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Compensating thermal drift of hybrid silicon and lithium niobate ring resonances Li Chen, Michael G. Wood, and Ronald M. Reano* Electroscience Laboratory, Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43212, USA *Corresponding author: [email protected] Received February 12, 2015; accepted March 1, 2015; posted March 12, 2015 (Doc. ID 234332); published March 31, 2015 We present low-power compensation of thermal drift of resonance wavelengths in hybrid silicon and lithium niobate ring resonators based on the linear electro-optic effect. Fabricated devices demonstrate a resonance wavelength tunability of 12.5 pm∕V and a tuning range of 1 nm. A capacitive geometry and low thermal sensitivity result in the compensation of 17°C of temperature variation using tuning powers at sub-nanowatt levels. The method establishes a route for stabilizing high-quality factor resonators in chip-scale integrated photonics subject to temperature variations. © 2015 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.6010) Sensors; (160.3730) Lithium niobate; (230.5750) Resonators; (200.4650) Optical interconnects. http://dx.doi.org/10.1364/OL.40.001599
The ever-growing demand for greater bandwidth in data communications has motivated the development of silicon-integrated photonics for optical interconnects [1–3]. Highly integrated silicon photonic circuits are enabled by the high-index contrast of silicon-on-insulator waveguides [4,5]. Among the many optical configurations demonstrated, silicon ring resonators enable compact and low-power on-chip optical filters, switches, and modulators for optical interconnects utilizing wavelength division multiplexing (WDM) [6–8]. The high-quality factor of ring resonators, combined with the plasma dispersion effect, allow for gigahertz speed modulators with femtojoule level power consumption [9,10]. While resonances produce high sensitivity and low power consumption, they also result in susceptibility to ambient temperature variations due primarily to the large thermo-optic coefficient (TOC) of silicon [11]. The thermal sensitivity of silicon ring resonators is in the range of 100 pm∕°C [11]. Consequently, devices are not practical without thermal compensation. Thermal challenges need to be resolved in order to advance resonator based photonics for future network-on-chip computing systems. One direction for addressing the thermal challenges focuses on passively reducing the thermal sensitivity of ring resonators, to achieve athermal operation, by incorporating materials with a negative TOC or by using compensating passive optical circuits [12–15]. Passive approaches generally lead to delocalized optical modes and large ring radii, reducing the benefit of high optical confinement in silicon waveguides, or requires larger footprint for interferometry. Although the techniques compensate for temperature drift of resonance wavelengths, appreciable detuning from target wavelengths remains inevitable due to manufacturing variation [16,17]. As a result, active thermal compensation methods are required to stabilize resonances to a particular wavelength. Local temperature control using integrated resistive heaters or tuning via pn junctions are examples of active techniques [9,18–22]. Thermal tuning of resonance wavelengths results in large tuning range; however, the method requires additional contacts, is relatively slow, and consumes 0146-9592/15/071599-04$15.00/0
significant power [11]. The power required to tune a free-spectral range (FSR) can be reduced to a few milliwatts using undercut-etched or backside-etched waveguides at the cost of enhanced optical bistability that limits the optical power handling [19,23,24]. Furthermore, thermal tuning is unidirectional. If the resonance is close to and higher than the operating wavelength, the resonance has to be red shifted by almost a FSR to the next order resonance. In contrast, wavelength stabilization based on carrier transport in pn junctions consumes power from microwatt to milliwatt levels [9,22]. For carrier injection-based devices, the tuning range is, however, limited by resistive heating [25]. For carrier depletion-based devices, the tuning is limited by doping concentration and junction breakdown. In both cases, the resonance lineshape is affected by carrier absorption [22]. While the required power consumption for the existing active approaches may be acceptable for current smallscale circuits, future VLSI photonic integrated circuits will demand power budgets that are not achievable onchip [8,26]. To advance the state of the art, a hybrid silicon and lithium niobate (LiNbO3 ) material system has been introduced to combine the dense integration of silicon photonics with the second-order susceptibility of LiNbO3 [27–30]. In this Letter, we demonstrate the compensation of thermal drift of resonance wavelengths in Si∕LiNbO3 ring resonators using sub-nanowatt power levels. Tuning is achieved by controlling the refractive index of LiNbO3 via the linear electro-optic effect. Resonance wavelengths are tuned over 1 nm by a DC voltage ranging from −30 V to 50 V, allowing for the compensation of 17°C of temperature increase. The voltages required for tuning the ring resonators can be delivered in a CMOS platform via capacitive charge pumps, similar to those used for micro-electromechanical systems and flash memory [31,32]. The tuning is capacitive with a static current below our measurement noise floor of 20 pA. The corresponding static power consumption is below 1 nW. A schematic of the tunable hybrid silicon and LiNbO3 microring resonator is shown in Fig. 1. The resonator © 2015 Optical Society of America
