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The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser
Applied Surface Science 254 (2007) 1101–1104 www.elsevier.com/locate/apsusc
The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser S.W. Kwon a, W.S. Yang b, H.M. Lee b, W.K. Kim b, H.-Y. Lee b, W.J. Jeong c, M.K. Song d, D.H. Yoon a,* a
School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea b Nano Bio-photonics Team, Korea Electronics Technology Institute, Bundang 463-816, Republic of Korea c School of Electrical and Computer Engineering, University of Seoul, Seoul 412-791, Republic of Korea d Department of Materials Engineering, Hankuk Aviation University, Goyang 412-791, Republic of Korea Available online 23 August 2007
Abstract Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxidedoped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 mm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 2.83% and demonstrated secondharmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved. # 2007 Elsevier B.V. All rights reserved. Keywords: Periodically poled magnesium lithium niobate; Ridge waveguide; Second-harmonic generation; Duty cycle
1. Introduction Efficient wavelength conversion devices, such as secondharmonic generation (SHG) and optical parametric oscillation (OPO), can be achieved by periodically poled domain inversion to obtain quasi-phase-matching (QPM) to propagate waves in the crystal [1,2]. Among many other advantages, the QPM process makes it possible to use the largest non-linear coefficient, d33, of the materials for the interaction between Z-polarized fields. It has been reported that non-doped periodically poled lithium niobate (PPLN) suffers from photorefractive damage at room temperature. Recently, MgO-doped LiNbO3 (MgO:LN) was proposed as an alternative due to its higher resistance to photorefractive damage and its large non-linear optical coefficient [3,4]. Waveguide fabrication has been applied mainly by Ti indiffusion [5] or annealed proton exchange (APE) [6]. In the case of a Ti in-diffused waveguide, although low propagation
* Corresponding author. Tel.: +82 31 290 7361; fax: +82 31 290 7371. E-mail address: [email protected] (D.H. Yoon). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.060
losses are obtainable, the waveguide exhibits a low conversion efficiency due to its weak confinement and photorefractive damage. APE waveguide devices that achieve strong confinement produce asymmetric mode profiles at corresponding wavelengths in the wavelength conversion, reducing modal overlap due to the mismatch of the mode peak positions. Ridge waveguide devices have been investigated in an attempt to resolve these problems. The advantages of ridge waveguide devices are their symmetric mode profiles and strong light confinement due to their step index profile. Moreover, this type of waveguide suffers neither degradation of its non-linear coefficient nor photorefractive damage. It is expected that a high conversion efficiency can be achieved by using ridge waveguide structures in QPM waveguide devices [7]. In this investigation, MgO-doped lithium niobate, which has a domain period of 6.8 mm, was fabricated by applying an external field and a specially designed ridge waveguide was fabricated by a precision lapping machine and neutron loop discharge (NLD) using periodically poled magnesium-doped lithium niobate (PPMgLN). The PPMgLN ridge waveguide was observed by optical microscope, scanning electron microscope and demonstrated SHG.
