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Abstract
We designed and fabricated a graded-index (GI) multicore fiber (MCF) compatible with both standard multimode and single-mode fiber for high density optical interconnect application in large-scale data centers. The proposed fiber supports long-distance multimode transmission at 850 nm as well as quasi-single mode transmission at 1310 nm and 1550 nm. The parameters of the GI-MCF have been optimized to obtain both a small differential mode delay at 850 nm and a small mode field diameter mismatch of less than 0.5 μm with single-mode fiber at 1310 nm and 1550 nm with a negligible inter-core crosstalk. In experiment, we successfully realized the multimode operation over 1 km-long GI-MCF at 850 nm and the quasi-single mode operation over 12.4 km-long GI-MCF at 1310 nm and 1550 nm at a data rate of 7×10-Gb/s. The multi-wavelengths multicore transmission was demonstrated for the first time. The experiment results imply that the proposed GI-MCF satisfies various requirements in such as operating wavelength, accessible distance and interconnect density of large-scale data center, and can effectively reduce the fiber numbers and system complexity.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Driven by the dramatic growth of the internet traffic, capacities of large-scale data centers are rapidly increasing [1]. Different kinds of optical fibers and a variety of multiplexing technologies have been used in large-scale data centers in order to meet the requirement on ever-growing capacity and to keep the system within a manageable power and cost budget at the same time. Generally, parallel links combined with graded-index (GI) multimode fibers (MMFs) and vertical-cavity surface-emitting lasers (VCESLs) are used for short-reach interconnects with a maximum length of hundreds of meters for their advantages in alignment tolerance and overall cost. Single-mode fibers (SMFs) and devices are usually adopted for longer reach applications.
Multicore fibers (MCFs), which are being intensively investigated in recent years as one of the key transmission media for space division multiplexing technology, are considered to be an attractive approach for their distinctive advantages in both improving the interconnect density and reducing the fiber numbers in large-scale data centers [2, 3]. The GI multimode seven-core fiber has been successfully demonstrated which transmitted the signal at a data rate of 70-Gb/s (7×10-Gb/s) over 100 m using VCSEL operating at 850 nm [4]. By using the VCSEL and photodiode (PD) arrays, an aggregate capacity of 120-Gb/s (6×20-Gb/s) with a 100 m-long single strand of six-core fiber has been achieved, which is desirable in data center applications because of its large potential in achieving high interconnect density and reducing system complexity [5]. A single-mode eight-core MCF which supports a data rate of 800 Gb/s/fiber over a 1.1 km-long distance at 1310 nm has also been demonstrated [6]. On the other hand, silicon photonics devices for optical interconnect applications are also being intensively studied because of their advantages in realizing high integration density and mature mass production technology [7, 8]. These devices operating in single-mode at 1310 nm or 1550 nm also use SMFs as transmission media.
The employment of both MMFs and SMFs in large-scale data center results in the needs of deployment and management of different types of fibers and devices which increase the system complexity and limits its future evolution capability. In order to simplify the data center infrastructures, an optimized single-core MMF called universal fiber has been proposed to simultaneously meet the requirement of multimode transmission at 850 nm and single mode transmission at 1310 nm and 1550 nm [9].
In this paper, we report the design and fabrication of a homogeneous GI-MCF with seven hexagonally arrayed cores which is compatible with the standard MMF and SMF for high density optical interconnect application in large-scale data centers. The proposed fiber supports both long-distance multimode transmission at 850 nm as well as quasi-single mode transmission at 1310 nm and 1550 nm. The major parameters of the GI-MCF have been optimized to obtain both a small differential mode delay (DMD) at 850 nm and a small mode field diameter (MFD) mismatch of less than 0.5 μm with the standard SMF at 1310 nm and 1550 nm. The preliminary results for multimode transmission at 850 nm and quasi-single mode transmission at 1310 nm at 10 Gb/s have been demonstrated in [10]. In this paper, we report on detailed design and evaluation of the proposed GI-MCF. The dependence of the inter-core crosstalk on core to core pitch and bending radius and the effect of the fiber design on the bending loss are investigated thoroughly. We also experimentally studied the effect of misalignment on insertion loss and the excitation of higher order modes that may be resulted from the use of fiber connectors. In transmission experiment, in addition to the multimode operation over 1 km-long GI-MCF at 850 nm and the quasi-single mode operation over 12.4 km-long GI-MCF at 1310 nm, multicore quasi-single-mode transmission at a data rate of 7×10 Gb/s with non-return-to-zero (NRZ) signals at 1550 nm were successfully demonstrated for the first time. The experiment results imply that the proposed GI-MCF satisfy various requirements such as operating wavelength, accessible distance and interconnect density of large-scale data centers, and can effectively reduce the fiber numbers and system complexity.
