristics weremeasured under 10-V constant bias voltage and the


0

E

W W

r Tr I

A,0~~~'-soljpm

~~~~~~I

[-1 pv~~/'-

sz5jimFig. 10. Electric field distributions of the even and odd modes inthe switching region for s = 1,3,5 gm, when a reverse-biased voltage

of 22.5 V is applied to the depletion layer in a ridge waveguide.

Fig. 11. Optical switch mounted on a ceramic stem with strip lines.

results are plotted as a function of modulation fre-quency for the OSM in Fig. 13. The theoretical valuescomputed from an equivalent circuit with some dis-tributed constants are also shown (the solid curve inFig. 13). Experimental results agree with theoreticalpredictions. It can be seen from Fig. 13 that thisswitch module has 1.6 GHz at 3-dB bandwidth. Thefrequency response in the lower frequency range (500

Fig. 12. Experimental setup. The 1.3-,um LD module is connectedto the optical switch module via an optical connector for polariza-tion-maintaining optical fibers. Output powers Po, and P02 aredetected by a Ge photodetector and displayed by a sampling oscillo-

scope after amplification.

-0

0a.mxx 5C s

-0 0.5 1.0 1.5Frequency (GHz)

20

Fig. 13. Frequency responses for small-signal modulation of theoptical switch module are plotted by the open circles. Theoreticalvalues computed from an equivalent circuit with some distributed

constants are also shown by the solid curve.

MHz) is slightly lower than the theoretical prediction.This may be due to the impedance mismatch fromparasitics such as bonding wire inductance and bond-ing pad capacitance.

The step-pulse response characteristics of the OSMwere studied in a similar experimental setup. Electri-cal step-pulse signals with 4-ns pulse width, 1-ns risetime, and 5-V amplitude were fed into the OSM undera 7.5-V constant bias voltage. The modulating step-pulse electrical signal and the modulated optical sig-nals are shown in Figs. 14(a) and (b), respectively. Itcan be clearly seen that the modulated optical signalsfrom the output ends Po, and P02 of the optical switchhad almost the same pulse shapes as the modulatingstep-pulse shape. Measured delays in the rise time ofthe modulated optical signal were <0.5 ns, which in-clude the time delays of the Ge photodiode and otherelectrical circuits used in the experiments.

V. Conclusions

GaAs directional coupler optical switches with low-loss ridge waveguides have been studied. These opti-cal switches are suitable for integration with otheroptical devices. Good agreement between theoreticaland experimental results has been obtained.

By changing the ridge step heights in the switchingand the guiding regions and by introducing smoothstructures at their boundaries, good switching charac-teristics and low-loss guiding properties have beenobtained. The total insertion loss was 10.7 dB includ-


00 0 0

-3 dB

I I I

5

-10- * *

12.5 V

Fig. 14. Step-pulse response characteristics of the optical switchmodule: (a) Modulating electrical step-pulse signal with 4-ns pulsewidth, 1-ns rise time, and 5-V amplitude under a 7.5-V constant biasvoltage. (b) Modulated optical output signals of Po, and P0 2. The

|o.l V delays in rise time are <0.5 ns, including time delays of the Gephotodiode and other electrical circuits used in the experiments.

I1o.iv

ing Fresnel reflection loss (4.0 dB) at the endfaces ofthe switch. Extinction ratios were more than 22 dB,both for the crossover (applied voltage V = 0 V) andstraight-through (V = 22.5 V) states.

These optical switches were mounted on ceramicstems with strip lines and jointed directly with polar-ization-maintaining optical fibers. The small-signalhigh-frequency response was 1.6 GHz at 3-dB band-width. Measured delays in the rise time of modulatedoptical signals were <0.5 ns.

The authors are grateful to K. Tada of the Universi-ty of Tokyo for valuable discussions. They wish tothank M. Nakamura of Hitachi, Ltd., Central Re-search Laboratory, H. Kodera of Hitachi, Ltd., FiberOptics Project Division, and S. Shimohori of HitachiCable, Ltd. for their helpful suggestions and encour-agement. Thanks are also due to T. Asai, H. Iizuka, A.Hongo, and T. Kadoi of Hitachi Cable, Ltd. for theiruseful discussions and assistance in the experiments.

H. Inoue is on leave from Hitachi, Ltd., Fiber OpticsProject Division, Totsuka, Yokohama 244, Japan.

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4. V. Evtuhov and A. Yariv, "GaAs and GaAlAs Devices for Inte-grated Optics," IEEE Trans. Microwave Theory Tech. MTT-23,44 (1975).

5. F. K. Reinhart and R. A. Logan, "Transmission Properties ofRib Waveguides Formed by Anodization of Epitaxial GaAs onAl 8Gal- 8As Layers," Appl. Phys. Lett. 24, 270 (1974).

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14. K. Tada, H. Yanagawa, and K. Hirose, "Design Theory of theCoupled-Waveguide Optical Modulator with pn Junction,"Trans. IECE Jpn. E01, 1 (1978).

15. R. C. Alferness, R. V. Schmidt, and E. H. Turner, "Characteris-tics of Ti-Diffused Lithium Niobate Optical Directional Cou-plers," Appl. Opt. 18,4012 (1979).

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2 ns

(a)2nsH

P.G.output

P01output

Po2output

(b)

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Switching characteristics of GaAs directional coupler optical switches