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Abstract
We propose a novel multimode fiber (MMF) with a 30 μm-core and fluorine-doped cladding for both high-speed short wavelength division multiplexing (SWDM) and coarse wavelength division multiplexing (CWDM) transmission. By optimizing the core size, the mode field diameter (MFD) mismatch between the proposed fiber and both the standard single-mode fiber (SMF) and MMF is minimized, which enables the quasi-single mode operation in the CWDM window and a compromised coupling loss with standard MMFs and SMFs. By adopting a fluorine-doped silica cladding, the bandwidth dependence on wavelength of the proposed fiber is minimized, which indicates that the modal bandwidth performance at the longer wavelength can be effectively improved without compromising modal bandwidth at 850 nm. The error-free 100 Gb/s (4×25.78 Gb/s) multimode transmission over 250 meter-long fiber is achieved using a commercially available VCSEL-based SWDM transceiver. The applicable distance can be extended to 300 meters when a biterror rate just below the forward error correction (FEC) threshold of 5×10 is acceptable. Besides, the 100 Gb/s error-free single-mode transmission over 10 km-long fiber was also demonstrated with a commercially available directly modulatedlaser (DML)-based CWDM transceiver. The results imply that the proposed MMF may be useful for large-scale data center applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
On the ever-growing demanding of the transmission capacity in large-scale data centers, short wavelength division multiplexing (SWDM) within the operation window of 850-940 nm is considered to be a promising technology to improve the transmission capacity in short reach optical interconnect without significantly increasing the complexity of the system [1]. Multimode fibers (MMFs) in cooperation of VCSEL are commonly employed in short reach optical interconnect as a mature and cost-effective solution. Since the performance of OM3/OM4 MMFs optimized at 850 nm severely degrades at longer wavelength, additional modal dispersion compensation technology has been studied to overcome the short comings of traditional MMF in multi-wavelength applications [2]. In recent years, OM5 fiber was proposed and standardized in TIA-492AAAE, which specifies the effective modal bandwidth (EMB) requirements of 4.7 and 2.47 GHzkm at 850 and 953 nm, respectively [3]. By shifting the EMB peak of OM4 fiber to longer wavelength, the 100 Gb/s SWDM error-free transmission with forward error correction (FEC) was demonstrated over 250 m-long OM5 fiber [4]. However, the bandwidth at 940 nm was improved with the compromise of degraded performance at 850 nm. Due to the dispersion characteristics of fiber core and cladding materials, the bandwidth of traditional MMF strongly depends on the operating wavelength, which makes it difficult to achieve balanced EMB performance at all the wavelengths in SWDM applications. Recently, a wideband MMF employing multiple dopants in the core area was proposed with improved performance competitive with OM4 MMFs in the whole SWDM operation window [5]. However, the multi-dopant technique could inevitably increase the fabrication complexity and cost.
Due to the limitation of modal bandwidth, MMFs combined with VCSEL applied in optical interconnects can only realize a maximum transmission distance of several hundred meters. The coarse wavelength division multiplexing (CWDM) technology employing single-mode fiber (SMF) has been widely applied to establish a large-capacity transmission link with a long reach distance [6]. However, the co-employment of MMF and SMF in large-scale data centers strongly increases the infrastructure complexity and the maintenance cost. In order to simplify the system configuration, MMFs with a reduced core size were proposed to support both multimode and single-mode transmission [7,8]. By reducing the core size of MMF, the 40 Gb/s SWDM transmission as well as 100 Gb/s CWDM transmission were demonstrated over 150 m and 2.7 km, respectively [9]. However, these MMFs were not been specifically optimized for SWDM applications and the bandwidth performance needs to be further improved to achieve larger capacity.
In this paper, a wideband MMF with a 30 μm-core and fluorine-doped cladding was proposed to carry out high-speed SWDM and CWDM transmissions in large-scale data centers. The core size of the proposed fiber is carefully designed to realize the mode field diameter (MFD) matching with the standard SMF at 1310 nm for enabling single-mode CWDM transmission, as well as to minimize the coupling loss between the proposed fiber and both MMF and SMF. Compared with traditional MMFs, the bandwidth of the proposed fiber employing fluorine dopant in the cladding region exhibits significantly smaller wavelength dependency, which implies a desirable performance in the SWDM operation window. Theoretical simulation results indicate that the bandwidth at 940 nm can be improved by 90% while maintaining a similar or even better bandwidth at 850 nm by doping 2 wt.% fluorine in silica cladding. In experiments, we successfully demonstrated error-free 100 Gb/s (4×25.78 Gb/s) SWDM transmission over 250 m-long fiber and the accessible distance can be extended to 300 m with a bit error rate (BER) below the FEC threshold (5×10). Besides, the error free 100 Gb/s transmission over 10 km was achieved using commercially available directly modulated laser (DML) based CWDM transceiver.
