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Universal fiber has an LP01 mode field diameter approximately matched to that of standard single mode fiber, while being a multimode fiber. We analyzed the dependence of the mode field diameter on the core diameter for different core delta values. Guided by the analysis, a universal fiber having a delta of 1.2% was fabricated, showing significantly reduced coupling loss of ~2.3 dB with conventional multimode fiber. We demonstrated that the fiber can transmit with full system reach in both single mode and VCSEL-based multimode transmissions, including 100G SR4, 40G sWDM, and 100G CWDM4 for the first time.
© 2016 Optical Society of America
1. Introduction
In data centers (DC), both multimode fibers (MMF) and single mode fibers are used with VCSEL-based multimode transceivers and single mode transceivers, respectively. With the emerging hyper-scale DC, single mode transmission is deployed more frequently to meet the need of longer system reach, while at the same time multimode fiber transmission continues to dominate in enterprise DC with system reach mostly less than 100 m [1]. Multimode transmission has long been a primary optical transmission technology in DC as a cost-effective solution. Novel approaches of using long wavelength MMF in longer system reaches have been explored [2,3]. Even in large scale DC, a portion of the optical transmission has short distances, less than tens of meters, where multimode transmission is more cost effective than single mode transmission. While it is feasible to use both MMF and single mode fiber in DC, it is desirable to use a uniform type of optical fiber that can accommodate both types of transmissions to simplify fiber cable management, and provide flexibility for future transceiver upgrades.
Although it is possible to launch the light into only the fundamental mode of MMF using various complicated mode expansion techniques [4–7], the solutions are too costly for cost-sensitive data center applications. One promising solution for the uniform fiber type in DC is called Universal Fiber (UF) as reported in [8]. It is a MMF with the fundamental LP01 mode approximately matching the mode field diameter (MFD) of a standard single mode fiber. We have reported preliminary results for this fiber and its performance. We have demonstrated its use in 850 nm VCSEL based multimode transmission at 10 Gb/s and 25 Gb/s over 100 m and 50 m and 1310 nm single mode 25 Gb/s NRZ and 44 Gb/s PAM4 transmission over 2 km. The main limitation of the fiber in [8] is high insertion loss of about 5 dB when it is coupled to either a VCSEL transmitter or a 50 μm core MMF. While it is still sufficient to cover a range of applications with a decent system reach, the elevated insertion loss poses a limitation for reaching the full potential. We also note here that a gradient-index multimode fiber that is designed for both 850nm VCSEL transmission and single mode transmission has been proposed before for home network applications [9]. The fiber has very high delta of 2.5-2.9% between the core and the cladding. The transmission experiments demonstrated single mode transmission using single mode laser at 10 Gb/s over 100 m and similar distance for 1.25 Gb/s multimode transmission. The UFs proposed in [8] and current paper have much lower delta and are close to that of existing high bandwidth MMFs. We have demonstrated very robust performance in both multimode and single mode transmissions at 10Gb/s and 25Gb/s data rates.
In this paper, we conduct a detailed study of the fiber parameters and show how they affect the LP01 MFD and coupling loss. As the result of the study, we fabricated an improved UF with significantly lower insertion loss. We demonstrated single mode and VCSEL-based multimode transmissions through the UF including 100G SR4, 40G short wavelength division multiplexing (sWDM) [10,11], and 100G CWDM4 [12] over the full system reach for the respective technology. Note that 100G SR4 and 100G CWDM4 are specified for 100 m OM4 transmission and 2 km SM transmission. For 40G sWDM, because it is a non-standard solution, we reference the reach with standard based 40G SR4 solution, which is specified for use at 150 m for OM4. These are the full system reaches that we compare with for transmission over the UF.