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2. Experimental procedure Z-cut, 500 mm thick and 3 in. diameter MgO-doped (4.9 mol%) LiNbO3 wafers were purchased from Crystal Technology Inc., USA. The wafers were surface cleaned by sonication using a solution mixture of acetone, NH4OH:H2O:H2O2 and HCl:H2O:H2O2. The Z face of the wafer was chemical–mechanically polished down to a thickness of 150 mm to minimize the electric field during the poling. A 6.8 mm period photoresist (PR) with a thickness of 1.5 mm thickness was deposited on the +Z face of LN. Post-baking was performed on the +Z faces at 140 8C. After slicing the wafers to dimensions of 30 mm 10 mm, the specimen was placed into the poling jig connected to the circuitry, as shown elsewhere. Poling was achieved by applying an external electric field [9,10]. Ridge waveguides were fabricated using a precision lapping machine and neutron loop discharge in PPMgLN. Fig. 1 shows a schematic of the NLD process used to the dry-etching of the PPMgLN ridge waveguide during the dry-etching process. In this equipment, a vacuum chamber made of a quartz crucible with a diameter of 300 mm is co-axially surrounded by three magnetic coil rings, with an RF antenna located near the central one. A neutral loop is created in the vacuum chamber in the same plane as the RF antenna, which generates an RF frequency of 13.56 MHz. The RF power generated at the RF antenna affects the plasma density, while keeping the electron temperature constant. Variations in the power of the central magnetic coil can control the diameter of the neutral loop and various etching characteristics. The NLD equipment used to dry-etch the ridge waveguide was used at a bias power of 300 J/ s and the C3F8/Ar ratio, antenna power and chamber pressure were fixed at 10/90, 600 J/s and 0.33 Pa, respectively [11].The second-harmonic spectrum was measured along the crosssection (X-direction) of the PPMgLN ridge waveguide with a domain-inverted period of 6.8 mm. The SHG experiment was performed using an laser diode (LD) with a wavelength of 1064 nm and a heater (Fig. 2).
Fig. 2. Experimental set-up for the measurement of the SH power.
3. Results and discussion An optical photograph of the etched domain-inverted structure on the +Z face of the PPMgLN with a domaininverted period of 6.8 mm is shown in Fig. 3. It was noted that no periodic deterioration of the +Z face or the domain damage was detected for the PPMgLN with a short domain-inverted period. To determine the duty cycle of the domain-inverted structure, the domain inverted widths on the +Z face were measured using an optical microscope. The duty cycle was estimated to be 51.9 2.83%. The phase-matching wavelength and the efficiency of a QPM device are mainly governed by the periodicity and 50% duty cycle of the domain-inverted structures. Maximum conversion efficiency can be achieved for a perfectly uniform duty cycle of 50% of the domaininverted structure [3,6,8]. It was found that the average duty cycle of the PPMgLN with a short domain-inverted period of 6.8 mm was close to the optimum value of 50%. Therefore, the
Fig. 3. (a–k) Etched +Z surface shape of fabricated PPMgLN with 6.8 mm period.
Fig. 1. A schematic of the NLD used for the PPMgLN ridge waveguide dryetching process.
Fig. 4. Process of PPMgLN ridge waveguide structure fabrication.
S.W. Kwon et al. / Applied Surface Science 254 (2007) 1101–1104
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Fig. 5. (a–c) Scanning electron microscope (SEM) and optical images of cross-section (X-axis) of fabricated PPMgLN ridge waveguide.
PPLN with a domain-inverted period of 6.8 mm is likely to be applicable to QPM devices. Fig. 4a–d shows the poling process with MgO-doped lithium niobate. The ridge waveguide fabrication is described in Fig. 4e– k. PPMgLN and Z-cut 1 mm thick congruent LN were attached by epoxy adhesive (Fig. 4e), and PPMgLN was polished down to a thickness of 10 mm by means of a lapping machine and a chemical–mechanical polisher, with a thickness deviation of 0.5 mm. As a dry-etching mask, Ni metal with a thickness of about 3 mm thick was evaporated on the sample by means of an E-beam evaporator, and the Ni-mask used to dry-etch the ridge waveguide during dry-etching was fabricated by a photolithography process and Ni-plating (Fig. 4f–i). The ridge waveguide was etched by NLD with C3F8/Ar gas (Fig. 4j and k). The PPMgLN ridge waveguide etched at a bias power of 300 J/s for 3600 s showed an etching depth of 6.5 mm and an etching angle of around (70p/180) rad. Because the etched side wall has a slope of around (70p/180) rad, the electron density is higher in the etching edge and the etching speed was slightly higher at the edge corner than in the bottom plane [11]. Fig. 5a–c shows the scanning electron microscope (SEM) and optical
images of the cross-section (X-axis) of the fabricated PPMgLN ridge waveguide. The width of the ridge waveguide was different at the top and bottom planes (Fig. 5b and c). In the case of the dryetching mask with a width of 3 mm, the widths at the top and bottom were 3 and 10 mm, respectively. This experimental result shows the effects of electron density and etching speed. In the ridge waveguide process, the size difference between the +X and X faces was generated by the lapping machine, chemical– mechanical polisher and NLD. This deteriorates conversion efficiency of the sample. This issue is currently being studied and will be addressed in a future paper. The dependence of the second-harmonic (SH) power on the SHG device temperature is shown in Fig. 6. The ridge waveguide height was fixed at 10 0.5 mm, and the width ranged of top and bottom was 3 0.5 and 10 0.5 mm, respectively, and the interaction length was 12 mm. This result showed that the largest output power at a wavelength of 532 nm was obtained at around 79 8C. The power of fundamental wavelength was 40 mW at a wavelength of 1064 nm and SH power was 0.2 mW at a wavelength of 532 nm. The low SH power was affected by the non-uniform waveguide size. This issue, which is currently being studied, will be also improved soon. The full width at half maximum (FWHM) phasematching bandwidth in terms of the device temperature deviation was 2.2 8C. 4. Conclusion
Fig. 6. The dependence of the SH power on the SHG device temperature.
