High-efficiency Si optical modulator using Cu travelling-wave electrode

We demonstrate a high-efficiency and CMOS-compatible silicon Mach-Zehnder Interferometer (MZI) optical modulator with Cu travelingwave electrode and doping compensation. The measured electro-optic bandwidth at Vbias = −5 V is above 30 GHz when it is operated at 1550 nm. At a data rate of 50 Gbps, the dynamic extinction ratio is more than 7 dB. The phase shifter is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (Vπ·Lπ) of ~18.5 V·mm. Such a Cu-photonics technology provides an attractive potentiality for integration development of silicon photonics and CMOS circuits on SOI wafer in the future. ©2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (230.4000) Microstructure fabrication; (250.7360) Waveguide modulators; (230.3990) Micro-optical devices. References and links 1. M. Paniccia, “Integrating silicon photonics,” Nat. Photonics 4(8), 498–499 (2010). 2. L. Tsybeskov, D. J. Lockwood, and M. Ichikawa, “Silicon photonics: CMOS going optical,” Proc. IEEE 97(7), 1161–1165 (2009). 3. Y. H. D. Lee and M. Lipson, “Back-end deposited silicon photonics for monolithic integration on CMOS,” IEEE J. Sel. Top. Quantum Electron. 19(2), 8200207 (2013). 4. Q. Fang, T. Y. Liow, J. F. Song, K. W. Ang, M. B. Yu, G. Q. Lo, and D. L. Kwong, “WDM multi-channel silicon photonic receiver with 320 Gbps data transmission capability,” Opt. Express 18(5), 5106–5113 (2010). 5. D. J. Thomson, F. Y. Gardes, J. M. Fedeli, S. Zlatanovic, Y. F. Hu, B. P. P. Kuo, E. Myslivets, N. Alic, S. Radic, G. Z. Mashanovich, and G. T. Reed, “50-Gb/s silicon optical modulator,” IEEE Photon. Technol. Lett. 24(4), 234–236 (2012). 6. X. N. Chen, Y. S. Chen, Y. Zhao, W. Jiang, and R. T. Chen, “Capacitor-embedded 0.54 pJ/bit silicon-slot photonic crystal waveguide modulator,” Opt. Lett. 34(5), 602–604 (2009). 7. K. Ogawa, K. Goi, Y. T. Tan, T. Y. Liow, X. G. Tu, Q. Fang, G. Q. Lo, and D. L. Kwong, “Silicon MachZehnder modulator of extinction ratio beyond 10 dB at 10.0-12.5 Gbps,” Opt. Express 19(26), B26–B31 (2011). 8. J. F. Ding, H. T. Chen, L. Yang, L. Zhang, R. Q. Ji, Y. H. Tian, W. W. Zhu, Y. Y. Lu, P. Zhou, and R. Min, “Low-voltage, high-extinction-ratio, Mach-Zehnder silicon optical modulator for CMOS-compatible integration,” Opt. Express 20(3), 3209–3218 (2012). 9. P. Dong, L. Chen, and Y.-K. Chen, “High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators,” Opt. Express 20(6), 6163–6169 (2012). 10. X. G. Tu, T. Y. Liow, J. F. Song, X. S. Luo, Q. Fang, M. B. Yu, and G. Q. Lo, “50-Gb/s silicon optical modulator with traveling-wave electrodes,” Opt. Express 21(10), 12776–12782 (2013). 11. X. G. Tu, T. Y. Liow, J. F. Song, M. B. Yu, and G. Q. Lo, “Fabrication of low loss and high speed silicon optical modulator using doping compensation method,” Opt. Express 19(19), 18029–18035 (2011). 12. L. Yang and J. F. Ding, “High-speed silicon Mach-Zehnder optical modulator with large optical bandwidth,” J. Lightwave Technol. 32(5), 966–970 (2014). 13. H. Xu, X. Y. Li, X. Xiao, Z. Y. Li, Y. D. Yu, and J. Z. Yu, “Demonstration and characterization of high-speed silicon depletion-mode Mach–Zehnder modulators,” IEEE J. Sel. Top. Quantum Electron. 20(4), 3400110 (2013). 14. M. Streshinsky, R. Ding, Y. Liu, A. Novack, Y. S. Yang, Y. J. Ma, X. G. Tu, E. K. S. Chee, A. E.-J. Lim, P. G.Q. Lo, T. Baehr-Jones, and M. Hochberg, “Low power 50 Gb/s silicon traveling wave Mach-Zehnder modulator near 1300 nm,” Opt. Express 21(25), 30350 (2013). 