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Abstract
We propose 400-Gb/s (16 × 25 Gb/s) optical transmitter and receiver modules for on-board interconnects using polymer waveguide (PWG) arrays. Four four-channel vertical cavity surface emitting laser and PIN photodiode arrays are used in the optical transmitter and receiver modules. A 16-channel PWG array with a length of 140 mm and a pitch of 62.5 μm is used as a short-reach optical signal transmission medium in the module. The 25-Gb/s optical eye diagrams of the all output channels utilize PWG transmissions and exhibit good performance in the transmitter module, with a signal-to-noise ratio (SNR) of over 8.01, peak-to-peak time jitter below 11.70 ps, and mask margin of over 31.5%. In the best-performance channel of the receiver module, the peak-to-peak time jitter and SNR of the eye diagram are 10.34 ps and 10.76, respectively, and the receiver sensitivity is approximately −2.6-dBm under a bit error rate of 10−12. The optical transmitter and receiver modules utilizing PWG transmissions were designed and fabricated to realize 400-Gb/s on-board interconnects.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In data centers and high performance computing environments, the need for high interconnection data rates and the power consumption of electrical interconnects are growing continuously [1–3]. To enable on-board optical interconnects to compete with electrical interconnects, the transmission speed, power consumption, channel density, and cost must have significant advantages. At high data rates, the signal integrity is also a key issue. For high-speed electrical interconnects, unmatched impedances, insufficient bandwidth, and frequency-dependent loss mechanisms lead to intersymbol interference (ISI) and signal distortion. However, optical interconnect technologies have the potential to overcome the bandwidth bottleneck, reduce power consumption, and provide higher channel densities. Therefore, research on short-reach optical interconnects has attracted significant attention over the past few years [4–7].
In an optical transceiver, the power consumption of the transmitter is generally higher than that of the receiver because of the high laser diode threshold current of transmitters. However, vertical-cavity surface-emitting laser (VCSEL) diodes have lower threshold currents in comparison with those of edge-emitting laser diodes. Therefore, VCSELs are used in 25-Gb/s short-reach optical transmission [8,9]. VCSEL and gallium arsenide (GaAs) PIN photodiode (PD) arrays are used in optical data communication modules to increase the channel density [10,11]. A polymer waveguide (PWG) is used for optical interconnects because the density of PWG arrays can be higher than that of optical fiber arrays. Therefore, optical interconnects using PWG arrays are expected to yield high-speed, low-power, and high-density on-board interconnects [12–20]. The longer PWG arrays, including mechanical transfer (MT) connectors, are designed to connect to the optical engines in transceiver modules [14,15]. Optical signals from optical sources or signals fed into detectors are directly coupled to the shorter PWG arrays with MT connectors for connecting to the optical fiber ribbon [17–20]. The PWG arrays can also be embedded in printed circuit boards (PCBs) [14–16]. Low-cost and easy-to-assemble optical transceivers that use PWG array transmissions need to be designed and developed for on-board interconnects.
Herein, 400-Gb/s (16 × 25 Gb/s) optical transmitter and receiver modules including VCSEL arrays, PD arrays, the related integrated circuits (ICs), and high-speed PCBs are designed and fabricated. The structure of the PWG array that interfaces with the optical transmitter and receiver will be also designed to obtain high-density on-board interconnects. A PWG array can easily be assembled with the optical transmitter and receiver modules.
2. Design of optical transmitter and receiver modules for on-board interconnects
illustrates a schematic of the 16-channel optical transmitter or receiver modules. Four four-channel high-speed VCSEL or PD arrays are placed on the PCB of the modules, with a distance of about 10 mm between adjacent VCSEL or PD arrays. Every four-channel VCSEL or PD array is connected with a laser diode driver or a receiver IC. A 16-channel PWG array is used as a short-reach optical signal transmission medium in the module. The lens is used to focus the light beam from the VCSEL array into the PWG array or from the PWG array into the PD array. The PWG array terminates in two twelve-channel MT connectors for connecting with the test equipment. The total length of the PWG array is about 140 mm.The optical transmitter and receiver modules are set in 400-Gb/s (16 × 25 Gb/s) transmission applications, so the PCB must be designed for 25-Gb/s single-channel transmissions. A six-layer PCB was used for the optical modules, with the top and bottom layers serving as high-frequency signal layers, the second and fifth layers as ground layers, and the third and fourth layers as low-frequency signal and power layers. The differential 100 Ω grounded coplanar waveguide was formed on the PCB by the top and ground layers and a 101.6-μm-thick dielectric material with a dielectric constant of 3.66 and loss tangent of 0.0037. The 3 dB bandwidth of high-frequency differential signal trace on the PCB is over 20 GHz. At the two ends of the module, the electrical input and output interfaces were designed as golden finger structures for QSFP28 pin definition.