The use of the ridge waveguide process to obtain symmetric mode profiles and strong light confinement was investigated by means of a poling process and dry-etching. MgO-doped lithium niobate with a domain period of 6.8 mm was fabricated by the application of an external field. The PPMgLN ridge waveguide was fabricated by means of a precision lapping machine and NLD using periodically poled magnesium-doped lithium niobate (PPMgLN). The experimental results demonstrate that the average duty cycle of PPMgLN was 51.9 2.83%. A PPMgLN ridge waveguide with dimensions of 3 mm (top plane) and 10 mm (bottom plane) width was fabricated by the dry-etching process.
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The analysis of the dependence of the SH power on the SHG device temperature confirmed that the largest output power at a wavelength of 532 nm was obtained at around 79 8C and the FWHM was 2.2 8C at an interaction length of 12 mm. By using an optimized period, a 50% duty cycle and precise fabrication of the devices, a high conversion efficiency can be obtained by using a PPMgLN ridge waveguide at room temperature. References [1] A. Bruner, D. Eger, J. Appl. Phys. 96 (2004) 7445. [2] T. Hatanaka, K. Nakamura, T. Taniuchi, H. Ito, Y. Furukawa, K. Kitamura, Appl. Phys. Lett. 25 (2000) 651.
[3] B. Vincent, A. Boudrioua, R. Kremer, P. Moretti, Opt. Commun. 247 (2005) 461. [4] Y. Cho, S. Kazuta, H. Ito, Appl. Phys. Lett. 79 (2001) 2955. [5] Y.L. Lee, H. Suche, Y.H. Min, J.H. Lee, W. Grundko¨tter, V. Quiring, W. Sohler, IEEE Photonics Technol. Lett. 15 (2003) 978. [6] M.L. Bortz, M.M. Fejer, Opt. Lett. 16 (1991) 1844. [7] S. Kurimura, Y. Kato, M. Maruyama, Y. Usui, H. Nakajima, Appl. Phys. Lett. 89 (2006) 191123. [8] Y. Kong, J. Xu, B. Li, S. Chen, Z. Huang, L. Zhang, S. Liu, W. Yan, H. Liu, X. Xie, L. Shi, X. Li, G. Zhang, Opt. Mater. 27 (2004) 471. [9] S.W. Kwon, Y.S. Song, W.S. Yang, H.M. Lee, W.K. Kim, H.Y. Lee, D.Y. Lee, Opt. Mater. 29 (2007) 923. [10] S.W. Kwon, Y.S. Song, W.S. Yang, H.M. Lee, W.K. Kim, H.Y. Lee, B.Y. Kim, M.H. Lee, D.Y. Lee, Thin Solid Films 515 (2006) 922. [11] W.S. Yang, H.-Y. Lee, W.K. Kim, D.H. Yoon, Opt. Mater. 27 (2005) 1642.