15. M. Ziebell, D. Marris-Morini, G. Rasigade, J. M. Fédéli, P. Crozat, E. Cassan, D. Bouville, and L. Vivien, “40 Gbit/s low-loss silicon optical modulator based on a pipin diode,” Opt. Express 20(10), 10591–10596 (2012). #221387 $15.00 USD Received 20 Aug 2014; revised 20 Oct 2014; accepted 28 Oct 2014; published 21 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029978 | OPTICS EXPRESS 29978 16. D. J. Thomson, F. Y. Gardes, Y. Hu, G. Mashanovich, M. Fournier, P. Grosse, J.-M. Fedeli, and G. T. Reed, “High contrast 40Gbit/s optical modulation in silicon,” Opt. Express 19(12), 11507–11516 (2011). 17. H. Yu, M. Pantouvaki, J. Van Campenhout, D. Korn, K. Komorowska, P. Dumon, Y. Li, P. Verheyen, P. Absil, L. Alloatti, D. Hillerkuss, J. Leuthold, R. Baets, and W. Bogaerts, “Performance tradeoff between lateral and interdigitated doping patterns for high speed carrier-depletion based silicon modulators,” Opt. Express 20(12), 12926–12938 (2012). 18. J. C. Rosenberg, W. M. J. Green, S. Assefa, D. M. Gill, T. Barwicz, M. Yang, S. M. Shank, and Y. A. Vlasov, “A 25 Gbps silicon microring modulator based on an interleaved junction,” Opt. Express 20(24), 26411–26423 (2012). 19. D. Edelstein, J. Heidenreich, R. Goldblatt, W. Cote, C. Uzoh, N. Lustig, P. Roper, T. Mcdevitt, W. Motsiff, A. Simon, J. Dukovic, R. Wachnik, H. Rathore, R. Schulz, L. Su, S. Luce, and J. Slattery, “Full copper wiring in a sub-0.25um CMOS ULSI technology,” Proceedings of IEEE International Electron Devices Meeting (IEEE, 1997), pp. 773–776. 20. X. Zhu, S. Santhanam, H. Lakdawala, H. Luo, and G. K. Fedder, “Copper interconnect low-K dielectric postCMOS micromachining,” Proceedings of 11th International Conference on Solid-State Sensors and Actuators (Munich, Germany, 2001), pp. 1548–1551. 21. Q. Jiang, Y. F. Zhu, and M. Zhao, “Copper metallization for current very large scale integration,” Recent Pat. Nanotechnol. 5(2), 106–137 (2011). 22. S. Assefa, S. Shank, W. Green, M. Khater, E. Kiewra, C. Reinholm, S. Kamlapurkar, A. Rylyakov, C. Schow, F. Horst, H. Pan, T. Topuria, P. Rice, D. M. Gill, J. Rosenberg, T. Barwicz, M. Yang, J. Proesel, J. Hofrichter, B. Offrein, X. Gu, W. Haensch, J. Ellis-Monaghan, and Y. Vlasov, ” A 90nm CMOS integrated nano-photonics technology for 25Gbps WDM optical communications applications,” Proceedings of IEEE International Electron Devices Meeting (IEEE, 2012), postdeadline session 33.8. pp. 809–811. 23. J. E. Cunningham, I. Shubin, H. D. Thacker, L. Jin-Hyoung, L. Guoliang, Z. Xuezhe, J. Lexau, R. Ho, J. G. Mitchell, L. Ying, Y. Jin, K. Raj, and A. V. Krishnamoorthy, “Scaling hybrid-integration of silicon photonics in Freescale 130nm to TSMC 40nm-CMOS VLSI drivers for low power communications,” Electronic Components and Technology Conference (ECTC) (IEEE 62nd, 2012), pp. 1518–1525. 24. T. Pinguet, P. M. D. Dobbelaere, D. Foltz, S. Gloeckner, S. Hovey, Y. Liang, M. Mack, G. Masini, A. Mekis, M. Peterson, T. Pinguet, S. Sahni, J. Schramm, M. Sharp, L. Verslegers, B. P. Welch, K. Yokoyama, and S. H. Yu, “25 Gb/s silicon photonic transceivers,” 2012 IEEE 9th International Conference on Group IV Photonics (GFP) (IEEE, 2012), pp.189–191. 25. R. H. Havemann and J. A. Hutchby, “High-performance interconnects: an integration overview,” Proc. IEEE 89(5), 586–601 (2001). 26. M. Matsumoto, K. Suzuki, T. Sakamoto, and A. Kamisawa, “Technology challenges for advanced Cu CMP using a new slurry free process,” Proceedings of IEEE international conference on interconnect technology (IEEE, 1999), pp. 92–94. 