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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 19 — Sep. 10, 2012
  • pp: 21114–21118
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Low-loss optical waveguides and Y-branch splitters in lithium niobate fabricated by MeV oxygen ions with low dose

Hui Hu, Fei Lu, Xue Lin Wang, Feng Chen, and Ke Ming Wang  »View Author Affiliations


Optics Express, Vol. 20, Issue 19, pp. 21114-21118 (2012)
http://dx.doi.org/10.1364/OE.20.021114


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Abstract

Single-mode optical waveguides in LiNbO3 substrate with loss as low as 0.17dB/cm were fabricated by a multi-energy low-dose ion implantation technology and cumulative annealing treatment. A waveguide Y-Branch splitter was demonstrated. Index profile in waveguide is described based on the ion implantation-induced damage profile, and propagation property in waveguide is simulated. Simulation results show a good consistence with the measured ones.

© 2012 OSA

In the present report channel waveguides with different widths and a Y-branch beam splitter with splitting ratio of 50:50 at 1.5µm are fabricated and characterized in x and z-cut LiNbO3. It is achieved by combining conventional photolithography pattern process and low dose O+ implantation, the same fabrication process as that in Si-semiconductor industry. By using a cumulative annealing treatment with stepwise temperature increment, the transmission loss of waveguide was reduced to 0.17dB/cm. The Y-branch beam splitter can provide equal splitting for both TM and TE mode. Y-branch splitter is a fundamental structure in many passive optical devices as well as in planar lightwave circuits (PLCs). The present Y-branch waveguide may achieve low-loss beam splitting function.

The x and z-cut LiNbO3 samples with the size of 10 × 20 × 1mm3 were firstly patterned on the surface with Cu mask by photolithography, forming open channels of width from 4 to 15μm for the ion irradiation. Then the implantation was carried out at room temperature. The experiments deal with the same dose of oxygen ions, which is around 8.5 × 1014 ions/cm2, but are carried out in two ways: one is a single implantation up to total dose with the same oxygen ion energy of 3.5 MeV; the other is a series of implantation step with ions in the sequence of energy of 4.0 MeV, 3.6 MeV and 3.0 MeV, accumulating to the same total dose. Thus, the formed waveguides has an only difference in the thickness of index barrier. The as-implanted samples were then measured using Metricon 2010 Prism Coupler at λ = 1.54 μm, and each of them showed only one distinct propagated TM mode. For TE mode, the m-line spectrum showed also a propagated mode but with poor light confinement. Since the waveguide properties depend only on the implanted-induced index profile, so the following results are all means on z-cut LiNbO3 samples with TM polarization. Each sample was then annealed in air ambient from 200°C to 280°C with temperature increment of 15~20°C every step, duration is from 20 min to 3 hours depending on the monitoring of the loss measurement.

Laser light with wavelength around 1.54 µm was edge-coupled into the waveguides by a 10 × objective lens. A 100 × 0.9 N.A. objective lens was used to collect the light out of the waveguide end face. A camera connected with a computer was used to record the near-field optical intensity pattern from end face of crystal. The propagation losses of the waveguides were measured using the Fabry-Perot resonance method at 1.54 µm wavelength. The sample is heated slowly to a temperature change of several degrees Celsius, and the transmitted light intensity is recorded. The loss is derived from the intensity contrast [13

13. R. Regener and W. Sohler, “Loss in low-finesse Ti:LiNbO3 optical waveguide resonators,” Appl. Phys. B 36(3), 143–147 (1985). [CrossRef]

].

The waveguides show different dark mode characteristics from that of waveguides formed by ion exchange or diffusion, as the latter guide the light due to the enhanced refractive index near sample surface. It is well known that the index profile in implanted waveguide has a typical “reduced index” layer, called “index barrier”, which acts as another waveguide boundary to confine light, shown in Fig. 1(a)
Fig. 1 (a) a schematic illustration of index profile in typical ion-implanted LiNbO3 waveguide; (b) the ne index profile in 7-µm width channel waveguide.
. For waveguide with no index profile, optical confinement is achieved between sample surface and the optical barrier. For ne, waveguide function comes from both the optical barrier and an increased index in waveguide region. In the present single-mode waveguide, the index profile in the waveguide by multi-energy ion implantation was reconstructed according to the damage profile, which is based on the theoretical model on index change in ion implant LiNbO3 [14

14. J. J. Yin, F. Lu, X. B. Ming, Y. J. Ma, and M. B. Huang, “Theoretical modeling of refractive index in ion implanted LiNbO3 waveguides,” J. Appl. Phys. 108(3), 033105–033109 (2010). [CrossRef]

], the index profile in channel waveguide and light propagation direction is shown in Fig. 1(b).