2. Fiber design and insertion loss
The key consideration of the UF design is for the fiber to be able to operate both for multimode transmission around 850 nm and single mode transmission around 1300 nm or 1550 nm [8]. A UF takes a simple alpha refractive index profile similar to conventional MMF, such as OM3 and OM4, as described by the following equation:
where n0 is the refractive index in the center of the core, a is the core radius, and is the relative refractive index change, where n1 is the refractive index of the cladding.In order to accommodate the single mode transmission it is necessary for the mode field diameter of the fundamental LP01 mode to approximately match that of standard single mode fiber, which is around 9.2 μm at 1310 nm. In our recent effort reported in [8], we fabricated a UF with 1% core delta, similar to that of OM3 and OM4 MMF, and a core diameter of 23 μm. We were able to demonstrate the 25 Gb/s single mode transmissions over 2 km and multimode transmission over 50 m using 100G SR4 transceiver, and over 100 m using 10G multimode transceiver. In that work, there was around 4.5 dB of coupling loss from conventional 50 micron core MMF to UF. The coupling from the VCSEL transmitter into the UF is also expected to have a similar coupling loss. In order to enhance the system performance, it is desirable to reduce the coupling loss further. In the current work, we have further explored the design space to guide us toward a more optimal design.
For multimode coupling from a 50 μm core fiber to a UF, the insertion loss is caused by the smaller core diameter of UF. For single mode coupling, the insertion loss is determined by the mismatch of MFD. If the core size increases further, the MFD of LP01 mode will also increase, which leads to a mismatch of MFD with the single mode fiber.
In addition to the core diameter, the core delta also affects the MFD. We calculated the LP01 MFD versus the core diameter for different core deltas at 1310 nm and 1550 nm. As shown in Fig. 1, when the core delta increases, the same LP01 MFD can be achieved with a larger core diameter. This allows us to increase the core diameter for better VCSEL-MMF coupling at 850 nm, without causing MFD mismatch at 1310 nm. We also observe in Fig. 1 that, when MFD is matched for single mode fiber at 1310 nm, it is also matched at 1550 nm. Note that the MFD of standard single mode fiber is around 10.4 μm at 1550 nm.
For single mode operation, a mismatch in MFD will cause insertion loss. To understand how large the core diameter can be for a given core delta, we calculated the insertion loss due to MFD mismatch at 1310 nm. Figure 2(a) shows the insertion loss as a function of MFD of UF when coupled to a standard single mode fiber. The MFD of the standard single mode fiber is assumed to be 9.2 μm. Figure 2(a) indicates that the MFD of UF does not need to match perfectly to 9.2 μm to have a low insertion loss. For insertion loss below 0.1 dB, the MFD can be as large as 10.7 μm. For single mode transmission using UF, one impairment is coupling to higher order modes, which could produce multipath interference (MPI) effects. For insertion loss below 0.1 dB, the power that is coupled to different higher order modes is below 0.1 dB and the power penalty due to MPI is minimal.
Fig. 2 (a) Insertion loss between standard single mode fiber and UF as a function of MFD of UF; (b) Insertion loss between a standard 50 μm MMF and UF as a function of relative etendue of UF.
For multimode operation, both the core diameter and core delta (or core numerical aperture) of UF affect the insertion loss to a standard MMF. To calculate the insertion loss from a 50 μm standard MMF to a UF, we performed numerical modeling by evaluating the overlap integrals between different modes. The encircled flux (EF) launch condition was assumed for launching into 50 μm MMF. UFs with different delta and core diameters were considered in the calculation. We found that the fiber etendue, which is proportional to NA2 × D2, could be used to gauge the insertion loss, where NA is the numerical aperture of the fiber core and D is the core diameter. Figure 2(b) shows modeled insertion loss as a function of etendue of UF normalized to that of the 50 μm standard MMF. The MMF insertion loss of UF can be reduced by increasing the etendue. We also measured insertion losses of 50 μm MMF to three different UFs and the results are also shown in Fig. 2(b). The three UFs have the core delta of 1, 1.2 and 2%, respectively, and the core diameter of 23, 31, and 35 μm, respectively. The measured insertion losses are in good agreement with the modeling results. The VCSEL used in getting the experimental data has a typical VCSEL launch condition in compliance of TIA standard and is from the VCSEL used in the experiment in Session 3. It has an integrated optical power of 12% at 4.5 micron radius and reaches 86% integrated optical power at 16.5 micron radius.