27. R. Chang and C. J. Spanos, “Dishing-radius model of copper CMP dishing effects,” IEEE Trans. Semicond. Manuf. 18(2), 297–303 (2005). 28. S. Lakshminarayanan, P. J. Wright, and J. Pallinti, “Electrical characterization of the copper CMP process and derivation of metal layout rules,” IEEE Trans. Semicond. Manuf. 16(4), 668–676 (2003). 29. R. Ding, Y. Liu, Q. Li, Y. S. Yang, Y. J. Ma, K. Padmaraju, A. E. J. Lim, G. Q. Lo, K. Bergman, T. B. Jones, and M. Hochberg, “Design and characterization of a 30-GHz bandwidth low-power silicon traveling-wave modulator,” Opt. Commun. 321, 124–133 (2014). 30. J. M. Liu, Photonic Devices (Cambridge University, 2005), Chap. 6.


Introduction
Silicon photonics devices have a promising future in the application of optical communications, due to their low cost, high performances and compatibility with the existing complementary metal-oxide-semiconductor (CMOS) technology [1][2][3][4].Silicon optical modulator is one key component for data communication related application, and significant progress has been achieved in the field of silicon photonics modulator over the past decade [5][6][7][8][9][10][11][12][13][14][15][16][17][18].Among all kinds of silicon optical modulators, silicon carrier-depletion-based modulator has proven itself to be the most prevailing solution for optical modulation on silicon because of its high performances, such as high speed and low power consumptions.
In most reported papers, aluminum is usually adopted as electrodes and contact/via plugs material in silicon optical modulators [6][7][8][9][10][11][12][13][14].For example, one kind of silicon slot photonic crystal modulator with 0.54 pJ/bit power consumption was formed with Al metal electrode [6].In 2011, one group from Fujikura achieved a silicon Mach-Zehnder Interferometer (MZI) modulator of extinction ratio beyond 10 dB at 10.0-12.5 Gbps [7].Also, a silicon MZI modulator with low power and data rate of 12.5 Gbps was reported [8], and Bell Labs demonstrated a single-drive push-pull silicon MZI modulator with data rate up to 50 Gbps in 2012 [9].Our group also presented a 50 Gbps silicon MZI modulator last year in which Al was also used as the metal electrode [10].These silicon modulators with high data rate and low power consumption based on Al electrode have been achieved in the past few years.However, Al material has its shortcomings, such as nano-pores-induced low density and high resistivity, which result in large delay.Large delay impedes further improvement of high speed devices.A good alternative to be used as electrode is Cu, which can replace Al in silicon photonics devices in the future because it has lower resistivity, higher conductivity and lower activation energy than Al [19][20][21].In principle, these advantages of Cu contribute to higher speed and lower power consumption for silicon photonics active devices and higher integration intensity for circuits.Currently, few silicon photonics devices with Cu electrode have been reported.IMEC reported one modulator with Cu traveling-wave electrode and contact filling material of W in 2012, which has a data rate up to 40 Gbps [17].IBM reported a 25 Gbps microring modulator using Cu electrode [18] and a 90 nm CMOS-photonics technology node for 25 Gbps transceiver [22] in 2012.Oracle labs reported hybrid-integration of silicon photonics using Cu electrode in 2012 [23].Luxtera reported a 4 × 25 Gbps transceiver [24].The stacked Ti/TiN/AlCu/Ti/TiN material utilized as electrodes are also reported to reduce the RF loss [15,16].
