We report Zr4+-doped Ti:LiNbO3 strip waveguide fabricated by Zr4+-diffusion-doping followed by diffusion of 8 μm wide, 100 nm thick Ti-strips on a Z-cut congruent substrate. Optical study shows that the waveguide well supports both TE and TM, is single-mode at the 1.5 μm wavelength, and has a loss ≤ 1.3/1.5 dB/cm for the TE/TM mode. Secondary ion mass spectrometry study shows that the Zr4+-profile part having a Zr4+-concentration above the threshold of photorefractive damage covers 60% (70%) ordinary (extraordinary) index profile in the waveguide. We conclude that the waveguide is optical-damage-resistant.
© 2015 Optical Society of America
LiNbO3 (LN) crystal suffers from serious photorefractive damage. This detrimental effect prevents development of new integrated optical devices. As we know, a congruent LN doped with >5 mol% MgO can effectively withstand this effect . However, heavy MgO doping causes either the difficulty in growing high optical-quality single-crystal when codoped with rare-earth ions or the material inhomogeneity that prevents the fabrication of microdomain gratings with required homogeneity. It is imperative to seek other dopant with a lower threshold concentration of photorefractive damage, named Cth. Besides the well-known Mg2+, people has reported some other photorefractive prohibitors, such as Zn2+ , Sc3+ , Tm3+ , In3+ , Hf4+ , Zr4+ ,  and Sn4+ . Among them, the Sc3+ or Zr4+ doping shows a lower threshold (Cth is ~2 mol% only). The low Cth enables to improve the material homogeneity and the optical quality of crystal when codoped with other luminescent ions, such as Er3+. The low Cth also enables to increase the diffusivity and solid solubility of the codoped rare-earth ions. Therefore, an LN doped with > 2 mol% Sc3+ or Zr4+ would be a more promising substrate for an active or passive device.
Ti-diffused LiNbO3 (Ti:LN) waveguide is a basic unit of a related device. Obviously, a simple way to fabricate a Ti:LN waveguide doped with Zr4+ or Sc3+ is to employ a bulk Zr4+ or Sc3+-doped substrate. However, such substrate is attained not as easily as a pure LN. As an alternative, the Zr4+ or Sc3+ can be incorporated into the crystal by diffusion method, and a Ti:Zr(Sc):LN waveguide is fabricated by either Ti4+/Zr4+ or Ti4+/Sc3+ co-diffusion or Ti4+ diffusion following Zr4+ (or Sc3+) doping. Previous study has shown that Zr4+ and Sc3+ diffuse slower than Ti4+ [10, 11 ]. In the viewpoint of optical-damage-resistant application, which requires that the Zr4+ or Sc3+ profile entirely covers the Ti4+ profile, only the two-step method is feasible. In the previous paper, we have reported the fabrication of the Ti:Sc:LN strip waveguide . As the Zr4+ and Sc3+ have different chemical valences, ionic radii (0.7 nm for Zr4+ and 0.8 nm for Sc3+) and atomic weights (91 for Zr and 45 for Sc), the two ions show considerable difference in diffusion properties, which are closely related to design and fabrication of related waveguide. It is thus essential to carry out an independent study on the fabrication of a Ti:Zr:LiNbO3 strip waveguide. In this work, we demonstrate the feasibility of using the two-step method to fabricate a Ti:LN single-mode strip waveguide doped with Zr4+. The waveguide features were characterized by end-fire coupling technique. The profiles of diffused Zr4+ and Ti4+ ions were studied by secondary ion mass spectrometry (SIMS). From the obtained Zr4+ concentration profile we conclude that the waveguide is optical-damage-resistant.
2. Experimental description
The Ti:Zr:LN waveguide was fabricated by Ti-diffusion following Zr4+-diffusion-doping.
(1) Zr4+-doping: A commercial 1-mm-thick Z-cut congruent LN plate with optical surface was used in present work. A 132 ± 2 nm thick ZrO2 (99.99%) film was coated onto 2/3 surface part. The other 1/3 part of surface was uncoated for reference. After ZrO2 coating, the plate was annealed in wet O2. The diffusion temperature/duration was 1050 °C/10 h.
