Open Access
Add to BibSonomy
Abstract
We demonstrate highly efficient lithium niobate thin film Michelson interferometer modulators with half-wave voltage length product of 1.4 V∙cm. Amorphous silicon grating couplers have been incorporated to achieve a 3.8-dB/port waveguide-fiber coupling loss. Devices with 1-mm phase shifter arms have a footprint of 2.5 mm × 1.7 mm. The demonstrated modulation data rates is up to 35 Gb/s.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical communication systems require high-speed electro-optic (E-O) modulators with low power consumption, low cost and compact footprint [1–3]. Lithium niobate (LN) with its strong Pockels effect, low optical absorption and refractive index of 2.1-2.2 enables high-speed E-O modulation with low loss. Conventional bulk LN E-O modulators, widely used in commercial optical fiber network [2], have weakly confined channel waveguides formed by proton exchange or Ti diffusion technologies. The gap between adjacent electrodes are > 30 μm to avoid metal absorption [4–6], resulting in low modulation efficiency and long electrode lengths [7]. Moreover, the small index contrast constrains the bending radius to few millimeters [8], therefore the device footprint is large. The developments of wafer bonding and ion slicing enable LN thin film directly bonded on silica [9,10]. As the refractive index contrast between LN and silica is ~0.7,tightly confined sub-micron-sized waveguides and small bending radius can be fabricated in the LN thin film platform. The sub-micron mode cross-section significantly enhances E-O and nonlinear optical effects [11–13]. Recently, the optimization of LN nano-fabrication process enables ultra-low loss waveguides of less than 0.03 dB/cm [14].
Mach-Zehnder interferometer (MZI) modulators on LN thin film have achieved extensive success, with high data speed exceeding 100 Gb/s [15,16], low half-wave voltage length product (Vπ∙l) of 1.8 V∙cm [17] and low insertion loss of less than 0.5 dB [16] having been experimentally demonstrated. However the voltage-length product values of LN modulators are still larger than those of Indium Phosphide [18,19] and polymer [20,21] devices.
Michelson interferometer modulator (MIM) is one of the schemes of folded modulators. With reflective mirrors on both arms, the interaction length between the light wave and modulating electrical field doubles. As a result, MIM can efficiently enhance modulation efficiency and/or reduce device size compared with MZI modulator [1,22,23]. Table 1 summarizes the figures of merit of demonstrated MZI modulators and MIMs, including what we report here, which is the first experimental demonstration of a high-efficiency MIM on LN thin film. Our MIM device, with 1-mm phase shifters, has a Vπ∙l of 1.4 V∙cm, 3-dB bandwidth of 12 GHz, and device insertion loss of 4 dB. With the integrated amorphous silicon (a-Si) gratings achieving a chip-fiber coupling loss of 3.8 dB per side, the fiber-to-fiber insertion loss is 11.6 dB.
Table 1. Demonstrated MZIs and MIMs of silicon and LN.
2. Design and fabrication
Figure 1 is a three-dimension (3-D) schematic view of our devices. The fiber input is coupled into the LN waveguide through a-Si - LN hybrid grating couplers designed for transverse-electric (TE) polarization [25]. Then the input light is 50:50 split by a 2 × 2 multimode interference (MMI) coupler. The split waves travel through the two equal-length phase shifter arms, reflected by the broadband loop mirrors, and travel back through the same phase shifter. The backward travelling lights from the two arms are combined through the same 2 × 2 MMI coupler. At the output grating coupler, the light switches on and off depending on the phase difference induced between the two arms. The loop mirror structures comprise a symmetric 1 × 2 MMI coupler, with the two output ports connected by a Euler waveguide bending. The minimum bending radius is 50 μm. The total length of the loop back mirror is 626.6 μm.
Fig. 1 The 3-D schematic overview of our MIM on LN thin film. Inset: the zoom-in drawing of a-Si grating coupler on LN waveguide.