In this paper, we demonstrate a high efficiency PN junction silicon optical modulator with Cu traveling-wave electrode on silicon-on-insulator (SOI) wafer with 220 nm-thick top Si layer.The phase shifter length is 3 mm.To reduce the optical transmission loss of phase shifter caused by ion implantation while keeping the modulation efficiency and switching speed at a high level, a doping compensation method is utilized to optimize the doping level on the depletion region of the phase shift [11].Since it is difficult to delineate Cu by subtractive etch due to the limited number of volatile Cu compounds, dual-damascene process was utilized to form the Cu traveling-wave electrode and the contact plugs [25].Cu deposition and chemical-mechanical polishing (CMP) process are included in dualdamascene process.To avoid the dishing caused by CMP process [26] on Cu surface, a latticed Cu surface pattern is designed.The simulated results show this kind of latticed Cu pattern does not degrade the speed of the modulator in the bandwidth range of 40 GHz.The measured results show the bandwidth of our modulator reaches above 30 GHz.Its modulation efficiency (V π ·L π ) is ~18.5 V·mm and the implantation-induced optical loss is ~1.3 dB/mm.The dynamic extinction ratio is 7.08 dB at a bias of −5 V and at a data rate of 50 Gbps, which is also close to the limit of our eye diagram measurement equipment.Cu application can further improve the integration of silicon photonics devices and CMOS circuits in the future.

Modulator design
Figure 1 shows the microscope image of the modulator.The silicon MZI modulator is based on a 3 mm-long PN junction phase shifter on a SOI wafer with 220 nm-thick top Si and 2 µmthick buried oxide (BOX).The waveguide width is 500 nm and the slab height of the ridge waveguide of the phase shifter is 100 nm.A 1 × 2 multimode interference (MMI) structure is used as the splitter and the combiner.It is an asymmetrical MZI structure and the arm length difference ∆L is 300 µm.The inset (a) of Fig. 1 shows the pattern of implantations.The implantation compensations are designed on both areas of the ridge corners for lower implantation-induced optical loss and higher modulation speed.The implantation compensation design is similar to our reported modulator [11].In our design, the Cu thickness is 2 µm.In order to reduce the dishing on Cu surface caused by CMP process [26][27][28], a latticed Cu pattern was used as Cu traveling-wave electrode of our silicon modulator.HFSS, a commercial simulation software, was used to evaluate the RF loss of the latticed Cu electrode.The inset (b) of Fig. 1 shows the latticed Cu pattern.The size of each dielectric slot pattern is 3 µm × 8 µm.The maximum Cu unit size in the electrode is 15 µm × 15 µm, which can effectively reduce the Cu dishing.Three kinds of metal electrodes are simulated, including the normal Al electrode, the normal Cu electrode and the latticed Cu electrode.Coplanar waveguide (CPW) models were adopted in this simulation.All models are with a Si substrate, which has permittivity of ε r = 11.9 and resistivity of ρ = 1000 Ω·cm.Between the Si substrate and the CPW layer, there was an oxide layer with 4 µm-thick.The thickness of the CPW layer is 2 µm.To obtain a 50-Ω impedance match, the width of the central signal CPW was set to be 10 µm, and the gap between the signal and ground was set to be 6.4 µm.Assuming that both materials do not have any defects, and the simulated result of insertion loss S21 and return loss S11 are shown in Fig. 2. The insertion loss and the return loss of the electrical signal in the latticed Cu pattern are quite close to that in the normal Cu electrode at 40GHz, which are both smaller than that in the normal Al electrode for different electrode lengths.These are caused by the lower resistivity of Cu (ρ Cu = 1.72e-6Ω·cm) than Al (ρ Al = 2.63e-6 Ω·cm).The RF 6.4-dB bandwidth is related to the electro-optic (EO) 3-dB bandwidth [29,30].The insertion loss of the latticed Cu electrode is less than 6.4 dB, and the return loss is less than −10 dB within 40 GHz when the electrode length is no more than 5 mm, which is longer than the length of latticed Cu electrode of our modulator.Therefore, this kind of latticed Cu electrode does not degrade the speed of our modulator within the range of 40 GHz.

