Open Access
Add to BibSonomy
Abstract
By using carbon ion implantation and rotating blade dicing, ridge waveguides have been produced in periodically poled MgO doped congruent LiNbO3 crystal. The guiding properties at wavelength of near-infrared waveband have been investigated. The quasi-phase-matched second harmonic generation in the ridge waveguides has been characterized. The depth profile of the d33 nonlinear coefficient in the implanted region has been evaluated by the reflected second-harmonic generation from angle-lapped samples.
© 2017 Optical Society of America
1. Introduction
LiNbO3 is one of the intensely studied ferroelectric crystals for optical guiding and nonlinear optical (NLO) signal processing due to its excellent electro-optical and nonlinear optical performance [1]. Structuring LiNbO3 in Micro and Nano-scale for integrated photonic applications has been an attracting theme during the last 10 years owing to the advancements of high quality etching for different integrated devices [2–4]. Among the LiNbO3 optical waveguide structures that have been reported, ridge waveguide structure provides the potential for highest conversion efficiency due to the tightest confinement. It would be strongly preferred in cases where high conversion efficiency and short waveguide length are required [5]. However, achieving ridges with smooth etching surface and high aspect ratio on LiNbO3 is a challenge. Many efforts have been dedicated to show the capability of HF acids for etching LiNbO3 [6–9]. As an alternative method, focused ion beam (FIB) milling has been utilized for the fabrication of arrays of photonic crystal structures [10–12]. However, this method is time-consuming and not appropriate to form layout with large area and deep etching depth. Techniques utilized in silicon integrated optics such as reactive ion etching (RIE), inductively coupled plasma reactive ion etching (ICP-RIE) have also been proposed for the etching of LiNbO3 [13,14]. So far, several methods have been investigated to form low-loss LiNbO3 ridge waveguides [15–17]. Another effective method is the optical grade dicing to structure LiNbO3 crystal which utilizes a rotating diamond blade [18–26]. Low loss ridge waveguide with high aspect ratio and smooth sidewall could be fabricated.
Before dicing the planar waveguide in lithium niobate can be formed by using diverse methods such as Ti-diffusion [17,18], proton exchange [19,20], smart cut [21], bonding/grinding [22–24] and ion implantation/irradiation [25,26]. As a well-known method to modify the surface properties of solids, ion implantation/irradiation has been utilized to form waveguide structures in many optical materials. Various functional integrated devices such as frequency converter [27], waveguide laser [28] and amplifier [29] have been demonstrated based on the ion implanted waveguides. Recently, carbon ion irradiation/implantation has been used to produce LiNbO3 ridge waveguides which preserve the NLO properties of LiNbO3 substrate to a reasonable extent [25,26]. Unlike the former ion implanted LiNbO3 waveguide, the carbon ion implanted waveguide shows a unique buried waveguide structure. The second harmonic generation (SHG) has been demonstrated by using the quasi-phase-matching (QPM) in the waveguide. However the effects of carbon ion implantation on the nonlinear coefficient d33 have not been investigated by direct measurement.
In this work, ridge waveguides with different widths were fabricated by the carbon ion implantation and rotating diamond blade dicing (optical grade dicing) on base of periodically poled LiNbO3 (PPLN). The formed ridge waveguides show excellent guiding and coupling performance. The SHG in the ridges together with the depth profiles of d33 coefficient are demonstrated.
2. Experiments in details
The z-cut 5% MgO doped periodically poled LiNbO3 (MgO:PPLN) wafer with periodically poled grating of 19.3 μm was bought from CQ Laser Technologies Co. (Nanjing, China). The poled grating was designed to fulfill QPM in bulk at wavelength of 1550 nm. The dimension of the wafer was 16 × 5 × 0.5 mm3. Before the implantation the upper surface of the sample was polished and cleaned to remove the etching grooves for examination of the poled grating.
2.1 Planar/ridge waveguide fabrication
The planar waveguide was prepared by the carbon ion implantations with two different energies (4 and 7.5-MeV) in one PPLN sample which is depicted in Fig. 1 (a). The beam flux was kept below 150 nA to prevent the charge build-up and heating. The fluence of both energies was 3 × 1014 ion/cm2. Another sample, x-cut congruent LiNbO3 (XLN) sample was implanted along with the PPLN sample. The energy deposition rate of carbon ion implantation was simulated by SRIM2008 Code [30].
