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Abstract
A few-mode transmission system is proposed using 850 nm single-mode VCSEL based transceivers over graded-index single-mode fibers for high data rate data center applications. A graded-index single-mode fiber that supports two mode groups at 850 nm window with a high modal bandwidth of 48.3 GHz·km is realized for the first time. 25 Gb/s transmission experiments using a 850 nm single-mode VCSEL over such fiber demonstrate that the system can support a link distance up to 1.5 km. Additionally, link model analysis provides more insights on how fiber and single-mode VCSEL parameters impact the system performance.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Cloud computing and internet of things are accelerating the expansion of data centers to mega and hyper scales in order to meet the bandwidth demand. In data centers, transmission using 850 nm VCSEL based multimode (MM) transceivers and multimode fibers (MMF) provides a cost-effective and power-efficient solution for short links within a few hundred meters [1]. However, the emerging mega and hyper-scale data centers prefer single-mode (SM) transmission systems, mainly due to the capability of higher system bandwidth and longer system reach. In such a SM system, non-VCSEL based SM transceivers at 1310 nm are typically used together with standard single-mode fibers to cover long transmission distances. However, such SM transceivers are more expensive and less power-efficient than MM VCSEL based transceivers [2]. To this end, a low-cost and power-efficient transmission system is highly desired using 850 nm SM VCSEL based transceivers over standard single-mode fibers to provide a cost-effective high bandwidth and long reach solution.
Significant progress has been made in developing 850 nm SM VCSELs [3,4]. SM operation in VCSELs can be achieved by reducing oxide aperture to support fewer lasing modes and employing an integrated mode filter to suppress higher order modes. Due to its smaller aperture than an MM VCSEL [3], an SM VCSEL can excite a smaller number of modes in an MMF to reduce the modal dispersion effect, and it is also suitable for transmission over fibers with smaller cores such as single-mode fibers. SM VCSELs operated at 25 Gb/s and above were demonstrated using a VCSEL with 5 μm oxide aperture and an integrated mode filter [5], as well as a VCSEL with ~3 μm oxide aperture [6]. In addition, reducing the aperture can also effectively narrow the VCSEL linewidth [3,7]. As a comparison, MM VCSELs with 11 µm aperture exhibit a linewidth of around 1 nm, while the linewidth of SM VCSELs with 3 µm aperture can be as narrow as 0.03 nm. The narrow linewidth of SM VCSELs reduces transmission penalty due to fiber chromatic dispersion, which in turn increases the system reach. At VCSEL level, SM VCSELs can be more power efficient than MM VCSELs as they have lower threshold current [3,4]. Although 1310 nm VCSELs can have similar power efficiency as 850 nm VCSELs, the higher costs associated with the material and process make it less attractive for data center applications.
A previous report has demonstrated 54 Gbit/s OOK transmission using SM VCSEL over 2.2 km MMF with digital equalization at the receiver [8]. However, in real system links with several connectors concatenated, the restricted launch condition cannot be guaranteed. An SM VCSEL with small spot size and low numerical aperture (NA) is suitable for transmission over standard single-mode fibers with low coupling loss using existing coupling technology adopted for SM transceiver. However, a conventional standard single-mode fiber with a step-index profile has very low modal bandwidth, owning to its large modal dispersion at 850 nm. To mitigate the modal dispersion effect, a fiber mode filter is proposed to convert LP11 mode to LP01 mode and was used in a 10 Gb/s error-free transmission experiment over 1 km of SMF-28 fiber [9]. Nevertheless, even with a mode filter, multi-path interference (MPI) can happen in a system with connectors and causes significant system performance degradation.
