We report the first successful demonstration of quadrature phase-shift keying (QPSK) modulation using two nested silicon Mach-Zehnder modulators. 50-Gb/s QPSK signal is generated with only 2.7-dB optical signal-to-noise ratio penalties from the theoretical limit at a bit-error ratio of 10−3. This result validates that silicon photonics could be a viable and powerful platform of photonic integrated circuits in coherent optical communications.
©2012 Optical Society of America
Global information communications demand optical transmission systems with high spectral efficiency, high per-channel data rate, and low cost. Advanced optical modulation formats, together with wavelength- and polarization-division multiplexing, are key enablers to increase the spectral efficiencies and the aggregation data rates within the limited spectrum bandwidth of available optical amplifiers [1, 2]. High capacity coherent optical transmitters and receivers require many advanced optical components, such as modulators, detectors, power splitters/combiners, polarization splitters, wavelength multiplexing filters, and so on. These components are ideally implemented on photonic integrated circuits (PICs) with the advantages of accurate optical path control, compact size, low power consumption, and potentially low packaging cost.
Emerging silicon photonic technologies promise powerful PIC platforms for the optical communications [3–6]. Silicon optoelectronic devices employ high-index silicon core within low-index silica cladding structures. These waveguide structures allow a tight bending radius down to ~1 μm, resulting in extremely compact optical components. Large-scale silicon PICs also promise high yields and uniform performance of optical elements by leveraging the mature complementary metal–oxide–semiconductor (CMOS) microelectronic fabrication infrastructures. Foundry service is available to reduce chip manufacture cost. Further integration of silicon PICs with CMOS driver circuits lead to more complex functions with low power consumption and low packaging cost.
Recently, silicon waveguide modulators were extensively investigated to improve the modulation rates and reduce the drive voltages [7–17]. Nevertheless, the performance of silicon modulators are usually still worse than LiNbO3 modulators, partially due to the weak electro-optic effect in the silicon modulators using the free-carrier induced index change . As results, most of the reported silicon modulators focused on on-off-keying (OOK) modulation format, geared for short-reach applications which are less demanding in extinction ratios and linearity. Theoretical studies of silicon quadrature phase-shift keying (QPSK) modulation have been reported previously [19–21]; experimental demonstration of 20-Gb/s QPSK signal was only reported recently by using silicon microring modulators . The design and fabrication of nested silicon Mach-Zehnder modulators (MZMs) for QPSK was reported in  without successful demonstration of QPSK constellations nor bit-error ratios (BERs). In this paper, a 50-Gb/s QPSK modulation is experimentally realized by using silicon MZMs. Although quadrature amplitude modulation (QAM) has been demonstrated by LiNbO3 modulators, III-V semiconductors modulators , and polymer modulators , to our best knowledge, this paper is the first to successfully generate advanced modulation formats utilizing silicon MZMs.
2. Silicon QPSK modulator
QAM is a modulation scheme with optical constellations made of multi-level amplitudes and phases to generate optical signals with high spectral efficiency. Among various QAM formats, QPSK has received significant attention in the optical communication industry as a de facto standard for 100-Gb/s polarization-division-multiplexed QPSK (PDM-QPSK) transponders in long-haul optical communication systems. QPSK transmits four phase shifts (spaced by π/2) with constant amplitudes at a symbol rate of half the aggregate bitrate. A QPSK transmitter can be implemented by two nested MZMs, each operating as a binary-phase modulator. A MZM can produce exact π-phase shift if biased around the zero transmission and driven by a push-pull scheme. To produce QPSK, two modulated fields from binary-phase MZMs are combined with a π/2 phase difference.
Push-pull operation of a MZM can be achieved by using either a dual-electrode configuration with two arms driven by differential data, or a single-electrode where both arms are connected together. While the dual-electrode configuration requires only half of the drive voltages for the single-electrode configuration, the latter may be very useful due to reduced number of RF inputs, especially in the applications where multiple modulators need to be integrated. Previously, we reported single-drive push-pull silicon MZMs [15, 17]. A low Vπ of 3.1 V was demonstrated at a modulation speed up to 30 Gb/s . The modulation mechanism in these MZMs is based on the carrier depletion of the silicon pn junction, embedded in the middle of silicon waveguides. A travelling-wave transmission-line electrode is loaded with the junction capacitor. In our modulator design, both modulator arms are symmetrically doped and share a highly n-doped (or p-doped) region in the center. Symmetric coplanar strips are connected with outside highly p-doped (or n-doped) regions of two MZM arms. Since the two junction capacitors in two MZM arms are connected in series, the loaded capacitance on the transmission line is approximately half of that for one arm. This leads to easier design of travelling-wave electrodes for high speeds, while allows both arms are driven in a push-pull fashion with a single input drive signal.