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consists of a 15-μm radius silicon ring and an 800-nmthick z-cut LiNbO3 thin film cladding that is bonded by benzocyclobutene (BCB). The LiNbO3 thin film, obtained from ion-slicing, is oriented with the c axis facing down [28]. The silicon rib waveguide is 500 nm wide with a 70-nm-thick slab and 180-nm rib height. A bottom aluminum electrode is formed on the silicon slab, together with nickel silicide, to provide an electrical path to the silicon ring waveguide that has a p-type background doping of 1015 cm−3 . A 125-nm-thick plasma-enhanced chemical vapor deposition (PECVD) SiO2 layer is deposited on top of the LiNbO3 thin film before patterning of the top aluminum electrode to provide optical isolation between the optical mode and the top electrode. The PECVD SiO2 layer is not required if a thicker LiNbO3 layer is used. The device is then capped with a 900nm-thick PECVD SiO2 film. Vias are etched through the SiO2 capping layer, and aluminum contact pads are patterned to allow access to the top and bottom electrodes. Finally, cantilever couplers are patterned for fiber-to-chip optical coupling [33,34]. Compared to the transverse electric (TE) mode, devices operating in the transverse magnetic (TM) mode have access to the r 33 electro-optic coefficient (31 pm V−1 at 1550 nm in bulk LiNbO3 [35]) and achieve large mode field overlap with the LiNbO3 . Beam propagation method simulations indicate that 40% of the optical mode power resides in the LiNbO3 for the TM mode, compared to 11% for the TE mode. The silicon waveguide functions as a transparent conductor that guides light and conducts electrons as well. In the capacitive structure, voltageinduced electric fields are highly confined to the LiNbO3 . As a result, the resonance is tuned efficiently via the linear electro-optic effect of the LiNbO3 . Compared to thermal tuning, the second order susceptibility response of LiNbO3 is fast, so that tuning speed is limited by electrical characteristics of the biasing arrangement. The silicon waveguide can be lightly doped to enable low RC time constant for high-speed operation [30]. The compensation of resonance wavelength shift by temperature variations is achieved by applying a DC voltage to the electrodes. AC switching or modulating voltages can be superimposed on the temperaturecompensating DC voltage using the same electrodes. The resistivity of bulk LiNbO3 crystal at room temperature is approximately 1 × 1015 Ω · cm [35], so DC current flow is negligible. Ideally, zero steady state tuning power is
Fig. 1. Schematic of Si∕LiNbO3 ring resonator. For clarity, the PECVD SiO2 top-cladding layer and electrical contact pads are not shown.
drawn in the absence of leakage current. The DC voltage can have either positive or negative polarity, enabling bidirectional temperature compensation. In the laboratory, the temperature sensitivity of the Si∕LiNbO3 ring resonator is characterized and compared to a silicon-on-insulator (SOI) ring resonator with similar radius. The SOI ring is clad with PECVD SiO2 and has the same core dimensions of the silicon in the Si∕LiNbO3 device. The device temperature is controlled by a thermoelectric cooler (TEC). Measured resonance wavelength detuning versus temperature is shown in Fig. 2. The initial ambient temperature is 22°C. The temperature sensitivity of the SOI ring is 103 pm∕°C, comparable to typical results in the literature [7]. In comparison, the temperature sensitivity of the Si∕LiNbO3 ring is 58 pm∕°C for TM polarization and 87 pm∕°C for TE polarization. The lower temperature sensitivity is a result of the negative TOC of BCB (−1.5 × 10−4 ∕°C) and smaller TOC of LiNbO3 (dno ∕dT 3.3 × 10−6 ∕°C and dne ∕dT 3.7 × 10−5 ∕°C) compared to silicon (1.86 × 10−4 ∕°C) at room temperature [11,35,36]. The lower sensitivity allows for the compensation of a larger temperature range with the same applied voltage. The TM optical transmission of the Si∕LiNbO3 ring is shown for various temperatures in Fig. 3 with voltage as parameter. The quality factor and free spectral range are 11,500 and 7.15 nm, respectively. The resonance is blue or red shifted, depending on the polarity of the voltage. A blue shift of resonance frequency is observed for increasingly positive voltage, indicating a decrease in refractive index, consistent with the orientation of the applied electric field and the z-axis of the LiNbO3 . Nominally, the extinction ratio of the resonance should not change with voltage and temperature. At a given temperature, the observed variation of extinction ratio with voltage is attributed to Fabry−Pérot fringes created between the two input/output fiber-to-chip coupling facets [30]. At different temperatures, the fiber-to-chip coupling is slightly modified. The slab and tapered waveguide combination in the fiber-to-chip coupler functions as a TM to TE mode converter [37]. Variations in fiber-to-chip coupling produce changes in mode conversion efficiency. Since the TE mode power is all-pass for the TM ring resonance, TE power variation produces changes in the extinction ratio for the TM output. The extinction ratio variations
Fig. 2.
