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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 7 — Apr. 4, 2005
  • pp: 2256–2262
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Investigation and analysis of a single-mode waveguide formed by multienergy-implanted LiNbO3

Fei Lu, Tingting Zhang, Gang Fu, Xuelin Wang, Keming Wang, Dingyu Shen, and Hongji Ma  »View Author Affiliations


Optics Express, Vol. 13, Issue 7, pp. 2256-2262 (2005)
http://dx.doi.org/10.1364/OPEX.13.002256


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Abstract

We demonstrate a single-mode waveguide in LiNbO3 by use of both prism and end-face coupling methods. The waveguide is formed by multienergy megaelectron-volt O2+ implantation at room temperature. A bright line is observed only when moderate postimplant annealing was performed. The possible index profile of the waveguide is constructed according to the damage profile of the lattice structure caused by the implantation. Although dark-mode measurement shows that the effective extraordinary index of LiNbO3 is raised in the waveguide layer, results of the analysis indicate that the single guiding mode could be supported by a synergetic effect from both the raised index layer and the low-index barrier. The reduced loss of the waveguide can be attributed to the widened low-index barrier from multienergy implantation.

© 2005 Optical Society of America

1. Introduction

2. Experimental results

Z-cut LN, with one polished upper surface and two polished end faces, were implanted by O2+ at room temperature. The energies of O2+ ions are 1.0, 1.5, and 2.0 MeV at doses of 1.2×1014, 1.5×1014, and 4×1014 ions/cm2, respectively. During implantation the LN samples were tilted 7O off normal to avoid channeling. After implantation, one sample was annealed in the ambient air at 250 °C for 30 min to remove the point defects and to decrease absorption loss. We then used prism coupling and end-face coupling to investigate the waveguide structure. A single narrow and deep dip in the dark-mode profile was observed by prism coupling for both as-implanted and annealed samples. The coexistence of a shallow and broad dip in the tail of the mode profile indicates that the waveguide exhibits some characteristics of a barrier-confined waveguide. Figure 1 shows the dark-mode profile obtained with prism coupling measurements (Model 2010, Metricon Corporation, Pennington, New Jersey). The inlet in the figure shows a comparison of the mode profile before and after annealing. The peak of the dip drifts slightly to the left after annealing, which means that the refractive index rises slightly after thermal annealing.

Fig. 1. Dark-mode profile obtained with prism coupling measurements.

End-face coupling was performed with a He-Ne laser at 633 nm. No detectable output was obtained from the as-implanted sample, which is attributed to the high absorption and scattering loss caused by the implantation process. A bright line was observed only in the annealed sample, which is shown in Fig. 2. Although the bright line does not show uniform brightness in the distribution, which is due mostly to inferior polish of the output end face of the sample, the brightness of the output line still indicates that the transmission efficiency in the waveguide is quite good.

Fig. 2. Bright line obtained by end-face coupling.

3. Analysis and discussion

Since the index profile in the waveguide cannot be constructed on the basis of one measured mode, we tried to describe the index profile with our theoretical model [8

8. K. E. Youden, S. W. James, R. W. Eason, P. J. Chandler, L. Zhang, and P. D. Townsend, “Photorefractive planar waveguides in BaTiO3 fabricated by ion-beam implantation,” Opt. Lett. 17, 1509–1511 (1992). [CrossRef] [PubMed]

]. According to the model, enhancement of the extraordinary index induced by the implantation can be a synthesized effect of four factors: degradation of spontaneous polarization, modification of molar polarization, molar volume change, and the optoelastic effect. Among these factors, spontaneous polarization dominates the increment of extraordinary index. Figure 3 is the lattice damage distribution in LN caused by O2+ implantation according to the transport of ions in matter code. Substituting known parameters of LN into parameters such as the electro-optic coefficient, the reconstructed extraordinary index profile is given in Fig. 4, where S is a variable parameter that represents the effect from the lattice damage. The results show that the extraordinary index profile is sensitive to the lattice damage. For a slightly damaged lattice (when the O2+ dose is low and S is small), the extraordinary index is enhanced in the whole guiding region, similar to the waveguide formed by indiffusion or the exchange method. As the ion dose increases, the index at the end of the O2+ track begins to decrease, mostly because the density of LN was diluted because of the aggregation of O2+ ions. When the O2+ dose is high enough, the index at the end of the ion track descends so significantly that it might be lower than that of the substrate. In this case, a low-index layer is formed to serve as an index barrier for a waveguide.

