Reverse proton exchange for buried waveguides in ... - OSA Publishing

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J. Opt. Soc. Am. A / Vol. 15, No. 7 / July 1998

Korkishko et al.

Reverse proton exchange for buried waveguides in LiNbO3 Yu. N. Korkishko, V. A. Fedorov, and T. M. Morozova Moscow Institute of Electronic Technology (Engineering University), Department of Chemistry, 103498, Moscow, Zelenograd, Russia

F. Caccavale, F. Gonella, and F. Segato Istituto Nazionale per la Fisica della Materia and Universita` degli Studi di Padova, Dipartimento di Fisica Galileo Galilei, Via Marzolo 8, 35131 Padova, Italy Received November 10, 1997; revised manuscript received March 2, 1998; accepted March 11, 1998 The reverse proton-exchange (RPE) process performed in different HxLi12xNbO3 crystalline phases of protonexchanged and annealed proton-exchanged LiNbO3 waveguides leads to remarkable optical and compositional modifications. Analysis of ordinary and extraordinary index profiles, correlated with concentration depth profiles in RPE samples subjected to different exchange and postexchange treatments, shows how buried-index optical waveguides in LiNbO3 with symmetric mode and reduced fiber–waveguide coupling losses can be realized. © 1998 Optical Society of America [S0740-3232(98)01507-5] OCIS codes: 230.7370, 130.3730, 310.3840, 130.3130.

1. INTRODUCTION Lithium niobate, LiNbO3, is a widely used ferroelectric crystal with various acoustical and integrated optical applications. One of the basic methods for creating optical waveguides in this crystal is proton exchange1 (PE). There are some exchange and annealing conditions for producing high-quality waveguides in LiNbO3 characterized by low optical losses, negligible decrease of electrooptical and nonlinear coefficients, and high-power handling capability. However, since the depth index profile for conventional PE LiNbO3 waveguides is asymmetric, the waveguide depth mode is also quite asymmetric. On the other hand, single-mode fibers have circularsymmetric mode profiles. Thus depth index profile symmetrizing is essential for reducing the fiber–waveguide coupling loss. This problem can be solved by making buried proton-exchanged waveguides. Such waveguides can be produced by the reverse proton-exchange (RPE) method. RPE is interesting for two reasons. First, it allows the realization of buried extraordinary waveguides, which presents several advantages: (1) better fiber–waveguide coupling because of the more symmetric shape of the waveguide modes, (2) higher conversion efficiency in nonlinear processes due to a better overlap between the interacting modes, and (3) lower losses due to a reduction of surface imperfections. Second, ordinary waveguides are created at the surface by using the PE region as an index barrier. Ten years ago, Korkishko and colleagues2 first reported that the reverse exchange Li1 → M1 (where M1 is a proton or metal ion) can be carried out in ionexchanged waveguides in LiNbO3. In other papers3,4 Korkishko and colleagues studied RPE process in LiNbO3 by using LiNO3 melt. Similar results were achieved 0740-3232/98/071838-05$15.00

later by Jackel and Johnson5 and Olivares and Cabrera,6,7 both groups using LiNO3 –NaNO3 –KNO3 melt. In a recent paper,8 correlations between PE and RPE conditions and second-harmonic generation capabilities of Z-cut LiNbO3 waveguides were obtained. Recently9–11 Korkishko, Fedorov, and colleagues extended the reverse exchange process to fabricate surface guiding layers of ordinary refractive index as well as buried guiding layers of extraordinary index in annealed PE (APE) LiNbO3 waveguides, as well as in LiTaO3 crystals.12 Moreover, our investigations11–16 allowed us to identify seven different crystallographic phases in HxLi12xNbO3 (a, k1 , k2 , b 1 , b 2 , b 3 , and b 4 phases) that can be realized in proton-exchanged layers, depending on exchange and annealing conditions. Now, knowing that many different crystalline phases exist in HxLi12xNbO3 waveguides, it is evident that the kinetics of the RPE processes will be different in different phases. In this paper we present the results of a study of RPE processes for some Hx Li12xNbO3 phases. Secondary ion mass spectrometry (SIMS) is used to recover, with good accuracy and great depth resolution, compositional profiles17 of PE and RPE LiNbO3 waveguides.