2. Optimized wideband multimode fiber
2.1. Fiber design
In short reach optical interconnect applications, the intermodal dispersion of MMF is the main factor that limits the transmission capacity. In order to minimize the intermodal dispersion, we adopted a graded-index (GI) profile design, which can be expressed as:
where r is the radial distance from the center of the core, R is the core radius, n0 is the refractive index of the center of the core, Δ is the relative refractive index difference between the core and cladding and the parameter α represents the shape of the refractive index profile. The intermodal dispersion of MMF is strongly dependent on the value of α. In order to determine the optimum value of α () of MMF within the window of 850-940 nm for SWDM application, we analyzed the wavelength dependency on intermodal dispersion and by using (Wentzel-Kramers-Brillouin) WKB approximation [10]. Assuming that all modes propagate in equal power, the root-mean-square (RMS) of pulse width broadening due to intermodal dispersion over a distance L can be calculated by [11]: whereHere, c refers to the speed of light in a vacuum, , N1 is the material group index and . It can be observed that the intermodal dispersion of MMF strongly depends on α value. In the case that the spectral width of the light source is limited and the chromatic dispersion is negligible in multimode transmission, the can be calculated by [11]:
Fig. 1 (a) The dependency of αopt on wavelength with different cladding dopants. (b) BWth as functions of wavelength with different cladding dopants.
The relationship between and the wavelength depends on the dispersion characteristics of core and cladding materials. Nowadays, germanium-doped silica and pure silica are the dominant materials forming the core and cladding of MMFs, respectively. However, the dispersion characteristics of germanium-doped silica and pure silica significantly diverge along with wavelength in the operation window of SWDM transceivers. As a result, the of traditional MMFs is sensitive to the wavelength and the bandwidth performance will be sharply degraded as the operating wavelength shifts from 850 nm to longer wavelength. It is an essential and promising solution that finding a desirable combination of core and cladding materials which exhibits a lower wavelength dependency on in a wide spectrum region. According to the theoretical analysis, the combination employing fluorine-doped silica as cladding and germanium-doped silica as core could be a practical and cost-effective solution. We investigate the wavelength dependency on with and without fluorine as a dopant in the silica cladding. The dispersion characteristics of different materials are approximated by Sellmeier dispersion formula. In the simulation, Δ was fixed to be 1% at the wavelength of 850 nm by simultaneously adjusting the concentration of germanium doped in the core. The result is shown in Fig. 1(a). It can be observed that the of the MMF with a pure silica cladding strongly depends on wavelength. By employing fluorine-doped silica as the cladding material instead, the dependence of the on the wavelength obviously mitigates. The change of in the window of 850-940 nm for the fiber with 2 wt.% fluorine doped in silica cladding is 0.0145, which is approximatively 57% as small as that for the fiber with pure silica cladding. The theoretical bandwidth (BW) with and without fluorine doped in cladding are also investigated by WKB method as shown in Fig. 1(b). Here, the BW of fibers with a fluorine-doped cladding was set to be as the same as that of fibers with pure silica cladding at 850 nm in order to enlarge the bandwidth at 940 nm as much as possible. It can be observed that the BW of the fluorine-doped fiber is remarkably improved in the operation window of 850-940 nm. With pure silica cladding, the MMF has a 99 nm operation window where the BW is beyond the bandwidth requirement of OM4 MMF of 4.7 GHz⋅km at 850 nm. By employing the fluorine-doped silica cladding with concentration of 2 wt.% and 1 wt.%, the operation windows are dramatically enlarged to be 187 nm and 143 nm, and the BW at 940 nm is improved by 90% and 47%, respectively. The simulation results indicate that the MMF with fluorine-doped silica cladding is capable of supporting a high-speed transmission in the SWDM operation window.
Fig. 2 The dependence of the (a) MFD at 1310 nm, coupling loss at (b) 1310 nm and (c) 850 nm on R and Δ.
