Open Access
Add to BibSonomyAbstract
We demonstrate a single-drive push-pull silicon Mach-Zehnder modulator (MZM) with a π-phase-shift voltage of 3.1 V and speed up to 30 Gb/s. The on-chip insertion loss is 9 dB due to the use of a 6 mm-long phase shifter. Higher switching speed up to 40-50 Gb/s is also demonstrated in devices with shorter phase shifters which require higher drive voltages but have lower insertion losses.
©2012 Optical Society of America
1. Introduction
Silicon-photonics based optoelectronic devices are very promising to provide high-bandwidth communications with low costs and low power consumption because of the compatibility with the proven CMOS manufacturing processes and the associated price-volume curve [1–5]. In an optical transponder, a significant portion of the power consumption comes from the electro-optic modulator and its electronic driver. Low-voltage silicon modulators could greatly reduce the power consumption. However, the small refractive index change from the weak electro-optic effect of silicon poses significant challenges to develop low-voltage silicon modulators. High-quality-factor silicon microcavity modulators [6–9] can achieve very low driving voltage with their tiny sizes but at the expense of reduced optical bandwidths. It is desirable to have high-performance silicon Mach-Zehnder modulators (MZMs) with low drive voltages and high speed.
The performance of a MZM can be determined by three most important parameters, i. e., speed, Vπ (the voltage swing required for a phase change of π between the two arms of MZMs), and insertion loss. Among the three parameters, Vπ is usually a tradeoff of speed and insertion loss, since longer phase shifters in MZMs require smaller Vπ but could result in slower speed and larger insertion loss. In principle, high extinction ratios (ER) can be obtained if the optical power is well balanced in the two arms of MZMs and if they are driven by a full Vπ. Therefore extinction ratios would be less important. Another figure of merit (FOM), commonly used for silicon MZMs, is the product of the voltage and phase-shifter length to achieve a π-phase change, i. e., Vπ⋅L. However, this FOM is an ‘internal’ parameter and is important during the design, but less important to characterize the performance of MZMs (But, Vπ⋅L is an important parameter if compact modulators are preferred).
High-speed modulation in silicon can be realized by free-carrier induced index change [10]. The carrier-density modulation in a silicon waveguide can be obtained with carrier injection in a forward-biased pin diode structure, carrier accumulation in a MOS capacitor structure, or carrier depletion in a reverse-biased pn diode structure [11]. Carrier injection is the most efficient modulation mechanism, but it is difficult to achieve high speed since it suffers from high junction capacitance and slow free-carrier recombination. By using complicated driving signals with pre-emphasis, 10 Gb/s data rate has been reported for carrier-injection silicon MZM devices [12]. Carrier accumulation in MOS capacitors has better modulation efficiency than carrier-depletion in a reverse-biased pn junction. However, the speed of MOS-type modulator is limited by the high capacitances from a thin oxide layer, and a 10-25 Gb/s switching speed has been reported [13–15]. The poly-Si layer often used in the MOS capacitor structure also creates high excess optical loss in the modulator. Carrier depletion has the worst modulation efficiency [16–27], yet comes with the best high-speed performance, as the junction capacitance can be reduced by optimizing doping profiles. Recently, high switching rates of 30-50 Gb/s have been demonstrated [17,24–27]. However, most of the reported high-speed silicon MZMs were demonstrated by using devices with rather short phase shifters with an extremely high or unachievable Vπ.
In this paper, we report single-drive push-pull carrier-depletion MZMs with low Vπ and high speed. The Vπ can be as low as 3.1 V at a high switching speed up to 30 Gb/s. We also demonstrate 40-50 Gb/s modulations for shorter devices with higher Vπ but with lower insertion loss. This type of MZMs has additional advantages including single drive signals without pre-emphasis and low chirp from push-pull operations.
