Drawing of single-crystal and glass-clad lithium ... - IUCr Journals

research papers Journal of

Applied Crystallography ISSN 0021-8898

Received 22 September 2009 Accepted 24 November 2009

Drawing of single-crystal and glass-clad lithium tantalate fibers by the laser-heated pedestal growth method S. C. Pei,a T. S. Ho,a T. M. Tai,b L. M. Lee,b J. C. Chen,b A. H. Kung,c F. J. Kaod and S. L. Huanga* a

Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan, Department of Photonics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, cInstitute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan, and dInstitute of Biophotonics Engineering, National Yang Ming University, Taipei 112, Taiwan. Correspondence e-mail: [email protected] b

# 2010 International Union of Crystallography Printed in Singapore – all rights reserved

Lithium tantalate (LT) single-crystalline fibers with various growth conditions were obtained using the laser-heated pedestal growth (LHPG) method. The fibers can be produced with a diameter down to 70 mm and tens of centimetres in length by using different pulling rates. Preliminary experiments also showed that the LT fibers can be electrically poled for quasi-phase matching. To facilitate wave propagation with low loss, glass-clad LT fibers were fabricated by a codrawing LHPG method for the first time. A vacuum apparatus was developed to eliminate the bubbles from incongruent evaporation of lithium oxide during the drawing of the glass-clad LT fibers. The glass-clad fibers are classified into two different categories by the measurements of refractive index profiles. One is a step-index fiber and the other is a graded-index fiber. Both fiber types are multimode at present, but single-mode LT fibers could be fabricated with the high-precision control of the LHPG systems.

1. Introduction Ferroelectric materials have been widely used in nonlinear optics because periodically poled structures can be fabricated for the quasi-phase-matching (QPM) technique. Various kinds of manufacturing processes have been developed, such as ion implantation, Ti diffusion (Miyazawa, 1979), proton exchange (Zhu et al., 1994), and the light-induced (Chen, Lu & Xiao, 2009) and electric-field poling methods (Yamada et al., 1993). Lithium tantalate (LiTaO3, LT) crystals are especially attractive in QPM devices for green and blue light generation because of their high photorefractive damage resistance and low coercive field at near-stoichiometric LiTaO3 composition (Barns & Carruthers, 1970; Ganesamoorthy et al., 2005; Nakamura et al., 2002). Many research groups have developed well defined periodic domain walls and obtained high conversion efficiency for second-harmonic or optical parametric generations with dopants such as Mg and Zn ions (Bruner et al., 2004; Lee, 2003; Hatanaka et al., 2000). For instance, an LT QPM device with 1 mm thickness and 35 mm length in a 1.0 mol% MgO–near-stoichiometric LT crystal has demonstrated a singly resonant optical parametric oscillator with a slope conversion efficiency of 65% (Yu et al., 2004). In these demonstrations, the double-crucible Czochralski method is still adopted for LT single-crystal growth. The optical properties of LixTa1xO3 single crystals have been widely studied (Kim et al., 2001). In the morphology, the

48

doi:10.1107/S0021889809050547

distribution of LT growth ridges and facets on crystal surfaces has also been studied (Hou, 1980). The fiber structure can have a long length with a small mode size; therefore, there is the potential to lower the required pump intensity for efficient nonlinear frequency conversion. Although, up until now, there have been many reports on waveguide techniques and manufacturing processes (Mignotte, 2005; Tascu et al., 2003), none have used the laser-heated pedestal growth (LHPG) system to grow glass-clad LT fibers. In this paper, a co-drawing cladding technique for LT crystal fibers is demonstrated. The co-drawing LHPG method has many advantages for the preparation of crystal fibers, such as no contamination from crucibles and a wide range of temperature control. Compared with LiNbO3 (LN), LT has a lower Curie temperature and a higher photorefractive damage threshold (Foulon et al., 1995; Lee et al., 2005). However, because of the incongruent nature of LT, which results in the evaporation of Li2O, there are no reports to date on LT fibers < 800 mm in fiber diameter (Burlot et al., 1998). In this paper, we will first describe the fabrication of our small-diameter LT fiber such that electric-field poling is possible. A cladding technique similar to that used for yttrium aluminium garnet was adopted (Lo et al., 2004, 2005; Lin et al., 2006). Both fused-silica and Pyrex capillaries were tested for the LT cladding through an inter-diffusion process. With a vacuumed capillary technique, the bubbles between the LT fiber and capillary were eliminated, resulting in successful drawing of clad LT fibers. J. Appl. Cryst. (2010). 43, 48–52