Measured resonance detuning versus temperature.
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Fig. 4. Measured TM-mode resonance wavelength versus applied voltage, together with linear fit, with temperature as parameter.
Fig. 3. TM-mode optical transmission of Si∕LiNbO3 ring resonator for applied voltage from −30 V to 50 V for a temperature increase of (a) 0°C, (b) 5°C, (c) 10°C, and (d) 15°C above ambient temperature.
can be minimized by optimizing the coupler design in the presence of the slab [33,34,37]. As shown in Fig. 4, the resonance red shifts by 0.9 nm for a temperature increase of 15°C. The device tunability is in the range of 12.5 pm∕V. The resonance is tuned by 1 nm for a voltage sweep from −30 V to 50 V, allowing for thermal compensation against a temperature fluctuation of up to 17°C. Feedback control can be applied to stabilize the resonance to a given wavelength [20–22]. The biasing current is measured with a Keithly 2400 source meter. The measured DC current is below the measurement noise floor of 20 pA over the applied voltage range, indicating the static power consumption is below 1 nW for a bias of 50 V. The amount of voltage that can be applied to the hybrid Si∕LiNbO3 device is limited by the coercive field (21 kV∕mm in bulk LiNbO3 ) and the breakdown fields of LiNbO3 (70 kV∕mm) and PECVD SiO2 , which is process dependent [38,39]. A decrease of tunability is observed as the voltage is increased above 45 V, suggesting possible DC drift and material degradation at high electric fields [40]. Lower voltage and larger range of temperature compensation can be achieved by optimizing both the passive athermal design and the active compensation, taking advantage of the negative TOC of BCB that confines a portion of the optical mode. In conclusion, we present the compensation of thermal drift of resonance wavelengths in a Si∕LiNbO3 ring resonator by tuning the refractive index of the LiNbO3 via the linear electro-optic effect. A tuning range of 1 nm
with sub-nanowatt static power consumption is demonstrated, enabling compensation of a 17°C temperature variation. In contrast to traditional tuning mechanisms based on resistive heating and carrier transport, the tuning in the Si∕LiNbO3 structure draws considerably lower current. The concept presented in this Letter can be applied to other ferroelectric materials and electro-optic polymers on silicon-on-insulator [41–43]. The approach paves a route for stabilizing resonators in integrated photonics subject to temperature variations. This work was supported by the Army Research Office (ARO) under grant number W911NF-12-1-0488. References 1. D. A. B. Miller, Proc. IEEE 88, 728 (2000). 2. R. A. Soref, IEEE J. Sel. Top. Quantum Electron. 12, 1678 (2006). 3. B. Jalali and S. Fathpour, J. Lightwave Technol. 24, 4600 (2006). 