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Feature Articles: Photonics-electronics Convergence Hardware Technology for Maximizing Network Performance

High-speed Electronic and Optical Device Technologies for Ultralarge-capacity Optical Transmission

Hiroshi Yamazaki, Munehiko Nagatani, Toshikazu Hashimoto, Hideyuki Nosaka, Akihide Sano, and Yutaka Miyamoto

Abstract

To increase the transmission capacity of optical fiber transmission systems, we have developed a multiplexer-digital-to-analogue converter and an optical time-division multiplexing modulator, which respectively generate high-speed signals in the electronic and optical domains. With these devices, we can significantly increase the capacity per wavelength channel.

Keywords: DAC, modulator, QAM

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1. Introduction

The demand for data transmission capacity is continuing to increase, and to meet this demand, optical transmission systems with per-channel rates exceeding 400 Gbit/s are desired. Several advanced technologies such as multicarrier transmission, high-order multilevel modulation, and high-symbol-rate transmission are expected to be key technologies for achieving a significant increase in capacity. In this article, we describe a multiplexer-digital-to-analog converter (MUX-DAC) and an optical time-division multiplexing (OTDM) modulator, which respectively double the symbol rates in the electronic and optical domains.

2. High-speed InP-HBT MUX-DAC

2.1 Circuit configuration

Multilevel modulation with a high baud rate is essential for achieving a channel capacity larger than 400 Gbit/s. The MUX-DAC combines the functions of a 2:1 multiplexer (MUX), which doubles the symbol rate, and a digital-to-analog converter (DAC), which converts the input binary (digital) signals to an output multilevel (analog) signal [1]. A functional-block diagram of the fabricated MUX-DAC with 6-bit resolution is shown in Fig. 1. The MUX-DAC operates as follows; first, the twelve input digital signals are synchronized with each other and reshaped by twelve D flip-flops (DFFs). Then, the outputs of the six pairs of DFFs are respectively multiplexed by six 2:1 MUXs, which generates six digital signals with a doubled symbol rate. Finally, the outputs of the MUXs are converted to an analog signal with a resolution of 6 bits. Since the frequency of the clock signal input to the DFFs and MUXs is half the symbol rate of the output analog signal, the MUX-DAC is suitable for high-symbol-rate applications. For the DAC section, we employed the R-2R ladder configuration to achieve high-speed operation with low power consumption.


Fig. 1. Block diagram of the MUX-DAC.

2.2 Fabrication and characteristics

We have developed fabrication technologies for indium-phosphide (InP) heterojunction bipolar transistor (HBT) devices, which are promising for high-speed applications in photonic networks [2]. We used our InP HBT process with a linewidth of 0.5 μm to fabricate the MUX-DAC. With this process, we can achieve a very high cutoff frequency (a metric of the operation speed of a transistor) of 290 GHz. Power consumption of the MUX-DAC integrated circuit (IC) is 3.4 W, with roughly 2.9 W consumed in the DFFs and MUXs and only 0.5 W used in the DAC section. The differential output swing voltage is 1 V. The 3-dB bandwidth of the frequency response of the MUX-DAC module exceeds 40 GHz, which is currently the largest value in the world. Eye diagrams of the output four-level (2-bit) signals with symbol rates of 50 and 75 Gbaud are shown in Figs. 2(a) and 2(b), respectively. Clear eye openings were achieved at a symbol rate as high as 75 Gbaud.


Fig. 2. Eye diagrams of output four-level signals with symbol rates of (a) 50 Gbaud and (b) 75 Gbaud.

3. OTDM modulator with a silica-LN hybrid configuration

3.1 Circuit configuration

OTDM technologies have been extensively studied with the objective of breaking the limit of the speed of electronics and generating ultrahigh-speed optical signals. In an OTDM transmitter, each tributary signal (signal to be multiplexed) is shaped into a pulsed waveform to avoid interference between the tributaries. Our modulator, which is designed for an OTDM with two tributaries, combines the functions of a pulse generator and two data modulators [3]. Each data modulator is a dual-polarization optical vector modulator, which is the standard modulator in currently deployed 100-Gbit/s digital coherent systems.

The OTDM modulator generates the output optical signal with a symbol rate that is double that of each input electronic signal. Unlike conventional OTDM setups, in which a pulsed light source and separate modulators are connected via fiber, the integrated OTDM modulator provides a much smaller footprint and higher stability of the relative optical phase between the tributaries.