(2) Fabrication of Ti-diffused strip waveguide: An array of Ti strips with a width 8 μm and a thickness 100 ± 2 nm were delineated on the 1/2 part of the Zr4+-diffused surface (the other 1/2 part of Zr4+-diffused surface was not coated for reference). The separation of two adjacent strips is equally 200 μm. The Ti strips orient along the Y-axis of crystal. The Ti diffusion was carried out at 1050 °C for 10 h. To suppress Li2O out-diffusion, the diffusion was also carried out in the wet O2 atmosphere.
The sample surface can be divided into three parts: undoped area, Zr4+-only doped part and Zr4+/Ti4+-diffused array waveguide region. After the fabrication, the surface refractive indices at all the three parts were measured. On the surface part with waveguides, the measurements were carried out in the case that the light beam propagates along the direction perpendicular to strip waveguide axis so as to avoid the excitation of waveguide mode, which may affect the result. The refractive index was measured at the 1553 nm wavelength using a Metricon 2010 prism coupler, which has a working principle of measurement on critical angle of total reflection. Note that the index measured by this method should be the value at the crystal surface because the total reflection occurs there. For a Z-cut LN, one can choose a transverse magnetic/electric (TM/TE) polarization scheme to measure the extraordinary/ordinary refractive index.
The end-fire technique was used to observe the near-field pattern of guided mode. To facilitate the end-fire coupling, the sample endfaces were optically polished. Figure 1 shows schematic of the experiment. It was carried out by launching into the strip waveguide the polarized light emitted from a 1547 nm single-frequency laser via endface butt-coupling between a section of polarization-maintaining fiber and one waveguide. A polarization controller was used to control the polarization state. The magnified near-field image of guided mode was projected onto an infrared CCD camera through a 40 × microscope objective lens. The camera has a spectral response range 1440-1605 nm and an effective pixel number 1392 × 1040 with a pixel pitch 3.2/4.3 µm in lateral/vertical direction. To increase the spatial resolution, a lens with a larger numerical aperture of 0.65 was used.
The profile characteristics of diffused Zr4+ and Ti4+ ions were analyzed by SIMS. A time-of-flight secondary ion mass spectrometer (ToF SIMS V, ION-TOF GmbH) was used to analyze the surface Ti4+ profile and the depth profiles of 91Zr, 48Ti, 6Li, 93Nb and 16O. The surface profile was obtained by doing surface mapping with a raster size of 131 × 131 μm2. For the depth analysis, a Cs+-beam of 30 nA at 3 keV was used to sputter a crater of 120 × 120 μm2 on the waveguide surface and a pulsed bismuth ion beam (pulsed current: 1 pA, pulsed energy: 25 keV) was used to analyze the yields of secondary ions 6Li, 93Nb, 16O, 91Zr and 48Ti as a function of time. Positive secondary ions were detected. Ions from a central area of 10 (width) × 20 (length along waveguide axis) μm2 inside the erosion crater rastered on the strip waveguide were detected. During the analysis, a low-energy pulsed electron gun was used to neutralize the positive charges and hence degrade the surface charge accumulation. For the same purpose, before the analysis a 30-nm thick Ag film was coated on the sample surface to be analyzed. The trace and depth of each erosion crater were measured by a Tencor Alpha Step 200 profilometer. The depth resolution was mainly determined by the roughness of crater and is better than 5 nm in our case.
3. Results and discussion
For a conventional Ti:LN waveguide, which is usually fabricated by diffusion of 6-10-µm -wide, ~100-nm-thick Ti-strip at 1030-1060 °C for 9 h, the surface Ti4+ concentration is around 12 × 1020 ions/cm3 and the Ti4+-induced increment of no (ne) is ~0.006 (0.012) at the 1.5 µm wavelength. It is unclear if the Zr4+ diffusion-doping contributes to the LN index, and if so, if the contribution is comparable to the Ti4+-induced increment. It is thus crucial to know the Zr4+ doping effect on the LN index. The effect can be determined by comparing the index values measured at Zr4+-only doped and undoped surface parts. In the earlier work, we have studied it and concluded that the effect is within the error and is small compared to above-mentioned index increment of usual Ti:LN waveguide . Present work provides further evidence for this argument. The index measurement, which has an error of ± 1 × 10−3, shows that after Ti diffusion the no values are 2.2113 and 2.2110 at the Zr4+-only doped and undoped surface parts, respectively, and the ne values are 2.1374 and 2.1375, respectively. We can see that Zr4+ contribution to the LN index is on the order of 10−4 for both cases of no and ne, showing that Zr4+ doping has little effect.
