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We demonstrate successful transmission of four 45 Gbps PAM4 single-channels through OM4 multimode fibers (MMFs) and wideband MMF using a PAM4 PHY chip and four vertical cavity surface emitting lasers (VCSELs) with wavelengths ranging over short wavelength division multiplexing (SWDM) grid. Real-time bit error ratios (BERs) < 2 × 10−4 were achieved for all four 45 Gbps PAM4 SWDM grid channels over 100 m, 200 m, and 300 m of wideband OM4 MMFs. All four channel received PAM4 optical eyes are shown after propagating through 100 m, 200 m, and 300 m of wideband OM4 as well as 100 m and 200 m conventional OM4 MMFs. The measured BERs as a function of the inner eye optical modulation amplitudes (OMAs) are shown for all four SWDM grid channels. Inner eye OMAs ranged from −16.2 dBm to −13.5 dBm for different channels over different OM4 MMF types at the KP4 BER threshold of 2 × 10−4.
© 2016 Optical Society of America
1. Introduction
Due to exponential growth of bandwidth demand, cost effective and large-scale installation of datacenter networks is critical. Several solutions have been proposed based on different transceiver technologies including: directly modulated laser (DML) [1], electro-absorption modulated laser (EML) [2], and silicon photonics (SiPh) [3]. However, the emerging solutions need to meet low-cost, power efficiency, small form-factor, and reliable uncooled operation requirements. In addition, to support the explosive demand in data traffic and cloud services, data centers are required to migrate to higher transmission rates, achieve higher capacity, extend reach, and maintain duplex connectivity.
Transmission through multimode fiber (MMF) with directly-modulated vertical cavity surface emitting lasers (VCSELs) has long been considered as a low cost and power efficient solution for short-reach communication within the datacenter [4]. In addition, transceivers with small footprint and reliable uncooled operation can be achieved using short wavelength VCSELs. However, MMF link bandwidth is inherently limited by the unavoidable modal dispersion of MMF and low VCSEL bandwidth as data rates increase. To solve this limitation, 100GBASE-SR4 has currently been standardized as a short reach Ethernet (IEEE P802.3bm) with a maximum reach of 100 m on four parallel OM4 fibers and lane rate of 25 Gbps using non-return-to-zero (NRZ) modulation format [5]. While the need to increase link capacity could be addressed though utilization of additional parallel fibers, this solution would increase the cost of the MMF cable plant and fiber management complexity. Currently, a combination of higher order modulation formats [6–9], such as four-level pulse amplitude modulation (PAM4) and digital signal processing (DSP) techniques [7–9] have been proposed to increase capacity over single mode fiber (SMF) at 1310 nm and a single conventional MMF at short wavelength. In addition, novel wideband MMF [10,11] has been studied in order to achieve 100 Gbps data rate over a single MMF with longer reach using short wavelength division multiplexing (SWDM) and four 25 Gbps NRZ modulation formats for datacenter networks [12]. This solution is predicted to capture a larger market share in the near future.
In this paper, the first real time four 45 Gbps PAM4 transmission is demonstrated over SWDM4 grid using a PHY. We show that combination of PAM4, DSP and SWDM4 as well as wideband MMF potentially enables to double the previous achieved capacity [8,9,12] and/or reach [8,9] over a single MMF. In addition, successful transmission of four 45 Gbps PAM4 VCSEL based technology (four 22.5 Gbaud PAM4 optical channels without using multiplexer and de-multiplexer) is demonstrated over 100 and 200 m of conventional OM4 fibers as well as 100 m, 200 m, and 300 m of wideband OM4 fibers optimized for SWDM in the 850 nm to 950 nm wavelength range. To extend the reach and improve performance, a 45 Gbps PAM4 commercial PHY is used for modal and chromatic dispersion equalization. We show the optical eye diagrams and measured bit error ratios (BERs) as a function of the inner eye optical modulation amplitude (OMA) over abovementioned MMFs for 851.9 nm, 882.0 nm, 912.1 nm, and 942.4 nm VCSELs. 300 m transmission was achieved for all four SWDM grid channels with BERs < 2 × 10−4 over the wideband OM4 fiber. The inner eye OMAs ranged from −16.2 dBm to −13.5 dBm for all fiber types and lengths and all channels at extinction ratio (ER) of ~3.0 dB and the KP4 BER threshold of 2 × 10−4. These results show that the combination of SWDM4 VCSEL technology, PAM4 modulation, and DSP methods as well as wideband OM4 fiber can increase the reach, achieve higher capacity, and support duplex connectivity for the next generation of datacenters with highly dense switches.