The OTDM modulator consists of a pulse generator, four optical vector modulators, and a polarization-multiplexing (PM) circuit, as shown in Fig. 3(a). Each of the two outputs of the pulse generator is bifurcated and sent to two of the four vector modulators. The outputs of each pair of vector modulators connected to different ports of the pulse generator are coupled, and the two coupled pairs are finally multiplexed in the PM circuit with orthogonal polarizations. The pulse generator consists of two Mach-Zehnder modulators (MZMs) connected in parallel with an output 2 × 2 coupler. To obtain a final output symbol rate of B, the two MZMs in the pulse generator are driven with sinusoidal clock signals with a frequency of B/4 and a relative delay of 1/B. Each vector modulator also consists of two parallel MZMs, which are driven with data signals with a symbol rate of B/2. As shown in Fig. 3(b), the intensity peak of the output from one port of the pulse generator always coincides with the extinction of the output from the other port and vice versa. Thus, we can transmit the two tributary signals without inter-symbol interference. Unlike the conventional OTDM with a pulsed light source, which is complicated and provides low spectral efficiency because of the broad spectral bandwidth of the pulse train, our OTDM modulator operates with a simple continuous-wave light source and offers higher spectral efficiency.


Fig. 3. (a) Optical-circuit diagram and (b) principle of the OTDM modulator.

3.2 Fabrication method and characteristics

To fabricate the OTDM modulator with the complex configuration shown in Fig. 3, we used a hybrid integration of silica planar lightwave circuits (PLCs) and a lithium niobate (LN) chip [4]. The hybrid integration enables us to obtain both high design flexibility for the silica PLCs and a broad electro-optic modulation bandwidth of the LN chip at the same time. The LN chip has ten push-pull pairs of straight phase modulators in an array, which corresponds to the ten MZMs: two for the pulse generator and eight for the four vector modulators (Fig. 4). All other passive components such as couplers, static phase shifters, and the PM circuit are fabricated in the PLCs. The pulse generator and the vector modulators are connected via U-turn waveguides to make the module compact. The modulator’s static insertion loss for both polarizations is 9.5 dB at a 1550-nm wavelength. All the MZMs have electro-optic 3-dB bandwidths of ~23 GHz. The half-wave voltage of each MZM is ~3.5 V.


Fig. 4. Configuration of the OTDM modulator.

4. Large-capacity transmission experiment using the MUX-DAC and OTDM modulator

Using the MUX-DACs and the OTDM modulator in combination, we can generate an ultralarge-capacity optical signal [5]. The experimental setup for a 125-Gbaud polarization-division-multiplexed (PDM) 16-level quadrature amplitude modulation (16QAM) signal generation using the two devices is shown in Fig. 5. The MUX-DACs converted 31.25-Gbaud binary signals (pseudo-random bit sequences) from bit-pattern generators (BPGs) into 62.5-Gbaud four-level electronic signals, which are used to drive the OTDM modulator to generate the 125-Gbaud 16QAM optical signal. Since only two MUX-DAC modules were available for the experiment, we drove only one polarization channel of the OTDM modulator and generated the PDM signal by using an external emulator. The optical signal was received by a coherent receiver with a 63-GHz real-time oscilloscope and demodulated offline to calculate the bit-error rate (BER).


Fig. 5. Experimental setup of signal generation.

The measured back-to-back BER versus optical signal-to-noise ratio (OSNR) curve is shown in Fig. 6. We obtained BERs below 2.7 × 10−2, which is the threshold of the forward error correction (FEC) code with 20% overhead, with an OSNR larger than 28.4 dB. We also tested transmission over 80-km standard single-mode fiber (SSMF) and obtained BERs below the threshold of 2.7 × 10−2 with a launched power smaller than +14 dBm. Since the PDM-16QAM signal conveys 8 bits of information per symbol, the gross bit rate of the 125-Gbaud PDM-16QAM signal is 1 Tbit/s. Based on an assumed use of the FEC with 20% overhead, the net data rate is 800 Gbit/s. These values are records for single-carrier optical transmission using a single modulator (with an external PDM emulator).


Fig. 6. Back-to-back BER curve and constellations of the 1-Tbit/s PDM-16QAM signal.

5. Summary

We developed a high-speed InP MUX-DAC and an integrated OTDM modulator with a silica-LN hybrid configuration, which double the respective symbol rates in electronic and optical domains. Using these devices, we successfully demonstrated the first 1-Tbit/s single-carrier optical transmission with a single modulator. These device technologies are promising for future ultralarge-capacity optical transmission systems.

