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Demonstration of Time- and Wavelength-Division Multiplexed Passive Optical Network Based on VCSEL Array
Demonstration of Time- and Wavelength-Division Multiplexed Passive Optical Network Based on VCSEL Array
ETRI Journal. 2016. Mar, 38(1): 9-17
Copyright © 2016, Electronics and Telecommunications Research Institute (ETRI)
  • Received : February 28, 2015
  • Accepted : November 11, 2015
  • Published : March 01, 2016
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About the Authors
Sil-Gu Mun
Eun-Gu Lee
Jie Hyun Lee
Heuk Park
Sae-Kyoung Kang
Han Hyub Lee
Kwangok Kim
Kyeong-Hwan Doo
Hyunjae Lee
Hwan Seok Chung
Jong Hyun Lee
Sangsoo Lee
Jyung Chan Lee

Abstract
We demonstrate a time- and wavelength-division multiplexed passive optical network system employing a vertical-cavity surface-emitting laser array–based optical line terminal transceiver and a tunable bidirectional optical subassembly–based optical network terminal transceiver. A packet error–free operation is achieved after a 40 km single-mode fiber bidirectional transmission. We also discuss an arrayed waveguide grating, a photo detector array based on complementary metal–oxide–semiconductor photonics technologies, and low-cost key devices for deployment in access networks.
Keywords
I. Introduction
With the advent of various mobile devices and content, such as smartphones and games, the amount of wired and wireless data traffic has dramatically increased. Moreover, it is expected that most 5G mobile data will be provided on the basis of cloud services and applications [1] . To accommodate these trends, a novel optical and mobile converged access network is needed. Among the available candidates, many mobile service providers are gradually preferring a cloud (or centralized) radio access network (C-RAN) architecture. In a C-RAN, base stations (BSs) are each separated into two different types of units — baseband units (BBUs) and remote radio units (RRUs). A BBU and an RRU are physically connected via an optical transport network. Until now, several research groups have been proposed for developing an advanced converged access network [2] [9] . One of them, Next-Generation Passive Optical Network Phase 2 (NG-PON2), is a good candidate to establish such a network [10] .
In this paper, we demonstrate a cost-effective NG-PON2 system with a time- and wavelength-division multiplexed passive optical network (TWDM-PON) for an optical-mobile converged access network. In particular, we use a low-cost vertical-cavity surface-emitting laser (VCSEL) array and low power consumption silicon photonic devices. We implement a transmitter optical subassembly (TOSA) and receiver optical subassembly (ROSA) module, and develop a transceiver for optical line terminal (OLT) and ONU supporting 40 Gb/s (4 channel × 10 Gb/s) downstream and 10 Gb/s (4 channel × 2.5 Gb/s) burst-mode BM upstream. The TWDM-PON media access control (MAC) is also implemented based on a field-programmable gate array, and a packet error–free operation is presented after bidirectional transmission over 40 G/10 G down/up stream through 40 km single-mode fiber (SMF).
II. Prototype TWDM-PON System and Performances
Since 2010, NG-PON2 has been studied by ITU-T Study Group 15 and the Full Service Access Network group [10] . Figure 1 shows the configuration of the TWDM-PON architecture. Four or eight wavelengths are used for down and upstream transmissions in a TWDM-PON. In this paper, we develop a multi-source agreement (MSA)-compatible optical transceiver for an OLT system. An OLT transceiver is equipped with a 10 Gb/s arrayed VCSEL transmitter and 2.5 Gb/s burst mode receivers including a wavelength multiplexer and demultiplexer. In addition, we investigate the use of integrated optical module technology based on complementary metal–oxide–semiconductor (CMOS) photonics for a low-cost solution.
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Simplified TWDM-PON architecture.