2. Experimental setup and results
The experimental setup comprised a TOSA, a ROSA, a variable optical attenuator (VOA), different MMF types and lengths, and a PHY chip as shown in Fig. 1. The PHY chip was responsible for performing the main functions including: 45 Gbps PAM4 clock and data recovery, pulse shaping at the transmitter, adaptive modal and chromatic dispersion equalization at the receiver, and real-time BER measurement. Using the PHY chip pre-emphasis functionality, all optical eyes were enhanced at TX. The detected signals were equalized at RX using a nine taps feed forward equalizer in all experiments. The tap coefficients were adapted automatically to compensate different channel degradation effects in each experiment with different fiber types and lengths as well as back-to-back (B2B). Two sets of MMF types and various fiber lengths were used for this experiment including: conventional OM4 fibers (100 and 200 m) and wideband OM4 fibers (100 m, 200 m, and 300 m).
Fig. 1 (a) Schematic of the experimental setup for receiver sensitivity measurement of each channel. Each channel operated individually and no multiplexer/de-multiplexer was used. Arrows show the direction of transmission. (b) 45 Gbps PAM4 electrical eye after DAC.
The wideband OM4 fiber [11] (trademark WideCap OM4) is manufactured by Prysmian Group. The measured effective modal bandwidth (EMB) of this fiber is shown in Fig. 2. The fiber is designed for peak EMB at ~900 nm so that the EMB is higher than the bandwidth requirement for OM4 performance [11] at all wavelengths from 850 to 950 nm. In comparison, the conventional OM4 fiber fails at wavelengths longer than 885 nm. The EMB of the wideband OM4 at 950 nm is 4300 MHz.km which is more than twice that of the conventional OM4 fiber (1900 MHz.km). The wideband OM4 is then expected to reduce system penalties for the two longest wavelengths compared to the conventional OM4 fiber.
Fig. 2 Measured effective modal bandwidth of the wideband OM4 (blue), OM4 (black), and bandwidth requirement for OM4 performance (red).
The 25G VCSELs used in this experiment were Finisar VCSELs. The 3-dB BWs of VCSELs were ranging from 18 GHz to 20 GHz for SWDM4 wavelengths. The 3-dB BW of bias tees were > 50 GHz. The measured VCSEL center wavelengths were 851.9, 882.0, 912.1, and 942.4 nm. Figures 3(a)-3(d) show the optical spectrums of these VCSELs. Root mean square (RMS) spectral bandwidths (SBWs) were 0.558, 0.370, 0.5011, and 0.527 nm from the short to the long wavelength, respectively. The measured average relative intensity noises (RINs) were ~-141 dB/Hz. In this study, a Finisar ROSA operating over the SWDM grid was used. The 3-dB BWs of PD and TIA were around 26 GHz. The PD responsivity was ~0.5 A/W over the SWDM grid. The ROSA input-referred noise density was ~11 pA/√(Hz). The 3-dB BW of the ROSA (including TIA) was 25 GHz. The optical power was adjusted using a mode preserving VOA. The receiver sensitivities were measured using the ROSAs with a limiting trans-impedance amplifiers (TIAs). Limiting TIAs properly operated in the linear region at the low average powers (inner eye OMA < −11.5 dBm) for all four PAM4 channels. Nonlinear distortion impact of limiting TIA on PAM4 signals was observed at inner eye OMAs > −11.5 dBm when the BERs start increasing.
45 Gbps PAM4 optical streams were generated by directly and differentially driving the VCSELs using 22.5-Gbaud scrambled pseudo-random bit sequences (PRBS) of length 231-1. These sequences were produced by integrated DACs, with ~0.8 Vpp electrical signal. The VCSELs were biased at 11 mA. The 11 mA DC bias point does not induce any non-linearity into the PAM4 eye. The PHY chip DSP provided functionality for pre-emphasis compensation. Figure 4 shows the measured received optical eye diagrams with the pre-emphasis compensation turned on at TX. The measured transmitter ERs ranged from 3.0 dB to 3.2 dB at four wavelengths. The first, second, third, and forth columns show the VCSEL eyes at 851.9 nm, 882.0 nm, 912.1 nm, and 942.4 nm, respectively. Different rows show VCSEL optical eyes after transmission over different MMF types and lengths as labelled in the figures. Open eyes were observed at 100 m OM4 as well as 100 m and 200 m WideCap OM4 for all four channels. Closed eyes were observed for 200 m conventional OM4 fibers for all four wavelengths. At 300 m WideCap OM4, the most open eyes were obtained at 882.0 nm and 912.1 nm wavelength channels where the EMB is the highest. Total eye closure was observed at 300 m WideCap OM4 for 942.4 nm wavelength channel.