2. Optimized graded-index multicore fiber
As shown in Fig. 1(a), we chose a hexagonally arrayed seven-core structure to achieve both desirable high interconnect density and fabrication simplicity. Here, D is the cladding diameter, R the core radius and Λ the core to core pitch. The refractive index profile of each individual core is exactly same and is shown in Fig. 1(b). A GI profile design was adopted in order to minimize DMD and achieve good compatibility with traditional OM2 and OM3 MMFs, which can be expressed as:
where, Δ is the relative refractive index difference between the core and cladding structure and the parameter α represents the shape of the refractive index profile of the core.
Fig. 1 Schematic of (a) the cross session of GI-MCF, and (b) the index profile of the core structure.
2.1. Fiber design
In order to realize multi-wavelength compatibility, the designed GI-MCF is expected to have a small DMD at 850 nm and have a similar performance compared with the SMF at 1310 nm and 1550 nm. By optimizing α, we may minimize the DMD of fiber and obtain a high modal bandwidth at 850 nm [11]. On the other hand, at 1310 nm and 1550 nm, the quasi-single mode transmission condition requires that there is a small mismatch of MFD of LP01 mode, which mainly depends on R and Δ, between SMF and the designed GI-MCF. Since DMD is not sensitive to R and Δ, we first studied the relationship between DMD and α at 850 nm using a finite element method.
In order to achieve the mode field match condition with SMF and to keep good compatibility with standard MMF, the values of R should be around 12 μm with a Δ of 1%. In the calculation, we fix R and Δ to be 12.0 μm and 1.1%, respectively. With the designed parameters, the individual core of the designed GI-MCF supports eight mode groups. The DMDs of each non-degenerated modes with respect to LP01 mode are shown in Fig. 2(a). It can be observed that the DMD can be minimized by adopting an optimized α value of 2.00 and a maximum DMD of less than 70 ps/km can be achieved for the first sixth mode groups. Since the power is mainly distributed in the lower order modes under the common center splicing condition, the designed GI-MCF is expected to have a smaller DMD in a long transmission distance at 850 nm. To meet the quasi-single mode operation condition, mode field mismatching is an important parameter and the dependence of MFD on R and Δ was investigated as shown in Fig. 2(b). The solid lines represent the R and Δ when the MFD is same as that of the SMF at 1310 nm and 1550 nm, respectively. The dashed lines represent the values of R and Δ when the MFD mismatch between the designed GI-MCF and the SMF is 0.5 μm. The optimal value of R and Δ should be in the optimal region (colored in yellow) between the two solid lines. Besides, the region marked with shadow represents the range of R and Δ in which the mode field mismatch with the SMF is less than 0.5 μm at both 1310 nm and 1550 nm. The dependence of DMD of the LP11 mode with respect to LP01 mode on R and Δ is also investigated and the results are shown in Fig. 2(c). It can be observed that the influence of R and Δ on DMD is small and the region in which the absolute value of the DMD is less than 10 ps/km in Fig. 2(c) overlaps with the optimal region in Fig. 2(b). As a result, the values of α, R and Δ were selected to be 2.00, 12.0 μm and 1.1%, respectively.
Fig. 2 (a) The relationship between the DMD and α, and the dependence of the (b) MFD, and (c) DMD of LP11 mode on R and Δ.
The inter-core crosstalk, which is a critical parameter of the MCF to guarantee the independent operation of individual cores, was calculated using coupled mode and coupled power theories. The mode coupling coefficient κmn and the average power coupling coefficient for homogeneous cores can be expressed as [12]:
where, ω is angular frequency of sinusoidally varying electromagnetic field, ε0 is permittivity of the medium, and uz represents outwardly directed unit vector. E and H represent the electric and magnetic fields, respectively. N2 denotes the refractive index distribution in the entire coupled region, Nn represents the refractive index distribution of core n. Kmn is the average value of κmn and κnm, βm is the propagation constant of core m, and Rb is the bending radius. The inter-core crosstalk in [dB] scale between two adjacent cores of MCF over a length of L can be calculated by: Besides, the center core with six adjacent cores has the worst crosstalk which can be expressed by: where Ncores is the number of nearest neighbor cores. The effective refractive index and the electric and magnetic fields distributions were obtained based on finite element method.We investigated the inter-core crosstalk as a function of Λ at different wavelengths. The bending radius in the calculation was assumed to be 80 mm, which is the same as that of the fiber spool. The results are shown in Fig. 3(a). It should be noted that the LP71 mode is the highest-order mode supported by the designed GI-MCF at 850 nm and has the larger crosstalk than the other modes. It can be observed that the inter-core crosstalk for designed GI-MCF is less than −100 dB for 1 km-long transmission at 850 nm or 10 km-long transmission at 1310 nm and 1550 nm even with a Λ as small as 40 μm which well satisfies the requirement of interconnect applications [13]. We also investigated the dependence of inter-core crosstalk on bending radius with a Λ of 42 μm at different wavelengths. As shown in Fig. 3(b), the inter-core crosstalk increases along with increasing of the bending radius. It is mainly because that a small bending radius results in a larger difference of propagation constant between the adjacent cores for homogeneous MCF.