Guided by modeling results of Figs. 1 and 2, we fabricated a new UF. The fiber has a core delta of 1.2%, a core diameter of 31 μm, and an alpha value of 2.09, which supports 12 mode groups. The larger core increases slightly the LP01 MFD mismatch to single mode fiber, but it also increases significantly the coupling efficiency to VCSEL or MMF. In addition, the higher numerical aperture of 0.225 further improves the coupling efficiency.
The fiber has attenuations of 2.2 dB/km, 0.42 dB/km and 0.23 dB/km at wavelengths of 850 nm, 1310 nm and 1550 nm respectively. The chromatic dispersion of the UF is similar to standard MMF as it is determined largely by material property. Specifically the chromatic dispersion around 850 nm is around −95 ps/nm-km and around 0 ps/nm-km at 1310 nm. We measured the overfill modal bandwidth of the UF at 850 nm and 1310 nm to be 1.1 GHz.km and 750 MHz.km, respectively. We found the fiber reaches the peak bandwidth 4.13 GHz.km at 935nm. We also show DMD chart of the UF as measured from 2.7 km of the sample in Fig. 3(a) and the centroid delay which is the measure of the location of the pulse as a function of single mode launch offset in Fig. 3(b). We observed that delay stays flat up to around 8 μm and becomes left tilt for higher radial offset. This is consistent with the observation the peak bandwidth is achieved at a higher wavelength. The bandwidth values and the DMD information reflect the results from the experiment fiber as an early stage effort. They can be further improved with fine tuning of the process.
3. Multimode transmission with 100G SR4 transceiver
We tested the new UF at 850 nm with a 100G SR4 VCSEL-based transceiver using the same setup as described in [8]. The transceiver is a commercial transceiver with QSFP form factor operating at 25 Gb/s in compliance with IEEE 802.3bm standard. Only one channel was used. In the experiment, because connectorized UFs were not available, a 60 m and a 110 m UF were spliced with a short 50 μm core MMF pigtail cable to connect with the transceiver MTP connector interface. The insertion loss was around 2.3 dB. We expect that when a connectorized UF is connected directly into the MTP connector interface, the additional insertion loss will be similar to the insertion loss between UF and 50 μm MMF. However, this additional insertion loss can be minimized or eliminated by changing the optical coupling design inside the VCSEL transceiver. Figure 4 shows measured BER as a function of received power. It can be seen that at 60 m the system is error free, while at 110 m the BER is 7.1 × 10−10, which is below the FEC threshold of 5 × 10−5 set by the IEEE 802.3bm standard. Note that the 110 m is longer than the 100 m system reach specified for OM4 fiber. For the 60 m length, even with the coupling loss, there is still 5 dB additional loss margin for error free transmission. In Fig. 5, we show the optical eye diagrams for back to back, 60 m and 110 m transmission lengths. The optical eyes are open for each case but they degrade over longer length. We believe the main limiting factor here for the system performance and reach is the modal bandwidth of this specific fiber.
4. Multimode transmission with VCSEL based 40G sWDM transceiver
Next, we tested the UF with another VCSEL-based 40G multimode transceiver that is designed for sWDM [10,11]. The transceiver utilizes four wavelengths as shown in Fig. 6, each operating at 10 Gb/s for an aggregate 40G data rate within a single fiber. Taking both directions into consideration, the transceiver only utilizes 2 fibers, which is in contrast to a more conventional parallel optics such as 40G SR4 using 8 fibers with an expected system reach of 150 m for OM4. It is a non-standard technology, but shares the standardized 40G QSFP electrical interface.
A UF fiber with a length of 150 m was used in the sWDM transmission test. It was spliced to 50 μm core MMF pigtails at both ends. The BER versus received power curves are shown in Fig. 7 for PRBS pattern with 231-1 bit sequence. To obtain these curves, we turned on one wavelength at a time. The measured optical power was properly calibrated for each wavelength channel. For all four channels, the system can get error free at optical power below −10.5 dBm. Moving from the 850 nm channel to the 940 nm channel, the system performance gets slightly better, which correlates with higher modal bandwidth at longer wavelengths for this UF. When all channels were turned on, we were also able to observe error free performance for more than 16 minutes. In Fig. 7, we also show the BER vs. received power for B2B conditions when only a 1 m short UF is used. Compared to B2B condition, the four channels show 1.5-2.7 dB power penalty when 150 m UF is engaged.