Modulator fabrication
This silicon modulator was fabricated on an 8-inch SOI wafer with top Si layer of 220 nm and BOX of 2 μm.After P and N compensation implantations were done, the waveguide was formed by double silicon dry etching processes.Figure 3 shows the scanning electron microscope (SEM) images of the silicon waveguide.Four more implantations and a rapid thermal annealing were performed for the formation of PN junction.Based on the actual implantation condition, the P and N doping levels in the PN junctions are estimated as ~4e17 cm −3 .And, the P and N doping levels in both compensation areas are estimated as ~3e16 cm −3 .Dual-damascene process was utilized to form the Cu traveling-wave electrode and the contact connection.After SiO 2 dielectric layer was deposited and polished, the trench of Cu electrode and the contact hole were formed in sequence.To avoid the diffusion of Cu into the Si/SiO 2 layer, a 250 Å-thick TaN barrier layer was deposited first.A 1500 Å-thick Cu seed layer was next deposited by physical vapor deposition (PVD) followed by 6 µm-thick Cu layer by electrochemical-plating (ECP).After removing the excess Cu by CMP, the Cu electrode and contact plugs were finally formed after annealing.The structures of the Cu electrode are presented in Fig. 4, with Fig. 4(a) showing the image of the Cu electrode surface after Cu CMP.A 5000 Å-thick SiO 2 was deposited as a dielectric layer over the Cu electrode subsequently.After the opening of the bond-pad, a thin Al layer was formed on the bond-pad pattern to avoid the oxidation of Cu electrode.Finally, more than 100 μm-deep Si trench was etched to hold optical lensed fiber for coupling with the nano-taper of Si waveguide.Figure 4(b) shows the transmission electron microscope (TEM) image of the phase shifter cross-section with Cu electrode and contact plugs.The inset shows the cross-section of the silicon ridge waveguide and the silicon slab height is ~100 nm.

Cu contact and Cu traveling-wave electrode characterization
Small signal microwave performance in the latticed Cu electrode with 3 mm-long phase shifter was measured through Agilent N4373C Lightwave Component Analyzer (LCA) which has a maximum bandwidth of 40 GHz.The signal is dependent on the PN junction of phase shifter by which the ground and signal Cu electrodes are connected.After the EE calibration of measurement setup, EE S21 signals of Cu electrode are measured with different DC biases which are used to avoid the PN junction effect, shown in Fig. 5(a).The EE bandwidth increases with the bias.This is caused by the carrier depletion out of the PN junction area under a reversed bias voltage, and the effect of PN junction on the Cu electrode reduces with increasing bias.The 6.4-dB bandwidth is more than 40 GHz when the bias is −18 V or more.This result proves that this latticed Cu electrode transmission speed is beyond 40 GHz and it does not degrade the speed of modulator within the range of 40 GHz.The Al traveling-wave electrode, which is laid over the same implanted 3 mm-long phase shifter, is also characterized to compare the bandwidth.The 6.4-dB bandwidth of microwave transmission in the Al electrode increases from 9.7 GHz at Vbias = 0 V to 21.1 GHz at Vbias = −18 V.It is verified that the Cu traveling-wave electrode can provide a higher bandwidth than Al.26.0 GHz @ -10 V 23.9 GHz @ -8 V 21.1 GHz @ -5 V 17.8 GHz @ -3 V 11.6 GHz @ 0 V

Cu EE S21 (dB)
Frequency (GHz)  Two lensed fibers with 2.5 µm focal-length were used to characterize the optical performance of the modulator.Figure 5(b) shows the result of the Cu-induced propagation loss of silicon waveguide.When the Cu-to-waveguide distance is more than 1 µm, the Cuinduced optical loss is less than 0.25 dB/cm.In our design, the Cu-to-waveguide distance is 4 µm as seen in Fig. 4(b).Therefore, the Cu-induced propagation loss in our modulator is negligible.The 2 µm-thick Cu sheet resistance is shown in Table 1 (left).It is 18.3 ± 0.3 mΩ/square, lower than Al sheet resistance of 25.3 ± 0.4 mΩ/square.It also reveals that Cu is better than Al as the electrode and contact material of silicon modulator for higher modulation speed.The Cu-to-Si contact resistivity was also measured and shown in Table 1 (right).The contact size of our modulator is 4 × 3000 µm 2 for both N-and P-contact.Based on the Cu-to-Si contact resistivity, the contact resistances of the modulator for both N-and P-contact are 15 mΩ and 19 mΩ, respectively.