Because Li out-diffusion usually accompanies ion diffusion into the LN crystal, it is essential to know the Li-compositions at the Zr4+-only doped and Ti-diffused surface parts of the studied sample. Here, the optical method of refractive index measurement was used to estimate the Li2O-contents at the undoped, Zr4+-only doped and waveguide array surface parts of studied sample on the basis of the Li2O-content-dependent birefringence , . The 10−3 error bar of index leads to 0.1 mol% Li2O-content uncertainty. For reference, estimation was also carried out for the as-grown plate from the measured index, no = 2.2112 and ne = 2.1374. As expected, it has a Li2O-content 48.6 ± 0.1 mol%, which is in good agreement with the nominal value. The Li2O-contents at the Zr4+-only doped and undoped surface parts are evaluated as 48.6 and 48.5 ± 0.1 mol%, respectively. Both are identical and similar to that of congruent plate, showing that Li2O out-diffusion is not measurable for the Zr4+-only doped and undoped surface parts.
It is impossible to accurately evaluate the Li2O-content on the surface of an individual strip waveguide on the basis of the index measurement by prism coupler. In principle, the prism-coupling method is also applicable to a strip waveguide. There is, however, a difficulty in getting accurate results with a commercial prism coupler, which is designed for the thin film and slab waveguide rather than the strip waveguide. Because the light spot is ~1 mm in diameter while the width of a strip waveguide is typically several micrometers only (8 μm here), the measured pattern of light reflected from sample surface is mainly contributed from the substrate surface instead of the narrow waveguide surface. In addition, the Ti4+ presence induces local index increase, making it impossible to obtain the correct Li composition information of strip waveguide. One has no choice but to evaluate it indirectly and roughly based on the index data measured from the whole surface part of waveguide array. As mentioned above, two adjacent 8-μm-wide strip waveguides separate by 200 μm. Such a large spacing results in that the area part of waveguide surface seen by the light spot is < 5%. Thus, the effect of strip waveguide presence on the result should be small. Based upon this consideration, we have done the measurement on the surface part of waveguide array. The results show that the refractive indices at the surface part of array waveguide region are no = 2.2110 and ne = 2.1386, yielding a Li2O-content of 48.4 ± 0.1 mol%, which has little difference from the values at the Zr4+-only doped and undoped surface parts, 48.6 and 48.5 ± 0.1 mol%, respectively. Thus, the array waveguide surface remains still the congruent composition.
It is concluded that Li2O out-diffusion is not serious for all the three surface parts of the sample studied here. This is possible from the point of view of charge equilibrium requirement. As we know, in the case of Ti4+ single diffusion in LN, Ti4+ enters into both Li and Nb sites. It is possible that the Li2O out diffusion is effectively suppressed as a large number of defects [1 mol% antisites (NbLi) and 4 mol% Li vacancies] ensure the charge equilibrium. The situation of Ti4+/Zr4+ co-doping is similar to that of Ti4+ single diffusion because both ions have the same chemical valence.
One can further conclude from above results and discussion that the refractive index increase in the waveguide is mainly contributed from the Ti4+ dopants while not from the Zr4+ dopants or the crystal composition reduction arising from Li2O out-diffusion.
The end-fire experiment shows that both the TE and TM modes are well guided. The waveguide is single-mode at the 1.5 μm wavelength. Figure 2(a) shows the morphology of the waveguide surface under a magnification of 1000 ×. Figure 2(b) shows the TE and TM mode patterns. Figures 2(c) and 2(d) show the TE-(red ball) and TM-mode (green balls) light intensity profiles along the width x and depth y directions of waveguide. The light intensity of the guided mode follows a Gauss function Axexp[-2(x/Wx)2] in the x direction and a Hermite-Gauss function Ayy2exp[-2(y/Wy)2] in the y direction. The black solid lines represent the Gauss or Hermite-Gauss fits to the experimental curves. As expected, the size of TE mode (Wx × Wy = 5.2 × 4.3 μm2) is larger than that of TM mode (Wx × Wy = 4.8 × 4.2 μm2) due to the weaker guiding for this polarization.