2. PREPARATION OF SAMPLES AND THEIR CHARACTERIZATION To realize PE waveguides, one can use either a simple H1 → Li1 exchange process by varying the temperature, the acidity of the bath, and the duration of the exchange, or a two-step process in which the proton exchange is followed by annealing, whose duration and temperature modify the waveguide parameters. © 1998 Optical Society of America

Korkishko et al.

Vol. 15, No. 7 / July 1998 / J. Opt. Soc. Am. A

PE waveguides were fabricated on integrated optical grade X-, Y-, and Z-cut LiNbO3 substrates. As sources of PE we used (1) a solution of KHSO4 in glycerine,18 (2) benzoic acid, whose acidity can be reduced by adding up to 5 wt.% of lithium benzoate, (3) pyrophosphoric acid, and (4) ammonium dihydrophosphate (NH4 H2 PO4 ). To find mode effective indices, we measured the excitation angles of dark m lines with a prism coupler setup at l 5 633 nm. The refractive-index profiles throughout the waveguide depth were reconstructed by the inverse Wentzel–Kramers–Brillouin (IWKB) technique proposed by White and Heidrich19 and improved by Dikaev et al.20 The calculation procedure proposed by Chiang21 was also used. The surface increments Dn e were determined as the average between values obtained by using these two methods. RPE was performed in eutectic melt of LiNO3 (37.5 mol.%)–KNO3 (44.5 mol.%)–NaNO3 (18.0 mol.%) (the melting point of this eutectic mixture is 120 °C) at temperatures varying from 250 °C to 330 °C (Refs. 9–11). SIMS measurements of the waveguides were performed with a CAMECA ims4f ion microscope equipped with a normal-incidence electron gun used to compensate the surface charge buildup during profiling of insulating samples such as LiNbO3 crystals. Depth profiles were obtained by use of 14.5 keV Cs1 bombardment and negative secondary ion detection. The erosion speed was evaluated by measuring the depth of the crater at the end of each analysis by means of a Tencor Alpha-step profilometer.

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Fig. 1. Typical transformation of the index profiles of initial b 2 and b 4 HxLi12xNbO3 PE waveguides (solid curves), and after RPE (dashed curves).

Fig. 2. Ordinary index profiles of RPE LiNbO3 waveguides. Zcut LiNbO3 exchanged in ammonium dihydrophosphate melt at 270 °C for 5 h with postexchange annealing at 330 °C for 3 h, RPE in LiNO3 melt. Solid curves represent RPE at 270 °C: 1, 85 h; 2, 125 h. Dashed curves represent RPE at 315 °C: 3, 22 h; 4, 41 h.

3. RESULTS AND DISCUSSION A. Reverse Proton Exchange in b 2 and b 4 Phases The b 2 and b 4 HxLi12xNbO3 phases are formed during the direct PE process in LiNbO3 (Refs. 14–16). These phases are characterized by the presence of interstitial protons, which quickly exchange with lithium ions at the first stage of RPE. As a result, b 1 – HxLi12xNbO3 waveguides on the extraordinary index are formed after a few minutes of RPE at 300 °C (Fig. 1). These guides are still on the surface; therefore it turns out that it is impossible to create buried waveguides when either the b 2 or the b 4 phase is presented in the exchanged layer. B. Reverse Proton Exchange in the b 1 Phase Z-cut PE LiNbO3 waveguides were prepared in ammonium dihydrophosphate melt at 270 °C for 5 h and annealed at 330 °C for 3 h until the b 1 phase was formed on the surface. X-ray rocking curves showed peaks corresponding to the b 1 phase. Figure 2 shows the ordinary index profile of the waveguide after the reverse exchange process, performed at 330 °C for different durations. From x-ray-diffraction measurements it results that in the buried region the b 1 – HxLi12xNbO3 phase still exists after the RPE treatment. The ordinary index profiles were approximated by the function