2. Device structure
The modulation of MZMs is based on the carrier depletion of the silicon pn junction embedded in the middle of silicon waveguides. A transmission-line electrode is loaded with the junction capacitor to provide a loaded characteristic impedance of 50 ohm with a phase velocity to match the optical group velocity in the silicon waveguide. Larger load capacitances make it more challenging to design the travelling-wave electrodes. A single-drive push-pull scheme, outlined in our previous paper [26], can be employed to effectively reduce the capacitance in half. In the current design, both modulator arms are symmetrically doped and share a highly n-doped region in the center. Symmetric coplanar strips are connected with outside highly p-doped regions of two MZM arms. The central highly n-doped region is connected separately for DC connection. Since the two junction capacitors in two MZM arms are connected in serial, the loaded capacitance on the transmission line is half of that for one arm (assume the capacitance are the same for both arms). This design allows both arms to be driven in a push-pull fashion with a single input drive signal. Furthermore, the push-pull scheme reduces the modulation-induced frequency chirp, which has been experimentally verified in [26].
In the current design, we do not use segmentally loaded transmission lines, which is different from that in Ref [26]. An asymmetric MZM is formed with a length difference of 40 μm between two arms, resulting in a free spectral range (FSR) of ~14.5 nm. Multimode interference couplers (MMIs) are used as input/output 3-dB couplers. The junction design and device fabrication are similar to previously reported silicon modulators presented in Refs [22]. and [26]. The device was fabricated on a 200-mm silicon-on-insulator (SOI) wafer with a buried oxide thickness of 3 μm and a top silicon thickness of 220 nm in a CMOS-compatible fab. The fabrication is mainly based on DUV (248 nm) optical lithography, plasma-assisted dry etching, ion implantation and rapid thermal annealing. The Si waveguides have a width of 0.5 μm and a slab thickness of 90 nm. The p and n doping levels in pn junctions are estimated as ~5e17 cm−3, leading to a junction capacitance around 0.3-0.4 fF/μm [26]. The metals in the transmission line are Al with a thickness of 2 μm and a width of 60 μm. Figure 1 shows a photograph of a fabricated MZM.
3. Experimental results
We first measured the optical transmission spectra for MZMs with different phase-shifter lengths. The recorded spectra in Fig. 2(a) were normalized to a reference waveguide. The peak transmission represents the on-chip insertion loss of MZMs, which includes the losses in MMIs and waveguide propagation loss in the phase shifters. For a 6-mm long device, the insertion loss is 9 dB. The device insertion loss comes mainly from the free-carrier losses in the doped pn-junction waveguides. The insertion loss is proportional to the device length as shown in Fig. 2(b) measured around 1550 nm. A linear fit shows that the propagation loss is about 12 dB/cm in the doped silicon waveguides, and a loss of 1.8 dB comes from the two 3-dB couplers. The ER of MZMs decreases with the longer device length, which may indicate the power imbalance between two MZM arms mainly comes from the waveguide loss variation rather than the power splitting/combining of MMI couplers.
Fig. 2 (a) Transmission spectra of silicon MZMs normalized to a reference waveguide. (b) On-chip insertion loss versus phase shifter length.
The modulation efficiency of the phase shifters is characterized by directly applying dc voltage on the striplines. Since the two diodes in two MZM arms are connected in serial in the opposite polarity, one of the two diodes is reverse-biased and the other is forward-biased, regardless of the voltage polarity. The applied voltage drops mostly on the reverse-biased diode. At a 10 V bias, the forward-biased diode only takes about 0.2 V from the measured current-voltage characteristics of a single test diode. Approximately, only the reversed-biased diode contributes the index change under the applied external dc voltage. Figure 3(a) -3(c) shows the normalized transmission of the MZMs versus the applied voltages while setting the input wavelength at the minimum transmission for 0 V. The extinction ratios under this dc condition are measured as 20 dB, 16 dB and 12.5 dB for the phase-shift length of 2 mm, 4 mm, and 6 mm, respectively. The Vπ is measured when the transmission reaches the maximum. The measured Vπ’s are 12 V, 5.2 V and 3.1 V for the phase-shifter length of 2 mm, 4 mm, and 6 mm, respectively. The calculated Vπ⋅L for different device lengths is summarized in Fig. 3(d). The Vπ⋅L product decreases with the device length. The depletion width of a pn junction has a square-root dependence of voltages [9], resulting in higher modulation efficiency in small-voltage regions (or longer phase shifters).
