Low-Driving-Voltage Silicon DP-IQ Modulator

Low-Driving-Voltage Silicon DP-IQ Modulator Kazuhiro Goi, 1 Norihiro Ishikura, 1 Haike Zhu, 1 Kensuke Ogawa, 1 Yuki Yoshida, 2 Ken-ichi Kitayama, 2, 3 Tsung-Yang Liow, 4 Xiaoguang Tu, 4 Guo-Qiang Lo, 4 and Dim-Lee Kwong 4 Silicon photonics is a promising technology to bring drastic reduction in cost and size for various kinds of optical devices. In this paper, we report a low-driving-voltage silicon dual polarization (DP) in-phase quadrature modulator especially for a digital coherent transmission system requiring low power consumption. The modulator includes a newly developed phase shifter and polarization multiplexing waveguide. The phase shifter employs a p-n junction formed in vertical direction, which enhances modulation efficiency and provides a low Vp of 2.5 V. The polarization multiplexing waveguide enables polarization rotation and combination at a low insertion loss of less than.5 db in a broad wavelength range of C- and L-bands. A packaged modulator has exhibited high-speed modulations such as DP 16- and 32-quadrature amplitude modulations, and achieved 2-km standard SMF transmission in DP quadrature phases shift keying well used in commercial optical fiber networks. 1. Introduction Worldwide spread of cloud services with high-speed IT equipment has generated huge amount of traffic in telecommunication networks. Led by these demands, digital coherent transmission has been introduced to long-haul networks and upgraded its capacity to 1 Gb/s/l. This technology is now expected to be applied to shorter reach networks such as metro and datacenter interconnects, where a coherent transceiver for the digital coherent transmission needs to be operated in small space with low power consumption 1). The standardization organization Optical Internetworking Forum, for example, published the specification for compact pluggable form-factor of coherent transceiver CFP2-ACO (C-form factor pluggable analog coherent optics), and announced that they were undertaking standardization of next-generation form-factors 2). A coherent transceiver for the digital coherent transmission uses complicated optical devices to deal with both amplitude and phase of light. For the nextgeneration coherent transceiver, the optical devices need to be improved in size and power consumption as well as electrical devices. Silicon photonics attracts attention as a promising technology giving drastic improvement to such optical devices. A high refractive index contrast waveguides made of silicon and silica confines optical light in small region, allowing high-density deployment of waveguide 1 Optical Device Research Department of Advanced Technology Laboratory 2 National Institute of Information and Communications Technology 3 The Graduate School for the Creation of New Photonics Industries 4 Institute of Microelectronics, Agency for Science, Technology and Research based optical devices. As silicon is semiconductor material, its optical characteristics can be varied by electrical signals. This technology realizes integration of various kinds of optical functions to a small chip of just a few square millimeters. Moreover, the chip can be fabricated at a low cost using a production process well-developed for electronic integrated circuits. We applied this technology to an optical modulator for the digital coherent transmission. The fabricated silicon DP-IQ modulator includes monolithically integrated high-speed phase shifters, a polarization multiplexing waveguide, and monitor PDs, achieving a small footprint of approximately one tenth of a conventional lithium niobate modulator 3) 4). Recently, we further focused on increasing modulation efficiency of the phase shifter and developed a low-driving-voltage silicon DP-IQ modulator package 5). This paper firstly describes details of major functions of the modulator: the phase shifter and the polarization multiplexing waveguide. And then, it shows demonstration of highspeed modulation including DP 16- and 32- quadrature amplitude modulations (QAMs) and long distance fiber transmission in DP- quadrature phases shift keying (QPSK) format widely used in commercial optical fiber networks. 2. Structure of DP-IQ modulator for digital coherent transmission Optical signals in the digital coherent transmission are generated using a DP-IQ modulator. This section describes a structure and modulation formats of the DP-IQ modulator with its fundamental elements: a Mach-Zehnder (MZ) modulator and an IQ modulator. 1

