A new high bandwidth bend-insensitive MMF optimized for 1310 nm is designed and characterized. 25 Gb/s transmission over a record 820 m length using a multimode launch from an integrated SiPh transceiver at 1310 nm through the new fiber is demonstrated with a power penalty of 3.4 dB at 10−12 BER. Detailed characteristics of the fiber and transceiver are presented along with BER measurements.
© 2014 Optical Society of America
Interest in using multimode fibers (MMFs) for high data rate short distance communications has increased significantly in recent years due to an increase in the number and size of data centers and enterprise networks, and the limitations of copper at higher line rates. The IEEE 802.3bm task force recently approved a new standard for 4x25 Gb/s 850 nm VCSEL transmission over 100 m of OM4 MMF. The 100 m maximum reach being considered is significantly shorter than the 550 m reach specified for 10 Gb/s transmission. To overcome the reach limitation encountered at 850 nm, single mode fibers have been suggested as the transmission media for the ~500 m interconnect needs in data center applications. However the small fiber core size and the required high alignment precision result in high packaging costs for laser to fiber coupling, leading to higher optical transceiver costs. In another effort to achieve extended reach over multimode fibers at 25 Gb/s, single mode or quasi single mode 850 nm VCSELs with a narrower laser spectral width have been used to reduce the chromatic dispersion related link penalty [1–3]. However, the combination of the high chromatic dispersion of the multimode fiber at 850 nm, the higher fiber attenuation at 850 nm, and the modal dispersion will still impose penalties to the data transmission.
We propose using a MMF optimized for high modal bandwidth at 1310 nm in conjunction with long wavelength sources such as integrated silicon-photonics (SiPh) transceivers . This system retains the advantage of low loss coupling and passive alignment of conventional 850 nm MMF systems. At the same time, the chromatic dispersion and attenuation of the fiber are much lower at 1310 nm. We note that this approach is different from experiments in which a long wavelength light source was launched into only the fundamental mode or a limited number of modes of the MMF with an alignment tolerance similar to single mode systems [5,6]. Therefore, it results in no advantage in coupling or alignment precision. Another experiment was reported on 40 Gb/s transmission over a 400 m MMF using a 1300 nm externally modulated external cavity laser as the light source under a restricted launch condition . While the eye diagram comparison was made with the help of an InGaAs PIN-photodiode, no quantified system performance assessment by bit error rate (BER) testing was performed using a proper receiver. The lateral alignment tolerance was only ± 2 μm, which is much more stringent than typical MMF systems.
In this paper, we demonstrate a record 820 m reach capability at 25 Gb/s transmission using a new MMF optimized for the 1310 nm window and an integrated SiPh transceiver . This reach is over 8x longer than the 100 m reach proposed in 4x25G standards for OM4 fibers at 850 nm and would be able to address future data centers’ critical needs for higher line rate transmission over longer distances.
2. Detailed technical results
2.1 MMF design and characterization
The key passive element in this system is a new MMF optimized for high bandwidth and low coupling losses in the 1310 nm window. The refractive index profile is shown in Fig. 1 and includes a core with a graded index or “alpha” profile,9,10], but also to tune the modal dispersion of the outer mode groups . In our optimized design shown in Fig. 1, the trench is located about 1.5 μm away from edge of the graded index core and has a relative refractive index delta of about −0.4%.
The calculated delays of the mode group centroids of 1310 nm-optimized MMFs with (BI-MMF) and without (Std-MMF) the trench are plotted in Fig. 2(a) versus the mode group number. The maximum number of propagating mode groups in both designs is twelve, however the outer two mode groups in the Std-MMF have high attenuation values, resulting in effectively only ten propagating mode groups. This can be seen in Fig. 2(b) where the calculated average loss of each mode is plotted. The average loss was calculated under a bend diameter of 80 mm to simulate practical cable deployment conditions. For the Std-MMF, the average loss for the lowest ten mode groups is nearly zero, while the mode groups eleven and twelve have an average loss of 27 and 264 dB/m, respectively. The addition of the trench in the BI-MMF greatly improves the guidance of the eleventh mode group, but the twelfth mode group has an average loss of 202 dB/m and will still be highly attenuated.
Figure 2(a) illustrates that the relative delays of the mode groups in the BI-MMF are all small, with a max-to-min spread less than 0.02 ps/m. In contrast, the mode group delays of the Std-MMF have significant curvature and higher max-to-min spread. This behavior is unavoidable because the core α has to be higher than the optimum value to compensate for the anomalous modal dispersion of the ninth and tenth mode groups . As a result, the mode group delays increase monotonically from groups one though eight and then fall steeply as the anomalous dispersion begins to dominate. The modeled overfilled bandwidths of the Std-MMF and BI-MMF designs are approximately 4 and 30 GHz-km, respectively, so an optimized trench is clearly an enabler of high bandwidth at 1310 nm.