References

[1] M. Nagatani, H. Wakita, H. Nosaka, K. Kurishima, M. Ida, A. Sano, and Y. Miyamoto, “75 GBd InP-HBT MUX-DAC Module for High-symbol-rate Optical Transmission,” Electron. Lett., Vol. 51, No. 9, pp. 710–712, 2015.
[2] N. Kashio, K. Kurishima, Y. K. Fukai, and S. Yamahata, “High-speed, High-reliability 0.5-μm-emitter InP-based Heterojunction Bipolar Transistors,” NTT Technical Review, Vol. 7, No. 12, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr200912le1.html
[3] H. Yamazaki, T. Goh, T. Hashimoto, A. Sano, and Y. Miyamoto, “Generation of 448-Gbps OTDM-PDM-16QAM Signal with an Integrated Modulator Using Orthogonal CSRZ Pulses,” Proc. of OFC (Optical Fiber Communication Conference and Exhibition) 2015, Th2A.18, Los Angeles, USA, Mar. 2015.
[4] S. Mino, H. Yamazaki, T. Goh, and T. Yamada, “Multilevel Optical Modulator Utilizing PLC-LiNbO3 Hybrid-integration Technology,” NTT Technical Review, Vol. 9, No. 3, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201103fa8.html
[5] H. Yamazaki, A. Sano, M. Nagatani, and Y. Miyamoto, “Single-carrier 1-Tb/s PDM-16QAM Transmission Using High-speed InP MUX-DACs and an Integrated OTDM Modulator,” Opt. Express, Vol. 23, No. 10, pp. 12866–12873, 2015.
Hiroshi Yamazaki
Distinguished Researcher, NTT Device Technology Laboratories and NTT Network Innovation Laboratories.
He received a B.S. in integrated human studies in 2003 and an M.S. in human and environmental studies in 2005, both from Kyoto University, and a Dr. Eng. in electronics and applied physics from Tokyo Institute of Technology in 2015. In 2005, he joined NTT Photonics Laboratories, where he researched optical waveguide devices for communication systems. He is currently with NTT Device Technology Laboratories and NTT Network Innovation Laboratories, where he is involved in research on devices and systems for optical transmission using advanced multilevel modulation formats. Dr. Yamazaki is a member of the Institute of Electronics, Information and Communication Engineers (IEICE).
Munehiko Nagatani
Researcher, NTT Device Technology Laboratories and NTT Network Innovation Laboratories.
He received an M.S. in electrical and electronics engineering from Sophia University, Tokyo, in 2007. He joined NTT Photonics Laboratories in 2007, where he worked on the research and development (R&D) of ultrahigh-speed mixed signal ICs for optical communications systems. He is a member of IEICE.
Toshikazu Hashimoto
Group Leader, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from Hokkaido University in 1991 and 1993. He joined NTT Photonics Laboratories in 1993. He has been researching hybrid integration of semiconductor lasers and photodiodes on silica-based planar lightwave circuits and conducting theoretical research and primary experiments on the wavefront matching method. He is a member of IEICE and the Physical Society of Japan.
Hideyuki Nosaka
Senior Research Engineer, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from Keio University, Kanagawa, in 1993 and 1995, and a Dr. Eng. in electrical and electronics engineering from Tokyo Institute of Technology in 2003. He joined NTT Wireless System Laboratories in 1995, where he engaged in R&D of monolithic microwave ICs and frequency synthesizers. He moved to NTT Photonics Laboratories in 1999, where he was involved in R&D of ultrahigh-speed mixed-signal ICs for optical communications systems. He received the 2001 Young Engineer Award and the 2012 Best Paper Award presented by IEICE. Dr. Nosaka is a member of the Institute of Electrical and Electronics Engineers (IEEE) and IEICE.
Akihide Sano
Senior Research Engineer, NTT Network Innovation Laboratories.
He received a B.S. and M.S. in physics and a Ph.D. in communication engineering from Kyoto University in 1990, 1992, and 2007. Since joining NTT in 1992, he has been engaged in R&D of high-speed, high-capacity, and long-haul fiber-optic communication systems. He received the Young Engineer’s Award and the Achievement Award from IEICE in 1999 and 2010, respectively, and the 56th Maejima Award from the Teishin Association in 2011. He is a member of IEEE and IEICE.
Yutaka Miyamoto
Senior Distinguished Researcher, Director of the Innovative Photonic Network Center, Group Leader of the Lightwave Signal Processing Research Group, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986 and 1988. In 1988, he joined NTT Transmission Systems Laboratories, where he engaged in research and development of high-speed optical communications systems including the 10-Gbit/s first terrestrial optical transmission system using EDFA inline repeaters. He was with NTT Electronics Technology Corporation between 1995 and 1997, where he was involved in the planning and product development of high-speed optical modules at data rates of 10 Gbit/s and beyond. Since 1997, he has been with NTT Network Innovation Labs, where he has been researching and developing optical transport technologies based on 40-Gbit/s and 100-Gbit/s channels. His current research interests include high-capacity optical transport systems with advanced modulation formats, digital signal processing, optical pre-processing, and space division multiplexing. He is a member of IEEE and a Fellow of IEICE.

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