DC measurement of silicon optical modulator
The measured output spectra of the silicon optical modulator under different reversed bias voltages are shown in Fig. 6(a).The bias is applied on one arm of the modulator.The free spectrum range (FSR) of the asymmetric MZI is 1.85 nm, which is dependent on ∆L.Without any bias, the optical extinction ratio of this modualtor is ~28 dB.With the reversed bias, the carrier is pumped out of the waveguide and the optical loss reduces.Thus, the optical extinction ratio decreases due to the unbalance of optical power in two modulator's arms with the increase of the reversed bias.The measured insertion loss of the modulator is ~9 dB as shown in Fig. 6(a), while the dynamic loss is shown in the inset of Fig. 6(a), which is the average measurement result.Based on the waveguide loss of 1.2 dB (undoped waveguide propagation loss ~0.2 dB/mm), 2 MMI loss of 0.6 dB and double fiber-to-waveguide coupling loss of 3.2 dB, the optical loss caused by implantation is 1.3 dB/mm.In Fig. 6(b), a π-phase shift can be realized under 6.0 V reversed voltage for a 3 mm-long phase shifter, which corresponds to a modulation efficiency (Vπ•Lπ) = 18.5 V•mm.With an increase in the applied reversed voltage from −2 V to −10 V, the efficiency is reduced from 11.1 V•mm to 21.5 V•mm, which is caused by the depletion of free carriers in the PN junction.In the deep depletion region, the modulation efficiency becomes lower because there are fewer free carriers left in the depletion region.The efficiency is improved compared with our previous Al-modulator [10] mainly due to the sheet resistance of Cu is 28% smaller than Al as shown in Table 1.Under the same DC bias measurement condition, the Cu-modulator PN junction experiences a higher DC voltage compared with the Al-modulator, therefore Cu-modulator has a larger phase shift.The small signal response of the silicon optical modulator with 3 mm-long phase shifter was measured using Agilent N4373C LCA.The input signal was adopted by a 67 G probe which was pinned on one end of Cu electrode.The 50-Ω matching impedance as a terminator was connected on the other end of Cu electrode by another 67 G probe to reduce the signal reflection.The measured EO bandwidth of silicon modulator is shown in Fig. 7(a).Under a V bias of −5 V, the 3-dB bandwidth of this modulator is up to 37 GHz.In order to get the eye diagram results, a high speed electrical signal coming from a 50/56-Gbps Anritsu Pattern Generator MP1822A was firstly amplified through a 67 G high speed driver.It was applied to the modulator through a 60 G DC bias tee and the input 67 G probe.A continuous-wave light coming from the 1550 nm tunable laser was firstly amplified through an erbium doped fibre amplifier (EDFA), and then it was modulated by adding a non-return-zero pseudorandom binary sequence (PRBS) 2 31 −1 signal under Vbias = −5.0V with Vpp = 3.5 V.The output optical signal was amplified again and collected by an Agilent Digital Communications Analyzer (DCA) after the optical filter.The data rate of the eye diagram reaches 50 Gbps with a dynamic extinction ratio of 7.08 dB, as shown in Fig. 7(b).A performance comparison of this work to other MZI modulators is shown in Table 2.

Conclusion
We have demonstrated a CMOS-compatible silicon MZI optical modulator enabled by Cu traveling-wave electrode and doping compensation.Cu electrode with low resistance provides higher bandwidth than Al electrode and does not show any undesirable influence on the EO performance of silicon modulator within the range of 40 GHz.The phase shifter of the modulator is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (V π ·L π ) of ~18.5 V·mm.The eye diagram of 50 Gbps data rate with dynamic extinction ratio of 7.08 dB is reached under Vbias = −5.0V with Vpp = 3.5 V.The measured EO bandwidth is up to above 30 GHz at V bias = −5.0V when it is operated at 1550 nm.Such a Cu-photonics application can further improve the integration of silicon photonics devices and CMOS circuits in the future.

Fig. 6 .
Fig. 6.Output spectra of silicon modulator with 3 mm-long phase shifter (a), Inset: dynamic insertion loss.Phase shift and efficiency VπLπ of the phase shifter under different applied reversed voltages of the 3 mm-long phase shifter (b).

Fig. 7 .
Fig. 7.The EO bandwidth of the silicon modulator (a) and eye diagram of the silicon modulator (b).