In practical applications, the designed MMF is usually connected with a commercial 50-μm core size MMF or SMF at 850 nm or 1310 nm, respectively. In order to realize the single-mode transmission, the MFD of the LP mode is expected to be reduced to a level similar with that of SMF to avoid unwelcome higher order modes excitation and power fluctuation at the output end. On the other hand, the coupling loss from commercial 50-μm core size MMF to the designed fiber increases along with the decrease of MFD mismatch with SMF, which would optimized to support single-mode transmission while keeping a low coupling loss from the 50-μm core size MMF to the designed fiber. Here, we investigated the dependence of MFD of the fundamental modeat 1310 nm on R and Δ using a finite element method. Besides, the coupling losses from 50-μm core size MMF to the designed fiber under over filled launch (OFL) condition at 850 nm and from the designed fiber to SMF under single-mode launch condition at 1310 nm were calculated using a beam propagation method. The results are shown in Fig. 2. It can be observed that the MFD is strongly dependent on both R and Δ. Since it is not practical to adjust Δ to a large value with the consideration of fabrication process, it is necessary to reduce the core size to enable the single-mode operation. On the other hand, it can be found from Fig. 2(c) that the coupling loss from 50-μm core size MMF to the designed fiber mainly depends on the core size with slight variation along with Δ. When the MFD of the designed fiber perfectly matches with that of SMF of 9.3 μm at 1310 nm, the coupling loss from 50-μm MMF to the designed fiber would increase to around 5 dB. Taking the tradeoff between small MFD mismatch at 1310 nm and low coupling loss at 850 nm into consideration, the R and Δ of the fiber were set to be appropriate values as marked in Fig. 2. By choosing a fiber core size of 30 μm, a small MFD mismatch of 1.2 μm and low coupling loss less than 0.1 dB at 1310 nm was achieved with the compromise of 2.5-dB coupling loss between 50-μm core size MMF and the designed fiber at 850 nm.
Fig. 3 (a) The normalized transfer function of the fabricated MMF at different wavelengths. (b) The measured EMB of the fabricated MMF in comparison with the specification of OM5 fiber.
2.2. Fiber fabrication and characteristics
We fabricated the MMF with a 30 μm-core and a fluorine-doped cladding using a plasma chemical vapor deposition (PCVD) method. By using cut-back method, the attenuations of the fiber were measured to be 2.2 and 0.39 dB/km at 850 and 1310 nm, respectively. The modal bandwidth is the dominant factor that limits the transmission capacity in multimode systems. Here, we employed a commercially available VCSEL based SWDM transceiver hosted by an evaluation board as the laser source and measured the EMB offabricated fiber at the wavelengths of 850, 880, 910 and 940 nm, respectively, using a vector network analyzer. A commercially available 50 μm-core MMF was used to connect the transceiver and fabricated fiber and realize an overfilled launch to the fabricated fiber. By employing the overfilled launch technique, the influence of the VCSEL spatial mode performance can be minimized. Figure 3(a) shows the measured transfer function of the fabricated fiber at four operating wavelengths of the SWDM transceiver. The EMB of the fiber is defined as the 3-dB bandwidth of the transfer function. As shown in Fig. 3(b), the EMBs were measured to be 6.5, 6.9, 4.9, and 4.0 GHzkm at the wavelength of 850, 880, 910, and 940 nm, respectively. By employing fluorine-doped cladding, the fabricated MMF well satisfies the minimal bandwidth requirement of 4700 and 2470 MHzkm at 850 and 953 nm, respectively, as suggested by TIA-492 AAAE for the OM5 fiber.
Fig. 5 The measured chromatic dispersion of the MMF with and without fluorine doped in silica cladding.
Differential mode delay (DMD) is another typical parameter to evaluate the modal dispersion properties of MMF. Here, we injected an optical pulse with a full width at half maximum (FWHM) of 45 ps into 4.4 km-long fabricated MMF with a scanning step of 1 μm from the core center to the edge of core. It should be noted that with a longer fiber length, the DMD plots will be more precise but some higher order modes may evanesce and not be observed at the output end. A distributed Bragg reflector (DBR) laser operating at 852 nm with a 5-μm core size single-mode pigtail was applied to realize single-mode launch condition. The measured DMD plots with an equivalent resolution of 0.01 ps/m are shown in Fig. 4. The DMD is determined by calculating the time delay between the fastest mode group and the lowest mode group over per unit length of fiber taking the pulse width into account, as described in [12]. The proposed MMF exhibits a negative DMD and the maximum DMD was measured to be 0.13 ps/m within a mask from 0 to 15 μm.