F S DG

Dn o ~ z ! 5 Dn o 0 exp 2

z

2 ADt

a

;

by varying parameters D and a, we tried to fit the calculated profiles to the experimental ones. For example, it was found that for RPE performed at temperature T 5 270 °C, D 5 7.56 3 1023 m m2/h and a 5 1.6; for T 5 315 °C, D 5 5.76 3 1022 m m2/h and a 5 1.2. Olivares and Cabrera6,7 reported that RPE guides on Z-cut LiNbO3 treated in LiNO3-KNO3-NaNO3 melt at 350 °C had a Gaussian-like (a 5 2) ordinary index profile. It is worth mentioning that RPE waveguides on X-cut LiNbO3 treated under the same conditions have a paraboliclike profile. Figure 3 shows the SIMS depth profiles of PE b 1 -phase waveguide on a Z-cut LiNbO3 sample, as-exchanged and after reverse exchange, respectively. In the asexchanged sample, the hydrogen depth profile consists of a plateau of constant concentration followed by a sharp decrease in the background substrate signal. The lithium depth distribution is different from that of the hydrogen one. This fact can be explained by the cationic vacancies model proposed by Ganshin and Korkishko.22 After RPE treatment, a buried hydrogen distribution is realized. At the same time, the lithium signal at the surface is similar to that in the bulk region; therefore one can expect that pure LiNbO3 is realized on the top of RPE LiNbO3 waveguide. Moreover, during the RPE process the hydrogen distribution at the diffusion front becomes graded as a result of the simultaneous annealing process carried with RPE.

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C. Reverse Exchange in Annealed Proton-Exchange LiNbO3 Waveguides Reverse exchange in APE waveguides (REAPE) process is effective in producing buried waveguides with graded refractive-index profiles. Figure 4 shows the evolution of the ordinary refractiveindex profile of the REAPE waveguide as a function of the reverse exchange time in LiNO3 –KNO3 –NaNO3 melt at 320 °C. Figure 5 shows the SIMS depth profiles in a Z-cut RPE LiNbO3 sample, first exchanged in a solution of KHSO4 in glycerine at 230 °C for 18 h (b 1 -phase waveguide), then annealed at 325 °C for 30 h, and then reverse exchanged at 320 °C for 4 h. The hydrogen depth distribution is graded both at the surface and in the buried region. As in the case of RPE in the as-exchanged sample, after the REAPE process the lithium signal at the surface is similar to that of the lithium signal in the bulk region; therefore one can again expect that pure LiNbO3 is realized on the top of REAPE LiNbO3 waveguides. By analysis of the RPE process in REAPE LiNbO3 waveguides, the relationship between ordinary and extraordinary refractive-index changes in k1 – and a – HxLi12xNbO3 phases can be obtained. With PE an increase in the extraordinary refractive index and a decrease in the ordinary index are observed with respect to the values of n e and n o of LiNbO3 substrates.23 Thus the prism-coupling technique and the

Korkishko et al.

Fig. 4. Time evolution of the ordinary index profile in an X-cut REAPE LiNbO3 waveguide with reverse exchange in LiNO3 –KNO3 –NaNO3 melt at 320 °C. The PE LiNbO3 waveguide was prepared in a solution of KHSO4 in glycerine (2g / l) at 225 °C for 50 h and annealed at 330 °C for 80 h.

Fig. 5. SIMS concentration profiles in Z-cut PE LiNbO3 samples, exchanged in a solution of KHSO4 in glycerine at 230 °C for 18 h (b 1 -phase waveguide), then annealed at 325 °C for 30 h, and then reverse exchanged at 320 °C for 4 h.

Fig. 3. SIMS depth profiles of Z-cut PE LiNbO3 samples, asexchanged in a solution of KHSO4 in glycerine (a) at 220 °C for 62 h (b 1 -phase waveguide) and (b) after reverse exchange at 320 °C for 2 h.

IWKB method can be used only to determine the extraordinary index profiles. To measure the ordinary refractive-index change in the b 1 -phase PE LiNbO3 waveguides with steplike index profiles, the dark-mode reflectivity technique has been used.24,25 However, interference analysis can not be performed on APE LiNbO3 waveguides, because these waveguides exhibit graded-index profiles. To measure the ordinaryindex increments in the APE LiNbO3 waveguides, we used the RPE technique. The method consists of preparing and analyzing samples by pairs. The initial exchange is identical on the two samples; one of them is reverse exchanged in a LiNO3 (37.5 mol.%)–KNO3 (44.5 mol.%)–NaNO3 (18.0 mol.%) melt (the melting point of this eutectic mixture is at 120 °C) or pure LiNO3 melts (melting point at 261 °C) at temperatures from 250 to 330 °C, while the second (reference sample) is annealed at the same temperature and for the same time t APE 5 t RPE . We characterized the surface guiding layers of both the RPE and the APE waveguides by using the prismcoupling technique. In this procedure we neglected the fact that the ordinary waveguide is due only to an index barrier and that the ordinary index increases again at a certain depth in the substrate. This is a good approximation when the barrier is deep enough to prevent significant evanescent tails of the ordinary modes from reaching the region of the substrate where the index increases again. We thus introduced a fictitious value of the ordi-