Fig. 3. Lattice damage profile in O2+ implanted LN.
Fig. 4. Reconstructed extraordinary index profile in a waveguide.

We now discuss the possible situation of the index profile in a waveguide. If the index profile in a waveguide can be assumed such as is the case in the first situation, where S is small (S<0.5) and the extraordinary index is raised throughout the ion range, such as that shown in Fig. 4, a hypothetic asymmetric slab waveguide might be a reasonable structure. An ideal model is shown in Fig. 5(a).

Fig. 5. Two models of extraordinary index profile in waveguides

According to waveguide theory, the cutoff and guidance conditions of the TM mode in an asymmetric dielectric slab waveguide are

β=k0n1sinθ=k0n2,
(1)
kd=mπ+tan1(n12pn22k)+tan1(n12qn02k),
(2)

where d is the depth of the waveguide; n 0, n 1, and n 2 are the refractive index in air, the guiding region, and the substrate, respectively; k 0 and k are the wave vectors in air and in the guiding region, respectively; and k=(k02n12 -β 2)1/2p=(β 2-k02n22 )1/2q=(β 2-k02n02 )1/2.

According to the measured TM mode, the effective extraordinary index is approximately 2.2116, which is larger than that of the LN substrate (2.2028). If we assume that n 0=1 (air), n 1=2.2150 (the index in the guiding region should be slightly larger than the measured effective extraordinary index; see the simulated result in Fig. 4), and n 2=2.2028 (LN substrate), by substituting m, n 0, n 1, n 2, and k into Eq. (2), the depth of a single-mode waveguide can be calculated at approximately 3λ, where λ=633nm. The result means that, if the waveguide is a complete index-raised layer, the depth of the guiding region should be at least 1.9µm. This value is much bigger than the simulated depth of the waveguide in Fig.4, which is only approximately 1.4µm.

If the second situation is reasonable (such as when S>1 in Fig. 4), the light is still confined between the surface (air) and the low-index barrier. If we assume that the index barrier is lower than that of a LN substrate by 0.5%, which is reasonable for most of the implanted waveguide with low dose implantation, the possible index profile can be described as Fig. 5(b). Because the barrier is thick enough compared with the depth of the waveguide, it is also reasonable to neglect the tunneling effect and to assume that an asymmetric slab is still available. In this case we determined that the depth of the waveguide is approximately 1.3λ, at approximately 0.82µm. The result is consistent with the simulated profile in Fig. 4, where the possible waveguide depth is approximately 0.8µm. The model in Fig. 5(b) is also supported by our waveguide measurement. If the waveguide has a structure such as that shown in Fig. 5(a), there should be no loss due to leakage in the waveguide. Broadening the index barrier would not have any effect on waveguide loss. However, we found that the bright output line can be obtained only in a multienergy implanted waveguide. By prism coupling a single-mode waveguide can also be found in the single energy implanted waveguide, but no bright line is observed. So the current low-loss waveguide can be attributed to the broadened index barrier, which plays a significant role in reducing leakage loss.