MMFs based on the above description were fabricated using the outside vapor deposition process. Figure 3 shows the measured macrobend losses of Std-MMF and BI-MMF samples for two wraps around a 15 mm diameter mandrel. The bend losses were measured using an approximation for an Encircled Flux Launch (EFL), obtained by launching an overfilled source into a 2 m length of Std-MMF with a 1x25 mm diameter mandrel wrap at the midpoint. Under these conditions, the macrobend loss of the BI-MMF is less than 0.2 dB while the Std-MMF exhibits more than 0.7 dB of loss. While MMF deployed in data centers may not be subjected to such tight bend conditions, the introduction of BI-MMF in 2009 enabled the utilization of smaller diameter, more flexible cables and more compact components that enable higher density and use less raw materials.
The measured differential mode delay (DMD) response of a 3200 m length of the 1310 nm -optimized BI-MMF is shown in Fig. 4, with the color scale indicating the optical intensity measured in the time domain as the probe is scanned across the fiber. There are uniform modal delays across the entire fiber core, illustrating insensitivity of the modal bandwidth to the launch condition.
A BI-MMF was cabled into an 8 fiber ribbon of 410 m in length. The transfer function of the BI-MMF was measured using a frequency sweeping method  and a controlled MM launch from a ModCon mode conditioner manufactured by Arden Photonics. The encircled flux of this MM launch is plotted using a solid line shown in Fig. 5, The modal bandwidth of an exemplary 410 m length of the BI-MMF using this launch condition is 28 GHz, which translates to a bandwidth-length product of 11.5 GHz-km, which we believe is a record value for a MMF at 1310 nm using a multimode launch. The high modal bandwidth, along with the low dispersion and attenuation of the fiber at 1310 nm, are the key enablers of long system reach.
2.2 Integrated SiPh transceiver
The SiPh transceiver is an integrated optical module that includes hybrid Si lasers, silicon modulators, photo-detectors, waveguides and high speed electronic circuitries . One 1310 nm transmitter channel operating at 25 Gb/s was used in this experiment. The transmitter (Tx) output has an extinction ratio of 4 dB with an average power of −3 dBm. The encircled flux of the optical output from the SiPh transceiver was measured using a variable aperture technique . The results plotted in Fig. 5 are similar to the MM launch from the mode conditioner.
2.3 Configuration of system testing
System performance of the SiPh module and the 1310 nm-optimized BI-MMF was evaluated in 25 Gb/s transmission experiments over 410 m and 820 m lengths. The schematic layout of the testing setup is shown in Fig. 6. The integrated transceiver module includes the SiPh based transmitter and receiver (Rx), as described above. An Agilent BERT system operating at 25 Gb/s was used for measuring bit error rate (BER). The controller (N4960A-CJ1) controls the pattern generator (N4951A-H32) and error detector (N4952A-E32). The controller also provides a clock signal to the pattern generator. In our testing, no clock recovery was used.
In addition to the BER testing, eye diagrams of the transmitter and the signal output from the fiber under test were characterized. An Agilent digital communication analyzer mainframe (86100D) with multimode optical receiver plugin (86105D) and a precision time base (86107A) was used to condition the time signal provided by the controller. From eye diagrams, signal rise times, extinction ratios, and jitters for the back to back and for different fiber lengths were determined.
2.4 System testing results
Figure 7 shows measured eye diagrams of the 25Gb/s signals obtained in the back-to-back and after 410 m and 820 m of BI-MMF. Table 1 provides the measured signal rise times, jitters and extinction ratios that were extracted from the eye diagrams. In the back to back condition, the eye diagram shown in Fig. 7 illustrates a 20% to 80% signal rise time of only 16.9 ps. There is some degradation of the signal rise time when 410 m and 820 m lengths of BI-MMFs were inserted into the system. Similar degradation is also observed from the RMS jitter and extinction ratio of the signals; however the eye remained open, even after transmission though 820 m of the BI-MMF, as shown in Fig. 7.
BER testing was conducted using a 231-1 PRBS data pattern. First the BER was measured vs. received optical power curve (BER curve) when connecting the SiPh transceiver Tx output to the optical receiver with a short piece of MMF. The received optical power was altered using an additional multimode variable optical attenuator (VOA), which introduced a minimum insertion loss of about 1.5 dB. Then the BER curves were measured with the 410 m and 820 m lengths of BI-MMFs inserted into the system. As shown in Fig. 8, the optical power penalties at 10−12 BER for 410 m and 820 m of BI-MMFs were 1.4 dB and 3.4 dB, respectively. It should be noted that all the above system results were achieved under a true multimode launch condition, which is different from those reported previously [5–7]. We believe the high performance integrated SiPh transceiver and the new BI-MMF optimized for high modal bandwidth at 1310 nm are the key enablers for the transmission of 25 Gb/s signals over this record 820 m length of MMF.
We designed and characterized a new high bandwidth BI-MMF optimized for 1310 nm. We performed 25G b/s system testing using the new BI-MMF and an integrated SiPh transceiver. We demonstrated a record transmission reach of 820 m. To the best of our knowledge, this is a longest reach of 25 Gb/s signals over MMF using a multimode launch as verified by direct BER system testing. The results illustrate that a carefully designed MMF combined with a high performance SiPh transceiver operating at 1310 nm can enable significantly longer system reach at higher data rates than 850 nm MMF systems while maintaining the ease of laser-fiber coupling.
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