For the optical transmitter modules, the wavelength for the GaAs VCSEL array ranges from 840 nm to 865 nm, with a 3 dB bandwidth of about 18 GHz, threshold current of about 4 mA, average operating current of 8 mA, and VCSEL die pitch of 250 μm. The VCSEL array driver included a high-performance clock and data recovery (CDR) and an equalizer, and the driver can adjust bias and modulation current. The typical power consumption of the driver IC is 0.495 mW/Gb/s for CDR bypassed, and 0.97 mW/Gb/s for all functions enabled. For the optical receiver modules, the detection wavelength for the GaAs PIN PD array ranges from 760 nm to 860 nm, with a 3 dB bandwidth of about 17 GHz, detection window diameter of 30 μm, and PD die pitch of 250 μm. The receiver IC included a high-sensitivity transimpedance amplifier (TIA), limiting amplifier, CDR, and output driver with an adjustable output swing. The typical power consumption of the receiver IC is 0.525 mW/Gb/s for CDR bypassed, and 0.855 mW/Gb/s for all functions enabled. On the PCB, the VCSEL or PD arrays are connected with the laser driver or receiver ICs through wire bonding. The total power consumption of the optical transmitter or receiver modules can be lower than the typical 10 mW/Gb/s [21].
The structure of the PWG array is shown in Fig. 2
. The PWG array is made of graded-index (GI) core waveguides. The material for the optical waveguide is polynorbornene, one of the cyclic olefin resins with a high refractive index and high optical transparency at 850 nm. The refractive indices of the core and cladding for the PWG array are about 1.553 and 1.536, respectively. The waveguide core size was 45 μm while the pitch was 62.5 μm. The density of the GI PWG was higher than that of standard twelve-channel ribbon fiber. The propagation loss of the PWG was 0.04 dB/cm at 850 nm, and its bandwidth coefficient was 40 GHz × m. The crosstalk between adjacent channels in the PWG array was below −30 dB. The pitch of the VCSEL or PD is four times that of the PWG, so every four waveguides with a 250 μm pitch serves as a waveguide set, while 45° mirrors in the waveguides were designed at the position of the VCSEL or PD arrays, as shown in Fig. 2. The 45° mirrors were fabricated by laser ablation using an excimer laser with a wavelength of 193 nm. Mirrors with total internal reflection (TIR) faces were used to collect the light beams from the VCSELs into the PDs. The insertion loss of the 45° mirror was about 2 dB, the −0.5 dB tolerance width was about ± 10 μm for the 45° mirror alignment, and the −2 dB tolerance width was about ± 20 μm [22]. The MT connectors for the PWG array were designed for measuring optical transmissions. For chip-to-chip interconnects, the optical signals can be linked through the PWG array without MT connectors.The 45° mirror can vertically couple light beams from the VCSEL to the PWG or from the PWG to the PD, as shown in Fig. 3
. The four VCSEL or PD arrays corresponded to four different waveguide sets. The high-speed electrical signal transmission uses PCB trace, laser driver, and receiver ICs. The electrical-to-optical and optical-to-electrical conversions were achieved through the VCSEL and PD arrays. The optical signal transmission was achieved through the lens array and PWG. The lens array was bought off the shelf and was fabricated by high-precision injection molding. Anti-reflective-coated lens arrays are also available. This lens array showed an insertion loss below 2 dB, crosstalk below −50 dB, and a lens position tolerance below 0.005 mm. In our modules, the component profile of the lens array was modified to fit the structure of the module. The lens array can improve the displacement tolerance for alignment between the VCSEL/PD arrays and the PWG array,.3. Measurement of 400-Gb/s transmissions using a PWG array
The transmission performance of the 400-Gb/s optical transmitter module was characterized using the experimental setup shown in Fig. 4