Figure 2(a) shows the cross-section of the phase shifter structure on one arm of the MIM and the intensity profile of the fundamental TE mode solved by a finite difference eigenmode (FDE) method at 1550 nm wavelength. The thickness of LN thin film is 600 nm. The top width of our LN waveguide is Wt = 1 μm and the etch depth is D = 300 nm. The thickness of radio frequency (RF) electrode is 600 nm, and the gap between adjacent electrodes is G = 5 μm. This gap width is chosen to achieve high electric field across the optical mode and to avoid severe optical absorption from the metal. Figure 2(b) shows the in-plane electric field (Ez) distribution with direct current (DC) voltage supplied which is numerically simulated by a finite element method (FEM). When the applied voltage is 1 V, the magnitude of Ez is in the order of ~105 V/m. The electric field interacts with the largest E-O tensor element (γ33) of LN to induce a linear refractive index change, and consequently gives rise to a phase modulation. Amplitude modulation is achieved by the interference between the two arms. The simulated half-wave voltage length product for the MIM is Vπ∙l = 1.14 V∙cm, which is about half of those in MZI modulators [15–17]. We design the signal electrode width of 13 μm and electrode thickness of 600 nm to achieve group velocity and impedance match. The calculated group refractive indices for optics (no = 2.29 at 1550 nm) and microwave (nm = 2.3 at 20 GHz) are about 2.3, and the characteristic impedance of travelling-wave electrode is around 50 Ω.
Fig. 2 (a) Lateral optical mode analysis of MIM by FDE method. The parameters are: Wt = 1 μm, D = 300 nm, G = 5 μm and the thickness of Au electrodes is 600 nm. (b) Numerical simulation of in-plane electric field (Ez) distribution with 1-V DC voltage supplied.
The devices are fabricated on an x-cut LNOI wafer from NANOLN. The 600-nm LN film is directly bonded on 2-μm thermal oxidized silica and the handling substrate is 400-μm silicon. A brief description of process flow is shown in Fig. 3(a). Steps 1-5 in Fig. 3(a) shows the LN waveguide devices and hybrid a-Si grating coupler processes, which are introduced in detail in our previous work [25]. An 850-nm-thick Hydrogen silsesquioxane (HSQ) is spin-coated on LN thin film as mask, and subsequent definition of waveguides patterns using E-beam lithography (EBL) system. The patterns are transferred 300 nm deep into LN with an optimized Argon plasma in an inductively coupled plasma (ICP) etching system. After LN sidewall cleaning and mask removal, a 220-nm a-Si layer is then deposited on the patterned LN waveguide. HSQ is spin-coated on a-Si for EBL. The grating patterns are transferred into the a-Si by ICP etching with HBr. A polymethyl methacrylate (PMMA) resist is spin-coated for EBL to define the electrode patterns. The electrodes comprising 10-nm Ti and 600-nm Au is deposited by E-beam evaporation (EBE) and finally completed with a lift-off process (step 6 in Fig. 3(a)). The microscope image shown in Fig. 3(b) is a MIM with 1-mm-long phase shifter arms, and the inset shows the a-Si grating coupler sitting on the terminal of the LN adiabatic taper waveguide. Our grating couplers have a simple periodic structure that can be further optimized in future by apodization designs [26,27].
Fig. 3 (a) Fabrication process of MIMs. (b) Microscope images of 1-mm-long MIM. Inset: zoom-in image of a-Si grating coupler.
3. Measurement
3.1. Optical insertion loss and drive voltage characterization
A high-precision external cavity tunable laser (Agilent 81600B) set at 1550 nm wavelength is used as the light source. A fiber polarization controller is used to maintain the input light to the TE polarization. The measured coupling loss of the a-Si grating coupler is ~3.8 dB per port. The total on-chip loss of the MIMs with 1-mm and 2-mm long arms are 4 dB and 5.1 dB respectively, which are measured at the maximum transmission with bias voltages of 9.7 V and 7.1 V respectively. The total on-chip loss results from a waveguide propagation loss of 4.6 dB/cm, a metal absorption loss of 2 dB/cm, and the insertion loss of the three MMI couplers of ~0.9 dB. Extinction ratios (ER) of 27.6 dB and 20 dB have been measured respectively. Figures 4(a) and 4(b) show the measured light transmission by applying a 100-kHz triangular wave with amplitude of 19 V, indicating Vπ of 14 V and 7.6 V for the two MIMs. Thus Vπ∙l values are 1.4 V∙cm and 1.5 V∙cm, respectively. The discrepancy between the measurement and simulation results may be due to the fabrication errors and frequency-dependent Vπ∙l, as we have measured 0.93 V∙cm and 0.84 V∙cm at DC voltage sweep.