Fig. 1 Fabrication process of the ridge waveguides and the optical characterizations. (a) Carbon ion implantation, (b) optical grade dicing, (c) end-face coupling setup and (d) geometry and orientation of the wedged x-cut LiNbO3 samples used for surface reflected SHG measurements.
The ridge formation process is depicted in Fig. 1(b). A resin blade bonded with diamond grains was chosen to dice the surface of the planar waveguide. The grit size and concentration of diamond grains embedded in the dicing saw were 0.5-1.0 μm and 21%, respectively. The rotation speed was set to 20,000 rpm with a feed speed of 0.1 mm/s. After dicing, the planar waveguide layer was structured with parallel air grooves, forming the ridge waveguides. The diced ridge waveguides exhibit widths from 5 μm to 17 μm. Before the performance characterizations, we observed all the ridges under the microscope. Selected for careful evaluation of optical performance were seven ridge waveguides owing to their being substantially free of dicing-induced cracks. The widths of these waveguides were 6.9 μm, 8.4 μm, 9.3 μm, 9.8 μm, 10.2 μm, 11.1 μm and 11.8 μm. The sample was then annealed at 300°C for 30 minutes in dry oxygen atmosphere. This thermal annealing process was used to remove the color centers generated by the carbon ion implantation and partially recover from the crystal lattice damage. The propagation loss could be reduced greatly at the same time. The two facets of the sample were polished in parallel before the forthcoming optical characterizations. The length of the sample was reduced to be 13 mm.
2.2 Linear/nonlinear optical characterization
The optical characterizations were performed with an end face coupling arrangement depicted in Fig. 1(c). A wavelength tunable semiconductor laser (TSL-210VF, Santec) was used as the pump. The whole tuning range of this laser was 1260 nm-1630 nm which was very suitable for our SHG measurements. The laser power before the fiber-waveguide coupling maintained 6.5 mW. A second laser, a visible LD at 657 nm was used as the guide for the coupling. During the optical measurements, the transmitted power of the ridge waveguides was detected by an InGaAs detector (OP-2 IR, Coherent) for pump and a Si detector (OP-2 VIS, Coherent) for SHG. A laser power meter (LabMax Top, Coherent) was used to record the transmitted power continuously. The mode pattern of the measured SHG light was collected by a CCD camera working at 400-1100 nm.
The Fabry-Pérot interference fringes were measured by varying the wavelength of the pump mildly in order to evaluate the propagation loss of the ridges [31,32]. The wavelength tuning step during the Fabry-Pérot fringe measurements was 0.002 nm. After the linear optical measurements, one facet of the PPLN waveguide was angle-polished to avoid the interference effect in the following nonlinear measurements. After angle polish the waveguide length ranged from 11.8 to 12.2mm. For the nonlinear performance measurements, the SHG signals were recorded simultaneously when the wavelength of the pump was tuning. During the SHG measurements, the temperature of the sample was kept at 23°C. The insets of the Fig. 1(c) show the picture of the end face of the ridge waveguide and the fiber-waveguide coupling. The mode profiles of the ridge waveguides were calculated by a Beam Propagation Method code packed in the Rsoft Suite [33].
2.3 Surface Refection SHG measurement
The d33 profile measurements were done on the XLN sample by a reflected SHG arrangement [34,35]. The implanted XLN samples were wedge-polished at an angle of 20 mrad (Fig. 1(d)). The 532 nm radiation from a Q-switched laser (DPS-532-A, Cnilaser) was focused onto the XLN surface to stimulate the reflected SH light. The repetition rate and the pulse width were 10 Hz and 10 ns, respectively. The energy of the pulse was attenuated to 5 μJ in order to avoid the optical damage to the sample surface. The photon energy of the 266 nm SHG is above the LiNbO3 band edge so that only the SH generated within a thin surface layer (about 50 nm) is observed. The focused spot of the pulsed laser was discretely scanned along the tilted XLN surface with a step of 10+/−0.25 μm, which equates to a depth resolution of 0.2 μm for the resulting d33 profile. The 266 nm SH signal reflected from different lateral position (corresponding to different depth) was detected by using a probing section comprised a dichroic mirror, a polarizer, a solid blind photomultiplier and a gated integrator (SR250 Boxcar, Stanford Research Systems, Inc.). The measured value of SH signal by the gated integrator was an average of 100 SH signal pulses from the photomultiplier. During the measurements the pump and SH fields were polarized parallel to the z axis of XLN samples so that d33 nonlinear coefficient of the surface region was coupled in the reflected SHG process.