. The power supply provided a 3.3-V DC voltage for the laser driver ICs to start the transmission of the VCSEL arrays in the transmitter module. A differential 25-Gb/s pseudo-random bit sequence (PRBS) signal with a pattern length of 231-1 was generated from the pattern generator module of a commercial signal quality analyzer (Anritsu MT1810A) and used to encode the VCSEL transmitter. The MT connector of the PWG array can be connected using an MT connector unto a twelve-channel ferrule connector (FC) fan-out fiber. The optical output signal from the FC port was received and analyzed using the optical module of a wide-bandwidth oscilloscope (Keysight 86100D). In Fig. 4, the bias-tees were used as DC blocks.Active alignment was used for every channel between the VCSEL array and the PWG array. Figure 5
shows the measured optical eye diagrams for the 16 25-Gb/s channels of the 400-Gb/s optical transmitter module. The PWG transmission lengths for the 1, 2, 3, and 4 optical signals in the VCSEL array were 110 mm, 120 mm, 130 mm, and 140 mm, respectively. The equalizers were turned on in the laser drivers to take the above measurements. Channel 4 of VCSEL array 4 showed the best performance in the eye diagrams, with a rise time of 15.04 ps, a fall time of 17.86 ps, a peak-to-peak time jitter of 10.43 ps, a signal-to-noise ratio (SNR) of 8.6, an extinction ratio (ER) of 3.74 dB, an average optical power of 1.03 mW, and a mask margin (MM) of 40.4% for the 100GBASE-SR4 standard. Channel 3 of VCSEL array 3 showed the worst performance: the eye diagram exhibited a rise time of 14.87 ps, a fall time of 17.35 ps, a peak-to-peak time jitter of 10.85 ps, an SNR of 8.01, an ER of 3.71 dB, an average optical power of 1.26 mW, and an MM of 31.5%. The 25-Gb/s optical eye diagrams of all output channels showed good performance in the optical transmitter module, with all SNR above 8.01, peak-to-peak time jitter below 11.70 ps, ER above 3.7 dB, MM above 31.5%, and average optical power from 0.86 mW to 1.28 mW. Therefore, the 400-Gb/s optical transmission through the 140 mm long PWG can be realized. Although active alignment was used for every channel, the good transmission performance observed here would still be expected if the misalignment displacement can be controlled within ± 12 μm in both x-axis and y-axis [19,20].The performance of the 400-Gb/s optical receiver module was characterized using the experimental setup shown in Fig. 6
. A commercial QSFP28 optical transceiver module was used as a testing light source that was modulated by a 25-Gb/s PRBS with a pattern length of 231-1. The power supply provided a 3.3-V DC voltage for the QSFP28 module and the 400-Gb/s optical receiver module. The optical PRBS signal was input into a twelve-channel FC fan-in fiber and then unto the MT connector, while the optical receiver module can receive the optical PRBS signal through the PWG array using MT connectors. The electrical output signal from the optical receiver module was received and analyzed using the electrical module of the wide-bandwidth oscilloscope.
Fig. 7 (a) The optical eye diagram from the QSFP28 module, (b) the electrical eye diagrams from 16 25-Gb/s channels in the 400-Gb/s optical receiver module, and (c) the electrical eye diagram showing the best-performing channel without the CDR function.
About half of all the channels showed bad performance in the electrical eye diagrams. Because the performance of PD array 4 with the longest PWG transmission was still good, the poor performance could be not due to the PWG transmission. Figure 7(c) shows the eye diagram for the best channel with the receiver IC bypassing CDR function, with a rise time of 16.24 ps, a fall time of 17.52 ps, a peak-to-peak time jitter of 17.01 ps, and an SNR of 8.81. The performance of the eye diagram without the CDR function was still better than that of the worst case, so the cause of some bad eye diagrams could be not due to the receiver IC. Because the rise and fall times were about 30 ps for the performance of the worst case and the bandwidth of the PD can be estimated to be about 11.7 GHz, the cause of some bad eye diagrams could be due to the insufficient bandwidth of some PDs or the insufficient bandwidth of the output buffer circuits in some receiver ICs. Therefore, if all PD arrays with sufficient bandwidth can be used in the receiver module, the 400-Gb/s optical reception using 140 mm long PWG transmissions could completely be realized. For high channel densities, all the components in the transceiver modules must have excellent uniformity, accuracy, and stability. If forward error correction technology was used in the 400-Gb/s transceiver module, the bit error rate (BER) demand can also be leveled down.