4. A. V. Krishnamoorthy, R. Ho, X. Zheng, H. Schwetman, J. Lexau, P. Koka, G. Li, I. Shubin, and J. E. Cunningham, Proc. IEEE 97, 1337 (2009). 5. I. A. Young, E. Mohammed, J. T. S. Liao, A. M. Kern, S. Palermo, B. A. Block, M. R. Reshotko, and P. L. D. Chang, IEEE J. Solid-State Circuits 45, 235 (2010). 6. D. G. Rabus, Integrated Ring Resonators, Springer Series in Optical Sciences (Springer, 2007). 7. W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. Kumar Selvaraja, T. Claes, P. Dumon, P. Bienstman, D. Van Thourhout, and R. Baets, Laser Photon. Rev. 6, 47 (2012). 8. B. E. Little, S. T. Chu, W. Pan, and Y. Kokubun, IEEE Photon. Technol. Lett. 12, 323 (2000). 9. E. Timurdogan, C. M. Sorace-Agaskar, J. Sun, E. Shah Hosseini, A. Biberman, and M. R. Watts, Nat. Commun. 5, 4008 (2014). 10. G. Li, X. Zheng, J. Yao, H. Thacker, I. Shubin, Y. Luo, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, Opt. Express 19, 20435 (2011). 11. K. Padmaraju and K. Bergman, Nanophotonics 13, 1 (2013). 12. J. Teng, P. Dumon, W. Bogaerts, H. Zhang, X. Jian, X. Han, M. Zhao, G. Morthier, and R. Baets, Opt. Express 17, 14627 (2009). 13. S. S. Djordjevic, K. Shang, B. Guan, S. T. S. Cheung, L. Liao, J. Basak, H. Liu, and S. J. B. Yoo, Opt. Express 21, 13958 (2013). 14. B. Guha, B. B. C. Kyotoku, and M. Lipson, Opt. Express 18, 3487 (2010).
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15. B. Guha and M. Lipson, Opt. Lett. 40, 103 (2015). 16. A. V. Krishnamoorthy, X. Zheng, G. Li, J. Yao, T. Pinguet, A. Mekis, H. Thacker, I. Shubin, Y. Luo, K. Raj, and J. E. Cunningham, IEEE Photon. J. 3, 567 (2011). 17. W. A. Zortman, D. C. Trotter, and M. R. Watts, Opt. Express 18, 23598 (2010). 18. P. Dong, R. Shafiiha, S. Liao, H. Liang, N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, Opt. Express 18, 10941 (2010). 19. P. Dong, W. Qian, H. Liang, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, Opt. Express 18, 20298 (2010). 20. K. Padmaraju, D. F. Logan, X. Zhu, J. J. Ackert, A. P. Knights, and K. Bergman, Opt. Express 21, 14342 (2013). 21. C. T. DeRose, M. R. Watts, D. C. Trotter, D. L. Luck, G. N. Nielson, and R. W. Young, in Proc. Conf. on Lasers and Electro-Optics (Optical Society of America, 2010), paper CThJ3. 22. K. Padmaraju, J. Chan, L. Chen, M. Lipson, and K. Bergman, Opt. Express 20, 27999 (2012). 23. J. E. Cunningham, I. Shubin, X. Zheng, T. Pinguet, A. Mekis, Y. Luo, H. Thacker, G. Li, J. Yao, K. Raj, and A. V. Krishnamoorthy, Opt. Express 18, 19055 (2010). 24. X. Zheng, Y. Luo, G. Li, I. Shubin, H. Thacker, J. Yao, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, Opt. Express 20, 11478 (2012). 25. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, Nature 435, 325 (2005). 26. D. A. B. Miller, Proc. IEEE 97, 1166 (2009). 27. Y. S. Lee, G.-D. Kim, W.-J. Kim, S.-S. Lee, W.-G. Lee, and W. H. Steier, Opt. Lett. 36, 1119 (2011). 28. L. Chen and R. M. Reano, Opt. Express 20, 4032 (2012).