Fig. 4 100-kHz triangular wave sweep with amplitude of 19 V for 1-mm- (a) and 2-mm- (b) long MIMs respectively. Blue line: triangular wave signal, red line: PD demodulation signal from modulators.
3.2. Small-signal characterization
We characterize the E-O bandwidth of the MIMs using the setup shown in Fig. 5(a). Port 1 of the network analyzer (Agilent, N5227A) is connected to a bias tee. The bias tee is set at the quadrature point voltage of MIMs, which are ~5.3 V and 5 V for the 1-mm and 2-mm devices respectively. A pair of 67 GHz microwave G-S-G probes (GGB, Model 67A) are in contact with the on-chip Au pads. One of probes is terminated with a standard 50 Ω terminal resistance. The modulated optical signal wave is amplified with an erbium-doped fiber amplifier (EDFA), and subsequently filtered by an optical band-past filter (OBPF). A photodiode with 100 GHz 3-dB bandwidth (Finisar) is used to capture the E-O response. We calibrate all the RF cables, bias tee, adapters and probes before testing our devices. The measured microwave group refractive index and characteristic impedance of the electrode are ~2.25 and 50 - 52 Ω at 20 GHz respectively.
Fig. 5 (a) Measurement setup for small signal characterization. (b) and (c) are the E-O and electrical S21 response of 1-mm- and 2-mm-long MIMs.
Figures 5(b) and 5(c) show the E-O S21 response of 1-mm and 2-mm MIMs, with 3-dB bandwidths values of 12 GHz and 9.5 GHz respectively. Notice that the 6-dB electrical bandwidth of the microwave electrode is larger than the measurement range, and extensively higher than the 3-dB E-O bandwidth. We believe that the velocity mismatch between the optics and microwave [22] limits the E-O bandwidth of the MIMs.
We model the MIM by unfolding the loop-back structure to a straight one as shown schematically in Fig. 6(a), where L and M are the waveguide lengths of the phase shifter and loop-back mirror respectively. Assuming an optical wave launched from the left input, we analyze its accumulated phase shift φ(t) at the right output. A microwave voltage signal v(t) = v0 cos(Ωt) with amplitude v0 and angular frequency Ω is applied at the junction between the phase shifter and the loop-back mirror. We also assume that the travelling-wave electrode is properly terminated without any reflections, which is the case in our devices. For the left phase shifter, the optical wave and microwave are counter-propagating, while for the right phase shifter, these two are co-propagating. We modify the established equations in [28], with time delay of the optical wave relative to microwave properly set to the output port reference. For the counter-propagating left phase shifter section, the contribution of the phase φ1(t) is given by
where g = π/(LVπ), c is the speed of light in free space, β1 = (no + nm)/c is the relative propagation constant between optical wave and microwave, no and nm are the group indices of optical wave and microwave, respectively. The φ2(t) contributed by the co-propagating right phase shifter iswhere β2 = (no−nm)/c. Introducing the microwave voltage signal v(t) = v0 cos(Ωt) to the integration in Eqs. (1) and (2) we have Then the total phase shift is given by φ(t) = φ1(t) + φ2(t). Notice that the results are sinusoidal with amplitude given by a normalized cardinal sine function (sinc function) defined as sinc(x) = sin(x/π)/(x/π), and relative phase given by the device length. Thuswhere A = sinc[Ωβ1L/(2π)], B = sinc[Ωβ2L/(2π)], θ = Ω(noM/c + β1L/2 + β2L/2). Then the total phase shift φ(t) has the amplitude of gLv0C(Ω), whereThe E-O response spectrum is then given bywhere J1[·] is the 1st order Bessel function of the first kind [29].Fig. 6 (a) Schematic of MIM by unfolding the loop-back structure to a straight one. (b) and (c) are the calculated E-O response of 1-mm and 2-mm MIM respectively.