research papers 2. Experimental The source rods were pure congruent x-cut LT crystals 0.5 0.5 mm in cross section and 40 mm in length. The growth chamber has a ZnSe window to support the inner cone that converts the CO2 laser to a donut-shaped beam as heat source, as shown in Fig. 1(a). The CO2 laser was also feedback controlled using the LabVIEW program (http://www.ni.com/ labview) to stabilize its power to within 0.5% fluctuation. In order to alleviate Li2O out-diffusion and evaporation, high growth speed was adopted. LT single-crystal fibers were fabricated with different core diameters as shown in Fig. 2. The molten zones were in the shape of a symmetric trapezoid once the equilibrium condition was achieved. Owing to its interfacial tension force in the molten zone, this shape will deviate as the growth condition changes. With a larger diameter reduction, as can be seen in Fig. 2, the contact angles at the air/melt/solid tri-junction became larger, while the molten zone length became shorter. This trend is inconsistent with the LHPG growth of a garnet single-crystal fiber (Chen et al., 2009). The smallest diameter we have achieved so far is about 70 mm with a growth power around 1.0 W. Fig. 1(b) shows the polished cross section of the LHPG-fabricated LT

and LN single-crystal fibers with a diameter of about 190 mm. The LT fiber has two crystal protuberances, which indicate a twofold symmetry (Hou, 1980). Because of its near-circular shape, LT has more potential in a wave-guided application than LN for mode matching with a single-mode fiber. To clad the LT single-crystalline fiber, a [100]-oriented crystal core with a diameter of 98 mm was inserted into a fused-silica or Pyrex capillary tube with inner diameter of 100 mm, and then the same LHPG system was used again to draw the LTinserted capillary tube. In this setup, it is essential to eliminate the bubbles from the gas mixture of air and an Li2O steam. Hence, we designed a low-vacuum facility as shown in Fig. 3 to solve this bubble problem. At a working pressure of around 100 Torr (1 Torr = 133.322 Pa), the bubbles can be eliminated. The softening points of the fused-silica and Pyrex capillaries are 1873 and 1093 K, respectively. The growth direction of both single crystals and clad fibers was along their x axes. The growth velocity was in the range 3–5 mm min1. The end facets of the glass-clad fibers were polished for optical measurement.

3. Results and discussion 3.1. Required CO2 power and periodic poling

Fig. 4 is a comparison of the required CO2 powers for LT and LN growth. Because of the difference in melting points between LT and LN, LT’s growth power is more than twice that of LN. Low growth power can be effective in reducing the Li2O out-diffusion during growth. A preliminary trial for in situ electric-field poling (Lee et al., 2005) is shown in Fig. 5 for a 190 mm-diameter LT fiber. The diameter reduction ratio is 3:1 from source materials to the grown LT fiber. The surface morphology of the LT periodical structure in the polished XY plane was obtained through second-harmonic confocal microscopy. A well defined 20 mm period on the XY plane can be seen on the 190 mm-diameter LT fiber. This method of fabricating LT QPM devices is fast and does not need a photolithography pattern. 3.2. Fused-silica cladding

Fused silica was used as the cladding in the co-drawing LHPG process because its softening point (1873–2273 K) is

Figure 2 Figure 1 (a) Illustration of the setup for the fabrication of an LT single-crystal fiber. (b) Polished cross-section image of the LT single-crystal fiber. J. Appl. Cryst. (2010). 43, 48–52

Images illustrate the melting zones of an LT single-crystal fiber with different diameters. Consecutive diameter reductions of source material to expected size.