Absorption and scattering can give rise to loss in ion implanted waveguide due to implantation-induced lattice disorder. However, since the implantation dose is very low (~1014ions/cm2), induced defects can be very low as well. It can be expected that by applying an annealing treatment at moderate temperature, a reduced waveguide loss and a recovery of lattice order can be achieved simultaneously. Another loss factor is light leakage, because light power can tunnel through the index barrier and be radiated into the substrate. An effective method to prevent light from leaking through index barrier is to broaden the width of the barrier, which can be met simply by multi-energy ion implantation. It has been demonstrated in our experiment that the loss of waveguide by multi-energy implantation is almost half the value of those by single-energy ion implantation. By comparing the near field patterns of output lights from channel waveguides, a better optical confinement was observed in multi-energy implanted waveguide.

After appropriate annealing, one of the measured Fabry-Perot transmission resonances/fringes in channel waveguide is shown in Fig. 2
Fig. 2 (left) the measured Fabry-Perot transmission resonances/fringes from 7-µm channel waveguide; (right) the measured losses of waveguides with different waveguide width under annealing condition of 200°C, 20min; 250°C, 40min and 270°C min.
(left). The measured losses of waveguides with different waveguide width are shown in Fig. 2 (right). The lowest loss 0.17dB/cm comes from the 7-µm wide channel waveguide.

For analyzing and evaluating the waveguide characteristics, we measured and simulated the light propagation in the implanted channel waveguide, which has the index profile given in Fig. 1(b). Figure 3
Fig. 3 (left) the measured near field pattern of output light from a 7-µm-wide channel waveguide in z-cut LiNbO3;(right) the simulated transverse field profile of TM mode after light propagate at 2000μm.
(left) shows a measured near field pattern of output light from a 7-µm-wide channel waveguide. By using Beam Propagation Method, the simulated transverse field profile of TM mode after light propagates at 2000µm is given in Fig. 3 (right). The calculated field profile has a good consistence with the experimental result. Since the index profile of each channel waveguide has a gradual decrease in both side boundaries, the measured field intensity doesn’t show a sharp decline as in simulated results.

The waveguide loss of 0.17dB/cm is the best result among all of the single mode channel waveguides formed by implantation, which is entirely comparable with that of waveguide devices currently used in market. Although both TE and TM mode can be transmitted in the same waveguide simultaneously, it is apparent that TM mode has a much low waveguide loss. The result is consistence with our former analysis and is preferable to application, since in the z-cut LiNbO3 a larger optic-electric coefficient is available for TM mode. The low propagation loss and low material composition contamination (‘clean process’) will highlight the implantation as a competitive method in forming waveguide for integrated photonic applications.

A useful waveguide device is the beam splitter, which divides the optical power in an input waveguide equally and sends them to out waveguides. Actually, such a “Y-branch” 
structure is the basis of many optoelectronic devices, such as modulators and couplers. Similar implantation parameters were employed to fabricate Y-branch waveguide structure. The angular separation of two divergent channels is 1.1°, and the separation of two parallel branches is 30μm. The width of each channel is 13 μm. The near field pattern of output light from Y-branch structure is shown in Fig. 4(a)
Fig. 4 (a) and (b) are the measured output near field pattern from Y-branch structure; (c) and (d) are the simulated output near field pattern and the light propagation path (waveguide width in (d) is normalized to 2μm for illustration). In the simulation, waveguide width of 13μm, angular separation of two divergent channels of 1.1° and the separation of two parallel branches of 30μm are used.
and 4(b), the measured splitting ratio is about 50:50. It is clear that light energy was divided into two parts by the Y-branch and guided out. A similar calculation is done to simulate the light propagation in Y-branch and the transverse field profile of TM mode after light propagate at 4500µm is shown in Fig. 4 (c) and (d). Since the waveguide is multi-mode, so the amplitude oscillation in Fig. 4(d) is due to mode beating.

