Water vapor effects on optical characteristics inTi:LiNbO3 channel waveguides
Toshinori Nozawa, Kazuto Noguchi, Hiroshi Miyazawa, and Kenji Kawano
This paper describes experimental studies on propagation loss and crosstalk of TM-polarized light in Ti-diffused Z-cut LiNbO3 channel waveguides as a function of the water vapor content in the diffusionatmosphere. The dependence of surface roughness and crystal quality on the waveguide fabrication atmo-sphere is taken into consideration. In this study it is found that waveguides with low propagation loss (<0.2dB/cm), low crosstalk (<-20 dB), and smooth surfaces can be fabricated by strictly controlling the watervapor content introduced into the oxygen or argon carrier gas. Key words: Optical waveguide, LiNbO3, Tidiffusion, propagation loss, crosstalk.
1. IntroductionLiNbO3 is a promising electrooptic material for the
fabrication of guided wave devices.' The most firmlyestablished waveguide fabrication technology in thematerial is the in-diffusion of Ti at -1000'C for severalhours.2
In waveguide fabrication, the diffusion conditionsmust be carefully controlled. Among the conditions,probably those relating to the diffusion atmosphereare the least understood, although there are manyreports in existence.
The addition of water vapor to the flowing carriergas during diffusion (wet atmosphere diffusion) hasgreatly improved waveguide quality. It has sup-pressed out-diffusion of Li (Ref. 3) and formation ofLiNb3O8.4 However, the waveguides on Z-cut sub-strates processed in wet atmosphere have also shownsevere surface roughness. 5 6 Such surface roughnessmay degrade optical characteristics of the waveguidesby increasing propagation loss and producing unde-sired crosstalk in a directional coupler.
An important study on light scattering in Ti-dif-fused Y-cut LiNbO3 has been performed by Armeniseet al.7 and Singh and De La Rue.8 They investigatedthe dependence of scattering levels on the waveguidefabrication parameters for dry atmosphere diffusion
The authors are with NTT Optoelectronics Laboratories, 3-1,Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-01, Japan.
and also discussed the major sources of it.McCaughan and Choquette9 reported that Ti concen-tration fluctuations degraded the crosstalk, which isconsidered to be limited by the processing technology.However, the water vapor effects on the optical charac-teristics-propagation loss and crosstalk in directionalcouplers-do not appear to have been published previ-ously.
It is our objective to understand the water vaporeffects on the optical characteristics of Ti-diffusedchannel waveguides and then to minimize both propa-gation loss and crosstalk.
11. Sample PreparationTo fabricate Ti-diffused waveguides successfully, it
is necessary to strictly control both crystal quality anddiffusion process conditions. Optical grade qualityand optically polished Z-cut LiNbO3 single crystalsubstrates with congruent composition were used forthe experiments. Refractive index deviation of thesubstrates was <1 X 10-4. Before fabrication, eachsubstrate was checked by x-ray topography to ensurethat it did not include any crucial defects. Wave-guides were formed on the (minus Z). An automaticfabrication process line for 7.6-cm (3-in.) wafers wasemployed for the Ti pattern formation. The photo-lithographic techniques were similar to those for sili-con VLSI fabrication. By using the process line in ourstandard process conditions, Ti strip linewidth with+0.1-,m deviation and Ti thickness with ±1.5% devi-ation were readily obtained with good reproducibility.
Propagation loss and crosstalk depend on how wellthe waveguide mode is confined, because the light of aweakly confined mode is more easily scattered out ofthe waveguide by imperfections. To fabricate well-
Ti THICKNESS (nm)Fig. 1. Normalized mode dispersion curve for waveguides withdiffusion lengths of 3.3 Am (lateral direction) and 3.7 Am (depth
direction).
confined single-mode waveguides, the Ti pattern di-mension must be determined by calculation using themodified step segment method.1 0 The calculated nor-malized mode dispersion curve for both the fundamen-tal mode and the first-order mode is shown in Fig. 1.In this experiment, the Ti width and thickness were 8,um and 75 nm, respectively.
The paired channel waveguides (i.e., directional cou-plers) used in this crosstalk study were twenty pairs ofwaveguides formed on a 10- X 25-mm chip with cou-pling lengths ranging from 1 to 20 mm. The center-center guide separation of the coupler was set at 13 Arm.