As for single-mode operation systems, chromatic dispersion is the main factor that results in the pulse broadening and limits the transmission capacity. Once the refractive index profile and the structure of the fiber are determined, the material dispersion plays an important role in chromatic dispersion. In order to investigate the influence of fluorine element doped in the cladding, we measured the chromatic dispersion of two 30 μm-core MMF samples with and without fluorine doped in silica cladding. As shown in Fig. 5, the zero-dispersion wavelength of the MMF with a pure silica cladding is around 1321.9 nm while it shifts to a shorter wavelength of 1315.6 nm with a fluorine-doped silica cladding. It is because that doping fluorine element into the cladding helps to reduce the concentration of germanium doped in the core with a certain Δ and then the zero-dispersion wavelength of fabricated fiber was shifted to be more closed to that of SMF [13].
2.3. Misalignment tolerance with MMF and SMF
In practical applications, the proposed MMF with a 30 μm-core may connects also with standard MMF and SMF using optical connectors. The employment of the optical connectors could lead to additional coupling loss and modal noise. As a result, we investigated the effect of misalignment on the excitation of higher order modes, coupling loss and modal noise under both multimode and single-mode operation condition. We simulated the misalignment by butt-coupling light from MMF and SMF into the fabricated fiber using an alignment equipment.
Fig. 6 The dependence of excess coupling loss and modal noise on coupling misalignment under multimode operation condition.
Under the multimode operation condition, the main influence of misalignment is the excess coupling loss. We employed the standard OM3 fiber to couple the light into and out from the fabricated fiber. Figure 6 shows the dependence of excess coupling loss and modal noise on coupling misalignment. When the light is center launched into the fabricated fiber, the coupling loss is about 2.7 dB, which coincidences with the simulated results. The excess coupling loss is less than 0.2 dB within the misalignment of ± 8 μm, and there is no significant modal noise resulted from the misalignment.
For the quasi-single mode operation condition, the misalignment has larger impact on the transmission performance due to the excitation of higher order modes. We first investigated the pulse response after 4.4 km transmission under different misalignment condition. A SMF was used to launch an optical pulse with a 45 ps FWHM to the fabricated fiber and a high-speed multimode photodetector was employed to detect the pulse response of different modes. It can be observed from Fig. 7(a) that within the misalignment of ± 2 μm, the excited higher order mode is limited and the quasi-single mode operation can be guaranteed. Moreover, the misalignment could result in a larger coupling loss and modal noise which means that the received signal may vary randomly due to the modal interference. As shown in Fig. 7(b), we analyzed the influence of the misaligned launch on the excess coupling loss and modal noise over a 10 km-long fabricated MMF. The coupling loss from a 30 μm-core MMF to SMF at the output end is about 0.3 dB under the center launch condition. Within the misalignment of ± 2 μm, the excess coupling loss is less than 0.3 dB and there is slight degradation of SNR resulted from the modal noise. The results show that the fabricated MMF has an alignment tolerance of ±
2 μm for quasi-single mode operation, which satisfy the requirement of practical applications.
Fig. 7 The dependence of (a) the pulse response, (b) the excess coupling loss and modal noise on coupling misalignment for quasi-single mode operation condition.
3. Transmission experiment
We carried out the multimode and single-mode transmission experiments using commercially available VCSEL based SWDM transceiver and DML based CWDM transceiver, respectively. Figure 8 shows the schematic of the experimental setup. The PRBS 2-1 pattern at 25.78 Gb/s is generated by a pulse pattern generator (PPG) and used to modulate the transceiver through an evaluation board. A variable optical attenuation (VOA) is employed to adjust the optical power received by transceivers. The output electricalsignals were detected by a BER tester (BERT).
Fig. 8 The experimental setup. PPG: pulse pattern generator; FUT: fiber under test; VOA: variable optical attenuator; BERT: bit error rate tester.
Fig. 10 The BER curves with different link distances at (a) 850 nm, (b) 880 nm, (c) 910 nm, and (d) 940 nm.