3. Results and discussion
All ridge waveguides transmitted no detectable pump light upon end-face coupling measurements prior to thermal annealing treatment. Therefore all the following results are measured after the thermal annealing treatment.
Figure 2(a) shows the transmission spectra of the 9.8 μm-wide ridge waveguide. As we can see, the interference inside the ridge waveguide induces evident oscillation when the wavelength is tuning. Considering the facet-air reflectivity of 0.136 and waveguide length of 12.2 mm, the evaluated propagation loss of the ridge waveguides in MgO:PPLN was ~0.27 dB/cm, which is comparable to PPLN waveguides produced by other techniques [15,20,22]. The evaluated group refractive index is about 2.2078. In addition, the propagation losses of other 6 ridge waveguides were well below 0.6 dB/cm except the ridge with width of 6.9 μm. The transmission power of 6.9 μm-wide ridge as a function of pump wavelength was depicted in Fig. 2(b). It is found that the transmitted power under 1250nm was 4.15mW, which means a total insertion loss of 1.86 dB. However the transmitted power monotonically deceased to 1.55 mW when the pump wavelength was tuned to 1630 nm (insertion loss of 6.1 dB) which means that the present ridge waveguide cannot support mode guiding in long wavelength effectively. However the total insertion loss under wavelength smaller than 1350 nm is still quite low.
Fig. 2 (a) Fabry-Pérot interference fringes measured in 9.8 μm-wide ridge waveguide and (b) transmitted light power of 6.9 μm-wide ridge as a function of wavelength.
Figure 3(a) shows the pattern of conversion efficiency versus the wavelength of the pump light of 9.8 μm-wide ridge waveguide. The maximum of SHG was found at 1615.1 nm. The FWHM of the curve was about 1 nm. The left inset figure is the SHG mode pattern which shows that the fundamental SHG mode was phase matched. It is also noted that the SHG light generated in all the seven selected ridges show a single-mode pattern corresponding to QPM only between the fundamental modes of pump and SHG light. The figure inset shows the observed quadratic dependence of SHG power on the pump power in the same waveguide. Under a pump power of 4.5 mW, the present process generates a signal of 7.2 μW. Figure 3(b) shows the relation between the phase-matching (PM) wavelength and the width of the ridges. The PM wavelength decreases when the ridge width gets large. The PM wavelength decreases from 1629 nm to 1611 nm as the ridge width increases from 7 to 12 microns.
Fig. 3 (a) Experimental SHG tuning curve. The insets are the SHG mode pattern of 9.8 μm-wide ridge waveguide and the measured SHG power vs. pump power; (b) phase matching wavelength versus the ridge width.
Figure 4(a) shows the measured depth profiles of reflected 266 nm SH signal power (a.u.) which are normalized to the SH signal from bulk LiNbO3. The energy deposition profile was also depicted in the figure for guidance. For the as-implanted case, the SH signal of first peak diminish to 13% of the substrate while the value of the second peak is 20% of the substrate. The two dips locate at depth of 2.6 μm and 4.2 μm. The SH signal from surface region is about 60% which means that the electronic energy deposition could reduce the nonlinear susceptibility as well. Comparing to the energy deposition profile, we can conclude that the two dips in the d33 profiles were due to the nuclear energy deposition of 4 and 7.5-MeV carbon ion implantations. The energy deposition during the carbon ion implantation can cause a microscopic depolarization, and hence, a decrease of the nonlinear susceptibilities. After the thermal annealing, the SH power from surface region increased to 90% of the bulk value. The SH value of the two dips were also recovered to 49% and 67%, respectively. The results indicate that the thermal annealing could recover the nonlinear performance partially. It should also be noted that the d33 depth profiles can be calculated by taking the square root of the normalized reflected SH power given in Fig. 4(a). Figure 4(b) shows the reconstructed ne refractive index profile of the planar waveguide at 1615 nm. The refractive index profiles of the planar waveguide were reconstructed by the method mentioned in previous studies [25]. As we can see, two index enhanced region were found around the depth of 2.6 μm and 4.1 μm. Figure 4(c) and 4(d) show the calculated mode profiles of pump and SHG light of 9.8 μm-wide ridge waveguide. The two dimensional index profile of the cross section of the ridge waveguide used in the BPM calculation was constructed via the index profile depicted in Fig. 4(b).