In conclusion, a multi-energy ion implantation technique is introduced to fabricate low-loss channel waveguides in LiNbO3 substrate. The ion implantation dose is very low (~1014ions/cm2), which will facilitate the application of ion implantation technique in integrated optics. The loss of the channel waveguides can be as low as 0.17dB/cm. A waveguide splitter is successfully fabricated. Splitting of light energy is observed.

Acknowledgments

The authors thank Prof. W. Sohler of University of Paderborn for providing the loss measurement setup. This work was supported by the National Natural Science Foundation of China (Grant No. 10735070 and No.61144001).

References and links

1.

R. Th. Kersten and H. Boroffka, “A strip-waveguide light-distribution structure made by ion implantation into fused quartz,” Opt. Quantum Electron. 8(3), 263–266 (1976). [CrossRef]

2.

G. L. Destefanis, P. D. Townsend, and J. Gailliard, “Optical waveguides in LiNbO3 formed by ion implantation of helium,” Appl. Phys. Lett. 32(5), 293–294 (1978). [CrossRef]

3.

T. Bremer, W. Heiland, B. Hellermann, P. Hertel, E. Krätzig, and D. Kollewe, “Waveguides in KNbO3 by He+ implantation,” Ferroelectr. Lett. 9(1), 11–14 (1988). [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 mega-electron-volt He-ion implantation,” Opt. Lett. 22(3), 163–165 (1997). [CrossRef] [PubMed]

5.

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

6.

F. Chen, X.-L. Wang, and K.-M. Wang, “Development of ion-implanted optical waveguides in optical materials: A review,” Opt. Mater. 29(11), 1523–1542 (2007). [CrossRef]

7.

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(22), 3759–3761 (2001). [CrossRef] [PubMed]

8.

X. Ming, F. Lu, J. Yin, M. Chen, S. Zhang, X. Liu, Z. Qin, and Y. Ma, “Optical confinement achieved in ZnO crystal by O+ ions implantation: analysis of waveguide formation and properties,” Opt. Express 19(8), 7139–7146 (2011). [CrossRef] [PubMed]

9.

G. Vázquez, J. Rickards, G. Lifante, M. Domenech, and E. Cantelar, “Low dose carbon implanted waveguides in Nd:YAG,” Opt. Express 11(11), 1291–1296 (2003). [CrossRef] [PubMed]

10.

L. Mutter, M. Jazbinsek, C. Herzog, and P. Günter, “Electro-optic and nonlinear optical properties of ion implanted waveguides in organic crystals,” Opt. Express 16(2), 731–739 (2008). [CrossRef] [PubMed]

11.

G. G. Bentini, M. Bianconi, M. Chiarini, L. Correra, C. Sada, P. Mazzoldi, N. Argiolas, M. Bazzan, and R. Guzzi, “Effect of low dose high energy O3+ implantation on refractive index and linear electro-optic properties in X-cut LiNbO3: Planar optical waveguide formation and characterization,” J. Appl. Phys. 92(11), 6477–6483 (2002). [CrossRef]

12.

X.-L. Wang, K.-M. Wang, F. Chen, G. Fu, S.-L. Li, H. Liu, L. Gao, D.-Y. Shen, H.-J. Ma, and R. Nie, “Optical properties of stoichiometric LiNbO3 waveguides formed by low dose oxygen ion implantation,” Appl. Phys. Lett. 86(4), 041103–041104 (2005). [CrossRef]

13.

R. Regener and W. Sohler, “Loss in low-finesse Ti:LiNbO3 optical waveguide resonators,” Appl. Phys. B 36(3), 143–147 (1985). [CrossRef]

14.

J. J. Yin, F. Lu, X. B. Ming, Y. J. Ma, and M. B. Huang, “Theoretical modeling of refractive index in ion implanted LiNbO3 waveguides,” J. Appl. Phys. 108(3), 033105–033109 (2010). [CrossRef]

15.

