Electrooptic diffraction modulation in Ti-diffused LiTaO3
G. L. Tangonan, D. L. Persechini, J. F. Lotspeich, and M. K. Barnoski
The design and fabriction of electrooptic Bragg diffraction modulators in Ti-diffused LiTaO3 waveguidesare reported. The modulators developed have demonstrated 98% deflection efficiency for visible and near-iroperation with extinction ratios ofbeams.
1. Introduction
Thin film optical waveguide devices, such as lightdeflectors, scanners, and modulators, are being devel-oped to be used in conjunction with conventional lasersor semiconductor lasers. Low electrical drive powerand high speed light switching are attractive featuresof the thin film approach. In this paper we describe thedesign and fabrication of thin film Bragg diffractionmodulators in Ti-diffused LiTaO3 waveguides. Themodulator performance is adequate for near-term sys-tems applications with a demonstrated diffraction ef-ficiency of 98% at visible and near-ir wavelengths, a highextinction ratio (>250:1), and a design bandwidth of -1GHz. LiTaO 3 was selected as the waveguide materialbecause of the much higher optical damage thresholdof waveguides formed by Ti-diffusion in LiTaO 3 thanin LiNbO 3.1
II. Theory
Beam diffraction, as a mechanism for intensitymodulation by electrooptic means in the thin films, isachieved by producing an electrically controlled phasegrating in the path of the propagating beam. The dif-fraction process results from a periodic perturbation ofthe refractive index transverse to the beam propagationdirection. A useful method for electrooptically gener-ating the desired phase grating is shown in Fig. 1. Themechanism for interaction relies on the fringing electricfields extending below the surface between interdigitalstrip electrodes formed on the crystal surface. The localfringing field strength should be reasonably uniformacross the guided beam and approximately sinusoidalin the plane of the guiding layer, transverse to the beam.
at least 250:1 for both deflected (m = 1) and nondeflected (m = 0)
This may be most readily achieved by applying an iso-lating lower-index layer above the guiding layer. Thisserves the added function of minmizing interaction ofthe optical beam evanescent tail with the lossy metallicsurfaces. Bragg diffraction involves introducing theinput beam at a specific angle OB, the Bragg angle, withrespect to the electrode array.2 3 Diffraction occursreflectively in a single output at twice the input anglewhen the Bragg condition is satisfied.
The phase change , in radians, induced by the elec-trical signal field over a pathlength L is
0 = [2rL)/XO] An, (1)
where An is the refractive-index increment caused bythe electrooptic effect, and X0 is the free-space wave-length. The strongest interaction in LiTaO3 andLiNbO3 occurs when the applied electric field and op-tical electric polarization are both parallel [or nearlyparallel2] to the crystalline c axis (optic axis). For thiscondition, the refractive-index increment is
An3 = /2n3r33E3, (2)
where n 3 is the extraordinary refractive index, r 33 is theappropriate electrooptic coefficient, and E 3 is the ap-plied electric field.4 Thus, the crystal must be cut withits c axis in the plane of the waveguide essentiallytransverse to the beam propagation direction, and thepropagating optical mode must have TE polarization.This polarization has the least loss characteristics inproximity to the metal electrode surfaces. Thus, thisminimizes the insertion loss of the modulator causd byabsorption.
Combining Eqs. (1) and (2) yields
0 = (7rLn3r33E3 )/Xo
The authors are with Hughes Research Laboratories, Malibu,California 90265.
Assume that the c-axis-oriented electric field in theregion of the guided layer is approximately sinusoidalin the transverse direction (a reasonable assumption fora region about a distance s below the surface). ForBragg diffraction, the zero- and first-order powers are
where K is a correction factor6 determined from theratio of electrode width to spacing wis. For LiTaO3 inthe clamped condition (which is expected to obtain overmost of the operating band), the capacitance is+An - -,Ln -
SUBSTRATE GUIDING LAYER
FRINGING FIELDS
Fig. 1. Phase grating formation by the electrooptic effect.
Table 1. Properties of Electrooptic Materials
LiTaO 3 LiNbO 3Quantity 0.53 Am 1.06, m 0.53 Am 1.06 Am
0.53 im, 3-W drive, 1.06 Am, 24-W drive,Parameter B = 0.7 GHz B = 1.4 GHz
K = , s = wE3 -L, V 1606 3732L, mm 2.5 5.0s, Am 4.6 6.9S, Am 18.4 27.5N 81 54C, pF 77 104R&, Q 6.0 2.2Q 11.0 20
proportional, respectively, to cos2(,P/2) and sin2(0/2).For modulation, corresponding to 100% depletion of thezero-order beam in the idealized case, the maximumrequired value of is r.
I1. Design
To design a suitable diffraction modulator, we needto determine the allowable dimensions of the electrodearray, based on bandwidth requirements and driverpower limitations. This can be done in a fairlystraightforward manner, and both the power and thecapacitance can be easily expressed in terms of the ratioof the electrode spacing to electrode length, s/L.
