We experimentally demonstrate real-time coherent transmission up to 400Gbps over 300m OM3 multimode fibers, showing resilience to connector offsets up to 3-6 µm and fiber mechanical shaking using rigorous TIA-455-203 procedures

Forghieri F.

Gaudino R.

Rizzelli G.

English

We experimentally demonstrate real-time coherent transmission up to 400Gbps over 300m OM3 multimode fibers, showing resilience to connector offsets up to 3-6 μm and fiber mechanical shaking using rigorous TIA-455-203 procedures

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

29 March 2023POLITECNICO DI TORINORepository ISTITUZIONALEExperimental Demonstration of Real-Time 400G Coherent Transmission over 300m OM3 MMF / Rizzelli, G.; Forghieri,F.; Gaudino, R.. - ELETTRONICO. - (2022), pp. 1-3. ((Intervento presentato al convegno Optical Fiber CommunicationConference, OFC 2022 tenutosi a San Diego, USA nel 06-10 March 2022 [10.1364/OFC.2022.W1G.2].OriginalExperimental Demonstration of Real-Time 400G Coherent Transmission over 300m OM3 MMFOptica Publishing Group (formely OSA) postprint/Author's Accepted ManuscriptPublisher:PublishedDOI:10.1364/OFC.2022.W1G.2Terms of use:openAccessPublisher copyright“© 2022 Optica Publishing Group. One print or electronic copy may be made for personal use only. Systematicreproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, ormodifications of the content of this paper are prohibited.”(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2975402 since: 2023-01-31T08:58:10ZOptica Publishing Group (formerly OSA)Experimental Demonstration of Real-Time 400GCoherent Transmission over 300m OM3 MMFG. Rizzelli1, F. Forghieri2, R. Gaudino11Politecnico di Torino, PhotoNext Center, Corso Duca degli Abruzzi 24, Torino, Italy2 Cisco Photonics Italy S.r.l., 20871 Vimercate, Italygiuseppe.rizzelli@polito.itAbstract: We experimentally demonstrate real-time coherent transmission up to 400Gbpsover 300m OM3 multimode fibers, showing resilience to connector offsets up to 3-6 µm andfiber mechanical shaking using rigorous TIA-455-203 procedures. © 2022 The Author(s)1. IntroductionMultimode fibers (MMF) are largely deployed in intra-data center (IDC) for links of up to 300 m, since theyenable using VCSEL-based transceivers at 850 nm, which have significantly lower costs than all other short reachtransceiver options. However, VCSEL+MMF transmission, even using newer OM4 and OM5 category fibers, haswell-known maximum capacity limits due to multimodal dispersion, that make today transmission at more than50 Gbit/s/λ and more than 100 m an extremely challenging task.In this paper, extending the work presented in [1, 2, 5], we experimentally investigate on the transmission ofcoherently detected PM-QPSK or PM-16QAM signals over MMF (CoH-MMF) under practical IDC conditions,showing excellent performances up to 300 m of OM3 MMF fiber, made possible by central-launching and by thewell-known properties of graded-index MMF, in which the LP01 fundamental mode does not couple significantlywith the higher order modes unless the MMF is strongly mechanically perturbed [4]. The novelty of this papercompared to the existing literature, and in particular to our previous work [5], is in that (i) we use a commercialcoherent transceiver to allow long-term real-time Bit Error Rate (BER) monitoring, (ii) we address the impact oflateral offsets in MMF connectors along the link and (iii) we rigorously tested resilience to MMF fiber shakingusing TIA-455-203 FOTP–203 procedures.Our results can be of practical interest for IDC situations having a large installed MMF infrastructure, whichcould be upgraded to much higher bit rates by changing the transceivers without substituting the fibers. Obvi-ously, the cost saving due to MMF re-use is traded-off by the significantly higher cost of coherent transceivers(that is anyway constantly decreasing, as shown for instance in the 400G-ZR scenario), but this techno-economicdiscussion is outside the scope of our paper. As a key result, we show reliable operation under continuous fibershaking of 200G PM-QPSK up to 6 µm lateral offset in MMF connectors, and 400G PM-16QAM up to 3 µm.2. Theoretical BackgroundSeveral previous papers, such as [4], have shown that in graded-index MMF, the LP01 fundamental mode does notcouple significantly with the higher order modes (unless the MMF is strongly mechanically perturbed), and alsothat a standard single-mode fiber (SMF) that is centrally connected to a MMF fiber couples most of its power tothe MMF LP01 mode [3]. As a consequence, an optical link obtained by cascading three SMF-MMF-SMF fibers,if all three fibers are centrally aligned in the connectors, appears to a first approximation as a ”quasi single-modesystem” and, moreover, has a low attenuation. In fact, we showed in [5] that using standard connectors, an SMF-MMF-SMF link used around 1550 nm has an extra attenuation (above the intrinsic length-dependent attenuationof each fiber spool) below 2 dB, and thus potentially enable the direct use of SMF transceivers centrally coupled toan MMF link. Even more importantly, this quasi-single mode propagation leads to a much lower impact