The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser S.W. Kwon a, W.S. Yang b, H.M. Lee b, W.K. Kim b, H.-Y. Lee b, W.J. Jeong c, M.K. Song d, D.H. Yoon a,* a
School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea b Nano Bio-photonics Team, Korea Electronics Technology Institute, Bundang 463-816, Republic of Korea c School of Electrical and Computer Engineering, University of Seoul, Seoul 412-791, Republic of Korea d Department of Materials Engineering, Hankuk Aviation University, Goyang 412-791, Republic of Korea Available online 23 August 2007
Abstract Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxidedoped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 mm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 2.83% and demonstrated secondharmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved. # 2007 Elsevier B.V. All rights reserved. Keywords: Periodically poled magnesium lithium niobate; Ridge waveguide; Second-harmonic generation; Duty cycle
1. Introduction Efficient wavelength conversion devices, such as secondharmonic generation (SHG) and optical parametric oscillation (OPO), can be achieved by periodically poled domain inversion to obtain quasi-phase-matching (QPM) to propagate waves in the crystal [1,2]. Among many other advantages, the QPM process makes it possible to use the largest non-linear coefficient, d33, of the materials for the interaction between Z-polarized fields. It has been reported that non-doped periodically poled lithium niobate (PPLN) suffers from photorefractive damage at room temperature. Recently, MgO-doped LiNbO3 (MgO:LN) was proposed as an alternative due to its higher resistance to photorefractive damage and its large non-linear optical coefficient [3,4]. Waveguide fabrication has been applied mainly by Ti indiffusion [5] or annealed proton exchange (APE) [6]. In the case of a Ti in-diffused waveguide, although low propagation
* Corresponding author. Tel.: +82 31 290 7361; fax: +82 31 290 7371. E-mail address: [email protected] (D.H. Yoon). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.060
losses are obtainable, the waveguide exhibits a low conversion efficiency due to its weak confinement and photorefractive damage. APE waveguide devices that achieve strong confinement produce asymmetric mode profiles at corresponding wavelengths in the wavelength conversion, reducing modal overlap due to the mismatch of the mode peak positions. Ridge waveguide devices have been investigated in an attempt to resolve these problems. The advantages of ridge waveguide devices are their symmetric mode profiles and strong light confinement due to their step index profile. Moreover, this type of waveguide suffers neither degradation of its non-linear coefficient nor photorefractive damage. It is expected that a high conversion efficiency can be achieved by using ridge waveguide structures in QPM waveguide devices [7]. In this investigation, MgO-doped lithium niobate, which has a domain period of 6.8 mm, was fabricated by applying an external field and a specially designed ridge waveguide was fabricated by a precision lapping machine and neutron loop discharge (NLD) using periodically poled magnesium-doped lithium niobate (PPMgLN). The PPMgLN ridge waveguide was observed by optical microscope, scanning electron microscope and demonstrated SHG.
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2. Experimental procedure Z-cut, 500 mm thick and 3 in. diameter MgO-doped (4.9 mol%) LiNbO3 wafers were purchased from Crystal Technology Inc., USA. The wafers were surface cleaned by sonication using a solution mixture of acetone, NH4OH:H2O:H2O2 and HCl:H2O:H2O2. The Z face of the wafer was chemical–mechanically polished down to a thickness of 150 mm to minimize the electric field during the poling. A 6.8 mm period photoresist (PR) with a thickness of 1.5 mm thickness was deposited on the +Z face of LN. Post-baking was performed on the +Z faces at 140 8C. After slicing the wafers to dimensions of 30 mm 10 mm, the specimen was placed into the poling jig connected to the circuitry, as shown elsewhere. Poling was achieved by applying an external electric field [9,10]. Ridge waveguides were fabricated using a precision lapping machine and neutron loop discharge in PPMgLN. Fig. 1 shows a schematic of the NLD process used to the dry-etching of the PPMgLN ridge waveguide during the dry-etching process. In this equipment, a vacuum chamber made of a quartz crucible with a diameter of 300 mm is co-axially surrounded by three magnetic coil rings, with an RF antenna located near the central one. A neutral loop is created in the vacuum chamber in the same plane as the RF antenna, which generates an RF frequency of 13.56 MHz. The RF power generated at the RF antenna affects the plasma density, while keeping the electron temperature constant. Variations in the power of the central magnetic coil can control the diameter of the neutral loop and various etching characteristics. The NLD equipment used to dry-etch the ridge waveguide was used at a bias power of 300 J/ s and the C3F8/Ar ratio, antenna power and chamber pressure were fixed at 10/90, 600 J/s and 0.33 Pa, respectively [11].The second-harmonic spectrum was measured along the crosssection (X-direction) of the PPMgLN ridge waveguide with a domain-inverted period of 6.8 mm. The SHG experiment was performed using an laser diode (LD) with a wavelength of 1064 nm and a heater (Fig. 2).