Fig. 4 (a) Reflected 266 nm SH intensity which is normalized to that from bulk LiNbO3 versus the depth into the 4/7.5 MeV carbon ion implanted XLN waveguide before (blue line with circle symbol) and after the thermal annealing (red line with diamond symbol). The SRIM simulated nuclear energy depositions were also depicted in the figure (dotted lines); (b) refractive index profile of planar waveguide at 1615 nm; calculated mode profiles of (c) pump light (1615 nm) and (d) SHG light (807.5 nm) of 9.8 μm-wide ridge waveguide.
The depth profiles of the d33 coefficients could be used to determine the normalized conversion efficiencies (η) for guided modes in second order nonlinear interactions. Assuming that all the nonlinear interactions were first-order quasi-phase-matching so that the interactions were occurred between TM00ω and TM002ω, the η is given by
Where dQPM = 2d33bulk/π = 14.9 pm/V, ‾d33implanted is d33implanted/d33bulk, and the fields of pump and SH were normalized to unity power. The calculated normalized conversion efficiencies of the seven ridge waveguides were show in Fig. 5. The perfect cases with un-degraded d33 coefficient (d33 perfect) were also depicted in the figure. As we can see, the conversion efficiency decreased when the ridge width get large due to the larger waveguide cross-sectional area. The highest conversion efficiency is 30.4%/W·cm2 (6.9 μm-wide). The perfect cases possess higher conversion efficiency compared to the d33 degraded cases. A residual SH performance of 64% was obtained by comparing the d33 degradation effects to the perfect cases.
Fig. 5 Calculated normalized conversion efficiencies of different ridges with (red line) or without (blue line) the consideration of the d33 profile mentioned in Fig. 4.
4. Conclusion
We have demonstrated a waveguide SH generator at the L waveband of telecommunication. The waveguide was fabricated by carbon ion implantation and precise blade dicing. The demonstrated advancement to reasonably low propagation loss and substantial conversion efficiency creates potential for adoption in applications involving nonlinear optical processing. The depth profiles of second order nonlinear susceptibility of the carbon ion implanted waveguide were also investigated. The results indicate that the energy deposition induces the degradation of the nonlinear performance. However, thermal annealing can recover the crystal damage so that the nonlinear performance can be partially restored.
Acknowledgements
This research is supported by the National Natural Science Foundation of China (NSFC, grants 11375105, 11535008, 11405098 and 11505105) and the Opening Foundation of State Key Laboratory of Nuclear Physics and Technology of China. The authors would like to show their sincere gratitude to D. Kip and C. E. Rüter for their help of diamond blade dicing.
Disclosures
The authors declare no competing financial interests.
References and links
1. F. Chen, “Photonic guiding structures in lithium niobate crystals produced by energetic ion beams,” J. Appl. Phys. 106(8), 11 (2009).
2. P. Rabiei and P. Gunter, “Optical and electro-optical properties of submicrometer lithium niobate slab waveguides prepared by crystal ion slicing and wafer bonding,” Appl. Phys. Lett. 85(20), 4603–4605 (2004).
3. W. Sohler, H. Hu, R. Ricken, V. Quiring, C. Vannahme, H. Herrmann, D. Büchter, S. Reza, W. Grundkötter, S. Orlov, H. Suche, R. Nouroozi, and Y. Min, “Integrated optical devices in lithium niobate,” Opt. Photonics News 19(1), 24–31 (2008).
4. I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, and R. Ito, “Absolute scale of second-order nonlinear-optical coefficients,” J. Opt. Soc. Am. B 14(9), 2268–2294 (1997).