The calculated E-O response of 1-mm and 2-mm MIMs are shown in Figs. 6(b) and 6(c), which are consistent with the measurement results we obtained in Figs. 5(b) and 5(c). Our calculation results show that the velocity mismatch is mainly attributed to the counter-propagating of microwave and optical wave and the time delay in loop mirror, which are inherent to MIMs.
3.3. Large-signal characterization
We characterize the large-signal performance of the MIMs using a setup similar to Fig. 5(a). We use an arbitrary waveform generator (AWG, Keysight M8195A) and a digital serial analyzer (DSA, Tektronix DSA8300) to measure the eye diagrams. A non-return-zero (NRZ) pseudorandom binary sequence (PRBS) signal with 29-1 pattern length is generated from the AWG, and subsequently amplified by a RF amplifier to obtain a 10-V and 5.6-V peak to peak signals for the 1- and 2-mm MIM respectively. The devices are bias at their quadrature point. The eye diagrams of the 1- and 2-mm MIM are collected with a 70-GHz optical sampling module of the DSA, and shown in Figs. 7(a) and 7(b). The open-eye diagrams are achieved at 20 Gb/s, 30 Gb/s and 35 Gb/s for 1-mm MIM, with ER of 11.4 dB, 6.4 dB and 6.4 dB, respectively. The open-eye diagrams are achieved at 10 Gb/s, 20 Gb/s and 23 Gb/s for 2-mm MIM, with ER of 9.5 dB, 7.8 dB and 6.5 dB, respectively.
Fig. 7 (a) The open-eye diagrams of 1-mm-long MIMs are achieved at 20 Gb/s (ER = 11.4 dB), 30 Gb/s (ER = 6.4 dB) and 35 GB/s (ER = 6.4 dB). (b) The open-eye diagrams of 2-mm-long MIMs are achieved at 10 Gb/s (ER = 9.5 dB), 20 Gb/s (ER = 7.8 dB) and 23 GB/s (ER = 6.5 dB).
4. Conclusion
In conclusion, we have experimentally demonstrated high-efficiency Michelson interferometer modulators on LN thin film. Low Vπ∙l of 1.4 V∙cm has been achieved, which is enhanced compare to the MZI modulators and resulting in smaller device footprint. Modulation bandwidth of 12 GHz has been achieved in the device with 1-mm long phase shifter arms, enabling data rates of 35 Gb/s with 6.4-dB ER of the open-eye diagram. We numerically analyze the total phase shift of the MIM and show that the limitation of E-O bandwidth is the velocity mismatch of the counter-propagation of optical wave and microwave and the time delay in the loop-back mirror. The total fiber-to-fiber loss of 11.6 dB is measured, consisting of a 4-dB on-chip loss and 3.8-dB chip-fiber coupling loss per port. The a-Si grating coupler has a higher coupling efficiency compare to end-fire coupling, and can be further improved by well-known apodization schemes. Further, reduction of electrode and waveguide losses is also possible by optimizing design and fabrication processes.
Funding
National Natural Science Foundation of China (NSFC) (61622510, 61575224, 11690031, U1701661, 61490715); Science and Technology Program of Guangzhou (201707020017); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X121).