Figure 2(a) shows the experimental setup used to demonstrate the TWDM-PON system. A transceiver for the OLT, based on a VCSEL array and avalanche photodiode (APD) array, is installed in the TWDM-PON OLT platform. Likewise, an optical network terminal (ONT) transceiver based on a tunable bidirectional optical subassembly (BOSA) is installed in the TWDM-PON ONT platform. The feeder fiber between the central office (CO) and remote node (RN) is an SMF of 40 km in length. Therefore, the loss is set to be 21 dB, which includes the insertion loss of the 40 km SMF, dispersion compensating fiber (DCF) for 40 km, and other excess losses. To check feasibility for VCSEL as an OLT optical source, we used a 4-channel VCSEL array operating at 1,579.52 nm to 1,582.02 nm for downstream; the ITU-T G. 989.2 standard’s recommendation is from 1,596 nm to 1,603 nm [10] . However, the effect of physical layer constraints, such as optical loss and dispersion, on VCSEL transmission could be successfully evaluated even at non-standard downstream wavelengths. The upstream wavelength was set to be 1,530.33 nm to 1,532.68 nm. Figure 2(b) shows a photograph of the OLT. The TWDM-PON OLT platform consists of a TWDM-PON line card, a switch fabric, a 10 G interface unit, and a power unit, all of which can be installed at a CO. Figure 2(c) shows a block diagram of an OLT line card. Ethernet data from the “Back plane” is connected to the input of the physical layer (PHY) chip by a 10GBASE-KR interface, and the TWDM-PON MAC [11] , [12] and PHY chip are connected via a 10 Gigabit Reduced Attachment Unit Interface (RXAUI). The optical transceiver and TWDM-PON MAC were connected with 16-bit low-voltage differential signaling (LVDS) through a serializer, and each 10 Gb/s electrical signal was converted to an optical signal by VCSEL.
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Experimental setup for TWDM-PON: (a) schematic diagram and wavelength for upstream/downstream transmission, (b) photograph of OLT platform, and (c) block diagram of line card.
To investigate the performances of the TWDM-PON system, we connected the OLT and ONU through 10-GbE traffic generator ports [13] . The transmission performance was measured by using a traffic generator and analyzer. The tester-generated L2 flow provided a packet size in a series of 64 bytes. Considering the mandatory forward error correction (FEC) overhead, the downstream packet rate is 80% load (8 Gb/s), and the upstream packet rate is 20% load (2 Gb/s). Figure 3 shows the captured test results for packet transmission over a 24-hour period. The total number of downstream packets is 1,028,571,428,572, and the total number of upstream packets is 257,142,857,143. We achieved a packet error–free transmission for a bidirectional transmission, as shown in Fig. 3 .
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Screenshot for transmission over 10-GbE traffic over 24 h through TWDM-PON system.
III. TWDM-PON Subsystem
This section describes in detail the TWDM-PON OLT transceiver and ONT transceiver used in Section II.
- 1. TWDM-PON OLT Transceiver
Figure 4 shows a functional block diagram of the TWDM-PON OLT transceiver. The interface is C form-factor pluggable (CFP) compatible. It consists of a VCSEL array–based TOSA and APD array–based ROSA. To realize cost efficiency, the VCSEL array and an arrayed waveguide grating (AWG) were butt-coupled to achieve a coupling efficiency of 50% without a lens array [14] , [15] . The optical transmitter is composed of a VCSEL array with a multichannel driver in a single TOSA and an optical receiver composed of an APD array with BM transimpedance amplifiers (TIAs) in a single ROSA in a subassembly form, respectively. There were several light sources for the OLT system, such as an electroabsorption-modulated laser (EML) and reflective electroabsorption modulator-semiconductor optical amplifier (REAM-SOA) [16] , [17] . Unlike previously reported light sources, VCSEL has low power consumption as well as cost-effectiveness. The total power consumption of the VCSEL array–based OLT transceiver (with 40 Gb/s capacity) is as low as 3 W with the help of a low threshold current of around 3 mA. Since a VCSEL has a possibility of a high yield and can be mass produced, an OLT light source could be implemented cost-effectively.
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Functional block diagram of TWDM-PON OLT transceiver.
Figures 5(a) and 5(b) show the structure of the VCSEL-based TOSA and building blocks of the metal optical bench (MOB) and flexible printed circuit board (FPCB) [15] . TOSA is composed of a VCSEL array, FPCB, laser diode driver (LDD), MOB, and spacer. The FPCB provides several degrees of freedom to design a compact OSA package to fit within the transceiver form factor. The FPCB in the VCSEL-based TOSA was used to orthogonally connect the light from the VCSEL array to the cross-section of the wavelength multiplexer. The FPCB has six layers and a total thickness of 265 μm. This allows the TOSA package to fit within the transceiver case. Similarly, ROSA is composed of an APD array, FPCB, BM TIAs, and an MOB, as shown in Fig. 5(c) . To reduce the cost of packaging, we utilized a direct-optical coupling technique without a lens array. However, the multiplexer cannot be in physical contact with the VCSEL array owing to the wiring used for the current injection. To avoid contact of the wire with the cross-section of the multiplexer, we inserted a glass block as a spacer between the MOB and mux/demux.