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nary substrate index n o b equal to the minimum measurable value of the ordinary index profile, observed at the turning point z o of the last ordinary polarization mode (Fig. 6). We assumed that essentially pure LiNbO3 is formed at the surface after RPE, which is confirmed by SIMS measurements showing that the hydrogen concentration at the surface is similar to that of the initial LiNbO3 (Fig. 5). This indicates that the surface ordinary index of the RPE waveguide can be taken as the bulk value so that n o 5 n o s . Therefore Dn o (z 5 z o ) 5 n o s 2 n ob. We found that the hydrogen concentration profile in the barrier region is not affected by the reverse exchange. Fig. 7. SIMS concentration profiles in reverse exchanged (dotted curve) and annealed (solid curve) Z-cut PE LiNbO3 samples, prepared under identical time and temperature conditions.

Fig. 8. Relationship between extraordinary and ordinary refractive-index changes (compared with those of pure LiNbO3) for different phases in PE LiNbO3 waveguides. The arrows show the directions in which proton concentration increases.

Fig. 9. Experimental distribution of optical guided radiation (near-field) in a straight REAPE waveguide formed in LiNO3 –KNO3 –NaNO3 melt. Fig. 6. Ordinary (1) and extraordinary (2) refractive-index profiles in RPE waveguides on Z-cut LiNbO3 prepared under the following conditions: (a) PE in ammonium dihydrophosphate melt at the T PE 5 220 °C, t PE 5 14 h, postexchanged annealing: T APE 5 330 °C, t APE 5 110 h; RPE in LiNO3 melt: T RPE 5 300 °C, t RPE 5 100 h. (b) PE in solution of KHSO4 in glycerine (C 5 1g/l) T PE 5 240 °C, t PE 5 50 h, RPE in LiNO3 melt: T RPE 5 300 °C, t RPE 5 50 h. Dashed lines show extraordinary refractive index profiles in ‘‘reference’’ sample.

The hydrogen concentration profiles and, as a consequence, the extraordinary-index profiles of the reference APE and RPE waveguides are identical (Fig. 7). In particular, we can say that at depth z o the influence of the RPE process is negligible and Dn APE (z 5 z o ) 5 Dn RPE (z e e 5 z o ) 5 Dn e (z 5 z o ).

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The dependence Dn o (z 5 z o ) on Dn e (z 5 z o ) obtained in this way (Fig. 8) is thought to be close to the general relationship between Dn o and Dn e in APE waveguides with graded refractive-index profiles. The APE LiNbO3 waveguides are more attractive for integrated optics applications, as they exhibit lower propagation loss compared with PE waveguides and are characterized by the restoration of the electro-optic and the nonlinear effects. Straight REAPE waveguides were also formed and characterized: An 8-mm-wide channel PE LiNbO3 waveguide was fabricated by use of a lithographically defined titanium mask. The technique used was APE, with 12 h of exchange at 215 °C in a solution of KHSO4 in glycerine with a concentration of 1 g/l, followed by 67 h of annealing at 320 °C. The straight REAPE waveguide was then fabricated by treatment of the obtained guide in eutectic LiNO3 –KNO3 –NaNO3 melt at 300 °C for 47 h. Figure 9 shows the mode profile, at l 5 1.55 m m, for the considered REAPE waveguide obtained by near-field measurement. It can be seen that the mode profile shape is very close to a circular one, as expected, with a consequent potential reduction of fiber–waveguide coupling losses. This technique is therefore suitable for making low-loss devices.

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4. CONCLUSION Reverse proton exchange is a simple and attractive technique for fabricating buried optical waveguides in LiNbO3. Depending on the crystal phase realized in the PE LiNbO3 waveguide and reverse proton-exchange conditions, buried waveguides with different properties can be realized. By analysis of the reverse exchange process in PE LiNbO3, the relationship between ordinary and extraordinary refractive-index changes in different HxLi12xNbO3 phases was determined.

ACKNOWLEDGMENT

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This research was supported by NATO Collaboration Research Grant HTECH.CRG.960338. 19.

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