The BER was also measured and the experimental setup was similar to that in Fig. 6. However, an optical attenuator was inserted between the QSFP28 module and the PWG array; the electrical output signal of the optical receiver module was input into the error bit detection module of the signal quality analyzer. Figure 8
shows the BER versus input optical power for PWG transmission for the best-performing channel. The input optical power means the incident optical power to the PWG array. The real received power of the receiver module must deduct the PWG losses and lens coupling losses from the input optical power. Under a BER of 10−12, the receiver sensitivity was about −2.6 dBm. Although only the BER of the best-performing channel was measured, the 10−12 BER is achievable if the SNR of the other channels is greater than seven.
Table 1. Comparison of the Performance of the Work Presented Herein with that of the Previously Reported Optical Transceiver Modules for On-Board Interconnects Using Polymer Waveguide (PWG) Arrays
4. Conclusions
Through PWG transmissions, we designed and fabricated 400-Gb/s (16 × 25 Gb/s) optical transmitter and receiver modules for on-board interconnects. Four four-channel VCSEL and PIN PD arrays were used in the optical transmitter and receiver modules. A 16-channel PWG array with a length of 140 mm and pitch of 62.5 μm was used as short-reach optical signal transmission medium in the modules. The 25-Gb/s optical eye diagrams of the optical transmitter module, the electrical eye diagrams of the optical receiver module, and the transmission BER were measured. Eye diagrams with good performance can be achieved if the bandwidth of the electrical devices is sufficient for 25-Gb/s transmissions. Optical transmitter and receiver modules with simple structures can be commercialized, and the PWG array can also be easily assembled with them. The PWG array without connectors can easily be designed as a transmission route with any shape to fit the position of the IC on the PCB and to realize high-speed chip-to-chip optical interconnects.
Funding
Taiwan Ministry of Science and Technology, (MOST 106-2218-E-005-001, MOST 106-2221-E-151-028, MOST 107-2218-E-992-304).
Acknowledgements
The authors would like to thank Sumitomo Bakelite Co. for supporting the PWG arrays.
References
1. H. Cho, P. Kapur, and K. C. Saraswat, “Power comparison between high-speed electrical and optical interconnects for interchip communication,” J. Lightwave Technol. 22(9), 2021–2033 (2004). [CrossRef]
2. C. F. Lam, H. Liu, B. Koley, X. Zhao, V. Kamalov, and V. Gill, “Fiber optic communication technologies: What’s needed for datacenter network operations,” IEEE Commun. Mag. 48(7), 32–39 (2010). [CrossRef]
3. W. Zhang, H. Wang, and K. Bergman, “Next-generation optically-interconnected high-performance data centers,” J. Lightwave Technol. 30(24), 3836–3844 (2012). [CrossRef]
4. H. Nasu, “Short-reach optical interconnects employing high-density parallel-optical modules,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1337–1346 (2010). [CrossRef]
5. F. Karinou, N. Stojanovic, A. Daly, C. Neumeyr, and M. Ortsiefer, “1.55-μm long-wavelength VCSEL-based optical interconnects for short-reach networks,” J. Lightwave Technol. 34(12), 2897–2904 (2016). [CrossRef]
6. M. Pantouvaki, P. De Heyn, R. Michal, P. Verheyen, S. Brad, A. Srinivasan, H. Chen, J. De Coster, G. Lepage, P. Absi, and J. Van Campenhout, “50Gb/s silicon photonics platform for short-reach optical interconnects,” in Opt. Fiber Commun. Conf. Exhib. (Optical Society of America, 2016), paper Th4H-4.
7. J. Ingham, “Future of short-reach optical interconnects based on MMF technologies,” in Opt. Fiber Commun. Conf. Exhib. (Optical Society of America, 2017), paper Tu2B-1.