According to the theoretical study, the bending loss of LP01 mode is negligible and far less than 1 dB/turn with a bending radius of 10 mm at 1310 nm and 1550 nm. At 850 nm, the bending loss of LP71 mode is about 3.5×10−3 dB/km with a bending radius of 30 mm. The core to core pitch and the number of cores are limited by the cladding diameter with the consideration of mechanical strength. The cladding diameter of MCFs should be smaller than around 230 μm to satisfy the limit of failure probability of 10−7 [14]. However, the confinement loss of the outer cores will increase when the cladding thickness is too small. With a Λ of 42 μm and a cladding diameter of 150 μm, the confinement loss of the outer core with the bending radius of 140 mm is sufficient low. With the consideration of both the inter-core crosstalk and the fabrication process of fan-in and fan-out (FIFO) devices, we selected a Λ of 42 μm and a cladding diameter of 150 μm for fabrication.
2.2. Fiber fabrication and characteristics
The fiber was successfully fabricated without obvious degradation on other characteristics using a plasma chemical vapor deposition (PCVD) method. The measured refractive index profile using a S14 optical fiber index profiler is shown in Fig. 4. The major parameters of the fabricated fiber are shown in Table 1, where the attenuations of each core were measured by using a cut-back method. We also measured the inter-core crosstalk over both 1 km-long GI-MCF at 850 nm, and 12.4 km-long GI-MCF at both 1310 nm and 1550 nm. The crosstalk was below the minimum detectable power of the optical powermeter (−110 dBm) as predicted by the theoretical results, which indicates that all the cores can operate simultaneously with negligible performance degradation.
Table 1. Main parameters of the fabricated GI-MCF
In practical applications, the GI-MCF and FIFO may be connected with other devices using connectors instead of fiber splicing. In order to simulate the condition of using connectors, we butt-coupled light from the SMF to the individual cores of the FIFO device made of the bridge fiber using an alignment equipment under the automatic cladding alignment mode. In comparison with the splicing loss of about 0.01 dB for all the three wavelengths, the measured coupling loss is about 0.38 dB, 0.18 dB, and 0.19 dB at 850 nm, 1310 nm, and 1550 nm, respectively. In order to investigate the effect of misalignment on both the coupling loss and the excitation of higher modes when use connectors, the bridge fiber was adopted as the launch fiber at 850 nm to simulate multimode operation, and the SMF was adopted as the launch fiber at 1310 nm and 1550 nm to simulate single-mode operation with a scanning step of 1 μm. It can be observed from Fig. 5 that within a misalignment of ±5 μm, the maximum variation in coupling loss is 0.24 dB, 0.19 dB and 0.53 dB at 850 nm, 1310 nm, and 1550 nm, respectively. Moreover, it can be observed that the minimum coupling loss for the multimode operation at a wavelength of 850 nm is not achieved under the center launch condition. This is may be resulted from the center core defect of the fabrication process [15]. The misalignment of the launch position will also result in the excitation of higher order modes. We injected an optical pulse with a FWHM of 45 ps into one core of the GI-MCF. Figure 6 show the dependence of impulse response on misalignment over a 1 km-long GI-MCF within a range of 0 to 12 μm and a step of 1 μm. It can be observed that the maximum DMD at 850 nm is about 0.1 ps/m calculated using the same method as [16]. On the other hand, the quasi-single mode transmission can be achieved within a misalignment of 2 μm at 1310 nm and 1550 nm.
Fig. 5 Misalignment tolerance of the GI-MCF at the wavelengths of (a) 850 nm, (b) 1310 nm, and (c) 1550 nm.
Fig. 6 The responses of the input pulse over 1-km long GI-MCF at (a) 850nm, (b) 1310 nm, and (c) 1550 nm.