References
1. J. Witzens, “High-speed silicon photonics modulators,” Proc. IEEE 106(12), 2158–2182 (2018). [CrossRef]
2. A. Rao and S. Fathpour, “Compact lithium niobate electrooptic modulators,” IEEE J. Sel. Top. Quantum Electron. 24(4), 1–14 (2018). [CrossRef]
3. K. Liu, C. R. Ye, S. Khan, and V. J. Sorger, “Review and perspective on ultrafast wavelength-size electro-optic modulators,” Laser Photonics Rev. 9(2), 172–194 (2015). [CrossRef]
4. J. Kondo, A. Kondo, K. Aoki, M. Imaeda, T. Mori, Y. Mizuno, S. Takatsuji, Y. Kozuka, M. Mu, and M. Minakata, “40-Gb/s X-Cut LiNbO3 Optical modulator with two-step back-slot structure,” J. Lightwave Technol. 20(12), 2110–2114 (2002). [CrossRef]
5. K. Aoki, J. Kondo, A. Kondo, T. Mori, Y. Mizuno, S. Shimodaira, M. Imaeda, Y. Kozuka, O. Mitomi, and M. Minakata, “High-performance optical modulator with a wide center electrode and thin x-cut LiNbO3 substrate,” IEEE Photonics Technol. Lett. 16(12), 2610–2612 (2004). [CrossRef]
6. K. Aoki, J. Kondo, A. Kondo, T. Ejiri, T. Mori, Y. Mizuno, M. Imaeda, O. Mitomi, and M. Minakata, “Single-drive x-cut thin-sheet LiNbO3 optical modulator with chirp adjusted using asymmetric CPW electrode,” J. Lightwave Technol. 24(5), 2233–2237 (2006). [CrossRef]
7. D. Maack, E. L. Wooten, K. M. Kissa, A. Yi-Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. V. Attanasio, D. J. Fritz, and G. J. Mcbrien, “A review of lithium niobate modulators for fiber-optic communications systems,” IEEE J. Sel. Top. Quantum Electron. 6(1), 69–82 (2000). [CrossRef]
8. P. Rabiei, J. Ma, S. Khan, J. Chiles, and S. Fathpour, “Heterogeneous lithium niobate photonics on silicon substrates,” Opt. Express 21(21), 25573–25581 (2013). [CrossRef] [PubMed]
9. M. Levy and A. M. Radojevic, “Single-crystal lithium niobate films by crystal ion slicing,” Appl. Phys. Lett. 73(16), 2293–2295 (1998). [CrossRef]
10. G. Poberaj, H. Hu, W. Sohler, and P. Guenter, “Lithium niobate on insulator (LNOI) for micro-photonic devices,” Laser Photonics Rev. 6(4), 488–503 (2012). [CrossRef]
11. C. Wang, X. Xiong, N. Andrade, V. Venkataraman, X. F. Ren, G. C. Guo, and M. Lončar, “Second harmonic generation in nano-structured thin-film lithium niobate waveguides,” Opt. Express 25(6), 6963–6973 (2017). [CrossRef] [PubMed]
12. R. Luo, Y. He, H. Liang, M. Li, and Q. Lin, “Highly tunable efficient second-harmonic generation in a lithium niobate nanophotonic waveguide,” Optica 5(8), 1006–1011 (2018). [CrossRef]
13. C. Wang, C. Langrock, A. Marandi, M. Jankowski, M. Zhang, B. Desiatov, M. M. Fejer, and M. Lončar, “Ultrahigh-efficiency wavelength conversion in nanophotonic periodically poled lithium niobate waveguides,” Optica 5(11), 1438–1441 (2018). [CrossRef]
14. M. Zhang, C. Wang, R. Cheng, A. Shams-Ansari, and M. Lončar, “Monolithic ultra-high-Q lithium niobate microring resonator,” Optica 4(12), 1536–1537 (2017). [CrossRef]
15. M. He, M. Xu, Y. Ren, J. Jian, Z. Ruan, Y. Xu, S. Gao, S. Sun, X. Wen, L. Zhou, L. Liu, C. Guo, H. Chen, S. Yu, L. Liu, and X. Cai, “High-performance hybrid silicon and lithium niobate Mach–Zehnder modulators for 100 Gbit s−1 and beyond,” Nat. Photonics 13(5), 359–364 (2019). [CrossRef]
16. C. Wang, M. Zhang, X. Chen, M. Bertrand, A. Shams-Ansari, S. Chandrasekhar, P. Winzer, and M. Lončar, “Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages,” Nature 562(7725), 101–104 (2018). [CrossRef] [PubMed]
17. C. Wang, M. Zhang, B. Stern, M. Lipson, and M. Lončar, “Nanophotonic lithium niobate electro-optic modulators,” Opt. Express 26(2), 1547–1555 (2018). [CrossRef] [PubMed]
18. G. Letal, K. Prosyk, R. Millett, D. Macquistan, S. Paquet, O. Thibaultmaheu, J. F. Gagné, P. L. Fortin, R. Dowlatshahi, and B. Rioux, “Low loss InP C-Band IQ modulator with 40 GHz bandwidth and 1.5 V Vπ,” in Optical Fiber Communications Conference and Exhibition (Optical Society of America, 2015), pp. 1–3.