Fig. 3 (a)-(c), Optical transmission under different applied voltages. (d) Modulation efficiency versus the phase shifter length.
We measured the small signal electro-optic (EO) response of the 6 mm-long modulator by using HP 8703A lightwave component analyzer which covers the frequency region of 130 MHz to 20 GHz. The RF signal from the analyzer was applied on the MZM through a high-speed probe. The end of the transmission line was terminated with a 50-ohm resistor from another high-speed probe. The modulated optical signal, amplified by an erbium-doped fiber amplifier (EDFA), was sent back to the analyzer. The measured 3-dB bandwidth exceeds 20 GHz when the MZM is biased more than 2 V, as shown in Fig. 4(a) . Figures 4(b)-4(c) show eye diagrams under a 231-1 Non-return-to-zero (NRZ) PRBS signal with a 3.5 V peak-to-peak driving voltage and 3.0 V dc bias. The wavelength is set at the quadrature point of the MZI. The measured eye diagrams exhibit excellent eye openings for both 20 Gb/s and 30 Gb/s, with a measured ER of 10 dB and 8.5 dB, respectively. The extinction ratios in eye measurement may be underestimated due to the noise in the 0 and 1 levels. In summery, we demonstrate 30 Gb/s operation for this MZM with a 6 mm-long phase shifter and a dc Vπ of 3.1 V.
Fig. 4 (a) Electro-optic response of the MZM with a 6 mm-long phase shifter under different dc biases. (b) and (c) eye diagrams under high-speed operation. The driving conditions are explained in the text.
For shorter devices with higher Vπ, higher switching speeds up to 40 and 50 Gb/s are achieved with lower insertion losses (Fig. 5 ). For the device with a 4 mm-long phase shifter with a Vπ of 5.2 V and an insertion loss of 6.6 dB, 40 Gb/s is demonstrated with an ER of 6 dB, as shown in the measured eye diagram in Fig. 5(b). For this measurement, the wavelength is set at the quadrature point of the MZI. For the 2 mm-long device, the switching speed reaches 50 Gb/s with an ER of 4.7 dB as shown in Fig. 5(d), which is limited by our 50 Gb/s measurement equipment with reduced drive voltage. Since the drive voltage is only ~4.5 V, which is far below the Vπ, we carried out this measurement with a wavelength close to the minimum transmission point.
Fig. 5 Eye diagrams for the MZMs with a 4 mm-long phase shifter (a-b) and 2 mm-long phase shifter (c-d). For all the measurement, the peak-to-peak drive voltage is about 4.5-5.5 V depending on the speed. The corresponding dc biases are (a) 3 V, (b) 5 V, (c) 4 V, and (d) 6 V, respectively.
4. Discussion and conclusion
Table 1 summarizes the modulation performance of the previously reported silicon MZMs with speed >25 Gb/s and our devices in this work. Most of the previously reported high-speed silicon MZM devices in Refs [15,17,25,27] were demonstrated using extremely short phase shifters, where a full π-phase modulation may not be achievable. For instance, the 40 Gb/s device in Ref [17]. requires at least 40 V based on the reported Vπ⋅L and the device length. The breakdown voltage for the pn junction may be much less than this voltage. Refs [24]. and [26] presented two MZMs with reasonable device lengths and modulation efficiencies so that Vπ is approximately 8-10 V. Our device with a 2 mm-long phase shifter has close insertion loss and Vπ as that in [26], but the speed can be much higher. Our device with a 4 mm-long phase shifter has close speed with that in [24], but with lower Vπ and lower insertion loss. Our device with a 6 mm-long phase shifter has the lowest Vπ and still demonstrates the high-speed operation up to 30 Gb/s.