5. H. Hu, R. Ricken, and W. Sohler, “Lithium niobate photonic wires,” Opt. Express 17(26), 24261–24268 (2009). [PubMed]
6. P. Sivarajah, C. A. Werley, B. K. Ofori-Okai, and K. A. Nelson, “Chemically assisted femtosecond laser machining for applications in LiNbO3 and LiTaO3,” Appl. Phys., A Mater. Sci. Process. 112(3), 615–622 (2013).
7. L. Wang, J. H. Zhao, and G. Fu, “Ridged LiNbO3 waveguide fabricated by a novel wet etching/MeV oxygen ion implantation method,” J. Lightwave Technol. 28(9), 1344–1348 (2010).
8. R. Geiss, A. Sergeyev, H. Hartung, A. S. Solntsev, A. A. Sukhorukov, R. Grange, F. Schrempel, E. B. Kley, A. Tünnermann, and T. Pertsch, “Fabrication of free-standing lithium niobate nanowaveguides down to 50 nm in width,” Nanotechnology 27(6), 065301 (2016). [PubMed]
9. R. Geiss, S. Saravi, A. Sergeyev, S. Diziain, F. Setzpfandt, F. Schrempel, R. Grange, E. B. Kley, A. Tünnermann, and T. Pertsch, “Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation,” Opt. Lett. 40(12), 2715–2718 (2015). [PubMed]
10. F. Wang, W. Yuan, O. Hansen, and O. Bang, “Selective filling of photonic crystal fibers using focused ion beam milled microchannels,” Opt. Express 19(18), 17585–17590 (2011). [PubMed]
11. R. Geiss, S. Diziain, M. Steinert, F. Schrempel, E. B. Kley, A. Tünnermann, and T. Pertsch, “Photonic crystals in lithium niobate by combining focused ion beam writing and ion-beam enhanced etching,” Phys. Status Solidi., A Appl. Mater. Sci. 211(10), 2421–2425 (2014).
12. T. Kovalevich, A. Ndao, M. Suarez, S. Tumenas, Z. Balevicius, A. Ramanavicius, I. Baleviciute, M. Häyrinen, M. Roussey, M. Kuittinen, T. Grosjean, and M. P. Bernal, “Tunable Bloch surface waves in anisotropic photonic crystals based on lithium niobate thin films,” Opt. Lett. 41(23), 5616–5619 (2016). [PubMed]
13. G. Sun, T. Gao, X. Zhao, and H. Zhang, “Fabrication of micro/nano dual-scale structures by improved deep reactive ion etching,” J. Micromech. Microeng. 20(7), 075018 (2010).
14. G. Ulliac, B. Guichardaz, J. Y. Rauch, S. Queste, S. Benchabane, and N. Courjal, “Ultra-smooth LiNbO3 micro and nano structures for photonic applications,” Microelectron. Eng. 88(8), 2417–2419 (2011).
15. H. Hu, R. Ricken, and W. Sohler, “Low-loss ridge waveguides on lithium niobate fabricated by local diffusion doping with titanium,” Appl. Phys. B 98(4), 677–679 (2010).
16. V. Dobrusin, S. Ruschin, and L. Shpisman, “Fabrication method of low-loss large single mode ridge Ti:LiNbO3 waveguides,” Opt. Mater. 29(12), 1630–1634 (2007).
17. N. Courjal, F. Devaux, A. Gerthoffer, C. Guyot, F. Henrot, A. Ndao, and M. P. Bernal, “Low-loss LiNbO3 tapered-ridge waveguides made by optical-grade dicing,” Opt. Express 23(11), 13983–13990 (2015). [PubMed]
18. N. Courjal, B. Guichardaz, G. Ulliac, J. Y. Rauch, B. Sadani, H. H. Lu, and M. P. Bernal, “High aspect ratio lithium niobate ridge waveguides fabricated by optical grade dicing,” J. Phys. D Appl. Phys. 44(30), 305101 (2011).