A. Majkić, M. Koechlin, G. Poberaj, and P. Günter, “Optical microring resonators in fluorineimplanted lithium niobate,” Opt. Express 16(12), 8769–8779 (2008). [CrossRef] [PubMed]

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(130.3730) Integrated optics : Lithium niobate
(230.7380) Optical devices : Waveguides, channeled

ToC Category:
Integrated Optics

History
Original Manuscript: July 12, 2012
Revised Manuscript: August 27, 2012
Manuscript Accepted: August 27, 2012
Published: August 30, 2012

Citation
Hui Hu, Fei Lu, Xue Lin Wang, Feng Chen, and Ke Ming Wang, "Low-loss optical waveguides and Y-branch splitters in lithium niobate fabricated by MeV oxygen ions with low dose," Opt. Express 20, 21114-21118 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-19-21114


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References

  1. R. Th. Kersten and H. Boroffka, “A strip-waveguide light-distribution structure made by ion implantation into fused quartz,” Opt. Quantum Electron.8(3), 263–266 (1976). [CrossRef]
  2. G. L. Destefanis, P. D. Townsend, and J. Gailliard, “Optical waveguides in LiNbO3 formed by ion implantation of helium,” Appl. Phys. Lett.32(5), 293–294 (1978). [CrossRef]
  3. T. Bremer, W. Heiland, B. Hellermann, P. Hertel, E. Krätzig, and D. Kollewe, “Waveguides in KNbO3 by He+ implantation,” Ferroelectr. Lett.9(1), 11–14 (1988). [CrossRef]
  4. 4F. 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 mega-electron-volt He-ion implantation,” Opt. Lett.22(3), 163–165 (1997). [CrossRef] [PubMed]
  5. P. D. Townsend and L. Zhang, Optical Effects of Ion Implantation (Cambridge University Press, 1994).
  6. F. Chen, X.-L. Wang, and K.-M. Wang, “Development of ion-implanted optical waveguides in optical materials: A review,” Opt. Mater.29(11), 1523–1542 (2007). [CrossRef]
  7. 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(22), 3759–3761 (2001). [CrossRef] [PubMed]
  8. X. Ming, F. Lu, J. Yin, M. Chen, S. Zhang, X. Liu, Z. Qin, and Y. Ma, “Optical confinement achieved in ZnO crystal by O+ ions implantation: analysis of waveguide formation and properties,” Opt. Express19(8), 7139–7146 (2011). [CrossRef] [PubMed]
  9. G. Vázquez, J. Rickards, G. Lifante, M. Domenech, and E. Cantelar, “Low dose carbon implanted waveguides in Nd:YAG,” Opt. Express11(11), 1291–1296 (2003). [CrossRef] [PubMed]
  10. L. Mutter, M. Jazbinsek, C. Herzog, and P. Günter, “Electro-optic and nonlinear optical properties of ion implanted waveguides in organic crystals,” Opt. Express16(2), 731–739 (2008). [CrossRef] [PubMed]
  11. G. G. Bentini, M. Bianconi, M. Chiarini, L. Correra, C. Sada, P. Mazzoldi, N. Argiolas, M. Bazzan, and R. Guzzi, “Effect of low dose high energy O3+ implantation on refractive index and linear electro-optic properties in X-cut LiNbO3: Planar optical waveguide formation and characterization,” J. Appl. Phys.92(11), 6477–6483 (2002). [CrossRef]
  12. X.-L. Wang, K.-M. Wang, F. Chen, G. Fu, S.-L. Li, H. Liu, L. Gao, D.-Y. Shen, H.-J. Ma, and R. Nie, “Optical properties of stoichiometric LiNbO3 waveguides formed by low dose oxygen ion implantation,” Appl. Phys. Lett.86(4), 041103–041104 (2005). [CrossRef]
  13. R. Regener and W. Sohler, “Loss in low-finesse Ti:LiNbO3 optical waveguide resonators,” Appl. Phys. B36(3), 143–147 (1985). [CrossRef]
  14. J. J. Yin, F. Lu, X. B. Ming, Y. J. Ma, and M. B. Huang, “Theoretical modeling of refractive index in ion implanted LiNbO3 waveguides,” J. Appl. Phys.108(3), 033105–033109 (2010). [CrossRef]
  15. A. Majkić, M. Koechlin, G. Poberaj, and P. Günter, “Optical microring resonators in fluorineimplanted lithium niobate,” Opt. Express16(12), 8769–8779 (2008). [CrossRef] [PubMed]

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