Table 1. , Performance comparison of previously reported high-speed silicon MZMs (>25 Gb/s) and devices in this work.
High-quality optical signals are required for long-haul optical communications. Driving MZMs with a full Vπ is preferred to achieve maximum extinction ratios and minimum insertion losses for NRZ signals. For many advanced modulation formats, such as quadrature phase-shift keying (QPSK), 2Vπ is usually used to drive push-pull MZMs to achieve prescribed phase/amplitude modulations. In these applications, low-Vπ MZMs are critical to reduce the voltage swing requirement of high-voltage drivers which are difficult to realize for broadband operations. The demonstrated Vπ of 3.1 V in this work is comparable to that of commercial LiNbO3 MZMs, and can be driven by commercial high-speed modulator drivers for long-reach optical link. Meanwhile, optical amplifiers can be used to compensate the high insertion loss. For short reach applications in which optical amplifiers are not allowed, the silicon MZMs with shorter device length and lower insertion loss can be employed. Driving such MZMs with a lower voltage than Vπ can still produce a high-ER modulation if the MZMs are biased close to the minimum-transmission wavelengths [17,25,27]. Therefore, the high-speed silicon MZMs presented in this work can be utilized for both long reach and short reach applications.
Acknowledgments
We thank Tsung-Yang Liow and Guo-Qiang Lo of the Institute of Microelectronics, Singapore on fabrication, Christopher R. Doerr and Pietro Bernasconi for helpful discussion on device characterizations, and David Neilson, Martin Zirngibl and Jeanette Fernandes of their supports.
References and links
1. R. A. Soref, “The past, present and future of silicon photonics,” IEEE. J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]
2. L. C. Kimerling, D. Ahn, A. B. Apsel, M. Beals, D. Carothers, Y.-K. Chen, T. Conway, D. M. Gill, M. Grove, C.-Y. Hong, M. Lipson, J. Liu, J. Michel, D. Pan, S. S. Patel, A. T. Pomerene, M. Rasras, D. K. Sparacin, K.-Y. Tu, A. E. White, and C. W. Wong, “Electronic–photonic integrated circuits on the CMOS platform,” Proc. SPIE 6125, 6–15 (2006).
3. B. Jalali, M. Paniccia, and G. Reed, “Silicon photonics,” IEEE Microw. Mag. 7(3), 58–68 (2006). [CrossRef]
4. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]
5. A. V. Krishnamoorthy, R. Ho, X. Zheng, H. Schwetman, J. Lexau, P. Koka, G. Li, I. Shubin, and J. E. Cunningham, “Computer systems based on silicon photonic interconnects,” Proc. IEEE 97(7), 1337–1361 (2009). [CrossRef]
6. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]
7. M. R. Watts, D. C. Trotter, R. W. Young, and A. L. Lentine, “Ultralow power silicon microdisk modulators and switches,” in Proceedings of 5th IEEE International Conference on Group IV Photonics (2008), pp 4–6.
8. P. Dong, S. Liao, D. Feng, H. Liang, D. Zheng, R. Shafiiha, C.-C. Kung, W. Qian, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Low Vpp, ultralow-energy, compact, high-speed silicon electro-optic modulator,” Opt. Express 17(25), 22484–22490 (2009). [CrossRef] [PubMed]
9. P. Dong, S. Liao, H. Liang, W. Qian, X. Wang, R. Shafiiha, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “High-speed and compact silicon modulator based on a racetrack resonator with a 1 V drive voltage,” Opt. Lett. 35(19), 3246–3248 (2010). [CrossRef] [PubMed]
10. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]
11. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]
12. W. M. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Ultra-compact, low RF power, 10 Gb/s silicon Mach-Zehnder modulator,” Opt. Express 15(25), 17106–17113 (2007). [CrossRef] [PubMed]
13. A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef] [PubMed]
14. http://www.ofcnfoec.org/conference_program/2009/images/09-DAndrea.pdf.