The waveguide loss was evaluated from the insertion loss measured at the 1547 nm wavelength. The fiber-to-fiber insertion loss of the 8-μm-wide, 1.5-cm-long waveguide was measured to be 6.0/6.6 dB under the TE/TM polarizations. From the obtained mode sizes, the coupling loss between the waveguide and a single-mode fiber (with a mode field diameter of 10.3 μm) is 1.9/2.0 dB for the TE/TM mode. The total reflection loss is 0.3 dB. With ignored etalon effect, the waveguide loss was conservatively evaluated as 1.3/1.5 dB/cm for the TE/TM mode. The loss is much larger than that of usual Ti:LN waveguide, which can be as low as 0.2 dB/cm , because of rough waveguide surface [see Fig. 2(a)] and the etalon effect, which cannot be determined accurately. Depending on the resonator length, the etalon effect gives rise to 0-2.0 dB loss in the resonator formed by the two waveguide endfaces and 0-0.5 dB loss in each air resonator formed by the endfaces of waveguide and fiber. The period of interference fringes, which is on the order of submicron, requires a measurement accuracy of at least a few tens of nanometer for both the waveguide and air resonator lengths. Such an accuracy cannot be attained. So, the above-mentioned waveguide losses are given as a result of conservative estimation.
The effective index of the TE/TM mode guided in the strip waveguide was further measured at the 1553 nm using the prism coupler. To achieve it, under the same condition we have fabricated an independent sample with a dense array of strip waveguides, which can be regarded as a quasi-planar waveguide and easily characterized using the prism coupler. The separation of two adjacent waveguides is 60 μm. One can see from Fig. 3(a) that the separation is large enough that the cross-talk does not occur. The measured mode index is given in Table 1 in comparison with substrate index.
Figure 3(a) shows the Ti4+ profile (magenta balls) on the waveguide surface. The data can be well fitted by a sum of two error functions. The fitting expression and parameter values are indicated (W is the initial Ti-strip width). The diffusion width dx is 9.0 ± 0.2 µm, yielding a lateral Ti4+ diffusivity 2.0 ± 0.1 μm2/h. Figure 3(b) shows the depth profiles of 6Li, 93Nb, 16O, 91Zr and 48Ti in the studied waveguide. One can see that both the Ti4+ and Zr4+ profiles can be well fitted by a Gaussian function, indicating that the diffusion reservoir, which is shared by Ti4+ and Zr4+, has been exhausted. The fitting expressions are indicated. The Ti4+- or Zr4+-concentration profile can be written as CTi or Zr(y) = C0·exp[-(y/dTi or Zr)2], where C0 is the surface Ti4+ or Zr4+ concentration and dTi (dZr) is the 1/e Ti4+ (Zr4+) diffusion depth that has a value of dTi = 5.3 ± 0.1 μm (dZr = 3.6 ± 0.1 μm), which yields a Ti4+ (Zr4+) bulk diffusivity of 0.70 ± 0.02 (0.16 ± 0.01) μm2/h. Note that Zr4+ diffuses much slower than Ti4+. It is due to the differences of two ions in ionic radius (0.7 Ǻ for Zr4+ and 0.6 Ǻ for Ti4+) and atomic mass (91 g/mol for Zr and 48 g/mol for Ti).
Based upon the law of mass conservation, the C0 is determined as (14.5 ± 0.6) × 1020 [(11.9 ± 1.0) × 1020] ions/cm3 for Ti4+ [Zr4+], equivalent to (7.8 ± 0.3) [(6.5 ± 0.3)] mol%.