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Fabricated TOSA and ROSA: (a) TOSA; (b) MOB and FPCB for VCSEL and RF interface; and (c) ROSA.
Figure 6(a) shows four-channel spectra from a single TOSA array. The output wavelength ranges from 1,579.52 nm to 1,582.02 nm. The VCSELs in the TOSA were controlled by temperature and current to emit four different wavelengths, spaced 100 GHz apart. The side-mode suppression ratio (SMSR) was over 40 dB for all channels. The VCSELs were modulated at 10.3 Gb/s with a 2 31 −1 pseudorandom bit sequence (PRBS) pattern. As shown in Fig. 6(b) , we can obtain a clear open eye. The extinction ratio of the VCSEL-based OLT transmitter was 6 dB, which is 2 dB lower than the extinction ratio defined in ITU-T G.989.2. Thus, there was 2 dB power penalty at a BER of 10 −10 . Figure 6(c) shows the measured BER performance of VCSEL after a 40 km transmission. Solid and hollow marks represent the measured BER with and without DCF, respectively. Since the VCSEL has a large chirp , we could not achieve an error-free transmission over 40 km of SMF without dispersion compensation. To compensate for chirp-induced pulse broadening, we use DCF for the OLT at the CO side. Thus, an optical distributed network (ODN) would not need to be modified to accommodate DCF in RN. For a more practical implementation, electronic dispersion compensation (EDC) could be used at the receiver. After a 40 km transmission with DCF at OLT, we achieved an error-free transmission.
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(a) Optical spectra of VCSEL array, (b) measured optical eye diagram, and (c) measured BER result.
Figure 7(a) shows the electrical characteristics of a TWDM-PON OLT BM receiver. The upper timing diagram is the optical signal measured at the input to the OLT receiver, and the lower timing diagram is the converted electrical signal measured at the output of BM ROSA. The measured recovered amplitude time is as fast as 8 ns, which is much lower than that specified in ITU-T G. 989.2 (25.6 ns). Figure 7(b) shows the four-channel BER for a 2.5 Gb/s transmission with a 2 31 −1 PRBS pattern. The measured sensitivity was −29.0 dBm at a BER of 10 −10 for all channels.
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Characteristics of BM receiver: (a) measured BM timing diagram and (b) measured BER for four channels.
- 2. TWDM-PON ONT Transceiver
Figures 8(a) and 8(b) show a photograph and block diagram of the implemented tunable BOSA for ONT transceiver, respectively. It consists of a cooled C-band distributed-feedback laser diode (DFB-LD), a thermally tunable filter (T-filter), an APD, a TIA, a monitoring photo detector (PD), and a WDM filter. We used wavelength-pairing of downstream wavelength and upstream wavelength. The wavelength-pairing T-filter has a cyclic property, where the free spectral range (FSR) of the filter is designed to transmit the upstream and downstream wavelengths simultaneously. By thermally tuning the pairing filter to the downstream wavelength, the upstream wavelength is automatically selected and the misaligned upstream wavelength is then blocked [18] . By thermally tuning the wavelength of both the laser diode (LD) and the T-filter, the BOSA could cover four wavelength channels on a 100 GHz grid from 1,530.33 nm to 1,532.68 nm with over a 26 °C temperature range, as shown in Fig. 8(c) . The sensitivity of the transmitter is about −33 dBm at a BER of 10 −10 after a 40 km SMF transmission for all four channels, as shown in Fig. 8(d) .
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Characteristics of tunable BOSA: (a) appearance, (b) block diagram, (c) wavelength-tunable characteristics, and (d) BM BER result for four channels.
IV. Silicon Photonics–Based ROSA Module
- 1. Hybrid Integrated ROSA
With the help of silicon photonics technology, we could further reduce the size and cost of the optical module. Thus, we implemented a prototype ROSA module composed of a silicon photonic integrated circuit (Si-PIC) chip, Germanium PIN-PD, and four-channel BM TIA, as illustrated in Fig. 9(a) . An AWG and four-channel PDs are monolithically integrated on the Si-PIC chip. Figure 9(b) shows the fabricated ROSA module with a lucent connector (LC) receptacle for the optical input, and an FPCB for the electrical output interface. The LC receptacle was assembled with a lensed fiber and optically coupled to the Si-PIC chip.
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Silicon photonics–based optical ROSA module: (a) block diagram and (b) fabricated module.
Figure 10 shows the measured optical-to-electrical (O/E) response of the fabricated ROSA module. An evaluation board was used to mount the ROSA module for the measurement. The 3 dB bandwidth transmission (S21) is about 1.62 GHz at a reverse bias of 1 V. The return loss (S11) is less than −7.5 dB at up to 2 GHz.