3.1. Multimode transmission with SWDM tranceiver
We first demonstrated the multimode transmission using a commercially available VCSEL based 100 Gb/s (4×25.78 Gb/s) SWDM transceiver (Finisar FTLC9152RGPL), which is compliant with the QSFP28 MSA and IEEE 802.3bm CAUI-4. The center wavelengths of the transceiver locate at 854, 883, 915, and 944 nm respectively and the optical spectrum is shown in Fig. 9. The linewidths of four VCSEL channels were measured to be 0.74, 0.78, 1.02, and 1.20 nm, respectively. A commercial multimode pigtail with 50-µm core size was used to connect the transceiver with other components. The output powers of the four channels are around 1.8 dBm. The length of the fiber under test (FUT) was set to be 150, 250, and 300 m, respectively. The BER curves of the back-to-back (B2B) link and transmission links are shown in Fig. 10. It should be noted that the performances of different SWDM channels are not exactly same. The fabricated fluorine-doped MMF can support a 150 m-long error free transmission with a power penalty less than 1.5 dB at all wavelengths. For a 250 m-long link, when all channels achieve error free (BER=10) transmission, the power penalty of all the channels varies from 2.3 to 3.2 dB and the received power is in the range of -6.2 and -4.9 dBm. This result indicates that the fabricated MMF exhibits a balanced performance for SWDM transmission. For a 300 m-long link, the BER of transmission link at all the four wavelengths can reach a level below the FEC threshold (5×10) suggested by IEEE 802.3bm standard. The channel of 880 nm shows the lowest power penalty among the four channels while the 910 nm channel achieves the lowest BER of 2.5×10. The SWDM system meets the FEC threshold requirement of (BER=5×10) over 300 m-long fabricated MMF with link margins of 6.9, 6.0, 6.7 and 5.3 dB at the wavelengths of 850, 880, 910, and 940 nm, respectively. The results indicate that the fabricated MMF with a 30 μm-core and fluorine-doped cladding successfully achieved a remarkable EMB in the window of 850-940 nm and can support 100 Gb/s SWDM transmission over 300 m.
3.2. Single-mode transmission with CWDM transceiver
We conducted the single-mode transmission experiment over 10 km-long MMF using a commercially available DML based 100 Gb/s QSFP28 CWDM transceiver in compliant with QSFP28 MSA and ITU-T G.694.2. The four channels operate with a separation of 20 nm between1270 and 1330 nm and the suggested transmission distance is up to 2 km. The optical spectrum of the transceiver with the center wavelengths of 1273, 1293, 1313, and 1333 nm is shown in Fig. 11. The experiment configuration is the same as described in Fig. 8. The transceiver has a port of two single-mode duplex LC connectors and the fabricated MMF was spliced to SMF to connect with other components. The insertion loss which contains the coupling loss and splicing loss from the fabricated MMF to SMF was measured to be 0.3 dB at 1310 nm, which is larger than the simulation results. This is probably because that the splicing loss between MMF and SMF is not negligible due to the large difference of Δ. The BER performances of different channels with and without 10-km transmission are presented in Fig. 12. It can be observed that all the channels achieve error free transmission over 10 km-long fabricated MMF with a received power of less than -5 dBm. The performances of the CWDM transceiver still exhibit a little difference among four channels and the received power varies from -7.5 to -5.5 dBm when the B2B link achieves error free transmission. The 1270 nm channel shows the largest power penalty of about 0.6 dB after 10-km transmission while there is nonoticeable power penalty of other three channels. It is because that the zero-dispersion wavelength of the designed MMF is around 1310 nm and the chromatic dispersion at 1270 nm is larger than other operation wavelengths, which coincides well with the dispersion measurement result in Fig. 5. As a result, the fabricated MMF successfully realized the single-mode transmission with a balanced performance and negligible compromise of optical performance compared to standard SMF in the CWDM window of 1270-1330nm. However, it is necessary to investigate the effect of variations in laser linewidth and output power of different transceivers and alignment tolerance of different connectors on the transmission properties of the proposed fiber in order to realize robust operation with a guaranteed access distance in real application.
4. Conclusion
We report on the design and fabrication of a wideband MMF with 30 μm-core and a fluorine-doped silica cladding for high-speed SWDM and CWDM applications. The core size of the MMF was optimized to realize single-mode operation in the window of CWDM transceivers with the consideration of minimizing the coupling loss from traditional MMF to the proposed fiber. Meanwhile, by employing fluorine-doped silica cladding, the wavelength dependency on is mitigated and modal bandwidth at the longer wavelength is effectively improved without compromising the bandwidth performance at 850 nm. According to the measurement results, the bandwidth of the proposed fiber is remarkably beyond the specification of OM5 fiber. In experiment, we successfully demonstrated the error free 100 Gb/s (4×25.78 Gb/s) SWDM transmission with a commercially available VCSEL based SWDM transceiver over 250 m-long proposed fiber. The accessible distance can be further increased to 300 meters with a BER just below the FEC threshold of (5×10). Besides, the error free 100 Gb/s CWDM transmission has been demonstrated by employing a commercially available DML based CWDM transceiver over 10 km-long proposed fiber. The results imply that the proposed fiber can be a potential candidate as the transmission media in large-scale data center.
Funding
National Natural Science Foundation of China (61775138, 61835006, and 61620106015).
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