Fig. 2. Experimental set-up for the measurement of the SH power.
3. Results and discussion An optical photograph of the etched domain-inverted structure on the +Z face of the PPMgLN with a domaininverted period of 6.8 mm is shown in Fig. 3. It was noted that no periodic deterioration of the +Z face or the domain damage was detected for the PPMgLN with a short domain-inverted period. To determine the duty cycle of the domain-inverted structure, the domain inverted widths on the +Z face were measured using an optical microscope. The duty cycle was estimated to be 51.9 2.83%. The phase-matching wavelength and the efficiency of a QPM device are mainly governed by the periodicity and 50% duty cycle of the domain-inverted structures. Maximum conversion efficiency can be achieved for a perfectly uniform duty cycle of 50% of the domaininverted structure [3,6,8]. It was found that the average duty cycle of the PPMgLN with a short domain-inverted period of 6.8 mm was close to the optimum value of 50%. Therefore, the
Fig. 3. (a–k) Etched +Z surface shape of fabricated PPMgLN with 6.8 mm period.
Fig. 1. A schematic of the NLD used for the PPMgLN ridge waveguide dryetching process.
Fig. 4. Process of PPMgLN ridge waveguide structure fabrication.
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Fig. 5. (a–c) Scanning electron microscope (SEM) and optical images of cross-section (X-axis) of fabricated PPMgLN ridge waveguide.
PPLN with a domain-inverted period of 6.8 mm is likely to be applicable to QPM devices. Fig. 4a–d shows the poling process with MgO-doped lithium niobate. The ridge waveguide fabrication is described in Fig. 4e– k. PPMgLN and Z-cut 1 mm thick congruent LN were attached by epoxy adhesive (Fig. 4e), and PPMgLN was polished down to a thickness of 10 mm by means of a lapping machine and a chemical–mechanical polisher, with a thickness deviation of 0.5 mm. As a dry-etching mask, Ni metal with a thickness of about 3 mm thick was evaporated on the sample by means of an E-beam evaporator, and the Ni-mask used to dry-etch the ridge waveguide during dry-etching was fabricated by a photolithography process and Ni-plating (Fig. 4f–i). The ridge waveguide was etched by NLD with C3F8/Ar gas (Fig. 4j and k). The PPMgLN ridge waveguide etched at a bias power of 300 J/s for 3600 s showed an etching depth of 6.5 mm and an etching angle of around (70p/180) rad. Because the etched side wall has a slope of around (70p/180) rad, the electron density is higher in the etching edge and the etching speed was slightly higher at the edge corner than in the bottom plane [11]. Fig. 5a–c shows the scanning electron microscope (SEM) and optical
images of the cross-section (X-axis) of the fabricated PPMgLN ridge waveguide. The width of the ridge waveguide was different at the top and bottom planes (Fig. 5b and c). In the case of the dryetching mask with a width of 3 mm, the widths at the top and bottom were 3 and 10 mm, respectively. This experimental result shows the effects of electron density and etching speed. In the ridge waveguide process, the size difference between the +X and X faces was generated by the lapping machine, chemical– mechanical polisher and NLD. This deteriorates conversion efficiency of the sample. This issue is currently being studied and will be addressed in a future paper. The dependence of the second-harmonic (SH) power on the SHG device temperature is shown in Fig. 6. The ridge waveguide height was fixed at 10 0.5 mm, and the width ranged of top and bottom was 3 0.5 and 10 0.5 mm, respectively, and the interaction length was 12 mm. This result showed that the largest output power at a wavelength of 532 nm was obtained at around 79 8C. The power of fundamental wavelength was 40 mW at a wavelength of 1064 nm and SH power was 0.2 mW at a wavelength of 532 nm. The low SH power was affected by the non-uniform waveguide size. This issue, which is currently being studied, will be also improved soon. The full width at half maximum (FWHM) phasematching bandwidth in terms of the device temperature deviation was 2.2 8C. 4. Conclusion
Fig. 6. The dependence of the SH power on the SHG device temperature.