To this end, a standard single-mode fiber with high modal bandwidth at 850 nm is desired for such a transmission system using SM VCSELs to avoid the complicated post signal process or mode filtering processes. In contrast to the conventional step-index profile, standard single-mode fibers can be designed with a graded-index profile to achieve high bandwidth. In a recent work [10], we have shown a preliminary study of a graded-index single-mode fiber and demonstrated 100 m transmission using a two-mode VCSEL at 850 nm. In this paper, we propose a few-mode transmission system using 850 nm SM VCSEL based transceivers over grade-index single-mode fibers to provide a cost-effective high bandwidth and long reach solution for data center applications. In Section 2, we discuss the design concept of graded-index single-mode fiber to achieve few-mode operation with high modal bandwith and present a graded-index fiber with a high modal bandwidth of 48.3 GHz·km. In Section 3, we demonstrate the system reach capability of the graded-index single-mode fibers with varying levels of modal bandwidth using a 850 nm SM VCSEL operated at 25 Gb/s. The transmission distance reaches over 1.5 km for the fiber with 48.3 GHz·km. To the best of our knowledge, such high modal bandwidth of the graded-index single-mode fiber and such long realized system reach are reported for the first time in such a transmission system. Additional link model analyses are described in Section 4 to provide more insights of SM VCSEL transmission over graded-index single-mode fiber around 850 nm as compared to MM VCSEL transmission over MMF similarly around 850 nm. Finally, the conclusions are presented in Section 5.
2. Graded-index single-mode fiber with high bandwidth at 850 nm
The refractive index profile of a graded-index fiber can be described by the parameter α:
where n0 is the refractive index in the center of the fiber core, a is the fiber core radius, is the relative index difference between the core center and the cladding, with n1 being the refractive index of the cladding. As can be seen, the profile shape is dependent on the parameter α. With the proper choice of fiber core and cladding parameters, a graded-index fiber can be designed with the mode field diameter, cable cutoff wavelength, zero-dispersion wavelength and other properties that are fully compatible with the ITU-T G.652 standard. Such fiber supports two modes in the wavelength window around 850 nm, LP01 and LP11. In addition, by choosing the right α value, the modal bandwidth at 850 nm can be maximized for few-mode transmissions. Figure 1 depicts the modal bandwidth for a graded-index single-mode fiber at wavelength of 850 nm as a function of α, with ∆ = 0.41% and . As shown in Fig. 1, the modal bandwidth can be maximized with , meaning that the two modes have similar propagation speed and the signals launched into the fiber would suffer little broadening due to modal dispersion.To illustrate the bandwidth benefit of graded-index profile designs, we made fiber samples, measured their modal bandwidths and compared with a step-index single-mode fiber. All the graded-index fiber samples and the step-index fiber are fully compliant with ITU-T G.652 standard for single-mode operation at 1310 nm. At 850 nm, they support two mode groups, LP01 and LP11. The modal bandwidth was measured using a frequency domain method [11]. A narrow linewidth continuous-wave light source at 850 nm is intensity modulated with the modulation frequency swept by the network analyzer. With proper calibration, a transfer function S21 can be obtained, from which the fiber modal bandwidth can be extracted. The measured transfer functions, S21 of one graded-index fiber and the step-index fiber when two fiber modes are nearly equally excited are depicted in Fig. 2, as well as the modeled transfer functions. As shown by the blue dots in Fig. 2(a), the transmission of graded-index fiber drops only by 1.1 dBe (electrical dB defined by 20·log operator) within a frequency range up to 22 GHz; the red line shows the fitting curve through modeling. The modeled transfer function is in excellent agreement with the experimental data. Extending the transfer function to higher frequency, we extracted the 3-dBo (optical dB defined by 10·log operator) modal bandwidth to be 48.3 GHz·km. As a comparison, the measured and modeled transfer function of the step-index fiber is shown in Fig. 2(b), yielding a 3-dBo bandwidth of 0.18 GHz·km, which is consistent with previous report [4].
In addition to the above fibers, the modal bandwidth of other two graded-index single-mode fibers were measured at 850 nm, showing modal bandwidths around 2.5 GHz·km and 3.93 GHz·km, respectively. Although the two fibers have lower bandwidth than the previous graded-index fiber, the effective bandwidths are high enough for short reach transmission of a few hundred meters because the chromatic dispersion effect is very low using SM VCSELs. All the four fibers are used in the transmission experiments in Section 3 to help understand the system transmission capability with different fiber modal bandwidth values.