8. C. P. Caputo, P. J. Decker, and S. E. Ralph, “VCSEL-based 100m 25Gb/s plastic optical fiber links,” in Opt. Fiber Commun. Conf. Expo. Natl. Fiber Opt. Eng. Conf. (Optical Society of America, 2011), paper OWB2. [CrossRef]
9. J. Tatum, “The Evolution of 850nm VCSELs from 10Gb/s to 25 and 56Gb/s,” in Opt. Fiber Commun. Conf. Exhib. (Optical Society of America, 2014), paper Th3C-1.
10. N. Bamiedakis, A. Hashim, R. V. Penty, and I. H. White, “A 40 Gb/s optical bus for optical backplane interconnections,” J. Lightwave Technol. 32(8), 1526–1537 (2014). [CrossRef]
11. J. Sangirov, G.-C. Joo, J.-S. Choi, D.-H. Kim, B.-S. Yoo, I. A. Ukaegbu, N. T. H. Nga, J.-H. Kim, T.-W. Lee, M. H. Cho, and H.-H. Park, “40 Gb/s optical subassembly module for a multi-channel bidirectional optical link,” Opt. Express 22(2), 1768–1783 (2014). [CrossRef] [PubMed]
12. T. Ishigure and Y. Nitta, “Polymer optical waveguide with multiple graded-index cores for on-board interconnects fabricated using soft-lithography,” Opt. Express 18(13), 14191–14201 (2010). [CrossRef] [PubMed]
13. M. Singh, K. Kitazoe, K. Moriya, and A. Horimoto, “High reliability and high density graded index polymer waveguides for optical interconnect,” Opt. Commun. 362, 33–35 (2016). [CrossRef]
14. K. Schmidtke, F. Flens, A. Worrall, R. Pitwon, F. Betschon, T. Lamprecht, and R. Krahenbuhl, “960 Gb/s optical backplane ecosystem using embedded polymer waveguides and demonstration in a 12G SAS storage array,” J. Lightwave Technol. 31(24), 3970–3975 (2013). [CrossRef]
15. M. Immonen, R. Zhang, M. Press, H. Tang, W. Lei, J. Wu, H. J. Yan, L. X. Zhu, and M. Serbay, “End-to-end optical 25Gb/s link demonstrator with embedded waveguides, 90° out-of-plane connector and on-board optical transceivers,” in 42nd European Conf. Opt. Commun. (2016), pp. 1154–1156.
16. J. Chen, N. Bamiedakis, P. P. Vasil’ev, T. J. Edwards, C. T. Brown, R. V. Penty, and I. H. White, “High-bandwidth and large coupling tolerance graded-index multimode polymer waveguides for on-board high-speed optical interconnects,” J. Lightwave Technol. 34(12), 2934–2940 (2016). [CrossRef]
17. T. Yagisawa, T. Shiraishi, M. Sugawara, Y. Miki, T. Kondou, M. Kabayashi, and K. Tanaka, “40-Gb/s card-edge connected optical transceiver using novel high-speed connector,” in IEEE 65th Electron. Compon. Technol. Conf. (2015), pp. 783–788.
18. H. Numata, M. Tokunari, and J. B. Heroux, “60-Micrometer pitch polymer waveguide array attached active optical flex,” in Opt. Fiber Commun. Conf. Exhib. (Optical Society of America, 2017), paper W1A.5. [CrossRef]
19. H. Nasu, N. Nishimura, Y. Nekado, and T. Uemura, “Polymer waveguide-coupled solderable optical modules for high-density optical interconnects,” in IEEE 66th Electron. Compon. Technol. Conf. (2016), pp. 1087–1092.
20. T. Yagisawa, T. Mori, R. Gappa, K. Tanaka, O. Daikuhara, T. Komiyama, and S. Ide, “Structure of 25-Gb/s optical engine for QSFP enabling high-precision passive alignment of optical assembly,” in IEEE 66th Electron. Compon. Technol. Conf. (2016), pp. 1099–1104.
21. M. A. Taubenblatt, “Optical interconnects for high-performance computing,” J. Lightwave Technol. 30(4), 448–457 (2012). [CrossRef]
22. Y. Morimoto and T. Ishigure, “Low-loss light coupling with graded-index core polymer optical waveguides via 45-degree mirrors,” Opt. Express 24(4), 3550–3561 (2016). [CrossRef] [PubMed]