The substrates were held in a quartz tube with a 10-cm i.d. Diffusion was carried out at 10000C for 10 h inoxygen or argon carrier gas, which was controlled at 1liter/min by a mass flowmeter. The heating rate was100 C/min, and the cooling rate limited by furnace heatcapacity was 5C/min. In the case of Ar carrier gas,the gas was changed to 02 1 h before the end of diffu-sion. Water vapor was introduced above 3500C. Theamount introduced into the carrier gas was controlledby bubbler temperature and determined by measuringthe reduced water amount in the bubbler.
Ill. Effects on Propagation LossThe propagation loss of the waveguide was obtained
by subtracting the coupling losses and the Fresnellosses at the fiber-waveguide interface from the fiber-waveguide-fiber insertion loss.
A 1.53-,um laser diode (LD) was used as the lightsource. The fibers were polarization maintaining andabsorption reducing (PANDA) single-mode fiberswith a measured full width at half-maximum (FWHM)of 5.6 ,m in mode power profile. The waveguide wasbutt-coupled to the fibers and the TM-polarized lightwas excited. The Fresnel losses were reduced to 0.15dB with refractive index matching fluid (n = 1.47).
The coupling losses were obtained by calculating theoverlap integral between 2-D mode field profiles of thefiber and the waveguide. The near field patterns(NFP) were measured by using the apparatus shown in
PANDASINGLE-MODEFIBER
LENS(X40) ATTENUATOR
Fig. 2. Schematic diagram of the near field pattern measuringapparatus.
3.0
C,,
o 2.0-J
12 1.00
a..
0.4O l 2 5 10 20
WATER VAPOR CONTENT(ml/hr)Fig. 3. Water vapor effects on the propagation loss for oxygen
carrier gas.
Fig. 2, namely, NFPs were imaged onto an infraredvidicon camera through a microscope objective (40X).The camera system was computer controlled and 2-Dmode field profiles were taken after calibration forboth field response and magnification. The uncer-tainties of the mode intensity profile measurementand the coupling loss were +0.1 um and +0.1 dB,respectively.
For oxygen carrier gas, the effects of water vapor onpropagation loss of Ti-diffused channel waveguidesare shown in Fig. 3. The loss increases with an in-crease in water vapor content. The SEM photographsshown in the figure, taken at an angle of 600 from thevertical axis, pertain to each plot. Surface undulationshaped in a triangle appears and increases with anincrease in water vapor content. The undulationseems to be similar to the hillocks reported by Canali etal.,5 Niizeki et al.,1 ' and Nassau et al.12 As can be seenin this figure, it seems the loss increase is due to scat-tering at the waveguide surface.
When using argon as a carrier gas, it was found thatthe effect was a little different from that of oxygen gas,especially at a low water vapor content, as shown inFig. 4. Namely, the loss decreases and eventuallyincreases with an increase in water vapor content, al-though the minimum propagation loss was almostequal to that when using oxygen carrier gas. Theincrease in propagation loss at low water vapor content
I I ----L-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~7~~~~~~~~~F
* L *1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
STRAIGHTTHROUGH
-ZTPo-Pc
+U~~PCCROSSOVER
Fig. 5. Schematic drawing of the directional coupler.
0
Fig. 4. Water vapor effects on the propagation loss for argon carriergas.
-J
I-
C,7U)
0
ix
Uo
seems to be due to scattering from round-shaped un-dulation.
In any event, it was found that low loss waveguides,below 0.2 dB/cm with a smooth surface, can be fabri-cated by strictly controlling the water vapor contentintroduced into each carrier gas.
IV. Effects on CrosstalkCrosstalk X in the directional coupler shown in Fig.
5 is defined as
X(dB) = -10 log 1o (1 - (1)
where Po and P, are input light power and coupledoutput light power at the cross-port, respectively.The P, is given by the expressions
Pe= PO[ 2 O 22K ) s {( o) + ( )]1} (2)
where L is the waveguide interaction length, Lo is thecoupling length for complete power transfer, A is thepropagation constant difference between paired wave-guides, and K is the coupling coefficient which de-scribes the strength of the interguide coupling. Typi-cally, K is related to Lo or separation between guides Gby14
K = = A * exp(-B -G), (3)2L,
where A and B are the constants that depend on thesubstrate material and waveguide parameters includ-ing the fabrication conditions.
From Eqs. (1) and (2), crosstalk X is given as
x = -10 loges(10 + ()2J)1] cos {( ) [1 + ( 2 )2]1/2}).