The use of the ridge waveguide process to obtain symmetric mode profiles and strong light confinement was investigated by means of a poling process and dry-etching. MgO-doped lithium niobate with a domain period of 6.8 mm was fabricated by the application of an external field. The PPMgLN ridge waveguide was fabricated by means of a precision lapping machine and NLD using periodically poled magnesium-doped lithium niobate (PPMgLN). The experimental results demonstrate that the average duty cycle of PPMgLN was 51.9 2.83%. A PPMgLN ridge waveguide with dimensions of 3 mm (top plane) and 10 mm (bottom plane) width was fabricated by the dry-etching process.
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S.W. Kwon et al. / Applied Surface Science 254 (2007) 1101–1104
The analysis of the dependence of the SH power on the SHG device temperature confirmed that the largest output power at a wavelength of 532 nm was obtained at around 79 8C and the FWHM was 2.2 8C at an interaction length of 12 mm. By using an optimized period, a 50% duty cycle and precise fabrication of the devices, a high conversion efficiency can be obtained by using a PPMgLN ridge waveguide at room temperature. References [1] A. Bruner, D. Eger, J. Appl. Phys. 96 (2004) 7445. [2] T. Hatanaka, K. Nakamura, T. Taniuchi, H. Ito, Y. Furukawa, K. Kitamura, Appl. Phys. Lett. 25 (2000) 651.
[3] B. Vincent, A. Boudrioua, R. Kremer, P. Moretti, Opt. Commun. 247 (2005) 461. [4] Y. Cho, S. Kazuta, H. Ito, Appl. Phys. Lett. 79 (2001) 2955. [5] Y.L. Lee, H. Suche, Y.H. Min, J.H. Lee, W. Grundko¨tter, V. Quiring, W. Sohler, IEEE Photonics Technol. Lett. 15 (2003) 978. [6] M.L. Bortz, M.M. Fejer, Opt. Lett. 16 (1991) 1844. [7] S. Kurimura, Y. Kato, M. Maruyama, Y. Usui, H. Nakajima, Appl. Phys. Lett. 89 (2006) 191123. [8] Y. Kong, J. Xu, B. Li, S. Chen, Z. Huang, L. Zhang, S. Liu, W. Yan, H. Liu, X. Xie, L. Shi, X. Li, G. Zhang, Opt. Mater. 27 (2004) 471. [9] S.W. Kwon, Y.S. Song, W.S. Yang, H.M. Lee, W.K. Kim, H.Y. Lee, D.Y. Lee, Opt. Mater. 29 (2007) 923. [10] S.W. Kwon, Y.S. Song, W.S. Yang, H.M. Lee, W.K. Kim, H.Y. Lee, B.Y. Kim, M.H. Lee, D.Y. Lee, Thin Solid Films 515 (2006) 922. [11] W.S. Yang, H.-Y. Lee, W.K. Kim, D.H. Yoon, Opt. Mater. 27 (2005) 1642.