3. SM VCSEL characteristics and transmission experiments around 850 nm
In order to explore the transmission capability of the graded-index single-mode fiber, we conducted 25Gb/s system transmission experiments using a SM VCSEL around 850 nm from V-I-Systems. Driven with 2.7 V DC voltage (~3.5 mA current), the free-space output power of the SM VCSEL is around 0.76 mW (−1.2 dBm), with a NA around 0.25 and an emission spot size approximately 3 µm. Packaged with a V-connector, the VCSEL is mounted on a plate followed by two sequential lenses in a cage system to enable the coupling of light from the VCSEL to the fiber, as shown in the dashed rectangle in Fig. 3 along with system setup, where lens 1 has a focal length of 8 mm and NA of 0.5, lens 2 has a focal length of 15.29 mm and NA of 0.16. The coupling system results in approximately 2.4 dB optical loss. The optical power output from the fiber is −3.6 dBm. The root mean square (RMS) linewidth of the VCSEL was measured to be 0.12 nm centered at 842 nm.
Fig. 3 Schematic of the experimental setup for system transmission test. PG: pattern generator; VOA: variable optical attenuator; ED: error detector.
In the transmission experiments, an Agilent BERT system is used to measure the bit error rate (BER). As shown in Fig. 3, a pattern generator (N4951B) with 5-tap de-emphasis is used to generate 27-1 PRBS non-return-to-zero (NRZ) signal with 1.4 Vpp at 25 Gb/s, which is then combined with the DC voltage through a bias-T (SHF 122C) to drive the VCSEL. After transmission through the single-mode fiber followed by a variable optical attenuator (VOA) to control the receiver power, the optical signal is detected by a Discovery Semiconductor’s 850 nm Lab Buddy optical receiver (R409) and an error detector (N4952A-E32). The optical receiver has a 15 GHz bandwidth around 850 nm. To improve the system performance, the de-emphasis feature of the pattern generator is used in the transmission experiment.
We first measured the transmission performance of the 48.3 GHz·km high modal bandwidth graded-index single-mode fiber. BERs at 25 Gb/s were measured under four configurations: back-to-back (1 m); 500 m; 1000 m; 1500 m graded-index single-mode fiber. BER as a function of received optical power was obtained by tuning the attenuation using the VOA, as shown in Fig. 4. Under the back-to-back condition, the system reached error-free performance with around −7.8 dBm power. No obvious power penalty was observed with the introduction of 500 m graded-index single-mode fiber, and the system can stay error free for four minutes with −5.7 dBm received optical power (without the VOA). In the case of 1000 m fiber, the system showed around 1.6 dB power penalty, while still achieving error-free performance for two minutes with −6.15 dBm received optical power (without the VOA). With 1500 m fiber, the system performance degraded significantly, but could still reach a BER of 2 × 10−6, which is better than the forward error correction threshold of 5 × 10−5 used for short distance optical communications. The eye diagrams for several testing configurations are shown in Figs. 5(a)–5(d). With 500 m and 1000 m fiber, the eye diagrams show slight degradation compared to the back-to-back case; while at 1500 m fiber, a noticeable degradation of the eye diagram is observed with higher noise and smaller eye opening, which is consistent with the measured BER.
Fig. 5 Eye diagrams captured with (a) back to back; (b) 500 m; (c) 1000 m and (d) 1500 m of the graded-index fiber with 48.3 GHz·km bandwidth.
Additionally, we conducted the transmission experiments using the two graded-index single-mode fibers with 2.5 GHz·km and 3.93 GHz·km modal bandwidth at 850 nm. For the fiber with 2.5 GHz·km, the measurements were done with lengths of 100 m, 150 m and 250 m, and the BERs vs. received power curves are shown in Fig. 6. As is shown, the system performs well at 100 m with little power penality compared to the back-to-back situation. With a longer length of 150 m, the performance degrades, showing around 1.7 dB power penalty; and with 250 m fiber, more power penality is observed. The system achieved error-free performance for 7 minutes at 100 m and 150 m by removing the VOA with −4.52 dBm and −4.77 dBm received power, respectively. At 250 m, the system could perform at a BER of 2.45x10−11 but could not be error free even when VOA was removed. The situation is similar for the fiber with 3.93 GHz·km modal bandwidth but at longer transmission distances, as shown in Fig. 7. With 200 m fiber, the system performs well and is essentially the same as the back-to-back situation. The BER performance degrades slightly with 285 m fiber and significantly with 385 m fiber, compared to the back-to-back and the 200 m case. Nevertheless, 5 minutes error-free performance was achieved in both the 285 m and 385 m cases by removing the VOA. It can be found by comparing the three fibers, when the modal bandwidth is increased from 2.5 GHz·km to 3.93 GHz·km and to 48.3 GHz·km, the system reach capability is also increased from around 150 m to around 300 m and 1000 m, while having relatively small power penalties.