of modaldispersion, since most of the power remains in the fundamental LP01 mode [3]. Anyway, the quasi-single modeand low-loss operation of an SMF-MMF-SMF link can degrade due to (i) lateral offsets in connectors along thelink, that do not ensure central launching conditions and (ii) mechanical stress applied to the MMF segment. Bothissues are clearly fundamental for IDC environments, where MMF connectors have their intrinsic mechanicaltolerances (typical offset is below 3 µ m) and the MMF can obviously be moved or continuously perturbed, forinstance when it passes close to fan coolers. Clearly, for increasing connector offsets and mechanical stress, theSMF-MMF-SMF system progressively departs from the quasi-single mode operation, thus introducing increasingloss and higher modal dispersion impact.In this paper, we experimentally address these two issues, by introducing in the MMF link MMF-to-MMF tran-sitions with a controlled offset, and by applying the same mechanical stress procedure introduced for testing IEEE10GBase-LRM systems, which requires inserting a specific instruments called ”MMF fiber shaker” to introduceup to 10 Hz standardized mechanical vibration to the fiber, according to TIA-455-203 procedures. Due to theresulting time constants (the MMF shaker continuously generates vibration with periodicity in the [0.1-1] secondrange), a typical off-line processing BER measurement approach would give unreliable results (since it acquiresonly very short time-windows) and this is why it was fundamental in our work to use a commercial coherent cardto enable long-term pre-FEC measurements.3. Experimental ResultsThe block diagram of our experimental setup is shown in Fig. 1a). A coherent transceiver inside a commercialline card is used to transmit up to 200 Gbps net bitrates using PM-QPSK, and up to 400 Gbps using PM-16QAM.The complete setup includes an SMF patchcord at the transmitter output connected to an MMF patchcord througha lab-grade mating sleeve, to ensure central launch conditions, an MPX-SR3 MMF fiber shaker (implementingTIA-455-203 specifications for MMF fiber testing) and a 300 m MMF fiber spool connected to the SMF patchcordat the receiver input through another mating sleeve. The MMF patchcord is composed of two MMF pigtailsfusion spliced together with a lateral offset to emulate non-perfect connections, such as introduced by IDC patchpanels. The four MMF elements in Fig. 1a) have actually been swapped to analyze in the following differentconfigurations: i) ’back-to-back’: no MMF in the optical path; ii) ’OM3 only’: the SMF patchcords are connectedto the OM3 fiber spool with zero offset and without using the shaker; iii) ’offset-OM3’: a 3 or 6 µm lateral offset isinduced on the MMF patchcord before the OM3 fiber spool; iv) ’offset-shaker-OM3’: the MMF shaker is insertedin between the offset MMF patchcord and the OM3 spool; v) ’offset-OM3-offset’: a 3 µm lateral offset is insertedbefore and after the OM3 fiber. Finally, a variable optical attenuator is inserted before the coherent receiver forRX sensitivity measurements.Fig. 1. a) Experimental setup. b), c) measured BER vs power budget margin for b) PM-QPSK andc) PM-16QAM modulation in different conditions: back-to-back (blue, circles), in ’OM3 only’ (red,crosses), ’3 µm offset-OM3’ (green, squares), ’6 µm offset-OM3’ (purple, plus signs) and ’3 µmoffset-OM3-3 µm offset’ (black, dots) configurations.Fig. 1b) shows the measured BER as a function of the power budget margin (PBM) for 100G and 200G PM-QPSK modulation in ’back-to-back’, ’OM3 only’, ’3 µm offset-OM3’, ’6 µm offset-OM3’ and ’3 µm offset-OM3-3 µm offset’ configurations. The PBM is defined here as the extra attenuation we introduced in the opticalpath via the VOA, which can thus be interpreted as the available system power margin. For both bitrates, the PBMpenalty due to transmission over 300 m OM3 fiber with respect to back-to-back conditions is negligible whencentral launch is ensured, and increases as the lateral offset increases due to higher-order modes excitation. Aspreviously mentioned, this penalty is caused by moving away from the ”quasi single mode” propagation condition,with the consequent increase in the average power loss in the SMF-MMF-SMF system and in the impact of modaldispersion. Nevertheless, very high margins can be obtained at a 10-2 BER threshold. For instance, at 200G morethan 30 dB extra loss can be tolerated in a system with 6 µm offset, and more than 28 dB in a system with twoseparate 3 µm offset connectors.The same type of results are shown in Fig. 1c) for 200G and 400G PM-16QAM modulation. The effect ofconnector offset on the performance becomes more evident, especially for large offsets and high bitrate. Forinstance, the PBM penalty of the ’3 µm offset-OM3-3 µm offset’ configuration is 4.5 dB at BER=10-2 withrespect to the ’OM3 only’ at 400G. However, achievable PBM is in the order of 20 dB for single connector offsetsup to 6 µm at BER=10-2 or even for two 3 µm separate offsets assuming a slightly stronger FEC is used with BERthreshold at 2 ·10-2.In order to test the system robustness to mechanical vibration and stresses, we