With optimized video peaking, the power requiredto drive a capacitance C over a bandwidth B with peakdriver voltage V is
P (V2 )/(2RS) = (/2)BCV2, (4)
where Rs l 1rBC is the shunt resistance needed todissipate the power and provide an RC-limited band-width B. The capacitance of an interdigated electrodearray having N finger pairs on an x- or y-cut uniaxialcrystal such as LiTaO3 is5
C = [eo + (E 3 )112 ]KLN, (5)
C = 3.8KLN pF. (6)
The number of electrode pairs is readily determinedfrom the total width of the electrode array. For aninput laser beam having a 1/e2 diameter D equal toabout 1 mm, we have found that is is sufficient to as-sume an electrode array width of 1.5D, which yields
N = (1.5D)/S = (0.15)/S, (7)
where S is the periodicity (expressed in centimeters).The applied electric field E3 in the active region of the
beam is estimated to be approximately5 6
E3 = (Vm)/(2s) (8)
when the distance below the surface is comparable tos. This is the assumed design condition that leads toa reasonably uniform field strength across the opticalbeam. It is convenient to express Vm in terms of thecommonly used electrooptic parameter E3-L, thefield-length product
Vm = 2(E 3 - L)(s/L), (9)
where
E3 -L = (XO)/(7rn'r3 3 ). (10)
Table I lists the relevant electrooptic and dielectriccharacteristics of LiTaO 3 and LiNbO3 at 0.53 Atm and1.06 ,im. These data are useful in the design of a dou-bled Nd:YAG communication link. The dielectricconstants (or specific permittivities) are indicated byEl/Eo (normal to the optical axis) and E3/E0 (parallel to theoptical axis), where Eo is the permittivity of free space.The changes in El and E3 for LiNbO 3 in going from theunclamped to clamped condition are quite large, a factwhich adversely affects the frequency response char-acteristics. Similarly, the change in r3 3 is substantiallygreater than that of LiTaO3, thus producing a strongereffect on the electrooptic frequency response.
We have calculated some specific design values fora modulator operating at both 0.53 Am and 1.06 Am forwls 1. A maximum interaction length of 2.5 mm waschosen for 0.53 Am to keep within reasonable limits ofoptical loss. In a previous study' we have found thatthe waveguide loss at 5145 A for Ti-diffused LiTaO 3guides was 3-5 dB/cm. For the 1.06-,m case, whereoptical losses are substantially lower (-1 dB/cm), alength of 5 mm was arbitrarily chosen as a reasonableupper limit.
Table II gives the results derived from the precedingequations for the two wavelengths of interest. Thetable includes a parameter Q defined by7
Q = (27rXoL)/(n3 S2 ), (11)
which describes the nature of the diffraction. Braggdiffraction occurs most efficiently when Q > 10.
Fig. 2. Series electrode modification for Bragg diffraction grating.
18.4 um- *
Fig. 3. Split electrode pattern of modulator.
Fig. 4. Transient decay of the diffracted intensity caused by leakage.Time scale: 5 sec/major division.
Examination of the values of shunt resistance R3 forthe two cases shown clearly indicates the need for im-pedance matching from a 50-Q driver source. Recentdevelopments in the design of wideband rf impedancetransformers have led to very wideband devices capableof operating from below 1 MHz to well above 500 MHzwith insertion losses of 0.5 dB and less, provided theimpedance ratios do not exceed about 3 or 4 to 1. Forlarger step-down ratios, the insertion losses are sub-stantially higher. As an alternative, a different elec-trode design may be used to provide matching to a 50-Qdriver. For the case of the 0.53-gm design the modifi-cation follows a scheme proposed in Ref. 3 in which theelectrode array is divided into several sections, say 3,each of length L/3, arranged in series both electricallyand optically. This device redues the capacitance bya factor of 9, which increases shunt resistance by thesame proportion. This modification is shown in Fig. 2.For the case of the 0.53-gm modulator design it is clearthat this yields a shunt resistance of 54 Q and a capaci-tance of 8.6 pf. The penalty paid by this approach isthat the driver voltage is increased by a factor of 3.
IV. Fabrication and Testing
Optical waveguides were formed in y-cut LiTaO3wafers by Ti-indiffusion following the processingtechnique described in Ref. 1. Interdigital electrodesemploying the design parameters in Table II for 0.53-gmoperation were fabricated on the wafers with the fielddirections aligned with the c axis. Diffraction efficiencymeasurements were made at 6328 A (He-Ne), 5145 A(Ar), and at 10,640 A (Nd:YAG). Diffraction mea-surements indicate that we have demonstrated the mostefficient electrooptic Bragg modulators to date: 98%efficiency with extinction ratios as high as 300:1. Pre-vious to this work, modulators with similar passivebuffer layers have achieved 70% efficiency.28
For the modulator structures fabricated, the electrodepatterns were formed by photoetching 1500-A Al filmsthat had been evaporated directly on the waveguidesample or on a buffer film of SiO2 (1500 A). Thisthickness of the buffer layer has been found to be ef-fective in providing the necessary isolation to preventdirect interactions of the optical field with the metalgrating. Figure 3 is a photograph of a portion of a splitelectrode design used to reduce the effective capacitanceby a factor of 9. The width-to-spacing ratio achievedwas close to 0.5 for all the samples studied.