Fig. 4 Received optical eye diagrams after transmission through different OM4 fibers. The first, second, third, and fourth columns are corresponding to 851.9 nm, 882.0 nm, 912.1 nm, and 942.4 nm optical channels. The first, second, third, fourth, fifth, and sixth rows are corresponding to the received eye diagrams for B2B, 100 m, 200 m, 300 m WideCap OM4 fibers as well as 100 m and 200 m conventional OM4 fibers.
To perform real-time BER measurement for PAM4 transmitted signals over different OM4 fiber spools and compare the receiver sensitivities, the detected signals were connected to the PHY through ROSA limiting TIA differential outputs. Figures 5(a)-5(d) show the BER measurements after adaptive equalization at RX as a function of inner eye OMAs using a pre-emphasis at TX. The KP4 (2 × 10−4) BER FEC threshold level determines the required inner eye OMA ( = average optical power-6.7 dB at ER of 3.0 dB) for 45 Gbps PAM4 modulation format through each MMF types and lengths. BER increase was observed for OMA > −11.5 dBm due to the nonlinear distortion of limiting TIA. Linear TIA is a potential solution for removing BER floors at high inner eye OMAs caused by the limiting TIA. Low latency FECs are preferred for data center applications and IEEE proposed such FECs with BER thresholds of KP4 (2 × 10−4) and KR4 (5 × 10−5) [13]. Table 1 shows the measured inner eye OMA receiver sensitivities at these IEEE BER threshold standards. The measured OMAs were −16.2 dBm, −16.2 dBm, −16.1 dBm, and −16.1 dBm for B2B at the KP4 BER threshold for 851.9 nm, 882.0 nm, 912.1 nm, and 942.4 nm VCSELs, respectively. The measured OMA penalties were ≤0.2 dB over 100 m WideCap OM4 fiber at the KP4 BER threshold for all four channelsin comparison with B2B OMAs. The OMA penalties were 0.6 dB and 1.1 dB for 912.1 nm and 942.4 nm VCSELs over 100 m conventional OM4 fiber at the KP4 BER threshold. While the measured OMA penalties over 200 m WideCap OM4 fiber were ≤ 1.0 dB, OMA penalty was ~2.5 dB over 200 m conventional OM4 fiber at KP4 level for 942.4 nm wavelength. Higher BER error floors were observed for 912.1 nm and 942.4 nm channels over 200 m conventional OM4 fiber. At high OMA values, 942.4 nm channel showed better sensitivity curve over 912.1 nm channel through 200 m conventional OM4 fiber. Our hypothesis is that the combination of OM4 fiber degradation effects and limiting TIA operation in the nonlinear region at high average optical power created this scenario. The depicted eye diagrams in Fig. 4 were also consistent with measured BERs when the limiting TIA operated in the linear region. The worst eye diagrams were captured for 912.1 nm and 942.4 nm channels over 200 m conventional fibers as well as 942.4 nm channel over 300 m wideband OM4 fiber. Eye diagram of 942.4 nm channel over 300 m wideband OM4 fiber looks the worst eye in comparison with the other two channels because lower optical power was coupled to the photodetector of digital communication analyzer for this channel during measurement. These results show the limitation of the conventional OM4 fibers for extending the reach over 100 m in SWDM applications. OMA penalties ≤ 2.2 dB were captured for all channels over 300 m WideCap OM4 fiber at KP4 level.
Fig. 5 Measured BER B2B (blue), 100 m (green), 200 m (purple), 300 m (yellow) of WideCap OM4 as well as 100 m (red) and 200 m (pink) conventional OM4 fibers for (a) 851.9 nm, (b) 882.0 nm, (c) 912.1 nm, and (d) 942.4 nm channels.
Table 1. Measured inner eye OMAs for B2B as well as conventional and WideCap OM4 fibers at KP4 (2 × 10−4) and KR4 (5 × 10−5)
3. Conclusion
Successful transmission of 4 × 45 Gbps directly PAM4 modulated VCSELs was demonstrated over 300 m of wideband OM4 fibers with BERs < 2 × 10−4 (KP4 FEC threshold). Receiver sensitivities were compared for diferent lengths of conventional and WideCap OM4 fibers at proposed IEEE FECs with BER thresholds of KP4 (2 × 10−4) and KR4 (5 × 10−5). These results show that the combination of SWDM4 VCSEL technology and PAM4 modulation as well as novel wideband OM4 fiber and DSP methods can increase the reach, achieve higher capacity, and support duplex connectivity for the next generation of data centers with highly dense switches.
Acknowledgment
We would like to acknowledge Dr. Julie Eng at Finisar. We also thank IC/DSP groups at Broadcom for their support of this research.
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