Next, attention is paid to the optical-damage-resistant issue of the Ti:Zr:LN waveguide under study. To get an optical-damage-resistant waveguide, the waveguide layer must be completely covered by the Zr4+ profile and the Zr4+ concentration at the 1/e waveguide depth should be above the threshold of photorefractive damage, named Cth, which is around 2 mol%. The Zr4+ profile in the waveguide under study covers ~70% Ti4+ profile as dZr = 3.6 ± 0.1 μm and dTi = 5.3 ± 0.1 μm. The Zr4+ concentration at the 1/e Ti4+ concentration depth is C0/e = 2.4 mol%, which is around Cth. It is worthy to mention that the above-mentioned former requirement should be strictly described as the Zr4+ profile entirely covers the index profile instead of the Ti4+ profile. The reason is given below. For a Ti4+-diffused waveguide fabricated on a congruent LN, the 1/e depth of index change, named do for the ordinary ray and de for the extraordinary ray, is not identical to the dTi because the Ti4+-induced index change ∆no,e and the Ti4+ concentration CTi follows a nonlinear relationship with a power index αo = 0.53-0.55 ,  for the ordinary ray, and a near or exact linearity with a power index αe = 0.83  or 1.0  for the extraordinary ray. For the waveguide under study, the relationship should remain still because Zr4+-doping has less effect on the LN refractive index. So, to get an optical-damage-resistant waveguide, it requires dZr ≥ dTi/αo,e 1/2. For the studied waveguide, dZr should be at least dTi/αo 1/2 = 7.0 μm for the ordinary ray and dTi/αe 1/2 = 5.3 μm for the extraordinary ray. One can see that the practical Zr4+ profile (dZr = 3.6 μm) is definitely narrower than the required for both cases of ordinary and extraordinary rays. Instead, the nonlinearity between ∆no,e and CTi does not affect the requirement on the surface Zr4+ concentration, i. e. C 0 ≥ eCth.
Although the waveguide under study is not entirely covered by the Zr4+ profile, the Zr4+ profile shows that the depth where the Zr4+ concentration is above Cth can be extended from dZr ( = 3.6 μm) to 4.0 μm. Accordingly, the Zr4+ profile covers ~60% no profile and 70% ne profile. Moreover, one can see from Fig. 2(d) that the depth range of 0.0-4.0 μm, where the Zr4+ concentration is above Cth, covers majority of field distribution for both TE and TM modes, showing that the waveguide is optical-damage-resistant.
Note that the above discussion assumes that the threshold Cth has a sharp value. Actually, it does not have a sharp value as some other factors such as temperature and light intensity may take effect on it, besides the crystal composition. So, the relevant quantification given above is an approximate consideration and is only for reference. Future work should concentrate on the optimization of fabrication parameters to improve the resistance to the optical damage. The optimization is mainly based on the above-mentioned two requirements: dZr ≥ dTi/αo,e 1/2 and C 0 ≥ eCth. By applying the law of mass conservation to the Gaussian Zr4+ profile, one can reach the following equation for the initial ZrO2 thickness τ to be coated: τ = eπ 0.5 dZrCth/(2Cs), where Cs = ρNA/M with ρ, NA and M representing mass density of ZrO2 (3.864 g/cm3), Avogadro’s number and molecular weight of the ZrO2, respectively. On the basis of the 1/e waveguide depth do,e = dTi/αo,e 1/2, one can determine the d Zr and τ. With known Zr4+ diffusivity one can further design the Zr4+ diffusion time required.
Finally, there is an issue to be clarified. As we know, a proton-exchanged LN waveguide with not very low H+ concentration suffers from the degradation of electro-optic (EO) coefficient. This shortcoming does not occur for the Ti:LN waveguide, verified by some already developed devices such as phase modulator, mode-locked and Q-switch Ti:Er:LN waveguide lasers. Previous study has shown that bulk Zr4+-doping has less effect on the EO properties of LN . One can anticipate that the possibility is small that Zr4+ diffusion-doping affects the EO properties of Ti:LN waveguide noticeably.
We have demonstrated Ti:Zr:LN strip waveguide fabricated by Zr4+-diffusion-doping followed by Ti diffusion. It was shown that Zr4+-doping has little effect on LN index, Li2O out-diffusion is not measurable, and index increment in the Zr4+/Ti4+-codoped waveguide layer is mainly contributed from Ti4+ dopants. Optical studies show that the waveguide well supports both the TE and TM modes, is single-mode at the 1.5μm wavelength, and has a loss less than 1.3/1.5 dB/cm for the TE/TM mode. SIMS study reveals that 60% ordinary index profile and 70% extraordinary index profile are covered by the Zr4+ profile part having the Zr4+ concentration above the threshold of optical damage, implying that the waveguide is optical-damage-resistant.
This work was supported by the National Natural Science Foundation of China under Project nos. 61377060, 61077039 and 50872089, and by the Research Grants Council of the Hong Kong Special Administrative Region, China, under Project no. 11211014.
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