This silicon QPSK PIC has a size of 6.5 mm x 1.4 mm, shown in Fig. 1(a) . The optical power splitters/combiners are implemented by 1x2 and 2x2 multimode interference couplers (MMIs). Thermal phase shifters are employed to the MZM arms to achieve zero-transmission bias (before RF modulation). They are also employed before the final power combiner to adjust 90-degree phase shift between two modulators. These phase shifters are implemented by doped Si-waveguide resistors. The segmentally loaded traveling-wave MZMs here are similar to those reported in Ref , with a Vπ of ~10 V, an on-chip insertion loss of ~4.2 dB, and a modulation rate up to 30 Gb/s. Detailed design and fabrication have been reported in Refs [12, 15, 17].
3. 50-Gb/s QPSK generation
The silicon PIC was wire-bonded to a printed circuit board (PCB) with high-speed transmission lines, DC metal lines, RF connectors, and DC connectors (Fig. 1(b)). Off-chip 50-ohm resistors are bonded to the end of the transmission lines of Si MZMs. Lensed fibers were used to couple the light into the chip.
We performed the QPSK generation using the measurement setup shown in Fig. 2 . The in-phase/quadrature (I/Q) signals use the same 25 Gb/s pseudorandom bit sequence (PRBS) of a length 215–1 from a pattern generator, delayed by 57 bits from each other (Fig. 2(a)). The signals are then amplified individually by two broadband electrical amplifiers to a peak-to-peak voltage of ~12 V. A continuous-wave laser of 1540.0 nm from an external cavity laser with ~1 MHz linewidth was fed to the device. The optical output power was adjusted by a variable optical attenuator (VOA) and then amplified by two erbium-doped fiber amplifiers (EDFAs) and filtered by a tunable optical filter, to achieve the desired optical signal-to-noise ratios (OSNRs) (Fig. 2(b)). The received optical signal was measured and analyzed by an optical spectrum analyzer (OSA), a high-speed sampling oscilloscope, and a real-time 80-GSamples/s digital sampling oscilloscope.
Proper tuning of three phase shifters is required in order to achieve QPSK. We first tuned the phase shifters within the MZMs such that a π-phase difference (zero-transmission point) between the MZM arms are obtained. Next, the phase shifter after one of the two MZMs are adjusted to achieve a π/2-phase difference between two MZM outputs. We then turned on the RF drive signals. The measured optical eye diagram and power spectrum are shown in Figs. 3(a) and 3(b), respectively. The three transition levels in the eye diagram and the suppression of the carrier in the spectrum exhibit the characteristics of the QPSK signal. The thick rails in the eye diagrams may come from: (1) the drive voltage is not high enough to achieve 2Vπ; and (2) the limited bandwidth of the modulators.
Next, we measured the optical signal with a real-time digital sampling oscilloscope at a sampling rate of 80 GSamples/s together with an optical coherent receiver. A homodyne detection scheme is utilized by using the local oscillator (LO) source from splitting the CW laser source launched into the device (Fig. 2(b)). The sampled waveforms are processed off-line with digital signal processing (DSP) codes including a digital finite impulse response (FIR) filter, optimized with the constant modulus algorithm (CMA), and Viterbi-Viterbi feedforward carrier recovery [26, 27]. Differential decoding is used in the detection and the BER of the received signal is calculated by direct error counting.
The recovered QPSK constellations after DSP are shown in Figs. 4(a) and (b) with OSNRs of 29.7 dB and 17.8 dB, respectively, where QPSK modulation are successfully generated by this device. Figure 5 depicts the measured BER versus OSNRs (noise in 0.1-nm bandwidth). At a BER of 1x10−3 (typical requirement in forward error correction (FEC) techniques), the required OSNR for the 50-Gb/s QPSK is about 12.9 dB, which is approximately 2.7-dB higher than the theoretical limit. This OSNR penalty is only about 1-dB higher than a commercial LiNbO3 QPSK modulator. The nonlinear BER curve in Fig. 5 indicates some noise floors from this device. Such BER behaviors typically happen in high-speed advanced modulation formats due to hardware limitations (from both electrical and optical components). FEC techniques are generally used to achieve error-free transmissions in coherent optical communications .