5. Single mode transmission with 100G CWDM4 transceiver
We also conducted detailed system testing on the UF for single mode transmission. Different from [8] where we used a 1310 nm CW source modulated by an intensity modulator as the transmitter, in this experiment we used a 100G CWDM4 transceiver. The CWDM4 transceiver [12] is a 100G or 4 × 25G single mode transceiver targeting DC applications with an expected system reach of up to 2 km. It operates at four wavelengths around 1300 nm with the optical spectrum shown in Fig. 8. Similar to the 40G sWDM transceiver, it utilizes duplex LC connectivity involving two fibers while having QSFP28 electric interface. By design, it is used with single mode fibers. In this experiment, we illustrate how a UF can work with CWDM4 transceiver directly.
In the testing, we used four Agilent N4960A serial BERTs to drive the four channels simultaneously at 25.78125Gb/s with 231-1 PRBS pattern. The system testing setup is similar to the 100G SR4 setup [8] except that the fibers were connected to the transceiver with two LC connectors. We also used the same 100G QSFP28 evaluation board to host the transceiver. A variable optical attenuator (VOA) was inserted right before the receiver to control the received power. A 2700 m long UF was used with each end spliced with a short standard single mode fiber pigtail cable due to same reason that connectorized UFs were not available. Because the UF has a MFD similar to that of single mode fiber, we expect that a connectorized UF will have similar performance when connected directly to the transceiver. The link loss was 2.1 dB. Taking fiber attenuation into consideration, we found that the insertion loss is around 0.9 dB. To test the BER vs. received optical power, we turned on optically one channel at a time and obtained the BER for different received optical power by dialing in different levels of attenuation in VOA. The results for all four channels are shown in Fig. 9. For all four channels the system can perform error free at a received optical power below −10.7 dBm. In addition, we compared transmission results from 2 km single mode fiber and found that, other than slight difference in insertion loss, the BER versus received power curves were very similar, which shows the transmission over the UF is single mode transmission in nature despite that the fiber is a MMF. We would note here that since only the fundamental mode is launched, the transmission practically enjoys unlimited bandwidth just like single mode fiber. Therefore the bandwidth concept for multimode fiber does not apply for single mode transmission here. We also noticed in our experiment, through normal splicing between single mode fiber and UF or connector junctions between UFs, there is no noticeable effect of the involvement of higher order mode coupling. We believe this is largely due to our choice of the UF parameters that result in minimum coupling or overlapping between fundamental mode and higher order modes with normal connector offsets. On the other hand, we noticed that when high delta version of UF was used, higher order modes were more easily excited. This may explain why in [9] the single mode transmission is only reported at 100 m for only 10 Gb/s transmission. The testing was done without FEC, which is specified to be used with 100G CWDM4 transceiver. If FEC is used, the system can have more robust performance and longer reach.
6. Conclusions
We have analyzed the dependence of the MFD of LP01 mode on the core diameter of the UF for different core delta values. It is found that higher core delta allows us to have a larger core diameter, while having an MFD approximately matched to that of standard single mode fibers. We also studied the insertion loss, as related to the fiber parameters, both through modeling and experimental measurements. We made a significantly improved UF with 1.2% core delta and 31 μm core diameter, guided by understanding of how MFD of the fundamental LP01 mode varies with the core diameter and core delta. The larger core size and higher NA result in much better coupling of the VCSEL light into UF with much reduced insertion loss. We further explored the use of UF for several major MM and SM transmission applications both at single wavelength and WDM. We successfully demonstrated 110 m, 150 m and 2700 m system reach, respectively with 100G SR4, 40G sWDM and 100G CWDM4 transceivers. In each case the system reach matches or exceeds the full system reach set for the respective technology. The results indicate that the UF may have the full potential to be universally used for both multimode and single mode transmissions in DC.
Acknowledgments
We thank FIT for providing us the 100G SR4 transceiver, Arista Network for providing 40G sWDM transceiver, and Lumentum for providing 100G CWDM4 transceiver for current work.
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