29. L. Chen, M. G. Wood, and R. M. Reano, Opt. Express 21, 27003 (2013). 30. L. Chen, Q. Xu, M. G. Wood, and R. M. Reano, Optica 1, 112 (2014). 31. M. Ker and S. Chen, IEEE Trans. Circuits Syst. II 54, 47 (2007). 32. A. Emira, M. Abdel Ghany, M. Elsayed, A. Elshurafa, S. Sedky, and K. Salama, IEEE Trans. Ind. Electron. 60, 4683 (2013). 33. P. Sun and R. M. Reano, Opt. Express 17, 4565 (2009). 34. M. Wood, P. Sun, and R. M. Reano, Opt. Express 20, 164 (2012). 35. K. K. Wong, Properties of Lithium Niobate (INSPEC, 2002). 36. M. F. Hossain, H. P. Chan, and M. A. Uddin, Opt. Express 18, 8896 (2010). 37. D. Dai, Y. Tang, and J. E. Bowers, Opt. Express 20, 13425 (2012). 38. S. Kim, V. Gopalan, and A. Gruverman, Appl. Phys. Lett. 80, 2740 (2002). 39. M. Luennemann, U. Hartwig, G. Panotopoulos, and K. Buse, Appl. Phys. B 76, 403 (2003). 40. E. L. Wooten, K. M. Kissa, A. Y. Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. Maack, D. V. Attanasio, D. J. Fritz, G. J. McBrien, and D. E. Bossi, IEEE J. Sel. Top. Quantum Electron. 6, 69 (2000). 41. M. Gould, T. Baehr-Jones, R. Ding, S. Huang, J. Luo, A. K.-Y. Jen, J.-M. Fedeli, M. Fournier, and M. Hochberg, Opt. Express 19, 3952 (2011). 42. S. Abel, T. Stöferle, C. Marchiori, C. Rossel, M. D. Rossell, R. Erni, D. Caimi, M. Sousa, A. Chelnokov, B. J. Offrein, and J. Fompeyrine, Nat. Commun. 4, 1671 (2013). 43. C.Xiong, W.H. Pernice, J. H. Ngai,J. W.Reiner, D. Kumarh, F.J. Walker, C. H. Ahn, and H. X. Tang, Nano Lett. 14, 1419 (2014).
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Compensating thermal drift of hybrid silicon and lithium niobate ring resonances Li Chen, Michael G. Wood, and Ronald M. Reano* Electroscience Laboratory, Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43212, USA *Corresponding author: [email protected] Received February 12, 2015; accepted March 1, 2015; posted March 12, 2015 (Doc. ID 234332); published March 31, 2015 We present low-power compensation of thermal drift of resonance wavelengths in hybrid silicon and lithium niobate ring resonators based on the linear electro-optic effect. Fabricated devices demonstrate a resonance wavelength tunability of 12.5 pm∕V and a tuning range of 1 nm. A capacitive geometry and low thermal sensitivity result in the compensation of 17°C of temperature variation using tuning powers at sub-nanowatt levels. The method establishes a route for stabilizing high-quality factor resonators in chip-scale integrated photonics subject to temperature variations. © 2015 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.6010) Sensors; (160.3730) Lithium niobate; (230.5750) Resonators; (200.4650) Optical interconnects. http://dx.doi.org/10.1364/OL.40.001599
The ever-growing demand for greater bandwidth in data communications has motivated the development of silicon-integrated photonics for optical interconnects [1–3]. Highly integrated silicon photonic circuits are enabled by the high-index contrast of silicon-on-insulator waveguides [4,5]. Among the many optical configurations demonstrated, silicon ring resonators enable compact and low-power on-chip optical filters, switches, and modulators for optical interconnects utilizing wavelength division multiplexing (WDM) [6–8]. The high-quality factor of ring resonators, combined with the plasma dispersion effect, allow for gigahertz speed modulators with femtojoule level power consumption [9,10]. While resonances produce high sensitivity and low power consumption, they also result in susceptibility to ambient temperature variations due primarily to the large thermo-optic coefficient (TOC) of silicon [11]. The thermal sensitivity of silicon ring resonators is in the range of 100 pm∕°C [11]. Consequently, devices are not practical without thermal compensation. Thermal challenges need to be resolved in order to advance resonator based photonics for future network-on-chip computing systems. One direction for addressing the thermal challenges focuses on passively reducing the thermal sensitivity of ring resonators, to achieve athermal operation, by incorporating materials with a negative TOC or by using compensating passive optical circuits [12–15]. Passive approaches generally lead to delocalized optical modes and large ring radii, reducing the benefit of high optical confinement in silicon waveguides, or requires larger footprint for interferometry. Although the techniques compensate for temperature drift of resonance wavelengths, appreciable detuning from target wavelengths remains inevitable due to manufacturing variation [16,17]. As a result, active thermal compensation methods are required to stabilize resonances to a particular wavelength. Local temperature control using integrated resistive heaters or tuning via pn junctions are examples of active techniques [9,18–22]. Thermal tuning of resonance wavelengths results in large tuning range; however, the method requires additional contacts, is relatively slow, and consumes 0146-9592/15/071599-04$15.00/0