used a procedure similar to theone used for IEEE 10GBase-LRM transceivers, investigating the effects introduced by the MMF fiber shaker.Moreover, we also manually changed the position of the MMF patchcord. Following a conservative approach,the MMF patchcord was rolled 4 times on a 10 cm circular bobbin, so the impact of the mechanical movementswas intentionally enhanced. It is worth mentioning that moving the MMF patchcord increases mode mixing bychanging the spatial distribution of the modes inside the fiber, thus increasing the power in the higher order modes,and consequently also increasing the net optical loss introduced by the offsets in the SMF-MMF-SMF system.We studied this effect in detailed numerical simulations (not shown here for space limitations), obtaining thatpropagation has a random characteristic due multi-path interference between the fundamental mode and all thehigher order modes. The information on the resulting net optical loss is included in Fig. 2 next to each curve. Fig.2a) shows three measurements of the ’6 µm offset-shaker-OM3’ configuration with 200G PM-QPSK modulation,each performed after moving the MMF patchcord in a different position. We actually performed many morerepeated measurements, and here we just show three cases corresponding to the worst and best cases, and a typicalintermediate one. PM-QPSK modulation exhibit good tolerance to modal noise even for a very large 6 µm offsetwith only about 1 dB PBM variation at BER=10-2, due mainly to the net loss variation. Fig 2b) highlights that 400GPM-16QAM modulation in the ’3 µm offset-shaker-OM3’ configuration is also quite robust to the fiber positionvariation, although the shaker somewhat affects the system performance at such high bitrate, inducing oscillationson a BER that is already very close to 10-2 in the considered PBM range. On the other hand, at BER=2 ·10-2 thethree curves are essentially superimposed, showing PBM better than 20 dB even when the MMF shaker is used.Lastly, Fig. 2c) shows the ’6 µm offset-shaker-OM3’ configuration with 400G PM-16QAM modulation. This isthe configuration where our stress-test is most critical. In ”good” modal noise conditions (black curve) the systemis capable of providing PBM in excess of 19.5 dB and 21.5 dB at BER=10-2 and BER=2 · 10-2, respectively.However, this solution is strongly dependent on the mode distribution and, in some worst-case conditions, doesnot provide sufficiently good performance below FEC threshold.Fig. 2. Repeated measurement on BER vs. power budget margin for different positions of the MMFpatchcord for a) ’6 µm offset-shaker-OM3’ configuration with 200G PM-QPSK modulation, b) ’3µm offset-shaker-OM3’ configuration with 400G PM-16QAM modulation and c) ’6 µm offset-shaker-OM3’ configuration with 400G PM-16QAM modulation. The number next to each curve isthe net optical loss introduced by the offset patchcord at each position.4. Discussion and ConclusionsWe demonstrated that commercial coherent transceivers can be used also on MMF fiber links up to much higherbit rates than those achieved by current VCSEL+direct detection based systems, provided that connections alongthe MMF have good quality (i.e. connectors with offsets in the 3µm to 6µm range). The ample system margin indB that we showed in the last graphs, even under strong mechanical stresses, confirms the practical feasibility ofthe proposed systems. In particular, we show that 200G PM-QPSK can tolerate one 6µm offset or two 3µm offsetswith ample margin at BER=10-2, while 400G PM-16QAM can tolerate one 3µm offset at BER=2 ·10-2.Acknowledgements: the authors thank the PhotoNext@POLITO Center at Politecnico di Torino, where theexperiments were performed, and LINKS Foundation researchers for the very fruitful scientific exchange.References1. Z. Wu et al., “Experimental Demonstration of MMF Fundamental-mode MMF Transmission Enabled by Mode Conver-sion,” in Asia Communications and Photonics Conference, OSA Technical Digest (CD) (Optical Society of America,2017), pp. 1-3.2. Y. Ma, Y. Tang and W. Shieh, “107 Gb/s transmission over multimode fiber with coherent optical OFDM using cen-ter launching technique,” in European Conference on Optical Communication, OSA Technical Digest (CD) (OpticalSociety of America, 2009), pp. 1-2.3. A. Amphawan et al., “Derivation of an Analytical Expression for the Power Coupling Coefficient for Offset LaunchInto Multimode Fiber,” IEEE J. Lightwave Technol. 28, 861-869 (2010).4. D. Donlagic and B. Culshaw,“Propagation of the fundamental mode in curved graded index multimode fiber and itsapplication in sensor systems,” IEEE J. Lightwave Technol., vol. 18, no. 3, 334-342 (2000).5. G. Rizzelli et al., “Experimental Demonstration of Coherent Transmission over MMF and of the Impact of ConnectorsOffset,” in 2020 22nd International Conference on Transparent Optical Networks (ICTON)

Experimental Demonstration of Real-Time 400G Coherent Transmission over 300m OM3 MMF

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Experimental Demonstration of Real-Time 400G Coherent Transmission over 300m OM3 MMF

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