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Measured O/E response of fabricated ROSA module.
Figure 11 shows the measured BER performance of the ROSA module at a data rate of 2.5 Gb/s. The receiver sensitivities of the four channels were measured to be about −10 dBm at a BER of 10 −12 with a back-to-back transmission. The inset of Fig. 11 shows the electrical output waveform from the ROSA at an optical input power of −3 dBm. The peak-to-peak jitter and rising and falling times were about 35 ps, and 262 ps and 200 ps, respectively.
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Measured BER performance (inset shows electrical output waveform of ROSA).
- 2. AWG
A silicon-based AWG has numerous benefits over a silica-based version. One benefit is the possibility of integration with active devices such as modulators and PDs. The small form factor of a silicon-based AWG is also an important merit because the package size of the transceiver module is continuously diminishing. However, the performance of a silicon-based AWG is far behind that of its silica-based counterpart.
One of challenging works for silicon photonics–based AWG is to reduce crosstalk. There are many known crosstalk mechanisms for an AWG. Among them, fabrication imperfection (FI) is known to be the most crucial source of crosstalk. Waveguide parameters such as the waveguide width and height, and the slab height, randomly deviate from design values owing to the non-ideal fabrication process. Since FI gives white noise, FI-induced crosstalk limits any attainable crosstalk values. FI-induced crosstalk is determined by many parameters of an AWG design. Thus, an analytical equation to predict FI-induced crosstalk was derived [19] . Using the equation, we could predict the FI-crosstalk values of the designed AWG, and could use the values as one of the constraints of the design process. When a predicted value is outside the range of a target value, we drop the design prior to fabrication; this saves precious time and money. Thus, we designed an AWG with appropriated parameters, and the implemented results are shown in Fig. 12 . The fabrication-induced crosstalk was less than 30 dB, and inter-channel crosstalk was 22 dB. According to the ITU-T G.supp.39 [20] , the power penalty due to 22 dB inter-channel crosstalk was as low as 0.1 dB, which is sufficient a value to use as a wavelength demultiplexer. The channel spacing of AWG is 100 GHz, and the total loss of AWG is about 19 dB. The coupling loss is 14 dB, which comes from the taper we used as an edge coupler, and the chip loss is 5 dB.
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Characteristic of silicon-AWG: (a) photograph of fabricated AWG chip and (b) transmission optical spectra.
V. Conclusion
We demonstrated a TWDM-PON system employing a low-cost VCSEL array and compact tunable BOSA. The OLT transmitter optical source used a VCSEL array with 40 Gb/s (4 channel × 10 Gb/s) for the downstream transmission, and the ONU transmitter optical source used the tunable BOSA with 10 Gb/s (4 channel × 2.5 Gb/s) for the upstream transmission. By using the prototype TWDM-PON system, we achieved a bidirectional error-free packet transmission over 40 km of SMF for 24 hours. The system allows a smooth pay-as-you-grow deployment with an aggregated capacity. In addition, the system could be used for an optical access network as well as for an optical-mobile converged access network.
This research was funded by the MSIP (Ministry of Science, ICT & Future Planning), Korea in the ICT R&D Program 2014 (14-000-05-002).
BIO
Corresponding Author exatto@etri.re.kr
Sil-Gu Mun received her PhD degree in electronics engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 2010. In 2010, she joined ETRI. Her research interests include high-speed optical access networks and optical transceivers.
eungulee@etri.re.kr
Eun-Gu Lee received his MS and PhD degrees in physics from Chungnam National University, Daejeon, Korea, in 2004 and 2009, respectively. His doctoral research included quantum dot laser diodes, semiconductor optical amplifiers, and integrated devices consisting of LDs and SOAs for booster amplifiers. After receiving his PhD, he joined ETRI as a senior researcher of the Optical Internet Research Department, in 2009. His current research interests include long-reach WDM-PONs and WDM/TDM hybrid PONs.
jhlee@etri.re.kr
Jie Hyun Lee received her MS degree in electronic engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 2003 and her PhD degree in electronic engineering from Chungnam National University, Daejeon, Rep. of Korea, in 2013. In 2004, she joined ETRI, where she currently works as a senior researcher. Her current research interests include high-speed modulation formats and tunable technologies for optical access networks and data centers. She has contributed to ITU-T and IEC standardizations.