significant power [11]. The power required to tune a free-spectral range (FSR) can be reduced to a few milliwatts using undercut-etched or backside-etched waveguides at the cost of enhanced optical bistability that limits the optical power handling [19,23,24]. Furthermore, thermal tuning is unidirectional. If the resonance is close to and higher than the operating wavelength, the resonance has to be red shifted by almost a FSR to the next order resonance. In contrast, wavelength stabilization based on carrier transport in pn junctions consumes power from microwatt to milliwatt levels [9,22]. For carrier injection-based devices, the tuning range is, however, limited by resistive heating [25]. For carrier depletion-based devices, the tuning is limited by doping concentration and junction breakdown. In both cases, the resonance lineshape is affected by carrier absorption [22]. While the required power consumption for the existing active approaches may be acceptable for current smallscale circuits, future VLSI photonic integrated circuits will demand power budgets that are not achievable onchip [8,26]. To advance the state of the art, a hybrid silicon and lithium niobate (LiNbO3 ) material system has been introduced to combine the dense integration of silicon photonics with the second-order susceptibility of LiNbO3 [27–30]. In this Letter, we demonstrate the compensation of thermal drift of resonance wavelengths in Si∕LiNbO3 ring resonators using sub-nanowatt power levels. Tuning is achieved by controlling the refractive index of LiNbO3 via the linear electro-optic effect. Resonance wavelengths are tuned over 1 nm by a DC voltage ranging from −30 V to 50 V, allowing for the compensation of 17°C of temperature increase. The voltages required for tuning the ring resonators can be delivered in a CMOS platform via capacitive charge pumps, similar to those used for micro-electromechanical systems and flash memory [31,32]. The tuning is capacitive with a static current below our measurement noise floor of 20 pA. The corresponding static power consumption is below 1 nW. A schematic of the tunable hybrid silicon and LiNbO3 microring resonator is shown in Fig. 1. The resonator © 2015 Optical Society of America
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consists of a 15-μm radius silicon ring and an 800-nmthick z-cut LiNbO3 thin film cladding that is bonded by benzocyclobutene (BCB). The LiNbO3 thin film, obtained from ion-slicing, is oriented with the c axis facing down [28]. The silicon rib waveguide is 500 nm wide with a 70-nm-thick slab and 180-nm rib height. A bottom aluminum electrode is formed on the silicon slab, together with nickel silicide, to provide an electrical path to the silicon ring waveguide that has a p-type background doping of 1015 cm−3 . A 125-nm-thick plasma-enhanced chemical vapor deposition (PECVD) SiO2 layer is deposited on top of the LiNbO3 thin film before patterning of the top aluminum electrode to provide optical isolation between the optical mode and the top electrode. The PECVD SiO2 layer is not required if a thicker LiNbO3 layer is used. The device is then capped with a 900nm-thick PECVD SiO2 film. Vias are etched through the SiO2 capping layer, and aluminum contact pads are patterned to allow access to the top and bottom electrodes. Finally, cantilever couplers are patterned for fiber-to-chip optical coupling [33,34]. Compared to the transverse electric (TE) mode, devices operating in the transverse magnetic (TM) mode have access to the r 33 electro-optic coefficient (31 pm V−1 at 1550 nm in bulk LiNbO3 [35]) and achieve large mode field overlap with the LiNbO3 . Beam propagation method simulations indicate that 40% of the optical mode power resides in the LiNbO3 for the TM mode, compared to 11% for the TE mode. The silicon waveguide functions as a transparent conductor that guides light and conducts electrons as well. In the capacitive structure, voltageinduced electric fields are highly confined to the LiNbO3 . As a result, the resonance is tuned efficiently via the linear electro-optic effect of the LiNbO3 . Compared to thermal tuning, the second order susceptibility response of LiNbO3 is fast, so that tuning speed is limited by electrical characteristics of the biasing arrangement. The silicon waveguide can be lightly doped to enable low RC time constant for high-speed operation [30]. The compensation of resonance wavelength shift by temperature variations is achieved by applying a DC voltage to the electrodes. AC switching or modulating voltages can be superimposed on the temperaturecompensating DC voltage using the same electrodes. The resistivity of bulk LiNbO3 crystal at room temperature is approximately 1 × 1015 Ω · cm [35], so DC current flow is negligible. Ideally, zero steady state tuning power is
Fig. 1. Schematic of Si∕LiNbO3 ring resonator. For clarity, the PECVD SiO2 top-cladding layer and electrical contact pads are not shown.