19. J. Sun, Y. Gan, and C. Xu, “Efficient green-light generation by proton-exchanged periodically poled MgO:LiNbO3 ridge waveguide,” Opt. Lett. 36(4), 549–551 (2011). [PubMed]
20. J. Sun and C. Xu, “466 mW green light generation using annealed proton-exchanged periodically poled MgO: LiNbO3 ridge waveguides,” Opt. Lett. 37(11), 2028–2030 (2012). [PubMed]
21. M. F. Volk, S. Suntsov, C. E. Rüter, and D. Kip, “Low loss ridge waveguides in lithium niobate thin films by optical grade diamond blade dicing,” Opt. Express 24(2), 1386–1391 (2016). [PubMed]
22. M. Iwai, T. Yoshino, S. Yamaguchi, M. Imaeda, N. Pavel, I. Shoji, and T. Taira, “High-power blue generation from a periodically poled MgO:LiNbO3 ridge-type waveguide by frequency doubling of a diode end-pumped Nd:Y3Al5O12 laser,” Appl. Phys. Lett. 83(18), 3659–3661 (2003).
23. S. Kurimura, Y. Kato, M. Maruyama, Y. Usui, and H. Nakajima, “Quasi-phase-matched adhered ridge waveguide in LiNbO3,” Appl. Phys. Lett. 89(19), 191123 (2006).
24. T. Nishikawa, A. Ozawa, Y. Nishida, M. Asobe, F. L. Hong, and T. W. Hänsch, “Efficient 494 mW sum-frequency generation of sodium resonance radiation at 589 nm by using a periodically poled Zn:LiNbO3 ridge waveguide,” Opt. Express 17(20), 17792–17800 (2009). [PubMed]
25. L. Wang, C. E. Haunhorst, M. F. Volk, F. Chen, and D. Kip, “Quasi-phase-matched frequency conversion in ridge waveguides fabricated by ion implantation and diamond dicing of MgO:LiNbO3 crystals,” Opt. Express 23(23), 30188–30194 (2015). [PubMed]
26. L. Ai, L. Wang, Y. Tan, S. Akhmadaliev, S. Zhou, and F. Chen, “Efficient second harmonic generation of diced ridge waveguides based on carbon ion irradiated periodically poled LiNbO3,” J. Lightwave Technol. 35(12), 2476–2480 (2017).
27. Q. Huang, P. Liu, T. Liu, L. Zhang, Y. F. Zhou, and X. L. Wang, “Second harmonic generation in periodically poled LiNbO3 waveguides formed by oxygen-ion implantation,” Phys. Status Solidi Rapid Res. Lett. 6(5), 205–207 (2012).
28. Y. Yao, Y. Tan, N. Dong, F. Chen, and A. A. Bettiol, “Continuous wave Nd:YAG channel waveguide laser produced by focused proton beam writing,” Opt. Express 18(24), 24516–24521 (2010). [PubMed]
29. Y. Tan, Z. Shang, S. K. Vanga, A. A. Bettiol, and F. Chen, “High-gain optical waveguide amplifier based on proton beam writing of Nd:YAG crystal,” Opt. Express 23(11), 14612–14617 (2015). [PubMed]
30. J. F. Ziegler, J. P. Biersack, and U. Littmark, “Stopping and Range of Ions in Matter,” http://srim.org/, (2008).
31. R. Regener and W. Sohler, “Loss in low-finesse Ti: LiNbO3 optical waveguide resonators,” Appl. Phys. B 36(3), 143–147 (1985).
32. S. Taebi, M. Khorasaninejad, and S. S. Saini, “Modified Fabry-Perot interferometric method for waveguide loss measurement,” Appl. Opt. 47(35), 6625–6630 (2008). [PubMed]
33. Rsoft Design Group, Computer software BeamProp. [online]. Available: http://www.rsoftdesign.com/, (1994).
34. M. L. Bortz, L. A. Eyres, and M. M. Fejer, “Depth profiling of the d33 nonlinear coefficient in annealed proton exchanged LiNbO3 waveguides,” Appl. Phys. Lett. 62(17), 2012–2014 (1993).
35. D. Fluck, T. Pliska, M. Küpfer, and P. Günter, “Depth profile of the nonlinear optical susceptibility of ion-implanted KNbO3 waveguides,” Appl. Phys. Lett. 67(6), 748–750 (1995).