S. C. Pei et al.

Drawing of single-crystal glass-clad lithium tantalate fibers

49

research papers fiber has a graded-index profile and can be applied to light guidance for fiber optic applications. Formation of the graded-index profile is due to the strong inter-diffusion between silica and LT because the softening point of fused silica is close to the melting point of an LT single crystal. The interdiffusion layer may serve as a buffer layer to mitigate the thermal strain induced by the large thermal expansion coefficient difference between LT and fused silica. Fig. 7(b) is the index profile of the same source rod fiber with a growth power that is 10% higher than that shown in Fig. 7(a). The peak index of 1.99 is slightly lower Figure 3 than that in Fig. 7(a). A relatively flat(a) Vacuum facility applied to the growth of glass-clad LT. (b) Images illustrate the formation process top core index was observed. This is an of LT glass cladding. indication that the LT-rich area has a smaller inter-diffusion rate than the close to the melting point of LT and its viscosity does not LT-deficient area, though the detailed mechanism is still to be change much with temperature (Knight et al., 1996). Fig. 6 elucidated. This also implies that we can control the amount of shows a bright-field cross-section image of the fused-silica-clad silica in the LT core by CO2 laser power, i.e. larger growth LT fiber made by the co-drawing LHPG method with core and power facilitates diffusion of more silica into the core. cladding diameters of 80 and 300 mm, respectively. To measure In order to examine the amount of silica in the core relative the wave-guiding property of the fiber, a 635 nm distributed to its silica cladding, energy-dispersive spectroscopy (EDS) feedback (DFB) laser-based confocal microscope was using an electron probe microanalyzer (Jeol JXA-8900R) was employed to map the index profile (Lo et al., 2002). We assume employed to examine the distribution of silicon in the core. a parabolic profile of the refractive index, n(r), at a distance r Although the EDS measurement cannot offer an absolute from the center, i.e. calibration of the silicon concentration, the line scan profile indicates the relative amount of silicon in the core and that in 2 nðrÞ ¼ no 1 ðA=2Þr ; ð1Þ the fused-silica cladding. As shown in Fig. 8, the amount of silicon in the core of a clad LT fiber with a growth power of where A is the gradient constant and no is the refractive index 1.40 W is half of that with 1.55 W growth power. The profile at the fiber center. Using Fig. 7(a), we find that no is 2.09 and the best fit for A is 2.39 104 for a fiber with 1.40 W growth power. Here no is less than the pure LT index, 2.17 at 635 nm. This implies that fused silica has entered the LT core. The clad

Figure 5 Figure 4 Comparison of the required CO2 powers for LT and LN growth.

50

S. C. Pei et al.

Domain period of a single LT crystal fiber with a diameter of 190 mm. Patterns are measured by second-harmonic confocal microscopy.


J. Appl. Cryst. (2010). 43, 48–52

research papers satisfies the form of diffusion equation in a pair of semi-infinite diffusion solids. 3.3. Pyrex cladding

Pyrex’s softening temperature is around 1073 K. Because of the large difference of the LT melting point and Pyrex’s softening point, the core of the Pyrex-clad fiber is still an LT single crystal. As confirmed by the refractive index profile measurement shown in Fig. 9, the 2.17 core index is comparable to the value of the bulk LT crystal. In the refractive index measurement, the power noise of the single-mode DFB laser is 0.5% and it results in an accuracy of n ’ 0.03 (Chen et al., 2005). After grinding and polishing, as shown in the inset of Fig. 9, cracks can be seen in the cladding, originating from the release of after-growth residual stress. The spatial distribution of cracks also indicates that the stress is typically concentrated near the two protuberances of the fiber. This implies that the two protuberant shapes in LT will lead to a stress-concentrated

Figure 6 End face image of the fused-silica-clad LT fiber.

Figure 8 Silicon line scan profiles of fused-silica-clad LT for different growth powers.

Figure 7 (a) Graded-index profile of a fused-silica-clad LT fiber. The maximum index of the core is 2.09 and 1.46 for the fused-silica cladding. (b) Gradedindex profile of a fused-silica-clad LT fiber. The maximum index of the core is 1.99 because of the higher growth power than in Fig. 7(a). J. Appl. Cryst. (2010). 43, 48–52

Figure 9 Index profile of a Pyrex-clad LT fiber. The maximum index is 2.17 measured by DFB laser with a wavelength of 635 nm. The inset is an optical microscopy image of a Pyrex-clad LT fiber.

S. C. Pei et al.


51

research papers area. On the other hand, the cracks in the Pyrex cladding were more severe than those in the fused-silica cladding as a result of the core index of LiTaO3–Pyrex cladding being closer to that of pure LT. This means that with the LiTaO3–Pyrexcladding structure it is easy to generate cracks during grinding and polishing. The numerical aperture (NA) of this waveguide calculated in the form NA = [no2 n(ro)2]1/2 is 1.60. This implies that all lights can be coupled in for any incident angles that satisfy the condition < nko, where and ko are the propagation constant parallel to the fiber axis and the free-space wavevector, respectively. A 1064 nm Yb fiber laser was used as the light source to measure the propagation loss of the glass-clad fibers. The absorption coefficient can be neglected because of the non-absorption of an LT crystal at this wavelength. By using the cutback method, the propagation loss that we obtained was 1.48 102 mm1, which is acceptable for various nonlinear optics applications. Some losses could be attributed to the two protuberances and the high index difference between the core and cladding. This Pyrex-cladding structure has the potential to be further integrated into multifunctional optical devices and optical integrated circuits.