3. Transmission experiment
We carried out transmission experiments using the fabricated GI-MCF at 850 nm, 1310 nm, and 1550 nm. Figure 7 shows the schematic of the experimental setup. A pair of FIFO devices were spliced to the GI-MCF for coupling purposes. The FIFO devices were fabricated by a wet etching technique [17] using a “bridge fiber” as the starting material. The “bridge fiber” has a single core structure and a standard cladding diameter of 125 μm. Both its core diameter and index profile coincide with that of the individual core of the GI-MCF. All the pigtails of the fan-out device were spliced with 2-m standard MMFs to be connected with other components. The insertion loss of a pair of FIFO was measured to be about 1 dB under the single mode launch condition. On the transmitter side, the light from the laser source was modulated at 10 Gb/s in a non-return-to-zero modulation format with a 27-1 pseudo-random binary sequence by a lithium niobate intensity modulator. On the receiver side, the transmitted optical signals were first attenuated by a variable optical attenuator (VOA) and then detected by a PD. The output electrical signals of the PD were amplified by a linear amplifier to meet the sensitivity requirement of the error detector (ED).
Fig. 7 The experiment setup. PPG: pulse pattern generator; Amp: amplifier; IM: intensity modulator; VOA: variable optical attenuator; MCF: multicore fiber; PD: photodiode; ED: error detector.
3.1. Multimode transmission
We first conducted the 10 Gb/s/core transmission at 850 nm over a 1 km-long GI-MCF. A distributed Bragg reflector (DBR) laser operating at 850 nm with an output power of 10 dBm was used as the light source. The measured bit error rate (BER) curves before and after transmission are shown in Fig. 8. Error free transmission was obtained for all the cores, with sensitivities of about −11.3 dBm. It can be observed that the power penalty after transmission in 1 km-long GI-MCF is about 1.8 dB for each core. The eye diagrams before and after transmission are also shown in Fig. 8. The eye diagrams after 1 km-long multimode transmission are clearly open and with negligible distortion, which implies that the designed GI-MCF has a small DMD as well as a large modal bandwidth. Moreover, the performance of different cores shows good consistency with only a power variation of less than 0.2 dB.
3.2. Quasi-single mode transmission
We also conducted the quasi-single mode transmission at 1310 nm and 1550 nm over a 12.4 km-long GI-MCF. Distributed feedback lasers (DFB) with center wavelength of 1310 nm and 1550 nm and a maximum output power of 11 dBm were used as the light source. In order to obtain a quasi-single mode operation condition, we spliced a single-mode fan-in device made by seven SMF pigtails and a single-mode MCF with the same cladding diameter and core-to-core pitch as the designed GI-MCF. The measured BER curves before and after transmission at 1310 nm and 1550 nm are shown in Fig. 9. After 12.4 km-long GI-MCF transmission at 1310 nm, error free transmission has been achieved for all seven cores with a maximum power penalty of less than 2 dB. There is no obvious distortion of the eye diagrams after 12.4 km-long transmission and the performance variations of different cores are negligible. At 1550 nm, the power penalty increases to around 2.7 dB, which is mainly resulted from the relative higher chromatic dispersion.
Fig. 9 The BER performance and eye diagrams over 12.4 km-long GI-MCF at (a) 1310 nm and (b) 1550 nm.
4. Conclusion
We designed and fabricated a GI-MCF compatible with both standard multimode and single-mode fiber for high density optical interconnect application in large-scale data centers. The proposed GI-MCF is capable of reducing the system complexity of the large-scale data centers. The major parameters of the GI-MCF have been optimized to obtain both a small DMD at 850 nm for multimode transmission, and a small MFD mismatch with SMF of less than 0.5 μm at 1310 nm and 1550 nm for quasi-single mode transmission. In experiment, the multimode operation over 1 km-long GI-MCF at 850 nm and the quasi-single mode operation over 12.4 km-long GI-MCF at 1310 nm and 1550 nm were achieved at a data rate of 7×10-Gb/s with NRZ signals. The experiment results imply that the proposed GI-MCF satisfies various demands on operating wavelength, accessible distance, and interconnect density and can effectively reduce the fiber numbers and system complexity of large-scale data center.
Funding
National Natural Science Foundation of China (NSFC) (61775138, 61620106015); Open Projects Foundation of Yangtze Optical Fiber and Cable Joint Stock Limited Company (YOFC) (SKLD1601).
Acknowledgments
The authors would like to thank Prof. Ming Tang of Huazhong University of Science and Technology for providing the FIFO devices.
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