drawn in the absence of leakage current. The DC voltage can have either positive or negative polarity, enabling bidirectional temperature compensation. In the laboratory, the temperature sensitivity of the Si∕LiNbO3 ring resonator is characterized and compared to a silicon-on-insulator (SOI) ring resonator with similar radius. The SOI ring is clad with PECVD SiO2 and has the same core dimensions of the silicon in the Si∕LiNbO3 device. The device temperature is controlled by a thermoelectric cooler (TEC). Measured resonance wavelength detuning versus temperature is shown in Fig. 2. The initial ambient temperature is 22°C. The temperature sensitivity of the SOI ring is 103 pm∕°C, comparable to typical results in the literature [7]. In comparison, the temperature sensitivity of the Si∕LiNbO3 ring is 58 pm∕°C for TM polarization and 87 pm∕°C for TE polarization. The lower temperature sensitivity is a result of the negative TOC of BCB (−1.5 × 10−4 ∕°C) and smaller TOC of LiNbO3 (dno ∕dT 3.3 × 10−6 ∕°C and dne ∕dT 3.7 × 10−5 ∕°C) compared to silicon (1.86 × 10−4 ∕°C) at room temperature [11,35,36]. The lower sensitivity allows for the compensation of a larger temperature range with the same applied voltage. The TM optical transmission of the Si∕LiNbO3 ring is shown for various temperatures in Fig. 3 with voltage as parameter. The quality factor and free spectral range are 11,500 and 7.15 nm, respectively. The resonance is blue or red shifted, depending on the polarity of the voltage. A blue shift of resonance frequency is observed for increasingly positive voltage, indicating a decrease in refractive index, consistent with the orientation of the applied electric field and the z-axis of the LiNbO3 . Nominally, the extinction ratio of the resonance should not change with voltage and temperature. At a given temperature, the observed variation of extinction ratio with voltage is attributed to Fabry−Pérot fringes created between the two input/output fiber-to-chip coupling facets [30]. At different temperatures, the fiber-to-chip coupling is slightly modified. The slab and tapered waveguide combination in the fiber-to-chip coupler functions as a TM to TE mode converter [37]. Variations in fiber-to-chip coupling produce changes in mode conversion efficiency. Since the TE mode power is all-pass for the TM ring resonance, TE power variation produces changes in the extinction ratio for the TM output. The extinction ratio variations
Fig. 2.
Measured resonance detuning versus temperature.
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Fig. 4. Measured TM-mode resonance wavelength versus applied voltage, together with linear fit, with temperature as parameter.
Fig. 3. TM-mode optical transmission of Si∕LiNbO3 ring resonator for applied voltage from −30 V to 50 V for a temperature increase of (a) 0°C, (b) 5°C, (c) 10°C, and (d) 15°C above ambient temperature.
can be minimized by optimizing the coupler design in the presence of the slab [33,34,37]. As shown in Fig. 4, the resonance red shifts by 0.9 nm for a temperature increase of 15°C. The device tunability is in the range of 12.5 pm∕V. The resonance is tuned by 1 nm for a voltage sweep from −30 V to 50 V, allowing for thermal compensation against a temperature fluctuation of up to 17°C. Feedback control can be applied to stabilize the resonance to a given wavelength [20–22]. The biasing current is measured with a Keithly 2400 source meter. The measured DC current is below the measurement noise floor of 20 pA over the applied voltage range, indicating the static power consumption is below 1 nW for a bias of 50 V. The amount of voltage that can be applied to the hybrid Si∕LiNbO3 device is limited by the coercive field (21 kV∕mm in bulk LiNbO3 ) and the breakdown fields of LiNbO3 (70 kV∕mm) and PECVD SiO2 , which is process dependent [38,39]. A decrease of tunability is observed as the voltage is increased above 45 V, suggesting possible DC drift and material degradation at high electric fields [40]. Lower voltage and larger range of temperature compensation can be achieved by optimizing both the passive athermal design and the active compensation, taking advantage of the negative TOC of BCB that confines a portion of the optical mode. In conclusion, we present the compensation of thermal drift of resonance wavelengths in a Si∕LiNbO3 ring resonator by tuning the refractive index of the LiNbO3 via the linear electro-optic effect. A tuning range of 1 nm
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