4. Conclusion Glass cladding on LT crystal fibers was tried with fused silica and Pyrex through silica inter-diffusion with LT during the LHPG process. The fused-silica-clad fiber shows a gradedindex waveguide structure though the core is a mixture of LT and silica. The Pyrex-cladding process has nearly no interdiffusion as evident from the fact that the LT core maintains its single-crystalline structure, demonstrated by its shape and index profile. This Pyrex-clad LT fiber has a propagation loss of 1.48 102 mm1, and may be used in various nonlinear optics applications. The fused-silica-cladding process could form a double-cladding structure with a single-crystalline core under a very stringent growth condition tolerance.

References Barns, R. L. & Carruthers, J. R. (1970). J. Appl. Cryst. 3, 395–399.

52

S. C. Pei et al.

Bruner, A., Eger, D. & Ruschin, S. (2004). J. Appl. Phys. 96, 7445– 7449. Burlot, R., Ferriol, M., Moncorge, R. & Boulon, G. (1998). Eur. J. Solid State Inorg. Chem. 35, 1–8. Chen, J. C., Lo, C. Y., Huang, K. Y., Kao, F. J., Tu, S. Y. & Huang, S. L. (2005). J. Cryst. Growth, 274, 522–529. Chen, P. Y., Chang, C. L., Huang, K. Y., Lan, C. W., Cheng, W. H. & Huang, S. L. (2009). J. Appl. Cryst. 42, 553–563. Chen, Y., Lu, Z. & Xiao, M. (2009). J. Appl. Cryst. 42, 265–267. Foulon, G., Ferriol, M., Brenier, A., Cohen-Adad, M. T. & Boulon, G. (1995). Chem. Phys. Lett. 245, 555–560. Ganesamoorthy, S., Nakamura, M., Takekawa, S., Kumaragurubaran, S., Terabe, K. & Kitamura, K. (2005). Mater. Sci. Eng. B, 120, 125– 129. Hatanaka, T., Nakamura, K., Taniuchi, T., Ito, H., Furukawa, Y. & Kitamura, K. (2000). Opt. Lett. 25, 651–653. Hou, C. Y. (1980). J. Cryst. Growth, 50, 757–760. Kim, I. G., Takekawa, S., Furukawa, Y., Lee, M. & Kitamura, K. (2001). J. Cryst. Growth, 229, 243–247. Knight, J. C., Birks, T. A., Russell, P. St J. & Atkin, D. M. (1996). Opt. Lett. 21, 1547–1549. Lee, G. H. (2003). Solid State Commun. 128, 351–354. Lee, L. M., Kuo, C. C., Chen, J. C., Chou, T. S., Cho, Y. C., Huang, S. L. & Lee, H. W. (2005). Opt. Commun. 253, 375–381. Lin, Y. S., Lai, C. C., Huang, K. Y., Chen, J. C., Lo, C. Y., Huang, S. L., Chang, T. Y., Ji, J. Y. & Shen, P. (2006). J. Cryst. Growth, 289, 515– 519. Lo, C. Y., Huang, K. Y., Chen, J. C., Chuang, C. Y., Lai, C. C., Huang, S. L., Lin, Y. S. & Yeh, P. S. (2005). Opt. Lett. 30, 129– 131. Lo, C. Y., Huang, K. Y., Chen, J. C., Tu, S. Y. & Huang, S. L. (2004). Opt. Lett. 29, 439–441. Lo, C. Y., Huang, P. L., Chou, T. S., Lee, L. M., Chang, T. Y. & Huang, S. L. (2002). Jpn J. Appl. Phys. 41, 1228–1231. Mignotte, C. (2005). Nucl. Instrum. Methods Phys. Res. Sect. B, 229, 55–59. Miyazawa, S. (1979). J. Appl. Phys. 50, 4599–4603. Nakamura, M., Takekawa, S., Terabe, K., Kitamura, K., Usami, T., Nakamura, K., Ito, H. & Furukawa, Y. (2002). Ferroelectrics, 273, 199–204. Tascu, S., Moretti, P., Kostritskii, S. & Jacquier, B. (2003). Opt. Mater. 24, 297–302. Yamada, M., Nada, N., Saitoh, M. & Watanabe, K. (1993). Appl. Phys. Lett. 62, 435–436. Yu, N. E., Kurimura, S., Normura, Y., Nakamura, M., Kitamura, K., Takada, Y., Sakuma, J. & Sumiyoshi, T. (2004). Appl. Phys. Lett. 85, 5134–5136. Zhu, Y. Y., Zhu, S. N., Hong, J. F. & Ming, N. B. (1994). Appl. Phys. Lett. 65, 558–560.


J. Appl. Cryst. (2010). 43, 48–52