Panel 1. Abbreviations, Acronyms, and Terms. OIF Optical Internetworking Forum CFP C Form-factor Pluggable DSP Digital Signal Processing IQ In-phase Quadrature OOK On-Off Keying BPSK Binary Phase Shift Keying QPSK Quadrature Phase Shift Keying QAM Quadrature Amplitude Modulation DP Dual Polarization MZ Mach-Zehnder Vp Half wave voltage TE Transverse Electric TM Transverse Magnetic TEC Thermoelectric Cooler PAM Pulse Amplitude Modulation AWG Arbitrary Waveform Generator VOA Variable Optical Attenuator EDFA Erbium Doped Fiber Amplifier BPF Band Pass Filter BER Bit Error Rate FEC Forward Error Correction Figure 1 illustrates a configurations of modulators with their typical modulation formats. The top row shows a MZ modulator, which consists of a single MZ interferometer with phase shifters on its two arms. In the phase shifter, an applied electrical signal changes velocity of propagating light, giving phase and amplitude modulation to an output light from the MZ interferometer. The modulated optical signals can be expressed in a constellation, in which electric fields of modulated signals are mapped onto symbols on a Structure Modulation Format MZ mod. CW Input electrical signals DATA: 111... Mod. signals out. (DATA: 11... ) Phase shifter Amplitude: OOK (On off keying) Im 1 BPSK (Binary phase shift keying) Im 1 Re 1 1 1 Re Amplitude: 1 1 1 1 1 18 18 18 Electric field Time Electric field Time MZ mod. 1 QPSK (Quadrature phase shift keying) xqam (Quadrature amplitude modulation) IQ mod. 9 MZ mod. 2 9 Phase shifter (16QAM) (32QAM) Pol. mux. waveguide DP-QPSK (Dual polarizatoin quadrature phase shift keying) DP-IQ mod. IQ mod. 1 IQ mod. 2 X-pol. Y-pol. Fig. 1. Modulator structures and modulation formats. Fujikura Technical Review, 217 11

complex field. The MZ modulator can generate on-off keying (OOK) or binary phase shift keying (BPSK), in which symbols are located on single axis in the constellation. The second row shows an IQ modulator, which is composed of two MZ modulators and a 9-degree phase shifter. This IQ modulator supports QPSK in which data is allotted to phase states differing by 9 degrees and QAM in which data is allotted to states differing in both phase and amplitude. In the bottom row, DP-IQ modulator was described. This modulator includes two IQ modulators and a polarization multiplexing waveguide. The polarization multiplexing waveguide combines two modulated signals from the two IQ modulators with controlling their polarization states to be orthogonal to each other. The modulator generates DP-QPSK or DP-QAM. The DP-IQ modulator thus has complicated structure including four conventional MZ modulators and additional functional elements such as the polarization multiplexing waveguide. In the early products, those elements are realized based on combination of plural materials and technologies, e.g. lithium niobate for the phase shifter, III-V materials for PD, and micro-optics for the polarization multiplexing function. The modulator tends to become considerably large and costly. Novel technology for the nextgeneration DP-IQ modulator is now emerging. 3. Functional elements for silicon DP-IQ modulator Silicon photonics provides integration of the various optical functions for the DP-IQ modulator to a single silicon chip 3) 4). In this section, we describe the two key functional elements: the phase shifter newly introduced for reduction of driving voltage and the polarization multiplexing waveguide providing high conversion efficiency over a broad wavelength range. 3.1 silicon phase shifter for low driving voltage operation A phase shifter controls phase of propagating light by electrical signals. A silicon modulator usually functions through carrier plasma dispersion, which is the phenomenon that a refractive index for light passing through silicon medium is affected by existence of free carriers 6). In C- and L-bands used for telecommunications, an increase in free carrier density decreases the refractive index; a decrease in free carrier density increases the refractive index 7). A change of the refractive index, i.e. a change of velocity of light propagating in a transmission medium, results in a change of phase of light output from the phase shifter. A structure for controlling free carrier density thereby provides function of a phase shifter. Figure 2 illustrates a schematic of a designed silicon phase shifter. The optical waveguide is a silicon-based rib-type waveguide that has a thick rib region at center of the core. In the silicon core, the p-n diode structure is formed and connected to the electrode located on the top of the waveguide through p+ and n+ regions. The electrode is patterned to be a coplanar-type traveling-wave electrode, in which electrical signals propagate with light in parallel to allow high-speed modulation. At the same time, operation under a reverse bias condition also helps the high-speed modulation because it does not include slow minority carrier recombination caused under a normal bias condition. In this design, the p-n junction is located in vertical direction. This implementation contributes to reduction of driving voltage due to following reasons. When the p-n junction is reversely biased, the depletion layer broadens and the free carrier density decreases. The broadening depletion layer covers major center area of the rib region as indicated in the simulated results in Fig. 3(a). On the other hand, as Fig. 3(b) shows, an optical field of light propagating in the silicon rib waveguide also localizes in the rib region. The vertical p-n junction structure thus causes the p+ Silisa(SiO 2 ) Electrode n Silicon substrate (a) Silicon p n+ Depletion layer Ground (b) Fig. 2. Schematics of silicon phase shifter : (a) cross section and (b) electrodes. No bias n p Rib region Depletion layer (a) (b) p Signal Reverse bias(-3v) Fig. 3. (a) Carrier distribution and (b) optical mode field. n 12