As a comparison, the system performance with the step-index single-mode fiber was also tested, which has a modal bandwidth of 0.18 GHz·km, much lower than the graded-index single-mode fibers. Two fiber samples with lengths of 15 m and 35 m were tested, and the measured BER vs. the received optical power is illustrated in Fig. 8. As shown, even with 15 m step-index fiber, a substantial power penalty of greater than 2 dB is observed compared to the back-to-back case. In addition, even with such a short length, the system cannot achieve error-free performance, consistent with the degraded eye diagrams shown in Fig. 9. In the case of 35 m step-index fiber, the eye diagram is very noisy with the eye essentially close.
Fig. 8 BER vs received optical power for the fiber with step-index fiber with 180 MHz·km of bandwidth.
Comparing the results of graded-index fibers with those of the step-index fiber, it can be concluded that graded-index fiber has far superior performance over the step-index fiber, which agrees with the modal bandwidth values obtained in Section 2.
4. Link model analysis of system reach capability of SM VCSEL transmission over graded-index single-mode fiber
In the above, we have conducted detailed experiments to demonstrate the transmission capability of a 25 G/s SM VCSEL over several graded-index single-mode fibers with different levels of modal bandwidth. In order to illustrate the benefits of such a transmission system in terms of both the graded index single-mode fiber and the SM VCSEL properties, we study the system reach capability using IEEE link model [12,13], and compare the results to the MM VCSEL over MMF system.
For MM VCSEL transmission over MMF at 25 Gb/s, IEEE 802.3bm task force defines the transmission capability through a link model. The system reach depends on a set of parameters related to the transmitter, receiver, MMF and connectivity attributes, which are used to specify MM VCSEL transmission over OM3 and OM4 fibers with 70 m and 100 m system reaches, respectively. Since the link model analysis reflects broad capability of many transceiver makers, we use it as a baseline to predict the VCSEL transmission capability using graded-index single-mode fiber. The main parameters we alter to match the SM VCSEL are the RMS linewidth and mode partitioning noise (MPN). The linewidth of a typical SM VCSEL is 0.1 nm, in contrast to 0.6 nm linewidth for a typical MM VCSEL. The reduced linewidth results in significant reduction of the chromatic dispersion penalty. Another major difference between SM VCSEL and MM VCSEL is that SM VCSEL is free of MPN as it only carries one transverse mode. Therefore, we set the MPN parameter, also referred to as ‘MPN k(OMA)’ in the link model, to be 0.3 for MM VCSEL and 0.0 for SM VCSEL in our link model analysis.
With the changes of the two transmitter attributes mentioned above, we can explore a few aspects of the system performance of SM VCSEL transmission over graded-index single-mode fiber at 850 nm. Figure 10 compares the system reach of an SM and an MM VCSEL transmission system as a function of the fiber modal bandwidth. For the SM VCSEL case, the linewidth is set to 0.1 nm and the MPN value is reduced to 0.0; while 0.6 nm linewidth and 0.3 MPN value are used for MM VCSEL transmission over MMF. Note that for the graded-index single mode fiber, the chromatic dispersion value at 850 nm is around −90 ps/(nm·km), slightly better than that for conventional 50 µm core MMF, which also helps reduce the chromatic dispersion penalty. For MM VCSEL over MMF transmission, the system has a reach of 102 m for OM4 level of modal bandwidth at 4.7GHz·km. As the modal bandwith increases further, the system reach extends moderately and saturates at 114 m. The limitation of such system performance primarily results from the chromatic dispersion penalty [14]. In the case of SM VCSEL transmission over graded-index single-mode fiber, the system reach is 182 m with 4700 MHz·km fiber modal bandwidth. As the modal bandwidth increases further, the system reach is 393 m at 48.3 GHz·km, which is the modal bandwidth value of one graded-index signle-mode fiber used in our experiments. Note that the 393 m distance is shorter than what we have demonstrated experimentally, mainly due to the fact that the link model utilizes conservertive parameters for the link power budget. As will be shown later in this section, by increasing the power budget the system reach can be extended significantly. The lower chromatic dispersion penalty and longer system reach for the SM VCSEL case highlights the benefits of the SM VCSEL over the MM VCSEL. In addition, through our experiments, we illustrate that graded-index single-mode fiber can potentially have much higher modal bandwidth than MMF, making it possible for the SM VCSEL over graded-index single-mode fiber system to have much higher overall transmission capability.