(4)
Here X has the minimum value when the ideal condi-tions A1/K = 0 and LILo = 1 are satisfied; however, itdegrades as A/3/K becomes larger.
To evaluate the quality of the directional coupler,tolerable coupling length AL is now introduced. This
0
-J
-10
-20
-30
COUPLING LENGTH (Arb.)Fig. 6. Schematic drawing of the crosstalk characteristics.
WATER VAPOR CONTENT (ml/hr)Fig. 7. Water vapor effects on crosstalk.
length indicates the coupling length range whichachieved crosstalk of better than -20 dB as shown inFig. 6, and then AL/Lo gives some indication of thefabrication tolerance required to make a low crosstalkdirectional coupler. When paired waveguides are fab-ricated ideally, the phase matching condition (i.e., A#3/K = 0) should be satisfied. In this case, the value of AL/Lo should be -13%.
Crosstalk was measured for TM-polarized light atthe 1.53-Mim wavelength. The water vapor effects onAL/Lo are shown in Fig. 7. Error bar limits shown inthis figure give the minimum and the maximum valuesfor eight chips. The AL/Lo values for dry atmospherediffusion samples degrade to <5%, while those for wet
WATER VAPOR CONTENT (ml/hr)Fig. 8. Water vapor effects on the quality of LiNbO3 substrates not
coated with Ti.
atmosphere diffusion samples are 10-15% and inde-pendent of the water vapor content. This tendencydoes not depend on the carrier gas. Comparing thisfigure with Figs. 3 and 4, the crosstalk in the direction-al coupler was found to be independent of the rough-ness at the waveguide surface.
Possible sources of the crosstalk include refractiveindex nonuniformity and crystal quality degradationinduced during Ti diffusion, except the surface rough-ness. McCaughan and Choquette9 investigated therefractive index fluctuations detected along the dif-fused waveguides and inferred that inhomogeneities inthe initial Ti film had been the source of index varia-tions and crosstalk degradation. In our experiment,however, Ti film thickness deviation was suppressedwithin 11.5% as described above, and fluctuation of Ticoncentration of waveguides was 2 X 10-2 wt% atmost15 even for dry atmosphere diffusion samples.Accordingly, refractive index nonuniformity is notconsidered to be the source of the crosstalk degrada-tion.
Crystal quality change during diffusion was investi-gated by the x-ray rocking curve (XRC) analysis usingthe double crystal method. The water vapor effects ona full width at half-maximum (FWHM) of the XRCobtained for LiNbO3 substrates not coated with Ti areshown in Fig. 8. Broadening of FWHM was observedonly for the dry atmosphere diffusion samples. Thissuggests that the crystal quality degrades when diffu-sion is performed in dry atmosphere. From this result,crosstalk degradation in the dry diffusion sample isinferred to have a close relation to LiNbO3 crystalquality degradation.
V. Conclusions
Our experimental studies have shown that propaga-tion loss and crosstalk in Ti-diffused Z-cut LiNbO3channel waveguides greatly depend on the water vaporcontent introduced into flowing oxygen or argon carri-er gas.
To prevent another process-induced fluctuation, op-tical grade quality substrates with congruent composi-
tion were employed and Ti stripe deviation was sup-pressed to less than 0.1 Atm in linewidth and to lessthan 1.5% in Ti thickness.
Propagation loss was found to be the result of scat-tering from the roughness at the waveguide surface.Low loss waveguides of 0.2 dB/cm with a smooth sur-face can be fabricated by strictly controlling the watervapor content. The optimum process condition forproducing low loss waveguides depends on the carriergas.
Crosstalk in a directional coupler was evaluated byconsidering tolerable coupling length AL, which indi-cated the coupling length range achieving better than-20-dB crosstalk. For wet atmosphere diffusion, thecrosstalk obtained was ideal, however, it degraded fordry atmosphere diffusion. Crosstalk degradation wasthought to be independent of the roughness at thewaveguide surface but to have a close relationship toLiNbO3 crystal quality degradation.
From this study it is believed that the optimumcondition for preparing ideal Ti:LiNbO3 waveguides isobtained in wet atmosphere diffusion.
The authors wish to express their deep appreciationto T. Ikegami, T. Sugeta, S. Miyazawa, J. Noda, and H.Jumonji for their encouragement. The authors wouldalso like to express their sincere gratitude to 0. Mi-tomi, T. Kitoh, M. Yanagibashi, and T. Suzuki fortheir constant cooperation and valuable discussions.
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