4. Summary

We have formed a single-mode waveguide in LiNbO3 by O2+ implantation at three different energies. We measured the waveguide by using both prism coupling and end-face coupling methods. We obtained a bright output line only in the annealed sample. The index profile in the waveguide is constructed according to our theory model. The analysis results indicate that the probable index profile in the waveguide should be a structure of the index-raised guiding region sandwiched between surface (air) and a low-index barrier. Since a broadened index barrier could prevent the light from leaking, a low-loss waveguide with an index-raised layer can be achieved by multienergy implantation. Because the implanted waveguide can be tailored by modulating the implantation parameters, the versatility of this method facilitates processes that are not available for conventional methods. In addition, the structure of an index-raised layer plus barrier confinement has another unique advantage in the fabrication of channel waveguides. Instead of conventional multistep implantation [11

11. B. Vincent, A. Boudrioua, J. C. Loulergue, P. Moretti, S. Tascu, B. Jacquier, G. Aka, and D. Vivien, “Channel waveguides in Ca4GdO(BO3)3 fabricated by He+ implantation for blue-light generation,” Opt. Lett. 28, 1025–1027 (2003). [CrossRef] [PubMed]

], the current method provides the possibility that channel waveguides can be fabricated simply by implantation.

Acknowledgments

This research is supported by the National Natural Science Foundation of China under grant 10375037.

References and Links

1.

A. Sjoberg, G. Arvidsson, and A. A. Lipovskii, “Characterization of waveguides fabricated by titanium diffusion in magnesium-doped lithium niobate,” J. Opt. Soc. Am. B 5, 285–291 (1988). [CrossRef]

2.

J. L. Jackel, C. E. Rice, and J. J. Veselka, “Proton exchange for high-index waveguides in LiNbO3,” Appl. Phys. Lett. 41, 607–608 (1982). [CrossRef]

3.

P. D. Townsend and L. Zhang, Optical Effects of Ion Implantation (Cambridge University Press, Cambridge, UK, 1994). [CrossRef]

4.

F. Lu, M.-Q. Meng, K.-M. Wang, X.-D. Liu, H.-C. Chen, and D.-Y. Shen, “Planar optical waveguide in Cu-doped potassium sodium strontium barium niobate crystal formed by megaelectron-volt He-ion implantation,” Opt. Lett. 22, 163–165 (1997). [CrossRef] [PubMed]

5.

M.-Q. Meng, F. Lu, K.-M. Wang, F.-X. Wang, W. Li, C.-Q. Ma, D.-H. Jiang, and Q.-M. Lu, “Property analysis of planar optical waveguide in NYAB formed by He+ ion implantation,” Electron. Lett. 33, 1045–1047 (1997). [CrossRef]

6.

E. E. Robertson, R. W. Eason, M. Kaczmarek, P. J. Chandler, and X. Huang, “Ion beam manipulation of the photorefractive properties of strontium barium niobate planar waveguides,” Opt. Lett. 21, 641–643 (1996). [CrossRef] [PubMed]

7.

L. Zhang, P. J. Chandler, and P. D. Townsend, “Detailed index profiles of ion-implanted wave-guides in KNbO3,” Ferroelectr. Lett. 11 (4), 89–97 (1990). [CrossRef]

8.

K. E. Youden, S. W. James, R. W. Eason, P. J. Chandler, L. Zhang, and P. D. Townsend, “Photorefractive planar waveguides in BaTiO3 fabricated by ion-beam implantation,” Opt. Lett. 17, 1509–1511 (1992). [CrossRef] [PubMed]

9.

H. Hu, F. Lu, F. Chen, B.-R. Shi, K.-M. Wang, and D.-Y. Shen, “Monomode optical waveguide in lithium niobate formed by MeV Si+ ion implantation,” J. Appl. Phys. 89, 5224–5226 (2001). [CrossRef]

10.

H. Hu, F. Lu, F. Chen, B.-R. Shi, K.-M. Wang, and D.-Y. shen, “Extraordinary refractive-index increase in lithium niobate caused by low dose ion implantation,” Appl. Opt. 40, 3759–3761 (2001). [CrossRef]

11.