4. Discussions and conclusion
Since the changes of the effective index of silicon waveguides with a reverse-biased pn junction have a nonlinear dependence on the voltage, residual chirps in the push-pull operation may be induced. Nevertheless, this chirp together with the dispersion during transmission can be compensated by proper DSP algorithms . The Vπ in the current device is about 10 V, which is significantly higher than that of LiNbO3 modulators. Based on our previously demonstrated single-drive push-pull silicon modulators of a Vπ of 3.1 V at 30 Gb/s , it is possible to further improve the current design to achieve low-voltage and high-performance silicon coherent transmitters.
In conclusion, we have demonstrated a 50-Gb/s QPSK modulator based on two nested single-drive push-pull silicon MZMs. This demonstration illustrates that silicon photonics could find applications in coherent optical communications. With further integration of polarization rotators and splitters/combiners in silicon photonics [28, 29], monolithic dual-polarization coherent transmitters could be implemented in silicon PICs. With many benefits such as low cost, compact size, high yield, and low power consumption, silicon photonic integrated circuits is a promising candidate to drive commercial high capacity coherent transmitters.
We thank Tsung-Yang Liow and Guo-Qiang Lo of the Institute of Microelectronics, Singapore on fabrication, Pietro Bernasconi, S. Chandrasekhar and Xiang Liu for helpful discussion on device characterizations, and David Neilson, Martin Zirngibl and Jeanette Fernandes for their support.
References and links
1. P. J. Winzer and R. Essiambre, “Advanced optical modulation formats,” Proc. IEEE 94(5), 952–985 (2006). [CrossRef]
2. T. Okoshi and K. Kikuchi, Coherent Optical Fiber Communications (Kluwer, 1998).
3. R. A. Soref, “The past, present and future of silicon photonics,” IEEE. J. Sel. Top. Quant. Electron. 12(6), 1678–1687 (2006). [CrossRef]
4. 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). [CrossRef]
5. B. Jalali, M. Paniccia, and G. Reed, “Silicon photonics,” IEEE Microw. Mag. 7(3), 56–68 (2006). [CrossRef]
6. 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]
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. 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]
11. 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]
12. 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]
13. 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]
14. 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]
15. 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.
16. D. Thomson, F. Gardes, J. Fedeli, S. Zlatanovic, Y. Hu, B. Kuo, E. Myslivets, N. Alic, S. Radic, G. Mashanovich, and G. Reed, “50Gbit/s silicon optical modulator,” IEEE Photon. Technol. Lett. 24, 234–236 (2012).
18. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]
19. L. Zhang, J.-Y. Yang, Y. Li, R. G. Beausoleil, and A. E. Willner, “Silicon microring-resonator-based modulation and demodulation of DQPSK Signals,” Optical Fiber Communication Conference (OFC 2008), (Optical Society of America, 2008), paper OWL5.
21. R. A. Integlia, L. Yin, D. Ding, D. Z. Pan, D. M. Gill, and W. Jiang, “Parallel-coupled dual racetrack silicon micro-resonators for quadrature amplitude modulation,” Opt. Express 19(16), 14892–14902 (2011). [CrossRef] [PubMed]
22. P. Dong, C. Xie, L. Chen, N. K. Fontaine, and Y.-K. Chen, “Experimental demonstration of microring quadrature phase-shift keying modulators,” Opt. Lett. 37(7), 1178–1180 (2012). [CrossRef] [PubMed]
23. K. Ogawa, K. Goi, H. Kusaka, K. Oda, T. Liow, X. Tu, G. Lo, and D. Kwong, “20-Gbps silicon photonic waveguide nested Mach-Zehnder QPSK modulator,” in Optical Fiber Communication Conference (Optical Society of America, 2012), paper JTh2A.20.
24. K. Prosyk, T. Brast, M. Gruner, M. Hamacher, D. Hoffmann, R. Millett, and K. Velthaus, “Tunable InP-based optical IQ modulator for 160 Gb/s,” in 37th European Conference and Exposition on Optical Communications (Optical Society of America, 2011), paper Th.13.A.5.
26. A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory 29(4), 543–551 (1983). [CrossRef]
27. S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]