parkh@etri.re.kr
Heuk Park received his PhD degree in physics from Seoul National University, Rep. of Korea, in 1995. He joined ETRI in 1995, where he has been involved in research on optical packet communication and optical transmission systems. His research interests include silicon photonics–based optical filters.
skkang35@etri.re.kr
Sae-Kyoung Kang received his PhD in information and communications engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 2006. In 2006, he joined ETRI, where he is currently working on high-speed Ethernet transceivers, optoelectronic packaging, and silicon photonic devices.
hanhyub@etri.re.kr
Han Hyub Lee received his MS and PhD degrees in physics from Chungnam National University, Daejeon, Rep. of Korea, in 2001 and 2005, respectively. His doctoral research included the application of the Raman fiber amplifier and gain-clamped SOA for WDM systems. From 2006 to 2007, he was a postdoctoral researcher at AT&T Laboratory, Middletown, NJ, USA, where he worked on extended hybrid WDM/TDM PONs using wideband optical amplifiers. In 2007, he joined ETRI as a senior researcher in the Optical Internet Research Department. He has worked on high-capacity WDM/TDM hybrid PONs and energy-efficient network technology, as well as contributing to ITU-T and IEC standardizations.
kwangok@etri.re.kr
Kwangok Kim received his BS degree in information and communication engineering from Chosun University, Gwangju, Rep. of Korea, in 1999 and his MS degree in electronic engineering from Chonnam University, Gwangju, Rep. of Korea, in 2001. Currently, he is both a senior researcher at ETRI and a PhD student at Chungnam National University, Daejeon, Rep. of Korea. His research interests include passive optical network technologies, such as TDMA-PON, WDM-PON, and OFDMA-PON.
khdoo@etri.re.kr
Kyeong-Hwan Doo received his BS and MS degrees in electronic engineering from Chonbuk National University, Jeonju, Rep. of Korea, in 1996 and 1998, respectively, and his PhD degree in electronic engineering from Chungnam National University, Daejeon, Rep. of Korea, in 2013. In 2000, he joined ETRI. His research interest includes scheduling in high-speed networks, passive optical networks, and optical OFDMs.
lhj@etri.re.kr
Hyunjae Lee received his PhD degree in physics from Sogang University, Seoul, Rep. of Korea, in 1990. He joined ETRI in 1992, where he has been involved in research on optical communication, optical transmission systems, and optical access networks. Currently he is researching space-division multiplexing, OFDMs, and network architecture for the future.
chung@etri.re.kr
Hwan Seok Chung received his PhD degree in electronics engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Rep. of Korea, in 2003. In 2003, he was a postdoctoral research associate with KAIST, where he worked on hybrid CWDM/DWDM systems for metro area networks. From 2004 to 2005, he was with KDDI R&D laboratories Inc., Saitama, Japan, where he was engaged in research on wavelength converters and regenerators. Since 2005, he has been with ETRI, where he is currently a director of the Optical Access Research Department. His current research interests include mobile fronthaul, high-speed PONs, and modulation formats. He has served as a technical committee member of OFC, OECC, COIN, ICOCON, and Photonic West. He was the recipient of the Best Paper Awards from the Optoelectronics and Communications Conference (OECC) in 2000 and 2003 as well as ETRI in 2011 and 2012. He is a senior member of the IEEE.
jlee@etri.re.kr
Jong Hyun Lee received his BS and MS degrees in electronic engineering from Sungkyunkwan University, Suwon, Rep. of Korea, in 1981 and 1983, respectively. He received his PhD degree in communication engineering from the same university in 1993. In 1983, he joined ETRI. His current research activities include optical communication systems and optical internet technology. He is currently the head of the Optical Internet Research Department of ETRI.
soolee@etri.re.kr
Sangsoo Lee received his BS and MS degrees in applied physics from Inha University, Incheon, Rep. of Korea, in 1988 and 1990, respectively. He received his PhD degree in the area of DWDM transmission technology from the same university in 2001. In 1990, he joined ETRI. His current research interests include optical access networks, WDM-PONs, and energy-efficient network technology.
jclee@etri.re.kr
Jyung Chan Lee received his MS degree in electronics engineering from Hanyang University, Seoul, Rep. of Korea, in 1999. From 1997 to 1999, he was a student research associate with the Korea Institute of Science and Technology, where he worked on high-speed very short pulse generation from fiber ring lasers and erbium-doped fiber amplifiers. Since 1999, he has been with ETRI. His current research interests include high-speed array technology of optical transceivers, electronic equalization, CMOS photonics, and OTDRs.
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