Fig. 10 The system reach as a function of modal bandwidth for two cases: SM VCSEL transmission and MM VCSEL transmission.
In Fig. 11, we explore the impact of the laser linewidth of the SM VCSELs for three individual cases of the fibers with modal bandwidth of 48.3 GHz·km, 4.7 GHz·km (OM4 MMF) and 2 GHz·km (OM3 MMF). Here, only the linewidth is altered from the baseline case, and MPN is set to 0.0. It can be found that as the laser linewidth increases from 0.1 nm to 0.6 nm, the chromatic dispersion penalty increasingly limit the system performance. The benefit of SM VCSELs with narrow laser linewidth is shown in all level of modal bandwiths from low to high.
In Fig. 12, we explore another aspect, the effect of transmitter power or power budget on system reach for several modal bandwidth values. The IEEE link model specifies the transceiver, fiber and connectivity attributes conservatively as they are the requirements for all manufactures to meet. In practice, several attributes can perform better. The overall power budget can be higher due to higher transmitter power, lower total connector loss and lower cable attenuation or a combination. Here we use modulated optical transmitter power, referered to as ‘Tx power OMA’ in the link model, as a simplified way to capture the potential system capability by having more power budget than specified. Note that the default ‘Tx power OMA’ is −3 dBm, if the power is increased to −1 dBm, it means the overall power budget is increased by 2 dB. It is observed that higher transmitter power leads to dramatic increase of the system reach, especially in the high modal bandwidth case. For the 48.3 GHz·km fiber, the system reach increases from 393 m to 693 m, with ‘Tx power OMA’ increasing from −3 dBm to −1 dBm. But for lower bandwidth fibers such as with 2 GHz·km and 4.7 GHz·km, the increase is very moderate, indicating the sytem performance is limited by bandwidth limitation or sometimes referred as intersymbol interference (ISI).
The link model analyses show that for widely accepted VCSEL parameters, SM VCSELs can have more robust transmission capability than MM VCSELs due to dramatically reduced laser linewidth and MPN. In addition, the possibility of graded-index single-mode fiber to have much higher modal bandwidth than traditional MMF can also enable longer system reach than MM-VCSEL/MMF ecosystem.
5. Conclusion
We have proposed a few-mode transmission system using 850 nm SM VCSEL based transceivers over graded-index single-mode fibers to provide a cost-effective high bandwidth and long reach solution for data centers and future high speed short distance communications. A graded-index single-mode fiber with high modal bandwidth of 48.3 GHz·km at 850 nm is reported. 25 Gb/s transmission experiments using a 850 nm SM VCSEL over such fiber demonstrate that the system can support a link distance up to 1.5 km without using post data processing, far superior to the step-index standard single-mode fiber. We also experimentally studied the system capability over a few graded-index single-mode fibers with 2.5 GHz·km and 3.93 GHz·km modal bandwidth to achieve incremental system reach of 250 m and 385 m in more practical cases. Additional link model anslyses provide more insights of key parameters such as fiber modal bandwidth, VCSEL linewidth and power on the system reaches using broadly accepted transceiver, fiber and connectivity attributes. Due to the reduced chromatic dispersion penalty from the narrower linewidth of the SM VCSELs and the potential higher modal bandwidth capability of graded-index single-mode fibers, such a system can enable longer system reach than the MM VCSEL/MMF system. The bandwidth capability and transmission performance of graded-index single-mode fiber opens the door to the possibility of using one type of standard single-mode fiber for both long reach 1310 nm SM transmission and short reach 850 nm VCSEL few-mode transmission to reduce costs and power consumption for hyper-scale data centers.
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