19. Y. Ogiso, J. Ozaki, Y. Ueda, N. Kashio, N. Kikuchi, E. Yamada, H. Tanobe, S. Kanazawa, H. Yamazaki, and Y. Ohiso, “Over 67 GHz bandwidth and 1.5 V Vπ InP-based optical IQ modulator with n-i-p-n heterostructure,” J. Lightwave Technol. 35, 1450–1455 (2017).
20. C. Kieninger, Y. Kutuvantavida, D. L. Elder, S. Wolf, H. Zwickel, M. Blaicher, J. N. Kemal, M. Lauermann, S. Randel, W. Freude, L. R. Dalton, and C. Koos, “Ultra-high electro-optic activity demonstrated in a silicon-organic hybrid modulator,” Optica 5(6), 739–748 (2018). [CrossRef]
21. S. Wolf, H. Zwickel, C. Kieninger, M. Lauermann, W. Hartmann, Y. Kutuvantavida, W. Freude, S. Randel, and C. Koos, “Coherent modulation up to 100 GBd 16QAM using silicon-organic hybrid (SOH) devices,” Opt. Express 26(1), 220–232 (2018). [CrossRef] [PubMed]
22. X. Li, X. Xiao, H. Xu, Z. Li, T. Chu, J. Yu, and Y. Yu, “Highly efficient silicon Michelson interferometer modulators,” IEEE Photonics Technol. Lett. 25(5), 407–409 (2013). [CrossRef]
23. D. Patel, V. Veerasubramanian, S. Ghosh, A. Samani, Q. Zhong, and D. V. Plant, “High-speed compact silicon photonic Michelson interferometric modulator,” Opt. Express 22(22), 26788–26802 (2014). [CrossRef] [PubMed]
24. P. Dong, X. Liu, S. Chandrasekhar, L. L. Buhl, R. Aroca, and Y. Chen, “Monolithic silicon photonic integrated circuits for compact 100 Gb/s coherent optical receivers and transmitters,” IEEE J. Sel. Top. Quantum Electron. 20(4), 150–157 (2014). [CrossRef]
25. J. Jian, P. Xu, H. Chen, M. He, Z. Wu, L. Zhou, L. Liu, C. Yang, and S. Yu, “High-efficiency hybrid amorphous silicon grating couplers for sub-micron-sized lithium niobate waveguides,” Opt. Express 26(23), 29651–29658 (2018). [CrossRef] [PubMed]
26. X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang, “Apodized waveguide grating couplers for efficient coupling to optical fibers,” IEEE Photonics Technol. Lett. 22(15), 1156–1158 (2010). [CrossRef]
27. J. Sun, C. Zhang, X. Xiao, W. Sun, X. Zhang, T. Chu, J. Yu, and Y. Yu, “High efficiency grating coupler for coupling between single-mode fiber and SOI waveguides,” Chin. Phys. Lett. 30(1), 014207 (2013). [CrossRef]
28. P. Dong, “Travelling-wave Mach-Zehnder modulators functioning as optical isolators,” Opt. Express 23(8), 10498–10505 (2015). [CrossRef] [PubMed]
29. C. Koos, “Nanophotonic Devices for Linear and Nonlinear Optical Signal Processing” (Universitätsverlag Karlsruhe, 2007).