large overlaps between the depletion layer and the optical field. That is because a small driving voltage is allowed to shift optical phase. Figure 4 indicates transmission characteristics of the fabricated MZ modulator with the vertical p-n junction phase shifter. Vp is defined as a difference of voltage from where the output power is minimum to where the output power is maximum. This parameter named half-wave voltage is one of the major indicators for efficiency of an optical modulator and represents a voltage required for 18-degree phase shift. The obtained Vp is as low as 2.5 V, which is less than a half of our previous design based on a conventional lateral 8) p-n junction. Figure 5 shows dependence on Transmittance (5dB/div) Vp (V) V p ~2.5V 1 2 3 4 5 6 Applied bias (V) Fig. 4. Applied bias vs. output power in MZ modulator. 3 2 1 153 155 157 159 161 Wavelength (nm) wavelength and temperature. No significant degradation of Vp was observed in the broad wavelength range over C- and L-bands, as well in the wide temperature range up to 15 degrees Celsius. These results conclude that the silicon modulator can be used in the broad wavelength range without a thermo-optic cooler consuming considerable electric power. 3.2 Polarization multiplexing waveguide for C- and L-bands A silicon waveguide devices formed on a silicon wafer is asymmetric between horizontal structure and vertical structure, causing birefringence; characteristics for transverse electric (TE) polarized light and that for transverse magnetic (TM) polarized light differ. The waveguide device is usually designed to function properly only for one polarization state. The modulator mentioned above affects TE polarized light and generates optical signals with TE polarization. For generating polarization multiplexed signals, a polarization multiplexing waveguide should have a function to rotate TE polarized light from a modulator to TM polarized light and combine the TM polarized light to the other TE polarized light from the other modulator. Our previous design of a polarization multiplexing waveguide utilized a directional coupler for the function of the combination. Since the directional coupler had wavelength dependence and it was optimized to C-band, the integrated DP-IQ modulator had limited characteristics in L-band 4) 9). In the new design of the polarization multiplexing waveguide, mode-evolution-type conversion is employed instead, bringing low wavelength dependence 1). Figure 6 illustrates the polarization multiplexing waveguide. The structure of the waveguide is like a transformation from a silicon rib waveguide. Because the two heights are designed to be the same as those of the silicon rib waveguide in the phase shifter, the polarization multiplexing waveguide can be fabricated at the same time with the phase shifter without any additional fabrication process. 3 Port 1 Region A Region B Vp (V) 2 1 TE Port 2 TE Cross section TE TE Port 3 TM+TE 2 4 6 8 1 12 14 16 Temperature (deg C) Fig. 5. Wavelength and temperature dependence of Vp. SiO 2 Si SiO 2 Si Fig. 6. Schematic view of polarization multiplexing waveguide. Fujikura Technical Review, 217 13

When TE polarized light is input from Port 1, it is converted to the first order TE polarization (TE ) in adjacent waveguide in Region A and then further converted to TM polarization in Region B. On the other hand, when TE polarized light is input from Port 2, it propagates as a TE polarized light in the waveguide and reaches to Port 3. This waveguide thus functions as both of a polarization rotator and a combiner. By connecting Ports 1 and 2 to two IQ modulators, polarization multiplexed modulated signals can be output from Port 3. Through this conversion process, the propagating lights stay in only single eigen mode at the waveguide; the lights do not split to the plural of eigen modes. This is mode-evolution-type conversion that has an advantage of low wavelength dependence. Measurement results of a fabricated device are shown in Fig. 7. The remarkable low conversion losses less than.5 db were observed in the broad wavelength range over C- and L-bands. 