The diffraction efficiency of modulators with andwithout electrode buffer layers were studied to deter-mine the degree of energy transfer from the m = 0 un-diffracted beam to the different grating orders. Elec-trical leakage can hinder device evaluation, and severeleakage currents were observed in several samples. Theleakage currents originate from incomplete oxidationof the SiO2. The effect of the leakage currents is topreclude dc testing in some samples. For instance, Fig.4 shows the diffracted beam intensity on a leaky mod-ulator structure as a function of time. The applied
Fig. 5. Measured diffracted power vs voltage at 5145 A.
160
140
E
LU
0c:a
LUup
120
100
80
60
40
20
0
Diffraction efficiency measurements were made at5145 A. This modulator had no SiO2 buffer layer on itand was used to determine the effects of the metalgrating. The metal grating induced a deflected spot attwice OB of intensity equal to 15-25% of the undeflected(m = 0) spot. The measured voltage for maximumdiffraction was 17.5 V, which is quite close to the cal-culated value of 17.0 V for 5145-A operation. The cal-culated value for doubled Nd:YAG operation (0.53,gm)is 17.7 V. The diffraction efficiency measured in thisexperiment was 95.3% as shown in Fig. 5, which is higherthan the best value of Lee and Wang 9 of 90% for a sim-ilar structure with no buffer layer. It is, however, im-portant to note that these results and the results of Leeand Wang are deceiving in that the measured diffractedpower results from depletion of both the m = 0 beamand the beam caused by the metal grating itself. In factin other experiments we have observed that the dif-fracted power (m = 1) may actually exceed that of theundeflected beam (m = 0).
The results of measurements made at 1.06 gtm areplotted in Fig. 6. The measured diffraction efficiencywas 98% with an extinction ratio of 300:1, or 24.7 dB.
Experiments were also performed to evaluate themodulator with SiO2 layers at 6328 A. Our results areshown in Fig. 7. A 98% diffraction efficiency wasmeasured with an extinction ratio of 250:1, or 24 dB.
1.0
0.8
LU
LLIL
z0C-)
U-U-
To
V
Fig. 6. Results of diffraction measurements at 1.06 jm showing 98%maximum first-order diffraction and a 300:1 extinction ratio.
voltage was simply turned on and kept on. It is clearfrom the trace that the effective field over the waveguidestructure goes to zero in a short time. These resultswere obtained for modulators with a sputtered SiO2buffer layer. These same samples were stripped of theAl electrode pattern and placed in an oven in an 02 at-mosphere at 5000C for a few hours. The samples werethen reprocessed and new modulator patterns fabri-cated on them. These samples were found to exhibitgood dc properties: no leakage was observed, andmodulation tests could be carried out.
Our results indicate that the design and fabricationtechniques described here have yielded Bragg diffrac-tion modulators of high efficiency. However, severalimportant materials processing questions remain wherefurther work must be done. In several samples, veryweak electrooptic effects were observed in which up to100 V were required at 6328 A to achieve 50% diffractionefficiency. The probable causes are incomplete polingor degradation of the electrooptic effect resulting fromdiffusion or heating (second-phase formation at thesurface, for instance). In addition, we have observeda drift of the maximum voltage with time. Initially, asample exhibits a diffraction efficiency that is sym-metric about zero voltage. However, after a prolongedperiod with the voltage on (typically 1 hr), an asym-metry of up to 3 V is observed. These large shifts seemto be related to both optical carrier generation andion-migration effects. Similar effects have been seenin Ti-diffused LiNbO 3 1
The authors acknowledge the technical advice andassistance of Nathan Hirsch of HRL in the photol-ithography processing of our modulators.
This work was supported by Air Force AvionicsLaboratory, Wright-Patterson Air Force Base, undercontract F33615-77-C-1007.
References1. G. L. Tangonan, M. K. Barnoski, J. F. Lotspeich, and A. Lee, Appl.
Phys. Lett. 30, 238 (1977).2. J. M. Hammer and W. Phillips, Appl. Phys. Lett. 24, 545
(1974).3. J. Noda, N. Uchida, and T. Saku, Appl. Phys. Lett. 25, 13
(1974).4. F. S. Chen, Proc. IEEE 58, 1440 (1970).5. S. G. Joshi and R. M. White, J. Acoust. Soc. 46, 17 (1969).6. M. A. R. P. de Barros and M. G. F. Wilson, Proc. IEEE 119, 807
(1972).7. W. R. Klein and B. D. Cook, IEEE Trans. Sonics Ultrason. 50-14,
123 (1967).8. J. M. Hammer, Integrated Optics, J. Tamir, Ed. (Springer-Verlag,
New York, 1975), p. 184.9. Y. K. Lee and S. Wang, Appl. Opt. 15, 1565 (1976).
10. M. Papuchon and B. Puech, Thomson-CSF, France; privatecommunication.
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