15. J. Fujikata, J. Ushida, and Y. Ming-Bin, Z. ShiYang, D. Liang, P. L. Guo-Qiang, D.-L. Kwong, and T. Nakamura, “25 GHz operation of silicon optical modulator with projection MOS structure,” in Optical Fiber Communication Conference (Optical Society of America, 2010), paper OMI3.
16. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007). [CrossRef] [PubMed]
17. L. Liao, A. Liu, D. Rubin, J. Basak, Y. Chetrit, H. Nguyen, R. Cohen, N. Izhaky, and M. Paniccia, “40 Gbit/s silicon optical modulator for high speed applications,” Electron. Lett. 43(22), 1196–1197 (2007). [CrossRef]
18. D. Marris-Morini, L. Vivien, J. M. Fédéli, E. Cassan, P. Lyan, and S. Laval, “Low loss and high speed silicon optical modulator based on a lateral carrier depletion structure,” Opt. Express 16(1), 334–339 (2008). [CrossRef] [PubMed]
19. S. J. Spector, M. W. Geis, G.-R. Zhou, M. E. Grein, F. Gan, M. A. Popovic, J. U. Yoon, D. M. Lennon, E. P. Ippen, F. Z. Kärtner, and T. M. Lyszczarz, “CMOS-compatible dual-output silicon modulator for analog signal processing,” Opt. Express 16(15), 11027–11031 (2008). [CrossRef] [PubMed]
20. D. M. Gill, S. S. Patel, M. Rasras, K.-Y. Tu, A. E. White, Y.-K. Chen, A. Pomerene, D. Carothers, R. Kamocsai, C. Hill, and J. Beattie, “CMOS compatible Si-ring assisted Mach-Zehnder interferometer with internal bandwidth equalization,” in Proceedings of 6th IEEE International Conference on Group IV Photonics (2009), paper PD 1.2.
21. M. R. Watts, W. A. Zortman, D. C. Trotter, R. W. Young, and A. L. Lentine, “Low voltage, compact, depletion-mode, silicon Mach-Zehnder modulator,” IEEE J. Sel. Top. Quantum Electron. 16(1), 159–164 (2010). [CrossRef]
22. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on SOI: monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 307–315 (2010). [CrossRef]
23. N.-N. Feng, S. Liao, D. Feng, P. Dong, D. Zheng, H. Liang, R. Shafiiha, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “High speed carrier-depletion modulators with 1.4V-cm V(π)L integrated on 0.25microm silicon-on-insulator waveguides,” Opt. Express 18(8), 7994–7999 (2010). [CrossRef] [PubMed]
24. 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). [CrossRef] [PubMed]
25. F. Y. Gardes, D. J. Thomson, N. G. Emerson, and G. T. Reed, “40 Gb/s silicon photonics modulator for TE and TM polarisations,” Opt. Express 19(12), 11804–11814 (2011). [CrossRef] [PubMed]
26. L. Chen, C. Doerr, P. Dong, and Y.-K. Chen, “Monolithic silicon chip with 10 modulator channels at 25 Gbps and 100-GHz spacing,” in 37th European Conference and Exposition on Optical Communications, (Optical Society of America, 2011), paper Th.13.A.1.
27. D. Thomson, F. Gardes, J. Fedeli, S. Zlatanovic, Y. Hu, B. Kuo, E. Myslivets, N. Alic, S. Radic, G. Z. Mashanovich, and G. T. Reed, “50Gbit/s silicon optical modulator,” IEEE Photon. Technol. Lett. 24, 234–236 (2012). [CrossRef]