4. High-speed modulation demonstrations with low-driving-voltage operation We evaluated high-speed modulations and fiber transmissions under low-driving-voltage operation conditions using the silicon DP-IQ modulator package shown in Fig. 8. Figure 9 shows the experimental setup. In the transmitter side, an arbitrary waveform generator (AWG) was used to supply electrical signals in pulse amplitude modulation (PAM) required for QAM generation. The output amplitudes were 1.4 Vppd or 2. Vppd (.7 Vpp or 1. Vpp for each of positive and negative ports) and applied directly to the modulator package without any driver amplifier. The line width of the light source was less than 1 khz and the wavelength was 155 nm. In the receiver side, the modulated signals were detected with the coherent receiver and real-time oscilloscope, and recovered by offline digital signal processing (DSP). In the transmission experiment, the circulating loop Transmittance (db) -.5-1 -1.5 Input Port1(TE) Output Port(TM) -.5-1 Input Port2(TE) Output Port(TE) -1.5 152 154 156 158 16 162 Wavelength (nm) Fig. 7. Transmission spectra of polarization multiplexing waveguide. Fig. 8. Silicon DP-IQ modulator package. CW laser BPF AWG EDFA back to back transmission SW SW 3-dB coupler EDFA 1-km SMF VOA EDFA BPF Coherent receiver EDFA: Erbium Doped Fiber Amplifier BPF: Band Pass Filter SW: Switch DSP: Digital Signal Processing Real-time oscilloscope Offline DSP Fig. 9. Experimental setup for fiber transmission. 14

16 Gbaud 32-QAM 32 Gbaud 16-QAM 32 Gbaud QPSK 48 Gbaud QPSK Back to back 1 km 2 km X-pol. X-pol. Y-pol. Y-pol. BER 5.9x1-3 1.9x1-2 ~1x1-5 4.4x1-3 BER <1-3 1.1x1-3 6.1x1-3 Fig.1. Constellations in various modulation formats. Fig.11. Constellations in 128-Gb/s DP-QPSK format. configuration was employed. Firstly, high-speed modulation formats including QAM were evaluated in the back-to-back configuration. Driving voltage applied from the AWG was 1.4 Vppd. Figure 1 shows constellations in various modulation formats. Symbols are aligned in line with no distortion, indicating that MZ modulator worked appropriately with the low-voltage differential drive. In each constellation, the bit error rate (BER) was less than forward error correction (FEC) limit of 2 1-2 11). We thus confirmed that the modulator provides highspeed modulations over 1 Gb/s with low driving voltage directly applied from the AWG. We next confirmed the long-distance transmission in 128-Gb/s DP-QPSK which is widely used in commercial networks. The driving voltage was 2 Vppd in this experiment. Signal degradation by chromatic dispersion of the fiber was compensated in the DSP. Figure 11 shows constellations in the back-to-back and after transmission. The noise in the constellation increases with an increase of the transmission length. However, even in after transmission of 2 km, the BER of 6.1 1-3, less than the FEC limit, was obtained. The 2-km SMF long distance transmission is also confirmed under the low-driving-voltage operation condition. 5. Conclusion We developed a low-driving-voltage silicon DP-IQ modulator for digital coherent transmission. The phase shifter with the vertical p-n junction structure increases modulation efficiency, achieving the low Vp of 2.5 V. In addition, the characteristics are stable in broad wavelength and temperature ranges. The integrated polarization multiplexing waveguide uses adiabatic conversion to enable low conversion loss of less than.5 db also in the broad wavelength range. Using the silicon DP-IQ modulator with low-drivingvoltage less than 2 Vppd, we demonstrated high-speed modulation such as in DP 16- and 32- QAM, and long distance SMF transmission in 2 km with 128 Gb/s DP-QPSK format. Reduction of a driving voltage of a DP-IQ modulator allows a direct drive from DSP-LSI, which facilitates large reduction both in space and power consumption currently used for a driver amplifier. The low-drivingvoltage silicon DP-IQ modulator will be an indispensable component for coherent transceivers applied to various optical communication fields. References 1) T. Saida, Emerging integrated devices for coherent transmission - digitally assisted analog optics, in Optical Fiber Communication Conference and Exhibition, Th3B.4, 217 2) Optical Networking Forum: http://www.oiforum.com/ 3) K. Goi et al., 128-Gb/s DP-QPSK using low-loss monolithic silicon IQ modulator integrated with partial-rib polarization rotator, in Optical Fiber Communication Conference and Exhibition, W1I.2, 214 4) K. Goi et al., 128-Gb/s Monolithic Silicon Optical Modulator for Digital Coherent Communication, Fujikura Technical Review, No.44, pp.33 4, 215 5) N. Ishikura et al., Transmission characteristics of 32-Gbaud PDM IQ monolithic silicon modulator operating with 2-Vppd drive voltage, in 42nd European Conference and Exhibition on Optical Communication, W.2.E.4, 216 6) G. T. Reed et al., Silicon optical modulators, Nature Photonics, Vol.4, No.8, pp.518 526, 21 7) R. A. Soref and B. R. Bennett, Electrooptical effects in silicon, IEEE J. Quantum Electron., Vol.23, No.1, pp.123 129, 1987 8) K. Goi et al., Low-loss high-speed silicon IQ modulator for QPSK/DQPSK in C and L bands, Optics Express, Vol.22, No.9, pp.173 179, 214 9) K. Goi et al., Low-loss partial rib polarization rotator consisting only of silicon core and silica cladding, Optics Letters, Vol.4, No.7, pp.141 1413, 215 1) A. Oka et al., Low-loss all-adiabatic silicon-waveguide polarization-division multiplexer in C and L bands, in OptoElectronics and Communication Conference and Australian Conference on Optical Fibre Technology 214, WE7E 1, 214 11) G. Tzimpragos et al., A survey on FEC codes for 1G and beyond optical networks, IEEE Communication Surveys & Tutorials, Vol.18, No.1, pp.29 221, 216 Fujikura Technical Review, 217 15