B. Vincent, A. Boudrioua, J. C. Loulergue, P. Moretti, S. Tascu, B. Jacquier, G. Aka, and D. Vivien, “Channel waveguides in Ca4GdO(BO3)3 fabricated by He+ implantation for blue-light generation,” Opt. Lett. 28, 1025–1027 (2003). [CrossRef] [PubMed]

OCIS Codes
(130.3730) Integrated optics : Lithium niobate
(160.3130) Materials : Integrated optics materials
(230.7390) Optical devices : Waveguides, planar

ToC Category:
Research Papers

History
Original Manuscript: February 3, 2005
Revised Manuscript: March 6, 2005
Published: April 4, 2005

Citation
Fei Lu, Tingting Zhang, Gang Fu, Xuelin Wang, Keming Wang, Dingyu Shen, and Hongji Ma, "Investigation and analysis of a single-mode waveguide formed by multienergy-implanted LiNbO3," Opt. Express 13, 2256-2262 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-7-2256


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References

  1. A. Sjoberg, G. Arvidsson, and A. A. Lipovskii, �??Characterization of waveguides fabricated by titanium diffusion in magnesium-doped lithium niobate,�?? J. Opt. Soc. Am. B 5, 285�??291 (1988). [CrossRef]
  2. J. L.Jackel, C. E. Rice, and J. J. Veselka, �??Proton exchange for high-index waveguides in LiNbO3,�?? Appl. Phys. Lett. 41, 607 �??608 (1982). [CrossRef]
  3. P. D. Townsend and L. Zhang, Optical Effects of Ion Implantation (Cambridge University Press, Cambridge, UK, 1994). [CrossRef]
  4. F. Lu, M.-Q. Meng, K.-M. Wang, X.-D. Liu, H.-C. Chen, and D.-Y. Shen, �??Planar optical waveguide in Cu-doped potassium sodium strontium barium niobate crystal formed by megaelectron-volt He-ion implantation,�?? Opt. Lett. 22, 163�??165 (1997). [CrossRef] [PubMed]
  5. M.-Q. Meng, F. Lu , K.-M. Wang , F.-X. Wang, W. Li, C.-Q. Ma, D.-H. Jiang, and Q.-M. Lu, �??Property analysis of planar optical waveguide in NYAB formed by He+ ion implantation,�?? Electron. Lett. 33, 1045�??1047 (1997). [CrossRef]
  6. E. E. Robertson, R. W. Eason, M. Kaczmarek, P. J. Chandler, and X. Huang, �??Ion beam manipulation of the photorefractive properties of strontium barium niobate planar waveguides,�?? Opt. Lett. 21, 641�??643 (1996). [CrossRef] [PubMed]
  7. L. Zhang, P. J. Chandler, and P. D. Townsend, �??Detailed index profiles of ion-implanted wave-guides in KNbO3,�?? Ferroelectr. Lett. 11 (4), 89�??97 (1990). [CrossRef]
  8. K. E. Youden, S. W. James, R. W. Eason, P. J. Chandler, L. Zhang, and P. D. Townsend, �??Photorefractive planar waveguides in BaTiO3 fabricated by ion-beam implantation,�?? Opt. Lett. 17, 1509�??1511 (1992). [CrossRef] [PubMed]
  9. H. Hu, F. Lu, F. Chen, B.-R. Shi, K.-M. Wang, and D.-Y. Shen, �??Monomode optical waveguide in lithium niobate formed by MeV Si+ ion implantation,�?? J. Appl. Phys. 89, 5224�??5226 (2001). [CrossRef]
  10. H. Hu, F. Lu, F. Chen,B.-R. Shi, K.-M. Wang, and D.-Y. Shen, "Extraordinary refractive-index increase in lithium niobate caused by low dose ion implantation,�?? Appl. Opt. 40, 3759�??3761 (2001). [CrossRef]
  11. B. Vincent, A. Boudrioua, J. C. Loulergue, P. Moretti, S. Tascu, B. Jacquier, G. Aka, and D. Vivien, �??Channel waveguides in Ca4GdO(BO3)3 fabricated by He+ implantation for blue-light generation,�?? Opt. Lett. 28, 1025�??1027 (2003). [CrossRef] [PubMed]

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